arXiv subjects

A. Airapetian

Publications and source records attributed to A. Airapetian.

At least 37 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.

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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.

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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.

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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.

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arXiv2010

Leading-Order Determination of the Gluon Polarization from high-$p_T$ Hadron Electroproduction

Longitudinal double-spin asymmetries of charged hadrons with high transverse momentum $p_T$ have been measured in electroproduction using the \hermes\ detector at \hera. Processes involving gluons in the nucleon have been enhanced relative to others by selecting hadrons with $p_T$ typically above 1 GeV. In this kinematic domain the gluon polarization has been extracted in leading order making use of the model embedded in the Monte Carlo Generator \Pythia\ 6.2. The gluon polarization obtained from single inclusive hadrons in the $p_T$ range 1 GeV $< p_T <$ 2.5 GeV using a deuterium target is $\frac{Δg}{g}(\langle x\rangle, \langle μ^2\rangle)=0.049\pm 0.034 (stat)\pm 0.010 (sys\textrm{-}exp)^{+0.126}_{-0.099}(sys\textrm{-}models)$ at a scale $\laμ^2\ra=1.35~{\rm GeV}^2$ and $\langle x\rangle = 0.22$. For different final states and kinematic domains, consistent values of \DGG\ have been found within statistical uncertainties using hydrogen and deuterium targets.

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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.

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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.

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arXiv2010

Nuclear-mass dependence of azimuthal beam-helicity and beam-charge asymmetries in deeply virtual Compton scattering

The nuclear-mass dependence of azimuthal cross section asymmetries with respect to charge and longitudinal polarization of the lepton beam is studiedfor hard exclusive electroproduction of real photons. The observed beam-charge and beam-helicity asymmetries are attributed to the interference between the Bethe-Heitler and deeply virtual Compton scattering processes. For various nuclei, the asymmetries are extracted for both coherent and incoherent-enriched regions, which involve different (combinations of) generalized parton distributions. For both regions, the asymmetries are compared to those for a free proton, and no nuclear-mass dependence is found.

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arXiv2009

Search for a Two-Photon Exchange Contribution to Inclusive Deep-Inelastic Scattering

The transverse-target single-spin asymmetry for inclusive deep-inelastic scattering with effectively unpolarized electron and positron beams off a transversely polarized hydrogen target was measured, with the goal of searching for a two-photon exchange signal in the kinematic range 0.007 < x_B < 0.9 and 0.25 GeV**2 < Q**2 < 20 GeV**2. In two separate regions Q**2 > 1 GeV**2 and Q**2 < 1 GeV**2, and for both electron and positron beams, the asymmetries are found to be consistent with zero within statistical and systematic uncertainties, which are of order 10**(-3) for the asymmetries integrated over x_B.

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arXiv2009

Measurement of azimuthal asymmetries associated with deeply virtual Compton scattering on an unpolarized deuterium target

Azimuthal asymmetries in exclusive electroproduction of a real photon from an unpolarized deuterium target are measured with respect to beam helicity and charge. They appear in the distribution of these photons in the azimuthal angle phi around the virtual-photon direction, relative to the lepton scattering plane. The extracted asymmetries are attributed to either the deeply virtual Compton scattering process or its interference with the Bethe-Heitler process. They are compared with earlier results on the proton target. In the measured kinematic region, the beam-charge asymmetry amplitudes and the leading amplitudes of the beam-helicity asymmetries on an unpolarized deuteron target are compatible with the results from unpolarized protons.

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arXiv2009

Separation of contributions from deeply virtual Compton scattering and its interference with the Bethe--Heitler process in measurements on a hydrogen target

Hard exclusive leptoproduction of real photons from an unpolarized proton target is studied in an effort to elucidate generalized parton distributions. The data accumulated during the years 1996--2005 with the HERMES spectrometer are analyzed to yield asymmetries with respect to the combined dependence of the cross section on beam helicity and charge, thereby revealing previously unseparated contributions from deeply virtual Compton scattering and its interference with the Bethe--Heitler process. The integrated luminosity is sufficient to show correlated dependences on two kinematic variables, and provides the most precise determination of the dependence on only the beam charge.

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arXiv2009

Exclusive rho-0 electroproduction on transversely polarized protons

The exclusive electroproduction of rho-0 mesons was studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. Spin density matrix elements for this process were determined from the measured production- and decay-angle distributions of the produced rho-0 mesons. These matrix elements embody information on helicity transfer and the validity of s-channel helicity conservation in the case of a transversely polarized target. From the spin density matrix elements, the leading-twist term in the single-spin asymmetry was calculated separately for longitudinally and transversely polarized rho-0 mesons. Neglecting s-channel helicity changing matrix elements, results for the former can be compared to calculations based on generalized parton distributions, which are sensitive to the contribution of the total angular momentum of the quarks to the proton spin.

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arXiv2009

Transverse momentum broadening of hadrons produced in semi-inclusive deep-inelastic scattering on nuclei

The first direct measurement of the dependence on target nuclear mass of the average squared transverse momentum of pi+, pi-, and K+ mesons from deep-inelastic lepton scattering is obtained as a function of several kinematic variables. The data were accumulated at the HERMES experiment at DESY, in which the HERA 27.6 GeV lepton beam was scattered off several nuclear gas targets. The average squared transverse momentum was clearly observed to increase with atomic mass number. The effect increases as a function of Q**2 and x and remains constant as a function of both the virtual photon energy nu and the fractional hadron energy z, except that it vanishes as z approaches unity.

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arXiv2009

Spin Density Matrix Elements in Exclusive rho^0 Electroproduction on 1H and 2H Targets at 27.5 GeV Beam Energy

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive rho^0 electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1 GeV^2 < Q^2 < 7 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t < 0.4 GeV^2. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q^2 and t are presented, as well as the longitudinal-to-transverse rho^0 electroproduction cross section ratio as a function of Q^2. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.

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arXiv2008

Measurement of Parton Distributions of Strange Quarks in the Nucleon from Charged-Kaon Production in Deep-Inelastic Scattering on the Deuteron

The momentum and helicity density distributions of the strange quark sea in the nucleon are obtained in leading order from charged-kaon production in deep-inelastic scattering on the deuteron. The distributions are extracted from spin-averaged K+/- multiplicities, and from K+/- and inclusive double-spin asymmetries for scattering of polarized positrons by a polarized deuterium target. The shape of the momentum distribution is softer than that of the average of the ubar and dbar quarks. In the region of measurement 0.02 < x < 0.6 and Q^2 > 1.0 GeV^2, the helicity distribution is zero within experimental uncertainties.

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arXiv2008

Evidence for a Transverse Single-Spin Asymmetry in Leptoproduction of pi+pi- Pairs

A single-spin asymmetry was measured in the azimuthal distribution of pi+pi- pairs produced in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target. For the first time, evidence is found for a correlation between the transverse target polarization and the azimuthal orientation of the plane containing the two pions.The corresponding single-spin asymmetry is expected to be related to the product of the little-known quark transversity distribution function and an unknown naive-T-odd chiral-odd dihadron fragmentation function.

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arXiv2008

Measurement of Azimuthal Asymmetries With Respect To Both Beam Charge and Transverse Target Polarization in Exclusive Electroproduction of Real Photons

Azimuthal asymmetries in exclusive electroproduction of real photons are measured for the first time with respect to transverse target polarisation, providing new constraints on Generalized Parton Distributions. From the same data set on a hydrogen target, new results for the beam-charge asymmetry are also extracted with better precision than those previously reported. By comparing model calculations with measured asymmetries attributed to the interference between the deeply virtual Compton scattering and Bethe-Heitler processes, a model-dependent constraint is obtained on the total angular momenta carried by up and down quarks in the nucleon.

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arXiv2007

Cross sections for hard exclusive electroproduction of pi+ mesons on a hydrogen target

The exclusive electroproduction of pi+ mesons was studied with the Hermes spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off an internal hydrogen gas target. The virtual-photon cross sections were measured as a function of the Mandelstam variable t and the squared four momentum -Q^2 of the exchanged virtual photon. A model calculation based on Generalized Parton Distributions is in fair agreement with the data at low values of |t| if power corrections are included. A model calculation based on the Regge formalism gives a good description of the magnitude and the t and Q^2 dependences of the cross section.

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