Primakoff effect in $ \eta$ -photoproduction off protons

We analyse data on forward h \eta -meson photoproduction off a proton target and extract the h \eta ? \rightarrow g \gamma g \gamma decay width utilizing the Primakoff effect. The hadronic amplitude that enters into our analysis is strongly constrained because it is fixed from a global fit to available g \gamma p ? \rightarrow p h \eta data for differential cross-sections and polarizations. We compare our results with present information on the two-photon h \eta -decay from the literature. We provide predictions for future PrimEx experiments at Jefferson Laboratory in order to motivate further studies.     

A hidden local symmetry approach to rho-meson photoproduction

We study the photoproduction of r \rho -mesons in a model of hidden local symmetry. For this purpose, the r \rho -meson is introduced as a hidden-gauge boson in a non-linear sigma model. For charged r \rho -meson photoproduction, the model takes into account the r \rho -meson magnetic moments from the three-point vertex in the kinetic terms. We show that the magnetic interaction of the charged r \rho -meson has a significant effect on the total cross-sections through the r \rho -meson exchange process, which is proportional to the energy of the photon. The t -channel dominance may be used for the study of structures of various unstable particles.     

K* 0 Σ+ Photoproduction with Evidence for the Kappa Meson Exchange at SPring-8/LEPS

Recently, LEPS collaboration reported the parity spin asymmetry for the γ p → K* ⁰Σ⁺ reaction using linearly polarized photons at beam energies from 1.85 to 2.96 GeV. The parity spin asymmetry ( ${P_{\sigma} = 2\rho^{1}_{1-1} - \rho^{1}_{00}}$ ) was obtained, with large positive asymmetry at forward angles. This supports that natural-parity exchange is dominant at forward angles for K* ⁰Σ⁺ photoproduction, which is consistent with κ meson-exchange. The differential cross sections for the γ p → K* ⁰Σ⁺ reaction are presented at forward angles averaged over all beam energies. There is good agreement with the differential cross sections from the CLAS and the CBELSA/TAPS collaborations.     

Tagging of $ \eta{^\prime}$ decay products to analyze chiral restoration

In case of chiral U _A(1) symmetry restoration the mass of the h￠ \eta{^\prime} boson (the ninth, would-be Goldstone boson) is decreased, thus its production cross-section is heavily enhanced. The h￠ \eta{^\prime} decays (through one of its decay channels) into five low-momentum pions. These pions will not contribute to measured Bose-Einstein correlations, thus the production enhancement changes the strength of two-pion correlation functions at low momentum. Preliminary results on Bose-Einstein correlation functions support the mass decrease of the h￠ \eta{^\prime} boson. In this paper we propose a method to select pions originating from h￠ \eta{^\prime} decays. We investigate the efficiency of the proposed kinematical cut in several collision systems and energies with several simulators. We prove that our method can be used in all investigated collision systems.     

Dependence of water viscosity on temperature and pressure

In the first approximation, the following formula is derived for water viscosity as a function of temperature and pressure:

The mixing of scalar mesons and the baryon-baryon interaction

By introducing the mixing of scalar mesons in the chiral SU(3) quark model, we dynamically investigate the baryon-baryon interaction. The hyperon-nucleon and nucleon-nucleon interactions are studied by solving the resonating group method (RGM) equation in a coupled-channel calculation. In our present work, the experimental lightest pseudoscalar p \pi, K,h \eta,h^￠ \eta^{{\prime}}_{} mesons correspond exactly to the chiral nonet pseudoscalar fields p \pi, K,h \eta,h^￠ \eta^{{\prime}}_{} in the chiral SU(3) quark model. The h \eta,h^￠ \eta^{{\prime}}_{} mesons are considered as the mixing of singlet and octet mesons, and the mixing angle q_ps \theta_{{ps}}^{} is taken to be -23^° . For scalar nonet mesons, we suppose that there exists a correspondence between the experimental lightest scalar f ₀(600) , k \kappa , a ₀(980) , f ₀(980) mesons and the theoretical scalar nonet s \sigma , k \kappa , s^￠ \sigma^{{\prime}}_{} , e \epsilon fields in the chiral SU(3) quark model. For scalar mesons, we consider two different mixing cases: one is the ideal mixing and another is the q_s \theta_{s}^{} = 19^° mixing. The masses of the s^￠ \sigma^{{\prime}}_{} and e \epsilon mesons are taken to be 980MeV, which are just the masses of the experimental a ₀(980) , f ₀(980) mesons. The mass of the s \sigma meson is an adjustable parameter and is decided by fitting the binding energy of the deuteron, the masses of 560MeV and 644MeV are obtained for the ideal mixing and the q_s \theta_{s}^{} = 19^° mixing, respectively. We find that, in order to reasonably describe the YN interactions, the mass of the k \kappa meson is near 780MeV for the ideal mixing. However, we must enhance the mass of the k \kappa meson for the q_s \theta_{s}^{} = 19^° mixing, the 1050MeV is favorably used in the present work. The experimental s \sigma and k \kappa scalar mesons are very strange, both have larger widths. Hence, no matter what kind of mixing is considered, all the masses of scalar mesons we used in the present work seem to be consistent with the present PDG information.     

Measurement of diffractive photoproduction of vector mesons at large momentum transfer at HERA

Elastic and proton–dissociative photoproduction of , and vector mesons (, , respectively) have been measured in interactions at HERA up to GeV, where t is the four-momentum transfer squared at the photon–vector–meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, , to values less than 0.01 GeV, and selects a average center-of-mass energy of = 94 GeV. Results for the differential cross sections, , for , and mesons are presented and compared to the results of recent QCD calculations. Results are also presented for the t-dependence of the pion-pair invariant-mass distribution in the mass region and of the spin-density matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of the W dependence of the elastic differential cross sections for both and mesons. Received: 16 October 1999 / Revised version: 17 January 2000 / Published online: 6 April 2000     

Properties of $ \alpha$ -decay to ground and excited states of heavy nuclei

Branching ratios and half-lives of a \alpha -decay to the ground-state rotational bands as well as the high-lying excited states of even-even nuclei have been calculated in the framework of the generalized liquid drop model (GLDM) and Royer’s formula that we improved very recently. The calculation covers the isotopic chains from Ra to No in the mass regions 222 ￡ \leq A ￡ \leq 252 and 88 ￡ \leq Z ￡ \leq 102 . The agreement between the calculated results and the experimental data indicates the reliability of investigating the properties of the unfavored a \alpha -decay with our method, especially the improved Royer’s formula, which is very valuable for the analysis of experimental data. In addition, the dependence of half-lives on excitation energies of daughter nuclei has been investigated. It is shown that the influence on half-lives becomes stronger and stronger with the increase of the excitation energies.     

Backward pion-nucleon scattering

A global analysis of the world data on differential cross-sections and polarization asymmetries of backward pion-nucleon scattering for invariant collision energies above 3GeV is performed in a Regge model. Including the N_a\ensuremath N_{\alpha} , N_g\ensuremath N_{\gamma} , D_d\ensuremath \Delta_{\delta} and D_b\ensuremath \Delta_{\beta} trajectories, we reproduce both angular distributions and polarization data for small values of the Mandelstam variable u , in contrast to previous analyses. The model amplitude is used to obtain evidence for baryon resonances with mass below 3GeV. Our analysis suggests a G₃₉\ensuremath G_{39} -resonance with a mass of 2.83GeV as member of the D_b\ensuremath \Delta_{\beta} -trajectory from the corresponding Chew-Frautschi plot.     

Temperature dependence of viscosity

A correction to the simple exponential temperature dependence of viscosity arises because of a difference between the real and ideal conditions for a viscous fluid flow. This correction is similar to those to pressure and volume in the real gas law. As a result, the expression for viscosity takes the form

Measurement of anti-deuteron photoproduction and a search for heavy stable charged particles at HERA

The cross section for anti-deuteron photoproduction is measured at HERA at a mean centre-of-mass energy of GeV in the range 0.2 < p _T /M < 0.7 and |y| < 0.4, where M, p _T and y are the mass, transverse momentum and rapidity of the anti-deuteron in the HERA laboratory frame, respectively. The numbers of anti-deuterons per event are found to be similar in photoproduction to those in central proton-proton collisions at the CERN ISR but much lower than those in central Au-Au collisions at RHIC. The coalescence parameter B ₂, which characterizes the likelihood of anti-deuteron production, is measured in photoproduction to be , which is much higher than in Au-Au collisions at a similar nucleon-nucleon centre-of-mass energy. No significant production of particles heavier than deuterons is observed and upper limits are set on the photoproduction cross sections for such particles.Received: 31 March 2004, Revised: 23 June 2004, Published online: 30 July 2004     

Photoproduction of φ mesons from protons and deuterons with linearly polarized photons at LEPS/SPring-8 experiment

Diffractive photoproduction of φ mesons near threshold involves Pomeron exchange, pseudoscalar exchanges and possible exotic components such as glueball and scalar-meson exchange. We measured φ-meson photoproduction from protons and deuterons at forward angles by using linearly polarized photons with E _γ = 1.5–2.4 GeV. The decay asymmetries show dominance of the natural-parity exchange process. The energy dependence of cross sections will help in determining the behavior of the Pomeron exchange near threshold. Correspondence: W. C. Chang, Institute of Physics, Academia Sinica, Taipei 11529, Taiwan     

Charm photo-and electroproduction at high energies

The differential and total cross sections for the photoproduction of vector D* mesons and for their production in deep-inelastic interactions at the HERA collider are estimated on the basis of a model motivated by perturbative calculations within QCD. The proposed model makes it possible to take into account higher twists in the meson transverse momentum at pT～m _c and to reproduce correctly the dominance of c-quark fragmentation for p _T m _c . The possibility of the hadronization of an octet c $\bar q$ state into a meson is considered, whereby good agreement with experimental data is obtained both for the case of D*-meson photoproduction and for the case of D*-meson formation in a deep-inelastic process.     

Total cross section for photon-nucleon interaction in the energy range $$\sqrt \mathcal{S} = 40 - 250 GeV$$

Results obtained by directly measuring the total cross section for photon-nucleon interaction through recording photoproduction processes at the Baksan underground scintillation telescope of the Institute for Nuclear Research (Russian Academy of Sciences, Moscow) are presented. The effect of a faster growth of photon-hadron cross sections in relation to hadron-hadron cross sections is confirmed in the energy range \(\sqrt \mathcal{S} = 40 - 130 GeV\). It is shown that an increase in the number of additive quarks in photon-hadronization products may be one of the reasons behind this effect. Experimental data on the cross sections for photon-nucleon and photon-photon interactions are subjected to a comparative analysis, and the status of the results obtained from direct and indirect cross-section measurements in the high-energy region is discussed.     

Radiative corrections to neutral pion-pair production

We calculate the one-photon loop radiative corrections to the neutral pion-pair photoproduction process p^-g ?p^-p⁰p⁰\pi^-\gamma \ensuremath \rightarrow\pi^-\pi^0\pi^0 . At leading order this reaction is governed by the chiral pion-pion interaction. Since the chiral p⁺ \pi^{+}_{} p^- \pi^{-}_{} ? \rightarrow p⁰ \pi^{0}_{} p⁰ \pi^{0}_{} contact vertex depends only on the final-state invariant mass it factors out of all photon loop diagrams. We give analytical expressions for the multiplicative correction factor R ~ a/2p\ensuremath R\sim \alpha/2\pi arising from eight classes of contributing one-photon loop diagrams. An electromagnetic counterterm has to be included in order to cancel the ultraviolet divergences generated by the photon loops. Infrared finiteness of the virtual radiative corrections is achieved (in the standard way) by including soft photon radiation below an energy cut-off l \lambda . The radiative corrections to the total cross-section vary between +2% and -2% for center-of-mass energies from threshold up to 7m_p\ensuremath 7m_{\pi} . We study also the radiative corrections to the p⁰p⁰\ensuremath \pi^0\pi^0 mass spectrum.     

Proton femtoscopy at STAR

The analysis of two-particle femtoscopy provides a powerful tool to study the properties of matter created in heavy ion collisions. Applied to identical and nonidentical hadron pairs, it makes the study of space-time evolution of the source in femtoscopic scale possible. Baryon femtoscopy allows extraction of the radii of produced sources which can be compared to those deduced from identical pion studies, providing additional information about source characteristics. In this paper we present the correlation functions obtained for protons and antiprotons for Au + Au collisions √s _NN = 62.4 and 200 GeV. On the other hand, as STAR experiment participates in Beam Energy Scan program, we present theoretical predictions of p − p, [`(p)] - [`(p)]\bar p - \bar p and p - [`(p)]p - \bar p femtoscopic measurements, based on UrQMD simulation for √s _NN = 5–39 GeV.     

Eta' photoproduction on the proton for photon energies from 1.527 to 2.227 GeV

Differential cross sections for the reaction gamma p --> eta' p have been measured with the CLAS spectrometer and a tagged photon beam with energies from 1.527 to 2.227 GeV. The results reported here possess much greater accuracy than previous measurements. Analyses of these data suggest for the first time the coupling of the eta'N channel to both the S11(1535) and P11(1710) resonances, known to couple strongly to the etaN channel in photoproduction on the proton, and the importance of J = 3/2 resonances in the process.     

DN interaction from meson exchange

A model of the DN interaction is presented which is developed in close analogy to the meson-exchange [`(K)] \bar{{K}} N potential of the Jülich group utilizing SU(4) symmetry constraints. The main ingredients of the interaction are provided by vector meson (r \rho , w \omega exchange and higher-order box diagrams involving D <sup>*</sup> N , D D \Delta , and D <sup>*</sup> D \Delta intermediate states. The coupling of DN to the p \pi L<sub>c</sub> \Lambda_{c}^{} and p \pi S<sub>c</sub> \Sigma_{c}^{} channels is taken into account. The interaction model generates the L<sub>c</sub> \Lambda_{c}^{}(2595) -resonance dynamically as a DN quasi-bound state. Results for DN total and differential cross sections are presented and compared with predictions of two interaction models that are based on the leading-order Weinberg-Tomozawa term. Some features of the L<sub>c</sub> \Lambda_{c}^{}(2595) -resonance are discussed and the role of the near-by p \pi S<sub>c</sub> \Sigma_{c}^{} threshold is emphasized. Selected predictions of the orginal [`(K)] \bar{{K}} N model are reported too. Specifically, it is pointed out that the model generates two poles in the partial wave corresponding to the L \Lambda(1405) -resonance.     

Quasi-particle model for lattice QCD: quark–gluon plasma in?heavy ion collisions

We propose a quasi-particle model to describe the lattice QCD equation of state for pure SU(3) gauge theory in its deconfined state, for T≥1.5T _c. The method involves mapping the interaction part of the equation of state to an effective fugacity of otherwise non-interacting quasi-gluons. We find that this mapping is exact. Using the quasi-gluon distribution function, we determine the energy density and the modified dispersion relation for the single particle energy, in which the trace anomaly is manifest. As an application, we first determine the Debye mass, and then the important transport parameters, viz., the shear viscosity, η, and the shear viscosity to entropy density ratio, h/S\eta/{\mathcal{S}}. We find that both η and h/S\eta/{\mathcal{S}} are sensitive to the interactions, and that the interactions significantly lower both η and h/S\eta/\mathcal{S}.