8,880 research outputs found
Lattice QCD with mixed actions
We discuss some of the implications of simulating QCD when the action used
for the sea quarks is different from that used for the valence quarks. We
present exploratory results for the hadron mass spectrum and pseudoscalar meson
decay constants using improved staggered sea quarks and HYP-smeared overlap
valence quarks. We propose a method for matching the valence quark mass to the
sea quark mass and demonstrate it on UKQCD clover data in the simpler case
where the sea and valence actions are the same.Comment: 15 pages, 10 figures some minor modification to text and figures.
Accepted for publicatio
Simulating chiral quarks in the epsilon-regime of QCD
We present simulation results for lattice QCD with chiral fermions in small
volumes, where the epsilon-expansion of chiral perturbation theory applies. Our
data for the low lying Dirac eigenvalues, as well as mesonic correlation
functions, are in agreement with analytical predictions. This allows us to
extract values for the leading Low Energy Constants F_{pi} and Sigma.Comment: 4 pages, talk presented by W.B. at Baryons04 (Paris, October 25 - 29,
2004); one Ref. adde
Improvements of L\"uscher's local bosonic fermion algorithm
We discuss the application of hybrid over-relaxation, even-odd
preconditioning and a modified local updating procedure to the local bosonic
fermion algorithm. Studies on autocorrelation times and the tuning of the
parameters of the algorithm are done on various lattice sizes simulating
SU(2)-LGT with 2 flavours of Wilson quarks. A substantial decrease of the
computational cost could be achieved.Comment: 20 pages, uuencoded compressed postscrip
How much do charm sea quarks affect the charmonium spectrum?
The properties of charmonium states are or will be intensively studied by the
B-factories Belle II and BESIII, the LHCb and PANDA experiments and at a future
Super-c-tau Factory. Precise lattice calculations provide valuable input and
several results have been obtained by simulating up, down and strange quarks in
the sea. We investigate the impact of a charm quark in the sea on the
charmonium spectrum, the renormalization group invariant charm-quark mass and
the scalar charm-quark content of charmonium. The latter is obtained by the
direct computation of the mass-derivatives of the charmonium masses. We do this
investigation in a model, QCD with two degenerate charm quarks. The absence of
light quarks allows us to reach very small lattice spacings down to 0.023 fm.
By comparing to pure gauge theory we find that charm quarks in the sea affect
the hyperfine splitting at a level below 2%. The most significant effects are
5% in the quark mass and 3% in the value of the charm quark content of the
eta_c meson. Given that we simulate two charm quarks these estimates are upper
bounds for the contribution of a single charm quark. We show that lattice
spacings <0.06 fm are needed for safe continuum extrapolations of the
charmonium spectrum with O(a) improved Wilson quarks. A useful relation for the
projection to the desired parity of operators in two-point functions computed
with twisted mass fermions is proven.Comment: 25 pages, 8 figure
Light dynamical fermions on the lattice: toward the chiral regime of QCD
Algorithmic and technical progress achieved over the last few years makes QCD
simulations with light dynamical quarks much faster than before. As a result
lattices with pions as light as 250--300 MeV can be simulated with the present
generation of computers. I review recent conceptual and numerical progress in
this field, with particular emphasis on results obtained and difficulties
encountered in simulations with significantly smaller quark masses with respect
to previous computations. I also attempt to compare physical results for pion
masses and decay constants available to date in the two-flavour theory with
expectations from chiral perturbation theory.Comment: Plenary talk given at XXIVth International Symposium on Lattice Field
Theory Lattice2006(plenary), Tucson, Arizona, 23-28 July 2006. Submitted to
PoS in October 200
Studying the Perturbative Reggeon
We consider the flavour non-singlet Reggeon within the context of
perturbative QCD. This consists of ladders built out of ``reggeized'' quarks.
We propose a method for the numerical solution of the integro-differential
equation for the amplitude describing the exchange of such a Reggeon. The
solution is known to have a sharp rise at low values of Bjorken-x when applied
to non-singlet quantities in deep-inelastic scattering. We show that when the
running of the coupling is taken into account this sharp rise is further
enhanced, although the Q^2 dependence is suppressed by the introduction of the
running coupling. We also investigate the effects of simulating
non-perturbative physics by introducing a constituent mass for the soft quarks
and an effective mass for the soft gluons exchanged in the t-channel.Comment: LaTeX, 21 pages, 16 figure
What is the W b anti-b, Z b anti-b or t anti-t b anti-b irreducible background to the light Higgs boson searches at LHC?
The W b anti-b, Z b anti-b and t anti-t b anti-b production at LHC are
irreducible backgrounds for possible observability of the Standard Model (SM)
and Minimal Supersymmetric Standard Model (MSSM) light Higgs boson respectively
in the associated WH, ZH and t anti-t H production followed by the H -> b
anti-b decay. We present a comparison of the background estimates as obtained
with (a) complete massive matrix element implemented in AcerMC Monte Carlo
generator and (b) PYTHIA implementation of the inclusive W, Z, and t anti-t
production, followed by the parton showering mechanism. Both approaches lead to
a production of the final state of interest, but differ in the approximations
used. We concentrate on the comparison of these approaches for the background
to the light Higgs boson searches at LHC.Comment: EPJC Referee's comments adde
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