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Probes of initial-state interactions in dilepton angular distributions

Description: We discuss the angular distribution of dileptons d sigma/d/sup 4/Qd OMEGA, emphasizing phase sensitivity as a probe of initial-state interactions in QCD. The coherent nature of Sudakov effects is discussed, along with the presence of imaginary parts related by analyticity. Angular-distribution structure functions which describe interference between longitudinal and transverse virtual photons, e.g., can be used to probe phase differences that depend on large momenta. These evolve according to exp(ic ln ln(Q/sup 2//lambda/sub QCD/sup 2/)) where Q/sup 2/ is a large scale. We report on a complete calculation at O(..cap alpha../sub s//sup 2/) of the q anti q ..-->.. ..gamma..* + gluons channel which confirms the cancellation of small (cutoff) scales, and describe a complementary experiment involving spin. We discuss the limit x ..-->.. 1 of the distribution d sigma/dQ/sup 2/dxdcos theta, and point out an unusual and interesting effect that a momentum-dependent phase can produce here.
Date: January 1, 1982
Creator: Ralston, J.P. & Pire, B.
Partner: UNT Libraries Government Documents Department

BFKL resummation effects in gamma* gamma* to rho rho

Description: We calculate the leading order BFKL amplitude for the exclusive diffractive process {gamma}*{sub L}(Q{sub 1}{sup 2}) {gamma}*{sub L}(Q{sub 2}{sup 2}) {yields} {rho}{sub L}{sup 0}{rho}{sub L}{sup 0} in the forward direction, which can be studied in future high energy e{sup +}e{sup -} linear colliders. The resummation effects are very large compared to the fixed-order calculation. We also estimate the next-to-leading logarithmic corrections to the amplitude by using a specific resummation of higher order effects and find a substantial growth with energy, but smaller than in the leading logarithmic approximation.
Date: August 11, 2005
Creator: Enberg, R.; Pire, B.; Szymanowski, L. & Wallon, S.
Partner: UNT Libraries Government Documents Department

Nucleon form factors '99

Description: The authors review recent progress in the experimental knowledge of and theoretical speculations about nucleon form factors, with special emphasis on the large Q{sup 2} region. There is now a long history of continuous progress in the understanding of electromagnetic form factors at large momentum transfer. After the pioneering works leading to the celebrated quark counting rules, the understanding of hard scattering exclusive processes has been solidly founded. A perturbative QCD subprocess is factorized from a wave function-like distribution amplitude {var_phi}(x{sub i},Q{sup 2}) (x{sub i} being the light cone fractions of momentum carried by valence quarks), the Q{sup 2} dependence of which is analyzed in the renormalization group approach. Although an asymptotic expression emerges from this analysis for the x dependence of the distribution, it was quickly understood that the evolution to the asymptotic Q{sub 2} is very slow and that indeed some non perturbative input is required to get reliable estimates of this distribution amplitude at measurable Q{sup 2}.
Date: June 1, 1999
Creator: Jager, Kees de & Pire, B.
Partner: UNT Libraries Government Documents Department

On the Physical Relevance of the Study of gamma* gamma -> pi pi at small t and large Q2

Description: We discuss the relevance of a dedicated measurement of exclusive production of a pair of neutral pions in a hard {gamma}*{gamma} scattering at small momentum transfer. In this case, the virtuality of one photon provides us with a hard scale in the process, enabling us to perform a QCD calculation of this reaction rate using the concept of Transition Distribution Amplitudes (TDA). Those are related by sum rules to the pion axial form factor F{sub A}{sup {pi}} and, as a direct consequence, a cross-section measurement of this process at intense beam electron-positron colliders such as CLEO, KEK-B and PEP-II, or Super-B would provide us with a unique measurement of the neutral pion axial form factor F{sub A}{sup {pi}0} at small scale. We believe that our models for the photon to meson transition distribution amplitudes are sufficiently constrained to give reasonable orders of magnitude for the estimated cross sections. Cross sections are large enough for quantitative studies to be performed at high luminosity e{sup +}e{sup -} colliders. After verifying the scaling and the {phi} independence of the cross section, one should be able to measure these new hadronic matrix elements, and thus open a new gate to the understanding of the hadronic structure. In particular, we argued here that the study of {gamma}*{gamma} {yields} {pi}{sup 0}{pi}{sup 0} in the TDA regime could provide with a unique experimental measurement of the {pi}{sup 0} axial form factor.
Date: August 26, 2010
Creator: Lansberg, J.P.; /SLAC; Pire, B.; /Ecole Polytechnique, CPHT; Szymanowski, L. & /Warsaw, Inst. Nucl. Studies
Partner: UNT Libraries Government Documents Department