arXiv subjects

B. G. Zakharov

Publications and source records attributed to B. G. Zakharov.

At least 91 records · Page 5Linked to original sources

arXiv2003

Lectures on Diffraction and Saturation of Nuclear Partons in DIS off Heavy Nuclei

The Lorentz contraction of ultrarelativistic nuclei entails a spatial overlap and fusion (recombination, saturation) of partons belonging to different nucleons at the same impact parameter. In these lectures we present a consistent description of the fusion of partons in terms of nuclear attenuation of color dipole states of the photon and collective Weizsäcker-Williams (WW) gluon structure function of a nucleus. The point that all observables for DIS off nuclei are uniquely calculable in terms of the nuclear WW glue amounts to a new form of factorization in the saturation regime. We start with the theory of multichannel propagation of color dipoles in a nuclear medium including the color-singlet to color-octet to color-octet transitions. We show how the Glauber-Gribov formulas are recovered from the multichannel formalism. Then we derive the two-plateau momentum distribution of final state (FS) quarks produced in deep inelastic scattering (DIS) off nuclei in the saturation regime. Then we comment on the signatures of saturation in exclusive diffractive DIS. A large body of these lectures is on the recent theory of jet-jet inclusive cross sections. We show that for hard dijets the decorrelation momentum is of the order of the nuclear saturation momentum $Q_A$. For minijets with the transverse momentum below the saturation scale we predict a complete disappearance of the azimuthal jet-jet correlation. We conclude with comment on a possible relevance of the decorrelation of jets to the experimental data from the STAR-RHIC Collaboration.

hep-ph↗

arXiv2003

Nonlinear kT factorization for Forward Dijets in DIS off Nuclei in the Saturation Regime

We develop the QCD description of the breakup of photons into forward dijets in small-x deep inelastic scattering off nuclei in the saturation regime. Based on the color dipole approach, we derive a multiple scattering expansion for intranuclear distortions of the jet-jet transverse momentum spectrum. A special attention is paid to the non-Abelian aspects of the propagation of color dipoles in a nuclear medium. We report a nonlinear $k_{\perp}$-factorization formula for the breakup of photons into dijets in terms of the collective Weizsäcker-Williams (WW) glue of nuclei as defined in ref. \cite{Saturation,NSSdijet}. For hard dijets with the transverse momenta above the saturation scale the azimuthal decorrelation (acoplanarity) momentum is of the order of the nuclear saturation momentum QA. For minijets with the transverse momentum below the saturation scale the nonlinear kT-factorization predicts a complete disappearance of the jet-jet correlation. We comment on a possible relevance of the nuclear decorrelation of jets to the experimental data from the STAR-RHIC Collaboration.

hep-ph↗

arXiv2002

High Density QCD, Saturation and Diffractive DIS

We review a consistent description of the fusion and saturation of partons in the Lorentz-contracted ultrarelativistic nuclei in terms of a nuclear attenuation of color dipole states of the photon and collective Weizsäcker-Williams (WW) gluon structure function of a nucleus. Diffractive DIS provides a basis for the definition of the WW nuclear glue. The point that all observables for DIS off nuclei are uniquely calculable in terms of the nuclear WW glue amounts to a new form of factorization in the saturation regime.

hep-ph↗

arXiv2002

High Density QCD and Saturation of Nuclear Partons

We review the recent finding of the two-plateau momentum distribution of sea quarks in deep inelastic scattering off nuclei in the saturation regime. The diffractive plateau which dominates for small $\bp$ measures precisely the momentum distribution of quarks in the beam photon, the rôle of the nucleus is simply to provide an opacity. The plateau for truly inelastic DIS exhibits a substantial nuclear broadening of the momentum distribution. Despite this nuclear broadening, the observed final state and initial state sea quark densities do coincide exactly. We emphasize how the saturated sea is generated from the nuclear-diluted Weizsäcker-Williams because of the anti-collinear splitting of gluons into sea quarks.

hep-ph↗

arXiv2002

Saturation of Nuclear Partons: the Fermi Statistics or Nuclear Opacity?

We derive the two-plateau momentum distribution of final state (FS) quarks produced in deep inelastic scattering (DIS) off nuclei in the saturation regime. The diffractive plateau which dominates for small transverse momentum p measures precisely the momentum distribution of quarks in the beam photon, the role of the nucleus is simply to provide an opacity. The plateau for truly inelastic DIS exhibits a substantial nuclear broadening of the FS momentum distribution. We discuss the relationship between the FS quark densities and the properly defined initial state (IS) nuclear quark densities.The Weizsäcker-Williams glue of a nucleus exhibits a substantial nuclear dilution, still soft IS nuclear sea saturates because of the anti-collinear splitting of gluons into sea quarks.

hep-ph↗

arXiv2002

Diffractive Hard Dijets and Nuclear Parton Distributions

Diffraction plays an exceptional role in DIS off heavy nuclei. First, diffraction into hard dijets is an unique probe of the unintegrated glue in the target. Second, because diffraction makes 50 per cent of total DIS off a heavy target, understanding diffraction in a saturation regime is crucial for a definition of saturated nuclear parton densities. After brief comments on the Nikolaev-Zakharov (NZ) pomeron-splitting mechanism for diffractive hard dijet production, I review an extension of the Nikolaev-Schafer-Schwiete (NSS) analysis of diffractive dijet production off nuclei to the definition of nuclear partons in the saturation regime. I emphasize the importance of intranuclear final state interactions for the parton momentum distributions.

hep-ph↗

arXiv2002

Coherent final state interaction in jet production in nucleus-nucleus collisions

We study the coherent final state interaction of an energetic parton produced in AA collisions caused by the change in the cutoff scale and running coupling constant from the vacuum to QGP. We demonstrate that the contribution of this new mechanism to the energy loss may be of the order of magnitude of the induced gluon radiation. However, an accurate evaluation of this medium effect is a difficult task because there is a strong cancellation between the cutoff and running coupling constant effects. The uncertainties in the contribution of the coherent final state interaction restrict strongly the accuracy of jet tomographic analyses of the matter density produced in AA reactions.

hep-ph↗

arXiv2000

On the Energy Loss of High Energy Quarks in a Finite-Size Quark-Gluon Plasma

We study within the light-cone path integral approach the induced gluon emission from a fast quark passing through a finite-size QCD plasma. We show that the leading log approximation used in previous studies fails when the gluon formation length becomes of the order of the length of the medium traversed by the quark. Calculation of the energy loss beyond the leading log approximation gives the energy loss which grows logarithmically with quark energy contrary to the energy independent prediction of the leading log approximation.

hep-ph↗

arXiv2000

Energy loss in perturbative QCD

We review the properties of energetic parton propagation in hot or cold QCD matter, as obtained in recent works. Advances in understanding the energy loss - collisional and radiative - are summarized, with emphasis on the latter: it features very interesting properties which may help to detect the quark-gluon plasma produced in heavy ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated in view of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly enhanced in hot compared to cold matter.

hep-ph↗

arXiv2000

Final state interactions in $^{4}$He(e,e$^\prime$p)$^{3}$H at large proton energy

At large proton energy, the final state interactions of the knocked out nucleon in $\eep$ reactions off nuclear targets can be described within the eikonal approximation, treating the spectator particles as a collection of fixed scattering centers. We use a generalization of this approach, suitable to take into account the possible occurrence of color transparency, to carry out an accurate calculation of the missing momentum distribution of the process $^{4}He(e,e^\prime p)^{3}H$. The pattern of final state interaction effect is analyzed for different kinematical setups in the domain corresponding to $2 \le Q^2 \le 20$ (GeV/c)$^2$.

nucl-th↗

arXiv1999

Nuclear-Medium Modification of the Rho(1S)- and Rhoprim(2S)-Mesons in Coherent Photo- and Electroproduction: Coupled Channel Analysis

We study medium modifications of the dilepton mass spectrum in coherent photo- and electroproduction of the Rho(1S)- and Rhoprim(2S)-meson resonances on nuclear targets. The analysis is performed within the coupled Rho, Rhoprim... channel formalism in which nuclear modifications derive from the off-diagonal rescatterings. We find that the effect of off-diagonal rescatterings on the shape of the dilepton mass spectrum in the Rho(1S)-meson mass region is only marginal but is very important in the Rhoprim mass region. The main off-diagonal contribution in the Rhoprim mass region comes from the sequential mechanism gamma* -> Rho -> Rhoprim(2S), which dominates the Rhoprim(2S) production for heavy nuclei. Our results show also that in the Rhoprim(2S) mass region there is a considerable effect of the interference of the Breit-Wigner tail of Rho(1S)-meson with the Rhoprim-meson.

hep-ph↗

arXiv1998

Azimuthal asymmetry as a new handle on $σ_{L}/σ_{T}$ in diffractive DIS

We propose a new method of the determination of $R^{D}=σ_{L}^{D}/σ_{T}^{D}$ from the dependence of the diffractive cross section on the azimuthal angle between the electron scattering and proton scattering planes. The method is based on our finding of the model independence of the ratio of the $LT$ interference and transverse diffractive structure functions. The predicted azimuthal asymmetry is substantial and can be measured at HERA. We show that the accuracy of our reconstruction of $R^{D}$ is adequate for a reliable test of an important pQCD prediction of $R^{D}\gsim 1$ for large $β$.

hep-ph↗

arXiv1998

Diffractive vector mesons beyond the s-channel helicity conservation

We derive a full set, and determine the twist, of helicity amplitudes for diffractive production of light to heavy vector mesons in deep inelastic scattering. For large $Q^{2}$ all helicity amplitudes but the double-flip are calculable in perturbative QCD and are proportional to the gluon structure function of the proton at a similar hard scale. We find a substantial breaking of the $s$-channel helicity conservation which must persist also in real photoproduction.

hep-ph↗

arXiv1998

Predictions for the forward cone in diffractive DIS

We calculate the diffraction slope $B_{D}$ for diffractive DIS. We find a counterintuitive rise of $B_{D}$ from exclusive diffractive excitation of vector mesons to excitation of continuum states with $M^{2} \sim Q^{2}$. For the small-mass continuum we predict a rapid variation of $B_{D}$ with $M^{2}$ on the scale $m_{V}^{2}$ and a sharp drop of $B_{D}$ for a small-mass continuum above the vector meson excitation.

hep-ph↗

arXiv1998

Light-cone path integral approach to the Landau-Pomeranchuk-Migdal effect

A new rigorous light-cone path integral approach to the Landau-Pomeranchuk-Migdal effect in QED and QCD is discussed. The rate of photon (gluon) radiation by an electron (quark) in a medium is expressed through the Green's function of a two-dimensional Schrodinger equation with an imaginary potential. In QED this potential is proportional to the dipole cross section for scattering of e+e- pair off an atom, in QCD it is proportional to the cross section of interaction of the color singlet quark-antiquark-gluon system with a medium constituent. In QED our predictions agree well with the photon spectrum measured recently at SLAC for 25 GeV electrons. In QCD for a sufficiently energetic quark produced inside a medium we predict the radiative energy loss proportional L^{2}, where L is the distance passed by the quark in the medium. It has a weak dependence on the initial quark energy. The L^{2} dependence transforms into L^{1} as the quark energy decreases. We also give new formulas for nuclear shadowing in hard reactions.

hep-ph↗

arXiv1998

Quark energy loss in an expanding quark-gluon plasma

We study the quark energy loss in an expanding quark-gluon plasma. The expanding plasma produced in high energy AA-collision is described by Bjorken's model. The dependence of the energy loss on the infrared cutoff for the radiated gluons, on the quark mass, and on the initial conditions of QCD plasma is investigated.

hep-ph↗

arXiv1998

Diffraction in Charged Current DIS

We present the QCD calculation of the diffractive structure function for charged current DIS. In particular we analyse the perturbatively tractable excitation of heavy quarks. We emphasize the peculiarities of the Regge factorization breaking in excitation of open charm.

hep-ph↗

arXiv1998

Light-cone path integral approach to the Landau-Pomeranchuk-Migdal effect and the SLAC data on bremsstrahlung from high energy electrons

The recent data of the SLAC E-146 collaboration on the Landau-Pomeranchuk-Migdal effect for bremsstrahlung from 8 and 25 GeV electrons are analyzed within a rigorous light-cone path integral approach. Numerical calculations have been carried out treating rigorously the Coulomb effects and including the inelastic processes. Comparison with the experimental data is performed taking into account multi-photon emission and photon absorption. For most of the targets the theoretical predictions are in excellent agreement with the experimental data.

hep-ph↗