Sea Quark Effects Research Articles

Lattice calculation of1−+hybrid mesons with improved Kogut-Susskind fermions

We report on a lattice determination of the mass of the exotic ${1}^{\ensuremath{-}+}$ hybrid meson using an improved Kogut-Susskind action. Results from both quenched and dynamical quark simulations are presented. We also compare with earlier results using Wilson quarks at heavier quark masses. The results on lattices with three flavors of dynamical quarks show effects of sea quarks on the hybrid propagators which probably result from coupling to two meson states. We extrapolate the quenched results to the physical light quark mass to allow comparison with experimental candidates for the ${1}^{\ensuremath{-}+}$ hybrid meson. The lattice result remains somewhat heavier than the experimental result, although it may be consistent with the ${\ensuremath{\pi}}_{1}(1600).$

Light hadron spectroscopy with two flavors ofO(a)-improved dynamical quarks

We present a high statistics study of the light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Numerical simulations are carried out using the plaquette gauge action and the O(a)-improved Wilson quark action at $\ensuremath{\beta}=5.2,$ where the lattice spacing is found to be $a=0.0887(11)\mathrm{fm}$ from the $\ensuremath{\rho}$ meson mass, on a ${20}^{3}\ifmmode\times\else\texttimes\fi{}48$ lattice. At each of five sea quark masses corresponding to ${m}_{\mathrm{PS}}{/m}_{\mathrm{V}}\ensuremath{\simeq}0.8--0.6,$ we generate 12 000 trajectories using a symmetrically preconditioned Hybrid Monte Carlo algorithm. Finite spatial volume effects are investigated employing ${12}^{3}\ifmmode\times\else\texttimes\fi{}48,$ ${16}^{3}\ifmmode\times\else\texttimes\fi{}48$ lattices. We also perform a set of simulations in quenched QCD with the same lattice actions at a similar lattice spacing to those for the full QCD runs. In the meson sector we find clear evidence of sea quark effects. The J parameter increases for lighter sea quark masses, and the full QCD meson masses are systematically closer to experiment than in quenched QCD. Careful finite-size studies are made to ascertain that these are not due to finite-size effects. Evidence of sea quark effects is less clear in the baryon sector due to larger finite-size effects. We also calculate light quark masses and find ${m}_{\mathrm{ud}}^{\overline{\mathrm{MS}}}(2\mathrm{}\mathrm{GeV}{)=3.223(}_{\ensuremath{-}0.069}^{+0.046})\mathrm{MeV}$ and ${m}_{s}^{\overline{\mathrm{MS}}}(2\mathrm{}\mathrm{GeV}{)=84.5(}_{\ensuremath{-}1.7}^{+12.0})\mathrm{MeV}$ which are about 20% smaller than in quenched QCD.

Light hadron spectrum with two flavors of O(a) improved dynamical quarks: final results from JLQCD

We present the final results of the JLQCD calculation of the light hadron spectrum and quark masses with two flavors of dynamical quarks using the plaquette gauge action and fully O( a)-improved Wilson quark action at β = 5.2. We observe that sea quark effects lead to a closer agreement of the strange meson and baryon masses with experiment and a reduction of quark masses by 20–30%.

Interaction of Wilson loops in a confining vacuum

Nonperturbative and perturbative interaction mechanisms of Wilson loops in gluodynamics are studied within the background field formalism. The first one operates when distance between minimal surfaces of the loops is small and may be important for sea quark effects and strong decay processes. The second mechanism -- perturbative interaction in nonperturbative confining background is found to be physically dominant for all loop configurations characteristic of scattering process. It reduces to perturbative gluon exchanges at small distances, while at larger distances it corresponds to the t-channel exchange of (reggeized) glueball states. Comparison to other approaches is made and possible physical applications are discussed.

Hadron spectroscopy from CP-PACS

We present recent results for the light hadron spectrum obtained by lattice QCD simulations on the CP-PACS computer. This article consists of two parts. We report in the first part results in two flavor full QCD for experimentally observed hadrons and discuss sea quark effects. In the second part, our attempt to study stability of a six quark state, the H dibaryon, in quenched QCD is presented.

Exploration of sea quark effects in two-flavor QCD with the O( a)-improved Wilson quark action

We explore sea quark effects in the light hadron mass spectrum in a simulation of two-flavor QCD using the nonperturbatively O( a)-improved Wilson fermion action. In order to identify finite-size effects, light meson masses are measured on 12 3 × 48, 16 3 × 48 and 20 3 × 48 lattices with a ∼ 0.1 fm. On the largest lattice, where the finite-size effect is negligible, we find a significant increase of the strange vector meson mass compared to the quenched approximation. We also investigate the quark mass dependence of pseudoscalar meson masses and decay constants and test the consistency with (partially quenched) chiral perturbation theory.

Light hadron spectroscopy with two flavors of dynamical quarks on the lattice

We present results of a numerical calculation of lattice QCD with two degenerate flavors of dynamical quarks, identified with up and down quarks, and with a strange quark treated in the quenched approximation. The lattice action and simulation parameters are chosen with a view to carrying out an extrapolation to the continuum limit as well as chiral extrapolations. Gauge configurations are generated with a renormalization-group improved gauge action and a mean field improved clover quark action at three values of $\beta$ and four sea quark masses. The sizes of lattice are chosen so that the physical spatial size is kept constant. Hadron masses, light quark masses and meson decay constants are measured at five valence quark masses. We also carry out complementary quenched simulations with the same improved actions. The quenched spectrum from this analysis agrees well in the continuum limit with the one of our earlier work using the standard action. We find the two-flavor full QCD meson masses in the continuum limit to be much closer to experimental meson masses than those from quenched QCD. We take these results as manifestations of sea quark effects in two-flavor full QCD. For baryon masses full QCD values for strange baryons are in agreement with experiment, while they differ increasingly with decreasing strange quark content, resulting in a nucleon mass higher than experiment. The pattern suggests finite size effects as a possible origin for this deviation. For light quark masses in the continuum limit we obtain values which are reduced by about 25% compared to the values in quenched QCD. We also present results for decay constants where large scaling violations obstruct a continuum extrapolation. Need for a non-perturbative estimate of renormalization factors is discussed.

QCD spectrum with three quark flavors

We present results from a lattice hadron spectrum calculation using three flavors of dynamical quarks --- two light and one strange---and quenched simulations for comparison. These simulations were done using a one-loop Symanzik improved gauge action and an improved Kogut-Susskind quark action. The lattice spacings, and hence also the physical volumes, were tuned to be the same in all the runs to better expose differences due to flavor number. Lattice spacings were tuned using the static quark potential, so as a by-product we obtain updated results for the effect of sea quarks on the static quark potential. We find indications that the full QCD meson spectrum is in better agreement with experiment than the quenched spectrum. For the ${0}^{++}$ ${(a}_{0})$ meson we see a coupling to two pseudoscalar mesons, or a meson decay on the lattice.

Decay constants ofBandDmesons from improved relativistic lattice QCD with two flavors of sea quarks

We present a calculation of the B and D meson decay constants in lattice QCD with two (Nf=2) flavours of light dynamical quarks, using an O(a)-improved Wilson action for both light and heavy quarks and a renormalization-group improved gauge action. Simulations are made at three values of lattice spacing a=0.22, 0.16, 0.11 fm and four values of sea quark mass in the range m_PS/m_V \~= 0.8-0.6. Our estimate for the continuum values of the decay constants are fBd = 208(10)(11) MeV, fBs = 250(10)(13)(^{+8}_{-0}) MeV, fDd = 225(14)(14) MeV, fDs = 267(13)(17)(^{+10}_{-0}) MeV for Nf=2 where the statistical and systematic errors are separately listed, and the third error for fBs and fDs show uncertainty of determination of strange quark mass. We also carry out a set of quenched simulations using the same action to make a direct examination of sea quark effects. Taking the ratio of results for Nf=2 and Nf=0, we obtain fb^{Nf=2}/fb^{Nf=0} = 1.11(6), fbs^{Nf=2}/fbs^{Nf=0} = 1.14(5), fd^{Nf=2}/\fd^{Nf=0} = 1.03(6), fds^{Nf=2}/\fds^{Nf=0} = 1.07(5). They show a 10-15% increase in the Nf=2 results over those of Nf=0 for the B meson decay constants, while evidence for such a trend is statistically less clear for the D meson decay constants.

Light hadron spectrum and light quark masses from lattice QCD

We present recent results on the light hadron spectrum and light quark masses in lattice QCD, both in the quenched approximation and with two flavors of dynamical quarks. Simulations are made with a renormalization-group improved gauge action and a tadpole-improved clover quark action. Our new results in the quenched simulation are in good agreement with our previous calculation using the standard lattice action. A comparison between the results in full and quenched QCD clearly shows the existence of sea quark effects in meson and quark masses.

Light hadron spectroscopy with two flavors of O( a) improved dynamical quarks

We report on our study of light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Simulations are made with the plaquette gauge action and the non-pertubatively O( a) improved Wilson quark action. We simulate 5 sea qaurk masses corresponding to m PS/ m V ⋍ 0.8-0.6 at β = 5.2 on 12 3 × 48, 16 3 × 48 and 20 3 × 48 lattices. A comparison with previous calculations in quenched QCD indicates sea quark effects in meson and quark masses.

Full QCD light hadron spectrum and quark masses: Final results from CP-PACS

We present the final results of the CP-PACS calculation of the light hadron spectrum and quark masses with two flavors of dynamical quarks. Simulations are made with a renormalization-group improved gauge action and a mean-field improved clover quark action for sea quark masses corresponding to m PS/ m V ≈ 0.8-0.6 and the lattice spacing a = 0.22-0.11 fm. For the meson spectrum in the continuum limit a clearly improved agreement with experiment is observed compared to the quenched case, demonstrating the importance of sea quark effects. For light quark masses we obtain m MS ud (2 GeV) = 3.44 +0.14 0.22 MeV and m MS s (2 GeV) = 88 +4 −6 MeV ( K-input) and m MS s (2 GeV) = 90 +5 −11 MeV (φ-input), which are reduced by about 25% compared to the values in quenched QCD.

Lattice simulations of the strange quark mass and Fritzsch texture

A number of numerical simulations of lattice gauge theory have indicated a low mass of strange quark in 100 MeV range at the scale of μ=2 GeV. In the unquenched case, which is improved over the simulation in the quenched approximation by the inclusion of u and d sea quark effects, one sees a further downward trend. Here the fermion mass spectrum of the Fritzsch texture is recalculated. In a single step supersymmetric GUT with M X ∼10 16 GeV such values of the strange quark mass can be obtained for low values of tan β. Experimental numbers m pole t =173±6 GeV and 4.1< m b ( m b )<4.4 GeV are used in this study. Since the scenario is supersymmetric, gaugino loop diagrams contribute to the masses in addition to usual tree level Yukawa contributions. Upper bound of the mixing parameter V cb is taken at 0.045.

Light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks

We present updated results of the CP-PACS calculation of the light hadron spectrum in N f = 2 full QCD. Simulations are made with an RG-improved gauge action and a tadpole-improved clover quark action for sea quark masses corresponding to m PS/ m V ≈ 0.8-0.6 and the lattice spacing a = 0.22-0.09 fm. A comparison of the N f = 2 QCD spectrum with new quenched results, obtained with the same improved action, shows clearly the existence of sea quark effects in vector meson masses. Results for light quark masses are also presented.

A study of sea-quark effects on the charmonium spectrum

We present results of a study of sea quark effects on the charmonium spectrum using configurations generated by the MILC collaboration with two flavors of dynamical staggered quarks.

Sea quark effects inBspectroscopy and decay constants

We present comprehensive results for the spectrum and decay constants of hadrons containing a single b quark. The heavy quark is simulated using an $O(1/M)$ NRQCD action and the light quark using the $O(a)$ tadpole-improved clover action on gauge configurations containing two degenerate flavours of sea quarks at $\beta^{n_f=2}=5.6$ provided by the HEMCGC collaboration. We present detailed results for the lower lying $S$ and $P$ wave $B$ meson states and the $\Lambda_b$ baryon. We find broad agreement with experiment. In addition, we present results for the pseudoscalar and, for the first time, the vector decay constants fully consistent to $O(\alpha/M): f_B = 186(5)(stat)(19)(pert)(9)(disc)(13)(NRQCD)(+50)(a^{-1})MeV, f_B^* = 181(6) (stat)(18)(pert)(9)(disc)(13)(NRQCD)(+55)(a^{-1})MeV$ and $f_{B_s}/f_B = 1.14(2)(stat)(-2)(\kappa_s)$. We present an investigation of sea quark effects in the $B$ spectrum and decay constants. We compare our results with those from similar quenched simulations at $\beta^{n_f=0}=6.0$. For the spectrum, the quenched results reproduce the experimental spectrum well and there is no significant difference between the quenched and $n_f=2$ results. For the decay constants, our results suggest that sea quark effects may be large. We find that $f_B$ increases by approximately 25% between $n_f=0$ and $n_f=2$.

Hadronic spectroscopy from the lattice: Glueballs and hybrid mesons

Lattice QCD determinations appropriate to hadron spectroscopy are reviewed with emphasis on the glueball and hybrid meson states in the quenched approximation. Hybrids are discussed for heavy and for light quarks. The effects of sea quarks (unquenching) are explored.

Light hadron spectroscopy withO(a)improved dynamical fermions

We present the first results for the static quark potential and the light hadron spectrum using dynamical fermions at $\ensuremath{\beta}=5.2$ using an $O(a)$ improved Wilson fermion action together with the standard Wilson plaquette action for the gauge part. Sea quark masses were chosen such that the pseudoscalar-vector mass ratio, ${m}_{\mathrm{PS}}{/m}_{V},$ varies from 0.86 to 0.67. Finite-size effects are studied by using three different volumes ${8}^{3}\ifmmode\times\else\texttimes\fi{}24,$ ${12}^{3}\ifmmode\times\else\texttimes\fi{}24,$ and ${16}^{3}\ifmmode\times\else\texttimes\fi{}24.$ Comparing our results to previous ones obtained using the quenched approximation, we find evidence for sea quark effects in quantities such as the static quark potential and vector-pseudoscalar hyperfine splitting.

Heavy quark physics in N f = 2 QCD

We present a preliminary analysis of the heavy-heavy spectrum and heavy-light decay constants in full QCD, using a tadpole-improved SW quark action and an RG-improved gauge action on a 16 3 × 32 lattice with four sea quark masses corresponding to m π/ m ρ ≈ 0.8, 0.75, 0.7, 0.6 and a −1 ≈ 1.3 GeV. We focus particularly on the effect of sea quarks on these observables.

Recent results from the CP-PACS collaboration

We present an overview of recent results from the CP-PACS computer on the quenched light hadron spectrum and an on-going two-flavour full QCD study. We find that our quenched hadron mass results are compatible with the mass formulae predicted by quenched chiral perturbation theory, which we adopt in our final analysis. Quenched hadron masses in the continuum limit show unambiguous and systematic deviations from experiment. For our two-flavour full QCD simulation we present preliminary results on the light hadron spectrum, quark masses and the static potential. The question of dynamical sea quark effects in these quantities is discussed.