Study of inclusive strange-baryon production and search for pentaquarks in two-photon collisions at LEP

Abstract

Measurements of inclusive production of the Λ, Ξ − and Ξ * (1530) baryons in two-photon collisions with the L3 detector at LEP are presented. The inclusive differential cross sections for Λ and Ξ − are measured as a function of the baryon transverse momentum, p t , and pseudo-rapidity, η. The mean number of Λ, Ξ − and Ξ * (1530) baryons per hadronic two-photon event is determined in the kinematic range 0.4 GeV < p t < 2.5 GeV, |η| < 1.2. Overall agreement with the theoretical models and Monte Carlo predictions is observed. A search for inclusive production of the pentaquark θ + (1540) in two-photon collisions through the decay θ + → pK 0 S is also presented. No evidence for production of this state is found.Submitted to Eur. Phys. J. C 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, for which the cross section is many orders of magnitude larger than the e + e − annihilation cross section. 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. At the LEP energies considered here, the negative four-momentum squared of the photons, Q 2 , has an average value of Q 2 ≃ 0.2 GeV 2 . Therefore, the photons may be considered as "quasi-real". In the Vector Dominance Model (VDM), each virtual photon can fluctuate into a vector meson, 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 are instead mainly produced by the QED process γγ → qq (direct process) or by QCD processes originating from the partonic content of the photon (resolved processes).Fragmentation mechanisms can be investigated in two-photon reactions. These processes are described phenomenologically. In the Lund string model [1], hadron production proceeds through the creation of quark-antiquark and diquark-antidiquark pairs during string fragmentation. Mesons and baryons are formed by colorless quark-antiquark and quark-diquark combinations, respectively. An extension of this model, the "simple popcorn mechanism" [2], includes the possibility of producing an additional meson between baryon-antibaryon pairs. The relative rate of occurrence of the baryon-meson-antibaryon configuration is governed by the so called "popcorn parameter." Many other parameters must be tuned to reproduce the measured hadron production rate, such as the strange-quark suppression factor, the diquark-to-quark production ratio or the spin-1 diquark suppression factor.In a statistical model approach [3], hadronisation is described with a reduced number of free parameters. Particles are produced in a purely statistical way from a massive colourless gas which, for hadron production in two-photon collisions, is completely specified by two parameters...

Present status on experimental search for pentaquarks

“…C. e + e − experiments The Θ + baryon was searched for at e + e − colliders via quarkonium resonance processes [74][75][76][77], Z resonance processes [78,79], continuum q q processes [75] and two-photon collisions [80]. No evidence was observed in any of these experiments.…”

“…with the same assumption for the branching ratio to pK 0 S . The L3 Collaboration searched for the Θ + through the two-photon collisions e + e − → e + e − γγ → e + e − + hadrons [80]. About 1.3 million hadronic events were selected with the two-photon center-of-mass energy greater than 5 GeV, and about 140 000 K 0 S events were selected to search for the Θ + .…”

Explanation why theΘ+is seen in some experiments and not in others

“…Especially impressive are the recent high-statistics null results of the CLAS Collaboration for Θ + -photoproduction in several final-states [20,21,22,23]. And yet, new ("after-CLAS") dedicated analyses have again provided both confirming [24,25,26,27,28] and null [29,30,31,32,33] publications. Moreover, a new suggested way [34] for data analysis may reveal presence of the Θ + even in the published CLAS results, through its interference with a known resonance.…”