Jizhi Wu

Dilepton as a probe of pion dynamics in heavy-ion collisions

Abstract Modeling heavy-ion collisions by an expanding fireball, we calculate the dilepton production cross section including contributions from both the nucleon-nucleon interactions and the pion-pion annihilations. For the pion, we take into account the medium polarization effect on its dispersion relation. We find that dilepton production at large invariant masses is dominated by the latter process and thus is a viable probe to the pion dynamics in heavy-ion collisions.

Dielectron production from nucleus-nucleus collisions

Abstract Dielectron pairs can be produced in nuclear reactions from many processes such as the proton-neutron bremsstrahlung, the delta decay, the annihilation of pions on nucleons, and the pion-pion annihilation. Based on the transport model, we calculate the dielectron-production cross section from nucleus-nucleus collisions and compare it to the available data.

Medium effect on kaon production from heavy-ion collisions

Abstract Both the nucleon-nucleon inelastic and the kaon-production cross sections in dense nuclear matter are evaluated in the one-pion exchange model. The medium effect is included by using the dressed pion propagator determined in the delta-hole model. Compared with the cross sections in free space, it is found that the nucleon-nucleon inelastic cross section is substantially enhanced at high nuclear-matter densities while the kaon-production cross section is only moderately increased. In the normal Vlasov-Uehling-Uhlenbeck model for heavy-ion collisions the total kaon yield is seen to be sensitive to the nuclear equation of state.

Landau pole effects and the parameter space of the minimal supergravity model

It is shown that analyses at the electroweak scale can be significantly affected due to Landau pole effects in certain regions of the parameter space. This phenomenon arises due to a large magnification of errors of the input parameters

Constraints on the minimal supergravity model from the b ---> s gamma decay

m_t

(27)3 YUKAWA COUPLINGS AND EMBEDDINGS OF DISCRETE GROUPS IN THE

,

CP^3\times CP^2/Z_3\times Z'_3

\alpha_G

MODEL

which have currently a 10 percent uncertainty in their determination. The influence of the Landau pole on the constraint that the scalar SUSY spectrum be free of tachyons is also investigated.It is found that this constraint is very strong and eliminates a large portion of the parameter space.Under the above constraint the trilinear soft SUSY breaking term at the electroweak scale is found to lie in a restricted domain.

Very large intermediate breaking scale in the Gepner/Schimmrigk three generation model

The constraints on the minimal supergravity model from the b → s decay are studied. A large domain in the parameter space for the model satisfies the CLEO bound, BR(b → s) 0, however, the branching ratio � � ,

τ→μμμ¯andτ→μee¯decays in string models withE6symmetry

It is pointed out that, besides the conventional embedding of the discrete group in the Schimmrigk model in which is embedded nontrivially, there exists an alternate embedding in which Z3 is nontrivially embedded. It is found that, for the Schimmrigk model deformed from the symmetric point by a single (2, 1)-form (z1z2z3), the deformation introduces six more nonvanishing independent (27)3 Yukawa couplings before flux breaking. The alternate embedding introduces many complex Yukawa couplings, even when the modulo is real, giving rise to automatic CP violation.