1,757 research outputs found
Higgs-regularized three-loop four-gluon amplitude in N=4 SYM: exponentiation and Regge limits
We compute the three-loop contribution to the N=4 supersymmetric Yang-Mills
planar four-gluon amplitude using the recently-proposed Higgs IR regulator of
Alday, Henn, Plefka, and Schuster. In particular, we test the proposed
exponential ansatz for the four-gluon amplitude that is the analog of the BDS
ansatz in dimensional regularization. By evaluating our results at a number of
kinematic points, and also in several kinematic limits, we establish the
validity of this ansatz at the three-loop level.
We also examine the Regge limit of the planar four-gluon amplitude using
several different IR regulators: dimensional regularization, Higgs
regularization, and a cutoff regularization. In the latter two schemes, it is
shown that the leading logarithmic (LL) behavior of the amplitudes, and
therefore the lowest-order approximation to the gluon Regge trajectory, can be
correctly obtained from the ladder approximation of the sum of diagrams. In
dimensional regularization, on the other hand, there is no single dominant set
of diagrams in the LL approximation. We also compute the NLL and NNLL behavior
of the L-loop ladder diagram using Higgs regularization.Comment: 45 pages, 9 figures; v3: major revision (more stringent tests,
discussion of order of limits in the Regge regime
Collective chemotactic dynamics in the presence of self-generated fluid flows
In micro-swimmer suspensions locomotion necessarily generates fluid motion,
and it is known that such flows can lead to collective behavior from unbiased
swimming. We examine the complementary problem of how chemotaxis is affected by
self-generated flows. A kinetic theory coupling run-and-tumble chemotaxis to
the flows of collective swimming shows separate branches of chemotactic and
hydrodynamic instabilities for isotropic suspensions, the first driving
aggregation, the second producing increased orientational order in suspensions
of "pushers" and maximal disorder in suspensions of "pullers". Nonlinear
simulations show that hydrodynamic interactions can limit and modify
chemotactically-driven aggregation dynamics. In puller suspensions the dynamics
form aggregates that are mutually-repelling due to the non-trivial flows. In
pusher suspensions chemotactic aggregation can lead to destabilizing flows that
fragment the regions of aggregation.Comment: 4 page
Field Strength Correlators For 2D Yang-Mills Over Riemann Surfaces
The path integral computation of field strength correlation functions for two
dimensional Yang-Mills theories over Riemann surfaces is studied. The
calculation is carried out by abelianization, which leads to correlators that
are topological. They are nontrivial as a result of the topological
obstructions to the abelianization. It is shown in the large N limit on the
sphere that the correlators undergo second order phase transitions at the
critical point. Our results are applied to a computation of contractible Wilson
loops.Comment: final version to appear in Int. Jour. Mod. Phys. A, minor
corrections, added a few comments on Wilson loops and non-abelian Stokes
theore
Multi-scale Renormalisation Group Improvement of the Effective Potential
Using the renormalisation group and a conjecture concerning the perturbation
series for the effective potential, the leading logarithms in the effective
potential are exactly summed for scalar and Yukawa theories.Comment: 19 pages, DIAS STP 94-09. Expanded to check large N limit, typo's
corrected, to appear in Phys Rev
A Note on the Picard-Fuchs Equations for N=2 Seiberg-Witten Theories
A concise presentation of the PF equations for N=2 Seiberg-Witten theories
for the classical groups of rank r with N_f massless hypermultiplets in the
fundamental representation is provided. For N_f=0, all r PF equations can be
given in a generic form. For certain cases with N_f\neq zero, not all equations
are generic. However, in all cases there are at least r-2 generic PF equations.
For these cases the classical part of the equations is generic, while the
quantum part can be formulated using a method described in a previous paper by
the authors, which is well suited to symbolic computer calculations.Comment: 25 pages, Latex; some new references adde
Effect of breastfeeding on gastrointestinal infection in infants: A targeted maximum likelihood approach for clustered longitudinal data
The PROmotion of Breastfeeding Intervention Trial (PROBIT) cluster-randomized
a program encouraging breastfeeding to new mothers in hospital centers. The
original studies indicated that this intervention successfully increased
duration of breastfeeding and lowered rates of gastrointestinal tract
infections in newborns. Additional scientific and popular interest lies in
determining the causal effect of longer breastfeeding on gastrointestinal
infection. In this study, we estimate the expected infection count under
various lengths of breastfeeding in order to estimate the effect of
breastfeeding duration on infection. Due to the presence of baseline and
time-dependent confounding, specialized "causal" estimation methods are
required. We demonstrate the double-robust method of Targeted Maximum
Likelihood Estimation (TMLE) in the context of this application and review some
related methods and the adjustments required to account for clustering. We
compare TMLE (implemented both parametrically and using a data-adaptive
algorithm) to other causal methods for this example. In addition, we conduct a
simulation study to determine (1) the effectiveness of controlling for
clustering indicators when cluster-specific confounders are unmeasured and (2)
the importance of using data-adaptive TMLE.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS727 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
From scalar to string confinement
We outline a connection between scalar quark confinement, a
phenomenologically successful concept heretofore lacking fundamental
justification, and QCD. Although scalar confinement does not follow from QCD,
there is an interesting and close relationship between them. We develop a
simple model intermediate between scalar confinement and the QCD string for
illustrative purposes. Finally, we find the bound state masses of scalar,
time-component vector, and string confinement analytically through
semi-classical quantization.Comment: ReVTeX, 9 pages, 5 figure
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