4,502 research outputs found

### The transverse structure of the QCD string

The characterization of the transverse structure of the QCD string is
discussed. We formulate a conjecture as to how the stress-energy tensor of the
underlying gauge theory couples to the string degrees of freedom. A consequence
of the conjecture is that the energy density and the longitudinal-stress
operators measure the distribution of the transverse position of the string, to
leading order in the string fluctuations, whereas the transverse-stress
operator does not. We interpret recent numerical measurements of the transverse
size of the confining string and show that the difference of the energy and
longitudinal-stress operators is the appropriate probe to use when comparing
with the next-to-leading order string prediction. Secondly we derive the
constraints imposed by open-closed string duality on the transverse structure
of the string. We show that a total of three independent `gravitational' form
factors characterize the transverse profile of the closed string, and obtain
the interpretation of recent effective string theory calculations: the square
radius of a closed string of length \beta, defined from the slope of its
gravitational form factor, is given by (d-1)/(2\pi\sigma)\log(\beta/(4r_0)) in
d space dimensions. This is to be compared with the well-known result that the
width of the open-string at mid-point grows as (d-1)/(2\pi\sigma) log(r/r_0).
We also obtain predictions for transition form factors among closed-string
states.Comment: 21 pages, 1 figur

### 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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