Quelques Aspects de la Spectrometrie des Dileptons Aux Energies Intermediaires

Franck Manso

In this dissertation, we have made a contribution to the study of dileptons (e+e- pairs) by analyzing data from the DLS and developing a RICH technique to identify electrons.

At intermediate energies (few GeV per nucleon), theoretical studies first assumed that the p-n hadronic bremsstrahlung is produced in an "elastic" channel, i.e. without simultaneous emission of pions (also, the p-p bremsstrahlung is neglected in comparison with the p-n bremsstrahlung in the dipole approximation). We have checked the validity of these approximations in studying the associated dilepton and hadron (protons, pions) production in the DLS, in p-p and p-d collisions at four energies between 1 and 5 GeV. A precise identification of hadrons has been achieved. The results of the analysis show that, with a fraction of associated dileptons and pions equal to 3 +/- 1% in the DLS and 40 +/- 20% in the whole phase space, a new formulation of the p-p and p-n bremsstrahlung is required. Other experimental results from the DLS collaboration and a theoretical study in progress on the concept of "inelastic" bremsstrahlung, led to the same conclusion.

The development of the dilepton study, envisioned by the HADES collaboration at GSI/Darmstad, requires the construction of a second generation spectrometer which can in particular operate in a high charged hadron multiplicity. In the framework of a spectrometer project (before HADES), we have studied a fast RICH detector consisting of an array of small photomultiplier tubes with quartz windows to identify electrons. We have worked out a ring identification algorithm, capable of separating two overlapping rings and computing their centers. With a 2X2 cm^2 segmentation of the detector, we show that spatial resolution is better than a centimeter and that high identification and background rejection can be achieved. A photomultiplier tube array would thus be an interesting alternative to presently used techniques.

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