Charm Estimate from the Dilepton Spectra.pdf

a r X i v : h e p - p h / 0 0 0 3 3 1 9 v 2 1 0 A p r 2 0 0 1 INSTITUT FU?R KERNPHYSIK, UNIVERSITA?T FRANKFURT D - 60486 Frankfurt, August–Euler–Strasse 6, Germany IKF–HENPG/02–00 Charm Estimate from the Dilepton Spectra in Nuclear Collisions Marek Gaz?dzickia,1 and Mark I. Gorensteinb,2 a CERN, Geneva, Switzerland, Institut fu?r Kernphysik, Universita?t Frankfurt, Frankfurt, Germany b Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine, Institut fu?r Theoretische Physik, Universita?t Frankfurt, Germany Abstract A validity of a recent estimate of an upper limit of charm production in central Pb+Pb collisions at 158 A·GeV is critically discussed. Within a simple model we study properties of the background subtraction procedure used for an extraction of the charm signal from the analysis of dilepton spectra. We demonstrate that a production asymmetry between positively and negatively charged background muons and a large multiplicity of signal pairs leads to biased results. Therefore the applicability of this procedure for the analysis of nucleus–nucleus data should be re- considered before final conclusions on the upper limit estimate of charm production could be drawn. 1E–mail: marek@ikf.physik.uni–frankfurt.de 2E–mail: goren@ap3.bitp.kiev.ua 1 Measurement of the invariant mass spectra of opposite–sign lepton pairs (dileptons) allow to extract information otherwise difficult or even impossible to obtain. Among interesting processes which contribute to dilepton production are decays of vector mesons (ρ, w, φ, J/ψ, ψ ′ ), Drell–Yan as well as thermal creation of dileptons, and decays of charm hadrons. Decays of pions and kaons are a dominant source of uninteresting (background) dileptons which should be subtracted before deconvolution of contributions from the interesting (signal) sources is performed. Recent analysis of dimuon spectrum measured in central Pb+Pb collisions at 158 A·GeV by NA50 Collaboration [1] suggests a significant enhancement of dilepto