arXiv subjects

N. N. Nikolaev

Publications and source records attributed to N. N. Nikolaev.

At least 55 records · Page 3Linked to original sources

arXiv2005

Nonlinear k_\perp-factorization for Gluon-Gluon Dijets Produced off Nuclear Targets

The origin of the breaking of conventional linear k_\perp-factorization for hard processes in a nuclear environment is by now well established. The realization of the nonlinear nuclear k_\perp-factorization which emerges instead was found to change from one jet observable to another. Here we report on an important technical progress, the evaluation of the four-gluon color dipole cross section operator. It describes the coupled seven-channel non-Abelian intranuclear evolution of the four-gluon color-singlet states. An exact diagonalization of this seven-channel problem is possible for large number of colors N_c and allows a formulation of nonlinear k_\perp-factorization for production of gluon-gluon dijets. The momentum spectra for dijets in all possible color representations are reported in the form of explicit quadratures in terms of the collective nuclear unintegrated glue. Our results fully corroborate the concept of universality classes.

hep-ph↗

arXiv2005

Nonlinear k_\perp-factorization for Quark-Gluon Dijet Production off Nuclei

The breaking of conventional linear k_\perp-factorization for hard processes in a nuclear environment is by now well established. Here we report a detailed derivation of the nonlinear k_\perp-factorization relations for the production of quark-gluon dijets. This process is of direct relevance to dijets in the proton hemisphere of proton-nucleus collisions at energies of the Relativistic Heavy Ion Collider (RHIC). The major technical problem is a consistent description of the non-Abelian intranuclear evolution of multiparton systems of color dipoles. Following the technique developed in our early work [ N.N. Nikolaev, W. Schäfer, B.G. Zakharov and V.R. Zoller, J. Exp. Theor. Phys.\ {\bf 97} (2003) 441], we reduce the description of the intranuclear evolution of the $qgg\bar{q}$ state to the system of three coupled-channel equations in the space of color singlet 4-parton states $\ket{3\bar{3}}, \ket{6\bar{6}}$ and $\ket{15\bar{15}}$ (and their large-N_c generalizations). At large number of colors N_c, the eigenstate $(\ket{6\bar{6}}-\ket{15\bar{15}})/\sqrt{2}$ decouples from the initial state $\ket{3\bar{3}}$. The resulting nuclear distortions of the dijet spectrum exhibit much similarity to those found earlier for forward dijets in Deep Inelastic Scattering (DIS). Still there are certain distinctions regarding the contribution from color-triplet $qg$ final states and from coherent diffraction excitation of dijets. To the large-N_c approximation, we identify four universality classes of nonlinear k_\perp-factorization for hard dijet production.

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arXiv2005

Nonuniversality Aspects of Nonlinear k_\perp-factorization for Hard Dijets

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arXiv2005

Breaking of k_\perp-factorization for Single Jet Production off Nuclei

The linear k_\perp-factorization is part and parcel of the pQCD description of high energy hard processes off free nucleons. In the case of heavy nuclear targets the very concept of nuclear parton density becomes ill-defined as exemplified by the recent derivation [2] of nonlinear nuclear k_\perp-factorization for forward dijet production in DIS off nuclei. Here we report a derivation of the related breaking of k_\perp-factorization for single-jet processes. We present a general formalism and apply it to several cases of practical interest: open charm and quark and gluon jet production in the central to beam fragmentation region of γ^*p,γ^*A, pp and pA collisions. We show how the pattern of k_\perp-factorization breaking and the nature and number of exchanged nuclear pomerons do change within the phase space of produced quark and gluon jets. As an application of the nonlinear k_\perp-factorization we discuss the Cronin effect. Our results are also applicable to the p_\perp-dependence of the Landau-Pomeranchuk-Migdal effect for, and nuclear quenching of, jets produced in the proton hemisphere of pA collisions.

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arXiv2005

Diffractive Vector Meson Production at HERA: from Soft to Hard QCD

Experimental results from HERA on diffractive vector meson production and their theoretical interpretation within microscopic QCD are reviewed with an emphasis on the BFKL color dipole and kt-factorization approaches.

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arXiv2005

Hard Scattering in a Nuclear Environment: Farewell to Linear $k_{\perp}$-Factorization

We discuss a dramatic change brought into the pQCD description of hard processes in a nuclear environment by a large thickness of heavy nuclei. It breaks the familiar linear $k_{\perp}$-factorization which must be replaced by a new concept of the nonlinear $k_{\perp}$-factorization introduced in \cite{Nonlinear}.We demonstrate the salient features of nonlinear $k_{\perp}$-factorization on several examples from hard dijet production in DIS off heavy nuclei to single-jet to dijet production in hadron-nucleus collisions >. We also comment briefly on the non-linear BFKL evolution for gluon density of nuclei.

hep-ph↗

arXiv2004

QCD physics with polarized antiprotons at GSI

A polarized antiproton beam at the Facility for Antiproton and Ion Research, proposed by the PAX collaboration, will open a window to new physics uniquely accessible at the new High Energy Storage Ring. Our proposal to realize an asymmetric collider, in which polarized protons with momenta of about 3.5 GeV/c collide with polarized antiprotons with momenta up to 15 GeV/c, is well--suited to perform a direct measurement of the transversity distribution function $h_1$. In this report we summarize the outcome of various working group meetings within the PAX collaboration. The overall machine setup at the HESR, proposed by the PAX collaboration, is described along with the associated PAX experimental program.

hep-ex↗

arXiv2004

Fate of k_perp-factorization for hard processes in nuclear environment

Large thickness of heavy nuclei brings in a new scale into the pQCD description of hard processes in nuclear environment. The familiar linear $k_{\perp}$-factorization breaks down and must be replaced by a new concept of the nonlinear $k_{\perp}$-factorization introduced in \cite{Nonlinear}. I demonstrate the salient features of nonlinear $k_{\perp}$-factorization on an example of hard dijet production in DIS off heavy nuclei. I also discuss briefly the non-linear BFKL evolution for gluon density of nuclei.

hep-ph↗

arXiv2004

Evolution of high-mass diffraction from the light quark valence component of the pomeron

We analyze the contribution from excitation of the $(q\bar q)(f\bar f),(q\bar q)g_1...g_n(f\bar f)$ Fock states of the photon to high mass diffraction in DIS. We show that the large $Q^2$ behavior of this contribution can be described by the DLLA evolution from the non-perturbative $f\bar f$ valence state of the pomeron. Although of higher order in pQCD, the new contribution to high-mass diffraction is comparable to that from the excitation of the $q\bar q g$ Fock state of the photon.

hep-ph↗

arXiv2004

Multiple exchanges in lepton pair production in high--energy heavy ion collisions

The recent analysis of nuclear distortions in DIS off nuclei revealed a breaking of the conventional hard factorization for multijet observable. The related pQCD analysis of distortion effects for jet production in nucleus-nucleus collisions is as yet lacking. As a testing ground for such an analysis we consider the Abelian problem of higher order Coulomb distortions of the spectrum of lepton pairs produced in peripheral nuclear collisions. We report an explicit calculation of the contribution to the lepton pair production in the collision of two photons from one nucleus with two photons from the other nucleus, $2γ+ 2γ\to \m{l}^+\m{l}^-$. The dependence of this amplitude on the transverse momenta has a highly nontrivial form the origin of which can be traced to the mismatch of the conservation of the Sudakov components for the momentum of leptons in the Coulomb field of the oppositely moving nuclei. The result suggests that the familiar eikonalization of Coulomb distortions breaks down for the oppositely moving Coulomb centers, which is bad news from the point of view of extensions to the pQCD treatment of jet production in nuclear collisions. On the other hand, we notice that the amplitude for the $2γ+ 2γ\to \m{l}^+\m{l}^-$ process has a logarithmic enhancement for the lepton pairs with large transverse momentum, which is absent for $nγ+ mγ\to \m{l}^+\m{l}^-$ processes with $m,n > 2$. We discuss the general structure of multiple exchanges and show how to deal with higher order terms which cannot be eikonalized.

hep-ph↗

arXiv2004

Color dipoles and k_t factorization for nuclei

We discuss applications of the color dipole approach to hard processes on nuclei. We focus on the relation to k_t--factorisation and the role of a nuclear unintegrated gluon distribution in single-- and two-- particle inclusive spectra in γ^*A and pA collisions. Linear k_t factorisation is broken for a wide class of observables, which we exemplify on the case of heavy quark p_t--spectra.

hep-ph↗

arXiv2004

Why Breakup of Photons and Pions into Forward Dijets Is so Different: Predictions from Nonlinear Nuclear k_t-factorization

Based on an approach to non-Abelian propagation of color dipoles in a nuclear medium we formulate a nonlinear k_t-factorization for the breakup of photons and pions into forward hard dijets in terms of the collective Weizsäcker-Williams (WW) glue of nuclei. We find quite distinct practical consequences of nonlinear nuclear k_t-factorization for interactions of pointlike photons and non-pointlike pions. In the former case the large transverse momentum p_t of jets comes from the intrinsic momentum of quarks and antiquarks in the photon and nuclear effects manifest themselves as an azimuthal decorrelation with an acoplanarity momentum of the order of the nuclear saturation momentum Q_A. In the breakup of pions off free nucleons to the leading order in pQCD the spectator parton has a small transverse momentum and the hard dijet cross section is suppressed. In the breakup of pions off heavy nuclei the forward hard jets are predicted to be entirely decorrelated. We comment on the sensitivity of the pionic dijet cross section to the pion distribution amplitude. The predicted distinction between the breakup of photons and pions can be tested by the sphericity and thrust analysis of the forward hadronic system in the COMPASS experiment at CERN.

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arXiv2004

Accessing transversity via J/Psi production in polarized p-pbar interactions

We discuss the possibility of a direct access to transversity distributions by measuring the double transverse spin asymmetry A_TT in p pbar --> J/Psi + X --> l^- l^+ + X processes at future GSI-HESR experiments with polarized protons and anti-protons. In the J/Psi resonance production region, with 30 < s < 45 GeV^2, both the cross-section and A_TT are expected to be sufficiently large to allow a measurement of h_1^q(x,M^2); numerical estimates are given.

hep-ph↗

arXiv2003

Impact of saturation on s-channel helicity nonconservation for diffractive vector mesons

As Glauber has shown in 1959, the spin-flip phenomena caused by the conventional spin-orbit interaction do vanish in the scattering off heavy, strongly absorbing nuclei. On the other hand, the origin of the s-channel helicity nonconservation (SCHNC) in diffractive DIS is of the origin different from simple spin-orbit interaction, and here we demonstrate that SCHNC in vector meson production survives strong absorption effects in nuclear targets. The intranuclear absorption often discussed in terms of the saturation effects introduces a new large scale $Q_A^2$ into the calculation of diffractive vector meson production amplitudes. Based on the color dipole approach, we show how the impact of the saturation scale $Q_A^2$ changes from the coherent to incoherent/quasifree diffractive vector mesons.

hep-ph↗

arXiv2003

Ultraperipheral photoproduction of vector mesons in the nuclear Coulomb field and the size of neutral vector mesons

We point out a significance of ultraperipheral photoproduction of vector mesons in the Coulomb field of nuclei as a means of measuring the radius of the neutral vector meson. This new contribution to the production amplitude is very small compared to the conventional diffractive amplitude, but because of large impact parameters inherent to the ultraperipheral Coulomb mechanism its impact on the diffraction slope is substantial. We predict appreciable and strongly energy dependent increase of the diffraction slope towards very small momentum transfer.The magnitude of the effect is proportional to the mean radius squared of the vector meson and is within the reach of high precision photoproduction experiments, which gives a unique experimental handle on the size of vector mesons.

hep-ph↗

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↗

arXiv2003

Possibility of the odderon discovery via observation of charge asymmetry in the diffractive pi+pi- production at HERA

The interference between the Pomeron and possible odderon mechanisms of diffractive pi+pi- photoproduction results in charge asymmetry of the produced pions. The observation of charge asymmetry of pions at moderate M_{pi+pi-} will be an undoubted signal of odderon existence. To make numeral estimates more definite, we limit ourselves by the region M_{pi+pi-} = 1.1 -- 1.5 GeV, where in the odderon mechanism of dipion production, the production via single f_2(1270) resonance is expected to be dominant. We find a very statistically significant effect of the odderon induced charge asymmetry even with very modest estimates for the f_2 photoproduction cross section (without referring to any particular model of the odderon).

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