arXiv subjects

N. N. Nikolaev

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

At least 91 records · Page 5Linked to original sources

arXiv2000

Coherent production of hard dijets on nuclei in QCD

We formulate the perturbative QCD approach to coherent diffractive dijet production in pion-nucleon and pion-nucleus collisions at high energy. For hard dijets the Pomeron splitting mechanism in which both helicity amplitudes are proportional to the unintegrated gluon structure function of the proton and the pion distribution amplitude is shown to dominate. In nuclear diffraction multiple Pomeron splitting components are found to give antishadowing contributions at large jet momentum. To leading twist there is an exact cancelation of effects of nuclear attenuation and antishadowing/broadening of multiple Pomeron splitting contributions. The next-to-leading higher twist correction driven by nuclear rescatterings is calculable in hard QCD and proves to be numerically very large. We argue that large higher twist effects do not preclude the determination of gross features of the pion distribution amplitude. Our results on the atomic mass number and momentum dependence of dijet cross sections agree well with the preliminary findings from the E791 experiment.

hep-ph↗

arXiv2000

Anatomy of the Proton Structure Functions in $κ$-factorization

We present the first experiment-based parameterization of differential gluon structure function, which is called upon in many applications. We compare $κ$-factorization and DGLAP approaches and analyze properties of DGSF, with special emphasis on soft-to-hard/hard-to-soft diffusion.

hep-ph↗

arXiv2000

Diffractive Vector Meson Production In A Unified $κ$-Factorization Approach

In the framework of the $κ$-factorization approach and on the basis of recently determined DGSF, we calculated the production rates of ground and excited (2S and D wave) states of vector mesons and investigated their various kinematical and spin dependencies. We also addressed the issue of S/D wave mixing in quarkonia.

hep-ph↗

arXiv2000

Anatomy of the differential gluon structure function of the proton from the experimental data on F_2p

The use of the differential gluon structure function of the proton ${\cal F}(x,Q^{2})$ introduced by Fadin, Kuraev and Lipatov in 1975 is called upon in many applications of small-x QCD. We report here the first determination of ${\cal F}(x,Q^{2})$ from the experimental data on the small-x proton structure function $F_{2p}(x,Q^{2})$. We give convenient parameterizations for ${\cal F}(x,Q^{2})$ based partly on the available DGLAP evolution fits (GRV, CTEQ & MRS) to parton distribution functions and on realistic extrapolations into soft region. We discuss an impact of soft gluons on various observables. The x-dependence of the so-determined ${\cal F}(x,Q^{2})$ varies strongly with Q^2 and does not exhibit simple Regge properties. None the less the hard-to-soft diffusion is found to give rise to a viable approximation of the proton structure function F_{2p}(x,Q^2) by the soft and hard Regge components with intercepts Δ_{soft}=0 and Δ_{hard}\sim 0.4.

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

The color dipole BFKL-Regge expansion: from DIS on protons to pions to rise of hadronic cross sections

As noticed by Fadin, Kuraev and Lipatov in 1975 incorporation of asymptotic freedom into the BFKL equation splits the QCD pomeron into a series of isolated poles in complex angular momentum plane. Following our earlier work on the proton structure function we explore the phenomenological consequences of the emerging BFKL-Regge factorized expansion for the small-$x$ structure function of the pion $F_{2π}(x,Q^2)$. We calculate $F_{2π}$ in a small-$x$ region and find good agreement with the recent H1 determination of $F_{2π}(x,Q^2)$. We also present the BFKL-Regge factorization based evaluation of the contribution from hard scattering to the observed rise of the $NN$, $πN$ and real photo-absorption $γN$ and $γπ$ total cross sections.

hep-ph↗

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↗

arXiv1999

Spin Dependence of Diffractive Vector Meson Production

I review the recent progress in the theory of s-channel helicity nonconservation (SCHNC) effects in diffractive DIS. SCHNC in diffractive vector meson production is an unique probe of the spin-orbit coupling and Fermi motion of quarks in vector mesons. Photo- and electroproduction of the $ϕ$ and its radial and angular excitations at Jlab is perfectly posed to probe SCHNC in QCD pomeron exchange. I also discuss the unitarity driven demise of the Burkhardt-Cottingham sum rule and large & scaling departure from the Wandzura-Wilczek relation.

hep-ph↗

arXiv1999

Helicity Flip in Diffractive DIS

I review new results on s-channel helicity nonconservation (SCHNC) in diffractive DIS. I discuss how by virtue of unitarity diffractive DIS gives rise to spin structure functions which were believed to vanish at x << 1. These include tensor polarization of sea quarks in the deuteron, strong breaking of the Wandzura-Wilczek relation and demise of the Burkhardt-Cottingham sum rule.

hep-ph↗

arXiv1999

Intrinsic $k_{\perp}$ in the pomeron

In a consistent QCD approach to diffractive DIS gluons in the pomeron have soft $z_{g}$ distribution and very broad intrinsic $k_{\perp}$ distribution. The latter has not yet been incorporated into diffractive Monte-Carlo generators (RAPGAP,....). I argue that the so-introduced bias in the diffractive jet analysis may be the reason why H1 Collaboration finds the very large and hard gluon content of the pomeron. I discuss possible direct experimental determinations of intrinsic $k_{\perp}$ in the pomeron.

hep-ph↗

arXiv1999

Spin dependence of diffractive DIS

I review the recent progress in the theory of s-channel helicity nonconservation (SCHNC) effects in diffractive DIS including the unitarity driven demise of the Burkhardt-Cottingham sum rule and strong scaling departure from the Wandzura-Wilczek relation.

hep-ph↗

arXiv1999

Diffraction driven steep rise of spin structure function $g_{LT}=g_{1}+g_{2}$ at small x and DIS sum rules

We derive a unitarity relationship between the spin structure function $g_{LT}(x,Q^{2})=g_{1}(x,Q^{2})+g_{2}(x,Q^{2})$, the LT interference diffractive structure function and the spin-flip coupling of the pomeron to nucleons. Our diffractive mechanism gives rise to a dramatic small-$x$ rise $g_{LT}(x,Q^{2}) \sim g^{2}(x,\bar{Q}^{2}) \sim (1/x)^{2(1+δ_{g})}$, where $δ_{g}$ is an exponent of small-$x$ rise of the unpolarized gluon density in the proton $g(x,\bar{Q}^{2})$ at a moderate hard scale $\bar{Q}^{2}$ for light flavour contribution and large hard scale $\bar{Q}^{2} \sim m_{f}^{2}$ for heavy flavour contribution. It invalidates the Burkhardt-Cottingham sum rule. The found small-$x$ rise of diffraction driven $g_{LT}(x,Q^{2})$ is steeper than given by the Wandzura-Wilczek relation under conventional assumptions on small-$x$ behaviour of $g_{1}(x,Q^{2})$.

hep-ph↗

arXiv1999

Diffractive S and D-wave vector mesons in deep inelastic scattering

We derive helicity amplitudes for diffractive leptoproduction of the S and D wave states of vector mesons. We predict a dramatically different spin dependence for production of the S and D wave vector mesons. We find very small $R=σ_{L}/σ_{T}$ and abnormally large higher twist effects in production of longitudinally polarized D-wave vector mesons.

hep-ph↗

arXiv1998

Precocious asymptopia for charm from the running BFKL

The running BFKL equation gives rise to a series of moving poles in the complex j-plane. Corresponding eigenfunctions (color dipole cross sections) are the oscillating functions of the color dipole size $r$. The first nodes for all sub-leading solutions (color dipole cross sections) accumulate at $r_1\sim 0.1 fm$. Therefore the processes dominated by the dipole sizes $r\sim r_1$ are free of sub-leading BFKL corrections. A practically important example - the leptoproduction of charm. In a wide range of $Q^2$ the calculated $F_2^{cc}(x,Q^2)$ is exhausted by the leading BFKL pole and gives a perfect description of the experimental data.

hep-ph↗

arXiv1998

The Running BFKL: Resolution of Caldwell's Puzzle

The HERA data on the proton structure function, $F_2(x,Q^2)$, at very small $x$ and $Q^2$ show the dramatic departure of the logarithmic slope, $\partial F_2/\partial\log Q^2$, from theoretical predictions based on the DGLAP evolution. We show that the running BFKL approach provides the quantitative explanation for the observed $x$ and/or $Q^2$ -dependence of $\partial F_2/\partial\log Q^2$.

hep-ph↗

arXiv1998

Do the E866 Drell Yan data change our picture of the chiral structure of the nucleon ?

We revisit the evaluation of the pionic mechanism of the $\bar{u}-\bar{d}$-asymmetry in the proton structure function. Our analysis is based on the extended AGK unitarity relation between contributions of different mechanisms to the inclusive particle production and the total photoabsorption cross-section (i.e. the proton structure function). We reanalyze the role of isovector reggeons in inclusive production of nucleons and Delta isobars in hadronic reactions. We find rather large contribution of reggeon-exchange induced production of Delta isobars. This leaves much less room for the pion-exchange induced mechanism of $Δ$ production and provides a constraint on the $πN Δ$ form factor. The production of leading pions in proton-proton collisions at ISR puts additional constraints on the $πNN$ vertex form factors. An extension of the AGK-rules to reggeon exchange suggests a negligible contribution to the proton structure function from DIS off the exchanged $ρ,a_2$ reggeons. All these constraints are used then to estimate the pion content of the nucleon and allow to calculate parameter-free the $x$-dependence of $\bar d - \bar u$. We discuss the violation of the Gottfried Sum Rule and $\bar d$-$\bar u$ asymmetry and compare to the one obtained from the E866 experiment at Fermilab. We estimate the background to the pion structure function being determined by the ZEUS and H1 collaborations at HERA from leading neutron experiments.

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↗