16,230 research outputs found
Instantons and the Spin of the Nucleon
Motivated by measurements of the flavor singlet axial coupling constant of
the nucleon in polarized deep inelastic scattering we study the contribution of
instantons to OZI violation in the axial-vector channel. We consider, in
particular, the f_1-a_1 meson splitting, the flavor singlet and triplet axial
coupling of a constituent quark, and the axial coupling constant of the
nucleon. We show that instantons provide a short distance contribution to OZI
violating correlation functions which is repulsive in the f_1 meson channel and
adds to the flavor singlet three-point function of a constituent quark. We also
show that the sign of this contribution is determined by positivity arguments.
We compute long distance contributions using numerical simulations of the
instanton liquid. We find that the iso-vector axial coupling constant of a
constituent quark is (g_A^3)_Q=0.9 and that of a nucleon is g_A^3=1.28, in good
agreement with experiment. The flavor singlet coupling of a quark is close to
one, while that of a nucleon is suppressed, g_A^0=0.77. However, this number is
larger than the experimental value g_A^0=(0.28-0.41).Comment: 36 pages, 16 figures, revised version (references added
Hadronic Wave Functions in the Instanton Model
We study hadronic wave functions using an instanton model for the QCD vacuum.
The wave functions are defined in terms of gauge invariant Bethe Salpeter
amplitudes which we have determined numerically using a Monte Carlo simulation
of the instanton ensemble. We find that the pion and the proton, as well as the
rho meson and the delta have very similar wave functions but observe a sizeable
splitting between mesons or baryons with different spin. We compare our results
with data obtained in lattice gauge simulations.Comment: 20 pages, uuencoded postscript file appended, SUNY-NTG-94-
Search for infrared counterparts of gamma-ray bursters
The result of two searches for infrared counterparts of Gamma-ray Bursters (GRB's) is reported. The first search was made using data from the Infrared Astronomy Satellite and covered 23 positions. The second search was made with the Kitt Peak 1.5 m telescope and covered 3 positions. In neither of these two searches was any infrared candidate detected
Gamma-ray burster recurrence timescales
Three optical transients have been found which are associated with gamma-ray bursters (GRBs). The deduced recurrence timescale for these optical transients (tau sub opt) will depend on the minimum brightness for which a flash would be detected. A detailed analysis using all available data of tau sub opt as a function of E(gamma)/E(opt) is given. For flashes similar to those found in the Harvard archives, the best estimate of tau sub opt is 0.74 years, with a 99% confidence interval from 0.23 years to 4.7 years. It is currently unclear whether the optical transients from GRBs also give rise to gamma-ray events. One way to test this association is to measure the recurrence timescale of gamma-ray events tau sub gamma. A total of 210 gamma-ray error boxes were examined and it was found that the number of observed overlaps is not significantly different from the number expected from chance coincidence. This observation can be used to place limits on tau sub gamma for an assumed luminosity function. It was found that tau sub gamma is approx. 10 yr if bursts are monoenergetic. However, if GRBs have a power law luminosity function with a wide dynamic range, then the limit is tau sub gamma 0.5 yr. Hence, the gamma-ray data do not require tau sub gamma and tau sub opt to be different
- …