Photoproduction of η\eta mesons off neutrons from a deuteron target

A formalism is developed for the partial wave analysis of data on meson photoproduction off deuterons and applied to photoproduction of $\eta$ and $\pi^0$ mesons. Different interpretations of a dip-bump structure of the $\eta$ photoproduction cross section in the 1670 MeV region are presented and discussed. Helicity amplitudes for two low-mass $S_{11}$ states are determined.Comment: 11 pages, 13 figure

Quark Model and Neutral Strange Secondary Production by Neutrino and Antineutrino Beams

The experimental data on $K^0$ and $\Lambda$ production by $\nu$ and $\bar{\nu}$ beams are compared with the predictions of quark model assuming that the direct production of secondaries dominates. Disagreement of these predictions with the data allows one to suppose that there exists considerable resonance decay contribution to the multiplicities of produced secondaries.Comment: 6 pages, no figures, 2 table

Chemical Freeze-out of Strange Particles and Possible Root of Strangeness Suppression

Two approaches to treat the chemical freeze-out of strange particles in hadron resonance gas model are analyzed. The first one employs their non-equillibration via the usual \gamma_s factor and such a model describes the hadron multiplicities measured in nucleus-nucleus collisions at AGS, SPS and RHIC energies with \chi^2/dof = 1.15. Surprisingly, at low energies we find not the strangeness suppression, but its enhancement. Also we suggest an alternative approach to treat the strange particle freeze-out separately, but with the full chemical equilibration. This approach is based on the conservation laws which allow us to connect the freeze-outs of strange and non-strange hadrons. Within the suggested approach the same set of hadron multiplicities can be described better than within the conventional approach with \chi^2/dof = 1.06. Remarkably, the fully equilibrated approach describes the strange hyperons and antihyperons much better than the conventional one.Comment: 6 pages, 5 figure

N∗N^{\bf *} decays to NωN\omega from new data on γp→ωp\gamma p\to \omega p

Data on the reaction $\gamma p\to \omega p$ with $\omega\to\pi^0\gamma$, taken with unpolarized or polarized beams in combination with an unpolarized or polarized proton-target, were analyzed within the Bonn-Gatchina (BnGa) partial wave analysis. Differential cross sections, several spin density matrix elements, the beam asymmetry $\Sigma$, the normalized helicity difference $E$, and the correlation $G$ between linear photon and longitudinal target polarization were included in a large data base on pion and photo-induced reactions. The data on $\omega$ photoproduction are used to determine twelve $N^*\to N\omega$ branching ratios; most of these are determined for the first time.Comment: 6 pages, 4 figures, 2 table

Hadron Resonance Gas Model with Induced Surface Tension

Here we present a physically transparent generalization of the multicomponent Van der Waals equation of state in the grand canonical ensemble. For the one-component case the third and fourth virial coefficients are calculated analytically. It is shown that an adjustment of a single model parameter allows us to reproduce the third and fourth virial coefficients of the gas of hard spheres with small deviations from their exact values. A thorough comparison of the compressibility factor and speed of sound of the developed model with the one and two component Carnahan-Starling equation of state is made. It is shown that the model with the induced surface tension is able to reproduce the results of the Carnahan-Starling equation of state up to the packing fractions 0.2-0.22 at which the usual Van der Waals equation of state is inapplicable. At higher packing fractions the developed equation of state is softer than the gas of hard spheres and, hence, it breaks causality in the domain where the hadronic description is expected to be inapplicable. Using this equation of state we develop an entirely new hadron resonance gas model and apply it to a description of the hadron yield ratios measured at AGS, SPS, RHIC and ALICE energies of nuclear collisions. The achieved quality of the fit per degree of freedom is about 1.08. We confirm that the strangeness enhancement factor has a peak at low AGS energies, while at and above the highest SPS energy of collisions the chemical equilibrium of strangeness is observed. We argue that the chemical equilibrium of strangeness, i.e. $\gamma_s \simeq 1$, observed above the center of mass collision energy 4.3 GeV may be related to the hadronization of quark gluon bags which have the Hagedorn mass spectrum, and, hence, it may be a new signal for the onset of deconfinement

Radiative decays of quarkonium states, momentum operator expansion and nilpotent operators

We present the method of calculation of radiative decays of composite quark-antiquark systems with different J^{PC}: (Q\bar Q)_{in} -> gamma (Q\bar Q)_{out}. The method is relativistic invariant, it is based on the double dispersion relation integrals over the masses of composite mesons, it can be used for the high spin particles and provides us with the gauge invariant transition amplitudes. We apply this method to the case when the photon is emitted by a constituent in the intermediate state (additive quark model). We perform the momentum operator expansion of the spin amplitudes for the decay processes. The problem of nilpotent spin operators is discussed.Comment: 21 pages, 1 figur