Faculty Publications: March, 2018

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Title:
Variational study of fermionic and bosonic systems with non-Gaussian states: Theory and applications
Authors:
Shi, Tao; Demler, Eugene; Ignacio Cirac, J.
Publication:
Annals of Physics, Volume 390, p. 245-302.
Publication Date:
03/2018
Origin:
ELSEVIER
Keywords:
Time-dependent variational principle, (Non-) Gaussian state, Electron-phonon interaction, Spin-boson model
Abstract Copyright:
(c) 2018 Elsevier Inc.
DOI:
10.1016/j.aop.2017.11.014
Bibliographic Code:
2018AnPhy.390..245S

Abstract

We present a new variational method for investigating the ground state and out of equilibrium dynamics of quantum many-body bosonic and fermionic systems. Our approach is based on constructing variational wavefunctions which extend Gaussian states by including generalized canonical transformations between the fields. The key advantage of such states compared to simple Gaussian states is presence of non-factorizable correlations and the possibility of describing states with strong entanglement between particles. In contrast to the commonly used canonical transformations, such as the polaron or Lang-Firsov transformations, we allow parameters of the transformations to be time dependent, which extends their regions of applicability. We derive equations of motion for the parameters characterizing the states both in real and imaginary time using the differential structure of the variational manifold. The ground state can be found by following the imaginary time evolution until it converges to a steady state. Collective excitations in the system can be obtained by linearizing the real-time equations of motion in the vicinity of the imaginary time steady-state solution. Our formalism allows us not only to determine the energy spectrum of quasiparticles and their lifetime, but to obtain the complete spectral functions and to explore far out of equilibrium dynamics such as coherent evolution following a quantum quench. We illustrate and benchmark this framework with several examples: a single polaron in the Holstein and Su-Schrieffer-Heeger models, non-equilibrium dynamics in the spin-boson and Kondo models, the superconducting to charge density wave phase transitions in the Holstein model.

 

Title:
Dynamical Signature of Fractionalization at the Deconfined Quantum Critical Point
Authors:
Ma, Nvsen; Sun, Guang-Yu; You, Yi-Zhuang; Xu, Cenke; Vishwanath, Ashvin; Sandvik, Anders W.; Meng, Zi Yang
Publication:
eprint arXiv:1803.01180
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons
Comment:
12 pages, 8 figures and 2 appendices
Bibliographic Code:
2018arXiv180301180M

Abstract

Deconfined quantum critical points govern continuous quantum phase transitions at which fractionalized (deconfined) degrees of freedom emerge. Here we study dynamical signatures of the fractionalized excitations in a quantum magnet (the easy-plane J-Q model) that realizes a deconfined quantum critical point with emergent O(4) symmetry. By means of large-scale quantum Monte Carlo simulations and stochastic analytic continuation of imaginary-time correlation functions, we obtain the dynamic spin structure factors in the $S^{x}$ and $S^{z}$ channels. In both channels, we observe broad continua as expected from the collective fluctuation of the deconfined excitations. We also provide field-theoretical calculations at the mean field level that explain the overall shapes of the computed spectra, while also pointing to the importance of interactions and gauge fluctuations to explain some aspects of the spectral-weight distribution. We make further comparisons with the conventional Landau paradigmatic O(2) transition in a different quantum magnet, at which no signature of fractionalization are observed. The distinctive spectral signatures of the deconfined quantum critical point suggest the feasibility of its experimental detection in neutron scattering and nuclear magnetic resonance experiments.

 

Title:
Duality between atomic configurations and Bloch states in twisted 2D bilayers
Authors:
Carr, Stephen; Massatt, Daniel; Luskin, Mitchell; Kaxiras, Efthimios
Publication:
eprint arXiv:1803.01242
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
6 pages, 4 figures, for submission to PRL
Bibliographic Code:
2018arXiv180301242C

Abstract

The stacking of individual layers of two-dimensional (2D) materials can be experimentally controlled with remarkable precision on the order of $0.1^\circ$. The relative orientation of successive layers introduces variations in the electronic properties that depend sensitively on the twist angle, creating moir\'e super-lattices. Here, we use simple theoretical models and accurate electronic structure calculations to predict that the electronic density in stacked 2D layers can vary in real space in a manner that replicates features of the band-structure in momentum-space. In particular, we draw a link between these banded real space electronic structures and the complexity of exact $k \cdot p$ models in the same energy range for each material we studied. A direct consequence of the patterns is the localization of electronic states. We demonstrate this effect in graphene, a semi-metal, and MoSe$_2$, a representative material of the transition metal dichalcogenide family of semiconductors. This effect can be useful in the design of localized electronic modes, arising from layer stackings, for experimental or technological applications.

 

Title:
Probing one-dimensional systems via noise magnetometry with single spin qubits
Authors:
Rodriguez-Nieva, Joaquin F.; Agarwal, Kartiek; Giamarchi, Thierry; Halperin, Bertrand I.; Lukin, Mikhail D.; Demler, Eugene
Publication:
eprint arXiv:1803.01521
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
14+4 pages, 5 figures
Bibliographic Code:
2018arXiv180301521R

Abstract

The study of exotic one-dimensional states, particularly those at the edges of topological materials, demand new experimental probes that can access the interplay between charge and spin degrees of freedom. One potential approach is to use a single spin probe, such as a Nitrogen Vacancy center in diamond, which has recently emerged as a versatile tool to probe nanoscale systems in a non-invasive fashion. Here we present a theory describing how noise magnetometry with spin probes can directly address several questions that have emerged in experimental studies of 1D systems, including those in topological materials. We show that by controlling the spin degree of freedom of the probe, it is possible to measure locally and independently local charge and spin correlations of 1D systems. Visualization of 1D edge states, as well as sampling correlations with wavevector resolution can be achieved by tuning the probe-to-sample distance. Furthermore, temperature-dependent measurements of magnetic noise can clearly delineate the dominant scattering mechanism (impurities vs. interactions) -- this is of particular relevance to quantum spin Hall measurements where conductance quantization is not perfect. The possibility to probe both charge and spin excitations in a wide range of length scales opens new pathways to bridging the large gap between atomic scale resolution of scanning probes and global transport measurements.

 

Title:
Fundamental precision bounds for three-dimensional optical localization microscopy with Poisson statistics
Authors:
Backlund, Mikael P.; Shechtman, Yoav; Walsworth, Ronald L.
Publication:
eprint arXiv:1803.01776
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Physics - Optics
Comment:
Updated to include Supplemental Material as ancillary file
Bibliographic Code:
2018arXiv180301776B

Abstract

Point source localization is a problem of persistent interest in optical imaging. In particular, a number of widely used biological microscopy techniques rely on precise three-dimensional localization of single fluorophores. As emitter depth localization is more challenging than lateral localization, considerable effort has been spent on engineering the response of the microscope in a way that reveals increased depth information. Here we consider the theoretical limits of such approaches by deriving the quantum Cram\'{e}r-Rao bound (QCRB). We show that existing methods for depth localization with single-objective detection exceed the QCRB by a factor $>\sqrt{2}$, and propose an interferometer arrangement that approaches the bound. We also show that for detection with two opposed objectives, established interferometric measurement techniques globally reach the QCRB.

 

Title:
Inflationary vs. Reionization Features from Planck 2015 Data
Authors:
Obied, Georges; Dvorkin, Cora; Heinrich, Chen; Hu, Wayne; Miranda, Vinicius
Publication:
eprint arXiv:1803.01858
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Astrophysics - Cosmology and Nongalactic Astrophysics
Comment:
10 pages, 11 figures, 1 table
Bibliographic Code:
2018arXiv180301858O

Abstract

Features during inflation and reionization leave corresponding features in the temperature and polarization power spectra that could potentially explain anomalies in the Planck 2015 data but require a joint analysis to disentangle. We study the interplay between these two effects using a model-independent parametrization of the inflationary power spectrum and the ionization history. Preference for a sharp suppression of large scale power is driven by a feature in the temperature power spectrum at multipoles $\ell \sim 20$, whereas preference for a component of high redshift ionization is driven by a sharp excess of polarization power at $\ell \sim 10$ when compared with the lowest multipoles. Marginalizing inflationary freedom does not weaken the preference for $z \gtrsim 10$ ionization, whereas marginalizing reionization freedom slightly enhances the preference for an inflationary feature but can also mask its direct signature in polarization. The inflation and reionization interpretation of these features makes predictions for the polarization spectrum which can be tested in future precision measurements especially at $10\lesssim \ell \lesssim 40$.

 

Title:
Magic-angle graphene superlattices: a new platform for unconventional superconductivity
Authors:
Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
Publication:
eprint arXiv:1803.02342
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Superconductivity
Comment:
18 pages, 9 figures (with Methods). In press in Nature; doi:10.1038/nature26160
Bibliographic Code:
2018arXiv180302342C

Abstract

The understanding of strongly-correlated materials, and in particular unconventional superconductors, has puzzled physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. Here we report on the realization of intrinsic unconventional superconductivity in a 2D superlattice created by stacking two graphene sheets with a small twist angle. For angles near $1.1^\circ$, the first `magic' angle, twisted bilayer graphene (TBG) exhibits ultra-flat bands near charge neutrality, which lead to correlated insulating states at half-filling. Upon electrostatic doping away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature $T_c$ up to 1.7 K. The temperature-density phase diagram shows similarities with that of the cuprates, including superconducting domes. Moreover, quantum oscillations indicate small Fermi surfaces near the correlated insulating phase, in analogy with under-doped cuprates. The relative high $T_c$, given such small Fermi surface (corresponding to a record-low 2D carrier density of $10^{11} \textrm{cm}^{-2}$ , renders TBG among the strongest coupling superconductors, in a regime close to the BCS-BEC crossover. These novel results establish TBG as the first purely carbon-based 2D superconductor and as a highly tunable platform to investigate strongly-correlated phenomena, which could lead to insights into the physics of high-$T_c$ superconductors and quantum spin liquids.

 

Title:
Simultaneous Broadband Vector Magnetometry Using Solid-State Spins
Authors:
Schloss, Jennifer M.; Barry, John F.; Turner, Matthew J.; Walsworth, Ronald L.
Publication:
eprint arXiv:1803.03718
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Quantum Physics, Condensed Matter - Mesoscale and Nanoscale Physics, Physics - Applied Physics, Physics - Instrumentation and Detectors
Comment:
13 pages, 5 figures, 1 table, Supplemental Material included as ancillary file
Bibliographic Code:
2018arXiv180303718S

Abstract

We demonstrate a vector magnetometer that simultaneously measures all Cartesian components of a dynamic magnetic field using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond. Optical NV-diamond measurements provide high-sensitivity, broadband magnetometry under ambient or extreme physical conditions; and the fixed crystallographic axes inherent to this solid-state system enable vector sensing free from heading errors. In the present device, multi-channel lock-in detection extracts the magnetic-field-dependent spin resonance shifts of NVs oriented along all four tetrahedral diamond axes from the optical signal measured on a single detector. The sensor operates from near DC up to a $12.5$ kHz measurement bandwidth; and simultaneously achieves $\sim\!50$ pT/$\sqrt{\text{Hz}}$ magnetic field sensitivity for each Cartesian component, which is to date the highest demonstrated sensitivity of a full vector magnetometer employing solid-state spins. Compared to optimized devices interrogating the four NV orientations sequentially, the simultaneous vector magnetometer enables a $4\times$ measurement speedup. This technique can be extended to pulsed-type sensing protocols and parallel wide-field magnetic imaging.

 

Title:
Laser Cooling of Optically Trapped Molecules
Authors:
Anderegg, Loïc; Augenbraun, Benjamin L.; Bao, Yicheng; Burchesky, Sean; Cheuk, Lawrence W.; Ketterle, Wolfgang; Doyle, John M.
Publication:
eprint arXiv:1803.04571
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Physics - Atomic Physics, Condensed Matter - Quantum Gases
Comment:
5 pages, 5 figures
Bibliographic Code:
2018arXiv180304571A

Abstract

Calcium monofluoride (CaF) molecules are loaded into an optical dipole trap (ODT) and subsequently laser cooled within the trap. Starting with magneto-optical trapping, we sub-Doppler cool CaF and then load $150(30)$ CaF molecules into an ODT. Enhanced loading by a factor of five is obtained when sub-Doppler cooling light and trapping light are on simultaneously. For trapped molecules, we directly observe efficient sub-Doppler cooling to a temperature of $60(5)$ $\mu\text{K}$. The trapped molecular density of $8(2)\times10^7$ cm$^{-3}$ is an order of magnitude greater than in the initial sub-Doppler cooled sample. The trap lifetime of 750(40) ms is dominated by background gas collisions.

 

Title:
Effective spin-spin interactions in bilayers of Rydberg atoms and polar molecules
Authors:
Kuznetsova, Elena; Rittenhouse, Seth T.; Beterov, I. I.; Scully, Marlan O.; Yelin, Susanne F.; Sadeghpour, H. R.
Publication:
eprint arXiv:1803.05358
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Quantum Physics, Condensed Matter - Quantum Gases
Bibliographic Code:
2018arXiv180305358K

Abstract

We show that indirect spin-spin interactions between effective spin-1/2 systems can be realized in two parallel 1D optical lattices loaded with polar molecules and/or Rydberg atoms. The effective spin can be encoded into low-energy rotational states of polar molecules or long-lived states of Rydberg atoms, tightly trapped in a deep optical lattice. The spin-spin interactions can be mediated by Rydberg atoms, placed in a parallel shallow optical lattice, interacting with the effective spins by charge-dipole (for polar molecules) or dipole-dipole (for Rydberg atoms) interaction. Indirect XX, Ising and XXZ interactions with interaction coefficients $J^{\bot}$ and $J^{zz}$ sign varying with interspin distance can be realized, in particular, the $J_{1}-J_{2}$ XXZ model with frustrated ferro-(antiferro-)magnetic nearest (next-nearest) neighbor interactions.

 

Title:
Jet Charge and Machine Learning
Authors:
Fraser, Katherine; Schwartz, Matthew D.
Publication:
eprint arXiv:1803.08066
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
High Energy Physics - Phenomenology, Statistics - Machine Learning
Comment:
17 pages, 8 figures, 1 table
Bibliographic Code:
2018arXiv180308066F

Abstract

Modern machine learning techniques, such as convolutional, recurrent and recursive neural networks, have shown promise for jet substructure at the Large Hadron Collider. For example, they have demonstrated effectiveness at boosted top or W boson identification or for quark/gluon discrimination. We explore these methods for the purpose of classifying jets according to their electric charge. We find that neural networks that incorporate distance within the jet as an input can provide significant improvement in jet charge extraction over traditional methods. We find that both convolutional and recurrent networks are effective and both train faster than recursive networks. The advantages of using a fixed-size input representation (as with the CNN) or a smaller input representation (as with the RNN) suggest that both convolutional and recurrent networks will be essential to the future of modern machine learning at colliders.

 

Title:
Finite-Temperature Scrambling of a Random Hamiltonian
Authors:
Vijay, Sagar; Vishwanath, Ashvin
Publication:
eprint arXiv:1803.08483
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Statistical Mechanics, High Energy Physics - Theory, Quantum Physics
Comment:
13 pages, 1 figure
Bibliographic Code:
2018arXiv180308483V

Abstract

We study the finite-temperature scrambling behavior of a quantum system described by a Hamiltonian chosen from a random matrix ensemble. This effectively (0+1)-dimensional model admits an exact calculation of various ensemble-averaged out-of-time-ordered correlation functions in the large-$N$ limit, where $N$ is the Hilbert space dimension. For a Hamiltonian drawn from the Gaussian unitary ensemble, we calculate the ensemble averaged OTOC at all temperatures. In addition to an early time quadratic growth of the averaged out-of-time-ordered commutator (OTOC), we determine that the OTOC saturates to its asymptotic value as a power-law in time, with an exponent that depends both on temperature, and on one of four classes of operators appearing in the correlation function, that naturally emerge from this calculation. Out-of-time-ordered correlation functions of operators that are distributed around the thermal circle take a time $t_{s}\sim \beta$ to decay at low temperatures. We generalize these exact results, by demonstrating that out-of-time-ordered correlation functions averaged over any ensemble of Hamiltonians that are invariant under unitary conjugation $H \rightarrow {U} H {U}^{\dagger}$, exhibit power-law decay to an asymptotic value. We argue that this late-time behavior is a generic feature of unitary dynamics with energy conservation. Finally, by comparing the OTOC with a commutator-anticommutator correlation function, we examine whether there is a time window over which a typical Hamiltonian behaves as a "coherent scrambler" in the language of Ref. \cite{Kitaev_IAS, Kitaev_Suh}.

 

Title:
Transport of neutral optical excitations using electric fields
Authors:
Cotlet, Ovidiu; Pientka, Falko; Schmidt, Richard; Zarand, Gergely; Demler, Eugene; Imamoglu, Atac
Publication:
eprint arXiv:1803.08509
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Quantum Gases, Quantum Physics
Comment:
18 + 11 pages, 4 figures
Bibliographic Code:
2018arXiv180308509C

Abstract

Mobile quantum impurities interacting with a fermionic bath form quasiparticles known as Fermi polarons. We demonstrate that a force applied to the bath particles can generate a drag force of similar magnitude acting on the impurities, realizing a novel, nonperturbative Coulomb drag effect. To prove this, we calculate the fully self-consistent, frequency-dependent transconductivity at zero temperature in the Baym-Kadanoff conserving approximation. We apply our theory to excitons and exciton polaritons interacting with a bath of charge carriers in a doped semiconductor embedded in a microcavity. In external electric and magnetic fields, the drag effect enables electrical control of excitons and may pave the way for the implementation of gauge fields for excitons and polaritons. Moreover, a reciprocal effect may facilitate optical manipulation of electron transport. Our findings establish transport measurements as a novel, powerful tool for probing the many-body physics of mobile quantum impurities.

 

Title:
A New Technique for Mapping Distances Across the Perseus Molecular Cloud Using CO Observations and Stellar Photometry
Authors:
Zucker, Catherine; Schlafly, Edward F.; Green, Gregory M.; Speagle, Joshua S.; Portillo, Stephen K. N.; Finkbeiner, Douglas P.; Goodman, Alyssa A.
Publication:
eprint arXiv:1803.08931
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Astrophysics - Astrophysics of Galaxies
Comment:
Submitted to The Astrophysical Journal
Bibliographic Code:
2018arXiv180308931Z

Abstract

We present a new technique to determine distances to major star-forming regions across the Perseus Molecular Cloud, using a combination of stellar photometry and $\rm ^{12} CO$ spectral-line data. We start by inferring the distance and reddening to thousands of stars across the complex from their Pan-STARRS1 and 2MASS photometry, using a technique presented in Green et al. (2014, 2015) and implemented in their 3D "Bayestar" dust map of three-quarters of the sky. We then refine their technique by using the velocity slices of a CO spectral cube as dust templates and modeling the cumulative distribution of dust along the line of sight towards these stars as a linear combination of the emission in the slices. Using a nested sampling algorithm, we fit these per-star distance-reddening measurements to find the distances to the CO velocity slices towards each star-forming region. We determine distances to the B5, IC348, B1, NGC1333, L1448, and L1451 star-forming regions and find that individual clouds are located between $\approx 280-310$ pc, with a per-cloud statistical uncertainty of 7 to 15 pc, or a fractional distance error between 2% and 5%. We find that on average the velocity gradient across Perseus corresponds to a distance gradient, with the eastern portion of the cloud (B5, IC348) about 30 pc farther away than the western portion (NGC1333, L1448). The method we present is not limited to the Perseus Complex, but may be applied anywhere on the sky with adequate CO data in the pursuit of more accurate 3D maps of molecular clouds in the solar neighborhood and beyond.

 

Title:
Origin of Mott insulating behavior and superconductivity in twisted bilayer graphene
Authors:
Po, Hoi Chun; Zou, Liujun; Vishwanath, Ashvin; Senthil, T.
Publication:
eprint arXiv:1803.09742
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Materials Science, Condensed Matter - Superconductivity
Comment:
Main text (16 pages, 4 figures, 1 table) + Appendices
Bibliographic Code:
2018arXiv180309742P

Abstract

A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat mini-band that arises at a `magic' twist angle. However, the nature of the Mott insulator, origin of superconductivity and an effective low energy model remain to be determined. We propose a Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry, and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hunds coupling. We also identify symmetry related features of the nearly flat band that are key to understanding the strong correlation physics and constrain any tight binding description. First, although the charge density is concentrated on the triangular lattice sites of the Moire pattern, the Wannier states of the tight-binding model must be centered on different sites which form a honeycomb lattice. Next, spatially localizing electrons derived from the nearly flat band necessarily breaks valley and other symmetries within any mean-field treatment, which is suggestive of a valley-ordered Mott state, and also dictates that additional symmetry breaking is present to remove symmetry-enforced band contacts. Tight binding models describing the nearly flat mini-band are derived, which highlight the importance of further neighbor hopping and interactions. We discuss consequences of this picture for superconducting states obtained on doping the valley ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future. We also describe a model for correlated states in trilayer graphene heterostructures and contrast it with the bilayer case.

 

Title:
Striking Isotope Effect on the Metallization Phase Lines of Liquid Hydrogen and Deuterium
Authors:
Zaghoo, Mohamed; Husband, Rachel; Silvera, Isaac F.
Publication:
eprint arXiv:1803.10692
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Other Condensed Matter, Condensed Matter - Materials Science
Comment:
12 pages with 3 figures plus Supplementary Information (14 pages and 7 figures)
Bibliographic Code:
2018arXiv180310692Z

Abstract

Liquid atomic metallic hydrogen is the simplest, lightest, and most abundant of all liquid metals. The role of nucleon motions or ion dynamics has been somewhat ignored in relation to the dissociative insulator-metal transition. Almost all previous experimental high-pressure studies have treated the fluid isotopes, hydrogen and deuterium, with no distinction. Studying both hydrogen and deuterium at the same density, most crucially at the phase transition line, can experimentally reveal the importance of ion dynamics. We use static compression to study the optical properties of dense deuterium in the pressure region of 1.2-1.7 Mbar and measured temperatures up to ~3000 K. We observe an abrupt increase in reflectance, consistent with dissociation-induced metallization, at the transition. Here we show that at the same pressure (density) for the two isotopes, the phase line of this transition reveals a prominent isotopic shift, ~700 K. This shift is lower than the isotopic difference in the free-molecule dissociation energies, but it is still large considering the high density of the liquid and the complex many-body effects. Our work reveals the importance of quantum nuclear effects in describing the metallization transition and conduction properties in dense hydrogen systems at conditions of giant planetary interiors, and provides an invaluable benchmark for ab-initio calculations.

 

Title:
Raman Spectrum of CrI$_3$: an ab-initio study
Authors:
Larson, Daniel T.; Kaxiras, Efthimios
Publication:
eprint arXiv:1803.10825
Publication Date:
03/2018
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science
Comment:
11 pages, 6 figures
Bibliographic Code:
2018arXiv180310825L

Abstract

We study the Raman spectrum of CrI$_3$, a material that exhibits magnetism in a single-layer. We employ first-principles calculations within density functional theory to determine the effects of polarization, strain, and incident angle on the phonon spectra of the 3D bulk and the single-layer 2D structure, for both the high- and low-temperature crystal structures. Our results are in good agreement with existing experimental measurements and serve as a guide for additional investigations to elucidate the physics of this interesting material.

 

Title:
Wang and Yau’s quasi-local energy for an extreme Kerr spacetime
Authors:
Miller, Warner A.; Ray, Shannon; Wang, Mu-Tao; Yau, Shing-Tung
Publication:
Classical and Quantum Gravity, Volume 35, Issue 5, article id. 055007 (2018). (CQGra Homepage)
Publication Date:
03/2018
Origin:
IOP
DOI:
10.1088/1361-6382/aaa625
Bibliographic Code:
2018CQGra..35e5007M

Abstract

There exist constant radial surfaces, S , that may not be globally embeddable in {R}3 for Kerr spacetimes with a>\sqrt{3}M/2 . To compute the Brown and York (B–Y) quasi-local energy (QLE), one must isometrically embed S into {R}3 . On the other hand, the Wang and Yau (W–Y) QLE embeds S into Minkowski space. In this paper, we examine the W–Y QLE for surfaces that may or may not be globally embeddable in {R}3 . We show that their energy functional, E[τ] , has a critical point at τ=0 for all constant radial surfaces in t=constant hypersurfaces using Boyer–Lindquist coordinates. For τ=0 , the W–Y QLE reduces to the B–Y QLE. To examine the W–Y QLE in these cases, we write the functional explicitly in terms of τ under the assumption that τ is only a function of θ. We then use a Fourier expansion of τ(θ) to explore the values of E[τ(θ)] in the space of coefficients. From our analysis, we discovered an open region of complex values for E[τ(θ)] . We also study the physical properties of the smallest real value of E[τ(θ)] , which lies on the boundary separating real and complex energies.

 

Title:
Measurement of quarkonium production in proton-lead and proton-proton collisions at 5.02 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2832 coauthors
Publication:
The European Physical Journal C, Volume 78, Issue 3, article id. #171, 32 pp. (EPJC Homepage)
Publication Date:
03/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-018-5624-4
Bibliographic Code:
2018EPJC...78..171A

Abstract

The modification of the production of J/ψ , ψ (2S), and Υ(nS) (n = 1, 2, 3) in p+Pb collisions with respect to their production in pp collisions has been studied. The p+Pb and pp datasets used in this paper correspond to integrated luminosities of 28 nb^{-1} and 25 pb^{-1} respectively, collected in 2013 and 2015 by the ATLAS detector at the LHC, both at a centre-of-mass energy per nucleon pair of 5.02 TeV. The quarkonium states are reconstructed in the dimuon decay channel. The yields of J/ψ and ψ (2S) are separated into prompt and non-prompt sources. The measured quarkonium differential cross sections are presented as a function of rapidity and transverse momentum, as is the nuclear modification factor, R_{p{Pb}} for J/ψ and Υ (nS). No significant modification of the J/ψ production is observed while Υ(nS) production is found to be suppressed at low transverse momentum in p+Pb collisions relative to pp collisions. The production of excited charmonium and bottomonium states is found to be suppressed relative to that of the ground states in central p+Pb collisions.

 

Title:
Measurement of differential cross-sections of a single top quark produced in association with a W boson at √{s}={13}{ {TeV}} with ATLAS
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2869 coauthors
Publication:
The European Physical Journal C, Volume 78, Issue 3, article id. #186, 29 pp. (EPJC Homepage)
Publication Date:
03/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-018-5649-8
Bibliographic Code:
2018EPJC...78..186A

Abstract

The differential cross-section for the production of a W boson in association with a top quark is measured for several particle-level observables. The measurements are performed using {36.1} {fb}^{-1} of pp collision data collected with the ATLAS detector at the LHC in 2015 and 2016. Differential cross-sections are measured in a fiducial phase space defined by the presence of two charged leptons and exactly one jet matched to a b-hadron, and are normalised with the fiducial cross-section. Results are found to be in good agreement with predictions from several Monte Carlo event generators.

 

Title:
Search for doubly charged Higgs boson production in multi-lepton final states with the ATLAS detector using proton-proton collisions at √{s}=13 {TeV}
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2893 coauthors
Publication:
The European Physical Journal C, Volume 78, Issue 3, article id. #199, 34 pp. (EPJC Homepage)
Publication Date:
03/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-018-5661-z
Bibliographic Code:
2018EPJC...78..199A

Abstract

A search for doubly charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses a proton-proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 {fb}^{-1} of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays H^{± ± }→ e^{± }e^{± }, H^{± ± }→ e^{± }μ ^{± } and H^{± ± }→ μ ^{± }μ ^{± }, fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section and consequently a lower limit on m(H^{± ± }) are derived at 95% confidence level. With ℓ ^{± }ℓ ^{± }=e^{± }e^{± }/μ ^{± }μ ^{± }/e^{± }μ ^{± }, the observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons varies from 770 to 870 GeV (850 GeV expected) for B(H^{± ± }→ ℓ ^{± }ℓ ^{± })=100% and both the expected and observed mass limits are above 450 GeV for B(H^{± ± }→ ℓ ^{± }ℓ ^{± })=10% and any combination of partial branching ratios.

 

Title:
A search for pair-produced resonances in four-jet final states at √{s}=13 {TeV} with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2869 coauthors
Publication:
The European Physical Journal C, Volume 78, Issue 3, article id. #250, 28 pp. (EPJC Homepage)
Publication Date:
03/2018
Origin:
SPRINGER
Abstract Copyright:
(c) 2018: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-018-5693-4
Bibliographic Code:
2018EPJC...78..250A

Abstract

A search for massive coloured resonances which are pair-produced and decay into two jets is presented. The analysis uses 36.7 fb^{-1} of √{s} = 13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the background prediction is observed. Results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is the top squark, \tilde{t}, which decays promptly into two quarks through R-parity-violating couplings. Top squarks with masses in the range 100 {GeV}<m_{\tilde{t}}<410 {GeV} are excluded at 95% confidence level. If the decay is into a b-quark and a light quark, a dedicated selection requiring two b-tags is used to exclude masses in the ranges 100 {GeV}<m_{\tilde{t}}<470 {GeV} and 480 {GeV}<m_{\tilde{t}}<610 {GeV}. Additional limits are set on the pair-production of massive colour-octet resonances.

 

Title:
Searches for heavy ZZ and ZW resonances in the ℓℓ qq and νν qq final states in pp 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 2886 coauthors
Publication:
Journal of High Energy Physics, Volume 2018, Issue 3, article id. #9, 53 pp.
Publication Date:
03/2018
Origin:
SPRINGER
Keywords:
Beyond Standard Model, Hadron-Hadron scattering (experiments)
Abstract Copyright:
(c) 2018: The Author(s)
DOI:
10.1007/JHEP03(2018)009
Bibliographic Code:
2018JHEP...03..009A

Abstract

This paper reports searches for heavy resonances decaying into ZZ or ZW using data from proton-proton collisions at a centre-of-mass energy of √{s}=13 TeV. The data, corresponding to an integrated luminosity of 36.1 fb-1, were recorded with the ATLAS detector in 2015 and 2016 at the Large Hadron Collider. The searches are performed in final states in which one Z boson decays into either a pair of light charged leptons (electrons and muons) or a pair of neutrinos, and the associated W boson or the other Z boson decays hadronically. No evidence of the production of heavy resonances is observed. Upper bounds on the production cross sections of heavy resonances times their decay branching ratios to ZZ or ZW are derived in the mass range 300-5000GeV within the context of Standard Model extensions with additional Higgs bosons, a heavy vector triplet or warped extra dimensions. Production through gluon-gluon fusion, Drell-Yan or vector-boson fusion are considered, depending on the assumed model. [Figure not available: see fulltext.]

 

Title:
Search for W W/W Z resonance production in ℓν qq final states in pp 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
ublication:
Journal of High Energy Physics, Volume 2018, Issue 3, article id. #42, 45 pp.
Publication Date:
03/2018
Origin:
SPRINGER
Keywords:
Hadron-Hadron scattering (experiments)
Abstract Copyright:
(c) 2018: The Author(s)
DOI:
10.1007/JHEP03(2018)042
Bibliographic Code:
2018JHEP...03..042A

Abstract

A search is conducted for new resonances decaying into a W W or W Z boson pair, where one W boson decays leptonically and the other W or Z boson decays hadronically. It is based on proton-proton collision data with an integrated luminosity of 36.1 fb-1 collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of √{s}=13 TeV in 2015 and 2016. The search is sensitive to diboson resonance production via vector-boson fusion as well as quark-antiquark annihilation and gluon-gluon fusion mechanisms. No significant excess of events is observed with respect to the Standard Model backgrounds. Several benchmark models are used to interpret the results. Limits on the production cross section are set for a new narrow scalar resonance, a new heavy vector-boson and a spin-2 Kaluza-Klein graviton. [Figure not available: see fulltext.]

 

Title:
Measurement of the Higgs boson coupling properties in the H → ZZ ∗ → 4ℓ decay channel at √{s}=13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2899 coauthors
Publication:
Journal of High Energy Physics, Volume 2018, Issue 3, article id. #95, 60 pp.
Publication Date:
03/2018
Origin:
SPRINGER
Keywords:
Hadron-Hadron scattering (experiments)
Abstract Copyright:
(c) 2018: The Author(s)
DOI:
10.1007/JHEP03(2018)095
Bibliographic Code:
2018JHEP...03..095A

Abstract

The coupling properties of the Higgs boson are studied in the four-lepton ( e, μ) decay channel using 36.1 fb-1 of pp collision data from the LHC at a centre-of-mass energy of 13 TeV collected by the ATLAS detector. Cross sections are measured for the main production modes in several exclusive regions of the Higgs boson production phase space and are interpreted in terms of coupling modifiers. The inclusive cross section times branching ratio for H → ZZ decay and for a Higgs boson absolute rapidity below 2.5 is measured to be 1. 73 - 0.23 + 0.24 (stat.) - 0.08 + 0.10 (exp.) ± 0.04(th.) pb compared to the Standard Model prediction of 1 .34±0 .09 pb. In addition, the tensor structure of the Higgs boson couplings is studied using an effective Lagrangian approach for the description of interactions beyond the Standard Model. Constraints are placed on the non-Standard-Model CP-even and CP-odd couplings to Z bosons and on the CP-odd coupling to gluons. [Figure not available: see fulltext.]

 

Title:
High-resolution magnetic resonance spectroscopy using a solid-state spin sensor
Authors:
Glenn, David R.; Bucher, Dominik B.; Lee, Junghyun; Lukin, Mikhail D.; Park, Hongkun; Walsworth, Ronald L.
Publication:
Nature, Volume 555, Issue 7696, pp. 351-354 (2018). (Nature Homepage)
Publication Date:
03/2018
Origin:
NATURE
Abstract Copyright:
(c) 2018: Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
DOI:
10.1038/nature25781
Bibliographic Code:
2018Natur.555..351G

Abstract

Quantum systems that consist of solid-state electronic spins can be sensitive detectors of nuclear magnetic resonance (NMR) signals, particularly from very small samples. For example, nitrogen–vacancy centres in diamond have been used to record NMR signals from nanometre-scale samples, with sensitivity sufficient to detect the magnetic field produced by a single protein. However, the best reported spectral resolution for NMR of molecules using nitrogen–vacancy centres is about 100 hertz. This is insufficient to resolve the key spectral identifiers of molecular structure that are critical to NMR applications in chemistry, structural biology and materials research, such as scalar couplings (which require a resolution of less than ten hertz) and small chemical shifts (which require a resolution of around one part per million of the nuclear Larmor frequency). Conventional, inductively detected NMR can provide the necessary high spectral resolution, but its limited sensitivity typically requires millimetre-scale samples, precluding applications that involve smaller samples, such as picolitre-volume chemical analysis or correlated optical and NMR microscopy. Here we demonstrate a measurement technique that uses a solid-state spin sensor (a magnetometer) consisting of an ensemble of nitrogen–vacancy centres in combination with a narrowband synchronized readout protocol to obtain NMR spectral resolution of about one hertz. We use this technique to observe NMR scalar couplings in a micrometre-scale sample volume of approximately ten picolitres. We also use the ensemble of nitrogen–vacancy centres to apply NMR to thermally polarized nuclear spins and resolve chemical-shift spectra from small molecules. Our technique enables analytical NMR spectroscopy at the scale of single cells.

 

Title:
Strong-coupling Bose polarons out of equilibrium: Dynamical renormalization-group approach
Authors:
Grusdt, Fabian; Seetharam, Kushal; Shchadilova, Yulia; Demler, Eugene
Publication:
Physical Review A, Volume 97, Issue 3, id.033612 (PhRvA Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevA.97.033612
Bibliographic Code:
2018PhRvA..97c3612G

Abstract

When a mobile impurity interacts with a surrounding bath of bosons, it forms a polaron. Numerous methods have been developed to calculate how the energy and the effective mass of the polaron are renormalized by the medium for equilibrium situations. Here, we address the much less studied nonequilibrium regime and investigate how polarons form dynamically in time. To this end, we develop a time-dependent renormalization-group approach which allows calculations of all dynamical properties of the system and takes into account the effects of quantum fluctuations in the polaron cloud. We apply this method to calculate trajectories of polarons following a sudden quench of the impurity-boson interaction strength, revealing how the polaronic cloud around the impurity forms in time. Such trajectories provide additional information about the polaron's properties which are challenging to extract directly from the spectral function measured experimentally using ultracold atoms. At strong couplings, our calculations predict the appearance of trajectories where the impurity wavers back at intermediate times as a result of quantum fluctuations. Our method is applicable to a broader class of nonequilibrium problems. As a check, we also apply it to calculate the spectral function and find good agreement with experimental results. At very strong couplings, we predict that quantum fluctuations lead to the appearance of a dark continuum with strongly suppressed spectral weight at low energies. While our calculations start from an effective Fröhlich Hamiltonian describing impurities in a three-dimensional Bose-Einstein condensate, we also calculate the effects of additional terms in the Hamiltonian beyond the Fröhlich paradigm. We demonstrate that the main effect of these additional terms on the attractive side of a Feshbach resonance is to renormalize the coupling strength of the effective Fröhlich model.

 

Title:
Implanted neural network potentials: Application to Li-Si alloys
Authors:
Onat, Berk; Cubuk, Ekin D.; Malone, Brad D.; Kaxiras, Efthimios
Publication:
Physical Review B, Volume 97, Issue 9, id.094106 (PhRvB Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.094106
Bibliographic Code:
2018PhRvB..97i4106O

Abstract

Modeling the behavior of materials composed of elements with different bonding and electronic structure character for large spatial and temporal scales and over a large compositional range is a challenging problem. Cases in point are amorphous alloys of Si, a prototypical covalent material, and Li, a prototypical metal, which are being considered as anodes for high-energy-density batteries. To address this challenge, we develop a methodology based on neural networks that extends the conventional training approach to incorporate pre-trained parts that capture the character of different components, into the overall network; we refer to this model as the "implanted neural network" method. We show that this approach works well for the Si-Li amorphous alloys for a wide range of compositions, giving good results for key quantities like the diffusion coefficients. The method is readily generalizable to more complicated situations that involve two or more different elements.

 

Title:
Dynamically induced many-body localization
Authors:
Choi, Soonwon; Abanin, Dmitry A.; Lukin, Mikhail D.
Publication:
Physical Review B, Volume 97, Issue 10, id.100301 (PhRvB Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.100301
Bibliographic Code:
2018PhRvB..97j0301C

Abstract

We show that a quantum phase transition from ergodic to many-body localized (MBL) phases can be induced via periodic pulsed manipulation of spin systems. Such a transition is enabled by the interplay between weak disorder and slow heating rates. Specifically, we demonstrate that the Hamiltonian of a weakly disordered ergodic spin system can be effectively engineered, by using sufficiently fast coherent controls, to yield a stable MBL phase, which in turn completely suppresses the energy absorption from external control field. Our results imply that a broad class of existing many-body systems can be used to probe nonequilibrium phases of matter for a long time, limited only by coupling to external environment.

 

Title:
Doping-dependent correlation effects in (Sr1-xLax) 3Ir2O7
Authors:
Affeldt, Gregory; Hogan, Tom; Denlinger, Jonathan D.; Vishwanath, Ashvin; Wilson, Stephen D.; Lanzara, Alessandra
Publication:
Physical Review B, Volume 97, Issue 12, id.125111 (PhRvB Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.125111
Bibliographic Code:
2018PhRvB..97l5111A

Abstract

We have measured the signatures of electronic energy scales and their doping evolution in the band structure of (Sr1-xLax) 3Ir2O7 using angle-resolved photoemission spectroscopy. While band splittings and positions corresponding to the bilayer splitting and spin-orbit coupling undergo only small changes, the Mott gap and effective mass of both the lower Hubbard band and conduction band exhibit strong variations with doping. These changes correspond to similar observations in the cuprate superconductors, and are likely connected to the changing effective Coulomb interaction upon addition of itinerant carriers.

 

Title:
From bosonic topological transition to symmetric fermion mass generation
Authors:
You, Yi-Zhuang; He, Yin-Chen; Vishwanath, Ashvin; Xu, Cenke
Publication:
Physical Review B, Volume 97, Issue 12, id.125112 (PhRvB Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.125112
Bibliographic Code:
2018PhRvB..97l5112Y

Abstract

A bosonic topological transition (BTT) is a quantum critical point between the bosonic symmetry-protected topological phase and the trivial phase. In this work, we investigate such a transition in a (2+1)-dimensional lattice model with the maximal microscopic symmetry: an internal SO (4 ) symmetry. We derive a description for this transition in terms of compact quantum electrodynamics (QED) with four fermion flavors (Nf=4 ). Within a systematic renormalization group analysis, we identify the critical point with the desired O (4 ) emergent symmetry and all expected deformations. By lowering the microscopic symmetry, we recover the previous Nf=2 noncompact QED description of the BTT. Finally, by merging two BTTs we recover a previously discussed theory of symmetric mass generation, as an SU (2 ) quantum chromodynamics-Higgs theory with Nf=4 flavors of SU (2 ) fundamental fermions and one SU (2 ) fundamental Higgs boson. This provides a consistency check on both theories.

 

Title:
Angle-resolved photoemission spectroscopy with quantum gas microscopes
Authors:
Bohrdt, A.; Greif, D.; Demler, E.; Knap, M.; Grusdt, F.
Publication:
Physical Review B, Volume 97, Issue 12, id.125117 (PhRvB Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: American Physical Society
DOI:
10.1103/PhysRevB.97.125117
Bibliographic Code:
2018PhRvB..97l5117B

Abstract

Quantum gas microscopes are a promising tool to study interacting quantum many-body systems and bridge the gap between theoretical models and real materials. So far, they were limited to measurements of instantaneous correlation functions of the form 〈O ̂(t ) 〉 , even though extensions to frequency-resolved response functions 〈O ̂(t ) O ̂(0 ) 〉 would provide important information about the elementary excitations in a many-body system. For example, single-particle spectral functions, which are usually measured using photoemission experiments in electron systems, contain direct information about fractionalization and the quasiparticle excitation spectrum. Here, we propose a measurement scheme to experimentally access the momentum and energy-resolved spectral function in a quantum gas microscope with currently available techniques. As an example for possible applications, we numerically calculate the spectrum of a single hole excitation in one-dimensional t -J models with isotropic and anisotropic antiferromagnetic couplings. A sharp asymmetry in the distribution of spectral weight appears when a hole is created in an isotropic Heisenberg spin chain. This effect slowly vanishes for anisotropic spin interactions and disappears completely in the case of pure Ising interactions. The asymmetry strongly depends on the total magnetization of the spin chain, which can be tuned in experiments with quantum gas microscopes. An intuitive picture for the observed behavior is provided by a slave-fermion mean-field theory. The key properties of the spectra are visible at currently accessible temperatures.

 

Title:
Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at √{s }=13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2851 coauthors
Publication:
Physical Review D, Volume 97, Issue 5, id.052010 (PhRvD Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: CERN
DOI:
10.1103/PhysRevD.97.052010
Bibliographic Code:
2018PhRvD..97e2010A

Abstract

A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2016, corresponding to 36.1 fb-1 of integrated luminosity at √{s }=13 TeV . Events with same-flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R -parity-conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with nonuniversal Higgs boson masses.

 

Title:
Search for long-lived, massive particles in events with displaced vertices and missing transverse momentum in √{s }=13 TeV p p collisions with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2871 coauthors
Publication:
Physical Review D, Volume 97, Issue 5, id.052012 (PhRvD Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: CERN
DOI:
10.1103/PhysRevD.97.052012
Bibliographic Code:
2018PhRvD..97e2012A

Abstract

A search for long-lived, massive particles predicted by many theories beyond the Standard Model is presented. The search targets final states with large missing transverse momentum and at least one high-mass displaced vertex with five or more tracks, and uses 32.8 fb-1 of √{s }=13 TeV p p collision data collected by the ATLAS detector at the LHC. The observed yield is consistent with the expected background. The results are used to extract 95% C.L. exclusion limits on the production of long-lived gluinos with masses up to 2.37 TeV and lifetimes of O (10-2)-O (10 ) ns in a simplified model inspired by split supersymmetry.

 

Title:
Scale-invariant instantons and the complete lifetime of the standard model
Authors:
Andreassen, Anders; Frost, William; Schwartz, Matthew D.
Publication:
Physical Review D, Volume 97, Issue 5, id.056006 (PhRvD Homepage)
Publication Date:
03/2018
Origin:
APS
Abstract Copyright:
2018: authors
DOI:
10.1103/PhysRevD.97.056006
Bibliographic Code:
2018PhRvD..97e6006A

Abstract

In a classically scale-invariant quantum field theory, tunneling rates are infrared divergent due to the existence of instantons of any size. While one expects such divergences to be resolved by quantum effects, it has been unclear how higher-loop corrections can resolve a problem appearing already at one loop. With a careful power counting, we uncover a series of loop contributions that dominate over the one-loop result and sum all the necessary terms. We also clarify previously incomplete treatments of related issues pertaining to global symmetries, gauge fixing, and finite mass effects. In addition, we produce exact closed-form solutions for the functional determinants over scalars, fermions, and vector bosons around the scale-invariant bounce, demonstrating manifest gauge invariance in the vector case. With these problems solved, we produce the first complete calculation of the lifetime of our Universe: 1 0139 years . With 95% confidence, we expect our Universe to last more than 1 058 years . The uncertainty is part experimental uncertainty on the top quark mass and on αs and part theory uncertainty from electroweak threshold corrections. Using our complete result, we provide phase diagrams in the mt/mh and the mt/αs planes, with uncertainty bands. To rule out absolute stability to 3 σ confidence, the uncertainty on the top quark pole mass would have to be pushed below 250 MeV or the uncertainty on αs(mZ) pushed below 0.00025.

 

 


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