Measurement of exclusive production in mid-virtuality two-photon interactions at LEP
Introduction
Recently, the L3 Collaboration measured the processes and , where one of the interacting photons, γ, is quasi-real and the other, , is off-mass-shell and has a virtuality in the range [1], [2]. The cross sections of these isospin-related reactions have a similar dependence on the two-photon centre-of-mass energy, , and are of similar magnitude, though the cross section is systematically higher than the one. These features of ρ pair-production at high are in contrast with the observed suppression, and different dependence, of production [3] with respect to [4], [5] in the data for and .
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 evolution of ρ pair-production between these two 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 in a kinematic region of intermediate values of the squared momentum transfer and for an invariant mass of the hadronic system, , in the interval
The data sample used was collected by the L3 detector [10] at LEP at centre-of-mass energies and corresponds to an integrated luminosity of . 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 , for azimuthal angles in the ranges and . When the electron with the largest scattering angle is detected by the VSAT, the maximum virtuality of the two photons, , is, to good approximation, equal to the transverse momentum squared, , of the final state hadron system where is the beam energy, and and 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 production cross section is determined as a function of and . 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 is identified by a scattered electron, , 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 events in the selected four-pion data sample, we consider non-interfering contributions from the processes To take into account production in the region , we also consider contributions from the processes 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 annihilation is negligible. Using 2 million Monte Carlo events of the reaction generated with the program LEP4F [18], the background contribution from this process is estimated to be 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 production rate, a maximum-likelihood fit of the data to the sum of the processes (6) and (7) is performed in intervals of and .
The parameter set, Ω, comprising the six two-pion masses in an event, namely the four neutral combinations and the two doubly-charged combinations , 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 ,
Results
The cross section, , of the process is measured in bins of and . 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 , derived from , is listed in Table 1. When evaluating the differential cross section, a correction based on the -dependence of the Monte Carlo sample is applied, so as to
Discussion
The cross section of the process as a function of 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 production. The measured cross section shows a broad enhancement at threshold. Fig. 5(b) and (c) compare the measured cross sections with those measured at high [1]. All cross sections decrease with and the variation with
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- 1
Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.
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Supported by the National Natural Science Foundation of China.
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Supported by the Hungarian OTKA fund under contract Nos. T019181, F023259 and T037350.
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Also supported by the Hungarian OTKA fund under contract No. T026178.
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Supported also by the Comisión Interministerial de Ciencia y Tecnología.
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Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.