Pion and Quark Annihilation Mechanisms of Dilepton Production in Relativistic Heavy Ion Collisions

Abstract

We revise the π + π − and qq annihilation mechanisms of dilepton production during relativistic heavy-ion collisions. We focus on the modifications caused by the specific features of intramedium pion states rather than by medium modification of the ρ-meson spectral density. The main ingredient emerging in our approach is a form-factor of the multi-pion (multi-quark) system. Replacing the usual delta-function the form-factor plays the role of distribution which, in some sense, "connects" the 4-momenta of the annihilating and outgoing particles. The difference between the c.m.s. velocities attributed to annihilating and outgoing particles is a particular consequence of this replacement and results in the appearance of a new factor in the formula for the dilepton production rate. We obtained that the form-factor of the multi-pion (multi-quark) system causes broadening of the rate which is most pronounced for small invariant masses, in particular, we obtain a growth of the rate for the invariant mass below two masses of the annihilating particles.Since leptons do not interact practically with the highly excited nuclear matter, they leave the reaction zone after their creation in a relativistic nucleus-nucleus collision without further rescattering. That is why, the dileptons (e + e − and µ + µ − pairs) observed in high-energy heavy ion collisions secure an excellent opportunity for obtaining information on the initial state and the evolution of the system created in a collision. The enhancement with an invariant mass of 200 ÷ 800 MeV observed by the CERES collaboration [1,2] in the production of dileptons has received recently a considerable attention and has been studied in the framework of various theoretical models (for the review, see Ref.[3]). It was found that a large part of the observed enhancement is due to the medium effects (see Refs. [4,5] and references therein). Meanwhile, pion annihilation is the main source of dileptons which come from the hadron matter [6,7]. That is why, the proper analysis of the dilepton spectra obtained experimentally gives important observables which probe the pion dynamics in the dense nuclear matter that exists at the early stage of the collision. The purpose of the present letter is to look once more on the π + π − annihilation mechanism of dilepton production from the hadron plasma by accounting the medium-induced modifications of the dilepton spectrum. In order to do this, we concentrate on the modifications which are due rather to intramedium pion states, than on the discussion of a modification of the ρ-meson spectral density. In accordance with our suggestions, the main features of a pion wave function follow from the fact that pions live a finite time in the *

Annihilation Mechanisms of e + e − and µ + µ − Production in Relativistic Nuclear Collisions

“…Note that the enhancement of the dilepton production rate for the low invariant mass region is much more sensitive to the variation in the spatial size of a manyparticle (pion, quark) system than to that in the system lifetime (see Fig. 3, right panel) [ 12].…”

An Effective Model for Glueballs and Dual Superconductivity at Finite Temperature

“…On the other hand, pion-pion annihilation serves as the principal source of dileptons (e − e + and μ − μ + ) produced from the hadron matter [27,28]. Thus, the proper study of the main source of dilepton spectra observed experimentally proposes significant observables which help to understand the pion dynamics in a dense nuclear matter that exist at the beginning of the collision [29].…”

An Effective Model for Glueballs and Dual Superconductivity at Finite Temperature

“…On the other hand, pion-pion annihilation serves as the principal source of dileptons (e − e + and µ − µ + ) produced from the hadron matter [22,23]. Thus, the proper study of the main source of dilepton spectra observed experimentally proposes significant observables which help to understand the pion dynamics in dense nuclear matter that exist at the beginning of the collision [24].…”