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

Exclusive electroproduction of <math><mi mathvariant="italic">ω</mi></math> mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of <math><mrow><msup><mi>Q</mi><mn>2</mn></msup><mo>&gt;</mo><mn>1.0</mn></mrow></math>  GeV <math><msup><mrow/><mn>2</mn></msup></math> , 3.0 GeV  <math><mrow><mo>&lt;</mo><mi>W</mi><mo>&lt;</mo></mrow></math>  6.3 GeV, and <math><mrow><mo>-</mo><msup><mi>t</mi><mo>′</mo></msup><mo>&lt;</mo><mn>0.2</mn></mrow></math>  GeV <math><msup><mrow/><mn>2</mn></msup></math> . Results on the angular distribution of the <math><mi mathvariant="italic">ω</mi></math> meson, including its decay products, are presented. The data were accumulated with the HERMES 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 <math><msup><mi>Q</mi><mn>2</mn></msup></math> or <math><mrow><mo>-</mo><msup><mi>t</mi><mo>′</mo></msup></mrow></math> . Violation of <math><mi>s</mi></math> -channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse <math><mi mathvariant="italic">ω</mi></math> production by longitudinal and transverse virtual photons, <math><mrow><msubsup><mi mathvariant="italic">γ</mi><mi>L</mi><mrow><mrow/><mo>∗</mo></mrow></msubsup><mo stretchy="false">→</mo><msub><mi mathvariant="italic">ω</mi><mi>T</mi></msub></mrow></math> and <math><mrow><msubsup><mi mathvariant="italic">γ</mi><mi>T</mi><mrow><mrow/><mo>∗</mo></mrow></msubsup><mo stretchy="false">→</mo><msub><mi mathvariant="italic">ω</mi><mi>T</mi></msub></mrow></math> , is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the <math><mrow><msubsup><mi mathvariant="italic">γ</mi><mi>T</mi><mo>∗</mo></msubsup><mo stretchy="false">→</mo><msub><mi mathvariant="italic">ω</mi><mi>T</mi></msub></mrow></math> transition dominates over the two natural-parity-exchange amplitudes describing the <math><mrow><msubsup><mi mathvariant="italic">γ</mi><mi>L</mi><mo>∗</mo></msubsup><mo stretchy="false">→</mo><msub><mi mathvariant="italic">ω</mi><mi>L</mi></msub></mrow></math> and <math><mrow><msubsup><mi mathvariant="italic">γ</mi><mi>T</mi><mo>∗</mo></msubsup><mo stretchy="false">→</mo><msub><mi mathvariant="italic">ω</mi><mi>T</mi></msub></mrow></math> 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

Bose–Einstein correlations in hadron-pairs from lepto-production on nuclei ranging from hydrogen to xenon

Bose–Einstein correlations of like-sign charged hadrons produced in deep-inelastic electron and positron scattering are studied in the HERMES experiment using nuclear targets of 1 H, 2 H, 3 He, 4 He, N, Ne, Kr, and Xe. A Gaussian approach is used to parametrize a two-particle correlation function determined from events with at least two charged hadrons of the same sign charge. This correlation function is compared to two different empirical distributions that do not include the Bose–Einstein correlations. One distribution is derived from unlike-sign hadron pairs, and the second is derived from mixing like-sign pairs from different events. The extraction procedure used simulations incorporating the experimental setup in order to correct the results for spectrometer acceptance effects, and was tested using the distribution of unlike-sign hadron pairs. Clear signals of Bose–Einstein correlations for all target nuclei without a significant variation with the nuclear target mass are found. Also, no evidence for a dependence on the invariant mass W of the photon-nucleon system is found when the results are compared to those of previous experiments