arXiv subjects

Xiao-Yong Jin

Publications and source records attributed to Xiao-Yong Jin.

34 records · Page 2Linked to original sources

arXiv2016

Strongly interacting dynamics and the search for new physics at the LHC

We present results for the spectrum of a strongly interacting SU(3) gauge theory with $N_f = 8$ light fermions in the fundamental representation. Carrying out non-perturbative lattice calculations at the lightest masses and largest volumes considered to date, we confirm the existence of a remarkably light singlet scalar particle. We explore the rich resonance spectrum of the 8-flavor theory in the context of the search for new physics beyond the standard model at the Large Hadron Collider (LHC). Connecting our results to models of dynamical electroweak symmetry breaking, we estimate the vector resonance mass to be about 2 TeV with a width of roughly 450 GeV, and predict additional resonances with masses below ~3 TeV.

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arXiv2015

Curvature of the critical line on the plane of quark chemical potential and pseudo scalar meson mass for three-flavor QCD

We investigate the phase structure of three-flavor QCD in the presence of finite quark chemical potential $μ/T\lesssim1.2$ by using the non-perturbatively $O(a)$ improved Wilson fermion action on lattices with a fixed temporal extent $N_{\rm t}=6$ and varied spatial linear extents $N_{\rm s}=8,10,12$. Especially, we focus on locating the critical end point that characterizes the phase structure, and extracting the curvature of the critical line on the $μ$-$m_π$ plane. For Wilson-type fermions, the correspondence between bare parameters and physical parameters is indirect. Hence we present a strategy to transfer the bare parameter phase structure to the physical one, in order to obtain the curvature. Our conclusion is that the curvature is positive. This implies that, if one starts from a quark mass in the region of crossover at zero chemical potential, one would encounter a first-order phase transition when one raises the chemical potential.

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arXiv2015

Phase structure of $N_{\rm f}=3$ QCD at finite temperature and density by Wilson-Clover fermions

We investigate the phase structure of 3-flavor QCD in the presence of finite quark chemical potential by using Wilson-Clover fermions. To deal with the complex action with finite density, we adopt the phase reweighting method. In order to survey a wide parameter region, we employ the multi-parameter reweighting method as well as the multi-ensemble reweighting method. Especially, we focus on locating the critical end point that characterizes the phase structure. It is estimated by the kurtosis intersection method for the quark condensate. For Wilson-type fermions, the correspondence between bare parameters and physical parameters is indirect, thus we present a strategy to transfer the bare parameter phase structure to the physical one. We conclude that the curvature with respect to the chemical potential is positive. This implies that, if one starts from a quark mass in the region of crossover at zero chemical potential, one would encounter a first-order phase transition when one raises the chemical potential.

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arXiv2015

Stealth Dark Matter: Dark scalar baryons through the Higgs portal

We present a new model of "Stealth Dark Matter": a composite baryonic scalar of an $SU(N_D)$ strongly-coupled theory with even $N_D \geq 4$. All mass scales are technically natural, and dark matter stability is automatic without imposing an additional discrete or global symmetry. Constituent fermions transform in vector-like representations of the electroweak group that permit both electroweak-breaking and electroweak-preserving mass terms. This gives a tunable coupling of stealth dark matter to the Higgs boson independent of the dark matter mass itself. We specialize to $SU(4)$, and investigate the constraints on the model from dark meson decay, electroweak precision measurements, basic collider limits, and spin-independent direct detection scattering through Higgs exchange. We exploit our earlier lattice simulations that determined the composite spectrum as well as the effective Higgs coupling of stealth dark matter in order to place bounds from direct detection, excluding constituent fermions with dominantly electroweak-breaking masses. A lower bound on the dark baryon mass $m_B \gtrsim 300$ GeV is obtained from the indirect requirement that the lightest dark meson not be observable at LEP II. We briefly survey some intriguing properties of stealth dark matter that are worthy of future study, including: collider studies of dark meson production and decay; indirect detection signals from annihilation; relic abundance estimates for both symmetric and asymmetric mechanisms; and direct detection through electromagnetic polarizability, a detailed study of which will appear in a companion paper.

hep-ph↗

arXiv2015

Direct Detection of Stealth Dark Matter through Electromagnetic Polarizability

We calculate the spin-independent scattering cross section for direct detection that results from the electromagnetic polarizability of a composite scalar baryon dark matter candidate -- "Stealth Dark Matter", that is based on a dark SU(4) confining gauge theory. In the nonrelativistic limit, electromagnetic polarizability proceeds through a dimension-7 interaction leading to a very small scattering cross section for dark matter with weak scale masses. This represents a lower bound on the scattering cross section for composite dark matter theories with electromagnetically charged constituents. We carry out lattice calculations of the polarizability for the lightest baryons in SU(3) and SU(4) gauge theories using the background field method on quenched configurations. We find the polarizabilities of SU(3) and SU(4) to be comparable (within about 50%) normalized to the baryon mass, which is suggestive for extensions to larger SU(N) groups. The resulting scattering cross sections with a xenon target are shown to be potentially detectable in the dark matter mass range of about 200-700 GeV, where the lower bound is from the existing LUX constraint while the upper bound is the coherent neutrino background. Significant uncertainties in the cross section remain due to the more complicated interaction of the polarizablity operator with nuclear structure, however the steep dependence on the dark matter mass, $1/m_B^6$, suggests the observable dark matter mass range is not appreciably modified. We briefly highlight collider searches for the mesons in the theory as well as the indirect astrophysical effects that may also provide excellent probes of stealth dark matter.

hep-ph↗

arXiv2015

Scalar correlators near the 3-flavor thermal critical point

We investigate screening masses at both sides of the first order finite temperature transition with 3 quark flavors using the nonperturbatively improved clover fermion action and the Iwasaki gauge action. We have developed the method of hierarchical truncations with stochastic probing to accelerate the noise estimator for evaluating quark loops at every spatial lattice slices. At parameter values we study, the flavor singlet scalar meson has a screening mass about half of the pion screening mass. It becomes lighter as the system approaches the critical endpoint.

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arXiv2014

Critical endpoint of finite temperature phase transition for three flavor QCD

We investigate the critical endpoint of finite temperature phase transition of $N_f=3$ QCD at zero chemical potential. We employ the renormalization-group improved Iwasaki gauge action and non-perturbatively $O(a)$-improved Wilson-clover fermion action. The critical endpoint is determined by using the intersection point of kurtosis for the temporal size $N_t$=4, 6, 8. Spatial sizes of $N_l$=6-16 ($N_t$=4), 10-24 ($N_t$=6), and 12-24 ($N_t$=8) are employed. We find that $N_t$=4 is out of the scaling region. Using results for $N_t$=6 and 8, and making linear extrapolations in $1/N_t^2$, we obtain $\sqrt{t_0}T_{\rm E}=0.0975(14)(8)$, $\sqrt{t_0}m_{\rm PS,E}=0.2254(52)(105)$ and $m_{\rm PS,E}/T_{\rm E}=2.311(63)(13)$, where the first error is statistical error, the second error is systematic error, and $m_{\rm PS}$ is the pseudo scalar meson mass. If one uses $1/\sqrt{t_0}=1.347(30)$ GeV reported by Borsanyi et al., one finds $T_{\rm E}=131(2)(1)(3)$ MeV, $m_{\rm PS,E}=304(7)(14)(7)$ MeV and $m_{\rm PS,E}/m_{\rm PS,E}^{\rm phys, sym}=0.739(17)(34)(17)$, where the third error comes from error of $\sqrt{t_0}$ and $m_{\rm PS}^{\rm phys, sym}=\sqrt{(m_π^2+2m_K^2)/3}$. Our current estimation of $\sqrt{t_0}m_{\rm PS,E}$ in the continuum limit is about 25% smaller than the SU(3) symmetric point.

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arXiv2014

Critical end point of Nf=3 QCD at finite temperature and density

We investigate the phase structure of 3-flavor QCD in the presence of finite quark chemical potential $aμ=0.1$ by using the Wilson-Clover fermion action. Especially, we focus on locating the critical end point that characterizes the phase structure. We do this by the kurtosis intersection method for the quark condensate. For Wilson-type fermions, the correspondence between bare parameters and physical parameters is indirect. Hence we present a strategy to transfer the bare parameter phase structure to the physical one.

hep-lat↗

arXiv2013

Zeros of QCD partition function from finite density lattices

Partition function zeros steer the critical behavior of a system. Studying four-flavor lattice QCD at finite temperature and density with the Wilson-clover fermion action and the Iwasaki gauge action using a phase-quenched fermion determinant, we combine statistics from multiple chemical potentials to improve sampling of the configuration space, and aim at unraveling the movement of zeros in finite systems. Preparing for further investigations, we discuss methods and criteria used to sieve through complex parameter space spanned by $(\text{Re}μ, \text{Im}μ)$ and $(\text{Re}μ, \text{Im}β)$, and present statistically robust zeros of the partition function.

hep-lat↗

arXiv2013

Finite size scaling study of $N_{\text{f}}=4$ finite density QCD on the lattice

We explore the phase space spanned by the temperature and the chemical potential for 4-flavor lattice QCD using the Wilson-clover quark action. In order to determine the order of the phase transition, we apply finite size scaling analyses to gluonic and quark observables including plaquette, Polyakov loop and quark number density, and examine their susceptibility, skewness, kurtosis and Challa-Landau-Binder cumulant. Simulations were carried out on lattices of a temporal size fixed at $N_{\text{t}}=4$ and spatial sizes chosen from $6^3$ up to $10^3$. Configurations were generated using the phase reweighting approach, while the value of the phase of the quark determinant were carefully monitored. The $μ$-parameter reweighting technique is employed to precisely locate the point of the phase transition. Among various approximation schemes for calculating the ratio of quark determinants needed for $μ$-reweighting, we found the Taylor expansion of the logarithm of the quark determinant to be the most reliable. Our finite-size analyses show that the transition is first order at $(β, κ, μ/T)=(1.58, 0.1385, 0.584\pm 0.008)$ where $(m_π/m_ρ, T/m_ρ)=(0.822, 0.154)$. It weakens considerably at $(β, κ, μ/T)=(1.60, 0.1371, 0.821\pm 0.008)$ where $(m_π/m_ρ, T/m_ρ)=(0.839, 0.150)$, and a crossover rather than a first order phase transition cannot be ruled out.

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arXiv2013

Lattice QCD with 12 Quark Flavors: A Careful Scrutiny

With a substantial amount of simulations, we have explored the system across a wide range of lattice scales. We have located a lattice artifact, first order bulk transition, have studied its properties, and found that the flavor-singlet scalar meson mass vanishes at the critical endpoint. We will discuss the lattice phase diagrams and the continuum limits for both a spontaneous chiral symmetry breaking phase and an infrared conformal phase, and compare results with other groups.

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arXiv2012

Finite size scaling for 4-flavor QCD with finite chemical potential

We explore the phase diagram spanned by the temperature and the chemical potential for 4-flavor QCD by the phase-reweighting approach. In order to determine the order of phase transition, we perform finite size scaling studies for various quantities, for example, susceptibility, kurtosis and Challa-Landau-Binder cumulant. At the parameter (beta=1.60, kappa=0.1371, c_sw=1.9655 and N_T=4), where the Kentucky group reported a first-order phase transition in their canonical simulation, we observe that the transition is consistent with being of first order.

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arXiv2012

Lattice QCD with 12 Degenerate Quark Flavors

We report on new data from additional zero temperature simulations of QCD with 12 flavors. This is a continuation of previous studies using the DBW2 gauge action and naive staggered fermions. With the use of the force gradient integrator and a multiple-quark-mass preconditioned HMC, we have done simulations with input quark masses from $m_q=0.003$ to $m_q=0.008$. We have observed a metastable, first order, bulk transition that occurs at small input quark masses. As the quark mass increases, this first order bulk transition ends at a second order critical point, and, for still heavier quark masses, becomes the cross-over we have previously reported. We present measurements of hadron masses, decay constants and other low energy observables in the small quark mass region on the weak coupling side of the bulk transition. Our results show that the behavior of the system is still consistent with spontaneously broken chiral symmetry. We also discuss a preliminary investigation into the behavior of the bulk transition itself. We have found that, as the system approaches the second order critical end point, the scalar singlet meson becomes lighter. Thus it appears that the critical endpoint corresponds to a continuum limit theory only involving scalars and, following known triviality arguments, this is likely a free field theory. The presence of this critical endpoint could influence scaling of lattice observables in the conventional continuum limit.

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arXiv2010

Evidence for a First Order, Finite Temperature Phase Transition in 8 Flavor QCD

As part of our ongoing investigations of QCD with many flavors of quarks, here we report on studies of the finite temperature phase transition for eight-flavor QCD with the DBW2 gauge action and naïve staggered fermions. We find a clear first order phase transition between the chirally asymmetric phase at zero temperature and the chirally symmetric phase at finite temperature, signaled by a two-state signal for $\langle\barψψ\rangle$ at a non-zero temperature. We see this signal at a gauge coupling of $β=0.54$, where, to set the scale, the zero temperature value for $f_π$, in the chiral limit, is 0.06661(92). This strong, first-order signal is seen for two different values of the quark mass, $m_q=0.007$ and 0.0195, at $N_τ=8$ and 6 respectively. Using $f_π(m_q)$ as the scale, the critical temperature is measured to be $T_c/f_π=1.638(93)$ at $m_π/f_π=3.329(30)$ for $m_q=0.007$, and $T_c/f_π=1.779(27)$ at $m_π/f_π=4.093(15)$ for $m_q=0.0195$. At a weaker coupling $β=0.56$, where at zero temperature and in the chiral limit we find $f_π=0.0312(10)$, the first order signal becomes numerically invisible to us for the $N_τ\leq 14$ lattices we have investigated so far.

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arXiv2009

Lattice QCD with 8 and 12 degenerate quark flavors

We compare extensive simulations of QCD with 8 and 12 flavors of degenerate quarks, using the DBW2 gauge action, naive staggered fermions, and the rational hybrid Monte Carlo algorithm. A variety of values of the coupling constant, quark mass, and lattice size have been used. Our data suggests that, as the bare coupling is decreased, a rapid cross-over, which dramatically changes the lattice scale, exists with both 8 and 12 flavors. The scale change across this cross-over is much larger with 12 flavors than it is with 8 flavors. All of the observables we have measured, in both the zero and finite temperature systems, are consistent with a chiral symmetry breaking phase for the zero temperature theory on the weak coupling side of the rapid cross-over.

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arXiv2008

Lattice QCD with Eight Degenerate Quark Flavors

We report on simulations of QCD with many flavors of degenerate quarks, the DBW2 gauge action and naive staggered fermions, using the rational hybrid Monte Carlo algorithm. We primarily focus on eight degenerate quark flavors where a variety of values of the coupling constant and quark mass have been used in the simulations. The scaling behavior of the hadron spectrum and the string tension of the heavy quark potential is studied, to probe whether the zero temperature, continuum limit of the theory breaks chiral symmetry.

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