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At least 271 records · Page 15Linked to original sources

arXiv2007

Numerical study of the mass spectrum in the 2D O(3) sigma model with a theta term

It has been conjectured that the mass spectrum of the O(3) non-linear sigma model with a theta term in 2 dimensions may possess an excited state, which decays when theta is lowered from pi below a critical value. Since the direct numerical investigation of the model is prevented by a sign problem, we try to infer some information on the mass spectrum at real theta by studying the model at imaginary theta via analytic continuation. A modified Swendsen-Wang cluster algorithm has been introduced to simulate the model with the theta term.

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arXiv2007

The three-loop beta-function of SU(N) lattice gauge theories with overlap fermions

We briefly report our calculation of the 2-loop coefficient of the coupling constant renormalization function Z_g in lattice perturbation theory. The quantity under study is defined through g_0 = Z_g g, where g_0 (g) is the bare (renormalized) coupling constant. The 2-loop expression for Z_g can be directly related to the 3-loop bare beta-function beta_L(g_0). Our calculation is performed using overlap fermions and Wilson gluons, and the background field technique has been chosen for convenience. Our results depend explicitly on the number of fermion flavors (N_f) and colors (N). Since the dependence of Z_g on the overlap parameter rho cannot be extracted analytically, we tabulate our results for different values of rho in the allowed range (0<rho<2), focusing on values which are being used most frequently in simulations. Plots of the 1- and 2-loop results for Z_g versus rho exhibit a nontrivial dependence on the overlap parameter. A longer write-up of this work may be found in 0709.4368.

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arXiv2007

Exploring the epsilon regime with twisted mass fermions

In this proceeding contribution we report on a first study in order to explore the so called epsilon regime with Wilson twisted mass (Wtm) fermions. To show the potential of this approach we give a preliminary determination of the chiral condensate.

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arXiv2007

QCD plasma instability and thermalisation at heavy ion collisions

Under suitable non-equilibrium conditions QCD plasma can develop plasma instabilities, where some modes of the plasma grow exponentially. It has been argued that these instabilities can play a significant role in the thermalisation of the plasma in heavy-ion collision experiments. We study the instability in SU(2) plasmas using the hard thermal loop effective lattice theory, which is suitable for studying real-time evolution of long wavelength modes in the plasma. We observe that under suitable conditions the plasma can indeed develop an instability which can grow to a very large magnitude, necessary for the rapid thermalisation in heavy-ion collisions.

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arXiv2007

Dirac b quark on the lattice

We perform the first study of treating b, c, and s quarks as Dirac fermions in lattice QCD with exact chiral symmetry. On a 32^3 60 lattice with 1/a ~ 7.68 GeV, we compute point-to-point quark propagators, and measure the time-correlation functions for mesons with quark contents b_bbar, c_bbar, s_bbar, and c_cbar. The lowest-lying meson mass spectra, the pseudoscalar decay constants, and the b and c quark masses are determined.

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arXiv2007

Hadron interactions from lattice QCD

Studies on hadron interactions from lattice QCD are reviewed. The $S$-wave $ππ$ scattering lengths of the I=0 and I=2 channels are extracted from various lattice determinations of low energy constants in $N_f=2$ chiral perturbation theory. The results agree with each other and agree also with other non-lattice estimates. Recently the $P$-wave $ππ$ scattering phase shift for the I=1 channel has been calculated. A preliminary estimate of the $ρ$ meson decay width from the phase shift is consistent with the experimental value. Two approaches to potentials between hadrons are discussed. One is a method using static quarks to define the distance between two hadrons. The other is a method to define a potential from a wave function of two hadrons. An application of the latter to the nucleon-nucleon ($NN$) potential turns out to reproduce qualitative features of the phenomenological $NN$ potential such as attraction at long distance and repulsion at short distance. Theoretical issues of this approach are also discussed.

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arXiv2007

Distribution Amplitudes of Vector Mesons

Results are presented for the lowest moment of the distribution amplitude for the K-star vector meson. Both longitudinal and transverse moments are investigated. We use two flavours of O(a) improved Wilson fermions, together with a non-perturbative renormalisation of the matrix element.

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arXiv2007

Relativistic corrections to the static potential at O(1/m) and O(1/m^2)

We investigate the relativistic corrections to the static potential, i.e. the O(1/m) potential and the O(1/m^2) velocity-dependent potentials, in SU(3) lattice gauge theory. They are important ingredients of potential nonrelativistic QCD for heavy quarkonium. Utilizing the multi-level algorithm, we obtain remarkably clean signals of these potentials up to r=0.9 fm. We observe long range nonperturbative contributions to these corrections.

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arXiv2007

Finite density simulations using a determinant estimator

Previous investigations have shown that the canonical approach to simulating QCD at finite density is promising. The algorithm we used in our earlier work employs an exact calculation of the fermionic determinant which limits the size of the lattices we can simulate. Interesting questions can only be answered if we simulate at larger volume. In this paper we explore an algorithm, Hybrid Noisy Monte Carlo, that employs a determinant estimator rather than an exact calculation. We first present the technical aspects of the estimator, check that the algorithm is correct by comparing it with our previous study, and then discuss its merits. We will also discuss the challenges faced when simulating larger lattice volumes.

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arXiv2007

New results using the canonical approach to finite density QCD

We present some new results regarding simulations of finite density QCD based on a canonical approach. A previous study has shown that such simulations are feasible, at least on small lattices. In the current study, we investigate some of the issues left open: we study the errors introduced by our approximation of the action and we show how to tune it to reduce the cost of the simulations while keeping the errors under control. To further reduce the cost of the simulations, we check the reliability of reweighting method with respect to the baryon number. Finally, using these optimizations, we carry out the simulations at larger densities than in our previous study to look for signals of a phase transition.

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arXiv2007

Standard Model parameters and heavy quarks on the lattice

I review recent progresses in heavy quarks physics on the lattice. I focus on decay constants and form factors relevant for the extraction of CKM matrix elements from experimental data. B-\bar{B} mixing is also discussed. In the last part of the paper I describe phenomenological applications of Heavy Quark Effective Theory (HQET) on the lattice, presenting in some detail the recent non-perturbative determination of the b-quark mass including O(1/m_b) corrections.

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arXiv2007

Elastic s-wave scattering phase shifts and |Vub| from lattice calculations of form factors for exclusive semileptonic decays

Omnes dispersion relations make a connection between form factors for exclusive semileptonic decays and phase shifts in the corresponding elastic scattering channels. We describe two applications. In the first, we use lattice form factor calculations to learn about phase shifts in elastic s-wave isospin-1/2 channels for K pi, B pi, D pi and D K scattering. The aim of the second application is to make the determination of the CKM matrix element magnitude |Vub| from exclusive semileptonic B to pi decays competitive with that from inclusive decays. Here we use many subtractions in an Omnes dispersion relation to motivate a simple fitting function, allowing data to constrain the q^2 shape of the differential decay rate and theory, primarily lattice results, to provide normalisation via form factor values.

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arXiv2007

Simulations of one-flavor QCD at finite temperature by RHMC

We simulate one-flavor QCD with standard Wilson fermions at finite temperature by the rational hybrid Monte Carlo algorithm. In the heavy quark region when we decrease the quark mass there is an endpoint which terminates the first order phase transition. We try to locate it by calculating the Binder cumulant of the Polyakov loop norm. We estimate the end-point to be kappa_c \sim 0.07-0.08.

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arXiv2007

Nf=2+1 dynamical Wilson quark simulation toward the physical point

We present preliminary results of the PACS-CS project which simulates 2+1 flavor lattice QCD toward the physical point with the nonperturbatively O(a)-improved Wilson quark action and the Iwasaki gauge action. Calculations are carried out at beta=1.9 on a 32^3x64 lattice with the use of the domain-decomposed HMC algorithm to reduce the up-down quark mass. The resulting pseudoscalar meson masses range from 730 MeV down to 210 MeV. We discuss the physical results including the chiral analysis in the pseudoscalar meson sector and the hadron spectrum. Some algorithmic issues are also discussed.

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arXiv2007

K to pi pi Amplitudes at Unphysical Kinematics Using Domain Wall Fermions

The use of chiral perturbation theory in extracting physical K to pi pi matrix elements from matrix elements calculated at unphysical kinematics is outlined. In particular, the possibility of utilizing pions with non-zero momentum in the final state, and of using partial quenching is discussed. Preliminary (not physically normalized) Delta I=3/2 (27,1) K to pi pi matrix elements are calculated on the RBC/UKQCD $24^3 \times 64$, $L_s=16$ lattices, using 2+1 dynamical flavors and domain wall fermions, with an inverse lattice spacing of $a^{-1}=1.729(28) GeV$. Effective mass plots are presented for a light sea quark mass of $m_l^{sea}=0.005$, and various valence quark masses. The plateaux are fit and $E_{ππ}-m_K$ is extracted.

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arXiv2007

The Spectral Structure of Correlator Matrices

In lattice QCD spectrum calculations, it is desirable to obtain multiple excited state energies in each symmetry channel. Typically, one constructs several interpolating operators for the symmetry channel of interest, forms the `correlator matrix' of all possible two-point functions, and uses the variational method to obtain as many energy levels as possible. We present a detailed look at this last step, starting from a discussion of the symmetry properties and spectral structure of the correlator matrix. We continue by motivating and describing the variational method, before discussing some conceptual and practical challenges concerning the light baryon sector. We conclude by mentioning some alternate spectrum extraction methods currently under study. Throughout, we attempt to quantify all approximations and assumptions, and we illustrate our points using a nucleon correlator matrix estimated on dynamical two-flavor lattice data.

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arXiv2007

Probing the chiral limit with clover fermions II: The baryon sector

Algorithmic progress in recent years made it possible to simulate QCD with Nf=2 flavours of O(a)-improved Wilson fermions at very light quark masses. We present the current results for baryon spectrum states, the nucleon axial coupling and the lowest moment of unpolarised nucleon structure functions. Special emphasis is given to a comparison of our calculations with results from chiral effective theories.

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arXiv2008

Note on the Lattice Fermion Chiral Symmetry Group

The group structure of the variant chiral symmetry discovered by Luscher in the Ginsparg-Wilson description of lattice chiral fermions is analyzed. It is shown that the group contains an infinite number of linearly independent symmetry generators, and the Lie algebra is given explicitly. CP is an automorphism of the chiral group, and the CP transformation properties of the symmetry generators is found. Features of the currents associated with these symmetries are discussed, including the fact that some different, non-commuting symmetry generators lead to the same Noether current. These strange features occur in all implementations of lattice fermions based on the Ginsparg-Wilson relation, including overlap, domain-wall, and perfect-action chiral fermions. The conclusions are illustrated in a solvable example, free overlap fermions.

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