Exclusive ρ0 production in deep inelastic scattering at HERA

Abstract

Exclusive ρ0 electroproduction at HERA has been studied with the ZEUS detector using 120 pb-1 of integrated luminosity collected during 1996-2000. The analysis was carried out in the ki nem ati c range of photon vir tuality 2 <Q2 < 160 GeV2, an d γ*p ce nt re-o f -m as s energy 32 <W < 180 GeV. The res ults include the Q2 and W dependence of the γ*p → ρ0p cross section and the distribution of the squared-four-momentum transfer to the proton.The helicity analysis of the decay-matrix elements of the ρ0 was us ed to study the ratio of the γ*p cross section for longitudinal and transverse photon as a function of Q2 an d W.Finally, an e ffective Pomeron trajectory was extracted. The re sults are compared to various the oretical predictions.PA CS Co des: 13.60.Hb, 13.60.Le IntroductionTwo of the most surprising aspects of high-energy deep inelastic scattering (DIS) observed at the HE RA ep collider have been the sharp rise of the proton structure function, F2, with decreasing val ue o f Bjorken x and the abundance of events with a large rapidity gap in the hadronic final state [1]. The latter are identified as due to diffraction in the deep inelastic regime. A contribution to the diffractive cross section arises from the exclusive production of vector mesons (VM). High-energy exclusive VM production in DIS has been postulated to proceed through twoglu on excha ng e [2,3], once the scale , us ual ly ta ken as the virtual ity Q2 of the ex changed photon, is la rg e eno ugh for perturbative Qu antum Chromody namics (pQCD) to be ap plicab le. The gluons in the proton, which lie at the origin of the sharp increase of F2, are also expected to cause the VM cro ss s ection to incre ase w ith increa sing p hoton proton centre -of-mas s energy, W, with th e ra te of increas e growin g wit h Q2 . Moreover, the effective size of the virtual photon decreases with increasing Q2, l eadi ng to a fl atter dis tribu tio n in t, th e fou r-momen tum-tran sfer sq uared a t This paper reports on an extensive study of the properties of exclusive ρ0-meson produ ction, γ*p → ρ0p, bas ed on a high sta tis tics da ta sa mp le co llected with the ZEU S detector during the perio d 1996-2000, corresponding to an integrated luminosity of about 120 pb-1. Theoretical backgroundCa lculations of the VM production cross s ection in DIS require knowledg e of the wave-function of the virtual photon, specified by QED and which depends on the polarisation of the virtual photon. For longi tudina lly pola rised photons, , pairs of s mall transverse size dominate [3 ].The o ppo site ho lds for transversel y po laris ed p hotons, , where config urati ons with l arge transverse size dominate. The favourable feature of exclusive VM production is that, at high Q2, the longitudina l component of the virtual photon is dominant. The interaction cross secti on i n this cas e can be fully ca lcula ted in pQCD [11], with two-gluon exchange a s the l eading process in the high-energy regi me. For heav y vecto r meso ns, su ch as the J/ψ or the ϒ, perturbative ca...

Color transparency inπ−-induced dilepton production on nuclei

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“…The default pion coherence length Eq. (7) calculated with ∆M 2 = 0.7 (color online) Transparency vs Q 2 for the (e, e ′ π + ) reaction on12 C, 27 Al, 63 Cu, and 197 Au targets (panels (a), (b), (c), and (d), respectively) in the collinear kinematics. Dashed lines -Glauber model, thick (thin) solid lines -quantum diffusion model with ∆M 2 = 0.7(1.4) GeV 2 .…”

Diffractive gauge bosons production beyond QCD factorization

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