Search results for "High Energy Physics - Lattice"
showing 10 items of 466 documents
Non-perturbative renormalization and O$(a)$-improvement of the non-singlet vector current with $N_{\mathrm{f}}=2+1$ Wilson fermions and tree-level Sy…
2018
In calculating hadronic contributions to precision observables for tests of the Standard Model in lattice QCD, the electromagnetic current plays a central role. Using a Wilson action with O($a$) improvement in QCD with $N_{\mathrm{f}}$ flavors, a counterterm must be added to the vector current in order for its on-shell matrix elements to be O($a$) improved. In addition, the local vector current, which has support on one lattice site, must be renormalized. At O($a$), the breaking of the SU($N_{\mathrm{f}}$) symmetry by the quark mass matrix leads to a mixing between the local currents of different quark flavors. We present a non-perturbative calculation of all the required improvement and re…
Towards the N_f=2 deconfinement transition temperature with O(a) improved Wilson fermions
2010
A lot of effort in lattice simulations over the last years has been devoted to studies of the QCD deconfinement transition. Most state-of-the-art simulations use rooted staggered fermions, while Wilson fermions are affected by large systematic uncertainties, such as coarse lattices or heavy sea quarks. Here we report on an ongoing study of the transition, using two degenerate flavours of nonperturbatively $\Ord(a)$ improved Wilson fermions. We start with $N_{t}=12$ and 16 lattices and pion masses of 600 to 450 MeV, aiming at chiral and continuum limits with light quarks.
Heavy-light mesons in the epsilon-regime
2009
We study the finite-size scaling of heavy-light mesons in the static limit. We compute two-point functions of chiral current densities as well as pseudoscalar densities in the epsilon-regime of heavy meson Chiral Perturbation Theory (HMChPT). As expected, finite volume dependence turns out to be significant in this regime and can be predicted in the effective theory in terms of the infinite-volume low-energy couplings. These results might be relevant for extraction of heavy-meson properties from lattice simulations.
Estimating the thermal photon production rate using lattice QCD
2021
We present results for the photon emission rate determined from the transverse channel vector correlator at fixed spatial momentum using two flavors of dynamical Wilson fermions at $T\sim$250 MeV. We estimate the transverse channel spectral function using the continuum extrapolated correlator by applying various fit ans\"atze with a smooth matching to the NLO perturbative result. We confront our estimate based on this channel with the latest results of our collaboration based on the difference of the transverse and longitudinal channels.
Lattice cutoff effects for F_Ds with improved Wilson fermions - a final lesson from the quenched case
2008
In view of the recent excitement about a tension between determinations of f_Ds from experiment and from simulations of lattice QCD with dynamical quarks, we try to clear up the picture of lattice determinations in the continuum limit of the quenched approximation. For O(a) improved Wilson quarks we see linear scaling in the squared lattice spacing a^2 only for a<~0.08fm. For coarser lattices we observe significant contaminations from higher order cutoff effects. As an aside we also study the scaling of the charm quark mass and the ratio of the vector to the pseudo-scalar decay constant and the spin-splitting.
Quark Contributions to Nucleon Momentum and Spin from Domain Wall fermion calculations
2011
We report contributions to the nucleon spin and momentum from light quarks calculated using dynamical domain wall fermions with pion masses down to 300 MeV and fine lattice spacing a=0.084 fm. Albeit without disconnected diagrams, we observe that spin and orbital angular momenta of both u and d quarks are opposite, almost canceling in the case of the d quark, which agrees with previous calculations using a mixed quark action. We also present the full momentum dependence of n=2 generalized form factors showing little variation with the pion mass.
Isovector Axial Vector Form Factors of the Nucleon from Lattice QCD with Nf=2+1 O(a)-improved Wilson Fermions
2021
We present the analysis of isovector axial vector nucleon form factors on a set of $N_f=2+1$ CLS ensembles with $\mathcal O(a)$-improved Wilson fermions and L\"uscher-Weisz gauge action. The set of ensembles covers a pion mass range of $130-353\,$MeV with lattice spacings between $0.05\,$fm and $0.09\,$fm. In particular, the set includes a $L/a=96$ ensemble at the physical pion mass. For the purpose of the form factor extraction, we employ both the summed operator insertion method (summation method) and explicit two-state fits in order to account for excited-state contributions to the nucleon correlation functions. To describe the $Q^{2}$-behavior of the form factors, we perform $z$-expansi…
Nucleon electromagnetic and axial form factors with N$_f$=2 twisted mass fermions at the physical point
2017
We present results for the nucleon electromagnetic and axial form factors using an N$_f$=2 twisted mass fermion ensemble with pion mass of about 131 MeV. We use multiple sink-source separations to identify excited state contamination. Dipole masses for the momentum dependence of the form factors are extracted and compared to experiment, as is the nucleon magnetic moment and charge and magnetic radii.
A variational method for spectral functions
2016
The Generalized Eigenvalue Problem (GEVP) has been used extensively in the past in order to reliably extract energy levels from time-dependent Euclidean correlators calculated in Lattice QCD. We propose a formulation of the GEVP in frequency space. Our approach consists of applying the model-independent Backus-Gilbert method to a set of Euclidean two-point functions with common quantum numbers. A GEVP analysis in frequency space is then applied to a matrix of estimators that allows us, among other things, to obtain particular linear combinations of the initial set of operators that optimally overlap to different local regions in frequency. We apply this method to lattice data from NRQCD. Th…
Lattice calculation of the pion transition form factor $��^0 \to ��^* ��^*$
2016
We calculate the $��^0\to ��^*��^*$ transition form factor ${\cal F}_{��^0��^*��^*}(q_1^2,q_2^2)$ in lattice QCD with two flavors of quarks. Our main motivation is to provide the input to calculate the $��^0$-pole contribution to hadronic light-by-light scattering in the muon $(g-2)$, $a_��^{\rm HLbL;��^0}$. We therefore focus on the region where both photons are spacelike up to virtualities of about $1.5~$GeV$^2$, which has so far not been experimentally accessible. Results are obtained in the continuum at the physical pion mass by a combined extrapolation. We reproduce the prediction of the chiral anomaly for real photons with an accuracy of about $8-9\%$. We also compare to various recen…