### Speaker

Ivana Hristova
(DESY)

### Description

Exclusive production of $\pi^+$ mesons was studied with the HERMES
spectrometer at the DESY laboratory by scattering $27.6\,\mathrm{GeV}$ positrons and
electrons off a transversely nuclear-polarised hydrogen target.
The spin-averaged cross section was measured for values of the
virtuality of the exchanged photon $Q^2 > 1\,\mathrm{GeV}^2$ and the invariant
mass of the photon-nucleon system $W^2 > 10\,\mathrm{GeV}^2$. The first measurement
was carried out of the single-spin azimuthal asymmetry.
The precision of the results was limited by the lack of detection of the
recoiling neutron in the process $ep \to e n \pi^+$ and the infeasibility
of taking data at different beam energies.
The leading contribution to the asymmetry was found to be consistent
with zero and with a recent model calculation, while one subleading
contribution was found to be large. The cross section results
are compared to model calculations based on the Regge formalism
and on generalised parton distributions. These distributions give a
three-dimensional representation of the hadron structure at the
partonic level.

### Primary author

Ivana Hristova
(DESY)