Low mass dimuon production in proton and ion induced interactions at the SPS

Abreu, M. C.; Alessandro, B.; Alexa, C.; Arnaldi, R.; Astruc, J.; Atayan, M.; Baglin, C.; Baldit, A.; Bedjidian, M.; Bellaiche, F.; Beole', Stefania; Boldea, V.; Bordalo, P.; Bussière, A.; Capony, V.; Casagrande, L.; Castor, J.; Chambon, T.; Chaurand, B.; Chevrot, I.; Cheynis, B.; Chiavassa, Emilio Luciano; Cicalò, C.; Comets, M. P.; Constans, N.; Constantinescu, S.; Cruz, J.; De Falco, A.; Dellacasa, G.; De Marco, N.; Devaux, A.; Dita, S.; Drapier, O.; Espagnon, B.; Fargeix, J.; Filippov, S. N.; Fleuret, F.; Force, P.; Gallio, Mauro; Gatignon, L.; Gavrilov, Y. K.; Gerschel, C.; Giubellino, P.; Golubeva, M. B.; Gonin, M.; Grigorian, A. A.; Grossiord, J. Y.; Guber, F. F.; Guichard, A.; Gulkanyan, H.; Hakobyan, R.; Haroutunian, R.; Idzik, M.; Jouan, D.; Karavitcheva, T. L.; Kluberg, L.; Kurepin, A. B.; Le Bornec, Y.; Lourenço, C.; Luquin, L.; Mac Cormick, M.; Macciotta, P.; Marzari Chiesa, A.; Masera, Massimo; Masoni, A.; Mehrabyan, S.; Monteno, M.; Mourgues, S.; Musso, A.; Ohlsson Malek, F.; Petiau, P.; Piccotti, A.; Pizzi, J. R.; Prado da Silva, W. L.; Prino, F.; Puddu, G.; Quintans, C.; Racca, C.; Ramello, L.; Ramos, S.; Rato Mendes, P.; Riccati, L.; Romana, A.; Ropotar, I.; Saturnini, P.; Scomparin, E.; Serci, S.; Shahoyan, R.; Silva, S.; Sitta, M.; Soave, C.; Sonderegger, P.; Tarrago, X.; Temnikov, P.; Topilskaya, N. S.; Usai, G. L.; Vercellin, Ermanno; Willis, N.

doi:10.1007/s100520000288

The low mass dimuon spectra collected in p-U collisions by the NA38 experiment significantly exceeds the total cross section expected from previous analysis, done by other experiments. The `excess' events have a harder p {T} distribution than the muon pairs from η and ω Dalitz decays, expected to dominate the mass window 0.4-0.65 GeV/ c2. We conjecture that the excess events may be due to q {q} annihilations, negligible at low p {T} but made visible by the m {T} cut applied in the NA38 data. Taking this assumption to parametrise the p-U spectra, we proceed with the corresponding analysis of the S-Cu, S-U and Pb-Pb data, collected by the NA38 and NA50 experiments, comparing the measured low mass dimuon spectra with the `expected cocktail'.