Searcharxiv⌕ Search

arXiv subjects

N. N. Nikolaev

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

At least 145 records · Page 8Linked to original sources
arXiv1994

Direct calculation of the triple-pomeron coupling for diffractive DIS and real photoproduction

We present a unified direct calculation of the triple-pomeron coupling $A_{3\Pom}(Q^{2})$ for diffractive real photoproduction ($Q^{2}=0$) and deep inelastic scattering at large $Q^{2}$ in the framework of the dipole approach to the generalized BFKL pomeron. The small phenomenological value of $A_{3\Pom}(0)\approx 0.16$\,GeV$^{2}$, which was a mystery, is related to the small correlation radius $R_{c}\approx 0.3$\,fm for the perturbative gluons. We confirm the early expectations of weak $Q^{2}$ dependence of the dimensionfull coupling $A_{3\Pom}(Q^{2})$ and predict that it rises by the factor $\sim 1.6$ from real photoproduction to deep inelastic scattering.

hep-ph↗
arXiv1994

Diffractive DIS from the generalized BFKL pomeron. Predictions for HERA

We present microscopic QCD calculation of the cross section of diffractive DIS and of the partonic structure of the pomeron from the dipole approach to the generalized BFKL pomeron. We show that the pomeron can not be treated as a particle with uniquely defined structure function and flux in the proton. We find strong factorization breaking which can approximately be described by the two-component structure function of the pomeron, each component endowed with the different flux of pomerons in the proton. We predict very weak $Q^{2}$ dependence of the diffractive contribution to the proton structure function.

hep-ph↗
arXiv1994

Final state interaction effects in $D(e,e'p)$ scattering

We present a systematic study of the final-state interaction (FSI) effects in $D(e,e'p)$ scattering in the CEBAF energy range with particular emphasis on the phenomenon of the angular anisotropy of the missing momentum distribution. We find that FSI effects dominate at missing momentum $p_m \gsim 1.5 $fm$^{-1}$. FSI effects in the excitation of the $S$-wave state are much stronger than in the excitation of the $D$-wave.

nucl-th↗
arXiv1994

Simple classification of final state interaction effects in $^4He(e,e'p)$ scattering

The radius of interaction between the struck proton and spectator nucleons is close to the radius of short-distance two-nucleon correlations in nuclear matter, which makes final state interaction(FSI) an important background to production of protons with large missing momentum. We present a simple classification of the dominant FSI effects in $^4He(e,e'p)$ scattering and identify parts of the phase space dominated by FSI. At large missing momentum, FSI leads to a striking angular anisotropy of the missing momentum distribution, which has a prominent peak in transverse kinematics and smaller, forward-backward asymmetric, peaks in parallel kinematics.

nucl-th↗
arXiv1994

Color transparency after the NE18 and E665 experiments: Outllok and perspectives at CEBAF

CEBAF is a high-luminocity factory of virtual photons with variable virtuality $Q^{2}$ and transverse size. This makes CEBAF, in particular after the energy upgrade to (8-12)GeV, an ideal facility for uncovering new phenomena, and opening new windows, at the interface of the perturbative and nonperturbative QCD. We discuss color transparency as the case for a broad program on electroproduction of vector mesons $ρ^{0},\,ω^{0},\,ϕ^{0}$ and their radial excitations $ρ',\,ω',\,ϕ'$ at CEBAF. We also comment on the second generation of experiments on color transparency in $^{4}He(e,e'p)$ scattering, which are also feasible at CEBAF. In 1994, we can make more reliable projections into future because our understanding of the onset of color transparency has greatly been augmented by two experiments completed in 1993:\\ i) no effect of CT was seen in the SLAC NE18 experiment on $A(e,e'p)$ scattering at virtualities of the exchanged photon $Q^{2} \lsim 7$ GeV$^{2}$, \\ ii) strong signal of CT was observed in the FNAL E665 experiment on exclusive $ρ^{0}$- meson production in deep inelastic scattering in the same range of $Q^{2}$. \\ We discuss the impact of these observations on the CEBAF experimental program. We argue they both are good news, both were anticipated theoretically, and both rule in the correct QCD mechanism of the onset of CT.

nucl-th↗
arXiv1994

Why more hadronlike photons produce less particles on nuclear targets

In hadron-nucleus interactions, the stronger is nuclear shadowing in the total cross section the higher is the multiplicity of secondary hadrons. In deep inelastic scattering, nuclear shadowing at small $x$ is associated with the hadronlike behaviour of photons as contrasted to the pointlike behaviour in the non-shadowing region of large $x$. In this paper we predict smaller mean multiplicity of secondary hadrons, and weaker fragmentation of the target nucleus, in deep inelastic leptoptoproduction on nuclei in the shadowing region of small $x$ as compared to the non-shadowing region of large $x$. This paradoxial conclusion has its origin in nuclear enhancement of the coherent diffraction dissociation of photons. We present numerical predictions for multiproduction in $μXe$ interactions studied by the Fermilab E665 collaboration.

hep-ph↗
arXiv1994

Scanning the BFKL pomeron in elastic production of vector mesons at HERA

Elastic production of vector mesons $γ^{*} N\to V N$ is the pomeron-exchange dominated diffractive reaction with much potential of probing the BFKL pomeron. The BFKL pomeron can conveniently be described in terms of the dipole cross section which is a solution of the generalized BFKL equation. In this paper we discuss, how the energy and $Q^{2}$ dependence of elastic production of vector mesons at HERA will allow scanning the dipole cross section as a function of dipole size $r$. We show that determinaton of the intercept of the BFKL pomeron requires measuring the $ρ^{0}$ and $J/Ψ$ production at $Q^{2} \sim (100-200)GeV^2$ and/or the quasireal photoproduction of the $Υ$. We present predictions for the effective intercept in the kinematic range of the forthcoming HERA experiments, which can shed much light on the nonperturbative component of the pomeron.

hep-ph↗
arXiv1994

Anomalous A-dependence of diffractive leptoproduction of radial excitation $ρ'(2S)$

We predict a strikingly different $A$ and $Q^{2}$ dependence of quasielastic leptoproduction of the $ρ^{0}(1S)$-meson and its radial excitation $ρ'(2S)$ on nuclei. Whereas for the $ρ^{0}$ production nuclear transparency $T_{A}$ decreases monotonically with $A$, for the $ρ'$ nuclear transparency $T_{A}$ can have the counterintuitive nonmonotonic $A$-dependence, having the minimum for light nuclei, and increasing with $A$ for medium and heavy nuclei. Strong enhancement of the $ρ' /ρ^{0}$ cross section ratio makes nuclear targets the $ρ'$-factory. The origin of the anomalous $A$-dependence is in the interplay of color transparency effects with the nodal structure of the $ρ'$ wave function. The predicted effects take place at moderate $Q^{2}\lsim (2-3)$GeV$^{2}$, which can be explored in high statistics experiments at CEBAF.

nucl-th↗
arXiv1994

Splitting the pomeron into two jets: a novel process at HERA

We study a novel property of large rapidity-gap events in deep inelastic scattering at HERA: splitting the pomeron into two jets in the photon-pomeron fusion reaction $γ^{*}\Pom \rightarrow q\bar{q}$. It gives rise to the diffraction dissociation of virtual photons into the back-to-back jets. We find that at large invariant mass $M$ of two jets, $M^{2}\gg Q^{2}$, the transverse momentum of jets comes from the intrinsic transverse momentum of gluons in the pomeron, and the photon-pomeron fusion direcly probes the diffrential gluon structure function of the proton $\partial G(x,q^{2})/\partial \log q^{2}$ at the virtuality $q^{2}\sim k^{2}$. We present estimates for the jet production cross section, which show the process is easily measurable at HERA.

hep-ph↗
arXiv1994

On determination of the large-${1\over x}$ gluon distribution at HERA

We discuss corrections to the Leading-Log$Q^{2}$ relationships between the gluon density $g(x,Q^{2})$ and $F_{L}(x,Q^{2}),\,\partial F_{T}(x,Q^{2}) /\partial \log Q^{2}$ in the HERA range of large ${1\over x}$. We find that the above quantities probe the gluon density $g(x,Q_{T,L}^{2})$ at $Q_{T,L}^{2}=C_{T,L}Q^{2}$, with the $Q^{2}$-rescaling factors $C_{T}\approx 2.2$ and $C_{L}\approx 0.42$. The possibility of treating charm as an active flavour is critically re-examined.

hep-ph↗
arXiv1994

How to measure the intercept of the BFKL pomeron at HERA

Determination of the intercept of the BFKL pomeron is one of the pressing issues in the high energy physics. Earlier we have shown that, at the dipole size $r=r_Δ=(0.1-0.2)$f, the dipole cross section $σ(x,r)$ which is a solution of the generalized BFKL equation, exhibits a precocious asymptotic behavior $σ(x,r)\propto \left({1\over x}\right)^{Δ_{\Pom}}$. In this paper we discuss how measuring $F_{L}(x,Q^{2})$ and $\partial F_{T}(x,Q^{2})/\partial \log Q^{2}$ at $Q^{2}= (10-40)$GeV$^{2}$ and $Q^{2}=(2-10)$GeV$^{2}$, respectively, one can probe $σ(x,r_Δ)$ and directly determine the intercept $Δ_{\Pom}$ of the BFKL pomeron in the HERA experiments.

hep-ph↗
arXiv1994

Leptoproduction of charm revisited

We calculate the energy--momentum distribution of the charmed quarks produced in neutrino reactions on protons, quantifying the importance of mass and current non--conservation effects. We study the strange and charm distributions probed in neutrino interactions in the presently accessible kinematical region. Some ambiguities inherent to the extraction of the parton densities from dimuon data are pointed out.

hep-ph↗
arXiv1994

The BFKL and GLDAP regimes for the perturbative QCD pomeron

The $S$-matrix of diffractive scattering is diagonalized in terms of the colour dipole-dipole cross section. Recently, we have shown that the dipole cross section satisfies the generalized BFKL equation. In this paper we discuss the spectrum and solutions of our generalized BFKL equation with allowance for the finite gluon correlation radius $R_{c}$. The latter is introduced in the gauge invariant manner. We present estimates of the intercept of the pomeron and find the asymptotic form of the dipole cross section. We consider the difference between the BFKL and GLDAP evolutions and conclude that the GLDAP evolution remains a viable description of deep inelastic scattering well beyond the kinematical range of the HERA experiments. We suggest methods of measuring the pomeron intercept in the HERA experiments.

hep-ph↗
arXiv1994

Theoretical Interpretation of the NE18 Experiment on Nuclear Transparency in A(e,e'p) Scattering

The spectral function, measured in $A(e,e'p)$ reactions, is distorted by the final-state interaction of the struck proton with the residual nucleus. This causes a broadening of the observed transverse-momentum distribution which is large even in the $d(e,e'p)$ reaction. We discuss the effects of this $p_{\perp}$-broadening on the nuclear transparency measured in the recent NE18 experiment. Within conventional Glauber theory we can describe the measurements. Transparency effects are thus small in agreement with our earlier predictions.

nucl-th↗
arXiv1994

Deep inelastic scattering at HERA and the BFKL pomeron

We present a BFKL phenomenology of structure functions in the framework of the generalized BFKL equation for the dipole cross section proposed by us recently. We show that the HERA measurements of excitation of charm at $Q^{2} \lsim $10 GeV$^{2}$ allow determination of the intercept of the BFKL pomeron. A comparison with the experimental data suggests a large contribution from the nonperturbative cross section for colour dipoles of large size $r$.

hep-ph↗
arXiv1994

BFKL evolution and universal structure function at very small $x$

The Balitskii-Fadin-Kuraev-Lipatov (BFKL) and the Gribov-Lipatov- Dokshitzer-Altarelli-Parisi (GLDAP) evolution equations for the diffractive deep inelastic scattering at ${1\over x} \gg 1$ are shown to have a common solution in the weak coupling limit: $F_{2}(x,Q^{2})\propto [α_{S}(Q^{2})]^{-γ} \left({1\over x}\right)^{Δ_{\Pom}}$. The exponent $γ$ and the pomeron intercept $Δ_{\Pom}$ are related by $γΔ_{\Pom}={4\over 3}$ for the $N_{f}=3$ active flavors. The existence of this solution implies that there is no real clash between the BFKL and GLDAP descriptions at very small $x$. We present derivation of this solution in the framework of our generalized BFKL equation for the dipole cross section, discuss conditions for the onset of the universal scaling violations and analyse the pattern of transition from the conventional Double-Leading-Logarithm approximation for the GLDAP evolution to the BFKL evolution at large ${1\over x}$.

hep-ph↗
arXiv1994

The spectrum and solutions of the generalized BFKL equation for total cross section

The colour dipole cross section is the principal quantity in the lightcone $s$-channel description of the diffractive scattering. Recently we have shown that the dipole cross section satisfies the generalized BFKL equation. In this paper we discuss properties and solutions of our generalized BFKL equation with allowance for the finite gluon correlation radius $R_{c}$. The latter is introduced in a gauge invariant manner. We present estimates of the intercept of the pomeron and find the asymptotic form of the dipole cross section.

hep-th↗