We present Gamow-Teller strength distributions from shell model Monte Carlo studies of fp-shell nuclei that may play an important role in the pre-collapse evolution of supernovae. We then use these strength distributions to calculate the electron-capture cross sections and rates in the zero-momentum transfer limit. We also discuss the thermal behavior of the cross sections. We find large differences in these cross sections and rates when compared to the naive single-particle estimates. These differences need to be taken into account for improved modeling of the early stages of type II supernova evolution

Lowest-order cross sections for qq production and annihilation can be approximately corrected for higher-order QCD effects by using a corrective Kfactor. For energies where quark masses cannot be ignored, the K-factor is dominated by the wave function distortion arising from the initial-or finalstate interaction between the quark and the antiquark. We evaluate this Kfactor for qq production and annihilation in a quark-gluon plasma by taking into account the effects of Debye screening through a color-Yukawa potential.We present the corrective K-factor as a function of dimensionless parameters which may find applications in other systems involving attractive or repulsive Yukawa interactions. Prominent peaks of the K-factor occur for an attractive q-q color-Yukawa interaction with Debye screening lengths of 0.835 and 3.23 times the Bohr radius, corresponding to two lowest s-wave qq bound states moving into the continuum to become qq resonances as the Debye screening length decreases. These resonances, especially the cc and the bb resonances, may be utilized to search for the quark-gluon plasma by studying the sys-1 tematics of the temperature dependence of heavy-quark pair production just above the threshold. PACS number(s): 25.75.+r, 24.85.+p, 12.38.Mh, 13.90.+i

In reactions with qq production or qq annihilation, initialand final-state interactions give rise to large corrections to the lowest-order cross sections. We evaluate the correction factor first for low relative kinetic energies by studying the distortion of the relative wave function. We then follow the procedure of Schwinger to interpolate this result with the well-known perturbative QCD vertex correction factors at high energies, to obtain an explicit semiempirical correction factor applicable to the whole range of energies. The correction factor predicts an enhancement for qq in color-singlet states and a suppression for color-octet states, the efFect increasing as the relative velocity decreases. Consequences on dilepton production in the quark-gluon plasma, the Drell-Yan process, and heavy quark production processes are discussed.PACS number(s): 25.75.+r, 24.85. +p, 12.38.Mh, 13.90.+i I. INTRODUCTIONThe possibility of hadron matter going through a phase transition into a deconfined quark-gluon plasma (QGP) state during high-energy heavy-ion collisions makes these collisions the focus of intense experimental and theoretical research [1]. In these collisions, the constituents of the possible QGP or the partons of the colliding nucleons can react at varying energies. Their reaction products such as dileptons and. photons provide the signals and the backgrounds for the detection of the quark-gluon plasma. The magnitudes of the signals depend on the cross sections for dilepton and photon production from the constituents of the plasma. The rates of approach to chemical equilibrium and thermal equilibrium of the plasma depend also on the cross sections for reactions among the constituents.When quarks, antiquarks, and gluons interact, they interact as constituents in the quark-gluon plasma or as partons in the colliding nucleon. However, the lowestorder Feynman diagrams and the next-to-leading-order Feynman diagrams are the same for the basic reaction processes involving q, q, and gluons, whether the reaction takes place in the environment of the quark-gluon plasma or in the environment, of partons in nucleon-nucleon collisions. In these basic reactions of quarks, antiquarks, and gluons, the next-to-leading-order diagrams, including the initialand final-state interactions and gluon radiations, give rise to large corrections to the lowest-order cross sections. It will be useful to develop an analytical semiempirical correction factor for the basic reaction processes over the whole range of relative energies so that in the next level of approximation the basic lowest-order reaction cross sections of quarks, antiquarks, and gluons can be corrected. on the same footing. Additional efFects and refinements such as the plasma screening and temperature can be added on in the future as our theoretical understanding is developed further.For processes involving qq production and annihilation