arXiv subjects

Karl Jansen

Publications and source records attributed to Karl Jansen.

At least 271 records · Page 15Linked to original sources

arXiv2011

Anomalous scaling in the random-force-driven Burgers equation: A Monte Carlo study

We present a new approach to determine numerically the statistical behavior of small-scale structures in hydrodynamic turbulence. Starting from the functional integral representation of the random-force-driven Burgers equation we show that Monte Carlo simulations allow us to determine the anomalous scaling of high-order moments of velocity differences. Given the general applicability of Monte Carlo methods, this opens up the possibility to address also other systems relevant to turbulence within this framework.

nlin.CD↗

arXiv2011

Two-flavor QCD correction to lepton magnetic moments at leading-order in the electromagnetic coupling

We present a reliable nonperturbative calculation of the QCD correction, at leading order in the electromagnetic coupling, to the anomalous magnetic moment of the electron, muon, and tau leptons using two-flavor lattice QCD. We use multiple lattice spacings, multiple volumes, and a broad range of quark masses to control the continuum, infinite-volume, and chiral limits. We examine the impact of the commonly ignored disconnected diagrams and introduce a modification to the previously used method that results in a well-controlled lattice calculation. We obtain 1.513(43) 10^(-12), 5.72(16) 10^(-8), and 2.650(54) 10^(-6) for the leading-order two-flavor QCD correction to the anomalous magnetic moment of the electron, muon, and tau, respectively, each accurate to better than 3%.

hep-lat↗

arXiv2011

Precision Study of Excited State Effects in Nucleon Matrix Elements

We present a dedicated analysis of the influence of excited states on the calculation of nucleon matrix elements. This calculation is performed at a fixed value of the lattice spacing, volume and pion mass that are typical of contemporary lattice computations. We focus on the nucleon axial charge, g_A, for which we use about 7,500 measurements, and on the average momentum of the unpolarized isovector parton distribution, _{u-d}, for which we use about 23,000 measurements. All computations are done employing N_f=2+1+1 maximally-twisted-mass Wilson fermions and using non-perturbatively calculated renormalization factors. Excited state effects are shown to be negligible for g_A whereas they lead to an O(10%) downward shift for _{u-d}.

hep-lat↗

arXiv2011

Resonance Parameters of the rho-Meson from Lattice QCD

We perform a non-perturbative lattice calculation of the P-wave pion-pion scattering phase in the rho-meson decay channel using two flavors of maximally twisted mass fermions at pion masses ranging from 480 MeV to 290 MeV. Making use of finite-size methods, we evaluate the pion-pion scattering phase in the center-of-mass frame and two moving frames. Applying an effective range formula, we find a good description of our results for the scattering phase as a function of the energy covering the resonance region. This allows us to extract the rho-meson mass and decay width and to study their quark mass dependence.

hep-lat↗

arXiv2011

A new moving frame to extract scattering phases in lattice QCD

We present a derivation of the finite-size formulae in a moving frame with total momentum P=(2π/L)(e1+e2). These formulae allow us to calculate the S-wave and P-wave scattering phases at more energies with a fixed lattice size and thus help us to determine the resonance parameters precisely.

hep-lat↗

arXiv2011

Leading-order hadronic contribution to g-2 from lattice QCD

We calculate the leading-order hadronic correction to the anomalous magnetic moments of each of the three charged leptons in the Standard Model: the electron, muon and tau. Working in two-flavor lattice QCD, we address essentially all sources of systematic error: lattice artifacts, finite-size effects, quark-mass extrapolation, momentum extrapolation and disconnected diagrams. The most significant remaining systematic error, the exclusion of the strange and charm quark contributions, will be addressed in our four-flavor calculation. We achieve a statistical accuracy of 2% or better for the physical values for each of the three leptons and the systematic errors are at most comparable.

hep-lat↗

arXiv2011

Nucleon matrix elements with $N_f=2+1+1$ maximally twisted fermions

We present the first lattice calculation of nucleon matrix elements using four dynamical flavors. We use the $N_f=2+1+1$ maximally twisted mass formulation. The renormalization is performed non-perturbatively in the RI$^\prime$-MOM scheme and results are given for the vector and axial vector operators with up to one-derivative. Our calculation of the average momentum of the unpolarized non-singlet parton distribution is presented and compared to our previous results obtained from the $N_f=2$ case.

hep-lat↗

arXiv2010

Continuum Limit of Overlap Valence Quarks on a Twisted Mass Sea

We study a lattice QCD mixed action with overlap valence quarks on two flavours of Wilson maximally twisted mass sea quarks. Employing three different matching conditions to relate both actions to each other, we investigate the continuum limit by using three values of the lattice spacing ranging from 0.05 fm to 0.08 fm. A particular emphasis is put on the effect on physical observables of the topological zero modes appearing in the valence overlap operator. We estimate the region of parameter space where the contribution from these zero modes is sufficiently small such that their effects can be safely controlled and a restoration of unitarity of the mixed action in the continuum limit is reached.

hep-lat↗

arXiv2010

Low lying baryon spectrum with $N_f = 2+1+1$ dynamical twisted quarks

We present first results on the octet and decuplet strange baryon spectrum with $N_f=2+1+1$ twisted mass quarks. We use an Osterwalder Seiler valence strange quark with a mass tuned to the kaon and compare the results with those obtained in the unitary setup. This comparison allows to perform a first study of the lattice artefacts introduced by the mixed action approach. We investigate the effect of the strange and charm quarks in the sea by using two lattice spacings and comparing with preceding $N_f = 2$ twisted mass fermion calculations.

hep-lat↗

arXiv2010

Pseudoscalar decay constants from N_f=2+1+1 twisted mass lattice QCD

We present first results for the pseudoscalar decay constants $f_K$, $f_D$ and $f_{D_s}$ from lattice QCD with N_f=2+1+1 flavours of dynamical quarks. The lattice simulations have been performed by the European Twisted Mass collaboration (ETMC) using maximally twisted mass quarks. For the pseudoscalar decay constants we follow a mixed action approach by using so called Osterwalder-Seiler fermions in the valence sector for strange and charm quarks. The data for two values of the lattice spacing and several values of the up/down quark mass is analysed using chiral perturbation theory.

hep-lat↗

arXiv2010

Leading order hadronic contribution to g-2 from twisted mass QCD

We calculate the leading order hadronic contribution to the muon anomalous magnetic moment using twisted mass lattice QCD. The pion masses range from 330 MeV to 650 MeV. We use two lattice spacings, a=0.079 fm and 0.063 fm, to study lattice artifacts. Finite-size effects are studied for two values of the pion mass, and we calculate the disconnected contributions for four ensembles. Particular attention is paid to the dominant contributions of the vector mesons, both phenomenologically and from our lattice calculation.

hep-lat↗

arXiv2010

Overlap Valence Quarks on a Twisted Mass Sea

We present the results of an investigation of a mixed action approach of overlap valence and maximally twisted mass sea quarks. Employing a particular matching condition on the pion mass, we analyze the continuum limit scaling of the pion decay constant and the role of chiral zero modes of the overlap operator in this process. We employ gauge field configurations generated by the European Twisted Mass Collaboration with linear lattice size $L$ ranging from 1.3 to 1.9 fm. The continuum limit is taken at a fixed value of $L=1.3$ fm, employing three values of the lattice spacing and two values of the pion mass constructed from sea quarks only.

hep-lat↗

arXiv2010

Effects of a potential fourth fermion generation on the upper and lower Higgs boson mass bounds

We study the effect of a potential fourth fermion generation on the upper and lower Higgs boson mass bounds. This investigation is based on the numerical evaluation of a chirally invariant lattice Higgs-Yukawa model emulating the same Higgs-fermion coupling structure as in the Higgs sector of the electroweak Standard Model. In particular, the considered model obeys a Ginsparg-Wilson version of the underlying ${SU}(2)_L\times {U}(1)_Y$ symmetry, being a global symmetry here due to the neglection of gauge fields in this model. We present our results on the modification of the upper and lower Higgs boson mass bounds induced by the presence of a hypothetical very heavy fourth quark doublet. Finally, we compare these findings to the standard scenario of three fermion generations.

hep-lat↗

arXiv2010

Kaon and D meson masses with N_f = 2+1+1 twisted mass lattice QCD

We discuss the computation of the kaon and D meson masses in the N_f = 2+1+1 twisted mass lattice QCD setup, where explicit heavy flavor and parity breaking occurs at finite lattice spacing. We present three methods suitable in this context and verify their consistency.

hep-lat↗

arXiv2010

Computing K and D meson masses with N_f = 2+1+1 twisted mass lattice QCD

We discuss the computation of the mass of the K and D mesons within the framework of N_f = 2+1+1 twisted mass lattice QCD from a technical point of view. These quantities are essential, already at the level of generating gauge configurations, being obvious candidates to tune the strange and charm quark masses to their physical values. In particular, we address the problems related to the twisted mass flavor and parity symmetry breaking, which arise when considering a non-degenerate (c,s) doublet. We propose and verify the consistency of three methods to extract the K and D meson masses in this framework.

hep-lat↗

arXiv2010

Light hadrons from lattice QCD with light (u,d), strange and charm dynamical quarks

We present results of lattice QCD simulations with mass-degenerate up and down and mass-split strange and charm (N_f = 2+1+1) dynamical quarks using Wilson twisted mass fermions at maximal twist. The tuning of the strange and charm quark masses is performed at two values of the lattice spacing a~0.078 fm and a~0.086 fm with lattice sizes ranging from L~1.9 fm to L~2.8 fm. We measure with high statistical precision the light pseudoscalar mass m_PS and decay constant f_PS in a range 270 < m_PS < 510 MeV and determine the low energy parameters f_0, l_3 and l_4 of SU(2) chiral perturbation theory. We use the two values of the lattice spacing, several lattice sizes as well as different values of the light, strange and charm quark masses to explore the systematic effects. A first study of discretisation effects in light-quark observables and a comparison to N_f=2 results are performed.

hep-lat↗

arXiv2010

Comparing topological charge definitions using topology fixing actions

We investigate both the hyperbolic action and the determinant ratio action designed to fix the topological charge on the lattice. We show to what extent topology is fixed depending on the parameters of these actions, keeping the physical situation fixed. At the same time the agreement between different definitions of topological charge - the field theoretic and the index definition - is directly correlated to the degree topology is fixed. Moreover, it turns out that the two definitions agree very well. We also study finite volume effects arising in the static potential and related quantities due to topology fixing.

hep-lat↗

arXiv2010

The pi+ pi+ scattering length from maximally twisted mass lattice QCD

We calculate the s-wave pion-pion scattering length in the isospin I=2 channel in lattice QCD for pion masses ranging from 270 Mev to 485 Mev using two flavors of maximally twisted mass fermions at a lattice spacing of 0.086 fm. Additionally, we check for lattice artifacts with one calculation at a finer lattice spacing of 0.067 fm. We use chiral perturbation theory at next-to-leading order to extrapolate our results. At the physical pion mass, we find m_pi a_pipi(I=2)=-0.04385(28)(38) for the scattering length, where the first error is statistical and the second is our estimate of several systematic effects.

hep-lat↗