In order to gain a better understanding of the dynamics inside the nucleon and of the non-perturbative regime of QCD, the nucleon excitation spectra and the properties of nucleon resonances are investigated. An essential experimental tool to achieve this goal is the study of different photoproduction reactions. Partial wave analyses are performed in order to obtain information about the...

The talk will give an overview of the current and planned research program of the A2@MAMI collaboration. Our research program utilises the intense energy-tagged real photon beam provided by the Mainz/Glasgow tagger in conjunction with the Crystal Ball/TAPS calorimeters, ancillary charged particle detector systems and a range of polarised and unpolarised cryogenic target systems. This...

Baryon spectroscopy is an essential tool in the study of nucleon resonances. The spectrum of broad and overlapping nucleon excitations can be greatly clarified by use of polarization observables. The N* program at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) includes experimental studies with linearly- and circularly-polarized tagged-photon beams, longitudinally- and...

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...

We will provide the theoretical description of $P_c$ states within coupled channel model. To provide information for the search of nucleon resonances with hidden charm $P_c$ for the on-going experiments at JLab, we make predictions by including the resonant amplitude of $\gamma p \to N^*_{c\bar{c}} \to J/\psi p$ calculated from all available theoretical models. The background is mainly from...

The current status of the $\eta$ photoproduction analysis within the Jülich-Bonn framework is presented. A structure in production on the proton at around $W\sim 1.68$ GeV is discussed. Furthermore, the extraction of resonance helicity couplings and their correlations from $\eta$ photoproduction is outlined, and how new experiment could help to further disentangle these quantities. Also, a...

We present the result of the combined analysis of the eta and eta-prime photoproduction data measured by A2,CB-ELSA and CLAS collaborations. The analysis confirms the existence of the N(1895)1/2- state and allowed us to define its properties with a good presicion.

A continuum approach to the three valence-quark bound-state problem in quantum field theory is used to compute the spectrum and Poincar\'e-covariant wave functions for all flavour-$SU(3)$ octet and decuplet baryons and their first positive-parity excitations. Such analyses predict the existence of nonpointlike, dynamical quark-quark (diquark) correlations within all baryons; and a uniformly...

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...

Results of the phenomenological analysis of $\eta$ and $\eta'$

photoproduction on the protons and the neutrons with updated version of

EtaMAID model are presented.The model includes 23 nucleon resonances parameterized with Breit-Wigner shapes, $t$-channel exchange of vector and axial-vector mesons with Regge cuts,and Born terms in $s$ and $u$ channels.

A new approach is discussed to avoid...

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...

In this present work, we study the scattering of a pseudoscalar

meson off one ground state octet baryon in covariant baryon

chiral perturbation theory up to the next-to-next-to-leading order. We

remove the power counting breaking terms with the extended-on-mass-shell

scheme. We perform the fi?rst combined study of the pion-nucleon

and kaon-nucleon scattering data and show that the covariant...

We adopt a covariant version of the naive dimensional analysis and construct the two-nucleon contact Lagrangian constrained by Lorentz, parity, charge conjugation, and Hermitian conjugation symmetries.We show that at O(q^0), O(q^2), and O(q^4), where q denotes a generic small momentum, there are 4, 13, and 23 terms, respectively.We find that by performing 1/mN expansions, the covariant...

Important insights into the excitation spectra of baryons are provided by measure-

ments of polarization observables in reactions that involve particles with spin. The

photoproduction of a single pseudoscalar meson constitutes an example-reaction that

has been under intense investigation recently.

In this talk, we present the basic method of moment-analysis for pseudoscalar...

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...

We carried out exclusive measurements of photo-production of an eta meson from a proton target with an egg-shape BGO calorimeter (BGOegg) and forward charged particle detectors at LEPS2/SPring-8, Japan. The differential cross-section at extreme backward angles were obtained and compared with the prediction from etaMAID2018. Possible evidence for a high spin N* resonance with a mass of 2.2 ~2.3...

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...

Radiative pion-photoproduction is studied in the framework of covariant SU(2) chiral perturbation theory up to loop order. The effects of the delta (1232) resonance are studied in the “$\epsilon$” power-counting scheme. Further, I will give an outlook how the magnetic dipole moment of the delta can be assessed with this reaction.

Pole parameters, as the minimally model dependent link between measurements and QCD approaches, were always extracted from experiment through procedures notably relying on elaborated microscopic theories. Partial waves were obtained by fitting free parameters of a microscopic model to experimental data, and poles were extracted by performing the analytic continuation of the result into the...

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 nonleptonic weak decay of Lambda_c->K0bar eta p is analyzed from the viewpoint of probing the N(1535) resonance, which has a big decay branching ratio to eta N. Up to an arbitrary normalization, the invariant mass distribution of eta p is calculated with both the chiral unitary approach and an effective Lagrangian model. Within the chiral unitary approach, the N(1535) resonance is...

The research work of Nuclear and Particle Physics group at University of Basel is centered around Hadron Physics sector. Photoproduction of Mesons provides an efficient tool for the study of decays of nucleon resonances and the excitation spectrum of hadrons tells us about the internal degrees of freedom. Thus to know the internal structural details of nucleons and mesons, investigation of...

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...

Highlights of recent progress in baryon calculations in lattice QCD are presented.

The high energy pp, p-Pb, and Pb-Pb collisions at the LHC offer a unique opportunity to study with ALICE the production of baryons, baryon bound systems, exotica and the corresponding anti-particle states.

Their production yields can be well described within a statistical-thermal model approach, in which the yields depend only the mass and quantum numbers of the state and a single temperature...

Based on the world's largest samples of $J/\psi$ and $\psi'$ events accumulated at the BESIII detector, the recent results on baryon decays and spectrosocopy will be presented. Also the perspectives on the baryon spectroscopy at BESIII will be discussed.

We give an overview of (some of) the different analysis tools and PWA approaches used to extract the spectrum of nucleon and Delta states from experimental data. Differences and similarities, e.g. in the construction of the amplitude or the data base, will be illustrated.

In addition, we show recent results of the Jülich-Bonn model, a unitary dynamical coupled-channel approach, and discuss the...

One of the greatest challenges within the Standard Model is to discover the source of visible mass. Indeed, this is the focus of a “Millennium Problem”, posed in 2000 by the Clay Mathematics Institute. The answer is hidden within quantum chromodynamics; and it is quite likely that revealing the origin of mass will also explain the nature of confinement. In addressing these issues, this...

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...

High-precision nucleon-resonance electroproduction data on a large kinematic domain of energy and momentum transfer have proven crucial in revealing novel features of strong interactions within the Standard Model and unfolding structural details of baryon excited states. Thus, in anticipation of new data reaching to unprecedented photon virtuality, we employ a quark-diquark approximation to...

We present calculations of $\gamma^\ast N \to N^\ast$

transition form factors, where $N$ is the nucleon

and $N^\ast$ and $N^\ast$ is a nucleon resonance,

based on a covariant quark model.

At high photon virtualities (large $Q^2$)

it is expected that the valence quark degrees of freedom

dominate the contributions to the transition form factors.

In these conditions, the quark model estimates...

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...

The nucleon excitation spectrum has traditionally been at the heart of strong interaction studies. Nucleon resonances and their transition form factors are interesting in a variety of processes: In addition to nucleon-pion scattering and meson photo- and electroproduction, from where they are experimentally extracted, nucleon resonances also appear as intermediate states in nucleon Compton...

Investigation of double pion electroproduction channel is a very efficient tool for the study of nucleon resonances.

Many experiments have already provided a lot of data on the cross sections of the reaction $\gamma_{v} p \rightarrow p'\pi^{+}\pi^{-}$. Their subsequent interpretation within the phenomenological model delivered valuable information on nucleon resonances...

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 study of electromagnetic transitions opens a window into the very nature of the strong interaction. And, indeed, such a study of how a ground-state nucleon transitions to an excited state, over a broad range of q$^2$, will provide keen insight into the evolution of how dynamically-generated masses emerge from the asymptotically-free, nearly massless quarks of perturbative QCD as well as...

The \pi^+\pi^-p photo- and electroproduction off proton channel is sensitive to the contributions from most excited nucleon states, offering an effective tool for the exploration of the nucleon resonance spectrum and structure. The data on nine independent one-fold differential \pi^+\pi^-p photo- and electroproduction cross sections have become available for the first time from the...

One important step in understanding the baryon spectrum is a precise knowledge of the excited states and their decays. In order to extract the contributing resonances from experimental data a partial wave analysis needs to be performed. To resolve ambiguities, the measurement of polarization observables is indispensable. In the regime of high mass baryon resonances multi-meson final states are...

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...

A dedicated programme to study electromagnetic baryonic transitions in the time-like

region has started using the pion beam and the HADES (High Acceptance Di-Electron

Spectrometer) set-up at GSI [1].

First measurements have been performed in the second resonance region, at a center-of-mass

energy of 1.49 GeV, using carbon and polyethylene targets, allowing for an analysis

of the inclusive...

The CLAS experiments have achieved major advances in the study of the N∗

region of the electroproduction spectrum, and the ata on electrocouplings of the many baryon resonances in the mass range up to 1.8 GeV showed consistency between the different meson channels. We present our theoretical studies of structure functions in view of the CLAS12 experiments planned in the near future, which are...

The study of virtual photon polarisation in time-like baryon electromagnetic transitions allows for the extraction of crucial information about the nature of particles and interactions at a fundamental level. Recent research has considered the transition $\pi^-p\rightarrow n e^+ e^-$ with respect to the polarisation of the virtual photon, which has an impact on the angular distribution of the...

In this talk I will give a general overview on recent results from several groups

on the spectrum and properties of baryons as obtained in the framework of

Dyson-Schwinger and Bethe-Salpeter equations.

I will discuss the spectrum of light baryons with focus on the comparison with

quark model expectations, the impact of dynamical mass generation and explain

the importance of relativistic...

Hadron spectroscopy has for many years been used to explore the excitation spectrum of the nucleon and the relevant degrees of freedom of the constituents. Despite the wealth of data there remain many "missing resonances" which are predicted by both quark models and Lattice QCD calculations, but are not observed experimentally.

Since the conception of the quark model, there have been...

Exploring time like transitions in pp and pi--p reactions with HADES

Piotr Salabura for the HADES collaboration

Radiative transition of an excited baryon to a nucleon with emission of a virtual massive photon decaying to electron-positron pair (R->Ne+e- Dalitz decays) provides important insight into baryon-photon vertex at low q2 =(me+e-)^2 in time-like region. A prominent enhancement...

Photoproduction of kaons can be regarded as the "golden channel" in the resonance region because of the opportunity to measure combinations of beam, target and recoil polarization. This talk will look at the impact of the recent results from CLAS and elsewhere, and in particular will describe the possibility of determining the weak decay parameter of the $\Lambda$. This study complements a...

The precise determination of neutrino properties in current and future accelerator-based oscillation experiments requires a good understanding and realistic modeling of neutrino interactions in the detectors: it is crucial to distinguish signal from background, reconstruct the neutrino energy and minimize systematic uncertainties. A significant fraction of the inelastic-scattering...

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...

Understanding of the non-trivial pattern of excited states of QCD at low energies is one of the main goals of the modern nuclear physics. One of the most interesting states is the so-called Lambda 1405, which is widely accepted as a dynamically generated state from the meson-baryon interaction.

Recently, high precision data on the energy shift and width of the Kaonic-hydrogen as well as the...

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 $\eta$/$\eta^{\prime}$-Meson is a unique tool to study low energy QCD phenomena and test models.

The isospin violating decay $\eta\rightarrow\pi^{+}\pi^{-}\pi^{0}$ is driven by the strong force and allows probing of the light quark masses, via measuring the corresponding decay amplitude. This amplitude is accessible either via a Dalitz-Plot or partial wave analysis. The latter one allows...

Low-energy data on the three charge states in $\gamma p \to K^+(\Sigma\pi)$ from CLAS at JLab, on $K^-p\to \pi^0\pi^0\Lambda$ and $\pi^0\pi^0\Sigma$ from the Crystal Ball at BNL, bubble chamber data on $K^-p\to\pi^-\pi^+\pi^{\pm}\Sigma^{\mp}$, low-energy $K^-p$ differential cross sections for elastic and charge exchange scattering, total cross sections on $K^-$ induced reactions, and data on...

The A2 Collaboration performs a manifold research program using real photons in the Crystal Ball/TAPS experiment at the MAMI accelerator facility in Mainz. The experiments take advantage of high-intensity unpolarized, linearly or circularly polarized photon beams, and unpolarized or polarized targets. The detector setup provides almost complete coverage in solid angle and is well suited for...

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...

A Σ∗ resonance with spin-parity JP=1/2− and mass in the vicinity of the K¯N threshold has been predicted in the unitary chiral approach and inferred from the analysis of CLAS data on the γp→K+π0Σ0 reaction. In this work, based on the dominant Cabibbo favored weak decay mechanism, we perform a study of Λ+c→π+π0Σ∗ with the possible Σ∗ state decaying into π−Σ+ through a triangle diagram. This...

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...

The study of baryonic excited states provides fundamental information on the internal structure of the nucleon and on the degrees of freedom that are relevant for QCD at low energies. N* are composite states and are sensitive to details of the how quark are confined.

A program has been approved at JLAB to search for new excited baryon states in the mass range from 1.8 GeV to 3 GeV with...

Since the discovery of the $\Lambda(1405)$, it remains poorly described by conventional constituent quark models, and it is a candidate for having an "exotic" meson-baryon or "penta-quark" structure, similar to states recently reported in the hidden charm sector.

The $\Lambda(1405)$ can be produced in the reaction $\gamma p \rightarrow K^+ \Lambda(1405)$. The pure I=0 decay mode into...

An experimental program has been approved at the Thomas Jefferson National Accelerator Facility to measure the (ep,e’K+)Y reactions using the CLAS12 setup in Hall B.

Data have been obtained using electron beams with energies of 6.5, 7.5, and 10.2 GeV, impinging upon a liquid hydrogen target in the CLAS12 center. Scattered electrons have been detected in an angle range of 2.5° to 4.5° by the...

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...

In the past decade, lots of hadrons were discovered especially in the heavy quarkonium mass ranges that cannot be fit into the level scheme provided by the traditional quark models. In the talk I will review some of the most popular proposals for the nature of these so-called XYZ states, including hadronic molecules, hadro-quarkonia and tetraquarks, as well as the non-trivial predictions that...

We explore the photo-production of possible exotic $N^*$, which are narrow pentaquark candidates containing light quarks or Charm or Beauty. The latter two are also known as $P_c$ and $P_b$ states in literatures. We analyzed the Compton scattering off the proton in the third resonance region in a coupled-channel effective Lagrangian model with K-matrix approximation. The evidence of exotic...

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...

We have investigated the cross section for the π−p→K0πΣ and pp→pK+πΣ reactions paying attention to a mechanism that develops a triangle singularity. The triangle diagram is realized by the decay of a N∗ to K∗Σ and the K∗ decay into πK, and the πΣ finally merges into Λ(1405). The mechanism is expected to produce a peak around 2140 MeV in the KΛ(1405) invariant mass. We found that a clear peak...

The spectroscopy of charmonium-like states together with the spectroscopy of charmed and strange baryons is discussed. It is a good testing tool for the theories of strong interactions, including: QCD in both the perturbative and non-perturbative regimes, LQCD, potential models and phenomenological models [1, 2, 3]. An understanding of the baryon spectrum is one of the primary goals of...

At Johannes Gutenberg University of Mainz, the new electron accelerator MESA (Mainz Energy-Recovering Superconducting Accelerator) for a new generation of fixed-target experiments, is currently under construction. In this talk we report on the status and the science case of MAGIX, which will be operated as an internal target experiment during the energy-recovery operation mode of MESA. The...

Recent theoretical results on spectroscopy, helicity amplitudes and strong decays will be reviewed.

We present calculations of the electromagnetic transitions of the nucleon,

to nucleon resonances $N^*$, namely the $\Delta(1232)\frac{3}{2}^+$,

$N(1520)\frac{3}{2}^-$ and $N(1535)\frac{1}{2}^-$.

Our results for these $\gamma^\ast N \to N^\ast$ reactions are

based on a covariant quark model. In this contribution, we focus

on the timelike regime --- where the square momentum transfer

is...