# The 12th International Workshop on the Physics of Excited Nucleons

Jun 10 – 14, 2019
Bonn, Campus Poppelsdorf
Europe/Zurich timezone

## Photoproduction of mesons off quasifree nucleons

Jun 13, 2019, 2:30 PM
30m
HS 5

### HS 5

Baryon spectrum through meson photoproduction

### Speaker

Bernd Krusche (University of Basel)

### Description

Photoproduction of mesons off the nucleon is a very efficient tool for the study
of the excitation spectrum of the nucleon related to the fundamental properties of the
strong interaction. Much progress has been made during the last decade in particular
due to the measurements of several polarization observables in particular with polarized
beams and polarized targets. Availability of such polarization observables allows
for much better constraint model analyses, because they are sensitive to small
interference terms in the amplitudes. However, up to now the experimental data base is
only much improved for reactions off free protons. The investigation of the isospin
degree of freedom requires also measurements with neutron targets. In the absence of free
neutron targets experiments must be done with quasifree nucleons bound in light
nuclei, in particular in the deuteron. Such experiments are complicated by the effects of
nuclear Fermi motion, which smears out all observed structures and by the presence of
Final State Interactions (FSI) which may significantly influence the reaction amplitudes
in comparison to free nucleons. However, also for this issue significant progress has
been achieved during the last few years. In particular at the Bonn CBELSA/TAPS experiment
and the Mainz Cball/TAPS experiment such reactions have been systematically investigated.
The effects of Fermi motion can be mostly removed when the center-of-mass energy of the
photon-participant-nucleon system is reconstructed from the final state kinematics of
the reaction products. Several attempts have been made to model in a better way FSI
effects, but also phenomenological approaches, which are based on the assumption that FSI
effects in the deuteron are for many reactions similar for recoil protons and neutrons,
have been used. For the measurement of polarization observables such effects cancel often
to a large extend because they effect different polarization states in the same way.
We will discuss recent results for the photoproduction of $\eta$- and $\pi^0$-mesons
and for $\pi\pi$- and $\pi\eta$-pairs. Reactions like photoproduction of neutral meson
pairs off quasifree neutrons are among the technically most difficult ones and can
practically only be measured when almost the complete solid angle is covered by an
electromagnetic calorimeter. Some results of these experiments, such as the narrow
structures observed in the excitation function of the $n\eta$ final state, where quite
surprising and are not yet fully understood.

Such experiments may also profit from
production mechanisms which only contribute to photoproduction from light nuclei
such as coherent production of mesons or meson pairs. This may partly help to disentangle
the isospin structure of the reaction amplitudes, but is also under discussion for
example for the possible manifestation of a di-baryon state in coherent photoproduction
of $\pi^0\pi^0$ pairs off the deuteron. Recent results will be discussed.

### Primary author

Bernd Krusche (University of Basel)