36,208 research outputs found
Lattice QCD and the two-photon decay of the neutral pion
Two-photon decays probe the structure of mesons and represent an important
contribution to hadronic light-by-light scattering. For the neutral pion, the
decay amplitude tests the effects of the chiral anomaly; for a heavy quarkonium
state, it measures the magnitude of its wavefunction at the origin. We rederive
the expression of the decay amplitude in terms of a Euclidean correlation
function starting from the theory defined on the torus. The derivation shows
that for timelike photons the approach to the infinite-volume decay amplitude
is exponential in the periodic box size.Comment: 18 pages, no figure
QCD at non-zero temperature from the lattice
I review the status of lattice QCD calculations at non-zero temperature.
After summarizing what is known about the equilibrium properties of strongly
interacting matter, I discuss in more detail recent results concerning the
quark-mass dependence of the thermal phase transition and the status of
calculations of non-equilibrium properties.Comment: 20 pages, 2 figures, proceedings of the Lattice 2015 conference in
Kobe, Japa
Lattice QCD and the Timelike Pion Form Factor
We present a formula that allows one to calculate the pion form factor in the
timelike region 2mpi <= sqrt{s} <= 4mpi in lattice QCD. The form factor
quantifies the contribution of two-pion states to the vacuum polarization. It
must be known very accurately in order to reduce the theoretical uncertainty on
the anomalous magnetic moment of the muon. At the same time, the formula
constitutes a rare example where, in a restricted kinematic regime, the
spectral function of a conserved current can be determined from Euclidean
observables without an explicit analytic continuation.Comment: 4 pages, 1 figure; corrects a factor 2 in Eq. (6) over the published
versio
The spectrum of SU(N) gauge theories in finite volume
We compute the spatial-volume dependence of the spectrum of 4D SU(3 <= N <=
6) gauge theories by lattice Monte-Carlo techniques. Setting the scale with the
string tension, the spatial volume is L^3 with 0.78fm <= L <= 2.3fm. The
Euclidean `time' direction is kept large enough to be considered infinite and
the boundary conditions are periodic in all four dimensions. We study the
mixing of torelon pairs with the scalar and tensor glueballs, using a 2x2
Hamiltonian based on large-N counting rules. Looking to the other symmetry
channels, finite-volume effects on the glueball spectrum are already
surprisingly small in SU(3), and they become rapidly smaller as N is increased:
several low-lying SU(6) states have no finite-volume corrections at the 1-2%
level, at least down to L=0.9fm. We discuss the relation of this work with
analytic calculations in small and intermediate volume, and with Eguchi-Kawai
reduction in the planar limit.Comment: 20 pages, 5 figures; major rewriting of section 2, and other minor
improvements: version accepted in JHE
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