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Register a new userProduction of Low Mass Electron Pairs Due to the Photon-Photon Mechanism in Central Collisions
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Kai Hencken n Universitaet Basel
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We calculate the probability for dilepton production in central relativistic heavy ion collisions due to the gamma-gamma mechanism. This is a potential background to more interesting mechanisms. We find that this mechanism is negligible in the CERES experiments. Generally, the contribution due to this mechanism is small in the central region, while it can be large for small invariant masses and forward or backward rapidities. A simple formula based on the equivalent photon approximation and applications to a possible scenario at RHIC are also given.
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Starting from a bound-state model of weakly bound quarks for ($q bar{q}$) mesons, we derive a formalism for computing the production or decay of such mesons, whatever the value of their internal orbital angular momentum L. That approach appears as a natural generalization of the Brodsky-Lepage formalism (valid only for L=0) that has been widely used in recent years for the computation of exclusive processes in perturbative QCD. We here apply it to the production, in photon-photon collisions, of: i) tensor-meson pairs; ii) pseudotensor-meson pairs; iii) hybrid pairs made of a pion and a pseudotensor meson. The numerical results we obtain allow for some hope of experimentally identifying such pairs, in the charged channels, at high-energy e^+e^- colliders of the next generation, provided integrated luminosities as high as $approx 10^{40} cm^{-2}$ can be reached.
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We study inclusive charged-hadron production in collisions of quasireal photons at NLO in perturbative QCD, using fragmentation functions recently extracted from PEP and LEP1 data. We superimpose the direct (DD), single-resolved (DR), and double-resolved (RR) gamma-gamma channels. First, we confront existing data taken by TASSO at PETRA and by MARK II at PEP with our NLO calculations. We also make comparisons with the neutral-kaon to charged-hadron ratio measured by MARK II. Then, we present NLO predictions for LEP2, a next-generation e+e- linear collider (NLC) in the TESLA design, and a Compton collider obtained by converting a NLC. We analyze transverse-momentum and rapidity spectra with regard to the scale dependence, the interplay of the DD, DR, and RR components, the sensitivity to the gluon density in the resolved photon, and the influence of gluon fragmentation. It turns out that the inclusive measurement of small-p_T hadrons at a Compton collider would greatly constrain the gluon density of the photon and the gluon fragmentation function.
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