arXiv subjects

Z. Akopov

Publications and source records attributed to Z. Akopov.

31 records · Page 2Linked to original sources

arXiv2011

Multidimensional Study of Hadronization in Nuclei

Hadron multiplicities in semi-inclusive deep-inelastic scattering were measured on neon, krypton and xenon targets relative to deuterium at an electron(positron)-beam energy of 27.6 GeV at HERMES. These ratios were determined as a function of the virtual-photon energy nu, its virtuality Q2, the fractional hadron energy z and the transverse hadron momentum with respect to the virtual-photon direction pt . Dependences were analysed separately for positively and negatively charged pions and kaons as well as protons and antiprotons in a two-dimensional representation. Compared to the one-dimensional dependences, some new features were observed. In particular, when z > 0.4 positive kaons do not show the strong monotonic rise of the multiplicity ratio with nu as exhibited by pions and negative kaons. Protons were found to behave very differently from the other hadrons.

hep-ex↗

arXiv2011

Inclusive Measurements of Inelastic Electron and Positron Scattering from Unpolarized Hydrogen and Deuterium Targets

Results of inclusive measurements of inelastic electron and positron scattering from unpolarized protons and deuterons at the HERMES experiment are presented. The structure functions $F_2^p$ and $F_2^d$ are determined using a parameterization of existing data for the longitudinal-to-transverse virtual-photon absorption cross-section ratio. The HERMES results provide data in the ranges $0.006\leq x\leq 0.9$ and 0.1 GeV$^2\leq Q^2\leq$ 20 GeV$^2$, covering the transition region between the perturbative and the non-perturbative regimes of QCD in a so-far largely unexplored kinematic region. They are in agreement with existing world data in the region of overlap. The measured cross sections are used, in combination with data from other experiments, to perform fits to the photon-nucleon cross section using the functional form of the ALLM model. The deuteron-to-proton cross-section ratio is also determined.

hep-ex↗

arXiv2011

Measurement of azimuthal asymmetries associated with deeply virtual Compton scattering on a longitudinally polarized deuterium target

Azimuthal asymmetries in exclusive electroproduction of a real photon from a longitudinally polarized deuterium target are measured with respect to target polarization alone and with respect to target polarization combined with beam helicity and/or beam charge. The asymmetries appear in the distribution of the real photons in the azimuthal angle $ϕ$ around the virtual photon direction, relative to the lepton scattering plane. The asymmetries arise from the deeply virtual Compton scattering process and its interference with the Bethe-Heitler process. The results for the beam-charge and beam-helicity asymmetries from a tensor polarized deuterium target with vanishing vector polarization are shown to be compatible with those from an unpolarized deuterium target, which is expected for incoherent scattering dominant at larger momentum transfer. Furthermore, the results for the single target-spin asymmetry and for the double-spin asymmetry are found to be compatible with the corresponding asymmetries previously measured on a hydrogen target. For coherent scattering on the deuteron at small momentum transfer to the target, these findings imply that the tensor contribution to the cross section is small. Furthermore, the tensor asymmetry is found to be compatible with zero.

hep-ex↗

arXiv2011

Ratios of Helicity Amplitudes for Exclusive rho-0 Electroproduction

Exclusive rho^0-meson electroproduction is studied in the HERMES experiment, using a 27.6 GeV longitudinally polarized electron/positron beam and unpolarized hydrogen and deuterium targets in the kinematic region 0.5 GeV^2 < Q^2 < 7.0 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t' < 0.4 GeV^2. Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T_{11} (γ^*_T --> ρ_T), T_{01} (γ^*_T --> ρ_L), T_{10} (γ^*_L --> ρ_T), and T_{1-1} (γ^*_{-T} -->ρ_T) to T_{00} (γ^*_L --> ρ_L) are extracted from the data. For the unnatural-parity-exchange amplitude U_{11}, the ratio |U_{11}/T_{00}| is obtained. The Q^2 and t' dependences of these ratios are presented and compared with perturbative QCD predictions.

hep-ex↗

arXiv2010

Effects of transversity in deep-inelastic scattering by polarized protons

Single-spin asymmetries for pions and charged kaons are measured in semi-inclusive deep-inelastic scattering of positrons and electrons off a transversely nuclear-polarized hydrogen target. The dependence of the cross section on the azimuthal angles of the target polarization (phi_S)and the produced hadron (phi) is found to have a substantial sin(phi+phi_S) modulation for the production of pi+, pi- and K+. This Fourier component can be interpreted in terms of non-zero transversity distribution functions and non-zero favored and disfavored Collins fragmentation functions with opposite sign. For pi0 and K- production the amplitude of this Fourier component is consistent with zero.

hep-ex↗

arXiv2010

Exclusive Leptoproduction of Real Photons on a Longitudinally Polarised Hydrogen Target

Polarisation asymmetries are measured for the hard exclusive leptoproduction of real photons from a longitudinally polarised hydrogen target. These asymmetries arise from the deeply virtual Compton scattering and Bethe-Heitler processes. From the data are extracted two asymmetries in the azimuthal distribution of produced real photons about the direction of the exchanged virtual photon: A_UL with respect to the target polarisation and A_LL with respect to the product of the beam and target polarisations. Results for both asymmetries are compared to the predictions from a generalised parton distribution model. The sin(phi) and cos(0*phi) amplitudes observed respectively for the A_UL and A_LL asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2*phi) modulation in the A_UL asymmetry with a magnitude similar to that of the sin(phi) modulation is observed.

hep-ex↗

arXiv2010

Study of the hadronization process in cold nuclear medium

The improved two-scale model is used to perform the fit to the semi-inclusive deep-inelastic scattering (SIDIS) data of HERMES experiment at DESY on nuclear targets. The ratio of hadron multiplicity on nuclear target to the deuterium one is chosen as observable, as usually. The two-parameter's fit gives satisfactory agreement with the data in term of $χ^2$ criterium. Best values of parameters are then used to calculate the nuclear multiplicity ratio for the hadrons not included in the fit procedure.

hep-ph↗

arXiv2008

Possible scaling behaviour of the multiplicities ratio in leptoproduction of charged pions in nuclear medium

In this paper it is demonstrated that based on two-dimensional distributions in semi-inclusive deep inelastic scattering (SIDIS) data, obtained recently by the HERMES experiment at DESY on different nuclei, which contains data for charged pions produced in $z$ slices as a function of $ν$, and in a $ν$ slices as a function of $z$, it it is possible to parametrise a ratio of multiplicities on nucleus and deuterium (per nucleon) $R_M^{h}$ in a form of a function of a single variable $τ$, which has the physical meaning of the formation time of hadron. We call this effect $τ$ scaling. $τ$ is a function of two variables $ν$ and $z$. It is also shown that $R_M^{h}$ can be presented in a form of a linear polynomial of $τ$, $a_{11}$ + $τa_{12}$, where parameters $a_{11}$ and $a_{12}$ do not depend on $ν$ and $z$.

hep-ph↗

arXiv2007

$A$-dependence of coherent electroproduction of $ρ^{0}$ mesons on nuclei in forward direction

This article presents the $A$-dependence of the differential cross section for the coherent electroproduction of vector mesons on nuclei in forward direction, at fixed values of longitudinal momentum transfer $q_{L}$. It is shown that such cross section has complicated behavior over the atomic mass number $A$ with local minimums and maximums. It is also shown that a ratio of the real to the imaginary parts of the forward coherent amplitude on nuclei $α_{A} = \Re e{f_{A}} / \Im m{f_{A}}$ has breaking points at some values of $A$. Comparison of the behaviors of the normalized cross section $\Big(\frac{dσ}{dΩ}\Big)_{A}\Big/\Big(\frac{dσ}{dΩ}\Big)_{N}$ and $α_{A}$ over $A$ shows that the location of minimums of the cross section are very close to the breaking points of $α_{A}$.

hep-ph↗

arXiv2007

Possible influence of the two string events on the hadron formation in a nuclear environment

One of the basic assumptions of the string model is that as a result of a DIS in nucleus a single string arises, which then breaks into hadrons. However the pomeron exchange considered in this work, leads to the production of two strings in the one event. The hadrons produced in these events have smaller formation lengths, than those with the same energy produced in the single string events. As a consequence, they undergo more substantial absorption in the nuclear matter.

hep-ph↗

arXiv2007

Simple parameterization of nuclear attenuation data

Based on the nuclear attenuation data obtained by the HERMES experiment on nitrogen and krypton nuclei, it is shown that the nuclear attenuation $R_M^{h}$ can be parametrised in a form of a linear polynomial $P_1=a_{11}$ + $τa_{12}$, where $τ$ is the formation time, which depends on the energy of the virtual photon $ν$ and fraction of that energy $z$ carried by the final hadron. Three widely known parameterizations for $τ$ were used for the performed fit. The fit parameters $a_{11}$ and $a_{12}$ do not depend on $ν$ and $z$.

hep-ph↗

arXiv2006

Nuclear Attenuation of high energy two-hadron system in the string model

Nuclear attenuation of the two-hadron system is considered in the string model. The two-scale model and its improved version with two different choices of constituent formation time and sets of parameters obtained earlier for the single hadron attenuation, are used to describe available experimental data for the $z$-dependence of subleading hadron, whereas satisfactory agreement with the experimental data has been observed. A model prediction for $ν$-dependence of the nuclear attenuation of the two-hadron system is also presented.

hep-ph↗

arXiv2004

Application of the Two-Scale Model to the HERMES Data on Nuclear Attenuation

The Two-Scale Model and its improved version were used to perform the fit to the HERMES data for $ν$ (the virtual photon energy) and z (the fraction of $ν$ carried by hadron) dependencies of nuclear multiplicity ratios for $π^+$ and $π^-$ mesons electro-produced on two nuclear targets ($^{14}$N and $^{84}$Kr). The quantitative criterium $χ^2$ was used for the first time to analyse the results of the model fit to the nuclear multiplicity ratios data. The two-parameter's fit gives satisfactory agreement with the HERMES data. Best values of the parameters were then used to calculate the $ν$- and $z$ - dependencies of nuclear attenuation for $π^0$, K$^+$, K$^-$ and $\bar{p}$ produced on $^{84}$Kr target, and also make a predictions for $ν$, z and the Q$^2$ (the photon virtuality) - dependencies of nuclear attenuation data for those identified hadrons and nuclea, that will be published by HERMES.

hep-ph↗