Single photo and electroproduction of pions at EBAC@JLAB

Within the Excited Baryon Analysis Center we have performed a dynamical coupled-channels analysis of the available p（e,e＇π）N data in the region of W ≤ 1.6 GeV and Q2 ≤1.45 （GeV/c）2. The channels included are γ＊N, π＊N, ηN, and ηN which has π△, pN, and σN components. With the hadronic parameters of the model determined in our previous investigations of πN→ πN reaction, we have found that the available data in the considered W ≤ 1.6 GeV region can be fitted well by only adjusting the bare γ＊N → N＊ helicity amplitudes for the lowest N＊ states in P33, P11, S11 and D13 partial waves. The meson cloud effect, as required by the unitarity conditions, on the γ＊N → N＊ form factors are examined.     

Survival of charged ρ condensation at high temperature and density

The charged vector ρ mesons in the presence of external magnetic fields at finite temperature T and chemical potential μ have been investigated in the framework of the Nambu-Jona-Lasinio model.We compute the masses of charged ρ mesons numerically as a function of the magnetic field for different values of temperature and chemical potential.The self-energy of the ρ meson contains the quark-loop contribution,i.e.the leading order contribution in 1/N_C expansion.The charged ρ meson mass decreases with the magnetic field and drops to zero at a critical magnetic field eB_c,which indicates that the charged vector meson condensation,i.e.the electromagnetic superconductor can be induced above the critical magnetic field.Surprisingly,it is found that the charged ρ condensation can even survive at high temperature and density.At zero temperature,the critical magnetic field just increases slightly with the chemical potential,which indicates that charged ρ condensation might occur inside compact stars.At zero density,in the temperature range 0.2 — 0.5 GeV,the critical magnetic field for charged ρ condensation is in the range of 0.2 — 0.6 GeV~2,which indicates that a high temperature electromagnetic superconductor might be created at LHC.     

PC Farms for Triggering and Online Reconstruction at HERA—B

The HERA-B data acquisition and triggering systems make use of Linux PC farms for triggering and online event reconstruction.We present in this paper the requirements,implementation and performance of both PC farms.They have been fully working during the year 2000 detector and trigger commissioning run.     

Parity Violation in Proton–Proton Scattering at High Energies

JETP Letters - A new approach has been proposed to describe parity violation in the scattering of polarized protons by protons at high energies. It has been shown that the P-odd proton- proton...     

Elastic photoproduction of ϱ0 mesons at HERA

The cross section for the elastic photoproduction of ϱ⁰ mesons (γp → ϱ⁰p) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. The lower energy point was measured by observing directly the ϱ⁰ decay giving a cross section of 9.1 ± 0.9 (stat.) ± 2.5 (syst.) μb. The logarithmic slope parameter of the differential cross section, dσ/dt, is found to be 10.9 ± 2.4 (stat.) ± 1.1 (syst.) GeV⁻². The ϱ⁰ decay polar angular distribution is found to be consistent with s-channel helicity conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be 13.6 ± 0.8 (stat.) ± 2.4 (syst.) μb.     

On the Coupling of φ Meson to Nucleons and Backward φ Production

Some analyses of the electromagnetic form factors of the nucleon and some analyses of the nucleon-nucleon potential favor a large coupling of the φ meson to nucleons. This is frequently quoted by advocates of a large ss component of the nucleon. It is shown that such large OZI-violating couplings are incompatible with data on φ production in backward direction via nucleon exchange.     

Photoproduction of K 0 and Λ at HERA and a comparison with deep inelastic scattering

Inclusive K ⁰ and Λ photoproduction has been investigated at HERA with the H1 detector at an average photon-proton center of mass energy of 200 GeV in the transverse momentum range 0.5 < p _t < 5 GeV. The production rates as a function of p _t and center of mass rapidity are compared to those obtained in deep inelastic scattering at 〈Q ²〉 = 23 GeV². A similar comparison is made of the rapidity spectra of charged particles. The rate of strangeness photoproduction is compared with $p\bar p$ measurements. The observations are also compared with next-to-leading order QCD calculations and the predictions of a Monte Carlo model.     

Analytic structure of ρ meson propagator at finite temperature

We analyse the structure of one-loop self-energy graphs for the ρ meson in real time formulation of finite temperature field theory. We find the discontinuities of these graphs across the unitary and the Landau cuts. These contributions are identified with different sources of medium modification discussed in the literature. We also calculate numerically the imaginary and the real parts of the self-energies and construct the spectral function of the ρ meson, which are compared with an earlier determination. A significant contribution arises from the unitary cut of the π ω loop, that was ignored so far in the literature.     

Factorisation breaking in diffractive dijet photoproduction at HERA?

Recent experimental data on dijet cross sections in diffractive photoproduction at HERA collider are analysed with an emphasis on QCD factorisation breaking effects. The possible sources of the different conclusions of H1 and ZEUS collaborations are studied.     

Measurement of inelastic J/ψ photoproduction at HERA

We present a measurement of the inelastic, non diffractive J/ψ photoproduction cross section in the reaction e ⁺ p → e ⁺J/ψ X with the ZEUS detector at HERA. The J/ψ was identified using both the μ ⁺ μ ^- and e ⁺ e ^- decay channels and events were selected within the range 0.4 < z < 0.9 (0.5 < z < 0.9) for the muon (electron) decay mode, where z is the fraction of the photon energy carried by the J/ψ in the proton rest frame. The cross section, the p _T ² and the z distributions, after having subtracted the contributions from resolved photon and diffractive proton dissociative processes, are given for the photon-proton centre of mass energy range 50 < W < 180 GeV; p _T ² is the square of the J/ψ transverse momentum with respect to the incoming proton beam direction. In the kinematic range 0.4 < z < 0.9 and p _T ² > 1 GeV², NLO calculations of the photon-gluon fusion process based on the colour-singlet model are in good agreement with the data. The predictions of a specific leading order colour-octet model, as formulated to describe the CDF data on J/ψ hadroproduction, are not consistent with the data.     

Factorisation breaking in diffractive dijet photoproduction at HERA

We discuss the factorisation breaking observed in diffractive dijet photoproduction by the H1 and ZEUS collaborations at HERA. By considering the effects of rapidity gap survival, hadronisation, migration and NLO contributions, we find that the observed data are compatible with theoretical expectations.     

Radiative decays of ρ and ω mesons

The results of the measurements of radiative decays of ρ and ω mesons with the Neutral Detector at thee ⁺ e ^? collider VEPP-2M are presented. The branching ratio of the decay ω→π ⁰γ was measured with higher than in previous experiments accuracy: $${\rm B}(\omega \to \pi ^0 \gamma ) = 0.0888 \pm 0.0062$$ . The ρ⁰→π ⁰ γ branching ratio was measured for the first time: $$B(\rho ^0 \to \pi ^0 \gamma ) = (7.9 \pm 2.0) \cdot 10^{ - 4} $$ . The decays ρ, ω→ηγ were studied. Their branching ratios with the assumption of constructive ρ?ω interference are: $$\begin{gathered} B(\omega \to \eta \gamma ) = (7.3 \pm 2.9) \cdot 10^{ - 4} , \hfill \\ B(\rho \to \eta \gamma ) = (4.0 \pm 1.1) \cdot 10^{ - 4} \hfill \\ \end{gathered} $$ . The branching ratios of ρ, ω→ηγ and ω→e ⁺ e ^? decays were also measured: $$\begin{gathered} B(\omega \to \pi ^ + \pi ^ - \pi ^0 ) = 0.8942 \pm 0.0062, \hfill \\ B(\omega \to e^ + e^ - ) = (7.14 \pm 0.36) \cdot 10^{ - 5} \hfill \\ \end{gathered} $$ . The upper limit for the ω→π ⁰ π ⁰ γ branching ratio was placed: B(ω→π ⁰ π ⁰ γ)<4·10^?4 at 90% confidence level.     

Anomalous quark-gluon chromomagnetic interaction and high-energy ρ-meson electroproduction

It is shown that existence of a large anomalous chromomagnetic moment of quark induced by nonperturbative structure of QCD leads to the additional contribution to exclusive ρ-meson electroproduction off proton target. The contribution coming from new type of quark-gluon interaction to the ρ-meson production cross section for both transversal and longitudinal polarization of virtual photon is found. Such nonperturbative contribution together with conventional perturbative two-gluon exchange allows us to describe the experimental data at low Q ² for transversal polarization. However, in the longitudinal polarization case there is still some discrepancy with the data. The possible source of this deviation is discussed.     

Reconciling J/ψ production at HERA, RHIC, Tevatron, and LHC with nonrelativistic QCD factorization at next-to-leading order

We calculate the cross section of inclusive direct J/ψ hadroproduction at next-to-leading order within the factorization formalism of nonrelativistic quantum chromodynamics, including the full relativistic corrections due to the intermediate 1S(0)[8], 3S(1)[8], and 3P(J)[8] color-octet states. We perform a combined fit of the color-octet long-distance matrix elements to the transverse-momentum (p(T)) distributions measured by CDF at the Fermilab Tevatron and H1 at DESY HERA and demonstrate that they also successfully describe the p(T) distributions from PHENIX at BNL RHIC and CMS at the CERN LHC as well as the photon-proton c.m. energy and (with worse agreement) the inelasticity distributions from H1. This provides a first rigorous test of nonrelativistic QCD factorization at next-to-leading order. In all experiments, the color-octet processes are shown to be indispensable.     

Electron-state bremsstrahlung processesep→γep(X) at HERA

The analytic formulae for the bremsstrahlung photon spectra in the processeslNlN andlNlX (taking into account -andZ-boson exchange diagrams) are derived. The energy and angular distributions of photons are computed at HERA energies. The very large sensitivity of spectra for the deep inelastic channel to the choice of the structure functions is revealed in the region of small angles and large energies of the emitted photons.     

Proton-deuteron elastic scattering at 135 MeV with BINA

Differential cross sections and vector analyzing powers for the p + d → p + d reaction have been measured using a new experimental setup (BINA) at KVI. The data are compared with published results and are found to be significantly different.     

Vector Meson Couplings to Nucleons and Backward ρ and ω Production

Data on the production of vector mesons ρ and ω in the backward direction via nucleon exchange areinvestigated. A better understanding of these data is achieved than hitherto, albeit at a semi-quantitative level. Thisis achieved partly by using more up-to-date values for the vector couplings and partly by using more precise theoreticalformalism. In view of this we conclude that the data do provide a useful constraint on the couplings.