70 research outputs found
Domain decomposition and multilevel integration for fermions
The numerical computation of many hadronic correlation functions is
exceedingly difficult due to the exponentially decreasing signal-to-noise ratio
with the distance between source and sink. Multilevel integration methods,
using independent updates of separate regions in space-time, are known to be
able to solve such problems but have so far been available only for pure gauge
theory. We present first steps into the direction of making such integration
schemes amenable to theories with fermions, by factorizing a given observable
via an approximated domain decomposition of the quark propagator. This allows
for multilevel integration of the (large) factorized contribution to the
observable, while its (small) correction can be computed in the standard way.Comment: 14 pages, 6 figures, v2: published version, talk presented at the
34th annual International Symposium on Lattice Field Theory, 24-30 July 2016,
University of Southampton, U
Local multiboson factorization of the quark determinant
We discuss the recently proposed multiboson domain-decomposed factorization
of the gauge-field dependence of the fermion determinant in lattice QCD. In
particular, we focus on the case of a lattice divided in an arbitrary number of
thick time slices. As a consequence, multiple space-time regions can be updated
independently. This allows to address the exponential degradation of the
signal-to-noise ration of correlation functions with multilevel Monte Carlo
sampling. We show numerical evidence of the effectiveness of a two-level
integration for pseudoscalar propagators with momentum and for vector
propagators, in a two active regions setup. These results are relevant to
lattice computation of the hadronic contributions to the anomalous magnetic
moment of the muon and to heavy meson decay form factors.Comment: 8 pages, 4 figures, talk presented at the 35th International
Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai
Testing the strength of the anomaly at the chiral phase transition in two-flavour QCD
We study the thermal transition of QCD with two degenerate light flavours by
lattice simulations using -improved Wilson quarks. Particular
emphasis lies on the pattern of chiral symmetry restoration, which we probe via
the static screening correlators. On volumes we observe that the
screening masses in transverse iso-vector vector and axial-vector channels
become degenerate at the transition temperature. The splitting between the
screening masses in iso-vector scalar and pseudoscalar channels is strongly
reduced compared to the splitting at zero temperature and is actually
consistent with zero within uncertainties. In this proceedings article we
extend our studies to matrix elements and iso-singlet correlation functions.
Furthermore, we present results on larger volumes, including first results at
the physical pion mass.Comment: 10 pages, 9 figures, invited contribution to the 9th International
Workshop on Chiral Dynamics, Sept. 17-21, 2018, Duke University, Durham, NC,
US
Deep inelastic scattering on the quark-gluon plasma
We provide an interpretation of the structure functions of a thermal medium such as the quark-gluon plasma in terms of the scattering of an incoming electron on the medium via the exchange of a spacelike photon. We then focus on the deep-inelastic scattering (DIS) regime, and formulate the corresponding moment sum rules obeyed by the structure functions. Accordingly, these moments are given by the thermal expectation value of twist-two operators, which is computable from first principles in lattice QCD for the first few moments. We also show how lattice QCD calculations can be used to probe how large the photon virtuality needs to be in order for the Bjorken scaling of structure functions to set in. Finally, we provide the parton-model interpretation of the structure functions in the Bjorken limit and test its consistency. As in DIS on the proton, the kinematic variable is proportional to the longitudinal momentum carried by the partons, however ranges from zero to infinity. Choosing the parton momentum parametrization to be where is the fluid four-velocity and its temperature in the rest frame, the parton distribution function for a plasma of non-interacting quarks is proportional to .We provide an interpretation of the structure functions of a thermal medium such as the quark-gluon plasma in terms of the scattering of an incoming electron on the medium via the exchange of a spacelike photon. We then focus on the deep-inelastic scattering (DIS) regime, and formulate the corresponding moment sum rules obeyed by the structure functions. Accordingly, these moments are given by the thermal expectation value of twist-two operators, which is computable from first principles in lattice QCD for the first few moments. We also show how lattice QCD calculations can be used to probe how large the photon virtuality needs to be in order for the Bjorken scaling of structure functions to set in. Finally, we provide the parton-model interpretation of the structure functions in the Bjorken limit and test its consistency. As in DIS on the proton, the kinematic variable x is proportional to the longitudinal momentum carried by the partons, however x ranges from zero to infinity. Choosing the parton momentum parametrization to be xT u where u is the fluid four-velocity and T its temperature in the rest frame, the parton distribution function for a plasma of non-interacting quarks is proportional to x log(1 + e)
The leading hadronic contribution to the running of the Weinberg angle using covariant coordinate-space methods
We present a preliminary study of the leading hadronic contribution to the
running of the Weinberg angle . The running is extracted
from the correlation function of the electromagnetic current with the vector
part of the weak neutral current using both the standard time-momentum
representation method and the Lorentz-covariant coordinate-space method
recently introduced by Meyer. Both connected and disconnected contributions
have been computed on non-perturbatively -improved
Wilson fermions configurations from the CLS initiative. Similar covariant
coordinate-space methods can be used to compute the leading hadronic
contribution to the anomalous magnetic moment of the muon and to
the running of the QED coupling .Comment: 7 pages, 2 figures, talk presented at The 36th Annual International
Symposium on Lattice Field Theory, July 22-28, 2018, East Lansing, MI, US
Aspects of chiral symmetry in QCD at T = 128 MeV
We investigate several aspects of chiral symmetry in QCD at a temperature of T=128 MeV. The study is based on a 24×963 lattice-QCD ensemble with O(a)-improved Wilson quarks and physical up, down and strange quark masses. The pion quasiparticle turns out to be significantly lighter than the zero-temperature pion mass, even though the corresponding static correlation length is shorter. We perform a quantitative comparison of our findings to predictions of chiral perturbation theory. Among several order parameters for chiral symmetry restoration, we compute the difference of the vector- and axial-vector time-dependent correlators and find it to be reduced by a factor ∼2/3 as compared to its vacuum counterpart
Photon emissivity of the quark-gluon plasma: a lattice QCD analysis of the transverse channel
We present results for the thermal photon emissivity of the quark-gluon
plasma derived from spatially transverse vector correlators computed in lattice
QCD at a temperature of 250 MeV. The analysis of the spectral functions,
performed at fixed spatial momentum, is based on continuum-extrapolated
correlators obtained with two flavours of dynamical Wilson fermions. We compare
the next-to-leading order perturbative QCD correlators, as well as the supersymmetric Yang-Mills correlators at infinite coupling, to the
correlators from lattice QCD and find them to lie within of each
other. We then refine the comparison, performing it at the level of filtered
spectral functions obtained model-independently via the Backus-Gilbert method.
Motivated by these studies, for frequencies GeV we use fit
ans\"atze to the spectral functions that perform well when applied to mock data
generated from the NLO QCD or from the strongly-coupled SYM spectral functions,
while the high-frequency part, GeV, is matched to NLO QCD.
We compare our results for the photon emissivity to our previous analysis of a
different vector channel at the same temperature. We obtain the most stringent
constraint at photon momenta around GeV, for which we find a
differential photon emission rate per unit volume of .Comment: 26 pages, 13 figures, 1 tabl
The leading hadronic contribution to from lattice QCD with flavours of O() improved Wilson quarks
The comparison of the theoretical and experimental determinations of the
anomalous magnetic moment of the muon constitutes one of the
strongest tests of the Standard Model at low energies. In this article, we
compute the leading hadronic contribution to using lattice QCD
simulations employing Wilson quarks. Gauge field ensembles at four different
lattice spacings and several values of the pion mass down to its physical value
are used. We apply the O() improvement programme with two discretizations of
the vector current to better constrain the approach to the continuum limit. The
electromagnetic current correlators are computed in the time-momentum
representation. In addition, we perform auxiliary calculations of the pion form
factor at timelike momenta in order to better constrain the tail of the
isovector correlator and to correct its dominant finite-size effect. For the
numerically dominant light-quark contribution, we have rescaled the lepton mass
by the pion decay constant computed on each lattice ensemble. We perform a
combined chiral and continuum extrapolation to the physical point, and our
final result is . It contains the contributions of quark-disconnected
diagrams, and the systematic error has been enlarged to account for the missing
isospin-breaking effects.Comment: 30 pages, 10 figure
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