Showing papers on "Explicit symmetry breaking published in 2016"
TL;DR: In this paper, the authors studied the quantum mechanical model of $N$ Majorana fermions with random interactions of a few Fermions at a time (Sachdev-Ye-Kitaev model) in the large N$ limit.
Abstract: The authors study in detail the quantum mechanical model of $N$ Majorana fermions with random interactions of a few fermions at a time (Sachdev-Ye-Kitaev model) in the large $N$ limit. At low energies, the system is strongly interacting and an emergent conformal symmetry develops. Performing technical calculations, the authors elucidate a number of properties of the model near the conformal point.
1,953 citations
TL;DR: In this paper, a two-dimensional dilaton gravity system was studied and the symmetry breaking was studied in terms of a Schwarzian derivative effective action for a reparametrization.
Abstract: We study a two dimensional dilaton gravity system, recently examined by Almheiri and Polchinski, which describes near extremal black holes, or more generally, nearly $AdS_2$ spacetimes. The asymptotic symmetries of $AdS_2$ are all the time reparametrizations of the boundary. These symmetries are spontaneously broken by the $AdS_2$ geometry and they are explicitly broken by the small deformation away from $AdS_2$. This pattern of spontaneous plus explicit symmetry breaking governs the gravitational backreaction of the system. It determines several gravitational properties such as the linear in temperature dependence of the near extremal entropy as well as the gravitational corrections to correlation functions. These corrections include the ones determining the growth of out of time order correlators that is indicative of chaos. These gravitational aspects can be described in terms of a Schwarzian derivative effective action for a reparametrization.
1,214 citations
TL;DR: The first experimental demonstration of optical anti-PT symmetry in a warm atomic-vapour cell was reported in this paper, where fast coherence transport via flying atoms leads to a dissipative coupling between two long-lived atomic spin waves, allowing for the observation of the essential features of anti-parity-time symmetry with unprecedented precision on the phase-transition threshold.
Abstract: The recently developed notion of parity–time (PT) symmetry in optical systems has spawned intriguing prospects. So far, most experimental implementations have been reported in solid-state systems. Here, we report the first experimental demonstration of optical anti-PT symmetry—the counterpart of conventional PT symmetry—in a warm atomic-vapour cell. Rapid coherence transport via flying atoms leads to a dissipative coupling between two long-lived atomic spin waves, allowing for the observation of the essential features of anti-PT symmetry with unprecedented precision on the phase-transition threshold, as well as refractionless light propagation. Moreover, we show that a linear or nonlinear interaction between the two spatially separated beams can be achieved. Our results advance non-Hermitian physics by bridging to the field of atomic, molecular and optical physics, where new phenomena and applications in quantum and nonlinear optics aided by (anti-)PT symmetry could be anticipated. Parity–time symmetry in optics is studied in a warm atomic vapour, where its counterpart, anti-parity–time symmetry, as well as refractionless propagation, can also be observed.
324 citations
TL;DR: Surprisingly, it is found that the helical edge states in this photonic topological insulator are protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time- reversal symmetries Tb.
Abstract: A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron's spin-1/2 (fermionic) time-reversal symmetry [Formula: see text] However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon's spin-1 (bosonic) time-reversal symmetry [Formula: see text] In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp ([Formula: see text]), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.
199 citations
TL;DR: The soft gluon theorem is shown, for massless theories at the semiclassical level, to be the Ward identity of a holomorphic two-dimensional G-Kac-Moody symmetry acting on these correlation functions.
Abstract: Scattering amplitudes of any four-dimensional theory with nonabelian gauge groupG may be recast as two-dimensional correlation functions on the asymptotic two-sphere at null innity. The soft gluon theorem is shown, for massless theories at the semiclassical level, to be the Ward identity of a holomorphic two-dimensionalG-Kac-Moody symmetry acting on these correlation functions. Holomorphic Kac-Moody current insertions are positive helicity soft gluon insertions. The Kac-Moody transformations are a CPT invariant subgroup of gauge transformations which act nontrivially at null innity and comprise the four-dimensional asymptotic symmetry group.
186 citations
169 citations
TL;DR: In this article, the existence of many-body localized (MBL) phases in the presence of global symmetries was shown not possible with symmetry groups that protect multiplets (e.g. all non-Abelian symmetry groups).
Abstract: We derive general constraints on the existence of many-body localized (MBL) phases in the presence of global symmetries, and show that MBL is not possible with symmetry groups that protect multiplets (e.g. all non-Abelian symmetry groups). Based on simple representation theoretic considerations, we derive general Mermin-Wagner-type principles governing the possible alternative fates of non-equilibrium dynamics in isolated, strongly disordered quantum systems. Our results rule out the existence of MBL symmetry protected topological phases with non-Abelian symmetry groups, as well as time-reversal symmetry protected electronic topological insulators, and in fact all fermion topological insulators and superconductors in the 10-fold way classification. Moreover, extending our arguments to systems with intrinsic topological order, we rule out MBL phases with non-Abelian anyons as well as certain classes of symmetry enriched topological orders.
111 citations
TL;DR: In this paper, a spontaneously broken global chiral Peccei-Quinn (PQ) symmetry arises as an accidental symmetry of an exact PQ symmetry, which can protect the Nambu-Goldstone boson against semiclassical gravity effects, thus suppressing gravitational corrections to the effective potential, while it can at the same time provide for the small explicit symmetry breaking term needed to make models with domain wall number (N}_{\mathrm{DW}}g1$, such as the popular Dine-Fischler-Srednicki-
Abstract: We consider extensions of the Standard Model in which a spontaneously broken global chiral Peccei-Quinn (PQ) symmetry arises as an accidental symmetry of an exact ${Z}_{N}$ symmetry. For $N=9$ or 10, this symmetry can protect the accion---the Nambu-Goldstone boson arising from the spontaneous breaking of the accidental PQ symmetry---against semiclassical gravity effects, thus suppressing gravitational corrections to the effective potential, while it can at the same time provide for the small explicit symmetry breaking term needed to make models with domain wall number ${N}_{\mathrm{DW}}g1$, such as the popular Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model (${N}_{\mathrm{DW}}=6$), cosmologically viable even in the case where spontaneous PQ symmetry breaking occurred after inflation. We find that $N=10$ DFSZ accions with mass ${m}_{A}\ensuremath{\approx}3.5--4.2\text{ }\text{ }\mathrm{meV}$ can account for cold dark matter and simultaneously explain the hints for anomalous cooling of white dwarfs. The proposed helioscope International Axion Observatory---being sensitive to solar DFSZ accions with mass above a few meV---will decisively test this scenario.
108 citations
TL;DR: In this paper, the authors derive positivity bounds for scattering amplitudes of particles with arbitrary spin using unitarity, analyticity and crossing symmetry, which imply the positivity of certain low-energy coefficients of the effective action that controls the dynamics of the light degrees of freedom.
Abstract: We derive positivity bounds for scattering amplitudes of particles with arbitrary spin using unitarity, analyticity and crossing symmetry. The bounds imply the positivity of certain low-energy coefficients of the effective action that controls the dynamics of the light degrees of freedom. We show that low-energy amplitudes strictly softer than $O(p^4)$ do not admit unitary ultraviolet completions unless the theory is free. This enforces a bound on the energy growth of scattering amplitudes in the region of validity of the effective theory. We discuss explicit examples including the Goldstino from spontaneous supersymmetry breaking, and the theory of a spin-1/2 fermion with a shift symmetry.
90 citations
TL;DR: In this paper, the strangeness of the S = − 2 baryon-baryon interaction was studied in chiral effective field theory up to next-to-leading order.
86 citations
TL;DR: In this article, the authors study the symmetry of the time-evolution operator of nonunitary quantum walks and provide a necessary and sufficient condition that the non-unitary walk retains symmetry even when parameters of the model depend on position.
Abstract: $\mathcal{PT}$ symmetry, that is, a combined parity and time-reversal symmetry, is a key milestone for non-Hermitian systems exhibiting entirely real eigenenergy. In the present work, motivated by a recent experiment, we study $\mathcal{PT}$ symmetry of the time-evolution operator of nonunitary quantum walks. We present the explicit definition of $\mathcal{PT}$ symmetry by employing a concept of symmetry time frames. We provide a necessary and sufficient condition so that the time-evolution operator of the nonunitary quantum walk retains $\mathcal{PT}$ symmetry even when parameters of the model depend on position. It is also shown that there exist extra symmetries embedded in the time-evolution operator. Applying these results, we clarify that the nonunitary quantum walk in the experiment does have $\mathcal{PT}$ symmetry.
TL;DR: In this article, a scale invariant model of quadratic gravity with a nonminimally coupled scalar field is studied and it is shown that scale invariance is spontaneously broken and a mass scale naturally emerges.
Abstract: We study a scale-invariant model of quadratic gravity with a nonminimally coupled scalar field. We focus on cosmological solutions and find that scale invariance is spontaneously broken and a mass scale naturally emerges. Before the symmetry breaking, the Universe undergoes an inflationary expansion with nearly the same observational predictions of Starobinsky's model. At the end of inflation, the Hubble parameter and the scalar field converge to a stable fixed point through damped oscillations and the usual Einstein-Hilbert action is recovered. The oscillations around the fixed point can reheat the Universe in various ways, and we study in detail some of these possibilities.
TL;DR: In this paper, the spontaneous chiral symmetry breaking in the vacuum and its restoration at finite temperature are correctly realized in the holographic QCD framework, for the first time, with proper profiles of the scalar potential and the dilaton field.
Abstract: With proper profiles of the scalar potential and the dilaton field, for the first time, the spontaneous chiral symmetry breaking in the vacuum and its restoration at finite temperature are correctly realized in the holographic QCD framework. In the chiral limit, a nonzero chiral condensate develops in the vacuum and decreases with temperature, and the phase transition is of the second order for a two-flavor case and of the first order for a three-flavor case. In the case of explicit chiral symmetry breaking, in the two-flavor case, the second-order phase transition turns into a crossover with any nonzero current quark mass, and in the three-flavor case, the first-order phase transition turns into a crossover at a finite current quark mass. The correct description of chiral symmetry breaking and restoration makes the holographic QCD models more powerful in dealing with nonperturbative QCD phenomena. This framework can be regarded as a general setup in an application of AdS/CFT to describe conventional Ginzburg-Landau-Wilson-type phase transitions, e.g. in condensed matter and cosmology systems.
TL;DR: In this paper, the effect of a T-brane on the membrane superpotential with monopole operators is investigated, and it is shown that a Tbrane is able to deform the membrane world-potential, which partially breaks the three-dimensional flavor symmetry, and reduce super-symmetry.
Abstract: T-branes are exotic bound states of D-branes, characterized by mutually non-commuting vacuum expectation values for the worldvolume scalars. The M/F-theory geometry lifting D6/D7-brane configurations is blind to the T-brane data. In this paper, we make this data manifest, by probing the geometry with an M2-brane. We find that the effect of a T-brane is to deform the membrane worldvolume superpotential with monopole operators, which partially break the three-dimensional flavor symmetry, and reduce super-symmetry from $$ \mathcal{N} $$
= 4 to $$ \mathcal{N} $$
= 2. Our main tool is 3d mirror symmetry. Through this language, a very concrete framework is developed for understanding T-branes in M-theory. This leads us to uncover a new class of $$ \mathcal{N} $$
= 2 quiver gauge theories, whose Higgs branches mimic those of membranes at ADE singularities, but whose Coulomb branches differ from their $$ \mathcal{N} $$
= 4 counterparts.
TL;DR: In this work, the symmetry group and similarity reductions of the two-dimensional generalized Benney system are investigated by means of the geometric approach of an invariance group, which is equivalent to the classical Lie symmetry method.
Abstract: In this work, the symmetry group and similarity reductions of the two-dimensional generalized Benney system are investigated by means of the geometric approach of an invariance group, which is equivalent to the classical Lie symmetry method. Firstly, the vector field associated with the Lie group of transformation is obtained. Then the point transformations are proposed, which keep the solutions of the generalized Benney system invariant. Finally, the symmetry reductions and explicitly exact solutions of the generalized Benney system are derived by solving the corresponding symmetry equations.
TL;DR: In this paper, the properties of media with chiral imbalance parametrized by chiral chemical potential were studied and it was shown that depending on the strength of interaction between constituents in the media, the chiral Chemical Potential either creates or enhances dynamical chiral symmetry breaking.
Abstract: In this paper, we study the properties of media with chiral imbalance parametrized by chiral chemical potential. It is shown that depending on the strength of interaction between constituents in the media the chiral chemical potential either creates or enhances dynamical chiral symmetry breaking. Thus, the chiral chemical potential plays the role of the catalyst of dynamical chiral symmetry breaking. Physically, this effect results from the appearance of the Fermi surface and additional fermion states on this surface, which take part in dynamical chiral symmetry breaking. An interesting conclusion which can be drawn is that at sufficiently small temperature chiral plasma is unstable with respect to condensation of Cooper pairs and dynamical chiral symmetry breaking even for vanishingly small interactions between constituents.
TL;DR: Above-threshold ionization of rare-gas atoms by a bicircular field with its two components counterrotating is theoretically investigated by means of the improved strong-field approximation.
Abstract: Above-threshold ionization of rare-gas atoms by a bicircular field with its two components counterrotating is theoretically investigated by means of the improved strong-field approximation. Both direct and rescattered electrons are considered and the quantum orbits that lead into a specific final state are calculated and depicted. The angle-dependent spectrum reflects the discrete rotational symmetry of the bicircular field. The backward-scattering contributions are very similar to those generated by a linearly polarized field; several such contributions are rotated one versus the other by the symmetry angle of the discrete rotational symmetry. The forward-scattering contributions dramatically affect the velocity map at comparatively low momenta. The direct-electron spectrum observes reflection symmetry about several symmetry axes determined by the field symmetry. This is broken by rescattering.
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TL;DR: In this paper, a two-dimensional dilaton gravity system was studied and the symmetry breaking was studied in terms of a Schwarzian derivative effective action for a reparametrization.
Abstract: We study a two dimensional dilaton gravity system, recently examined by Almheiri and Polchinski, which describes near extremal black holes, or more generally, nearly $AdS_2$ spacetimes. The asymptotic symmetries of $AdS_2$ are all the time reparametrizations of the boundary. These symmetries are spontaneously broken by the $AdS_2$ geometry and they are explicitly broken by the small deformation away from $AdS_2$. This pattern of spontaneous plus explicit symmetry breaking governs the gravitational backreaction of the system. It determines several gravitational properties such as the linear in temperature dependence of the near extremal entropy as well as the gravitational corrections to correlation functions. These corrections include the ones determining the growth of out of time order correlators that is indicative of chaos. These gravitational aspects can be described in terms of a Schwarzian derivative effective action for a reparametrization.
TL;DR: In this article, the superconducting instabilities in the vicinity of the quantum-critical point of an inversion symmetry breaking order are analyzed. But unlike many well-known examples, the selection of the pairing symmetry of the condensate is independent of the momentum-space structure of the collective mode that mediates the pairing interaction.
Abstract: We analyze the superconducting instabilities in the vicinity of the quantum-critical point of an inversion symmetry breaking order. We first show that the fluctuations of the inversion symmetry breaking order lead to two degenerate superconducting (SC) instabilities, one in the $s$-wave channel, and the other in a time-reversal invariant odd-parity pairing channel (the simplest case being the same as the of $^{3}\mathrm{He}\ensuremath{-}\mathrm{B}$ phase). Remarkably, we find that unlike many well-known examples, the selection of the pairing symmetry of the condensate is independent of the momentum-space structure of the collective mode that mediates the pairing interaction. We found that this degeneracy is a result of the existence of a conserved fermionic helicity $\ensuremath{\chi}$, and the two degenerate channels correspond to even and odd combinations of SC order parameters with $\ensuremath{\chi}=\ifmmode\pm\else\textpm\fi{}1$. As a result, the system has an enlarged symmetry $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$, with each $U(1)$ corresponding to one value of the helicity $\ensuremath{\chi}$. Because of the enlarged symmetry, this system admits exotic topological defects such as a fractional quantum vortex, which we show has a Majorana zero mode bound at its core. We discuss how the enlarged symmetry can be lifted by small perturbations, such as the Coulomb interaction or Fermi surface splitting in the presence of broken inversion symmetry, and we show that the resulting superconducting state can be topological or trivial depending on parameters. The $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$ symmetry is restored at the phase boundary between the topological and trivial SC states, and allows for a transition between topologically distinct SC phases without the vanishing of the order parameter. We present a global phase diagram of the superconducting states and discuss possible experimental implications.
TL;DR: In this article, the existence of a second order quantum phase transition between a massless fermion phase and a massive one was shown to arise due to dynamics without the need for spontaneous symmetry breaking.
Abstract: Using large scale Monte Carlo calculations in a simple three dimensional lattice fermion model, we establish the existence of a second order quantum phase transition between a massless fermion phase and a massive one, both of which have the same symmetries. This shows that fermion masses can arise due to dynamics without the need for spontaneous symmetry breaking. Universality suggests that this alternate origin of the fermion mass should be of fundamental interest.
TL;DR: In this article, the optimal symmetry breaking region in Caffarelli-Kohn-Nirenberg inequalities is characterized and sharp estimates for the principal eigenvalue of Schrodinger operators on some non-flat non-compact manifolds are obtained.
Abstract: This paper is motivated by the characterization of the optimal symmetry breaking region in Caffarelli-Kohn-Nirenberg inequalities. As a consequence, optimal functions and sharp constants are computed in the symmetry region. The result solves a longstanding conjecture on the optimal symmetry range. As a byproduct of our method we obtain sharp estimates for the principal eigenvalue of Schrodinger operators on some non-flat non-compact manifolds, which to the best of our knowledge are new. The method relies on generalized entropy functionals for nonlinear diffusion equations. It opens a new area of research for approaches related to carre du champ methods on non-compact manifolds. However, key estimates depend as much on curvature properties as on purely nonlinear effects. The method is well adapted to functional inequalities involving simple weights and also applies to general cylinders. Beyond results on symmetry and symmetry breaking, and on optimal constants in functional inequalities, rigidity theorems for nonlinear elliptic equations can be deduced in rather general settings.
TL;DR: In this paper, a simple model for Dirac neutrinos where the smallness of neutrino mass follows from a parameter κ whose absence enhances the symmetry of the theory is proposed.
Journal Article•
TL;DR: It is shown that Coulomb repulsion or an external Zeeman field suppresses the s-wave pairing and promotes the odd-parity superconducting state, which opens a route to novel topological superconductivity.
Abstract: We study superconductivity in spin-orbit-coupled systems in the vicinity of inversion symmetry breaking. We find that, because of the presence of spin-orbit coupling, fluctuations of the incipient parity-breaking order generate an attractive pairing interaction in an odd-parity pairing channel, which competes with the s-wave pairing. We show that Coulomb repulsion or an external Zeeman field suppresses the s-wave pairing and promotes the odd-parity superconducting state. Our work provides a new mechanism for odd-parity pairing and opens a route to novel topological superconductivity.
TL;DR: In this article, the authors proposed a new dynamics of the electroweak symmetry breaking in a classically scale invariant version of the standard model, where scale invariance is broken by the condensations of additional fermions under a strong coupling dynamics.
TL;DR: In this paper, a model of radiative charged lepton and neutrino masses with A 4 symmetry was proposed and the soft breaking of A 4 to Z 3 lepton triality was accomplished by dimension-three terms.
TL;DR: In this paper, it was shown that neutrino mass models using either the type T3 or type T1-iii one-loop topologies do not give viable minimal dark matter candidates.
TL;DR: In this paper, an S3 symmetry was imposed on the quark fields under which two of three quarks transform like a doublet and the remaining one as a singlet, and used a scalar sector with the same structure of SU(2) doublets.
TL;DR: In this article, the authors investigated relativistic quantum effects on a scalar particle in background of the Lorentz symmetry violation defined by a tensor field, and they showed that harmonic-type and linear-type confining potentials can stem from the breaking of the symmetry.
TL;DR: In this paper, it was shown that a discrete symmetry remains preserved at high energies and only becomes dynamically broken at relatively late times in the cosmological evolution, i.e., after the dynamical breaking of SUSY.
Abstract: Spontaneously broken supersymmetry (SUSY) and a vanishingly small cosmological constant imply that $R$ symmetry must be spontaneously broken at low energies. Based on this observation, we suppose that, in the sector responsible for low-energy $R$ symmetry breaking, a discrete $R$ symmetry remains preserved at high energies and only becomes dynamically broken at relatively late times in the cosmological evolution, i.e., after the dynamical breaking of SUSY. Prior to $R$ symmetry breaking, the Universe is then bound to be in a quasi--de Sitter phase---which offers a dynamical explanation for the occurrence of cosmic inflation. This scenario yields a new perspective on the interplay between SUSY breaking and inflation, which neatly fits into the paradigm of high-scale SUSY: inflation is driven by the SUSY-breaking vacuum energy density, while the chiral field responsible for SUSY breaking, the Polonyi field, serves as the inflaton. Because $R$ symmetry is broken only after inflation, slow-roll inflation is not spoiled by otherwise dangerous gravitational corrections in supergravity. We illustrate our idea by means of a concrete example, in which both SUSY and $R$ symmetry are broken by strong gauge dynamics and in which late-time $R$ symmetry breaking is triggered by a small inflaton field value. In this model, the scales of inflation and SUSY breaking are unified, the inflationary predictions are similar to those of F-term hybrid inflation in supergravity, reheating proceeds via gravitino decay at temperatures consistent with thermal leptogenesis, and the sparticle mass spectrum follows from pure gravity mediation. Dark matter consists of thermally produced winos with a mass in the TeV range.
TL;DR: In this paper, the singular-drift problem associated with the appearance of nearly zero eigenvalues of the Dirac operator is avoided by deforming the action with a fermion bilinear term.
Abstract: In many interesting physical systems, the determinant which appears from integrating out fermions becomes complex, and its phase plays a crucial role in the determination of the vacuum. An example of this is QCD at low temperature and high density, where various exotic fermion condensates are conjectured to form. Another example is the Euclidean version of the type IIB matrix model for 10d superstring theory, where spontaneous breaking of the SO(10) rotational symmetry down to SO(4) is expected to occur. When one applies the complex Langevin method to these systems, one encounters the singular-drift problem associated with the appearance of nearly zero eigenvalues of the Dirac operator. Here we propose to avoid this problem by deforming the action with a fermion bilinear term. The results for the original system are obtained by extrapolations with respect to the deformation parameter. We demonstrate the power of this approach by applying it to a simple matrix model, in which spontaneous symmetry breaking from SO(4) to SO(2) is expected to occur due to the phase of the complex fermion determinant. Unlike previous work based on a reweighting-type method, we are able to determine the true vacuum by calculating the order parameters, which agree with the prediction by the Gaussian expansion method.