Measurement of exclusive [formula omitted] production in high-[formula omitted] two-photon collisions at LEP

P Achard,A Aloisio,J Alcaraz,J Allaby,G Alemanni,M G Alviggi ,M Aguilar-Benı́tez ,О Адриани ,F Linde

doi:10.1016/j.physletb.2004.07.013

Abstract

Exclusive ρ+ρ− production in two-photon collisions involving a single highly-virtual photon is studied for the first time with data collected by the L3 experiment at LEP at centre-of-mass energies 89<s<209GeV with a total integrated luminosity of 854.7 pb−1. The cross section of the process γγ*→ρ+ρ− is determined as a function of the photon virtuality, Q2, and the two-photon centre-of-mass energy, Wγγ, in the kinematic region: 1.2<Q2<30GeV2 and 1.1<Wγγ<3GeV. The ρ+ρ− production cross section is found to be of the same magnitude as the cross section of the process γγ*→ρ0ρ0, measured in the same kinematic region by L3, and to have similar Wγγ and Q2 dependences.

Highlights

In this Letter, we present the first measurement of the process: e+e− → e+e−γγ∗ → e+e−ρ+ρ− (1)in a kinematic region of large momentum transfer, obtained with data collected by the L3 detector [1] at LEP
The interest in exclusive production of hadron pairs in two-photon interactions at high momentum transfer is due to recently developed methods for calculating the cross section of such processes in the framework of perturbative QCD [5]
The production cross section is determined in the restricted Wγγ-region (5), which contains 287 events, of which 195 events are at the Z pole and 92 events are at high energy

Summary

Introduction

In this Letter, we present the first measurement of the process: e+e− → e+e−γγ∗ → e+e−ρ+ρ−. Is identified by a scattered beam electron, etag, detected in the luminosity monitor, two charged pions measured in the tracking chamber, and energy clusters from the two-photon decays of the π0’s deposited in the electromagnetic calorimeter. These events are accepted by several independent triggers with major contributions coming from a charged-particle trigger [11], with different features for the Z-pole and high-energy data-taking periods, and an energy trigger demanding a large energy deposition in the luminosity monitor in coincidence with at least one track [12]. These features of the two-particle mass correlations give evidence for a signal from ρ+ρ− intermediate states

Background estimation

Production model

Fit method

Results

Discussion