Beam-helicity and beam-charge asymmetries in the hard exclusive leptoproduction of real photons from an unpolarised hydrogen target by a 27.6 GeV lepton beam are extracted from the HERMES data set of ...2006-2007 using a missing-mass event selection technique. The asymmetry amplitudes extracted from this data set are more precise than those extracted from the earlier data set of 1996-2005 previously analysed in the same manner by HERMES. The results from the two data sets are compatible with each other. Results from these combined data sets are extracted and constitute the most precise asymmetry amplitude measurements made in the HERMES kinematic region using a missing-mass event selection technique.
Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive \(\pi^+\) electroproduction reaction \(\gamma^* p \to n \pi^+\). The results were obtained from ...scattering of 6 GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The kinematic range covered is \(1.1<W<3\) GeV and \(1<Q^2<6\) GeV\(^2\). Results were obtained for about 6000 bins in \(W\), \(Q^2\), \(\cos(\theta^*)\), and \(\phi^*\). Except at forward angles, very large target-spin asymmetries are observed over the entire \(W\) region. Reasonable agreement is found with phenomenological fits to previous data for \(W<1.6\) GeV, but very large differences are seen at higher values of \(W\). A GPD-based model is in poor agreement with the data. When combined with cross section measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of \(Q^2\), for resonances with masses as high as 2.4 GeV.
Phys. Rev. C 98, 015207 (2018) Exclusive photoproduction cross sections have been measured for the process
$\gamma p \rightarrow p\pi^0(e^+e^-(\gamma))$ with the Dalitz decay final state
using tagged ...photon energies in the range of $E_{\gamma} = 1.275-5.425$ GeV.
The complete angular distribution of the final state $\pi^0$, for the entire
photon energy range up to large values of $t$ and $u$, has been measured for
the first time. The data obtained show that the cross section $d\sigma/dt$, at
mid to large angles, decreases with energy as $s^{-6.89\pm 0.26} $. This is in
agreement with the perturbative QCD quark counting rule prediction of $s^{-7}
$. Paradoxically, the size of angular distribution of measured cross sections
is greatly underestimated by the QCD based Generalized Parton Distribution
mechanism at highest available invariant energy $s=11$ GeV$^2$. At the same
time, the Regge exchange based models for $\pi^0$ photoproduction are more
consistent with experimental data.
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.
Results are presented for the first measurement of the double-polarization helicity asymmetry E for the \(\eta\) photoproduction reaction \(\gamma p \rightarrow \eta p\). Data were obtained using the ...FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the J\"ulich model to examine the case for the existence of a narrow \(N^*\) resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the E observable using that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.
It has been found that three of the spin-density matrix elements (SDMEs) in Fig. 6 of the published paper were incorrectly labeled. The labels have been fixed, while none of the results or ...conclusions have changed.
Hard exclusive electroproduction of \(\omega\) mesons is studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized ...hydrogen target. The amplitudes of five azimuthal modulations of the single-spin asymmetry of the cross section with respect to the transverse proton polarization are measured. They are determined in the entire kinematic region as well as for two bins in photon virtuality and momentum transfer to the nucleon. Also, a separation of asymmetry amplitudes into longitudinal and transverse components is done. These results are compared to a phenomenological model that includes the pion pole contribution. Within this model, the data favor a positive \(\pi\omega\) transition form factor.
Double-spin asymmetries in exclusive electroproduction of real photons from a transversely polarized hydrogen target are measured with respect to the product of target polarization with beam helicity ...and beam charge, and with respect to the product of target polarization with beam helicity alone. The asymmetries arise from the deeply virtual Compton scattering process and its interference with the Bethe--Heitler process. They are related to the real part of the same combination of Compton form factors as that determining the previously published transverse target single-spin asymmetries through the imaginary part. The results for the double-spin asymmetries are found to be compatible with zero within the uncertainties of the measurement, and are not incompatible with the predictions of the only available GPD-based calculation.
Hadron multiplicities in semi-inclusive deep-inelastic scattering were measured on neon, krypton and xenon targets relative to deuterium at an electron(positron)-beam energy of 27.6 GeV at HERMES. ...These ratios were determined as a function of the virtual-photon energy nu, its virtuality Q2, the fractional hadron energy z and the transverse hadron momentum with respect to the virtual-photon direction pt . Dependences were analysed separately for positively and negatively charged pions and kaons as well as protons and antiprotons in a two-dimensional representation. Compared to the one-dimensional dependences, some new features were observed. In particular, when z > 0.4 positive kaons do not show the strong monotonic rise of the multiplicity ratio with nu as exhibited by pions and negative kaons. Protons were found to behave very differently from the other hadrons.
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