The quark condensate in nuclear matter is studied. We relate it to the nuclear sigma commutator, and then treat it as sigma commutator for quasi- particles in a self-consistent way. We find that the deviation from the linear expression is large at high density.

Abstract.

Even if dark matter particles were unambiguously discovered in experiments, there is no clear reason to expect that this would resolve the dark matter problem. It is very easy to provide examples of dark matter scenarios (e.g. in supersymmetric models) in which nearly identical detector signals correspond to extremely different relic densities. The density of the discovered particles, therefore, must be determined before their cosmological relevance can be established. In this paper, I present a general method that utilizes both dark matter and hadron collider experimental data (once they become available) to estimate the local density of dark matter particles. These results were obtained in collaboration with Gordon Kane at the University of Michigan.

Using the factorization scheme for the nonleptonic D → > VV₀ weak amplitudes, we classify all diagrams which arise in D → Vγ decays and calculate them with the help of the hybrid model which combines the heavy quark effective theory and the chiral Lagrangian approach. Thus we determine the long distance contribution to the amplitudes of Cabibbo allowed and Cabibbo suppressed D → Vγ decays. The calculation of the expected range of the branching ratios of nine different D → Vγ channels is compared with results of other approaches. The present work establishes an increase of the parity violating contribution in these decays in comparison with previous analyses.

Results are presented on p + ω,ø and J/ψ production in p-W and ³²S-W interactions at 200GeV/c/nucleon measured via the dimuon decay in a large kinematic region. The data are normalized to the charged particle multiplicity in the same rapidity interval. They have been collected using the HELIOS/3 muon spectrometer at the CERN SPS. The ratio Bσ_ø/ (Bσ_ρ + Bσ_ω), where B is the relevant resonance µµ branching fraction, increases between proton and sulphur projectiles, and is somewhat enhanced going from peripheral to central S-W interactions. This results from an increase in the number of produced ø’s per charged particle. The ratio is measured in different intervals of pt and rapidity. It is not clearly dependent on pt, but is larger at higher rapidities. J/Ψ production, likewise normalized to charged multiplicity, is significantly lower in S-W compared to p-W interactions.

Abstract.

Charged Higgs boson production via the gluon-bottom quark mode, gb → $tH^{\pm}$ , followed by its decay into a chargino and a neutralino has been investigated. The calculations are based on masses and couplings given by the Minimal Supersymmetric Standard Model (MSSM) for a specific choice of MSSM parameters. The signature of the signal is characterized by three hard leptons, a substantial missing transverse energy due to the decay of the neutralino and the chargino and three hard jets from the hadronic decay of the top quark. The possibility of detecting the signal over the Standard Model (SM) and non-SM backgrounds was studied for a set of $\tan\beta$ and m_A. The existence of 5- $\sigma$ confidence level regions for $H^{\pm}$ discovery at integrated luminosities of 100 fb^-1 and 300 fb^-1 is demonstrated, which cover also the intermediate region 4 $\lesssim$ $\tan\beta$ $\lesssim$ 10 where $H^{\pm}$ decays to SM particles cannot be used for $H^{\pm}$ discovery.

Interactions of the type ep → eXY are studied, where the component X of the hadronic final state contains two jets and is well separated in rapidity from a leading baryonic system Y. Analyses are performed of both resolved and direct photoproduction and of deep-inelastic scattering with photon virtualities in the range 7.5 < Q² < 80 GeV². Cross sections are presented where Y has mass M_Y < 1.6 GeV, the squared four-momentum transferred at the proton vertex satisfies |t| < 1 GeV² and the two jets each have transverse momentum p_T^jet > 5 GeV relative to the photon direction in the rest frame of X. Models based on a factorisable diffractive exchange with a gluon dominated structure, evolved to a scale set by the transverse momentum $$\hat p_T $$ of the outgoing partons from the hard interaction, give good descriptions of the data. Exclusive $$q\bar q$$ production, as calculated in perturbative QCD using the squared proton gluon density, represents at most a small fraction of the measured cross section. The compatibility of the data with a breaking of diffractive factorisation due to spectator interactions in resolved photoproduction is investigated.

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (ϰ_γ^OBS) and pomeron (β^OBS) momentum participating in the production of the dijet system. The observed ϰ_γ^OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section da/dβ^OBS increases as β^OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

We study ππ correlations in the exclusive reaction $$\bar pp \to 2\pi ^ + 2\pi ^ - \pi ^0 $$ at rest with complete reconstruction of the kinematics for each event. Inclusive and differential distributions properly normalized show an enhancement for small invariant masses M₊₊,M_-- of like-pion pairs. An even stronger enhancement is seen for the double differential distributions for low pion-pair masses. The signal is strongest when the π₀ energy is large and much weaker when this energy is small. Dynamical models with resonances in the final state are confronted with the data. For the kinematic situation where the ρ3π channel is important the simulation predicts a large pion correlation for the double differential density, qualitatively explaining the observed bahaviour. The stochastic HBT mechanism is not supported by these findings for the exclusive annihilation reaction studied.

The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off-equilibrium version allowing for partial strangeness saturation is significantly better. The chemical freeze-out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in e⁺ + e⁻, p + p and $$p + \bar p$$ collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor γs increases from about 0.45 for p + p interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions indicating that the strangeness enhancement in heavy ion collisions cannot be fully attributed to the increased system size. The quark strangeness suppression factor As is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding system.

The structure of the QCD gluonic cascade in configuration space is investigated. The explicit form of the inclusive single particle density matrix and single particle density in configuration space transverse coordinates is derived in the double logarithmic approximation (DLA) of QCD. The possible simpli-fication of the multiparton density matrix formalism for DLA approach is found and discussed.