83,034 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
Unifying Requirements and Code: an Example
Requirements and code, in conventional software engineering wisdom, belong to
entirely different worlds. Is it possible to unify these two worlds? A unified
framework could help make software easier to change and reuse. To explore the
feasibility of such an approach, the case study reported here takes a classic
example from the requirements engineering literature and describes it using a
programming language framework to express both domain and machine properties.
The paper describes the solution, discusses its benefits and limitations, and
assesses its scalability.Comment: 13 pages; 7 figures; to appear in Ershov Informatics Conference, PSI,
Kazan, Russia (LNCS), 201
Density, short-range order and the quark-gluon plasma
We study the thermal part of the energy density spatial correlator in the
quark-gluon plasma. We describe its qualitative form at high temperatures. We
then calculate it out to distances approx. 1.5/T in SU(3) gauge theory lattice
simulations for the range of temperatures 0.9<= T/T_c<= 2.2. The
vacuum-subtracted correlator exhibits non-monotonic behavior, and is almost
conformal by 2T_c. Its broad maximum at r approx. 0.6/T suggests a dense medium
with only weak short-range order, similar to a non-relativistic fluid near the
liquid-gas phase transition, where eta/s is minimal.Comment: 4 pages, 4 figure
Lattice Gauge Theory Sum Rule for the Shear Channel
An exact expression is derived for the thermal correlator of
shear stress in SU() lattice gauge theory. I remove a logarithmic
divergence by taking a suitable linear combination of the shear correlator and
the correlator of the energy density. The operator product expansion shows that
the same linear combination has a finite limit when . It
follows that the vacuum-subtracted shear spectral function vanishes at large
frequencies at least as fast as and obeys a sum rule. The
trace anomaly makes a potential contribution to the spectral sum rule which
remains to be fully calculated, but which I estimate to be numerically small
for . By contrast with the bulk channel, the shear channel
spectral density is then overall enhanced as compared to the spectral density
in vacuo.Comment: 11 pages, no figure
Cutoff Effects on Energy-Momentum Tensor Correlators in Lattice Gauge Theory
We investigate the discretization errors affecting correlators of the
energy-momentum tensor at finite temperature in SU() gauge
theory with the Wilson action and two different discretizations of
. We do so by using lattice perturbation theory and
non-perturbative Monte-Carlo simulations. These correlators, which are
functions of Euclidean time and spatial momentum , are the
starting point for a lattice study of the transport properties of the gluon
plasma. We find that the correlator of the energy has much
larger discretization errors than the correlator of momentum . Secondly, the shear and diagonal stress correlators ( and
) require \Nt\geq 8 for the point to be in the scaling
region and the cutoff effect to be less than 10%. We then show that their
discretization errors on an anisotropic lattice with \as/\at=2 are comparable
to those on the isotropic lattice with the same temporal lattice spacing.
Finally, we also study finite correlators.Comment: 16 pages, 5 figure
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
Top Quark Production
Recent measurements of top quark pair and single top production are
presented. The results include inclusive cross sections as well as studies of
differential distributions. Evidence for single top quark production in
association with a W-boson in the final state is reported for the first time.
Calculations in perturbative QCD up to approximate next-to-next-to-leading
order show very good agreement with the data.Comment: Physics in Collision, Slovakia, 2012 PSNUM 0
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