High Energy Physics
Lattice QCD confronts experiment
For almost thirty years precise numerical studies of nonperturbative QCD, formulated on a space-time lattice, have been stymied by an inability to include the effects of realistic quark vacuum polarization. The MILC, HPQCD, UKQCD, and Fermilab Lattice collaborations have presented detailed evidence of a breakthrough that may now permit a wide variety of high-precision, nonperturbative QCD calculations, including high-precision B and D meson decay constants, mixing amplitudes, and semi-leptonic form factors—all quantities of great importance in current experimental work on heavy-quark physics. The breakthrough comes from a new discretization for light quarks: Symanzik-improved staggered quarks. The researchers compared a wide variety of nonperturbative calculations in QCD with experiment, and found agreement to within statistical and systematic errors of 3% or less.
C. T. H. Davies, E. Follana, A. Gray, G. P. Lepage, Q. Mason, M. Nobes, J. Shigemitsu, H. D. Trottier, M. Wingate, C. Aubin, C. Bernard, T. Burch, C. DeTar, S. Gottlieb, E. B. Gregory, U. M. Heller, J. E. Hetrick, J. Osborn, R. Sugar, D. Toussaint, M. Di Pierro, A. El-Khadra, A. S. Kronfeld, P. B. Mackenzie, D. Menscher, and J. Simone, “High-precision lattice QCD confronts experiment,” Phys. Rev. Lett. (submitted, 2003), hep-lat/0304004. HEP, NSF, PPARC
Two-color QCD with four continuum flavors
Recently there has been a re-evaluation of the old idea that quark pairs
might condense, giving rise to a transition to a color superconducting state
at high baryon-number density. Kogut et al. approached this question by examining
the spectrum of two-color lattice QCD with four continuum flavors at a finite
chemical potential (µ) for quark-number, on a 123 × 24 lattice.
They found evidence that the system undergoes a transition to a state with
a diquark condensate, which spontaneously breaks quark number at µ =
m
/2, and that this transition is mean field in nature. They then examined
the three states that would be Goldstone bosons at µ = 0 for zero Dirac
and Majorana quark masses.
J. B. Kogut, D. Toublan, and D. K. Sinclair, “The pseudo-Goldstone spectrum of 2-color QCD at finite density,” Argonne National Laboratory report ANL-HEP-PR-03-022 (May 2003), hep-lat/0305003. HEP, NSF, HFS