Objective
The objective is to upgrade the NA50 (former NA38) dimuon experiment, after 10 years of high statistics exploration of very rare phenomena, by adding a vertex (target) spectrometer capable of measuring hundreds of emitted particles, and of recognizing the two muons among these. The resulting improvement in resolution will give access to new physics.
The first goal is to measure separately cross sections and shapes of rho, omega and phi vector mesons produced in Pb-Pb collisions. The physics at stake is the lifetime of the strong interacting matter, and its effect on the properties of vector mesons. Hadron masses might be modified by the medium, much like electron masses in solid state physics. But these hadrons are detected after exiting from the hot matter and thereby recovering their standard properties. The in-medium properties can be measured only using dilepton decays, since electrons and muons do not interact with the medium. The rho meson with its short lifetime is the most likely to be affected. The NA45/CERES dielectron experiment has found an intriguing excess of events at and below the rho mass.
Both experiments aim now at a mass resolution good enough to compare directly the rho and the practically unmodified omega and phi shapes. While the present proposal may well fare better in statistical errors and cost than the CERES upgrade, the main challenge is to approach such potentially groundbreaking physics by two independent experiments, with different backgrounds and systematic errors.
Measuring some 500 charged particles produced in head-on Pb-Pb collisions within the 25 cm between target and hadron absorber has become possible thanks to the new, fast pixel detector (cell size 50x500 micron) introduced in 1996 in the WA97 experiment. The setup proposed here comprises 131000 such cells. A version less prone to radiation damage is expected to become operational in 1998. It will allow to run the NA50 low mass configuration at maximal luminosity and measure also the D and Dbar mesons cross sections. [The pixel spectrometer is not suitable, and of little use for the high mass configuration, which studies the J/Psi suppression.] Combining the spectrometers before and after the hadron absorber allows to measure the muons' angles before they suffer multiple scattering in the absorber, and hereby to reach a mass resolution of 18 MeV/c^2.
Typically, 50 % of the muon pairs produced in an average Pb-Pb collision and identified in the dimuon spectrometer, are recognized in the pixel spectrometer, by track matching, among all the pions and kaons. The signal to background ratio is much improved, since few muons resulting from meson decays satisfy the matching criteria.
The responsibilities for the pixel spectrometer are shared as follows: mechanics, IPN Lyon; special PCB's and cables, tests, INR and YerPhI; data acquisition, LAPP Annecy; software and simulations, IPN Lyon and YerPhI; data taking and analysis, all. The LIP-Lisbon group has long ago specialized in low mass spectra analysis and shall perform the overall hardware compatibility check for upgraded setup.
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Coordinator
91128 Palaiseau
France
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