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Prompt Contributions to the Dilepton Yield in Heavy Ion Collisions
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Prompt Contributions to the Dilepton Yield
in Heavy Ion Collisions
J. Hu?fnera,b, Yu.P. Ivanova,b,c, B.Z. Kopeliovichb,c and J. Raufeisena,b
a
Institut fu?r Theoretische Physik der Universita?t, Philosophenweg 19, D-69120 Heidelberg, Germany
b
Max-Planck Institut fu?r Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany
c
Joint Institute for Nuclear Research, Dubna, 141980 Moscow Region, Russia
February 9, 2008
Abstract
The mass spectrum is calculated for those dileptons which are produced in the
early phase of a heavy ion collisions via the direct production NN → l+l?X and via
the Compton process GN → l+l?X with prompt gluons radiated in preceding NN
interactions. Both mechanisms produce a mass spectrum which decreases steeply with
invariant mass of the l+l? pair and which is below the CERES data for Pb-Au collisions
by about one order of magnitude.
1 Introduction
Dileptons with invariant mass below 1 GeV have been measured in proton-nucleus [1] and
heavy ion collisions [2, 3] at the CERN SpS accelerator. For heavy ions (laboratory energy
158 A GeV) a sizable enhancement of the order of a factor three has been observed over
what is calculated from the e+e? decay of the hadrons in the final state. Also the observed
shape is very different from the calculated one. The data have not yet found an unambiguous
explanation. Most theories locate the origin for the observed dileptons in the hot and dense
phase of hadrons [4]-[12]. This phase is rather late in the time evolution of a heavy ion
collision.
In this note, we take the opposite point of view and investigate the mass spectrum of the
dileptons which arise from the very early stages of the heavy ion reaction in which partons
are the relevant degrees of freedom. We calculate (i) the direct production NN → l+l?X
in the light cone approach and in a parton model and (ii) the lepton production via a
gluonic Compton process GN → l+l?X from prompt gluo
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