Abstract
Exclusive ρ+ρ− production in two-photon collisions between a quasi-real photon, γ, and a mid-virtuality photon, γ∗, is studied with data collected at LEP at centre-of-mass energies 183 GeV⩽s⩽209 GeV with a total integrated luminosity of 684.8 pb−1. The cross section of the γγ∗→ρ+ρ− process 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 GeV2⩽Q2⩽0.85 GeV2 and 1.1 GeV⩽Wγγ⩽3 GeV. These results, together with previous L3 measurements of ρ0ρ0 and ρ+ρ− production, allow a study of the γγ∗→ρρ process over the Q2-region 0.2 GeV2⩽Q2⩽30 GeV2.
Highlights
The L3 Collaboration has recently measured the exclusive production of ρ0ρ0 [1,2] and ρ+ρ− [3] pairs in the two-photon fusion process: e+e− → e+e−γ γ ∗ → e+e−ρρ, (1)where the beam electrons7 radiate virtual photons which interact and produce a hadronic final state
When evaluating the differential cross section, a correction based on the Q2-dependence of the ρ+ρ− Monte Carlo sample is applied, so as to assign the cross section value to the centre of the corresponding Q2-bin [19]
The cross section σγ γ is derived from the measured cross section using the relation σγ γ = 1σee/LT T
Summary
The L3 Collaboration has recently measured the exclusive production of ρ0ρ0 [1,2] and ρ+ρ− [3] pairs in the two-photon fusion process: e+e− → e+e−γ γ ∗ → e+e−ρρ,. The two measurements [1,3] done at large virtualities, 1.2 GeV2 6 Q2 6 30 GeV2, provide a testing ground for a recently-developed QCD-based model [4] This model describes well the Q2-dependence of the ρ0ρ0 production at large momentum transfer [5]. The ρ+ρ− cross section is systematically higher than the ρ0ρ0 one This is in contrast with the suppression and different Wγ γ dependence of ρ+ρ− production [6] with respect to ρ0ρ0 [7] observed in data with Q2 ≈ 0 and Wγ γ 6 2 GeV. When the electron with the largest scattering angle is detected in the VSAT, the virtuality of the photon it radiated is, within 1% precision, equal to the transverse momentum squared, pt, of the final state hadron system: Q2 = 2EbEs (1 − cos θs ) ≈ EbEs θs2 ≈ pt2,. The VSAT is not used to directly measure Q2, but rather to select exclusive final states by correlating the direction of the transverse momentum vector of the tagged electron with the detected hadron system
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