Production de Dileptons Dans Les Collisions p+Be A 4.9GeV, Multiplicite Associee, Comparaison A p+p Et p+d

M. Bougteb

Dileptons appear to be a clean probe of dense nuclear matter. Dilepton pairs can be produced in nuclear reactions from nay processes such as the N-N bremsstrahlung, the pi+pi- annihilation, the vector mesons (rho, omega, phi) decay.

We have measured the dilepton production cross section in p-Be collisions at 4.9 GeV. The experiment was carried out at the Lawrence Berkeley Laboratory (LBL) using the DiLepton Spectronmeter (DLS). The results have allowed us first of all to specify a correction for the electronic's dead time observed in the previous data (1986).

The dilepton invariant mass spectrum is in quite good agreement with the more recent Haglin and Gale's model, except in the intermediate mass region around 400-500 MeV. The transverse momentum distribution, as well as the observed dependence between invariant mass and transverse momentum, correspond to hadronic physics data at higher energies.

The charged multiplicity from the reaction is measured by an appropriate detector. The average total multiplicity for the dielectron is 2.8.

The study of the results obtained from p-p, p-d, and p-Be at 4.9 GeV shows a similarity in the shape of the dilepton invariant mass and transverse momentum spectra. The pd/pp and pBe/pp ratios of the cross sections are 1.9 +/- 0.1 and 6.7 +/- 0.6 respectively. The measured cross sections in pp, pd, and pBe are well reproduced by the usual parametrisation sigma \prop A^alpha, where alpha = 0.87 +/- 0.26.

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