Featured Researches

High Energy Physics Lattice

Chiral properties of (2+1)-flavor QCD in strong magnetic fields at zero temperature

We present lattice QCD results for masses and magnetic polarizabilities of light and strange pseudo-scalar mesons, chiral condensates, decay constants of neutral pion and neutral kaon in the presence of background magnetic fields witheBranging up to around 3.35 GeV2(∼70 M2π) in the vacuum. The computations were carried out in (2+1)-flavor QCD mostly on323×96lattices using the Highly Improved Staggered Quarks (HISQ) action withMπ≈220 MeV at zero temperature. We find that the masses of neutral pseudo-scalar mesons monotonously decrease as the magnetic field strength grows and then saturate at a nonzero value, while there exists a non-monotonous behavior of charged pion and kaon masses in the magnetic field. We observe aqBscaling of the up and down quark flavor components of neutral pion mass, neutral pion decay constant as well as the quark chiral condensates at 0.05≲eB≲3.35 GeV2. We show that the correction to the Gell-Mann-Oakes-Renner relation involving neutral pion is less than 6% and the correction for the relation involving neutral kaon is less than 30% ateB≲3.35 GeV2. We also derive the Ward-Takahashi identities for QCD in the magnetic field in the continuum formulation including the relation between integrated neutral pseudo-scalar meson correlators and chiral condensates.

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High Energy Physics Lattice

Chiral symmetry and taste symmetry from the eigenvalue spectrum of staggered Dirac operators

We investigate general properties of the eigenvalue spectrum for improved staggered quarks. We introduce a new chirality operator[γ5⊗1]and a new shift operator[1⊗ξ5], which respect the same recursion relation as theγ5operator in the continuum. Then we show that matrix elements of the chirality operator sandwiched between two eigenstates of the staggered Dirac operator are related to those of the shift operator by the Ward identity of the conservedU(1)Asymmetry of staggered fermion actions. We perform a numerical study in quenched QCD using HYP staggered quarks to demonstrate the Ward identity. We introduce a new concept of leakage patterns which collectively represent the matrix elements of the chirality operator and the shift operator sandwiched between two eigenstates of the staggered Dirac operator. The leakage pattern provides a new method to identify zero modes and non-zero modes in the Dirac eigenvalue spectrum. This method is as robust as the spectral flow method but requires much less computing power. Analysis using a machine learning technique confirms that the leakage pattern is universal, since the staggered Dirac eigenmodes on normal gauge configurations respect it. In addition, the leakage pattern can be used to determine a ratio of renormalization factors as a by-product. We conclude that it might be possible and realistic to measure the topological chargeQusing the Atiya-Singer index theorem and the leakage pattern of the chirality operator in the staggered fermion formalism.

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High Energy Physics Lattice

Chiral-spin symmetry of the meson spectral function aboveTc

Recently, via calculation of spatial correlators ofJ=0,1isovector operators using a chirally symmetric Dirac operator withinNF=2QCD, it has been found that QCD at temperaturesTc−3Tcis approximatelySU(2)CSandSU(4)symmetric. The latter symmetry suggests that the physical degrees of freedom are chirally symmetric quarks bound by the chromoelectric field into color singlet objects without chromomagnetic effects. This regime of QCD has been referred to as a Stringy Fluid. Here we calculate correlators for propagation in time direction at a temperature slightly aboveTcand find the same approximate symmetries. This means that the meson spectral function is chiral-spin andSU(4)symmetric.

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High Energy Physics Lattice

Classifying Topological Charge in SU(3) Yang-Mills Theory with Machine Learning

We apply a machine learning technique for identifying the topological charge of quantum gauge configurations in four-dimensional SU(3) Yang-Mills theory. The topological charge density measured on the original and smoothed gauge configurations with and without dimensional reduction is used as inputs for the neural networks (NN) with and without convolutional layers. The gradient flow is used for the smoothing of the gauge field. We find that the topological charge determined at a large flow time can be predicted with high accuracy from the data at small flow times by the trained NN; for example, the accuracy exceeds99%with the data att/a2≤0.3. High robustness against the change of simulation parameters is also confirmed with a fixed physical volume. We find that the best performance is obtained when the spatial coordinates of the topological charge density are fully integrated out in preprocessing, which implies that our convolutional NN does not find characteristic structures in multi-dimensional space relevant for the determination of the topological charge.

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High Energy Physics Lattice

Clear correlation between monopoles and the chiral condensate in SU(3) QCD

We study spontaneous chiral-symmetry breaking in SU(3) QCD in terms of the dual superconductor picture for quark confinement in the maximally Abelian (MA) gauge, using lattice QCD Monte Carlo simulations with four different lattices of164,244,243×6atβ=6.0(i.e., the spacinga≃0.1 fm), and324atβ=6.2(i.e.,a≃0.075 fm), at the quenched level. First, in the confinement phase, we find Abelian dominance and monopole dominance in the MA gauge for the chiral condensate in the chiral limit,using the two different methods of i) the Banks-Casher relation with the Dirac eigenvalue density and ii) finite quark-mass calculations with the quark propagator and its chiral extrapolation. In the high-temperature deconfined phase, the chiral restoration is observed also for the Abelian and the monopole sectors. Second, we investigate local correlation between the chiral condensate and monopoles, which topologically appear in the MA gauge. We find that the chiral condensate locally takes a quite large value near monopoles. As an interesting possibility, the strong magnetic field around monopoles is responsible to chiral symmetry breaking in QCD, similarly to the magnetic catalysis.

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High Energy Physics Lattice

Cold Atom Quantum Simulator for String and Hadron Dynamics in Non-Abelian Lattice Gauge Theory

We propose an analog quantum simulator for simulating real time dynamics of(1+1)-d non-Abelian gauge theory well within the existing capacity of ultracold atom experiments. The scheme calls for the realization of a two-state ultracold fermionic system in a 1-dimensional bipartite lattice, and the observation of subsequent tunneling dynamics. Being based on novel loop string hadron formalism of SU(2) lattice gauge theory, this simulation technique is completely SU(2) invariant and simulates accurate dynamics of physical phenomena such as string breaking and/or pair production. The scheme is scalable, and particularly effective in simulating the theory in weak coupling regime, and also bulk limit of the theory in strong coupling regime up to certain approximations. This paper also presents a numerical benchmark comparison of exact spectrum and real time dynamics of lattice gauge theory to that of the atomic Hamiltonian with experimentally realizable range of parameters.

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High Energy Physics Lattice

Collins-Soper Kernel for TMD Evolution from Lattice QCD

The Collins-Soper kernel relates transverse momentum-dependent parton distribution functions (TMDPDFs) at different energy scales. For small parton transverse momentumqT∼ΛQCD, this kernel is non-perturbative and can only be determined with controlled uncertainties through experiment or first-principles calculations. This work presents the first exploratory determination of the Collins-Soper kernel using the lattice formulation of Quantum Chromodynamics. In a quenched calculation, theNf=0kernel is determined at scales in the range 250 MeV<qT<2GeV, and an analysis of the remaining systematic uncertainties is undertaken.

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High Energy Physics Lattice

Color Screening in Quantum Chromodynamics

We review lattice studies of the color screening in the quark-gluon plasma. We put the phenomena related to the color screening into the context of similar aspects of other physical systems (electromagnetic plasma or cold nuclear matter). We discuss the onset of the color screening and its signature and significance in the QCD transition region, and elucidate at which temperature and to which extent the weak-coupling picture based on hard thermal loop expansion, potential nonrelativistic QCD, or dimensionally-reduced QCD quantitatively captures the key properties of the color screening. We discuss the different regimes pertaining to the color screening and thermal dissociation of the static quarks in depth for various spatial correlation functions that are studied on the lattice, and clarify the status of their asymptotic screening masses. We finally discuss the screening correlation functions of dynamical mesons with a wide range of flavor and spin content, and how they conform with expectations for low- and high-temperature behavior.

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High Energy Physics Lattice

Color dependence of tensor and scalar glueball masses in Yang-Mills theories

We report the masses of the lightest spin-0 and spin-2 glueballs obtained in an extensive lattice study of the continuum and infinite volume limits ofSp(Nc)gauge theories forNc=2,4,6,8. We also extrapolate the combined results towards the large-Nclimit. We compute the ratio of scalar and tensor masses, and observe evidence that this ratio is independent ofNc. Other lattice studies of Yang-Mills theories at the same space-time dimension provide a compatible ratio. We further compare these results to various analytical ones and discuss them in view of symmetry-based arguments related to the breaking of scale invariance in the underlying dynamics, showing that a constant ratio might emerge in a scenario in which the0++glueball is interpreted as a dilaton state.

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High Energy Physics Lattice

Compact Gauge Fields on Causal Dynamical Triangulations: a 2D case study

We discuss the discretization of Yang-Mills theories on Dynamical Triangulations in the compact formulation, with gauge fields living on the links of the dual graph associated with the triangulation, and the numerical investigation of the minimally coupled system by Monte Carlo simulations. We provide, in particular, an explicit construction and implementation of the Markov chain moves for 2D Causal Dynamical Triangulations coupled to eitherU(1)orSU(2)gauge fields; the results of exploratory numerical simulations on a toroidal geometry are also presented for both cases. We study the critical behavior of gravity related observables, determining the associated critical indices, which turn out to be independent of the bare gauge coupling: we obtain in particularν=0.496(7)for the critical index regulating the divergence of the correlation length of the volume profiles. Gauge observables are also investigated, including holonomies (torelons) and, for theU(1)gauge theory, the winding number and the topological susceptibility. An interesting result is that the critical slowing down of the topological charge, which affects various lattice field theories in the continuum limit, seems to be strongly suppressed (i.e., by orders of magnitude) by the presence of a locally variable geometry: that may suggest possible ways for improvement also in other contexts.

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