Faculty Publications: April, 2018

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Title:
Measurement of radon-induced backgrounds in the NEXT double beta decay experiment
Authors:
NEXT Collaboration; Novella, P.; Palmeiro, B.; Simón, A.;... Guenette, R.;... and 75 coauthors
Publication:
eprint arXiv:1804.00471
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Instrumentation and Detectors, High Energy Physics - Experiment, Nuclear Experiment
Comment:
27 pages, 10 figures, 5 tables
Bibliographic Code:
2018arXiv180400471N

Abstract

The measurement of the internal $^{222}$Rn activity in the NEXT-White detector during the so-called Run-II period with $^{136}$Xe-depleted xenon is discussed in detail, together with its implications for double beta decay searches in NEXT. The activity is measured through the alpha production rate induced in the fiducial volume by $^{222}$Rn and its alpha-emitting progeny. The specific activity is measured to be $(38.1\pm 2.2~\mathrm{(stat.)}\pm 5.9~\mathrm{(syst.)})$~mBq/m$^3$. Radon-induced electrons have also been characterized from the decay of the $^{214}$Bi daughter ions plating out on the cathode of the time projection chamber. From our studies, we conclude that radon-induced backgrounds are sufficiently low to enable a successful NEXT-100 physics program, as the projected rate contribution should not exceed 0.2~counts/yr in the neutrinoless double beta decay sample.

 

Title:
The M-Theory S-Matrix From ABJM: Beyond 11D Supergravity
Authors:
Chester, Shai M.; Pufu, Silviu S.; Yin, Xi
Publication:
eprint arXiv:1804.00949
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
High Energy Physics - Theory
Comment:
45 pages
Bibliographic Code:
2018arXiv180400949C

Abstract

We show that by studying the flat spacetime limit of the Mellin amplitude associated with the four-point correlation function of scalar operators in the stress tensor multiplet of ABJM theory, one can produce the momentum expansion of the M-theory four-graviton S-matrix elements. Using CFT data previously obtained from the supersymmetric localization method, we carry out this procedure explicitly to the second nontrivial order in the momentum expansion, and recover precisely the known $R^4$ contribution to the scattering amplitude of super-gravitons in M-theory in eleven dimensions.

 

Title:
Elliptical polarization for molecular Stark shift compensation in deep optical traps
Authors:
Rosenband, Till; Grimes, David D.; Ni, Kang-Kuen
Publication:
eprint arXiv:1804.01030
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Atomic Physics
Comment:
6 pages, 2 figures
Bibliographic Code:
2018arXiv180401030R

Abstract

In optical dipole traps, the excited rotational states of a molecule may experience a very different light shift than the ground state. For linear $^1\Sigma$ molecules in deep traps, the differential shift can be nulled by choice of elliptical polarization. When one component of the polarization vector is $\pm i\sqrt{2}$ times the orthogonal component, the light shift for a sublevel of excited rotational states asymptotically approaches that of the ground state, as the trap depth increases. In this case, fluctuating trap intensity need not limit coherence between ground and excited rotational states.

 

Title:
The Factorization Problem in Jackiw-Teitelboim Gravity
Authors:
Harlow, Daniel; Jafferis, Daniel
Publication:
eprint arXiv:1804.01081
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
High Energy Physics - Theory, General Relativity and Quantum Cosmology
Comment:
32 pages, 8 figures
Bibliographic Code:
2018arXiv180401081H

Abstract

In this note we study the $1+1$ dimensional Jackiw-Teitelboim gravity in Lorentzian signature, explicitly constructing the gauge-invariant classical phase space and the quantum Hilbert space. We also semiclassically compute the Hartle-Hawking wave function in two different bases of this Hilbert space. We then use these results to illustrate the gravitational version of the factorization problem of AdS/CFT: the Hilbert space of the two-boundary system tensor-factorizes on the CFT side, which appears to be in tension with the existence of gauge constraints in the bulk. In this model the tension is acute: we argue that JT gravity is a sensible quantum theory, based on a well-defined Lorentzian bulk path integral, which has no CFT dual. In bulk language, it has wormholes but it does not have black hole microstates. It does however give some hint as to what could be added to to rectify these issues, and we give an example of how this works using the SYK model. Finally we suggest that similar comments should apply to pure Einstein gravity in $2+1$ dimensions, which we'd then conclude also cannot have a CFT dual, consistent with the results of Maloney and Witten.

 

Title:
21-cm Fluctuations from Charged Dark Matter
Authors:
Muñoz, Julian B.; Dvorkin, Cora; Loeb, Abraham
Publication:
eprint arXiv:1804.01092
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - Instrumentation and Methods for Astrophysics, High Energy Physics - Phenomenology
Comment:
6 pages, 4 figures. Comments welcome
Bibliographic Code:
2018arXiv180401092M

Abstract

The epoch of the formation of the first stars, known as the cosmic dawn, has emerged as a new arena in the search for dark matter. In particular, the first claimed 21-cm detection exhibits a deeper global absorption feature than expected, which could be caused by a low baryonic temperature. This has been interpreted as a sign for electromagnetic interactions between baryons and dark matter. However, in order to remain consistent with the rest of cosmological observations, only part of the dark matter is allowed to be charged, and thus interactive. This hypothesis has a striking prediction: large temperature anisotropies sourced by the velocity-dependent cooling of the baryons. Here we compute, for the first time, the 21-cm fluctuations caused by a charged component of the dark matter, including both the pre- and post-recombination evolution of all fluids. We find that, for the same parameters that can explain the anomalous 21-cm absorption signal, any fraction $f_{\rm dm}$ of charged dark matter larger than 2 % would cause 21-cm fluctuations with pronounced acoustic oscillations, and with an amplitude above any other known effects. These fluctuations would be observable at high significance with interferometers such as LOFAR and HERA, thus providing an additional probe of dark matter at cosmic dawn.

 

Title:
Confinement transition of $\mathbb{Z}_2$ gauge theories coupled to massless fermions: emergent QCD$_3$ and $SO(5)$ symmetry
Authors:
Gazit, Snir; Assaad, Fakher F.; Sachdev, Subir; Vishwanath, Ashvin; Wang, Chong
Publication:
eprint arXiv:1804.01095
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons
Comment:
31 pages, 11 figures
Bibliographic Code:
2018arXiv180401095G

Abstract

We study a model of fermions on the square lattice at half-filling coupled to an Ising gauge theory, that was recently shown in Monte Carlo simulations to exhibit $\mathbb{Z}_2$ topological order and massless Dirac fermion excitations. On tuning parameters, a confining phase with broken symmetry (an antiferromagnet in one choice of Hamiltonian) was also established, and the transition between these phases was found to be continuous, with co-incident onset of symmetry breaking and confinement. While the confinement transition in pure gauge theories is well understood in terms of condensing magnetic flux excitations, the same transition in the presence of gapless fermions is a challenging problem owing to the statistical interactions between fermions and the condensing flux excitations. The conventional scenario then proceeds via a two step transition, involving a symmetry breaking transition leading to gapped fermions followed by confinement. In contrast, here, using large scale quantum Monte Carlo simulations, we provide further evidence for a direct, continuous transition and also find numerical evidence for an enlarged $SO(5)$ symmetry rotating between antiferromagnetism and valence bond solid orders proximate to criticality. Guided by our numerical finding, we develop a field theory description of the direct transition involving an emergent non-abelian ($SU(2)$) gauge theory and a matrix Higgs field. We contrast our results with the conventional Gross--Neveu--Yukawa transition.

 

Title:
Electron drift properties in high pressure gaseous xenon
Authors:
NEXT Collaboration; Simón, A.; Felkai, R.; Martínez-Lema, G.;... Guenette, R.;... and 70 coauthors
Publication:
eprint arXiv:1804.01680
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Instrumentation and Detectors, High Energy Physics - Experiment, Nuclear Experiment
Comment:
22 pages, 17 figures
Bibliographic Code:
2018arXiv180401680N

Abstract

Gaseous time projection chambers (TPC) are a very attractive detector technology for particle tracking. Characterization of both drift velocity and diffusion is of great importance to correctly assess their tracking capabilities. NEXT-White is a High Pressure Xenon gas TPC with electroluminescent amplification, a 1:2 scale model of the future NEXT-100 detector, which will be dedicated to neutrinoless double beta decay searches. NEXT-White has been operating at Canfranc Underground Laboratory (LSC) since December 2016. The drift parameters have been measured using $^{83m}$Kr for a range of reduced drift fields at two different pressure regimes, namely 7.2 bar and 9.1 bar. The results have been compared with Magboltz simulations. Agreement at the 5% level or better has been found for drift velocity, longitudinal diffusion and transverse diffusion.

 

Title:
Calibration of the NEXT-White detector using $^{83m}\mathrm{Kr}$ decays
Authors:
NEXT Collaboration; Martínez-Lema, G.; Hernando Morata, J. A.; Palmeiro, B.;... Guenette, R.;... and 72 coauthors
Publication:
eprint arXiv:1804.01780
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Instrumentation and Detectors, High Energy Physics - Experiment
Comment:
20 pages, 18 figures
Bibliographic Code:
2018arXiv180401780N

Abstract

The NEXT-White (NEW) detector is currently the largest radio-pure high pressure gas xenon time projection chamber with electroluminescent readout in the world. NEXT-White has been operating at Laboratorio Subterr\'aneo de Canfranc (LSC) since October 2016. This paper describes the calibrations performed with $^{83m}\mathrm{Kr}$ decays during a long run taken from March to November 2017 (Run II). Krypton calibrations are used to correct for the finite drift-electron lifetime as well as for the dependence of the measured energy on the event position which is mainly caused by variations in solid angle coverage. After producing calibration maps to correct for both effects we measure an excellent energy resolution for 41.5 keV point-like deposits of (4.55 $\pm$ 0.01) % FWHM in the full chamber and (3.88 $\pm$ 0.04) % FWHM in a restricted fiducial volume. Using naive 1/$\sqrt{E}$ scaling, these values translate into FWHM resolutions of (0.592 $\pm$ 0.001) % FWHM and (0.504 $\pm$ 0.005) % at the $Q_{\beta\beta}$ energy of xenon double beta decay (2458 keV), well within range of our target value of 1%.

 

Title:
The Next White (NEW) detector
Authors:
Monrabal, F.; Gómez-Cadenas, J. J.; Toledo, J. F.; Álvarez, V.; ... Guenette, R.;... and 71 coauthors
Publication:
eprint arXiv:1804.02409
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Instrumentation and Detectors
Bibliographic Code:
2018arXiv180402409M

Abstract

Conceived to host 5 kg of xenon at a pressure of 15 bar in the fiducial volume, the NEXT- White (NEW) apparatus is currently the largest high pressure xenon gas TPC using electroluminescent amplification in the world. It is also a 1:2 scale model of the NEXT-100 detector scheduled to start searching for $\beta\beta 0\nu$ decays in 136Xe in 2019. Both detectors measure the energy of the event using a plane of photomultipliers located behind a transparent cathode. They can also reconstruct the trajectories of charged tracks in the dense gas of the TPC with the help of a plane of silicon photomultipliers located behind the anode. A sophisticated gas system, common to both detectors, allows the high gas purity needed to guarantee a long electron lifetime. NEXT-White has been operating since October 2017 at the Canfranc Underground Laboratory (LSC), in Spain. This paper describes the detector and associated infrastructures.

 

Title:
Ionization Electron Signal Processing in Single Phase LArTPCs II. Data/Simulation Comparison and Performance in MicroBooNE
Authors:
MicroBooNE collaboration; Adams, C.; An, R.; Anthony, J.;... Guenette, R.;... and 166 coauthors
Publication:
eprint arXiv:1804.02583
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Instrumentation and Detectors, High Energy Physics - Experiment, Nuclear Experiment
Comment:
54 pages, 36 figures; the first part of this work can be found at arXiv:1802.08709
Bibliographic Code:
2018arXiv180402583M

Abstract

The single-phase liquid argon time projection chamber (LArTPC) provides a large amount of detailed information in the form of fine-grained drifted ionization charge from particle traces. To fully utilize this information, the deposited charge must be accurately extracted from the raw digitized waveforms via a robust signal processing chain. Enabled by the ultra-low noise levels associated with cryogenic electronics in the MicroBooNE detector, the precise extraction of ionization charge from the induction wire planes in a single-phase LArTPC is qualitatively demonstrated on MicroBooNE data with event display images, and quantitatively demonstrated via waveform-level and track-level metrics. Improved performance of induction plane calorimetry is demonstrated through the agreement of extracted ionization charge measurements across different wire planes for various event topologies. In addition to the comprehensive waveform-level comparison of data and simulation, a calibration of the cryogenic electronics response is presented and solutions to various MicroBooNE-specific TPC issues are discussed. This work presents an important improvement in LArTPC signal processing, the foundation of reconstruction and therefore physics analyses in MicroBooNE.

 

Title:
Fermionic formalism for driven-dissipative multi-level systems
Authors:
Shchadilova, Yulia; Roses, Mor M.; Dalla Torre, Emanuele G.; Lukin, Mikhail D.; Demler, Eugene
Publication:
eprint arXiv:1804.03543
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Condensed Matter - Strongly Correlated Electrons, Quantum Physics
Comment:
14 pages, 7 figures, 3 appendices
Bibliographic Code:
2018arXiv180403543S

Abstract

We present a fermionic description of non-equilibrium few-level systems. Our approach uses the Keldysh path integral formalism and allows us to take into account periodic drives, as well as dissipative channels. The technique is based on the Majorana fermion representation of spin-1/2 models which follows earlier applications in the context of spin and Kondo systems. We demonstrate that our technique provides a convenient and powerful framework for analyzing generalized Dicke models with many few-level atoms coupled to a single cavity. We consider a concrete example of the non-equilibrium dynamics of such a system. We compare our theoretical predictions with recent experiments and point out the features of a lasing transition.

 

Title:
Atomic reconstruction at van der Waals interface in twisted bilayer graphene
Authors:
Yoo, Hyobin; Zhang, Kuan; Engelke, Rebecca; Cazeaux, Paul; Sung, Suk Hyun; Hovden, Robert; Tsen, Adam W.; Taniguchi, Takashi; Watanabe, Kenji; Yi, Gyu-Chul; Kim, Miyoung; Luskin, Mitchell; Tadmor, Ellad B.; Kim, Philip
Publication:
eprint arXiv:1804.03806
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science
Bibliographic Code:
2018arXiv180403806Y

Abstract

Interfaces between crystalline materials have been an essential engineering platform for modern electronics. At the interfaces in two-dimensional (2D) van der Waals (vdW) heterostructures, the twist-tunability offered by vdW crystals allows the construction of a quasiperiodic moir\'e superlattice of tunable length scale, leading to unprecedented access to exotic physical phenomena. However, these interfaces exhibit more intriguing structures than the simple moir\'e pattern. The vdW interaction that favors interlayer commensurability competes against the intralayer elastic lattice distortion, causing interfacial reconstruction with significant modification to the electronic structure. Here we demonstrate engineered atomic-scale reconstruction at the vdW interface between two graphene layers by controlling the twist angle. Employing transmission electron microscopy (TEM), we find local commensuration of Bernal stacked graphene within each domain, separated by incommensurate structural solitons. We observe electronic transport along the triangular network of one-dimensional (1D) topological channels as the electronic bands in the alternating domains are gapped out by a transverse electric field. The atomic scale reconstruction in a twisted vdW interface further enables engineering 2D heterostructures with continuous tunability.

 

Title:
$Z_2$ fractionalized phases of a solvable, disordered, $t$-$J$ model
Authors:
Fu, Wenbo; Gu, Yingfei; Sachdev, Subir; Tarnopolsky, Grigory
Publication:
eprint arXiv:1804.04130
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, High Energy Physics - Theory
Comment:
30 pages, 9 figures
Bibliographic Code:
2018arXiv180404130F

Abstract

We describe the phases of a solvable $t$-$J$ model of electrons with infinite-range, and random, hopping and exchange interactions, similar to those in the Sachdev-Ye-Kitaev models. The electron fractionalizes, as in an `orthogonal metal', into a fermion $f$ which carries both the electron spin and charge, and a boson $\phi$. Both $f$ and $\phi$ carry emergent $\mathbb{Z}_2$ gauge charges. The model has a phase in which the $\phi$ bosons are gapped, and the $f$ fermions are gapless and critical, and so the electron spectral function is gapped. This phase can be considered as a toy model for the underdoped cuprates. The model also has an extended, critical, `quasi-Higgs' phase where both $\phi$ and $f$ are gapless, and the electron operator $\sim f \phi$ has a Fermi liquid-like $1/\tau$ propagator in imaginary time, $\tau$. So while the electron spectral function has a Fermi liquid form, other properties are controlled by $\mathbb{Z}_2$ fractionalization and the anomalous exponents of the $f$ and $\phi$ excitations. This `quasi-Higgs' phase is proposed as a toy model of the overdoped cuprates. We also describe the critical state separating these two phases.

 

Title:
Dirac Electrons in a Dodecagonal Graphene Quasicrystal
Authors:
Ahn, Sung Joon; Moon, Pilkyung; Kim, Tae-Hoon; Kim, Hyun-Woo; Shin, Ha-Chul; Hye Kim, Eun; Cha, Hyun Woo; Kahng, Se-Jong; Kim, Philip; Koshino, Mikito; Son, Young-Woo; Yang, Cheol-Woong; Real Ahn, Joung
Publication:
eprint arXiv:1804.04261
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Comment:
20 pages, 4 figures
Bibliographic Code:
2018arXiv180404261A

Abstract

Quantum states of quasiparticles in solids are dictated by symmetry. Thus, a discovery of unconventional symmetry can provide a new opportunity to reach a novel quantum state. Recently, Dirac and Weyl electrons have been observed in crystals with discrete translational symmetry. Here we experimentally demonstrate Dirac electrons in a two-dimensional quasicrystal without translational symmetry. A dodecagonal quasicrystal was realized by epitaxial growth of twisted bilayer graphene rotated exactly 30 degree. The graphene quasicrystal was grown up to a millimeter scale on SiC(0001) surface while maintaining the single rotation angle over an entire sample and was successfully isolated from a substrate, demonstrating its structural and chemical stability under ambient conditions. Multiple Dirac cone replicated with the 12-fold rotational symmetry were observed in angle resolved photoemission spectra, showing its unique electronic structures with anomalous strong interlayer coupling with quasi-periodicity. Our study provides a new way to explore physical properties of relativistic fermions with controllable quasicrystalline orders.

 

Title:
Building one molecule from a reservoir of two atoms
Authors:
Liu, L. R.; Hood, J. D.; Yu, Y.; Zhang, J. T.; Hutzler, N. R.; Rosenband, T.; Ni, K.-K.
Publication:
eprint arXiv:1804.04752
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Physics - Atomic Physics
Comment:
doi:10.1126/science.aar7797
Bibliographic Code:
2018arXiv180404752L

Abstract

Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combine exactly two atoms into a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms (one sodium and one cesium) in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photoassociation forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.

 

Title:
Solid-state magnetic traps and lattices
Authors:
Knörzer, Johannes; Schuetz, Martin J. A.; Giedke, Geza; Huebl, Hans; Weiler, Mathias; Lukin, Mikhail D.; Cirac, J. Ignacio
Publication:
eprint arXiv:1804.07644
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Quantum Physics, Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
18 pages, 8 figures
Bibliographic Code:
2018arXiv180407644K

Abstract

We propose and analyze magnetic traps and lattices for electrons in semiconductors. We provide a general theoretical framework and show that thermally stable traps can be generated by magnetically driving the particle's internal spin transition, akin to optical dipole traps for ultra-cold atoms. Next we discuss in detail periodic arrays of magnetic traps, i.e. magnetic lattices, as a platform for quantum simulation of exotic Hubbard models, with lattice parameters that can be tuned in real time. Our scheme can be readily implemented in state-of-the-art experiments, as we particularize for two specific setups, one based on a superconducting circuit and another one based on surface acoustic waves.

 

Title:
The Minkowski formula and the quasi-local mass
Authors:
Chen, Po-Ning; Wang, Mu-Tao; Yau, Shing-Tung
Publication:
eprint arXiv:1804.08216
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Mathematics - Differential Geometry
Comment:
15 pages
Bibliographic Code:
2018arXiv180408216C

Abstract

In this article, we estimate the quasi-local energy with reference to the Minkowski spacetime [16,17], the anti-de Sitter spacetime [4], or the Schwarzschild spacetime [3]. In each case, the reference spacetime admits a conformal Killing-Yano 2-form which facilitates the application of the Minkowski formula in [15] to estimate the quasi-local energy. As a consequence of the positive mass theorems in [9,13] and the above estimate, we obtain rigidity theorems which characterize the Minkowski spacetime and the hyperbolic space.

 

Title:
Surface Topological Order and a new 't Hooft Anomaly of Interaction Enabled 3+1D Fermion SPTs
Authors:
Fidkowski, Lukasz; Vishwanath, Ashvin; Metlitski, Max A.
Publication:
eprint arXiv:1804.08628
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons
Comment:
15 pages
Bibliographic Code:
2018arXiv180408628F

Abstract

Symmetry protected topological (SPT) phases are well understood in the context of free fermions and in the context of interacting but essentially bosonic models. Recently it has been realized that intrinsically fermionic SPTs exist which only appear in interacting models. Here we show that the 3+1 dimensional realizations of these phases have surface states characterized by a new 't Hooft anomaly, captured by a $H^3(G, Z_2)$ class. This is encoded in the anomalous action of symmetry on the surface states with topological order, which must necessarily permute the anyons. We discuss in detail an example with symmetry group $G = Z_2 \times Z_4$. Using a network model of the surface we derive a candidate surface topological order given by a $Z_4$ gauge theory. We relate our findings to anomalies valued in $H^3$ with various coefficients introduced previously in both bosonic and fermionic settings, and describe a general framework that unifies these various anomalies.

 

Title:
Opening the black box of neural nets: case studies in stop/top discrimination
Authors:
Roxlo, Thomas; Reece, Matthew
Publication:
eprint arXiv:1804.09278
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
High Energy Physics - Phenomenology, Statistics - Machine Learning
Bibliographic Code:
2018arXiv180409278R

Abstract

We introduce techniques for exploring the functionality of a neural network and extracting simple, human-readable approximations to its performance. By performing gradient ascent on the input space of the network, we are able to produce large populations of artificial events which strongly excite a given classifier. By studying the populations of these events, we then directly produce what are essentially contour maps of the network's classification function. Combined with a suite of tools for identifying the input dimensions deemed most important by the network, we can utilize these maps to efficiently interpret the dominant criteria by which the network makes its classification. As a test case, we study networks trained to discriminate supersymmetric stop production in the dilepton channel from Standard Model backgrounds. In the case of a heavy stop decaying to a light neutralino, we find individual neurons with large mutual information with $m_{T2}^{\ell\ell}$, a human-designed variable for optimizing the analysis. The network selects events with significant missing $p_T$ oriented azimuthally away from both leptons, efficiently rejecting $t\overline{t}$ background. In the case of a light stop with three-body decays to $Wb{\widetilde \chi}$ and little phase space, we find neurons that smoothly interpolate between a similar top-rejection strategy and an ISR-tagging strategy allowing for more missing momentum. We also find that a neural network trained on a stealth stop parameter point learns novel angular correlations.

 

Title:
JUNIPR: a Framework for Unsupervised Machine Learning in Particle Physics
Authors:
Andreassen, Anders; Feige, Ilya; Frye, Christopher; Schwartz, Matthew D.
Publication:
eprint arXiv:1804.09720
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
High Energy Physics - Phenomenology, Statistics - Machine Learning
Comment:
37 pages, 24 figures
Bibliographic Code:
2018arXiv180409720A

Abstract

In applications of machine learning to particle physics, a persistent challenge is how to go beyond discrimination to learn about the underlying physics. To this end, a powerful tool would be a framework for unsupervised learning, where the machine learns the intricate high-dimensional contours of the data upon which it is trained, without reference to pre-established labels. In order to approach such a complex task, an unsupervised network must be structured intelligently, based on a qualitative understanding of the data. In this paper, we scaffold the neural network's architecture around a leading-order model of the physics underlying the data. In addition to making unsupervised learning tractable, this design actually alleviates existing tensions between performance and interpretability. We call the framework JUNIPR: "Jets from UNsupervised Interpretable PRobabilistic models". In this approach, the set of particle momenta composing a jet are clustered into a binary tree that the neural network examines sequentially. Training is unsupervised and unrestricted: the network could decide that the data bears little correspondence to the chosen tree structure. However, when there is a correspondence, the network's output along the tree has a direct physical interpretation. JUNIPR models can perform discrimination tasks, through the statistically optimal likelihood-ratio test, and they permit visualizations of discrimination power at each branching in a jet's tree. Additionally, JUNIPR models provide a probability distribution from which events can be drawn, providing a data-driven Monte Carlo generator. As a third application, JUNIPR models can reweight events from one (e.g. simulated) data set to agree with distributions from another (e.g. experimental) data set.

 

Title:
Density functional theory beyond the Born-Oppenheimer approximation: exact treatment of the ionic zero-point motion
Authors:
Kolesov, Grigory; Kaxiras, Efthimios; Manousakis, Efstratios
Publication:
eprint arXiv:1804.10852
Publication Date:
04/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science, Condensed Matter - Statistical Mechanics
Comment:
22 pages, 5 figures; + few corrections and references
Bibliographic Code:
2018arXiv180410852K

Abstract

We introduce a method to carry out zero-temperature calculations within density functional theory (DFT) but without relying on the Born-Oppenheimer (BO) approximation for the ionic motion. Our approach is based on the finite-temperature many-body path-integral formulation of quantum mechanics by taking the zero-temperature limit and treating the imaginary-time propagation of the electronic variables exactly in the context of DFT. This goes beyond the familiar BO approximation and is limited from being an exact treatment of both electrons and ions only by the approximations involved in the DFT component. We test our method in two simple molecules, H$_2$ and benzene. We demonstrate that the method produces a difference from the results of the BO approximation which is significant for many physical systems, especially those containing light atoms such as hydrogen; in these cases, we find that the fluctuations of the distance from its equilibrium position, due to the zero-point-motion, is comparable to the interatomic distances.

 

Title:
Last electroweak WIMP standing: pseudo-dirac higgsino status and compact stars as future probes
Authors:
Krall, Rebecca; Reece, Matthew
Publication:
Chinese Physics C, Volume 42, Issue 4, article id. 043105 (2018).
Publication Date:
04/2018
Origin:
IOP
DOI:
10.1088/1674-1137/42/4/043105
Bibliographic Code:
2018ChPhC..42d3105K

Abstract

Electroweak WIMPs are under intense scrutiny from direct detection, indirect detection, and collider experiments. Nonetheless the pure (pseudo-Dirac) higgsino, one of the simplest such WIMPs, remains elusive. We present an up-to-date assessment of current experimental constraints on neutralino dark matter. The strongest bound on pure higgsino dark matter currently may arise from AMS-02 measurements of antiprotons, though the interpretation of these results has sizable uncertainty. We discuss whether future astrophysical observations could offer novel ways to test higgsino dark matter, especially in the challenging regime with order MeV mass splitting between the two neutral higgsinos. We find that heating of white dwarfs by annihilation of higgsinos captured via inelastic scattering could be one useful probe, although it will require challenging observations of distant dwarf galaxies or a convincing case to be made for substantial dark matter content in ω Cen, a globular cluster that may be a remnant of a disrupted dwarf galaxy. White dwarfs and neutron stars give a target for astronomical observations that could eventually help to close the last, most difficult corner of parameter space for dark matter with weak interactions.

Supported by NSF (PHY-1415548) and NASA ATP (NNX16AI12G)

 

Title:
Calabi-Yau Volumes and Reflexive Polytopes
Authors:
He, Yang-Hui; Seong, Rak-Kyeong; Yau, Shing-Tung
Publication:
Communications in Mathematical Physics, Online First
Publication Date:
04/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: The Author(s)
DOI:
10.1007/s00220-018-3128-6
Bibliographic Code:
2018CMaPh.tmp.1276H

Abstract

We study various geometrical quantities for Calabi-Yau varieties realized as cones over Gorenstein Fano varieties, obtained as toric varieties from reflexive polytopes in various dimensions. Focus is made on reflexive polytopes up to dimension 4 and the minimized volumes of the Sasaki-Einstein base of the corresponding Calabi-Yau cone are calculated. By doing so, we conjecture new bounds for the Sasaki-Einstein volume with respect to various topological quantities of the corresponding toric varieties. We give interpretations about these volume bounds in the context of associated field theories via the AdS/CFT correspondence.

 

Title:
Search for heavy ZZ resonances in the ℓ ^+ℓ ^-ℓ ^+ℓ ^- and ℓ ^+ℓ ^-ν \bar{ν } final states using proton-proton collisions at √{s}= 13 {TeV} with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2897 coauthors
Publication:
The European Physical Journal C, Volume 78, Issue 4, article id. #293, 34 pp. (EPJC Homepage)
Publication Date:
04/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-018-5686-3
Bibliographic Code:
2018EPJC...78..293A

Abstract

A search for heavy resonances decaying into a pair of Z bosons leading to ℓ ^+ℓ ^-ℓ ^+ℓ ^- and ℓ ^+ℓ ^-ν \bar{ν } final states, where ℓ stands for either an electron or a muon, is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 {TeV} corresponding to an integrated luminosity of 36.1 fb^{-1} collected with the ATLAS detector during 2015 and 2016 at the Large Hadron Collider. Different mass ranges for the hypothetical resonances are considered, depending on the final state and model. The different ranges span between 200 and 2000 {GeV}. The results are interpreted as upper limits on the production cross section of a spin-0 or spin-2 resonance. The upper limits for the spin-0 resonance are translated to exclusion contours in the context of Type-I and Type-II two-Higgs-doublet models, while those for the spin-2 resonance are used to constrain the Randall-Sundrum model with an extra dimension giving rise to spin-2 graviton excitations.

 

Title:
The Weak Gravity Conjecture and emergence from an ultraviolet cutoff
Authors:
Heidenreich, Ben; Reece, Matthew; Rudelius, Tom
Publication:
The European Physical Journal C, Volume 78, Issue 4, article id. #337, 33 pp. (EPJC Homepage)
Publication Date:
04/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: The Author(s)
DOI:
10.1140/epjc/s10052-018-5811-3
Bibliographic Code:
2018EPJC...78..337H

Abstract

We study ultraviolet cutoffs associated with the Weak Gravity Conjecture (WGC) and Sublattice Weak Gravity Conjecture (sLWGC). There is a magnetic WGC cutoff at the energy scale e G_N^{-1/2} with an associated sLWGC tower of charged particles. A more fundamental cutoff is the scale at which gravity becomes strong and field theory breaks down entirely. By clarifying the nature of the sLWGC for nonabelian gauge groups we derive a parametric upper bound on this strong gravity scale for arbitrary gauge theories. Intriguingly, we show that in theories approximately saturating the sLWGC, the scales at which loop corrections from the tower of charged particles to the gauge boson and graviton propagators become important are parametrically identical. This suggests a picture in which gauge fields emerge from the quantum gravity scale by integrating out a tower of charged matter fields. We derive a converse statement: if a gauge theory becomes strongly coupled at or below the quantum gravity scale, the WGC follows. We sketch some phenomenological consequences of the UV cutoffs we derive.

 

Title:
Improved retention of phosphorus donors in germanium using a non-amorphizing fluorine co-implantation technique
Authors:
Monmeyran, Corentin; Crowe, Iain F.; Gwilliam, Russell M.; Heidelberger, Christopher; Napolitani, Enrico; Pastor, David; Gandhi, Hemi H.; Mazur, Eric; Michel, Jürgen; Agarwal, Anuradha M.; Kimerling, Lionel C.
Publication:
Journal of Applied Physics, Volume 123, Issue 16, id.161524 (JAP Homepage)
Publication Date:
04/2018
Origin:
AIP
Abstract Copyright:
2017: Author(s)
DOI:
10.1063/1.4999210
Bibliographic Code:
2018JAP...123p1524M

Abstract

Co-doping with fluorine is a potentially promising method for defect passivation to increase the donor electrical activation in highly doped n-type germanium. However, regular high dose donor-fluorine co-implants, followed by conventional thermal treatment of the germanium, typically result in a dramatic loss of the fluorine, as a result of the extremely large diffusivity at elevated temperatures, partly mediated by the solid phase epitaxial regrowth. To circumvent this problem, we propose and experimentally demonstrate two non-amorphizing co-implantation methods; one involving consecutive, low dose fluorine implants, intertwined with rapid thermal annealing and the second, involving heating of the target wafer during implantation. Our study confirms that the fluorine solubility in germanium is defect-mediated and we reveal the extent to which both of these strategies can be effective in retaining large fractions of both the implanted fluorine and, critically, phosphorus donors.

 

Title:
High level active n+ doping of strained germanium through co-implantation and nanosecond pulsed laser melting
Authors:
Pastor, David; Gandhi, Hemi H.; Monmeyran, Corentin P.; Akey, Austin J.; Milazzo, Ruggero; Cai, Yan; Napolitani, Enrico; Gwilliam, Russell M.; Crowe, Iain F.; Michel, Jurgen; Kimerling, L. C.; Agarwal, Anuradha; Mazur, Eric; Aziz, Michael J.
Publication:
Journal of Applied Physics, Volume 123, Issue 16, id.165101 (JAP Homepage)
Publication Date:
04/2018
Origin:
AIP
Abstract Copyright:
2018: Author(s)
DOI:
10.1063/1.5012512
Bibliographic Code:
2018JAP...123p5101P

Abstract

Obtaining high level active n+ carrier concentrations in germanium (Ge) has been a significant challenge for further development of Ge devices. By ion implanting phosphorus (P) and fluorine (F) into Ge and restoring crystallinity using Nd:YAG nanosecond pulsed laser melting (PLM), we demonstrate 1020 cm-3 n+ carrier concentration in tensile-strained epitaxial germanium-on-silicon. Scanning electron microscopy shows that after laser treatment, samples implanted with P have an ablated surface, whereas P + F co-implanted samples have good crystallinity and a smooth surface topography. We characterize P and F concentration depth profiles using secondary ion mass spectrometry and spreading resistance profiling. The peak carrier concentration, 1020 cm-3 at 80 nm below the surface, coincides with the peak F concentration, illustrating the key role of F in increasing donor activation. Cross-sectional transmission electron microscopy of the co-implanted sample shows that the Ge epilayer region damaged during implantation is a single crystal after PLM. High-resolution X-ray diffraction and Raman spectroscopy measurements both indicate that the as-grown epitaxial layer strain is preserved after PLM. These results demonstrate that co-implantation and PLM can achieve the combination of n+ carrier concentration and strain in Ge epilayers necessary for next-generation, high-performance Ge-on-Si devices.

 

Title:
Exploring cosmic origins with CORE: Survey requirements and mission design
Authors:
Delabrouille, J.; de Bernardis, P.; Bouchet, F. R.;... Dvorkin, C.;... ; and 199 coauthors
Publication:
Journal of Cosmology and Astroparticle Physics, Issue 04, article id. 014 (2018). (JCAP Homepage)
Publication Date:
04/2018
Origin:
IOP
DOI:
10.1088/1475-7516/2018/04/014
Bibliographic Code:
2018JCAP...04..014D

Abstract

Future observations of cosmic microwave background (CMB) polarisation have the potential to answer some of the most fundamental questions of modern physics and cosmology, including: what physical process gave birth to the Universe we see today? What are the dark matter and dark energy that seem to constitute 95% of the energy density of the Universe? Do we need extensions to the standard model of particle physics and fundamental interactions? Is the ΛCDM cosmological scenario correct, or are we missing an essential piece of the puzzle? In this paper, we list the requirements for a future CMB polarisation survey addressing these scientific objectives, and discuss the design drivers of the COREmfive space mission proposed to ESA in answer to the "M5" call for a medium-sized mission. The rationale and options, and the methodologies used to assess the mission's performance, are of interest to other future CMB mission design studies. COREmfive has 19 frequency channels, distributed over a broad frequency range, spanning the 60–600 GHz interval, to control astrophysical foreground emission. The angular resolution ranges from 2' to 18', and the aggregate CMB sensitivity is about 2 μKṡarcmin. The observations are made with a single integrated focal-plane instrument, consisting of an array of 2100 cryogenically-cooled, linearly-polarised detectors at the focus of a 1.2-m aperture cross-Dragone telescope. The mission is designed to minimise all sources of systematic effects, which must be controlled so that no more than 10-4 of the intensity leaks into polarisation maps, and no more than about 1% of E-type polarisation leaks into B-type modes. COREmfive observes the sky from a large Lissajous orbit around the Sun-Earth L2 point on an orbit that offers stable observing conditions and avoids contamination from sidelobe pick-up of stray radiation originating from the Sun, Earth, and Moon. The entire sky is observed repeatedly during four years of continuous scanning, with a combination of three rotations of the spacecraft over different timescales. With about 50% of the sky covered every few days, this scan strategy provides the mitigation of systematic effects and the internal redundancy that are needed to convincingly extract the primordial B-mode signal on large angular scales, and check with adequate sensitivity the consistency of the observations in several independent data subsets. COREmfive is designed as a "near-ultimate" CMB polarisation mission which, for optimal complementarity with ground-based observations, will perform the observations that are known to be essential to CMB polarisation science and cannot be obtained by any other means than a dedicated space mission. It will provide well-characterised, highly-redundant multi-frequency observations of polarisation at all the scales where foreground emission and cosmic variance dominate the final uncertainty for obtaining precision CMB science, as well as 2' angular resolution maps of high-frequency foreground emission in the 300–600 GHz frequency range, essential for complementarity with future ground-based observations with large telescopes that can observe the CMB with the same beamsize.

 

Title:
Exploring cosmic origins with CORE: Cosmological parameters
Authors:
Di Valentino, E.; Brinckmann, T.; Gerbino, M.;... Dvorkin, C.;... and 124 coauthors
Publication:
Journal of Cosmology and Astroparticle Physics, Issue 04, article id. 017 (2018). (JCAP Homepage)
Publication Date:
04/2018
Origin:
IOP
DOI:
10.1088/1475-7516/2018/04/017
Bibliographic Code:
2018JCAP...04..017D

Abstract

We forecast the main cosmological parameter constraints achievable with the CORE space mission which is dedicated to mapping the polarisation of the Cosmic Microwave Background (CMB). CORE was recently submitted in response to ESA's fifth call for medium-sized mission proposals (M5). Here we report the results from our pre-submission study of the impact of various instrumental options, in particular the telescope size and sensitivity level, and review the great, transformative potential of the mission as proposed. Specifically, we assess the impact on a broad range of fundamental parameters of our Universe as a function of the expected CMB characteristics, with other papers in the series focusing on controlling astrophysical and instrumental residual systematics. In this paper, we assume that only a few central CORE frequency channels are usable for our purpose, all others being devoted to the cleaning of astrophysical contaminants. On the theoretical side, we assume ΛCDM as our general framework and quantify the improvement provided by CORE over the current constraints from the Planck 2015 release. We also study the joint sensitivity of CORE and of future Baryon Acoustic Oscillation and Large Scale Structure experiments like DESI and Euclid. Specific constraints on the physics of inflation are presented in another paper of the series. In addition to the six parameters of the base ΛCDM, which describe the matter content of a spatially flat universe with adiabatic and scalar primordial fluctuations from inflation, we derive the precision achievable on parameters like those describing curvature, neutrino physics, extra light relics, primordial helium abundance, dark matter annihilation, recombination physics, variation of fundamental constants, dark energy, modified gravity, reionization and cosmic birefringence. In addition to assessing the improvement on the precision of individual parameters, we also forecast the post-CORE overall reduction of the allowed parameter space with figures of merit for various models increasing by as much as ~ 107 as compared to Planck 2015, and 105 with respect to Planck 2015 + future BAO measurements.

 

itle:
On geometric classification of 5d SCFTs
Authors:
Jefferson, Patrick; Katz, Sheldon; Kim, Hee-Cheol; Vafa, Cumrun
Publication:
Journal of High Energy Physics, Volume 2018, Issue 4, article id. #103, 75 pp.
Publication Date:
04/2018
Origin:
SPRINGER
Keywords:
Conformal Field Models in String Theory, Differential and Algebraic Geometry, Field Theories in Higher Dimensions, Supersymmetry and Duality
Abstract Copyright:
(c) 2018: The Author(s)
DOI:
10.1007/JHEP04(2018)103
Bibliographic Code:
2018JHEP...04..103J

Abstract

We formulate geometric conditions necessary for engineering 5d superconformal field theories (SCFTs) via M-theory compactification on a local Calabi-Yau 3-fold. Extending the classification of the rank 1 cases, which are realized geometrically as shrinking del Pezzo surfaces embedded in a 3-fold, we propose an exhaustive classification of local 3-folds engineering rank 2 SCFTs in 5d. This systematic classification confirms that all rank 2 SCFTs predicted using gauge theoretic arguments can be realized as consistent theories, with the exception of one family which is shown to be non-perturbatively inconsistent and thereby ruled out by geometric considerations. We find that all rank 2 SCFTs descend from 6d (1,0) SCFTs compactified on a circle possibly twisted with an automorphism together with holonomies for global symmetries around the Kaluza-Klein circle. These results support our conjecture that every 5d SCFT can be obtained from the circle compactification of some parent 6d (1,0) SCFT.

 

Title:
Observational signature of high spin at the Event Horizon Telescope
Authors:
Gralla, Samuel E.; Lupsasca, Alexandru; Strominger, Andrew
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 475, Issue 3, p.3829-3853 (MNRAS Homepage)
Publication Date:
04/2018
Origin:
OUP
Astronomy Keywords:
black hole physics, gravitational lensing: strong
Abstract Copyright:
2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
DOI:
10.1093/mnras/sty039
Bibliographic Code:
2018MNRAS.475.3829G

Abstract

We analytically compute the observational appearance of an isotropically emitting point source on a circular, equatorial orbit near the horizon of a rapidly spinning black hole. The primary image moves on a vertical line segment, in contrast to the primarily horizontal motion of the spinless case. Secondary images, also on the vertical line, display a rich caustic structure. If detected, this unique signature could serve as a `smoking gun' for a high spin black hole in nature.

 

Title:
A quantum dipolar spin liquid
Authors:
Yao, N. Y.; Zaletel, M. P.; Stamper-Kurn, D. M.; Vishwanath, A.
Publication:
Nature Physics, Volume 14, Issue 4, p.405-410
Publication Date:
04/2018
Origin:
NATURE
Abstract Copyright:
2018: The Author(s)
DOI:
10.1038/s41567-017-0030-7
Bibliographic Code:
2018NatPh..14..405Y

Abstract

Quantum spin liquids are a class of magnetic ground states reliant on non-local entanglement. Motivated by recent advances in the control of ultracold polar molecules and the development of dipolar quantum materials, we show that dipolar interactions between S = 1/2 moments stabilize spin liquids on the triangular and kagome lattices. In the latter case, the moments spontaneously break time-reversal, forming a chiral spin liquid with robust edge modes and emergent semions. We propose a simple route toward synthesizing a dipolar Heisenberg antiferromagnet from lattice-trapped polar molecules using only a single pair of rotational states and a constant electric field.

 

Title:
Unconventional superconductivity in magic-angle graphene superlattices
Authors:
Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
Publication:
Nature, Volume 556, Issue 7699, pp. 43-50 (2018). (Nature Homepage)
Publication Date:
04/2018
Origin:
NATURE
Abstract Copyright:
(c) 2018: Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
DOI:
10.1038/nature26160
Bibliographic Code:
2018Natur.556...43C

Abstract

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

 

Title:
Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
Authors:
Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Ashoori, Ray C.; Jarillo-Herrero, Pablo
Publication:
Nature, Volume 556, Issue 7699, pp. 80-84 (2018). (Nature Homepage)
Publication Date:
04/2018
Origin:
NATURE
Abstract Copyright:
(c) 2018: Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
DOI:
10.1038/nature26154
Bibliographic Code:
2018Natur.556...80C

Abstract

A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the ‘twist’ angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moiré pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically, when this angle is close to the ‘magic’ angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moiré pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.

 

Title:
A search for resonances decaying into a Higgs boson and a new particle X in the XH → qqbb final state with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2876 coauthors
Publication:
Physics Letters B, Volume 779, p. 24-45.
Publication Date:
04/2018
Origin:
ELSEVIER
Abstract Copyright:
(c) 2018 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2018.01.042
Bibliographic Code:
2018PhLB..779...24A

Abstract

arch for heavy resonances decaying into a Higgs boson (H) and a new particle (X) is reported, utilizing 36.1 fb-1 of proton-proton collision data at √{ s } = 13TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The particle X is assumed to decay to a pair of light quarks, and the fully hadronic final state XH → qqbar‧ b b bar is analysed. The search considers the regime of high XH resonance masses, where the X and H bosons are both highly Lorentz-boosted and are each reconstructed using a single jet with large radius parameter. A two-dimensional phase space of XH mass versus X mass is scanned for evidence of a signal, over a range of XH resonance mass values between 1 TeV and 4 TeV, and for X particles with masses from 50 GeV to 1000 GeV. All search results are consistent with the expectations for the background due to Standard Model processes, and 95% CL upper limits are set, as a function of XH and X masses, on the production cross-section of the XH → qqbar‧ b b bar resonance.

 

Title:
Proposal for the detection of magnetic monopoles in spin ice via nanoscale magnetometry
Authors:
Kirschner, Franziska K. K.; Flicker, Felix; Yacoby, Amir; Yao, Norman Y.; Blundell, Stephen J.
Publication:
Physical Review B, Volume 97, Issue 14, id.140402 (PhRvB Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.140402
Bibliographic Code:
2018PhRvB..97n0402K

Abstract

We present a proposal for applying nanoscale magnetometry to the search for magnetic monopoles in the spin ice materials holmium and dysprosium titanate. Employing Monte Carlo simulations of the dipolar spin ice model, we find that when cooled to below 1.5 K these materials exhibit a sufficiently low monopole density to enable the direct observation of magnetic fields from individual monopoles. At these temperatures we demonstrate that noise spectroscopy can capture the intrinsic fluctuations associated with monopole dynamics, allowing one to isolate the qualitative effects associated with both the Coulomb interaction between monopoles and the topological constraints implied by Dirac strings. We describe in detail three different nanoscale magnetometry platforms (muon spin rotation, nitrogen-vacancy defects, and nanoscale arrays of superconducting quantum interference devices) that can be used to detect monopoles in these experiments and analyze the advantages of each.

 

Title:
Exploring the anisotropic Kondo model in and out of equilibrium with alkaline-earth atoms
Authors:
Kanász-Nagy, Márton; Ashida, Yuto; Shi, Tao; Moca, Cǎtǎlin Paşcu; Ikeda, Tatsuhiko N.; Fölling, Simon; Cirac, J. Ignacio; Zaránd, Gergely; Demler, Eugene A.
Publication:
Physical Review B, Volume 97, Issue 15, id.155156 (PhRvB Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.155156
Bibliographic Code:
2018PhRvB..97o5156K

Abstract

We propose a scheme to realize the Kondo model with tunable anisotropy using alkaline-earth atoms in an optical lattice. The new feature of our setup is Floquet engineering of interactions using time-dependent Zeeman shifts, that can be realized either using state-dependent optical Stark shifts or magnetic fields. The properties of the resulting Kondo model strongly depend on the anisotropy of the ferromagnetic interactions. In particular, easy-plane couplings give rise to Kondo singlet formation even though microscopic interactions are all ferromagnetic. We discuss both equilibrium and dynamical properties of the system that can be measured with ultracold atoms, including the impurity spin susceptibility, the impurity spin relaxation rate, as well as the equilibrium and dynamical spin correlations between the impurity and the ferromagnetic bath atoms. We analyze the nonequilibrium time evolution of the system using a variational non-Gaussian approach, which allows us to explore coherent dynamics over both short and long timescales, as set by the bandwidth and the Kondo singlet formation, respectively. In the quench-type experiments, when the Kondo interaction is suddenly switched on, we find that real-time dynamics shows crossovers reminiscent of poor man's renormalization group flow used to describe equilibrium systems. For bare easy-plane ferromagnetic couplings, this allows us to follow the formation of the Kondo screening cloud as the dynamics crosses over from ferromagnetic to antiferromagnetic behavior. On the other side of the phase diagram, our scheme makes it possible to measure quantum corrections to the well-known Korringa law describing the temperature dependence of the impurity spin relaxation rate. Theoretical results discussed in our paper can be measured using currently available experimental techniques.

 

Title:
Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2850 coauthors
Publication:
Physical Review D, Volume 97, Issue 7, id.072003 (PhRvD Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
2018: CERN
DOI:
10.1103/PhysRevD.97.072003
Bibliographic Code:
2018PhRvD..97g2003A

Abstract

A search for the associated production of the Higgs boson with a top quark pair (t t ¯H ) is reported. The search is performed in multilepton final states using a data set corresponding to an integrated luminosity of 36.1 fb-1 of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy √{s }=13 TeV at the Large Hadron Collider. Higgs boson decays to W W*, τ τ , and Z Z* are targeted. Seven final states, categorized by the number and flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the t t ¯H production cross section is σ (t t ¯H )=79 0-210+230 fb , in agreement with the Standard Model prediction of 50 7-50+35 fb . The combination of this result with other t t ¯H searches from the ATLAS experiment using the Higgs boson decay modes to b b ¯, γ γ and Z Z*→4 ℓ, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the t t ¯H production mode.

 

Title:
Search for the standard model Higgs boson produced in association with top quarks and decaying into a b b macr pair in p p collisions at √{s }=13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2844 coauthors
Publication:
Physical Review D, Volume 97, Issue 7, id.072016 (PhRvD Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
2018: CERN
DOI:
10.1103/PhysRevD.97.072016
Bibliographic Code:
2018PhRvD..97g2016A

Abstract

A search for the standard model Higgs boson produced in association with a top-quark pair, t t ¯H , is presented. The analysis uses 36.1 fb-1 of p p collision data at √{s }=13 TeV collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. The search targets the H →b b ¯ decay mode. The selected events contain either one or two electrons or muons from the top-quark decays, and are then categorized according to the number of jets and how likely these are to contain b -hadrons. Multivariate techniques are used to discriminate between signal and background events, the latter being dominated by t t ¯+jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured t t ¯H signal cross-section to the standard model expectation is found to be μ =0.8 4-0.61+0.64. A value of μ greater than 2.0 is excluded at 95% confidence level (C.L.) while the expected upper limit is μ <1.2 in the absence of a t t ¯H signal.

 

Title:
New light Higgs boson and short-baseline neutrino anomalies
Authors:
Asaadi, J.; Church, E.; Guenette, R.; Jones, B. J. P.; Szelc, A. M.
Publication:
Physical Review D, Volume 97, Issue 7, id.075021 (PhRvD Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
2018: authors
DOI:
10.1103/PhysRevD.97.075021
Bibliographic Code:
2018PhRvD..97g5021A

Abstract

The low-energy excesses observed by the MiniBooNE experiment have, to date, defied a convincing explanation under the standard model even with accommodation for nonzero neutrino mass. In this paper we explore a new oscillation mechanism to explain these anomalies, invoking a light neutrinophilic Higgs boson, conceived to induce a low Dirac neutrino mass in accord with experimental limits. Beam neutrinos forward scattering off of a locally overdense relic neutrino background give rise to a novel matter effect with an energy-specific resonance. An enhanced oscillation around this resonance peak produces flavor transitions which are highly consistent with the MiniBooNE neutrino- and antineutrino-mode data sets. The model provides substantially improved χ2 values beyond either the no-oscillation hypothesis or the more commonly explored 3 +1 sterile neutrino hypothesis. This mechanism would introduce distinctive signatures at each baseline in the upcoming short-baseline neutrino program at Fermilab, presenting opportunities for further exploration.

 

Title:
Search for High-Mass Resonances Decaying to τ ν in p p Collisions at √{s }=13 TeV with the ATLAS Detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2893 coauthors
Publication:
Physical Review Letters, Volume 120, Issue 16, id.161802 (PhRvL Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
2018: CERN
DOI:
10.1103/PhysRevLett.120.161802
Bibliographic Code:
2018PhRvL.120p1802A

Abstract

A search for high-mass resonances decaying to τ ν using proton-proton collisions at √{s }=13 TeV produced by the Large Hadron Collider is presented. Only τ -lepton decays with hadrons in the final state are considered. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 36.1 fb-1. No statistically significant excess above the standard model expectation is observed; model-independent upper limits are set on the visible τ ν production cross section. Heavy W' bosons with masses less than 3.7 TeV in the sequential standard model and masses less than 2.2-3.8 TeV depending on the coupling in the nonuniversal G (221 ) model are excluded at the 95% credibility level.

 

Title:
Relaxation to a Phase-Locked Equilibrium State in a One-Dimensional Bosonic Josephson Junction
Authors:
Pigneur, Marine; Berrada, Tarik; Bonneau, Marie; Schumm, Thorsten; Demler, Eugene; Schmiedmayer, Jörg
Publication:
Physical Review Letters, Volume 120, Issue 17, id.173601 (PhRvL Homepage)
Publication Date:
04/2018
Origin:
APS
Abstract Copyright:
(c) 2018: us
DOI:
10.1103/PhysRevLett.120.173601
Bibliographic Code:
2018PhRvL.120q3601P

Abstract

We present an experimental study on the nonequilibrium tunnel dynamics of two coupled one-dimensional Bose-Einstein quasicondensates deep in the Josephson regime. Josephson oscillations are initiated by splitting a single one-dimensional condensate and imprinting a relative phase between the superfluids. Regardless of the initial state and experimental parameters, the dynamics of the relative phase and atom number imbalance shows a relaxation to a phase-locked steady state. The latter is characterized by a high phase coherence and reduced fluctuations with respect to the initial state. We propose an empirical model based on the analogy with the anharmonic oscillator to describe the effect of various experimental parameters. A microscopic theory compatible with our observations is still missing.

 

Title:
Imaging electron flow from collimating contacts in graphene
Authors:
Bhandari, S.; Lee, G. H.; Watanabe, K.; Taniguchi, T.; Kim, P.; Westervelt, R. M.
Publication:
2D Materials, Volume 5, Issue 2, article id. 021003 (2018). (IOP Homepage)
Publication Date:
04/2018
Origin:
IOP
DOI:
10.1088/2053-1583/aab38a
Bibliographic Code:
2018TDM.....5b1003B

Abstract

The ballistic motion of electrons in graphene opens exciting opportunities for electron-optic devices based on collimated electron beams. We form a collimating contact in a hBN-encapsulated graphene hall bar by adding zigzag contacts on either side of an electron emitter that absorb stray electrons; collimation can be turned off by floating the zig-zag contacts. The electron beam is imaged using a liquid-He cooled scanning gate microscope (SGM). The tip deflects electrons as they pass from the collimating contact to a receiving contact on the opposite side of the channel, and an image of electron flow can be made by displaying the change in transmission as the tip is raster scanned across the sample. The angular half width Δθ of the electron beam is found by applying a perpendicular magnetic field B that bends electron paths into cyclotron orbits. The images reveal that the electron flow from the collimating contact drops quickly at B  =  0.05 T when the electron orbits miss the receiving contact. The flow for the non-collimating case persists longer, up to B  =  0.19 T, due to the broader range of entry angles. Ray-tracing simulations agree well with the experimental images. By fitting the fields B at which the magnitude of electron flow drops in the experimental SGM images, we find Δθ  =  9° for electron flow from the collimating contact, compared with Δθ  =  54° for the non-collimating case.

 


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