Pion (Kaon) and Sigma polarizabilities

We report the results of the working group on ``Pion (Kaon) and Sigma Polarizabilities''. Interesting possibilities to measure these polarizabilities include the radiative pion photoproduction in the MAMI experiment at Mainz, as well as at the GRAAL facility (actually the latter is being considered for an experimental determination of the pion polarizabilities here for the first time), the experimental plans on Primakoff effect at FNAL, and the measurements at the Frascati $\Phi$-factory DA$\Phi$NE.Comment: Rapporteur Talk, Workshop on Chiral Dynamics, A. Bernstein and B. Holstein organizers, (M.I.T., July 25-29, 1994), working group conducted together with R. Baldin

Hard exclusive processes with photons

Virtual photons have proven to be very efficient probes of the hadronic structure, mostly through deep inelastic scattering and related processes. The advent of high luminosity lepton beams has allowed to enlarge the studied processes to hard exclusive reactions, such as deeply virtual Compton scattering and the electroproduction of mesons. We discuss theoretical progress which has lately been quite remarkable in this domain and first much encouraging experimental data.Comment: 3 pages, to be published in the proceedings of the International Conference on the Structure and Interactions of the Photon (Photon 2007) Paris, july 200

HERA Inclusive Diffraction and Factorisation Tests

HERA measurements of diffractive ep scattering - the quasi-elastic scattering of the photon in the proton colour field - are summarised. Emphasis is placed on the most recent data.Comment: 9 pages, proceedings of PHOTON'0

Evidence for a narrow baryonic state decaying to K0Sp and KOSp in deep inelastic scattering at HERA

A resonance search has been made in the K0Sp and KOSp invariant-mass spectrum measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb−1. The search was performed in the central rapidity region of inclusive deep inelastic scattering at an ep centre-of-mass energy of 300–318 GeV for exchanged photon virtuality, Q2, above 1 GeV2. Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K+n and K0Sp, interpreted as a pentaquark. The results presented here support the existence of such state, with a mass of 1521.5±1.5(stat.)+2.8−1.7(syst.) MeV and a Gaussian width consistent with the experimental resolution of 2 MeV. The signal is visible at high Q2 and, for Q2>20 GeV2, contains 221±48 events. The probability of a similar signal anywhere in the range 1500–1560 MeV arising from fluctuations of the background is below 6×10−5