Elsevier

Physics Letters B

Volume 604, Issues 1–2, 16 December 2004, Pages 48-60
Physics Letters B

Measurement of exclusive ρ0ρ0 production in mid-virtuality two-photon interactions at LEP

https://doi.org/10.1016/j.physletb.2004.10.049Get rights and content

Abstract

Exclusive ρ0ρ0 production in two-photon collisions between a quasi-real and a mid-virtuality photon is studied with data collected at LEP at centre-of-mass energies 183<s<209GeV with a total integrated luminosity of 684.8pb−1. The cross section of the process γγ*ρ0ρ0 is determined as a function of the photon virtuality, Q2, and the two-photon centre-of-mass energy, Wγγ, in the kinematic region: 0.2<Q2<0.85GeV2 and 1.1<Wγγ<3GeV.

Introduction

Recently, the L3 Collaboration measured the processes γγ*ρ0ρ0 and γγ*ρ+ρ, where one of the interacting photons, γ, is quasi-real and the other, γ*, is off-mass-shell and has a virtuality in the range 1.2<Q2<30GeV2 [1], [2]. The cross sections of these isospin-related reactions have a similar dependence on the two-photon centre-of-mass energy, Wγγ, and are of similar magnitude, though the ρ+ρ cross section is systematically higher than the ρ0ρ0 one. These features of ρ pair-production at high Q2 are in contrast with the observed suppression, and different Wγγ dependence, of ρ+ρ production [3] with respect to ρ0ρ0 [4], [5] in the data for Q20 and Wγγ<2GeV.

The observed behaviour of ρ pair-production at large momentum transfer is well described by the QCD-based model developed in Ref. [6], as shown by the analysis of the L3 data presented in Ref. [7]. On the other hand, ρ pair-production by quasi-real photons is still not well understood, despite a wide range of theoretical models [8], [9]. Thus, the study of the Q2 evolution of ρ pair-production between these two Q2 regimes is an important task in the experimental investigation of vector meson pair-production in two-photon interactions. This Letter presents results on the measurement of the process e+ee+eγγ*e+eρ0ρ0 in a kinematic region of intermediate values of the squared momentum transfer 0.2<Q2<0.85GeV2 and for an invariant mass of the hadronic system, Wγγ, in the interval 1.1<Wγγ<3GeV.

The data sample used was collected by the L3 detector [10] at LEP at centre-of-mass energies 183<s<209GeV and corresponds to an integrated luminosity of 684.8pb−1. Scattered beam electrons7 which have radiated photons with virtualities in the range (2) can be detected (“tagged”) by the Very Small Angle Tagger (VSAT) [11]. The VSAT is an electromagnetic calorimeter made of BGO crystals installed around the beam line on opposite sides of the L3 detector, at 8.05 m from the interaction point. Its geometrical acceptance covers the polar angle range 5mrad<θ<10mrad, for azimuthal angles in the ranges 1.25rad<ϕ<1.25rad and π1.25rad<ϕ<π+1.25rad. When the electron with the largest scattering angle is detected by the VSAT, the maximum virtuality of the two photons, Q2, is, to good approximation, equal to the transverse momentum squared, pt2, of the final state hadron system Q2=2EbEs(1cosθs)EbEsθs2pt2, where Eb is the beam energy, and Es and θs are the energy and the scattering angle of the tagged electron, respectively. The VSAT provides a means to ensure selection of exclusive final states by correlating the scattered electron and the detected hadron system.

The ρ0ρ0 production cross section is determined as a function of Wγγ and Q2. The results are compared to the generalised vector dominance model (GVDM) [12]. A measurement of process (1) in a similar kinematic region was performed at lower centre-of-mass energy by the PLUTO Collaboration [5]. The present measurement represents a tenfold increase of the statistics compared to that measurement.

Section snippets

Event selection

The reaction (1), contributing to the process e+ee+etagπ+ππ+π, is identified by a scattered electron, etag, detected in the VSAT and four charged pions measured in the tracking chamber. These events are collected by two independent track-triggers [13]. The trigger efficiency is determined from the data itself, making use of the redundancy of the triggers, and is around 94%.

Single-tagged events are selected by requiring one electromagnetic cluster in the VSAT. This cluster must have energy

Monte Carlo modelling and studies

To estimate the number of ρ0ρ0 events in the selected four-pion data sample, we consider non-interfering contributions from the processes γγ*ρ0ρ0,γγ*ρ0π+π,γγ*π+ππ+π,non-resonant. To take into account f2(1270) production in the region Wγγ>2.1GeV, we also consider contributions from the processes γγ*f2f2,γγ*f2ρ0,γγ*f2π+π. Monte Carlo samples of processes (6) and (7) are generated with the EGPC [14] program. About 4 million events are produced for each of the processes (6), about 3

Background estimation

The contribution to the selected sample from e+e annihilation is negligible. Using 2 million Monte Carlo events of the reaction e+ee+eτ+τ generated with the program LEP4F [18], the background contribution from this process is estimated to be 0.6±0.3 events and is neglected. The background is mainly due to partially reconstructed events from two-photon interactions with higher particle multiplicities in the final state, when tracks or photons escape detection. Another background

Fit method

In order to determine the differential ρ0ρ0 production rate, a maximum-likelihood fit of the data to the sum of the processes (6) and (7) is performed in intervals of Q2 and Wγγ.

The parameter set, Ω, comprising the six two-pion masses in an event, namely the four neutral combinations π+π and the two doubly-charged combinations π±π±, provides a complete kinematic description of a four-pion event in our model of isotropic production and decay. For each data event, i, with measured variables Ωi,

Results

The cross section, Δσee, of the process e+ee+eρ0ρ0 is measured in bins of Q2 and Wγγ. The results are listed in Table 1, Table 2, together with the efficiencies and the background fractions. The statistical uncertainties, listed in Table 1, Table 2, are those of the fit. The differential cross section dσee/dQ2, derived from Δσee, is listed in Table 1. When evaluating the differential cross section, a correction based on the Q2-dependence of the ρ0ρ0 Monte Carlo sample is applied, so as to

Discussion

The cross section of the process γγ*ρ0ρ0 as a function of Wγγ is plotted in Fig. 5(a), together with the sum of the cross sections of the other contributing processes. The shoulder in the latter is due to the contribution of the subprocesses involving f2(1270) production. The measured ρ0ρ0 cross section shows a broad enhancement at threshold. Fig. 5(b) and (c) compare the measured cross sections with those measured at high Q2 [1]. All cross sections decrease with Q2 and the variation with Q2

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    1

    Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.

    6

    Supported by the National Natural Science Foundation of China.

    2

    Supported by the Hungarian OTKA fund under contract Nos. T019181, F023259 and T037350.

    3

    Also supported by the Hungarian OTKA fund under contract No. T026178.

    4

    Supported also by the Comisión Interministerial de Ciencia y Tecnología.

    5

    Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.

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