Flavors Of Sea Quarks Research Articles

Hadronic decay of a vector meson from the lattice

We explore the decay of a vector meson to two pseudoscalar mesons on the lattice with Nf=2 flavours of sea quark. Although we are working with quark masses that do not allow a physical decay, we show how the transition rate can be evaluated from the amplitude for ρ→ππ and from the annihilation component of ππ→ππ. We explore the decay amplitude for two different pion momenta and find consistent results. The coupling strength we find is in agreement with experiment. We also find evidence for a shift in the ρ mass caused by mixing with two-pion states.

Lattice calculation of heavy-light decay constants with two flavors of dynamical quarks

We present results for ${f}_{B},$ ${f}_{{B}_{s}},$ ${f}_{D},$ ${f}_{{D}_{s}}$ and their ratios in the presence of two flavors of light sea quarks ${(N}_{f}=2).$ We use Wilson light valence quarks and Wilson and static heavy valence quarks; the sea quarks are simulated with staggered fermions. Additional quenched simulations with nonperturbatively improved clover fermions allow us to improve our control of the continuum extrapolation. For our central values the masses of the sea quarks are not extrapolated to the physical u, d masses; that is, the central values are ``partially quenched.'' A calculation using ``fat-link clover'' valence fermions is also discussed but is not included in our final results. We find, for example, ${f}_{B}{=190(7)(}_{\ensuremath{-}17}^{+24}{)(}_{\ensuremath{-}2}^{+11}{)(}_{\ensuremath{-}0}^{+8})\mathrm{MeV},$ ${f}_{{B}_{s}}{/f}_{B}{=1.16(1)(2)(2)(}_{\ensuremath{-}0}^{+4}),$ ${f}_{{D}_{s}}{=241(5)(}_{\ensuremath{-}26}^{+27}{)(}_{\ensuremath{-}4}^{+9}{)(}_{\ensuremath{-}0}^{+5})\mathrm{MeV},$ and ${f}_{B}{/f}_{{D}_{s}}{=0.79(2)(}_{\ensuremath{-}4}^{+5}{)(3)(}_{\ensuremath{-}0}^{+5}),$ where in each case the first error is statistical and the remaining three are systematic: the error within the partially quenched ${N}_{f}=2$ approximation, the error due to the missing strange sea quark and to partial quenching, and an estimate of the effects of chiral logarithms at small quark mass. The last error, though quite significant in decay constant ratios, appears to be smaller than has been recently suggested by Kronfeld and Ryan, and Yamada. We emphasize, however, that as in other lattice computations to date, the lattice $u,d$ quark masses are not very light and chiral log effects may not be fully under control.

Flavour singlet mesons in full QCD on the lattice

We apply spectral methods to compute the OZI-rule suppressed loop-loop correlators in the pseudocalar meson flavour singlet channel. Using SESAM configurations (obtained with two degenerate sea quark flavours on 16 3 × 32 lattices at β = 5.6, with standard Wilson action), we find for the first time clear evidence for mass plateau formation in the η′ channel of this theory. As a consequence, we observe a clear signal of a mass gap persistent under chiral extrapolation. This sets the stage for a more realistic two-channel approach, where partially quenched strange quarks would be included, in addition to the u, d sea quarks.

Detailed balance and sea-quark flavor asymmetry of proton

In this study, the proton is taken as an ensemble of quark-gluon Fock states. Using the principle of detailed balance, the probabilities of finding every Fock states of the proton are obtained without any parameter. A new origin of the light flavor sea quark asymmetry, i.e., $\bar{u} \not= \bar{d}$, is given as a pure statistical effect. It is found that $\bar{d}-\bar{u} \approx 0.124$, which is in surprisingly agreement with the experimental observation.

Zero temperature string breaking in lattice quantum chromodynamics

The separation of a heavy quark and antiquark pair leads to the formation of a tube of flux, or ``string,'' which should break in the presence of light quark-antiquark pairs. This expected zero-temperature phenomenon has proven elusive in simulations of lattice QCD. We study mixing between the string state and the two-meson decay channel in QCD with two flavors of dynamical sea quarks. We confirm that mixing is weak and find that it decreases at level crossing. While our study does not show direct effects of internal quark loops, our results, combined with unitarity, give a clear confirmation of string breaking.

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.

High precision study of theQQ¯potential from Wilson loops at large distances

For lattice QCD with two sea quark flavors we study the static quark-antiquark potential $V(R)$ in the regime where string breaking is expected. In order to increase the statistics, we make full use of the lattice information by including all lattice vectors $\mathbf{R}$ to any given separation $R=|\mathbf{R}|$ in the infrared regime. The corresponding paths between the lattice points are constructed by means of a generalized Bresenham algorithm as known from computer graphics. As a result, we achieve a determination of the Wilson loops in the range $0.8--1.5 \mathrm{fm}$ with hitherto unknown precision. Finally, we discuss the impact of this approach on the signal of the transition matrix element between two- and four-quark states.

Erratum: Polarized light–sea-quark flavor asymmetry in polarized semi-inclusive processes [Phys. Rev. D61, 057501 (2000)

Erratum: Polarized light–sea-quark flavor asymmetry in polarized semi-inclusive processes [Phys. Rev. D61, 057501 (2000)

Heavy-light decay constants from clover heavy quark action in QCD with two flavors of dynamical quarks

We present results on an analysis of the decay constants f Bd and f Bs with two flavours of sea quark. The calculation has been carried out on 3 different bare gauge couplings and 4 sea quark masses at each gauge coupling, with m π / m ϱ ranging from 0.8 to 0.6. We employ the Fermilab formalism to perform calculations with heavy quarks whose mass is in the range of the b-quark. A detailed comparison with a quenched calculation using the same action is made to elucidate the effects due to the sea quarks.

Polarized light–sea-quark flavor asymmetry in polarized semi-inclusive processes

We propose new formulas for extracting the difference of the polarized light--sea-quark density $\ensuremath{\Delta}\overline{d}(x)\ensuremath{-}\ensuremath{\Delta}\ifmmode \bar{u}\else \={u}\fi{}(x)$ from polarized deep-inelastic semi-inclusive data. We have estimated the value of it from the present experimental data measured by the SMC and HERMES groups. Although the data suggest a violation of polarized light--sea-quark flavor symmetry, the precision of the present data is not enough for confirming it.

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 $n_f=2$.

Light and strange hadron spectroscopy with dynamical Wilson fermions

We present the final analysis of the light and strange hadron spectra from a full QCD lattice simulation with two degenerate dynamical sea quark flavours at $\beta = 5.6$ on a $16^3 \times 32$ lattice. Four sets of sea quark masses corresponding to the range $.69 \leq m_\pi/m_\rho \leq .83$ are investigated. For reference we also ran a quenched simulation at $\beta_{\sf eff} = 6.0$, which is the point of equal lattice spacing, $a_{\rho}^{-1}$. In the light sector, we find the chiral extrapolation to physical u- and d- masses to present a major source of uncertainty, comparable to the expected size of unquenching effects. From linear and quadratic fits we can estimate the errors on the hadron masses made from light quarks to be on a 15 % level prior to the continuum extrapolation. For the hadrons with strange valence quark content, the $N_F = 2$ approximation to QCD appears not to cure the well-known failure of quenched QCD to reproduce the physical $K-K^*$ splitting.

Scanning the topological sectors of the QCD vacuum with the hybrid Monte Carlo algorithm

We address a long standing issue and determine the decorrelation efficiency of the hybrid Monte Carlo algorithm (HMC), for full QCD with Wilson fermions, with respect to vacuum topology. On the basis of five large QCD vacuum field ensembles (with 3000 to 5000 trajectories each and ${m}_{\ensuremath{\pi}}{/m}_{\ensuremath{\rho}}$- ratios in the range \ensuremath{\geqslant}0.69, for two sea quark flavors), we are able to establish that HMC provides sufficient tunneling between the different topological sectors of QCD. This will have an important bearing on the prospect to determine, by lattice techniques, the topological susceptibility of the vacuum, and topology sensitive quantities such as the spin content of the proton, or the ${\ensuremath{\eta}}^{\ensuremath{'}}$ mass.

Implementing the generalised hybrid Monte-Carlo algorithm

UKQCD's dynamical fermion project uses the Generalised Hybrid Monte Carlo (GHMC) algorithm to generate QCD gauge configurations for a non-perturbatively O(a) improved Wilson action with two degenerate sea-quark flavours. We describe our implementation of the algorithm on the Cray-T3E, concentrating on issues arising from code verification and performance optimisation, such as parameter tuning, reversibility, the effect of precision, the choice of matrix inverter, and the behaviour of different molecular dynamics integration schemes.

QCD with dynamical Wilson fermions—First results from SESAM

First results of a recently started simulation of full QCD with two flavours of sea-quarks at a coupling of β = 5.6 on a 163 × 32 lattice are presented. Emphasis is laid on the statistical significance that can be achieved by an “integrated luminosity” of 140 TFlop×hrs, for Hybrid Monte Carlo simulations at four intermediate values of mπmρ. The simulation takes place on the Quadrics QH2 at DESY/Zeuthen and DFG/Bielefeld. The performance is optimized by means of BiCGStab and the chronological inversion method of Brower et al. We discuss the systematic errors arising from lack of the molecular dynamic's reversibility on the 32-bit QH2. For plaquette and meson correlators we find integrated autocorrelation times of < 20 units of molecular dynamics time and exponential autocorrelation times of about 50 units. Using these results we perform preliminary measurements of the central potential and π and ρ correlators on independent configurations and obtain first estimates of the lattice spacings at three values of the dynamical hopping parameter.

Simple matrix elements with dynamical fermions

We report on studies of simple matrix elements from simulations with two flavors of sea quarks, both staggered and Wilson. We show the decay constants of vector aseudoscalar mesons. The effects of sea quarks are small. These simulations are done at relatively large lattice spacing compared to most quenched studies.