High Energy Physics Lattice

We calculate the low-lying spectrum of charmed baryons in lattice QCD on the323×64,Nf=2+1PACS-CS gauge configurations at the almost physical pion mass of∼156MeV/c2. By employing a set of interpolating operators with different Dirac structures and quark-field smearings for the variational analysis, we extract the ground and first few excited states of the spin-1/2and spin-3/2, singly-, doubly-, and triply-charmed baryons. Additionally, we study theΞc-Ξ′cmixing and the operator dependence of the excited states in a variational approach. We identify several states that lie close to the experimentally observed excited states of theΣc,ΞcandΩcbaryons, including some of theΞcstates recently reported by LHCb. Our results for the doubly- and triply-charmed baryons are suggestive for future experiments.

We report the current status of a study of charmonium contribution toB→Kℓ+ℓ−on the lattice. Our lattice calculation tests the factorization approximation for this contribution. In order to control the problem of the artificial divergence, we focus on the lowq2region with a small b-quark mass. We also take into account the renormalization constants of relevant four-quark operators calculated through the temporal moments. Results suggest a violation of the factorization approximation.

We have performed the firstnf=2+1+1lattice QCD computations of the properties (masses and decay constants) of ground-state charmonium mesons. Our calculation uses the HISQ action to generate quark-line connected two-point correlation functions on MILC gluon field configurations that includeu/dquark masses going down to the physical point, tuning thecquark mass fromMJ/ψand including the effect of thecquark's electric charge through quenched QED. We obtainMJ/ψ−Mηc(connected) = 120.3(1.1) MeV and interpret the difference with experiment as the impact onMηcof its decay to gluons, missing from the lattice calculation. This allows us to determineΔMannihilnηc=+7.3(1.2) MeV, giving its value for the first time. Our result offJ/ψ=0.4104(17) GeV, givesΓ(J/ψ→e+e−)=5.637(49) keV, in agreement with, but now more accurate than experiment. At the same time we have improved the determination of thecquark mass, including the impact of quenched QED to givem¯¯¯¯¯c(3GeV)= 0.9841(51) GeV. We have also used the time-moments of the vector charmonium current-current correlators to improve the lattice QCD result for thecquark HVP contribution to the anomalous magnetic moment of the muon. We obtainacμ=14.638(47)×10−10, which is 2.5σhigher than the value derived using moments extracted from some sets of experimental data onR(e+e−→hadrons). This value foracμincludes our determination of the effect of QED on this quantity,δacμ=0.0313(28)×10−10.

We present the first lattice investigation of coupled-channelDD¯andDsD¯sscattering in theJPC=0++and2++channels. The scattering matrix for partial wavesl=0,2and isospin zero is determined using multiple volumes and inertial frames via Lüscher's formalism. Lattice QCD ensembles from the CLS consortium withmπ≃280MeV,a≃0.09fm andL/a=24, 32are utilized. The resulting scattering matrix suggests the existence of three charmonium-like states withJPC=0++in the energy region ranging from slightly below2mDup to 4.13 GeV. We find a so far unobservedDD¯bound state just below threshold and aDD¯resonance likely related toχc0(3860), which is believed to beχc0(2P). In addition, there is an indication for a narrow0++resonance just below theDsD¯sthreshold with a large coupling toDsD¯sand a very small coupling toDD¯. This resonance is possibly related to the narrowX(3915)/χc0(3930)observed in experiment also just belowDsD¯s. The partial wavel=2features a resonance likely related toχc2(3930). We work with several assumptions, such as the omission ofJ/ψω,ηcηand three-particle channels. Only statistical uncertainties are quantified, while the extrapolations to the physical quark-masses and the continuum limit are challenges for the future.

We study the behaviour of a universal combination of susceptibility and correlation length in the Ising model in two and three dimensions, in presence of both magnetic and thermal perturbations, in the neighbourhood of the critical point. In three dimensions we address the problem using a parametric representation of the equation of state. In two dimensions we make use of the exact integrability of the model along the thermal and the magnetic axes. Our results can be used as a sort of "reference frame" to chart the critical region of the model. While our results can be applied in principle to any possible realization of the Ising universality class, we address in particular, as specific examples, three instances of Ising behaviour in finite temperature QCD related in various ways to the deconfinement transition. In particular, in the last of these examples, we study the critical ending point in the finite density, finite temperature phase diagram of QCD. In this finite density framework, due to well know sign problem, Montecarlo simulations are not possible and thus a direct comparison of experimental results with QFT/Statmech predictions like the one we discuss in this paper may be important. Moreover in this example it is particularly difficult to disentangle "magnetic-like" from "thermal-like" observables and thus universal quantities which do not need a precise identification of the magnetic and thermal axes, like the one we address in this paper, can be particularly useful.

Lattice techniques are the most reliable ones to investigate non-perturbative aspects of quantum chromodynamics (QCD) such as its phase diagram in the temperature-baryon density plane. They are, however, well-known to be beset with a variety of problems as one increases the density. We address here the old question of placing the baryonic (quark) chemical potential on the lattice. We point out that it may have important consequences for the current and future experimental searches of QCD critical point.

We describe a proposal for constructing a lattice theory that we argue may be capable of yielding free Weyl fermions in the continuum limit. The model employs reduced staggered fermions and uses site parity dependent Yukawa interactions of Fidkowski-Kitaev type to gap a subset of the lattice fermions without breaking symmetries. The possibility for such symmetric mass generation is tied to the cancellation of certain discrete anomalies arising in the continuum limit. The latter place strong constraints on the number of lattice fermions -- constraints that are satisfied by this model. We present numerical results for the model in two dimensions which support this sc

Large momentum effective field theory (LaMET) enables the extraction of parton distribution functions (PDFs) directly on a Euclidean lattice through a factorization theorem that relates the computed quasi-PDFs to PDFs. We apply chiral perturbation theory (ChPT) to LaMET to further separate soft scales, such as light quark masses and lattice size, to obtain leading model independent extrapolation formulas for extrapolations to physical quark masses and infinite volume. We find that the finite volume effect is reduced when the nucleon carries a finite momentum. For nucleon momentum greater than1GeV and the lattice sizeLand pion massmπsatisfyingmπL≥3, the finite volume effect is less than1%and is negligible for the current precision of lattice computations. This can be interpreted as a Lorentz contraction of the nucleon size in thez-direction which makes the lattice size effectively larger in that direction. We also find that the quark mass dependence in the infinite volume limit computed with non-zero nucleon momentum reproduces the previous result computed at zero momentum, as expected. Our approach can be generalized to other parton observables in LaMET straight forwardly.

We study chiral properties of eigenvalue spectrum for staggered quarks. We present a new method to identify would-be zero modes and nonzero modes using their symmetry and chiral properties. Here, we review the traditional method with HYP improved staggered quarks, and extend it to a completely new method which uses the chiral Ward identities and leakage patterns to achieve the goal.

We investigate the chiral phase structure of three flavor QCD in a backgroundU(1)magnetic field using the standard staggered action and the Wilson plaquette gauge action. We perform simulations on lattices with a temporal extent ofNτ=4and four spatial extents ofNσ=8,16,20and 24. We choose a smaller-than-physical quark mass in lattice spacing asam=0.030such that there exists a crossover transition at vanishing magnetic fields, and adopt two values of magnetic field strength in lattice spacingaeB−−−√≃1.5and 2. We find that the transition becomes stronger in the presence of a background magnetic field, and turns into a first order as seen from the volume scaling of the order parameter susceptibility as well as the metastable states in the time history of the chiral condensate. On the other hand, the chiral condensate and transition temperature always increase withBeven within the regime of a first order phase transition. This suggests that the discrepancy in the behavior of chiral condensates and transition temperature as a function ofBbetween earlier lattice studies using larger-than-physical pion masses with standard staggered fermions and those using physical pions with improved staggered fermions is mainly due to lattice cutoff effects.