A comprehensive collection of results on longitudinal double-spin asymmetries is presented for charged pions and kaons produced in semi-inclusive deep-inelastic scattering of electrons and positrons ...on the proton and deuteron, based on the full HERMES data set. The dependence of the asymmetries on hadron transverse momentum and azimuthal angle extends the sensitivity to the flavor structure of the nucleon beyond the distribution functions accessible in the collinear framework. No strong dependence on those variables is observed. In addition, the hadron charge-difference asymmetry is presented, which under certain model assumptions provides access to the helicity distributions of valence quarks.
The beam-helicity asymmetry in associated electroproduction of real photons, \(ep\to e\gamma \pi N\), in the \(\Delta\)(1232)-resonance region is measured using the longitudinally polarized HERA ...positron beam and an unpolarized hydrogen target. Azimuthal Fourier amplitudes of this asymmetry are extracted separately for two channels, \(ep\to e\gamma \pi^0 p\) and \(ep\to e\gamma \pi^+ n\), from a data set collected with a recoil detector. All asymmetry amplitudes are found to be consistent with zero.
Evidence for a positive longitudinal double-spin asymmetry 〈
A
1
ρ
〉=0.24±0.11
stat±0.02
syst in the cross section for exclusive diffractive
ρ
0(770) vector–meson production in polarised ...lepton–proton scattering was observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA positron beam was scattered off a longitudinally polarised pure hydrogen gas target. The average invariant mass of the photon–proton system has a value of 〈
W〉=4.9 GeV, while the average negative squared four-momentum of the virtual photon is 〈
Q
2〉=1.7 GeV
2. The ratio of the present result to the corresponding spin asymmetry in inclusive deep-inelastic scattering is in agreement with an early theoretical prediction based on the generalised vector–meson dominance model.
Exclusive electroproduction of $\omega $ mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of $Q^2> 1.0$ GeV$^2$ , 3.0 GeV $< W
The dependence on
Q
2 (the negative square of the 4-momentum of the exchanged virtual photon) of the generalised Gerasimov–Drell–Hearn integral for the proton has been measured in the range 1.2 GeV
...2<
Q
2<12 GeV
2 by scattering longitudinally polarised positrons on a longitudinally polarised hydrogen gas target. The contributions of the nucleon-resonance and deep inelastic regions to this integral have been evaluated separately. The latter has been found to dominate for
Q
2>3 GeV
2, while both contributions are important at low
Q
2. The total integral shows no significant deviation from a 1/
Q
2 behaviour in the measured
Q
2 range, and thus no sign of large effects due to either nucleon-resonance excitations or nonleading twist.
A measurement of the proton spin structure function
g
1
p(
x,
Q
2) in deep-inelastic scattering is presented. The data were taken with the 27.6 GeV longitudinally polarised positron beam at HERA ...incident on a longitudinally polarised pure hydrogen gas target internal to the storage ring. The kinematic range is 0.021<
x<0.85 and 0.8 GeV
2<
Q
2<20 GeV
2. The integral
∫
0.021
0.85
g
1
p
(x)
dx
evaluated at
Q
0
2 of 2.5 GeV
2 is 0.122±0.003(stat.)±0.010(syst.).
Exclusive electroproduction of $\omega$ mesons on unpolarized hydrogen and
deuterium targets is studied in the kinematic region of Q$^2$>1.0 GeV$^2$, 3.0
GeV < W < 6.3 GeV, and -t'< 0.2 GeV$^2$. ...Results on the angular distribution of
the $\omega$ 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 Q$^2$ or -t'. Violation of s-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 $\omega$ production by longitudinal
and transverse virtual photons, $\gamma^{*}_{L} \to \omega_{T}$ and
$\gamma^{*}_{T} \to \omega_{T}$, is determined for the first time. A hierarchy
of helicity amplitudes is established, which mainly means that the
unnatural-parity-exchange amplitude describing the $\gamma^*_T \to \omega_T$
transition dominates over the two natural-parity-exchange amplitudes describing
the $\gamma^*_L \to \omega_L$ and $\gamma^*_T \to \omega_T$ 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.
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