Spin density matrix elements in exclusive ω electroproduction on ¹H and ²H targets at 27.5 GeV beam energy

Spin density matrix elements in exclusive ω electroproduction on ¹H and ²H targets at 27.5 GeV beam energy

Exclusive electroproduction of. mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of Q(2) > 1.0 GeV2, 3.0 GeV < W < 6.3 GeV, and -t ' < 0.2 GeV2. Results on the angular distribution of the omega meson, including its decay products, are presented...

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Hauptverfasser: Airapetian, A, Akopov, N, Akopov, Z, Augustyniak, W, Avetissian, A, Blok, HP, Borissov, A, Bryzgalov, V, Capiluppi, M, Capitani, GP, Cisbani, E, Ciullo, G, Contalbrigo, M, Dalpiaz, PF, Deconinck, W, De Leo, R, De Sanctis, E, Diefenthaler, M, Di Nezza, P, Düren, M, Ehrenfried, M, Elbakian, G, Ellinghaus, F, Etzelmüller, E, Fabbri, R, Felawka, L, Frullani, S, Gabbert, D, Gapienko, G, Gapienko, V, Garibaldi, F, Gavrilov, G, Gharibyan, V, Hartig, M, Hasch, D, Holler, Y, Hristova, I, Ivanilov, A, Jackson, HE, Joosten, S, Kaiser, R, Karyan, G, Keri, T, Kinney, E, Kisselev, A, Korotkov, V, Kozlov, V, Kravchenko, P, Krivokhijine, VG, Lagamba, L, Lapikás, L, Lehmann, I, Lenisa, P, Lorenzon, W, Ma, BQ, Mahon, D, Manaenkov, SI, Mao, Y, Marianski, B, Marukyan, H, Movsisyan, A, Murray, M, Naryshkin, Y, Nass, A, Nowak, WD, Pappalardo, LL, Perez-Benito, R, Petrosyan, A, Reimer, PE, Reolon, AR, Riedl, C, Rith, K, Rostomyan, A, Ryckbosch, Dirk, Schäfer, A, Schnell, G, Schüller, KP, Seitz, B, Shibata, TA, Stahl, M, Stancari, M, Statera, M, Steffens, E, Steijger, JJM, Taroian, S, Terkulov, A, Truty, R, Trzcinski, A, Tytgat, Michael, Van Haarlem, Y, VanHulse, C, Vikhrov, V, Vilardi, I, Wang, S, Yaschenko, S, Yen, S, Zeiler, D, Zihlmann, B, Zupranski, P
Format: Artikel
Sprache:eng
Schlagworte:
PARTON DISTRIBUTIONS
Physics and Astronomy
QCD
VECTOR-MESON LEPTOPRODUCTION
VIRTUAL COMPTON-SCATTERING
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Zusammenfassung:Exclusive electroproduction of. mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of Q(2) > 1.0 GeV2, 3.0 GeV < W < 6.3 GeV, and -t ' < 0.2 GeV2. Results on the angular distribution of the omega meson, including its decay products, are presented. The data were accumulated with the HER-MES forward spectrometer during the 1996-2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of Q(2) or -t '. Violation of s-channel helicity conservation is observed for some of these elements. Asizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse omega production by longitudinal and transverse virtual photons,gamma(L)* -> omega(T) and gamma(T)* -> omega(T), is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the gamma(T)* -> omega(T) transition dominates over the two natural-parity-exchange amplitudes describing the gamma(L)* -> omega(L) and gamma(T)* -> omega(T) transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.
ISSN:1434-6044