Spin-dependent parton distributions and structure functions

Nuclear parton distributions and structure functions are determined in an effective chiral quark theory. We also discuss an extension of our model to fragmentation functions.     

Spin-dependent structure functions for the proton and neutron

We present a phenomenological model for the spin-dependent structure functions g₁(x) of the proton and neutron. The model is an extension of the one proposed by Carlitz and Kaur. We use improved unpolarized structure functions and include effects due to the mass difference between up and down quark and due to the mass difference between spin $\text{1}\text{2}$ and $\text{3}\text{2}$ baryons. Our results for the proton agree with the data.     

Comparing real bremsstrahlung photons from electrons and positrons and virtual photons in electron-nuclear and positron-nuclear reactions for studies of giant resonances

The validity of determining the contributions attributable to bremsstrahlung photons from positrons using data with bremsstrahlung photons from electrons for identical radiators when studying E1 giant resonance by means of quasimonochromatic annihilation photons is confirmed. Photonuclear reactions induced by bremsstrahlung photons and electrons or positrons are compared with one another to determine the possibility of isolating the contributions from isovector E1 and E2 giant resonances.     

Spin-dependent processes involving a real photon

We calculate the helicity difference quark and gluon distributions in a polarized real photon using the Altarelli-Parisi equations. The corresponding helicity difference fragmentation functions to produce a real photon are dealt with in the same way. Both sets of results are presented in the form of simple parametrizations and the former used to investigate spin-spin asymmetries in the photoproduction of two high transverse momentum jets. These asymmetries are classed as “four jet” or “three jet” depending on whether any photon fragments are present in the final state along the beam axis. We find that the four-jet asymmetry should provide an experimental test for the presence of the anomalous polarized photon structure function while the three-jet asymmetry should provide new information about the spin structure of a polarized proton and in particular its gluon component.     

Pomeron structure function and diffraction dissociation of virtual photons in perturbative QCD

We study scaling properties of the diffraction dissociation of virtual photons in a deep inelastic scattering. We concentrate on the total diffraction dissociation rate, diffraction excitation mass spectrum and the pomeron structure function to the lowest order in perturbative QCD. We calculate the valence structure function and the strangeness and charm content of the pomeron and estimate the ocean structure function using the pomeron factorization property. We find that quarks carry ≈ 10% of pomeron's momentum. Differential cross section of the (virtual) photon-pomeron scattering is found to exhibit features typical of the hadronic two-body reactions, supporting a treatment of the Pomeron as a particle, whereas the flavor dependence of structure functions does not support the particle treatment of the pomeron. Diffraction dissociation of photons is predicted to make ≈ 15% of the total deep inelastic scattering rate at smallx and largeQ ². Detailed predictions for the mass spectrum and angular distribution of jets produced on the valence component of the pomeron are presented.     

A universal Regge pole model for all vector meson exclusive photoproduction by real and virtual photons

A model based on a dipole pomeron framework for exclusive vector meson photoproduction by real and virtual photons shows very good agreement with the experimental data. This model does not violate unitarity constraints and describes in a universal manner all the available data for , , , and vector meson photoproduction in the region of energies and photon virtualities . Received: 18 December 2001 / Published online: 25 October 2002 RID="a" ID="a" e-mail: martynov@bitp.kiev.ua RID="b" ID="b" e-mail: predazzi@to.infn.it RID="c" ID="c" e-mail: prokudin@to.infn.it     

Rho production by virtual photons

The reaction γ_Vp → pπ⁺π^? was studied in the W, Q² region 1.3–2.8 GeV, 0.3–1.4 GeV² using the streamer chamber at DESY. A detailed analysis of rho production viaγ_Vp→?⁰p is presented. Near threshold rho production has peripheral and non-peripheral contributions of comparable magnitude. At higher energies (W > 2 GeV) the peripheral component is dominant. The Q² dependence of σ(γ_Vp→?⁰p) follows that of the rho propagator as predicted by VDM. The slope of dσ/dt at 〈Q²〉 = 0.4 and 0.8 GeV² is within errors equal to its value at Q² = 0. The overall shape of the ?⁰ is t dependent as in photoproduction, but is independent of Q². The decay angular distribution shows that longitudinal rhos dominate in the threshold region. At higher energies transverse rhos are dominant. Rho production by transverse photons proceeds almost exclusively by natural parity exchange, σ_T^N ? (0.83 ± 0.06) σ_T for 2.2 < W < 2.8 GeV. The s-channel helicity-flip amplitudes are small compared to non-flip amplitudes. The ratio R = σ_L/σ_T was determined assuming s-channel helicity conservation. We find R = ξ²Q²/M_?² with ξ² ≈ 0.4 for 〈W〉 = 2.45 GeV. Interference between rho production amplitudes from longitudinal and transverse photons is observed. With increasing energy the phase between the two amplitudes decreases. The observed features of rho electroproduction are consistent with a dominantly diffractive production mechanism for W > 2 GeV.     

Jet production by virtual photons

The production of jets is studied in collisions of virtual photons, and , specifically for applications at HERA and LEP2. Photon flux factors are convoluted with matrix elements involving either direct or resolved photons and, for the latter, with parton distributions of the photon. Special emphasis is put on the range of uncertainty in the modeling of the resolved component. The resulting model is compared with existing data. Received: 7 July 1999 / Published online: 27 January 2000     

Spin-dependent isoscalar response functions and interpretation of polarization-transfer measurements

The correlations of tensor character are found to affect the integrated isoscalar spin-isospin response functions as much as the isovector ones, but with opposite signs. These effects can modify the interpretation of the recent polarization-transfer measurements in (p, p′) reactions.     

Including virtual photons in strong interactions

In the perturbative field-theoretical models we investigate the inclusion of the electromagnetic interactions into the purely strong theory that describes hadronic processes. In particular, we study the convention for splitting electromagnetic and strong interactions and the ambiguity of such a splitting. The issue of the interpretation of the parameters of the low-energy effective field theory in the presence of electromagnetic interactions is addressed, as well as the scale and gauge dependence of the effective theory couplings. We hope, that the results of these studies are relevant for the electromagnetic sector of ChPT.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.40.Ks Electromagnetic corrections to strong- and weak-interaction processes - 13.40.Dk Electromagnetic mass differences - 11.30.Rd Chiral symmetries - 11.10.Hi Renormalization group evolution of parameters     

Gluon distributions in real and virtual photon and the photon structure function

Gluon distributions in real and virtual photons are calculated using evolution equations in the NLO approximation. The quark distributions in the photon determined on the basis of the QCD sum rule approach in [1] are taken as an input. It is shown that gluon distribution in the photon can be reliably determined up tox=0.03÷0.05, much lower than the corresponding values in the case of quark distributions. Two variants of the calculations are considered: (1) it is assumed that there are no intrinsic gluons in the photon at some low normalization pointQ ²=Q ₀ ² ∼1GeV²; (2) it is assumed that gluonic content of the photon at lowQ ₀ ² is described by gluonic content of vector mesonsρ, ω, ϕ. The gluon distributions in these two variants appear to be different. This fact permits one to clarify the origin of nonperturbative gluonic content of the photon by comparing the results with experiment. Structure functionsF ₂(x) for real and virtual photon are calculated and it is shown that in the regionx≥0.2 where QCD approach is valid, there is a good agreement with experiment.