Search results for "Lattice QCD"
showing 10 items of 241 documents
Two- and Three-Pion Finite-Volume Spectra at Maximal Isospin from Lattice QCD.
2019
We present the three-pion spectrum with maximal isospin in a finite volume determined from lattice QCD, including excited states in addition to the ground states across various irreducible representations at zero and nonzero total momentum. The required correlation functions, from which the spectrum is extracted, are computed using a newly implemented algorithm which speeds up the computation by more than an order of magnitude. On a subset of the data we extract a nonzero value of the three-pion threshold scattering amplitude using the $1/L$ expansion of the three-particle quantization condition, which consistently describes all states at zero total momentum. The finite-volume spectrum is p…
Hamiltonian lattice QCD at finite density: equation of state in the strong coupling limit
2001
The equation of state of Hamiltonian lattice QCD at finite density is examined in the strong coupling limit by constructing a solution to the equation of motion corresponding to an effective Hamiltonian describing the ground state of the many body system. This solution exactly diagonalizes the Hamiltonian to second order in field operators for all densities and is used to evaluate the vacuum energy density from which we obtain the equation of state. We find that up to and beyond the chiral symmetry restoration density the pressure of the quark Fermi sea can be negative indicating its mechanical instability. Our result is in qualitative agreement with continuum models and should be verifiabl…
Abelian projection and studies of gauge-variant quantities in lattice QCD without gauge fixing
1996
We suggest a new (dynamical) Abelian projection of the lattice QCD. It contains no gauge condition imposed on gauge fields so that Gribov copying is avoided. Configurations of gauge fields that turn into monopoles in the Abelian projection can be classified in a gauge invariant way. In the continuum limit, the theory respects the Lorentz invariance. A similar dynamical reduction of the gauge symmetry is proposed for studies of gauge-variant correlators (like a gluon propagator) in lattice QCD. Though the procedure is harder for numerical simulations, it is free of gauge fixing artifacts, like the Gribov horizon and copies.
Nucleon isovector charges and twist-2 matrix elements with Nf=2+1 dynamical Wilson quarks
2019
We present results from a lattice QCD study of nucleon matrix elements at vanishing momentum transfer for local and twist-2 isovector operator insertions. Computations are performed on gauge ensembles with nonperturbatively improved ${N}_{f}=2+1$ Wilson fermions, covering four values of the lattice spacing and pion masses down to ${M}_{\ensuremath{\pi}}\ensuremath{\approx}200\text{ }\text{ }\mathrm{MeV}$. Several source-sink separations (typically $\ensuremath{\sim}1.0$ to $\ensuremath{\sim}1.5\text{ }\text{ }\mathrm{fm}$) allow us to assess excited-state contamination. Results on individual ensembles are obtained from simultaneous two-state fits across all observables and all available sou…
Numerical Stochastic Perturbation Theory and the Gradient Flow
2013
We study the Yang-Mills gradient flow using numerical stochastic perturbation theory. As an application of the method we consider the recently proposed gradient flow coupling in the Schr\"odinger functional for the pure SU(3) gauge theory.
Correlators of left charges and weak operators in finite volume chiral perturbation theory
2002
We compute the two-point correlator between left-handed flavour charges, and the three-point correlator between two left-handed charges and one strangeness violating \Delta I=3/2 weak operator, at next-to-leading order in finite volume SU(3)_L x SU(3)_R chiral perturbation theory, in the so-called epsilon-regime. Matching these results with the corresponding lattice measurements would in principle allow to extract the pion decay constant F, and the effective chiral theory parameter g_27, which determines the \Delta I = 3/2 amplitude of the weak decays K to \pi\pi as well as the kaon mixing parameter B_K in the chiral limit. We repeat the calculations in the replica formulation of quenched c…
A new representation of the Adler function for lattice QCD
2013
We address several aspects of lattice QCD calculations of the hadronic vacuum polarization and the associated Adler function. We implement a representation derived previously which allows one to access these phenomenologically important functions for a continuous set of virtualities, irrespective of the flavor structure of the current. Secondly we present a theoretical analysis of the finite-size effects on our particular representation of the Adler function, based on the operator product expansion at large momenta and on the spectral representation of the Euclidean correlator at small momenta. Finally, an analysis of the flavor structure of the electromagnetic current correlator is perform…
Extraction of K --> pi pi matrix elements with Wilson fermions
2001
We present the status of a lattice calculation for the K-->pipi matrix elements of the (delta S=1) effective weak Hamiltonian, directly with two pion in the final state. We study the energy shift of two pion in a finite volume both in the I=0 and I=2 channels. We explain a method to avoid the Goldstone pole contamination in the computation of renormalization constants for (delta I=3/2) operators. Finally we show some preliminary results for the matrix elements of (delta I=1/2) operators. Our quenched simulation is done at beta=6.0, with Wilson fermions, on a (24^3 X 64) lattice.
Covariant approximation averaging
2015
We present a new class of statistical error reduction techniques for Monte-Carlo simulations. Using covariant symmetries, we show that correlation functions can be constructed from inexpensive approximations without introducing any systematic bias in the final result. We introduce a new class of covariant approximation averaging techniques, known as all-mode averaging (AMA), in which the approximation takes account of contributions of all eigenmodes through the inverse of the Dirac operator computed from the conjugate gradient method with a relaxed stopping condition. In this paper we compare the performance and computational cost of our new method with traditional methods using correlation…
Precision measurements of B(D+→μ+νμ), the pseudoscalar decay constant fD+, and the quark mixing matrix element |Vcd|
2014
We report a measurement of the branching fraction B(D+ -> mu(+)nu(mu)) = [3.71 +/- 0.19(stat) +/- 0.06(sys)] x 10(-4) based on 2.92 fb(-1) of data accumulated at root s = 3.773 GeV with the BESIII detector at the BEPCII collider. This measurement, in conjunction with the Cabibbo-Kobayashi-Maskawa matrix element vertical bar V-cd vertical bar determined from a global Standard Model fit, implies a value for the weak decay constant f(D+) = (203.2 +/- 5.3 +/- 1.8) MeV. Additionally, using this branching fraction measurement together with a lattice QCD prediction for f(D+), we find vertical bar V-cd vertical bar 0.2210 +/- 0.0058 +/- 0.0047. In either case, these are the most precise results for…