arXiv subjects
Publications and source records attributed to Yu. A. Golubkov.
The exclusive photoproduction reaction gamma p -> Upsilon(1S) p has been studied with the ZEUS detector in ep collisions at HERA using an integrated luminosity of 468 pb^-1. The measurement covers the kinematic range 60 Upsilon(1S) p cross section.
A search for single-top production, $ep \rightarrow etX$, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of $0.37\fbi$. No evidence for top production was found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50fb-1. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of $\sqrt{s}=315\gev$.
The exclusive electroproduction of two pions in the mass range 0.4 < Mππ < 2.5 GeV has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb-1. The analysis was carried out in the kinematic range of 2 < Q2 < 80 GeV2, 32 < W < 180 GeV and |t| < 0.6 GeV2, where Q2 is the photon virtuality, W is the photon-proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The two-pion invariant-mass distribution is interpreted in terms of the pion electromagnetic form factor, |F(Mππ)|, assuming that the studied mass range includes the contributions of the ρ, ρ' and ρ" vector-meson states. The masses and widths of the resonances were obtained and the Q2 dependence of the cross-section ratios σ(ρ' \rightarrow ππ)/σ(ρ) and σ(ρ" \rightarrow ππ)/σ(ρ) was extracted. The pion form factor obtained in the present analysis is compared to that obtained in e+e- \rightarrow π+π-.
Photoproduction of beauty and charm quarks in events with at least two jets has been measured with the ZEUS detector at HERA using an integrated luminosity of 133 $pb^{-1}$. The fractions of jets containing b and c quarks were extracted using the invariant mass of charged tracks associated with secondary vertices and the decay-length significance of these vertices. Differential cross sections as a function of jet transverse momentum, $p_{T}^{\text{jet}}$, and pseudorapidity, $η^{\text{jet}}$, were measured. The data are compared with previous measurements and are well described by next-to-leading-order QCD predictions.
The production of beauty quarks in ep interactions has been studied with the ZEUS detector at HERA for exchanged four-momentum squared Q^2 > 10 GeV^2, using an integrated luminosity of 363 pb^{-1}. The beauty events were identified using electrons from semileptonic b decays with a transverse momentum 0.9 < p_T^e < 8 GeV and pseudorapidity |eta^e| < 1.5. Cross sections for beauty production were measured and compared with next-to-leading-order QCD calculations. The beauty contribution to the proton structure function F_2 was extracted from the double-differential cross section as a function of Bjorken-x and Q^2.
Beauty production in deep inelastic scattering with events in which a muon and a jet are observed in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb^-1. The fraction of events with beauty quarks in the data was determined using the distribution of the transverse momentum of the muon relative to the jet. The cross section for beauty production was measured in the kinematic range of photon virtuality, Q^2 > 2 Gev^2, and inelasticity, 0.05 < y < 0.7, with the requirement of a muon and a jet. Total and differential cross sections are presented and compared to QCD predictions. The beauty contribution to the structure function F_2 was extracted and is compared to theoretical predictions.
We performed a measurement of differential and integral jet shapes in proton-carbon, proton-tungsten and proton-aluminium collisions at 920 GeV/c proton momentum with the HERA-B detector at HERA for the jet transverse energies in the range 4 < E_T(jet) < 12 GeV. Jets were identified using the k_T-clustering algorithm. The measurements were performed for the hardest jet in the event, directed towards the opposite side with respect to the trigger direction. Jets become narrower with increasing transverse energy and measured distributions agree well with predictions of the PYTHIA 6.2 model. We do not observe any significant difference in the jet shape for the carbon and the aluminium targets. Nevertheless, the transverse energy flow at small and large radii for the tungsten sample is slightly less than for light nuclei. This observation indicates some influence of the nuclear environment on the formation of jets in heavy nuclei, especially at lower transverse energies, 5 < E_T(jet) < 6 GeV.
We calculated the contribution of the photon hadronic component in the charmed structure function of the leptoproduction. The contribution comes from scattering of the c quarks of the virtual photon on the quarks and gluons of the proton. Comparison of our calculations with the measurements of the charm production in $μ^+p$-scattering by EMC shows, that contribution of resolved photon can explain the excess of EMC data over the predictions of the photon-gluon fusion model at large momentum transfers. Thus, one does not need to use a non-perturbative admixture of the charmed quarks in the proton wave function (''intrinsic charm'') to describe such an excess in EMC data.
Using a statistical approach in the framework of non-covariant perturbation theory the distributions for light and charmed quarks in the hadron have been derived, taking into account the mass of the charmed quark. The parameters of the model have been extracted from the comparison with NA3 data on hadroproduction of J/psi particles. A reanalysis of the EMC data on charm production in muon-nucleon scattering has been performed. It has been found in comparison with the conventional source of charmed quarks from photon-gluon fusion, that the EMC data indicate the presence of an additional contribution from deep-inelastic scattering on charmed quarks at large x. The resulting admixture of the Fock states, containing charmed quarks in the decomposition of the proton wave function is of the order of 1%. The approach presented for the excitation of the Fock states with charmed quarks can also be applied to states with beauty quarks as well as to the hadronic component of the virtual photon (resolved photon component).
The existence of antimatter domains in baryon asymmetrical Universe can appear as the cosmological consequence of particle theory in inflationary models with non-homogeneous baryosynthesis. Such a domain can survive in the early Universe and form globular cluster of antimatter stars in our Galaxy. The model of antimatter pollution of Galaxy and annihilation with matter gas is developed. The proton-antiproton annihilation gamma flux is shown to reproduce the observed galactic gamma background measured by EGRET. From comparison with observational data the estimation on the maximally allowed amount of antimatter stars, possibly present in our Galaxy, is found.
The all-sky survey in high-energy gamma rays (E > 30 MeV) carried out by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma-Ray Observatory provides a unique opportunity to examine in detail the diffuse gamma-ray emission. The observed diffuse emission has a Galactic component arising from cosmic-ray interactions with the local interstellar gas and radiation as well an almost uniformly distributed component that is generally believed to originate outside the Galaxy. The results of the observations have been interpreted as the extragalactic high-energy gamma-ray emission arising primarily from unresolved gamma-ray-emitting blazars. We considered another possible origin of the diffuse gamma-ray flux, namely, as originating from the annihilation of the antiprotons with the interstellar medium.
The 4th generation of fermions predicted by the phenomenology of heterotic string models can possess new strictly conserved charge, which leads, in particular, to the hypothesis of the existence of the 4th generation massive stable neutrino. The compatibility of this hypothesis with the results of underground experiment DAMA searching for weakly interactive particles of dark matter and with the EGRET measurements of galactic gamma--background at energies above 1 GeV fixes the possible mass of the 4th neutrino at the value about 50 GeV. The possibility to test the hypothesis in accelerator experiments is considered. Positron signal from the annihilation of relic massive neutrinos in the galactic halo is calculated and is shown to be accessible for planned cosmic ray experiments.
The AMS--Shuttle experiment is shown to be sensitive to test the hypothesis on the existence of antimatter globular cluster in our Galaxy. The hypothesis follows from the analysis of possible tests for the mechanisms of baryosynthesis and uses antimatter domains in the matter dominated Universe as the probe for the physics underlying the origin of the matter. The total mass for the antimatter objects in our Galaxy is fixed from the below by the condition of antimatter domain survival in the matter dominated Universe and from above by the observed gamma ray flux. For this mass interval the expected fluxes of antinuclei can lead to up to ten antihelium events in the AMS-Shuttle experiment.
The AMS experiment is shown to be sensitive to test the hypothesis on the existence of antimatter globular cluster in our Galaxy. The hypothesis follows from the analysis of possible tests for the mechanisms of baryosynthesis and uses antimatter domains in the matter dominated Universe as the probe for the physics underlying the origin of the matter. The interval of masses for the antimatter in our Galaxy is fixed from below by the condition of antimatter domain survival in the matter dominated Universe and from above by the observed gamma ray flux. For this interval the expected fluxes of antihelium-3 and antihelium-4 are calculated with the account for their interaction with the matter in the Galaxy.
We carry out a detailed analysis of fluxes of cosmic ray antiprotons, positrons, electrons and photons to be expected from the annihilation of relic heavy neutrinos in the galactic halo. The spectra of particles are evaluated by Monte Carlo simulation. The results of calculations show that the investigation of cosmic ray positron spectra at high energies could provide a distinctive signal for annihilation of very heavy neutrinos in the Galaxy and give an important information on parameters of dark matter particles.
Using the statistical approach additional light partons have been introduced in the Intrinsic Charm model. Explicit expressions are obtained for charmed and light quark distributions. A comparison is made with standard IC model predictions. It is shown that such a modification of the IC model leads to better agreement with the $pN$ and $πN$ data on $\JP$ production at low energies and allows to avoid a contradiction to experimental data at high energy.