210 research outputs found

    Timelike Compton scattering: exclusive photoproduction of lepton pairs

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    We investigate the exclusive photoproduction of a heavy timelike photon which decays into a lepton pair, gamma p -> l+ l- p. This can be seen as the analog of deeply virtual Compton scattering, and we argue that the two processes are complementary for studying generalized parton distributions in the nucleon. In an unpolarized experiment the angular distribution of the leptons readily provides access to the real part of the Compton amplitude. We estimate the possible size of this effect in kinematics where the Compton process should be dominated by quark exchange.Comment: 31 pages, 17 figure

    Hard Pomeron in exclusive meson production at ILC

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    We calculate the exclusive process gamma* gamma* ->rho rho at high energy. The Born order estimate and the leading (LLA) and next to leading order (NLLA) BFKL resummation effects show the feasibility of experimental detection in a quite large range of Q2 values at future high energy e+e- linear colliders.Comment: 4 pages, Presented at Photon2005, International Conference on the Structure and Interactions of the Photon, Warsaw 31.08-04.09.2005, by Samuel Wallo

    Revealing transversity GPDs through the photoproduction of a photon and a ρ\rho meson

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    Photoproduction of a pair of particles with large invariant mass is a natural extension of collinear QCD factorization theorems which have been much studied for deeply virtual Compton scattering and deeply virtual meson production. We discuss the production of a photon and a meson, where the wide angle Compton scattering on a meson subprocess factorizes from generalized parton distribution. We calculate at dominant twist and leading order in αs\alpha_s, the production cross-section of a transversely polarized ρ\rho meson which is sensitive to chiral-odd GPDs, and show that it may be measurable in near future JLab experiments.Comment: 6 pages, 3 figures. Contribution to the proceedings of the 6th International Conference on Physics Opportunities at an Electron-Ion Collider (POETIC6), September 7-11, 2015, Palaiseau, France. The numerical plot (fig.3) has been corrected. arXiv admin note: text overlap with arXiv:1511.0437

    Transversity GPD in photo- and electroproduction of two vector mesons

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    The chiral-odd generalized parton distribution (GPD), or transversity GPD, of the nucleon can be accessed experimentally through the photo- or electroproduction of two vector mesons on a polarized nucleon target, \gamma^(*) N --> \rho_1 \rho_2 N', where \rho_1 is produced at large transverse momentum, \rho_2 is transversely polarized, and the mesons are separated by a large rapidity gap. We predict the cross section for this process for both transverse and longitudinal \rho_{2} production. To this end we propose a model for the transversity GPD H_T(x,\xi,t), and give an estimate of the relative sizes of the transverse and longitudinal \rho_{2} cross sections. We show that a dedicated experiment at high energy should be able to measure the transversity content of the proton.Comment: 15 pages, 9 figures. Extended discussion of experimental prospects in the conclusions. Version to appear in EPJ

    Off-Forward Parton Distributions in 1+1 Dimensional QCD

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    We use two-dimensional QCD as a toy laboratory to study off-forward parton distributions (OFPDs) in a covariant field theory. Exact expressions (to leading order in 1/NC1/N_C) are presented for OFPDs in this model and are evaluated for some specific numerical examples. Special emphasis is put on comparing the x>ζx>\zeta and x<ζx<\zeta regimes as well as on analyzing the implications for the light-cone description of form factors.Comment: Revtex, 6 pages, 4 figure

    Hadron Helicity Violation in Exclusive Processes: Quantitative Calculations in Leading Order QCD

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    We study a new mechanism for hadronic helicity flip in high energy hard exclusive reactions. The mechanism proceeds in the limit of perfect chiral symmetry, namely without any need to flip a quark helicity. The fundamental feature of the new mechanism is the breaking of rotational symmetry of the hard collision by a scattering plane in processes involving independent quark scattering. We show that in the impulse approximation there is no evidence for of the helicity violating process as the energy or momentum transfer Q2Q^2 is increased over the region 1 GeV^2 < Q^2 < 100 GeV^2. In the asymptotic region Q^2> 1000 GeV^2, a saddle point approximation with doubly logarithmic accuracy yields suppression by a fraction of power of Q^2. ``Chirally--odd" exclusive wave functions which carry non--zero orbital angular momentum and yet are leading order in the high energy limit, play an important role.Comment: uuencoded LaTeX file (21 pages) and PostScript figure

    The Proton Electromagnetic Form Factor F2F_2 and Quark Orbital Angular Momentum

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    We analyze the proton electromagnetic form factor ratio R(Q2)=QF2(Q2)/F1(Q2)R(Q^{2})=QF_2(Q^{2})/F_1(Q^{2}) as a function of momentum transfer Q2Q^{2} within perturbative QCD. We find that the prediction for R(Q2)R(Q^{2}) at large momentum transfer QQ depends on the exclusive quark wave functions, which are unknown. For a wide range of wave functions we find that $ QF_2/F_1 \sim\ const$ at large momentum transfer, in agreement with recent JLAB data.Comment: 8 pages, 2 figures. To appear in Proceedings of the Workshop QCD 2002, IIT Kanpur, 18-22 November (2002

    Indication of asymptotic scaling in the reactions ddp3dd\to p^3H, ddn3dd\to n^3He and dpdpdp\to dp

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    It is shown that the differential cross sections of the reactions dd3Hendd\to ^3He n and dd3Hpdd\to ^3H p measured at c.m.s.scattering angleθcm=60\theta_{cm}=60^\circ in the interval of the deuteron beam energy 0.5 - 1.2 GeV demonstrate the scaling behaviour,dσ/dts22d\sigma/d t\sim s^{-22}, which follows from constituent quark counting rules. It is found also that the differential cross section of the elastic dpdpdp\to dp scattering at θcm=125135\theta_{cm}=125^\circ-135^\circ follows the scaling regime s16\sim s^{-16} at beam energies 0.5 - 5 GeV. These data are parameterized here using the Reggeon exchange.Comment: 6 pages, Latex, 2 eps figures; final version accepted by Pis'ma v ZHETF, corrected and completed reference

    Femto-Photography of Protons to Nuclei with Deeply Virtual Compton Scattering

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    Developments in deeply virtual Compton scattering allow the direct measurements of scattering amplitudes for exchange of a highly virtual photon with fine spatial resolution. Real-space images of the target can be obtained from this information. Spatial resolution is determined by the momentum transfer rather than the wavelength of the detected photon. Quantum photographs of the proton, nuclei, and other elementary particles with resolution on the scale of a fraction of a femtometer is feasible with existing experimental technology.Comment: To be published in Physical Review D. Replaces previous version with minor changes in presentatio
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