/ Experiment-HEP hep-ex/0406052


Hard exclusive electroproduction of pi+ pi- pairs


Airapetian, A. ; Akopov, N. ; Akopov, Z. ; Amarian, M. ; Ammosov, V.V. ; Andrus, A. ; Aschenauer, E.C. ; Augustyniak, W. ; Avakian, R. ; Avetissian, A. ; Avetissian, E. ; Bailey, P. ; Balin, D. ; Baturin, V. ; Beckmann, M. ; Belostotski, S. ; Bernreuther, S. ; Bianchi, N. ; Blok, H.P. ; Bottcher, H. ; Borissov, A. ; Borysenko, A. ; Bouwhuis, M. ; Brack, J. ; Brull, A. ; Bryzgalov, V. ; Capitani, G.P. ; Chen, T. ; Ciullo, G. ; Contalbrigo, M. ; Dalpiaz, P.F. ; De Leo, R. ; Demey, M. ; De Nardo, L. ; De Sanctis, E. ; Devitsin, E. ; Di Nezza, P. ; Duren, M. ; Ehrenfried, M. ; Elalaoui-Moulay, A. ; Elbakian, G. ; Ellinghaus, F. ; Elschenbroich, U. ; Fabbri, R. ; Fantoni, A. ; Fechtchenko, A. ; Felawka, L. ; Frullani, S. ; Gapienko, G. ; Gapienko, V. ; Garibaldi, F. ; Garrow, K. ; Garutti, E. ; Gavrilov, G. ; Gharibyan, V. ; Graw, G. ; Grebeniouk, O. ; Greeniaus, L.G. ; Gregor, I.M. ; Griffioen, K.A. ; Hafidi, K. ; Hartig, M. ; Hasch, D. ; Heesbeen, D. ; Henoch, M. ; Hertenberger, R. ; Hesselink, W.H.A. ; Hillenbrand, A. ; Hoek, M. ; Holler, Y. ; Hommez, B. ; Iarygin, G. ; Ivanilov, A. ; Izotov, A. ; Jackson, H.E. ; Jgoun, A. ; Kaiser, R. ; Kinney, E. ; Kisselev, A. ; Kopytin, M. ; Korotkov, V. ; Kozlov, V. ; Krauss, B. ; Krivokhijine, V.G. ; Lagamba, L. ; Lapikas, L. ; Laziev, A. ; Lenisa, P. ; Liebing, P. ; Linden-Levy, L.A. ; Lipka, K. ; Lorenzon, W. ; Lu, H. ; Lu, J. ; Lu, S. ; Ma, B.-Q. ; Maiheu, B. ; Makins, N.C.R. ; Mao, Y. ; Marianski, B. ; Marukyan, H. ; Masoli, F. ; Mexner, V. ; Meyners, N. ; Mikloukho, O. ; Miller, C.A. ; Miyachi, Y. ; Muccifora, V. ; Nagaitsev, A. ; Nappi, E. ; Naryshkin, Y. ; Nass, A. ; Negodaev, M. ; Nowak, W.-D. ; Oganessyan, K. ; Ohsuga, H. ; Osborne, A. ; Pickert, N. ; Potashov, S. ; Potterveld, David H. ; Raithel, M. ; Reggiani, D. ; Reimer, P.E. ; Reischl, A. ; Reolon, A.R. ; Riedl, C. ; Rith, K. ; Rosner, G. ; Rostomyan, A. ; Rubacek, L. ; Rubin, J. ; Ryckbosch, D. ; Salomatin, Y. ; Sanjiev, I. ; Savin, Igor A. ; Schafer, A. ; Schill, C. ; Schnell, G. ; Schuler, K.P. ; Seele, J. ; Seidl, R. ; Seitz, B. ; Shanidze, R. ; Shearer, C. ; Shibata, T.-A. ; Shutov, V. ; Sinram, K. ; Sommer, W. ; Stancari, M. ; Statera, M. ; Steffens, E. ; Steijger, J.J.M. ; Stenzel, H. ; Stewart, J. ; Stinzing, F. ; Tait, P. ; Tanaka, H. ; Taroian, S. ; Tchuiko, B. ; Terkulov, A. ; Tkabladze, A. ; Trzcinski, A. ; Tytgat, M. ; Vandenbroucke, A. ; van der Nat, P. ; ven der Steenhoven, G. ; van Haarlem, Y. ; Vetterli, M.C. ; Vikhrov, V. ; Vincter, M.G. ; Vogel, C. ; Vogt, M. ; Volmer, J. ; Weiskopf, C. ; Wendland, J. ; Wilbert, J. ; Ybeles Smit, G. ; Ye, Y. ; Ye, Z. ; Yen, S. ; Zihlmann, B. ; Zupranski, P.

Published in: Phys.Lett.
Year: 2004
Vol.: B599
Page No: 212-222
Pages: 9

Abstract: Hard exclusive electroproduction of $\pi^+\pi^-$ pairs off hydrogen and deuterium targets has been studied by the HERMES experiment at DESY. Legendre moments $<P_1>$ and $<P_3>$ of the angular distributions of $\pi^+$ mesons in the center-of-mass frame of the pair have been measured for the first time. Their dependence on the $\pi^+\pi^-$ invariant mass can be understood as being due to the interference between relative $P$-wave (isovector) and $S,D$-wave (isoscalar) states of the two pions. % The increase in magnitude of $<P_1>$ as Bjorken $x$ increases is interpreted in the framework of Generalized Parton Distributions as an enhancement of flavour {non-singlet} $q\bar{q}$ exchange for larger values of $x$, which leads to a sizable admixture of isoscalar and isovector pion pairs. % In addition, the interference between {$P$-wave} and {$D$-wave} states separately for transverse and longitudinal pion pairs has been studied. The data indicate that at {$\la Q^2 \ra = 3 $ GeV$^2$} higher twist effects can be as large as the leading-twist longitudinal component

Web-Page: http://www.adsabs.harvard.edu/abs/2004PhLB..599..212H; http://dx.doi.org/10.1016/j.physletb.2004.08.059

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