HERMES

Exclusive $\rho^0$-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV$^2$ < Q$^2$ < 7.0 GeV$^2$, 3.0 GeV < W < 6.3 GeV, and -t' < 0.4 GeV$^2$. Using an unbinned maximum-likelihood method, 25 parameters are extracted. [...]

An overview of more recent and important results from the HERMES experiment is presented in this paper. HERMES collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. [...]

The predicted high sensitivity of the nuclear modification factor for $K^-$ in SIDIS in respect to the medium-induced flavour conversion is studied at HERMES experiment. Unlike $\pi^+$, $\pi^-$ and $K^+$ nuclear modification factor for $K^-$ is assumed to increase at high value of Bjorken variable x and a hadron fraction energy z. [...]

Results from the study of exclusive $\rho^0$-meson electroproduction by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, are presented. In analysis of the angular dependence of the distribution of the decay pions, 25 parameters are extracted, which determine the real and imaginary parts of the ratios of several helicity amplitudes describing $\rho^0$-meson production by a virtual photon. [...]

Three decades ago it became that the simple quark model approach for the proton failed in describing the spin structure — the spin contribution from quarks, as measured by the European Muon Collaboration (EMC), came out small, by far insufficient to explain the proton spin of 1/2. The experiment utilized the polarized muon beam from CERN’s SPS, which is naturally polarized originating from the weak decay of mainly pions. [...]

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. [...]