What happens when a proton is subjected to an electric field? The simple answer from first-year physics is that it would move. The less simple answer is that it would also deform due to the internal structure of the proton. Such is also the case in a magnetic field, and similarly so for the neutron, which obviously would have otherwise remained at rest. These responses to electric and magnetic...
In 2017, the polarized target system from Mainz/Dubna and Bonn were combined for data taking in Bonn. After testing the combined system, the experiment with a polarized frozen-spin target and the upgraded Crystal Barrel detector started in winter 2017. First data with a transversal proton target were already taken. In the meantime, several developments to improve the figure of merit are...
Linearly polarized photons are an invaluable tool for disentangling the nucleon spectrum. Used in a number of experiments both past and present, a range of techniques have been developed to determine the polarization degree of the photons. Many recent measurements have revealed the need to have a more accurate measurement of the degree of polarization, the systematic error of which was often...
Two-pion production is a very rich inelastic channel in nucleon-nucleon collisions in the few energy range, as it carries unique information both on $\pi^+\pi^-$ dynamics and on single and double baryon excitation. Precise data are therefore crucial to improve the description of hadronic or nuclear collisions which is mostly based on one meson production channels. Two pion production channels...
The search for non-strange $B=2$ (dibaryon) bound/resonance states has a long history. The dibaryon state is of interest, which can be a molecule consisting of two baryons or a spatially compact hexaquark object. The $\gamma d\to \pi^0\pi^0 d$ reaction has been experimentally investigated at incident energies ranging from 0.58 to 1.2 GeV to study non-strange dibaryons. The angular...
We have been carrying out photoproduction experiments by using linearly polarized tagged photon beams with energies of 1.5-2.9 GeV at SPring-8/LEPS. Charged pions and kaons were detected at forward angles. We studied the $\gamma p$ -> $\pi^{-}\Delta^{++}$, $\pi^{+}n$, and $K^{+}\Lambda$($\Sigma^{0}$) reactions. In the final states of these reactions, $u\bar{u}$, $d\bar{d}$, and $s\bar{s}$...
The unexpected nodal structure of the beam asymmetry recently re-
ported by the GRAAL collaboration in η' photoproduction very close
to threshold could be explained by a previously unobserved very nar-
row resonance. Therefor, the measurement is important to be inde-
pendently confirmed.
This possibility is offered by the BGO-OD experiment. It is well suited
for the detection of forward going...
We report on our investigation of low-energy pion-nucleon scattering from lattice QCD with Wilson-type fermions.
Our focus is on the isospin $I=3/2$ channel, which couples to the Delta resonance.
We discuss our calculations aiming at the extraction of the Delta resonance mass, width and effective coupling at pion mass
$250$ MeV and the physical point, using the L\"uscher finite-volume method...
Allowing arbitrary dressing mechanisms for both pions and nucleons, it is shown
that combining the respective fully dressed propagators with fully dressed
electromagnetic currents in a consistent manner completely removes all
dressing effects from half-on-shell combinations of the form
(propagator)$\times$(current). The key to this exact result is the realization
that the coupling...
Lattice QCD calculations provide an ab initial access to hadronic process. These calculations are usually performed in a small cubic volume with periodic boundary conditions. The infinite volume extrapolations for three-body systems are indispensable to understand many systems of high current interest. We derive the three-body quantization condition in a finite volume using an effective field...
The High Acceptance DiElectron Spectrometer (HADES) [1], installed at GSI Helmholtzzentrum in Darmstadt, was designed to measure emissivity of dense and hot baryonic matter. The microscopic description of the emissivity requires understanding of baryon-virtual photon couplings which can be studied in NN and $\pi$-N reactions. The elementary collisions, especially those with pion beams, also...
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...
The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the Crystal Barrel/MiniTAPS detector system at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This work has been motivated by the recently claimed observation of a narrow structure around an excitation energy of 1678 MeV [1]. Invariant mass distributions and...
The nucleon-like member $N(1685)$ of the speculative baryon antidecuplet denotes one possible explanation for the narrow peak-structure around $W = 1.68$ GeV observed in the total cross section of $\eta$-photoproduction off the neutron. If this baryon existed, it would likely to be seen in other reactions as well. While the aforementioned peak, whatever its nature is, was confirmed by several...
Since the beginning of the 1960's, the internal structure of the nucleon, in particular its spin structure, and the excitation spectra of protons and neutrons have been a central issue for many theoretical models and experiments of nuclear and particle physics.
Photon-induced reactions, like meson photoproduction, allow to excite the nucleon, to have access to many different polarisation...
We present a simultaneous analysis of s- and p-waves of the S = −1 meson-baryon scattering amplitude using low-energy experimental data. For the first time differential cross section data are included for chiral unitary coupled-channel models. From this model s- and p-wave amplitudes are extracted and we observe both well-known I(J P ) = 0(1/2 −) s-wave states as well as a new I(J P ) = 1(1/2...
For a deeper insight into the mechanisms of non-perturbative QCD it is essential to understand the excitation pattern of baryons. Up to now only the nucleon excitation spectrum has been subject to systematic experimental studies, while very little is known on excited states of double or triple strange baryons.
In studies of antiproton-proton collisions the $\overline{\mathrm{P}}$ANDA...
