Abstract
Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive ρ 0 electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1<Q 2<7 GeV2, 3.0<W<6.3 GeV, and −t<0.4 GeV2. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q 2 and t are presented, as well as the longitudinal-to-transverse ρ 0 electroproduction cross-section ratio as a function of Q 2. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.
Highlights
Exclusive electroproduction of vector mesons on nucleons offers a rich source of information on the mechanisms that produce these mesons, see e.g., Refs. [1,2]
Exclusive ω electroproduction is studied at HERMES using a longitudinally polarized lepton beam and unpolarized hydrogen and deuterium targets in the kinematic region Q2 > 1.0 GeV2, 3.0 GeV < W < 6.3 GeV, and −t < 0.2 GeV2
The kinematic dependences of all 23 Spin density matrix elements (SDMEs) are presented for proton and deuteron data
Summary
Exclusive electroproduction of vector mesons on nucleons offers a rich source of information on the mechanisms that produce these mesons, see e.g., Refs. [1,2]. The latter are included at subleading twist in the subprocess γ ∗ f → M f , where M denotes the meson, while the partons are still emitted and reabsorbed by the nucleon collinear to the nucleon momentum By using this approach, the pQCD-inspired phenomenological “GK model” can describe existing data on cross sections, SDMEs and spin asymmetries in exclusive vector-meson production for values of Bjorken-x, xB, below about 0.2 [12,13,14]. The pQCD-inspired phenomenological “GK model” can describe existing data on cross sections, SDMEs and spin asymmetries in exclusive vector-meson production for values of Bjorken-x, xB, below about 0.2 [12,13,14] It can describe exclusive leptoproduction of pseudoscalar mesons by including the full contribution to the electromagnetic form factor from the pion, in contrast to earlier studies at leading-twist, which took into account only the relatively small perturbative contribution to this form factor SDMEs in exclusive ω electroproduction were studied for 1.6 GeV2 < Q2 < 5.2 GeV2 by CLAS [19] and it was found that the exchange of the pion Regge trajectory dominates exclusive ω production, even for Q2 values as large as 5 GeV2
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