Nuclear Target Effects Research Articles

Quasifreeηphotoproduction from nuclei and medium modifications of resonances

This paper establishes the case that the process of quasifree $\ensuremath{\eta}$ photoproduction from nuclei is an important tool to study medium modifications and changes to the elementary process $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\eta}N$ in the nuclear medium. We investigate the sensitivity of the differential cross section, recoil nucleon polarization, and the photon asymmetry to changes in the elementary amplitude, medium modifications of the resonance ${(S}_{11}{,D}_{13})$ masses, as well as nuclear target effects. All calculations are performed within a relativistic plane-wave impulse approximation formalism resulting in analytical expressions for all observables. Our results indicate that polarization observables are largely insensitive to nuclear target effects. Depending on the type of coupling, the spin observables do display a sensitivity to the magnitude of the $\ensuremath{\eta}\mathrm{NN}$ coupling constant. The polarization observables are identified to be the prime candidates to investigate the background processes and their medium modifications in the elementary process such as the ${D}_{13}$ resonance. Moreover, as a consequence of the large dominance in the differential cross section of the ${S}_{11}$ resonance, the quasifree differential cross section provides an exceptional instrument to study medium modifications to the ${S}_{11}$ resonance in such a manner that helps to distinguish between various models that attempt to understand the ${S}_{11}$ resonance and its distinctive position as the lowest lying negative parity state in the baryon spectrum.

Novel QCD Effects in the Production and Decay of Quarkonium

There are many outstanding discrepancies comparing the predictions of perturbative QCD and measurements of the rate of production and decay of heavy quark systems. The problems include the J/ψ → ρπ puzzle, leading charmed particle effects, the anomalous behavior of the heavy quark sea components of structure functions, anomalous nuclear target effects, and the large rates observed for single and double quarkonium production at large xF and large pT. I argue that these anomalies may be associated with nonperturbative effects in the higher Fock structure of hadron wavefunctions.

Nuclear-target effects in J/ ψ production in p̄ and π − interactions

The cross sections for J/ ψ hadroproduction in Be, Cu, and W targets are presented as functions of Feynman-x (x F) and transverse momentum ( pT). A suppression of the forward cross section per nucleon in heavy targets is observed; the suppression increases with increasing x F and decreases with increasing pT. The data are compared with other J/ ψ hadroproduction data and data from the Drell-Yan production of muon pairs, J/ ψ photoproduction and open charm hadroproduction.

Nuclear-target effects in J/ psi production in 125-GeV/c antiproton and pi - interactions.

The production of the J/psi resonance in 125-GeV/c p-bar and ..pi../sup -/ interactions with Be, Cu, and W targets has been measured. The cross section per nucleon for J/psi production is suppressed in W interactions relative to the lighter targets, especially at large values of Feynman x, which is opposite to the expectation from the various explanations of the European Muon Collaboration effect. Models incorporating modifications of the gluon structure functions in heavy targets show qualitative agreement with the data.

The energy calibration of the Sydney air shower array

The predicted of giant air showers are calculated using a comprehensive set of hadron interaction models. The possible existence of nuclear target effects, increasing interaction cross section and heavy primaries is considered by treating them as operators on other, more basic sets of model parameters. Comparison with the latest experimental results on large showers leads to a realistic relationship between observed muon size Nmu and the total energy of the primary particle E0. In addition, the range of acceptable models determines the uncertainty in the Nmu -E0 conversion. The results are used to produce an integral energy spectrum from the observations of the Sydney array.