Abstract
The exclusive electroproduction of two pions in the mass range 0.4<M ππ <2.5 GeV has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb−1. The analysis was carried out in the kinematic range of 2<Q 2<80 GeV2, 32<W<180 GeV and |t|<0.6 GeV2, where Q 2 is the photon virtuality, W is the photon-proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The two-pion invariant-mass distribution is interpreted in terms of the pion electromagnetic form factor, |F(M ππ )|, assuming that the studied mass range includes the contributions of the ρ, ρ′ and ρ′′ vector-meson states. The masses and widths of the resonances were obtained and the Q 2 dependence of the cross-section ratios σ(ρ′→ππ)/σ(ρ) and σ(ρ′′→ππ)/σ(ρ) was extracted. The pion form factor obtained in the present analysis is compared to that obtained in e + e −→π + π −.
Highlights
The analyzed data were collected with the ZEUS detector at the HERA collider in the years 1998–2000, when 920 GeV protons collided with 27.5 GeV electrons or positrons
The results shown here for the ρ region are in excellent agreement with the values given in an earlier ZEUS paper [2]
They are equal within errors for MV +5ΓV dMππ BWV (Mππ) > 1.8 GeV
Summary
The only radially excited 2S triplet qqstate studied at HERA so far has been the ψ(2S) state [18]. Only the photoproduction reaction was investigated and the low cross-section ratio of ψ(2S) to the ground-state J /ψ supported the existence of a suppression effect, expected if a node in the ψ(2S) wave-function is present. Other excited vector-meson states, in particular those consisting of light quarks, can be used to study the effect caused by changing the scanning size. The two-pion decay mode of these resonances is related [25, 26] to the pion electromagnetic form factor, Fπ (Mππ ). A study of exclusive electroproduction of two pions, γ ∗p → π+π−p,. The resonances are described by the pion form factor. The contributions of the three vector-mesons ρ, ρ and ρ are extracted and their relative rates as a function of Q2 are discussed in terms of QCD expectations
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