Associated multiplicities in μ-pair events at the ISR

A study of the dependence of the charged hadronic multiplicity 〈nch〉 in μ-pair events on the dynamic parameters of the dimuon (dimuon massm and transverse momentumpT, and total hadronic energyMh) is presented for proton-proton collisions at √s=63 and 44 GeV. The full data collected in the R209 experiment at the ISR is used. With respect to already published data at √s=63 GeV, the present work has about a factor of four more statistics and includes data at two energies. In addition, we have corrected the data for background and we present absolute multiplicities. J/ψ events (m<4.5 GeV/c2) and Drell-Yan events (m>4.5 GeV/c2) are studied separately, since the production dynamics is expected to be different in the two cases. 〈nch〉 is found to decrease slowly with increasingm and to be rather insensitive toMh. On the other hand, 〈nch〉 grows withpT in a different way in J/ψ and Drell-Yan events. The effect might be indicative of a low-multiplicity jet recoiling against the dimuon in J/ψ events. A model analysis of thepT distribution of the pair in the transverse event plane is reported, showing that a possible contamination of μ-pairs from c\(\bar c\) semileptonic decays should be small at all masses.