167 research outputs found
Idealized Multigrid Algorithm for Staggered Fermions
An idealized multigrid algorithm for the computation of propagators of
staggered fermions is investigated.
Exemplified in four-dimensional gauge fields, it is shown that the
idealized algorithm preserves criticality under coarsening.
The same is not true when the coarse grid operator is defined by the Galerkin
prescription.
Relaxation times in computations of propagators are small, and critical
slowing is strongly reduced (or eliminated) in the idealized algorithm.
Unfortunately, this algorithm is not practical for production runs, but the
investigations presented here answer important questions of principle.Comment: 11 pages, no figures, DESY 93-046; can be formatted with plain LaTeX
article styl
The Chiral Extension of Lattice QCD
The chiral extension of Quantum Chromodynamics (XQCD) adds to the standard
lattice action explicit pseudoscalar meson fields for the chiral condensates.
With this action, it is feasible to do simulations at the chiral limit with
zero mass Goldstone modes. We review the arguments for why this is expected to
be in the same universality class as the traditional action. We present
preliminary results on convergence of XQCD for naive fermions and on the
methodology for introducing counter terms to restore chiral symmetry for Wilson
fermions.Comment: 7 pages, LATTICE 94 talk by R. Brower: Latex file with 2 postscript
figures for encapsulatio
Magnetic Monopole Content of Hot Instantons
We study the Abelian projection of an instanton in as a
function of temperature (T) and non-trivial holonomic twist () of the
Polyakov loop at infinity. These parameters interpolate between the circular
monopole loop solution at T=0 and the static 't Hooft-Polyakov
monopole/anti-monopole pair at high temperature.Comment: 3 pages, LATTICE98(confine), LaTeX, PostScript figures include
Extrapolation Methods for the Dirac Inverter in Hybrid Monte Carlo
In Hybrid Monte Carlo(HMC) simulations for full QCD, the gauge fields evolve
smoothly as a function of Molecular Dynamics (MD) time. Thus we investigate
improved methods of estimating the trial solutions to the Dirac propagator as
superpositions of the solutions in the recent past. So far our best
extrapolation method reduces the number of Conjugate Gradient iterations per
unit MD time by about a factor of 4. Further improvements should be forthcoming
as we further exploit the information of past trajectories.Comment: latex file with espcrc2 styl
Heavy quark potential in the instanton liquid model
We study the heavy quark potential in the instanton liquid model by carefully
measuring Wilson loops out to a distance of order 3. A random instanton
ensemble with a fixed radius = 1/3 generates a potential
growing very slowly at large . In contrast, a more realistic size
distribution growing as at small and decaying as at
large , leads to a potential which grows linearly with . The string
tension, however, is only about 1/10 of the phenomenological value.Comment: LATTICE98(confine
Instanton Distribution in Quenched and Full QCD
In order to optimize cooling as a technique to study the instanton content of
the QCD vacuum, we have studied the effects of alternative algorithms, improved
actions and boundary conditions on the evolution of single instantons and
instanton anti-instanton pairs. Using these results, we have extracted and
compared the instanton content of quenched and full QCD.Comment: 3 pages, LaTeX file + 3 figures included, uses epsfig.sty and
espcrc2.sty. Talk presented at LATTICE96(topology
Polarized DIS in N=4 SYM: Where is spin at strong coupling?
Using the AdS/CFT correspondence, we calculate the polarized structure
functions in strongly coupled N=4 supersymmetric Yang-Mills theory deformed in
the infrared. We find that the flavor singlet contribution to the g_1 structure
function is vanishingly small, while the flavor non-singlet contribution shows
the Regge behavior at small-x with an intercept slightly less than 1. We
explicitly check that the latter satisfies the moment sum rule. We discuss the
`spin crisis' problem and suggest that at strong coupling the spin of a hadron
entirely comes from the orbital angular momentum.Comment: 24 page
Iterated amplitudes in the high-energy limit
We consider the high-energy limits of the colour ordered four-, five- and
six-gluon MHV amplitudes of the maximally supersymmetric QCD in the multi-Regge
kinematics where all the gluons are strongly ordered in rapidity. We show that
various building blocks occurring in the Regge factorisation (the Regge
trajectory, the coefficient functions and the Lipatov vertex) satisfy an
iterative structure very similar to the Bern-Dixon-Smirnov (BDS) ansatz. This
iterative structure, combined with the universality of the building blocks,
enables us to show that in the Euclidean region any two- and three-loop
amplitude in multi-Regge kinematics is guaranteed to satisfy the BDS ansatz. We
also consider slightly more general kinematics where the strong rapidity
ordering applies to all the gluons except the two with either the largest or
smallest rapidities, and we derive the iterative formula for the associated
coefficient function. We show that in this kinematic limit the BDS ansatz is
also satisfied. Finally, we argue that only for more general kinematics - e.g.
with three gluons having similar rapidities, or where the two central gluons
have similar rapidities - can a disagreement with the BDS ansatz arise.Comment: Version corresponding to the Erratum sent to JHEP on October 16th
200
Glauber - Gribov approach for DIS on nuclei in N=4 SYM
In this paper the Glauber-Gribov approach for deep-inelastic scattering (DIS)
with nuclei is developed in N=4 SYM. It is shown that the amplitude displays
the same general properties, such as geometrical scaling, as is the case in the
high density QCD approach. We found that the quantum effects leading to the
graviton reggeization, give rise to an imaginary part of the nucleon amplitude,
which makes the DIS in N=4 SYM almost identical to the one expected in high
density QCD. We concluded that the impact parameter dependence of the nucleon
amplitude is very essential for N=4 SYM, and the entire kinematic region can be
divided into three regions which are discussed in the paper. We revisited the
dipole description for DIS and proposed a new renormalized Lagrangian for the
shock wave formalism which reproduces the Glauber-Gribov approach in a certain
kinematic region. However the saturation momentum turns out to be independent
of energy, as it has been discussed by Albacete, Kovchegov and Taliotis. We
discuss the physical meaning of such a saturation momentum and argue
that one can consider only within the shock wave approximation.Comment: 40pp.,9 figures in eps file
On The Pomeron at Large 't Hooft Coupling
We begin the process of unitarizing the Pomeron at large 't Hooft coupling.
We do so first in the conformal regime, which applies to good accuracy to a
number of real and toy problems in QCD. We rewrite the conformal Pomeron in the
-plane and transverse position space, and then work out the eikonal
approximation to multiple Pomeron exchange. This is done in the context of a
more general treatment of the complex -plane and the geometric consequences
of conformal invariance. The methods required are direct generalizations of our
previous work on single Pomeron exchange and on multiple graviton exchange in
AdS space, and should form a starting point for other investigations. We
consider unitarity and saturation in the conformal regime, noting elastic and
absorptive effects, and exploring where different processes dominate. Our
methods extend to confining theories and we briefly consider the Pomeron kernel
in this context. Though there is important model dependence that requires
detailed consideration, the eikonal approximation indicates that the Froissart
bound is generically both satisfied and saturated.Comment: 63 pages, 7 figures; published version: references updated and
several typos correcte
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