arXiv subjects

Yanwen Shang

Publications and source records attributed to Yanwen Shang.

14 recordsLinked to original sources

arXiv2013

On the Potential for General Relativity and its Geometry

The unique ghost-free mass and nonlinear potential terms for general relativity are presented in a diffeomorphism and local Lorentz invariant vierbein formalism. This construction requires an additional two-index Stuckelberg field, beyond the four scalar fields used in the metric formulation, and unveils a new local SL(4) symmetry group of the mass and potential terms, not shared by the Einstein-Hilbert term. The new field is auxiliary but transforms as a vector under two different Lorentz groups, one of them the group of local Lorentz transformations, the other an additional global group. This formulation enables a geometric interpretation of the mass and potential terms for gravity in terms of certain volume forms. Furthermore, we find that the decoupling limit is much simpler to extract in this approach; in particular, we are able to derive expressions for the interactions of the vector modes. We also note that it is possible to extend the theory by promoting the two-index auxiliary field into a Nambu-Goldstone boson nonlinearly realizing a certain space-time symmetry, and show how it is "eaten up" by the antisymmetric part of the vierbein.

hep-th↗

arXiv2013

On two dimensional non-abelian chiral lattice gauge theories in Ginsparg-Wilson formalism

Defining chiral lattice gauge theories in the Ginsparg-Wilson formalism is complicated by the so-called fermion measure problem. It has been proven for the abelian theories that smooth well-behaved fermion measure exists if and only if the anomaly-free condition is granted, and the same was shown to hold in perturbative theories for non-abelian gauge groups, but the non-perturbative proof is absent. In this paper, we consider a simpler problem in 2-d and present a proof for the existence of smooth and gauge invariant fermion measure on the gauge field configuration space with zero field strengths for arbitrary compact Lie groups, provided the anomaly-free conditions are satisfied. It is conjectured that such consideration is sufficient for the unknown full proof.

hep-lat↗

arXiv2012

Correlation functions in the holographic replica method

Disorder has long been a difficult subject in condensed matter systems and the The replica method is a well-known tool in this field. Implementing the replica method the AdS/CFT correspondence has been proposed and discussed in literatures. We point out, for any CFT that has a holographic dual and to the leading order of the large-N expansion, the corrections due to the presence of random disorder to any connected correlation functions vanish identically, provided that the disorder strength is normalized as discussed in literatures and that the symmetry among replicas is unbroken. Same must hold true to any observables that are determined by the connected correlation functions through a linear relation. This behavior resembles strongly that of a free theory where disorder is coupled to the fundamental field. We demonstrate this by both the means of holographic principle and field theory analysis in a toy model. We also propose ways of evaluating the non-zero sub-leading effects perturbatively in terms of the disorder strength and discuss a novel possibility of defining a new holographic dual if we adopt a different normalization for the disorder strength.

hep-th↗

arXiv2011

On Lorentz violation in Horava-Lifshitz type theories

We show that coupling the Standard Model to a Lorentz symmetry violating sector may co-exist with viable phenomenology, provided that the interaction between the two is mediated by higher-dimensional operators. In particular, if the new sector acquires anisotropic scaling behavior above a "Horava-Lifshitz" energy scale L_HL and couples to the Standard Model through interactions suppressed by M_P, the transmission of the Lorentz violation into the Standard Model is protected by the ratio L_HL^2/M_P^2. A wide scale separation, L_HL<<M_P, can then make Lorentz-violating terms in the Standard Model sector within experimental bounds without fine-tuning. We first illustrate our point with a toy example of Lifshitz-type neutral fermion coupled to photon via the magnetic moment operator, and then implement similar proposal for the Hořava-Lifshitz gravity coupled to conventional Lorentz-symmetric matter fields. We find that most radiatively induced Lorentz violation can be controlled by a large scale separation, but the existence of instantaneously propagating non-Lifshitz modes in gravity can cause a certain class of diagrams to remain quadratically divergent above L_HL. Such problematic quadratic divergence, however, can be removed by extending the action with terms of higher Lifshitz dimension, resulting in a completely consistent setup that can cope with the stringent tests of Lorentz invariance.

hep-th↗

arXiv2010

On 1-loop diagrams in AdS space and the random disorder problem

We study the complex scalar loop corrections to the boundary-boundary gauge two point function in pure AdS space in Poincare coordinates, in the presence of a boundary quadratic perturbation to the scalar. These perturbations correspond to double trace perturbations in the dual CFT and modify the boundary conditions of the bulk scalars in AdS. We find that, in addition to the usual UV divergences, the 1-loop calculation suffers from a divergence originating in the limit as the loop vertices approach the AdS horizon. We show that this type of divergence is independent of the boundary coupling, and making use of which we extract the finite relative variation of the imaginary part of the loop via Cutkosky rules as the boundary perturbation varies. Applying our methods to compute the effects of a time-dependent impurity to the conductivities using the replica trick in AdS/CFT, we find that generally an IR-relevant disorder reduces the conductivity and that in the extreme low frequency limit the correction due to the impurities overwhelms the planar CFT result even though it is supposedly $1/N^2$ suppressed. Comments on the effect of time-independent impurity in such a system are presented.

hep-th↗

arXiv2010

Chiral Lattice Gauge Theories Via Mirror-Fermion Decoupling: A Mission (im)Possible?

This is a review of the status and outstanding issues in attempts to construct chiral lattice gauge theories by decoupling the mirror fermions from a vectorlike theory. In the first half, we explain why studying nonperturbative chiral gauge dynamics may be of interest, enumerate the problems that a lattice formulation of chiral gauge theories must overcome, and briefly review our current knowledge. We then discuss the motivation and idea of mirror-fermion decoupling and illustrate the desired features of the decoupling dynamics by a simple solvable toy model. The role of exact chiral symmetries and matching of 't Hooft anomalies on the lattice is also explained. The second, more technical, half of the article is devoted to a discussion of the known and unknown features of mirror-decoupling dynamics formulated with Ginsparg-Wilson fermions. We end by pointing out possible directions for future studies.

hep-lat↗

arXiv2009

Solar Gamma Rays Powered by Secluded Dark Matter

Secluded dark matter models, in which WIMPs annihilate first into metastable mediators, can present novel indirect detection signatures in the form of gamma rays and fluxes of charged particles arriving from directions correlated with the centers of large astrophysical bodies within the solar system, such as the Sun and larger planets. This naturally occurs if the mean free path of the mediator is in excess of the solar (or planetary) radius. We show that existing constraints from water Cerenkov detectors already provide a novel probe of the parameter space of these models, complementary to other sources, with significant scope for future improvement from high angular resolution gamma-ray telescopes such as Fermi-LAT. Fluxes of charged particles produced in mediator decays are also capable of contributing a significant solar system component to the spectrum of energetic electrons and positrons, a possibility which can be tested with the directional and timing information of PAMELA and Fermi.

hep-ph↗

arXiv2009

Lattice chirality, anomaly matching, and more on the (non)decoupling of mirror fermions

We study 't Hooft anomaly matching in lattice models with strong Yukawa or multi-fermion interactions. Strong non-gauge interactions among the mirror fermions in a vectorlike lattice gauge theory are introduced with the aim to obtain, in a strong-coupling symmetric phase, a long-distance unbroken gauge theory with chiral fermions in a complex representation. We show how to use exact lattice chirality to analyze the anomaly matching conditions on chiral symmetry current correlators at finite lattice spacing and volume. We perform a Monte Carlo study of the realization of anomaly matching in a toy two-dimensional model with an anomalous mirror-fermion content at strong mirror Yukawa coupling. We show that 't Hooft anomaly matching is satisfied, in most of the phase diagram, via the minimal solution in either the massless fermion or "Goldstone" mode, while in some cases there are extra massless vectorlike mirror fermions. The mirror spectrum at strong coupling is thus consistent with long-distance unitarity. We discuss the implications of our results for future studies of the most interesting case of the decoupling of anomaly-free mirror-fermion sectors.

hep-lat↗

arXiv2008

General Massive Spin-2 on de Sitter Background

We study linearized massive gravity on the de Sitter background. With a small-enough graviton mass this may have had relevance to inflation, or the present-day cosmic acceleration. Higuchi has shown that this theory has a ghost as long as the dS curvature exceeds the graviton mass, 2H^2 > m^2; this would entail rapid instabilities. In this work we extend the model and observe that the helicity-0 mode which is a ghost, can be turned into a positive energy state via kinetic mixing with the conformal mode. The latter gets restricted here by the requirement that the Bianchi identities be satisfied. This eliminates the ghost from the linearized theory. However, the spectrum still contains scalar tachyonic instabilities with the time scale ~1/H. This would have been problematic for the early univesre, however, may be acceptable for the present-day accelerated expansion as the tachyon instability would take the age of the universe to develop.

hep-th↗

arXiv2008

"Light from chaos" in two dimensions

We perform a Monte-Carlo study of the lattice two-dimensional gauged XY-model. Our results confirm the strong-coupling expansion arguments that for sufficiently small values of the spin-spin coupling the ``gauge symmetry breaking" terms decouple and the long-distance physics is that of the unbroken pure gauge theory. We find no evidence for the existence, conjectured earlier, of massless states near a critical value of the spin-spin coupling. We comment on recent remarks in the literature on the use of gauged XY-models in proposed constructions of chiral lattice gauge theories.

hep-lat↗

arXiv2007

Lattice chirality and the decoupling of mirror fermions

We show, using exact lattice chirality, that partition functions of lattice gauge theories with vectorlike fermion representations can be split into "light" and "mirror" parts, such that the "light" and "mirror" representations are chiral. The splitting of the full partition function into "light" and "mirror" is well defined only if the two sectors are separately anomaly free. We show that only then is the generating functional, and hence the spectrum, of the mirror theory a smooth function of the gauge field background. This explains how ideas to use additional non-gauge, high-scale mirror-sector dynamics to decouple the mirror fermions without breaking the gauge symmetry--for example, in symmetric phases at strong mirror Yukawa coupling--are forced to respect the anomaly-free condition when combined with the exact lattice chiral symmetry. Our results also explain a paradox posed by a recent numerical study of the mirror-fermion spectrum in a toy would-be-anomalous two-dimensional theory. In passing, we prove some general properties of the partition functions of arbitrary chiral theories on the lattice that should be of interest for further studies in this field.

hep-th↗

arXiv2006

Resonance in Asymmetric Warped Geometry

We study the spectrum of an asymmetric warped braneworld model with different AdS curvatures on either side of the brane. In addition to the RS-like modes we find a resonance state. Its mass is proportional to the geometric mean of the two AdS curvature scales, while the difference between them determines the strength of the resonance peak. There is a complementarity between the RS zero-mode and the resonance: making the asymmetry stronger weakens the zero-mode but strengthens the resonance, and vice versa. We calculate numerically the braneworld gravitational potential and discuss the holographic correspondence for the asymmetric model.

hep-th↗

arXiv2005

Quantum Cosmology of Classically Constrained Gravity

In hep-th/0506040 we discussed a classically constrained model of gravity. This theory contains known solutions of General Relativity (GR), and admits solutions that are absent in GR. Here we study cosmological implications of some of these new solutions. We show that a spatially-flat de Sitter universe can be created from ``nothing''. This universe has boundaries, and its total energy equals to zero. Although the probability to create such a universe is exponentially suppressed, it favors initial conditions suitable for inflation. Then we discuss a finite-energy solution with a nonzero cosmological constant and zero space-time curvature. There is no tunneling suppression to fluctuate into this state. We show that for a positive cosmological constant this state is unstable -- it can rapidly transition to a de Sitter universe providing a new unsuppressed channel for inflation. For a negative cosmological constant the space-time flat solutions is stable.

hep-th↗

arXiv2005

Classically Constrained Gauge Fields and Gravity

We study gauge and gravitational field theories in which the gauge fixing conditions are imposed as constraints on classical fields. Quantization of fluctuations can be performed in a BRST invariant manner, while the main novelty is that the classical equations of motion admit solutions that are not present in the standard approach. Although the new solutions exist for both gauge and gravitational fields, one interesting example we consider in detail is constrained gravity endowed with a nonzero cosmological constant. This theory, unlike General Relativity, admits two maximally symmetric solutions one of which is a flat space, and another one is a curved-space solution of GR. We argue that, due to BRST symmetry, the classical solutions obtained in these theories are not ruined by quantum effects. We also comment on massive deformations of the constrained models. For both gauge and gravity fields we point out that the propagators of the massive quanta have soft ultraviolet behavior and smooth transition to the massless limit. However, nonlinear stability may require further modifications of the massive theories.

hep-th↗