arXiv subjects
Publications and source records attributed to N. N. Nikolaev.
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.
Recent experimental and theoretical developments in the understanding of high energy diffraction, presented in the working group on diffraction at DIS98 in Brussels, are summarised. A template, giving the definition of the most commonly used kinematical variables in diffraction, which was provided in the working group sessions, is reproduced as a appendix. References to original papers may be found within the individual contributions.
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.
We present our study of the diffraction in charged current DIS. We analyse the perturbatively tractable excitation of heavy quarks, emphasizing the peculiarities of the Regge factorization breaking in excitation of open charm.
We develop the color dipole gBFKL phenomenology of a diffraction cone for photo- and electroproduction $γ^{*}N \to VN$ of heavy vector mesons (charmonium & bottonium) at HERA and in fixed target experiments. We predict a substantial shrinkage of the diffraction cone from the CERN/FNAL to the HERA range of c.m.s. energy $W$. The $Q^{2}$-controlled selectivity to the color dipole size (scanning phenomenon) is shown to lead to a decrease of the diffraction slope with $Q^{2}$ (which is supported by the available experimental data). We predict an approximate flavor independence of the diffraction slope in the scaling variable $Q^{2}+m_{V}^{2}$. For diffractive production of the radially excited $2S$ states ($Ψ',Υ'$) the counterintuitive inequality of diffraction slopes $B(2S) \lsim B(1S)$ is predicted, which defies the common wisdom that diffraction slopes are larger for reactions with larger size particles.
The QCD hardness scale for secondary particles ($h$) production in semi-inclusive deep inelastic scattering (DIS), $e p \to e' X h$, gradually decreases from $Q^{2}$, the photon virtuality which determines the hard scale in the virtual photon (current) fragmentation region to a soft, hadronic, scale in the proton fragmentation region. This suggests similarity of the inclusive spectra of leading protons and neutrons, $h=p,n$, in high energy hadron-proton and virtual photon-proton collisions. We explore this similarity extending to the DIS regime the nonperturbative peripheral mechanisms of inelastic scattering traditionally used in hadronic interactions to explain fast nucleons production. While the production of leading neutrons is known to be exhausted by DIS off charged pions, the production of leading protons by DIS off neutral pions must be supplemented by a substantial contribution from isoscalar reggeon ($f_{0}$) exchange extrapolated down to moderate values of $x_{L}$. We comment on the $x$ and $Q^{2}$ dependence of leading proton production as a probe of a universal pattern of the $x,Q^{2}$ evolution of the nucleon and meson (reggeon) structure functions at small $x$.
In this letter we report the direct perturbative QCD evaluation of twist-4 effects in diffractive DIS. They are large and have a strong impact on the $Q^2$ dependence of diffractive structure functions at large $β$. Based on the AGK rules, we comment on the possible contribution from diffractive higher twists to $\propto {1 \over Q^{2}}$ corrections to proton structure function at small $x$. These corrections to the longitudinal structure function $F_{L}$ may be particularly large.
We compaWe compare the absorptive corrections to the one pion exchange in $pp\to Xn$ and $ep\to e'Xn$ reactions. It is shown that the absorption is stronger in the case of $pp$ collision. The difference in strength of the absorption for the $pp\to Xn$ and $ep\to e'Xn$ reactions breaks the factorization peculiar to the pure one pion exchange. We evaluate the emerging model dependence of extraction of the small-$x$ pion structure function from an analysis of the HERA data on the neutron production at physical values of $t$.
We calculate the Regge trajectories of the subleading BFKL singularities and eigenfunctions for the running BFKL pomeron in the color dipole representation. We obtain a viable BFKL-Regge expansion of the proton structure function $F_{2p}(x,Q^2)$ in terms of several rightmost BFKL singularities. We find large subleading contributions to $F_{2p}(x,Q^2)$ in the HERA kinematical region which explains the lack of a predictive power of GLDAP-extrapolations of $F_{2p}(x,Q^2)$ to a domain of small $x$. We point out the relation of our early finding of the precocious BFKL asymptotics to the nodal structure of subleading BFKL eigenfunctions.
We propose two new sources of a substantial tensor polarization of sea partons in the deuteron: the diffractive nuclear shadowing which depends on the alignment of nucleons in the polarized deuteron and the nuclear excess of pions in the deuteron which is sensitive to the spin state of the deuteron. The corresponding tensor structure function $b_{2}(x,Q^{2})$ rises towards small x and we predict an about one per cent tensor asymmetry $A_{2}(x,Q^{2})=b_{2}(x,Q^{2})/ F_{2d}(x,Q^{2})$ which by almost two orders in magnitude exceeds the effect evaluated earlier in the impulse approximation. The both mechanisms for tensor structure function break the sum rule $\int dx b_{1}(x,Q^{2})=0$ suggested by Close and Kumano. We comment on the impact of tensor polarization on the determination of the vector spin structure function $g_{1d}(x,Q^{2})$ for the deuteron.
The diffractive production of vector mesons in ep interactions at low x is a subject of heated debates. This chapter consists of four contributions written by the original authors and expresses the possible scenarios which are to be investigated experimentally.
We discuss the factorization breaking effects caused by the contribution to large rapidity gap events from DIS on secondary reggeons. Based on the triple-Regge phenomenology of hadronic diffraction dissociation, we present estimates for the flux and structure function of the f reggeon. The kinematical x_Pom-beta correlation is shown to modify substantially the observed x_Pom dependence of the diffractive structure function. The secondary reggeon and x_Pom-beta correlation effects explain the recent H1 finding of the factorization breaking and resolve the apparent contradiction between the preliminary H1 results and predictions from the color dipole gBFKL approach. We suggest further tests of predictions for diffractive DIS from the gBFKL approach.
We discuss the implications of a strong enhancement of diffraction for multiproduction in DIS off nuclear targets. The predicted effects are large and observable at HERA. We present the prediction of a large tensor structure function b_2(x) of the deuteron which does not vanish at small x.
We explore the missing momentum dependence of the CT effects in quasielastic (e,e'p) scattering. We develop the coupled-channel multiple-scattering theory (CCMST) description of final-state interaction including both the coherent and incoherent rescatterings of the ejectile state. We demonstrate that the contribution of the off-diagonal incoherent rescattering does not vanish at low Q^{2} which is a novel correction to the conventional Glauber theory evaluation of nuclear transparency. We comment on the nontrivial impact of this correction on the onset of CT. The sensitivity of the onset of CT to the 3q-nucleon reggeon amplitudes is discussed for the first time. We present numerical results for nuclear transparency as a function of the missing momentum for exclusive (e,e'p) reaction in the kinematical region of Q^{2} \le 40 GeV^{2} and p_{m} \le 250$ MeV/c. Our evaluations show that at Q^{2}\sim 10 CT effects are substantial only in antiparallel kinematics at p_{m,z}\sim -250 MeV/c. The effect is enhanced on light nuclei and could be observed in a high precision experiment.
We develop the Glauber theory description of initial- and final-state interactions (IFSI) in quasielastic A(p,2p) scattering. We study the IFSI-distortion effects both for the inclusive and exclusive conditions. In inclusive reaction the important new effect is an interaction between the two sets of the trajectories which enter the calculation of IFSI-distorted one-body density matrix for inclusive (p,2p) scattering and are connected with incoherent elastic rescatterings of the initial and final protons on spectator nucleons. We demonstrate that IFSI-distortions of the missing momentum distribution are large over the whole range of missing momentum both for inclusive and exclusive reactions and affect in a crucial way the interpretation of the BNL data on (p,2p) scattering. Our numerical results show that in the region of missing momentum $p_{m}\lsim 100-150$ MeV/c the incoherent IFSI increase nuclear transparency by 5-10\%. The incoherent IFSI become dominant at $p_{m}\gsim 200$ MeV/c.
We review the recent progress in the theory of diffractive DIS, focusing on predictions of strong breaking of the Ingelman-Schlein-Regge factorization and the related breaking of the GLDAP evolution for the diffractive structure function.
We develop the color dipole phenomenology of diffractive photo- and electroproduction $γ^{*}\,N\rightarrow V(V')\,N$ of light vector mesons ($V(1S) = ϕ^0, ω^0, ρ^0$) and their radial excitations ($V'(2S) = ϕ', ω', ρ'$). The node of the radial wave function of the $2S$ states in conjunction with the energy dependence of the color dipole cross section is shown to lead to a strikingly different $Q^2$ and $ν$ dependence of diffractive production of the $V(1S)$ and $V'(2S)$ vector mesons. We discuss the restoration of flavor symmetry and universality properties of production of different vector mesons as a function of $Q^{2}+m_{V}^{2}$. The color dipole model predictions for the $ρ^{0}$ and $ϕ^{0}$ production are in good agreement with the experimental data from the EMC, NMC, ZEUS and H1 collaborations. We present the first direct evaluation of the dipole cross section from these data.
We present the color dipole phenomenology of diffractive photo- and electroproduction $γ^{*} N\to V(V') N$ of vector mesons ($V(1S) = ϕ^0, ω^0, ρ^0, J/Ψ, Υ$) and their radial excitations ($V'(2S) = ϕ', ω', ρ', Ψ', Υ'$). The main emphasis is related to light vector mesons. We discuss how the energy dependence of the color dipole cross section in conjuction with the node of the radial wave function of the $2S$ states can lead to an anomalous $Q^2$ and energy dependence of diffractive production of $V'(2S)$ vector mesons. The color dipole model predictions for $V(1S)$ light vector meson production are compared with the experimental data from the EMC, NMC, ZEUS and H1 collaborations.