Unquenched Lattice QCD Research Articles

Nucleon structure from lattice QCD with domain wall fermions

We report the current status of RBCK calculations on nucleon structure with both quenched and unquenched lattice QCD. The combination of domain wall fermions and DBW2 gauge action works well for quantities like isovector vector and axial charges, moments of structure functions 〈χ〉q, di, and 〈1〉σq, and nucleon decay matrix elements. Good chiral symmetry is observed, and translates into continuum-like behavior of non-perturabtive renormalization where explicitly checked. Pion form factor calculation in the same framework is also reported.

Unquenched QCD with light quarks

We present recent results in unquenched lattice QCD with two degenerate light sea quarks using the truncated determinant approximation (TDA). In the TDA the infrared modes contributing to the quark determinant are computed exactly up to some cutoff in quark off-shellness (typically $2{\ensuremath{\Lambda}}_{\mathrm{QCD}}).$ This approach allows simulations to be performed at much lighter quark masses than possible with conventional hybrid Monte Carlo techniques. Results for the static energy and topological charge distributions are presented using a large ensemble generated on very coarse ${(6}^{4})$ but physically large lattices. Preliminary results are also reported for the static energy and meson spectrum on ${10}^{3}\ifmmode\times\else\texttimes\fi{}20$ lattices (lattice scale ${a}^{\ensuremath{-}1}=1.15\mathrm{GeV})$ at quark masses corresponding to pions of mass $<~200\mathrm{MeV}.$ Using multiboson simulation to compute the ultraviolet part of the quark determinant the TDA approach becomes an exact algorithm with essentially no increase in computational effort. Some preliminary results using this fully unquenched algorithm are presented.

Continuum extrapolation of moments of nucleon quark distributions in full QCD

Moments of light cone quark density, helicity, and transversity distributions are calculated in unquenched lattice QCD at β = 5.5 and β = 5.3 using Wilson fermions on 163 x 32 lattices. These results are combined with earlier calculations at β = 5.6 using SESAM configurations to study the continuum limit.

Moments of nucleon light cone quark distributions calculated in full lattice QCD

Moments of the quark density, helicity, and transversity distributions are calculated in unquenched lattice QCD. Calculations of proton matrix elements of operators corresponding to these moments through the operator product expansion have been performed on $16^3 \times 32$ lattices for Wilson fermions at $\beta = 5.6$ using configurations from the SESAM collaboration and at $\beta = 5.5$ using configurations from SCRI. One-loop perturbative renormalization corrections are included. At quark masses accessible in present calculations, there is no statistically significant difference between quenched and full QCD results, indicating that the contributions of quark-antiquark excitations from the Dirac Sea are small. Close agreement between calculations with cooled configurations containing essentially only instantons and the full gluon configurations indicates that quark zero modes associated with instantons play a dominant role. Naive linear extrapolation of the full QCD calculation to the physical pion mass yields results inconsistent with experiment. Extrapolation to the chiral limit including the physics of the pion cloud can resolve this discrepancy and the requirements for a definitive chiral extrapolation are described.

Temperature dependence of instantons in QCD

We investigate the temperature dependence of the instanton content of gluon fields, using unquenched lattice QCD and the cooling method. The topological susceptibility arising from instantons decreases rapidly below the phase-transition temperature ${T}_{c}$ (\ensuremath{\approx}150 MeV), becoming practically zero at $T\ensuremath{\approx}{T}_{c}.$ The dominant caloron size parameter deduced from the correlation function decreases from about 0.9 fm below ${T}_{c}$ to 0.4 fm at ${1.3T}_{c}.$ The instanton charge distribution is Poissonian above ${T}_{c}$ with deviations at low temperature.

Evidence for η′-π splitting in unquenched lattice QCD

We perform an extrapolation from negative to positive flavour numbers of full QCD lattice estimates of the η′ mass. The extrapolations are carried out by keeping ϱ and π masses at fixed values. We find an η′-π splitting which shows a flavour dependence consistent with the Witten-Veneziano formula based on the U(1) anomaly. The quantitative splitting is consistent with the estimates made in the quenched approximation.

Hadrons in unquenched lattice QCD

Incorporating dynamical Kogut-Susskind fermions ( N F = 4) into a Monte Carlo simulation of QCD, we have analyzed the masses of low lying hadrons, chiral symmetry breaking and the interquark potential. We used a 24 × 12 3 lattice for two couplings β = 5.20 and 5.35; quarkk masses varied from m = 0.075 down to 0.010. We find good agreement between the results from our pseudofermion method and those reported from a hybrid simulation at [ β, m] = [5.20, 0.050].

A hadron mass spectrum calculation in unquenched lattice QCD at β = 5.70

We report on a calculation of the hadronic mass spectrum and Q Q ̃ - potential which incorporates the effects of the virtual quark-antiquark pairs, at a coupling of β = 5.7, quark masses of .10, .05 and .02, on a 10 3 × 32 lattice. The effects of the dynamical quarks were implemented with the pseudofermion algorithm. The results will be compared with those obtained in the quenched approximation.