A gas analyzer for the internal polarized target of the HERMES experiment

A gas analyzer has been developed for the internal polarized target of the HERMES experiment at DESY in order to determine the relative amount of atomic and molecular hydrogen or deuterium in a gas sample. The precise quantitative knowledge of this ratio is crucial because the nucleons in atoms and molecules contribute differently to the average nuclear polarization of the target gas. A new calibration technique used to derive the relative sensitivity to atoms and molecules is presented. As an example, it is shown how the gas analyzer is used within the HERMES environment to divide the molecules in the gas sample into an unpolarized and a potentially polarized fraction

DVCS measurements at HERMES

Deeply Virtual Compton Scattering (DVCS) is currently one of the most promising processes that provides information about the structure of the nucleon in the framework of Generalized Parton Distributions (GPDs). During its last years the deeply virtual Compton scattering process was extensively studied at the HERMES experiment through measurements of cross-section asymmetries. To the virtue of unique experimental conditions, HERMES had collected a wealth of data on scattering a longitudinally polarized lepton (electron/positron) beam off unpolarized, longitudinally and transversely polarized hydrogen targets, as well as off unpolarized and longitudinally polarized deuterium targets. During the last two years of HERA operation a recoil detector around the target area allowed for the full kinematic reconstruction of the exclusive DVCS event, leading to a clean measurement of the beam-helicity asymmetry with background well below the 1% level. The recoil detector also allowed for first measurement of the related process of associated DVCS, in which the proton is excited to a Delta resonance, giving access to transition GPDs. In this contribution these recent results using the recoil detector are presented

HERMES results on transverse target single-spin asymmetries in inclusive electroproduction of charged pions and kaons

Single-spin asymmetries were investigated in inclusive electroproduction of charged pions and kaons from transversely polarised protons at the HERMES experiment. The asymmetries were studied as a function of the azimuthal angle Y around the beam direction between the target-spin direction and the hadron production plane, the transverse hadron momentum $P_T$ relative to the direction of the incident beam, and the Feynman variable $x_F$ . The $\sin\psi$ amplitudes are positive for $\pi^+$ and $K^+$, slightly negative for $\pi^-$ and consistent with zero for $K^-$, with particular $P_T$ but weak $x_F$ dependences. Large asymmetries are observed for two small subsamples of events, where also the scattered electron was recorded by the spectrometer