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At least 253 records · Page 14Linked to original sources

arXiv2007

2+1 flavour Domain Wall Fermion simulations by the RBC and UKQCD collaborations

We review simulations of dynamical domain wall fermions at a fixed inverse lattice spacing of 1.73GeV and with pion masses as light as 330MeV and spatial dimensions as large as 2.7fm performed by the RBC and UKQCD collaborations. These results include pseudoscalar masses and decay constants and low energy constants of the chiral effective lagrangian. We also review results for the neutral kaon mixing amplitude $B_K$, the Kl3 form factor, pseudoscalar meson structure, and vector meson decay constants. In the baryon sector we review results for the spectrum, and nucleon form factors and structure functions. Highlights of our programme include preliminary quark masses, and determinations of $V_{us}$ from both $f_K/f_π$ and from Kl3, and an updated result for $B_K$. We find significant finite volume effects in the nucleon axial charge $g_A$ for our $m_π=330$ MeV ensemble on a $(2.7 {\rm fm})^3$ lattice, and highlight the importance of large physical volumes for non-trivial nucleon physics.

hep-lat↗

arXiv2007

Baryon masses with improved staggered quarks

The MILC collaboration's simulations with improved staggered quarks are being extended with runs at a lattice spacing of 0.06 fm with quark masses down to one tenth the strange quark mass. We give a brief introduction to these new simulations and the determination of the lattice spacing. Then we combine these new runs with older results to study the masses of the nucleon and the Omega minus in the continuum and chiral limits.

hep-lat↗

arXiv2007

On the phase diagram of QCD at finite isospin density

Using a canonical formalism, we determine the equation of state and the phase diagram of eight-flavour QCD, as a function of temperature and isospin density. Two mechanisms are at work: Bose condensation of pions at high density, and deconfinement at high temperature. We study their interplay and find that on our small and coarse lattice the first order deconfinement transition appears to end at a critical point at finite density. We investigate the strength of the overlap and of the sign problems and discuss implications for the baryonic density case.

hep-lat↗

arXiv2007

QCD finite T transition -- Comparison between Wilson and staggered results

A quantitative comparison between the finite temperature behaviour of the staggered and Wilson fermion formulations are performed. The comparison is based on a physical quantity that is expected to be quite sensitive to the fermionic features of the action. For that purpose we use the height of the peak for $dχ_s/dT$, where $χ_s$ is the quark number susceptibility.

hep-lat↗

arXiv2007

High precision study of B*B pi coupling in unquenched QCD

The B* B pi coupling is a fundamental parameter of chiral effective Lagrangian with heavy-light mesons and can constrain the B->pi l nu form factor in the soft pion limit which will be useful for precise determination of |Vub|. We compute the B* B pi coupling with the static heavy quark and the O(a)-improved Wilson light quark. Simulations are carried out with n_f=2 unquenched 12^3x24 lattices at beta=1.80 generated by CP-PACS collaboration. Following the quenched study by Negishi et al., we employ the all-to-all propagator with 200 low eigenmodes as well as HYP smeared link to improve the statistical accuracy.

hep-lat↗

arXiv2008

A QCD chiral critical point at small chemical potential: is it there or not?

For a QCD chiral critical point to exist, the parameter region of small quark masses for which the finite temperature transition is first-order must expand when the chemical potential is turned on. This can be tested by a Taylor expansion of the critical surface (m_{u,d},m_s)_c(mu). We present a new method to perform this Taylor expansion numerically, which we first test on an effective model of QCD with static, dense quarks. We then present the results for QCD with 3 degenerate flavors. For a lattice with N_t=4 time-slices, the first-order region shrinks as the chemical potential is turned on. This implies that, for physical quark masses, the analytic crossover which occurs at mu=0 between the hadronic and the plasma regimes remains crossover in the mu-region where a Taylor expansion is reliable, i.e. mu less than or similar to T. We present preliminary results from finer lattices indicating that this situation persists, as does the discrepancy between the curvature of T_c(m_c(mu=0),mu) and the experimentally observed freeze-out curve.

hep-lat↗

arXiv2007

QCD Thermodynamics

Recent results on QCD thermodynamics are presented. The nature of the T>0 transition is determined, which turns out to be an analytic cross-over. The absolute scale for this transition is calculated. The temperature dependent static potential is given. The results were obtained by using a Symanzik improved gauge and stout-link improved fermionic action. In order to approach the continuum limit four different sets of lattice spacings were used with temporal extensions N_t=4, 6, 8 and 10 (they correspond to lattice spacings a \sim 0.3, 0.2, 0.15 and 0.12 fm). A new technique is presented, which --in contrast to earlier methods-- enables one to determine the equation of state at very large temperatures.

hep-lat↗

arXiv2007

A Schur Complement Approach to Chiral Fermions

Lattice chiral fermions are synonymous to the Ginsparg-Wilson relation. Indeed, this relation is satisfied by the overlap, domain wall and perfect action fermion kernel. In a recent work we have shown that it is possible to take a direct RG approach for fermions in the presence of gauge fields. This is due to an algebraically implicit blocking technique which yields a Schur-complementary coarse Dirac operator. Using a Schur complement approximation which is stable and regular, the scheme can be iterated to the fixed point. In this talk, we elaborate more on the direct RG approach and show how to get highly improved chiral fermions on the coarse lattice with the gauge fields remaining on the fine lattice. We give numerical examples in the case of lattice QCD using QCDLAB {\tt http://phys.fshn.edu.al/qcdlab.html}

hep-lat↗

arXiv2007

Revisiting the pentaquark episode for lattice QCD

This work revisits the pentaquark episode, particularly in reference to lattice QCD, to collect in one place the lessons that have been or should be learnt. This also examines whether, and if yes, where, there might be any prospect for future studies of pentaquarks and the role of lattice QCD in it. Tests such as volume dependence, and hybrid boundary conditions to distinguish bound and scattering states are identified as the positives to take forward from this episode. On the other hand it is also observed that pentaquarks with heavier quark content and in the $SU(3)_f$ limit may still be worth not forgetting, and in that case lattice QCD may have an important role to play.

hep-lat↗

arXiv2007

Distributions of individual Dirac eigenvalues for QCD at non-zero chemical potential: RMT predictions and lattice results

For QCD at non-zero chemical potential $μ$, the Dirac eigenvalues are scattered in the complex plane. We define a notion of ordering for individual eigenvalues in this case and derive the distributions of individual eigenvalues from random matrix theory (RMT). We distinguish two cases depending on the parameter $α=μ^2 F^2 V$, where $V$ is the volume and $F$ is the familiar low-energy constant of chiral perturbation theory. For small $α$, we use a Fredholm determinant expansion and observe that already the first few terms give an excellent approximation. For large $α$, all spectral correlations are rotationally invariant, and exact results can be derived. We compare the RMT predictions to lattice data and in both cases find excellent agreement in the topological sectors $ν=0,1,2$.

hep-lat↗

arXiv2007

Recent lattice results on finite temperature and density QCD, part I

We discuss recent progress made studies of bulk thermodynamics of strongly interacting matter through lattice simulations of QCD with an almost physical light and strange quark mass spectrum. We present results on the QCD equation of state at vanishing and non-vanishing quark chemical potential and show first results on baryon number and strangeness fluctuations, which might be measured in event-by-event fluctuations in low energy runs at RHIC as well as at FAIR.

hep-lat↗

arXiv2007

Recent lattice results on finite temerature and density QCD, part II

We discuss recent progress in studies of QCD thermodynamics with almost physical light quark masses and a physical value of the strange quark mass. We summarize results on the transition temperature in QCD and analyze the relation between deconfinement and chiral symmetry restoration.

hep-lat↗

arXiv2007

Progress in hadron structure physics on the lattice

This is a review of progress in hadron structure physics from lattice QCD. Recent results on the structure of the nucleon and the pion in terms of (transition) form factors, moments of distribution amplitudes and (generalized) parton distribution functions are presented. These observables allow us to investigate a number of fundamental physics questions related to e.g. the distribution of charge and momentum in hadrons, the spin structure of the nucleon and the pion, and correlations between spin, orbital angular momentum and coordinate degrees of freedom. Chiral extrapolations of selected lattice results are presented and compared to results from experiment and phenomenology. We conclude that lattice simulations already today strongly contribute to our understanding of the structure of hadrons.

hep-lat↗

arXiv2007

Towards 4-loop NSPT result for a 3-dimensional condensate-contribution to hot QCD pressure

Thanks to dimensional reduction, the contributions to the hot QCD pressure coming from so-called soft modes can be studied via an effective three-dimensional theory named Electrostatic QCD (spatial Yang-Mills fields plus an adjoint Higgs scalar). The poor convergence of the perturbative series within EQCD suggests to perform lattice measurements of some of the associated gluon condensates. These turn out, however, to be plagued by large discretization artifacts. We discuss how Numerical Stochastic Perturbation Theory can be exploited to determine the full lattice spacing dependence of one of these condensates up to 4-loop order, and sharpen our tools on a concrete 2-loop example.

hep-lat↗

arXiv2007

Progress in kaon physics on the lattice

CKM-unitarity, direct and indirect CP-violation and the ΔI=1/2 rule in full lattice QCD are the focus of this talk. To this end I will discuss and compare recent lattice results for leptonic, semi-leptonic and non-leptonic decays of the kaon and neutral kaon mixing and I will motivate current best estimates f_K/f_π=1.198(10), f_+^{Kπ}(0)=0.964(5) and \hat{B}_K=0.720(39). Moreover new theoretical advances that will improve the quality of these computations in the future will be discussed.

hep-lat↗

arXiv2007

Finite temperature Lattice QCD with two flavors of improved Wilson fermions

We present results of finite temperature QCD from N_f=2 flavors of non-perturbatively improved Wilson fermions on lattices with N_t=8, 10 and 12. The transition temperature T_c is determined for the first time at lattice spacings as low as a = 0.08 fm, albeit at pion masses 1.3 < r_0 m_π. We furthermore compute the screening masses in various color channels at T>T_c.

hep-lat↗

arXiv2007

Extraction of alpha_s and m_Q from Onia

We briefly review how precise determinations of the strong coupling constant and of the heavy quark masses may be obtained from heavy quarkonium. Such determinations are competitive with heavy quark masses extraction from other systems and give an accurate value for the strong coupling constant at a relatively low energy scale. In particular we report about a recent determination of $\als$ from $Γ(Υ(1S) \to γX)/Γ(Υ(1S) \to X)$ with CLEO data which includes color octet contributions and avoids model dependence in the extraction. The obtained value is $\als (M_{Υ(1S)})= 0.184^{+0.015}_{-0.014}$, which corresponds to $\als(M_Z) = 0.119^{+0.006}_{-0.005}$. Future prospects for more precise extractions of the heavy quark masses and $\als$ are discussed.

hep-ph↗

arXiv2007

Z(3)-symmetric effective theory of hot QCD

We study a three dimensional Z(3)-symmetric effective theory of high temperature QCD. The exact lattice-continuum relations, needed in order to perform lattice simulations with physical parameters, are computed to order O(a^0) in lattice perturbation theory. Lattice simulations are performed to determine the phase structure of a subset of the parameter space.

hep-lat↗