Richard C. Brower

String-gauge dual description of deep inelastic scattering at small-x

The AdS/CFT correspondence in principle gives a new approach to deep inelastic scattering as formulated by Polchinski and Strassler. Subsequently Brower, Polchinski, Strassler and Tan (BPST) computed the strong coupling kernel for the vacuum (or Pomeron) contribution to total cross sections. By identifying deep inelastic scattering with virtual photon total cross section, this allows a self consistent description at small-x where the dominant contribution is the vacuum exchange process. Here we formulate this contribution and compare it with HERA small-x DIS scattering data. We find that the BPST kernel along with a very simple local approximation to the proton and current “wave functions” gives a remarkably good fit not only at large Q2 dominated by conformal symmetry but also extends to small Q2, supplemented by a hard-wall cut-off of the AdS in the IR. We suggest that this is a useful phenomenological parametrization with implications for other diffractive processes, such as double diffractive Higgs production.

QCD as a quantum link model

QCD is constructed as a lattice gauge theory in which the elements of the link matrices are represented by non-commuting operators acting in a Hilbert space. The resulting quantum link model for QCD is formulated with a fifth Euclidean dimension, whose extent resembles the inverse gauge coupling of the resulting four-dimensional theory after dimensional reduction. The inclusion of quarks is natural in Shamir{close_quote}s variant of Kaplan{close_quote}s fermion method, which does not require fine-tuning to approach the chiral limit. A rishon representation in terms of fermionic constituents of the gluons is derived and the quantum link Hamiltonian for QCD with a U(N) gauge symmetry is expressed in terms of glueball, meson and constituent quark operators. The new formulation of QCD is promising both from an analytic and from a computational point of view. {copyright} {ital 1999} {ital The American Physical Society}

QCD at fixed topology

Abstract Since present Monte Carlo algorithms for lattice QCD may become trapped in a fixed topological charge sector, it is important to understand the effect of calculating at fixed topology. In this Letter, we show that although the restriction to a fixed topological sector becomes irrelevant in the infinite volume limit, it gives rise to characteristic finite-size effects due to contributions from all θ -vacua. We calculate these effects and show how to extract physical results from numerical data obtained at fixed topology.

Magnetic monopole loop for the Yang-Mills instanton

We investigate {close_quote}t Hooft{endash}Mandelstam monopoles in QCD in the presence of a single classical instanton configuration. The solution to the maximal Abelian projection is found to be a circular monopole trajectory with radius R centered on the instanton. At zero loop radius, there is a marginally stable (or flat) direction for loop formation to O(R{sup 4}lnR). We argue that loops will form, in the semiclassical limit, due to small perturbations such as the dipole interaction between instanton{endash}anti-instanton pairs. As the instanton gas becomes a liquid, the percolation of the monopole loops may, therefore, provide a semiclassical precursor to the confinement mechanism. {copyright} {ital 1997} {ital The American Physical Society}

DECOUPLING OF THE POMERANCHON REGGE TRAJECTORY.

Abstract Recently it has been shown that an isolated Pomeranchon Regge pole with unit intercept decouples from many processes at zero momentum transfer. Here we show that the vanishing of the Pomeranchon-Reggeon-particle vertex implies the decoupling of the Pomeranchon from total cross sections. The nontrivial Toller angle and signature properties are essential to the argument. This inconsistency with constant sections raises serious questions about the physical significance of such a Pomeranchon.

Quantum link models with many rishon flavors and with many colors

Abstract Quantum link models are a novel formulation of gauge theories in terms of discrete degrees of freedom. These degrees of freedom are described by quantum operators acting in a finite-dimensional Hilbert space. We show that for certain representations of the operator algebra, the usual Yang-Mills action is recovered in the continuum limit. The quantum operators can be expressed as bilinears of fermionic creation and annihilation operators called rishons. Using the rishon representation the quantum link Hamiltonian can be expressed entirely in terms of color-neutral operators. This allows us to study the large N tc limit of this model. In the t Hooft limit we find an area law for the Wilson loop and a mass gap. Furthermore, the strong coupling expansion is a topological expansion in which graphs with handles and boundaries are suppressed.

Study of instanton contributions to moments of nucleon spin-dependent structure functions

Instantons are the natural mechanism in non-perturbative QCD to remove helicity from valence quarks and transfer it to gluons and quark-antiquark pairs. To understand the extent to which instantons explain the so-calledspin crisisin the nucleon, we calculate moments of spin-dependent structure functions in quenched QCD and compare them with the results obtained with cooled configurations from which essentially all gluon contributions except instantons have been removed. Preliminary results are presented.Abstract Instantons are the natural mechanism in non-perturbative QCD to remove helicity from valence quarks and transfer it to gluons and quark-antiquark pairs. To understand the extent to which instantons explain the so-called “spin crisis” in the nucleon, we calculate moments of spin-dependent structure functions in quenched QCD and compare them with the results obtained with cooled configurations from which essentially all gluon contributions except instantons have been removed. Preliminary results are presented.

Lattice study of ChPT beyond QCD

Thomas Appelquista, Adam Avakianb, Ron Babichb, Richard C. Browerb, Michael Chengc, Michael A. Clark f g, Saul D. Cohenb, George T. Fleminga, Joseph Kiskisd , Ethan T. Neil∗a, James C. Osborne, Claudio Rebbib, David Schaichb, Pavlos Vranasc a Department of Physics, Sloane Laboratory, Yale University, New Haven, CT 06520 b Department of Physics, Boston University, Boston, MA 02215 c Physical Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA

Progress Report on Staggered Multigrid

Multigrid methods have become the optimal solvers for Lattice QCD, first for the Wilson-clover discrete representation and more recently for the domain wall formulation. However, at present, the ensembles with the largest lattices use a third discretization, staggered. At the physical pion mass, propagator inversions with the staggered operator require

National Computational Infrastructure for Lattice Gauge Theory SciDAC-2 Closeout Report

mathcal{O}(10,000)