Inclusive Lambda production in two-photon collisions at LEP

Abstract

The reactions e + e − → e + e − ΛX and e + e − → e + e − ΛX are studied using data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 209 GeV. Inclusive differential cross sections are measured as a function of the lambda transverse momentum, p t , and pseudo-rapidity, η, in the ranges 0.4 GeV < p t < 2.5 GeV and |η| < 1.2. The data are compared to Monte Carlo predictions. The differential cross section as a function of p t is well described by an exponential of the form A exp(−p t / p t ).Submitted to Phys. Lett. B IntroductionTwo-photon collisions are the main source of hadron production in high-energy e + e − interactions at LEP via the process e + e − → e + e − γ * γ * → e + e − hadrons. The outgoing electron and positron carry almost the full beam energy and their transverse momenta are usually so small that they escape, undetected, along the beam pipe. In this process, the negative fourmomentum squared of the photons, Q 2 , has an average value Q 2 ≃ 0.2 GeV 2 and they can therefore be considered as quasi-real. In the Vector Dominance Model (VDM), each photon can transform into a vector meson with the same quantum numbers, thus initiating a strong interaction process with characteristics similar to hadron-hadron interactions. This process dominates in the "soft" interaction region, where hadrons are produced with a low transverse momentum, p t . Hadrons with high p t may also be produced by the QED process γγ → qq (direct process) or by QCD processes originating from the partonic content of the photon (resolved processes).The L3 Collaboration has previously studied inclusive π 0 , K from quasi-real photons is studied for a centre-of-mass energy of the two interacting photons, W γγ , greater than 5 GeV. The results are expressed in bins of transverse momentum, p t , and pseudo-rapidity, η, in the ranges 0.4 GeV < p t < 2.5 GeV and |η| < 1.2. The η range is further divided in two subsamples with 0.4 GeV < p t ≤ 1 GeV and 1 GeV < p t < 2.5 GeV. The data sample corresponds to a total integrated luminosity of 610 pb Monte Carlo simulationThe process e + e − → e + e − hadrons is modelled with the PYTHIA [6] and PHOJET [7] event generators with two times more luminosity than the data. Both generators include the VDM, direct and resolved processes. In PYTHIA, including these processes for each photon leads to six classes of interactions. A smooth transition between these classes is then obtained by introducing a transverse momentum scale. The two-photon luminosity function is based on the EPA approximation [8] with a cut at the mass of the rho meson. The SaS-1D parametrisation is used for the photon structure [9]. PHOJET relies on the Dual Parton Model [10], with a soft and a hard component. The two-photon luminosity functions is calculated in the formalism of Reference 8. The leading-order GRV parametrisation is used for the photon structure [11]. For both programs, matrix elements are calculated at the leading order and higher-order terms are simulated by parton shower in t...