Study of the η\etaN scattering amplitude through the associated photoproduction of ϕ\phi- and η\eta-mesons in the region of the N*(1535) resonance

The $\gamma p \to \phi \eta p$ reaction is studied in the kinematic region where the $\eta p$ final state originates dominantly from the decay of the N*(1535) resonance. The threshold laboratory photon energy for this reaction (at the peak of the S11 resonance) is $E_\gamma^{Lab} =$3 GeV. We will discuss it somewhat above threshold, at $E_\gamma^{Lab}\simeq 4-5$ GeV, in order to reach lower (absolute) values of the squared 4-momentum transfer from the initial photon to the final $\phi$-meson. In these conditions, we expect the t-channel $\pi^0$- and $\eta$-meson exchanges to drive the dynamics underlying the $\gamma p \to \phi \eta p$ process. The initial photon dissociates into the final $\phi$-meson and a virtual pseudoscalar meson ($\pi^0$ or $\eta$). The virtual pseudoscalar meson scatters from the proton target to produce the final $\eta p$ state. The $\pi^0 p \to \eta p$ and $\eta p \to \eta p$ amplitudes are derived in the framework of a coupled-channel effective field theory of meson-baryon scattering. We found the $\eta$-meson exchange to be largely dominant. The $\eta$-$\pi^0$ interference is of the order of $20 - 30$. The sign of this term is not known and has a significant influence on the results. The $\pi N\to \eta N$ amplitude being largely determined by data on the $\pi^- p \to \eta n$ reaction, we found that the $\gamma p \to \phi \eta p$ reaction cross section is rather directly related to the $\eta$-nucleon scattering amplitude in the N*(1535) resonance region. Accurate data on the $\gamma p \to \phi \eta p$ process would therefore put additional constraints on this still poorly known amplitude.Comment: 13 pages, 8 figure

Quantum interferences in the γN→e+e−N\gamma N \to e^+e^- N reaction close to the vector meson production threshold

The exclusive photoproduction of $e^+e^-$ pairs from nucleons close to the vector meson production threshold ($1.4<\sqrt s <1.8$ GeV) results from two main processes: the emission of Bethe-Heitler pairs and the photoproduction of $\rho^0$- and $\omega$-mesons decaying into $e^+e^-$ pairs. The Bethe-Heitler amplitudes are purely electromagnetic and reflect mostly the nucleon magnetic structure. The $\gamma N\to e^+e^- N$ amplitudes arising from vector meson production and decay are derived from $\gamma N\to \rho^0 N$ and $\gamma N \to \omega N$ amplitudes supplemented by the Vector Meson Dominance assumption. The vector meson photoproduction amplitudes are calculated using a relativistic and unitary coupled-channel approach to meson-nucleon scattering. They depend sensitively on the coupling of vector fields to baryon resonances. The $\gamma N \to e^+e^- N$ differential cross sections display interference patterns. The interference of Bethe-Heitler pair production with vector meson $e^+e^-$ decay is quite small in the domain of validity of our model for all angles of the emitted $e^+e^-$ pair. The interference of $\rho^0$- and $\omega$-mesons in the $e^+e^-$ channel can be large. It is constructive for the $\gamma p \to e^+e^- p$ reaction and destructive for the $\gamma n \to e^+e^- n$ reaction. We discuss the shape and magnitude of the $e^+e^-$ pair spectra produced in the $\gamma p \to e^+e^- p$ and $\gamma n \to e^+e^- n$ reactions as functions of the pair emission angle and of the total center of mass energy $\sqrt s$.Comment: 28 pages, 12 figure

Axial-vector form factors of the baryon octet and chiral symmetry

We consider the axial-vector form factors of the baryon octet in flavor-SU(3) chiral perturbation theory. The baryon octet and decuplet and the pseudoscalar-meson octet are included as explicit degrees of freedom. We explore the use of on-shell meson and baryon masses in the one-loop contributions to the axial-vector form factors and focus on a consistent treatment in terms of chiral power counting. The convergence properties of such an approach are scrutinized. We discuss the potential for comparison to upcoming QCD lattice data.Comment: 57 pages, 4 figures, in the revised manuscript we extended the presentation considering now all relevant counterterm

Chiral symmetry and strangeness at SIS energies

In this talk we review the consequences of the chiral SU(3) symmetry for strangeness propagation in nuclear matter. Objects of crucial importance are the meson-baryon scattering amplitudes obtained within the chiral coupled-channel effective field theory. Results for antikaon and hyperon-resonance spectral functions in cold nuclear matter are presented and discussed. The importance of the Sigma(1385) resonance for the subthreshold antikaon production in heavy-ion reaction at SIS is pointed out. The in-medium properties of the latter together with an antikaon spectral function based on chiral SU(3) dynamics suggest a significant enhancement of the pi \Lambda -> bar K N reaction in nuclear matter.Comment: 10 pages, 2 figures, invited talk at Erice 200

On chiral excitations with exotic quantum numbers

We consider the flavour sextet of charmed meson resonances with Jᴾ = 1⁺ quantum numbers that is predicted by the leading order chiral Lagrangian with up, down and strange quarks. The effect of chiral correction terms as determined previously from QCD lattice data is worked out. Pole masses in the complex energy plane are derived. The most promising signal from such states accessible in experiments like Belle, LHCb and PANDA is foreseen in the s-wave π D* phase shift and the η D* invariant mass distribution. For physical quark masses a rapid variation of the phase shift in-between the η D* and the K D¯* s thresholds is predicted

Study of the Σ\Sigma(1385) and Λ\Lambda(1405) resonances in K+^+ photoproduction processes

The $\gamma p \to K^+ \pi^0 \Lambda$ and $\gamma p \to K^+ \pi \Sigma$ reactions are studied in the kinematic region where the $\pi^0 \Lambda$(1116) and $\pi\Sigma$(1192) pairs originate dominantly from the decay of the $\Sigma$(1385) and $\Lambda$(1405) resonances. We consider laboratory photon energies around 2 GeV, i.e. total center of mass energies above the known resonance region. We compute the t-channel kaon-exchange contribution to these reactions using $K^- p \to \pi^0 \Lambda$ and $K^- p \to \pi \Sigma$ amplitudes calculated in the framework of a chiral coupled-channel effective field theory of meson-baryon scattering. We extract from the calculated cross section the gauge-invariant double kaon pole term. We find this term to be large and likely to drive significantly the $\gamma p \to K^+ \pi^0 \Lambda$ and $\gamma p \to K^+ \pi \Sigma$ reactions in the kinematics under investigation. Accurate measurements of t-distributions for these processes, in progress or planned at ELSA and at SPring-8, are needed to confirm this expectation and assess the possibility of studying antikaon-nucleon dynamics just below threshold through these reactions.Comment: 8 pages, 4 figures, Proceedings of the Int. School on Nuclear Physics, 26th Course, "Lepton scattering and the structure of hadrons and nuclei", Erice (Sicily), September 16th-24th, 200

Radiative and isospin-violating decays of Ds mesons in the hadrogenesis conjecture

The masses and decays of the scalar D_{s0}^*(2317) and axial-vector D_{s1}^*(2460) charmed strange mesons are calculated consistently in the hadrogenesis conjecture. These mesons decay either strongly into the isospin-violating pi^0 D_s and pi^0 D_s^* channels or electromagnetically. They are generated by coupled-channel dynamics based on the leading order chiral Lagrangian. The effect of chiral corrections to chiral order Q_\chi^2 is investigated. We show that taking into account large-N_c relations to determine the strength of these correction terms implies a measurable signal for an exotic axial-vector state in the eta D* invariant mass distribution. The one-loop contribution to the electromagnetic decay amplitudes of scalar and axial-vector states is calculated. The Lagrangian describing electromagnetic interactions is obtained by gauging the chiral Lagrangian for hadronic interactions and adding gauge-invariant correction terms to chiral order Q_chi^2. In addition the role of light vector meson degrees of freedom is explored. We confront our results with measured branching ratios. Once the light vector mesons are included, a natural explanation of all radiative decay parameters is achieved.Comment: 102 pages, 7 figures, further improved presentatio

Quantum interference of rho0- and omega-mesons in the pi N -> e+e- N reaction

The study of the $\pi N\to \rho^0 N$ and $\pi N \to \omega N$ amplitudes below and close to the vector meson production threshold ($1.4<\sqrt s <1.8$ GeV)reveals a rich structure arising from the presence of baryon resonances in this energy range. These resonances are reflected in the interference pattern of the $e^+e^-$ decays of the $\rho^0$- and $\omega$-mesons produced in $\pi^- p$ and $\pi^+ n$ reactions. We discuss the shape and magnitude of the $\rho^0$-$\omega$ interference in the $\pi^-p \to e^+e^- n$ and $\pi^+n \to e^+e^- p$ reaction cross sections as functions of the total center of mass energy $\sqrt s$. We find contrasted results: the interference is largely destructive for the $\pi^-p \to e^+e^- n$ cross section but constructive for the $\pi^+n \to e^+e^- p$ cross section. An experimental study of these reactions would provide significant constraints on the coupling of vector meson-nucleon channels to low-lying baryon resonances.Comment: 25 pages, 22 figure

The associated photoproduction of positive kaons and π0Λ\pi^0 \Lambda or πΣ\pi \Sigma pairs in the region of the Σ\Sigma(1385) and Λ\Lambda(1405) resonances

The $\gamma p \to K^+ \pi^0 \Lambda$ and $\gamma p \to K^+ \pi \Sigma$ reactions are studied in the kinematic region where the $\pi^0 \Lambda$(1116) and $\pi\Sigma$(1192) pairs originate dominantly from the decay of the $\Sigma$(1385) and $\Lambda$(1405) resonances. We consider laboratory photon energies around 2 GeV, significantly above the threshold for producing the $K^+ \Sigma$(1385) and $K^+ \Lambda$(1405) final states. We compute for both reactions the process in which the ingoing photon dissociates into a real K$^+$ and a virtual K$^-$, the off-shell K$^-$ scattering subsequently off the proton target to produce the $\pi^0 \Lambda$ or $\pi \Sigma$ pair. The $K^- p \to \pi^0 \Lambda$ and $K^- p \to \pi \Sigma$ amplitudes are calculated in the framework of a chiral coupled-channel effective field theory of meson-baryon scattering. The structure of the amplitudes reflects the dominance of the $\Lambda(1405)$ in the $\pi \Sigma$ channel and of the $\Sigma(1385)$ in the $\pi \Lambda$ channel. The full pion-hyperon final state interaction is included in these amplitudes. We extract from the calculated cross section the gauge-invariant double kaon pole term. We found this term to be large and leading to sizeable cross sections for both the $\gamma p \to K^+ \pi^0 \Lambda$ and $\gamma p \to K^+ \pi \Sigma$ reactions, in qualitative agreement with the scarce data presently available. Accurate measurements of these cross sections should make it possible to extract the contribution of the double kaon pole and hence to assess the possibility of studying kaon-nucleon dynamics ust below threshold through these reactions.Comment: 16 pages, 8 figure

The CHEMDNER corpus of chemicals and drugs and its annotation principles

The automatic extraction of chemical information from text requires the recognition of chemical entity mentions as one of its key steps. When developing supervised named entity recognition (NER) systems, the availability of a large, manually annotated text corpus is desirable. Furthermore, large corpora permit the robust evaluation and comparison of different approaches that detect chemicals in documents. We present the CHEMDNER corpus, a collection of 10,000 PubMed abstracts that contain a total of 84,355 chemical entity mentions labeled manually by expert chemistry literature curators, following annotation guidelines specifically defined for this task. The abstracts of the CHEMDNER corpus were selected to be representative for all major chemical disciplines. Each of the chemical entity mentions was manually labeled according to its structure-associated chemical entity mention (SACEM) class: abbreviation, family, formula, identifier, multiple, systematic and trivial. The difficulty and consistency of tagging chemicals in text was measured using an agreement study between annotators, obtaining a percentage agreement of 91. For a subset of the CHEMDNER corpus (the test set of 3,000 abstracts) we provide not only the Gold Standard manual annotations, but also mentions automatically detected by the 26 teams that participated in the BioCreative IV CHEMDNER chemical mention recognition task. In addition, we release the CHEMDNER silver standard corpus of automatically extracted mentions from 17,000 randomly selected PubMed abstracts. A version of the CHEMDNER corpus in the BioC format has been generated as well. We propose a standard for required minimum information about entity annotations for the construction of domain specific corpora on chemical and drug entities. The CHEMDNER corpus and annotation guidelines are available at: http://www.biocreative.org/resources/biocreative-iv/chemdner-corpus