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M. G. Ryskin

Publications and source records attributed to M. G. Ryskin.

At least 271 records · Page 15Linked to original sources
arXiv1999

Pomeron in diffractive processes $γ^*(Q^2)p\toρ^0 p$ and $γ^*(Q^2)p\toγ^*(Q^2) p$ at large Q^2: the onset of pQCD

We study the reactions $γ^*(Q^2)p\toρ^0 p$ and $γ^*(Q^2)p\toγ^*(Q^2) p$ at large Q^2 and $W^2/Q^2$ and small momentum transfer, $κ^2_\perp$, to the nucleon where the pomeron exchange dominates. At large Q^2 the virtual photon selects a hard $q\bar q$ pair, thus selecting the hard pomeron component (the BFKL pomeron). The amplitudes for both transverse and longitudinal polarizations of the initial photon and outgoing $ρ$-meson (photon) are calculated in the framework of the BFKL pomeron exchange. Our calculations show that one cannot expect the early onset of the pure perturbative regime in the discussed diffractive processes: the small interquark distances, $ρ_{q\bar q} <0.2$ fm, start to dominate not earlier than at $Q^2 \simeq 100 GeV^2, W^2/Q^2 \simeq 10^7$ in $γ^*(Q^2)p\toρ^0 p$ and $Q^2 \simeq 50 GeV^2, W^2/Q^2 \simeq 10^6$ in $γ^*(Q^2)p\toγ^*(Q^2) p$.

hep-ph↗
arXiv1999

Upsilon photoproduction at HERA compared to estimates of perturbative QCD

We estimate the cross section for gamma p -> Upsilon p by two independent methods. First, by studying the corrections to the naive leading-order QCD formula and, second, by using parton-hadron duality. The estimates are in good agreement with each other and with the recent measurements of the cross section at HERA.

hep-ph↗
arXiv1998

F_2 at low Q^2

We analyse the data for the proton structure function $F_2$ over the entire $Q^2$ domain, including especially low $Q^2$, in terms of perturbative and non-perturbative QCD contributions. The small distance configurations are given by perturbative QCD, while the large distance contributions are given by the vector dominance model and, for the higher mass $q \overline{q}$ states, by the additive quark approach.

hep-ph↗
arXiv1998

Self-consistent treatment of the quark condensate and hadrons in nuclear matter

We calculate the contribution of pions to the $\bar qq$-expectation value $κ(ρ)= $ in symmetric nuclear matter. We employ exact pion propagator renormalized by nucleon-hole and isobar-hole excitations. Conventional straightforward calculation leads to the "pion condensation" at unrealistically small values of densities, causing even earlier restoration of chiral symmetry. This requires a self-consistent approach, consisting in using the models, which include direct dependence of in-medium mass values on $κ(ρ)$, e.g. the Nambu-Jona-Lasinio-model. We show, that in the self-consistent approach the $ρ$-dependence of the condensate is described by a smooth curve. The "pion condensate " point is removed to much higher values of density. The chiral restoration does not take place at least while $ρ<2.8ρ_0$ with $ρ_0$ being the saturation value. Validity of our approach is limited by possible accumulation of heavier baryons (delta isobars) in the ground state of nuclear matter. For the value of effective nucleon mass at the saturation density we found $m^*(ρ_0)=0.6m$, consistent with nowadays results of other authors.

hep-ph↗
arXiv1998

Phenomenological description of the gamma* p cross section at low Q2

Low Q2 photon-proton cross sections are analysed using a simple, QCD-motivated parametrisation $σ_{γ^\star p}\propto 1/(Q^2+Q_0^2)$, which gives a good description of the data. The Q2 dependence of the gamma* p cross section is discussed in terms of the partonic transverse momenta of the hadronic state the photon fluctuates into.

hep-ph↗
arXiv1998

The description of $F_2$ at low $Q^2$

We analyse the data for the proton structure function $F_2$ over the entire $Q^2$ domain, including especially low $Q^2$, in terms of perturbative and non-perturbative QCD contributions. The small distance configurations are given by perturbative QCD, while the large distance contributions are given by the vector dominance model and, for the higher mass $q \bar{q}$ states, by the additive quark approach. The interference between states of different $q\bar{q}$ mass (in the perturbative contribution) is found to play a crucial role in obtaining an excellent description of the data throughout the whole $Q^2$ region, including photoproduction.

hep-ph↗
arXiv1998

Higher twists in deep inelastic scattering

We perform an exploratory study of higher twist contributions to deep inelastic scattering. We estimate the size of two major sources of higher twist, namely absorptive corrections and the vector meson dominance (VMD) contribution. We find that they give a sizeable higher twist component of F_2. For example at x = 0.01 it is about 8% (17%) at Q^2 = 10 GeV^2 (4 GeV^2), reaching up to 27% at x = 10^{-4} and Q^2 = 4 GeV^2. At the smaller x value the largest contribution comes from absorptive corrections, while at the larger values of x the VMD term dominates.

hep-ph↗
arXiv1997

Elastic ρ' and ϕMeson Photo- and Electroproduction with Non-Resonant Background

The backgrounds due to the direct diffractive dissociation of the photon into the $π^+π^-$ as well as $K^+K^-$ pairs to the "elastic" diffractive $ρ'$ and $ϕ$ mesons production in electron-proton collisions are calculated. It is shown that the role of background and background-resonant interference contributions is very important in experimental distribution of $M_{π^+ π^-}$ in the $ρ'$ mass region. The amplitude for the background process is proportional to the $π$-meson - proton or K-meson - proton cross sections. Therefore, describing the HERA data, one can estimate $σ(πp)$ and $σ(Kp)$ at energies higher than the region of direct measurements.

hep-ph↗
arXiv1997

QCD sum rules as a tool for investigation of the baryon properties at finite densities

We analyse the calculations of deep inelastic structure functions of free nucleons by QCD sum rules method,carried out by others.We present our results of calculation of the distribution of valence quarks in nucleon placed into the nuclear matter.We show that the change in distribution has typical EMC shape.We discuss possible application of the method to investigation of other aspects of deep-inelastic processes.We analyse also the limits of possibilities of the approach.

hep-ph↗
arXiv1997

The effect of off-diagonal parton distributions in diffractive vector meson electroproduction

We present a simple physical description of the off-diagonal gluon distribution g (x,x^\prime,Q^2) and splitting function P_{gg}(x,x^\prime) which enter the amplitude for diffractive vector meson production. We study the off-diagonal effects both on the evolution in Q^2 and on the input distribution. We predict the ratio $R$ of the off-diagonal to the diagonal gluon density, x^\prime g(x,x^\prime,Q^2)/xg(x, Q^2), as a function of the kinematic variables.

hep-ph↗
arXiv1997

Vector Meson Production at a Polarized HERA Collider

A successful interpretation of the cross sections for elastic and inelastic vector meson production can be made if one assumes the underlying dynamics to be governed by hard partonic subprocesses. Extending these partonic approaches to longitudinally polarized electron-proton collisions, we estimate the expected production asymmetries at the HERA collider. The anticipated statistical errors on these asymmetries mostly turn out to be larger than the asymmetries themselves, such that an experimental observation of these asymmetries at HERA looks not feasible.

hep-ph↗
arXiv1997

Charm in Deep Inelastic Scattering

We outline the existing descriptions of the charm component of the deep inelastic proton structure function F2. We discuss recent approaches to include charm mass effects in the parton evolution equations and the coefficient functions.

hep-ph↗
arXiv1997

Diffractive rho-meson Leptoproduction on Polarized Nucleon as a Way to Measure the Polarized Gluon Distribution

The amplitude of diffractive $ρ$ (and open quark) leptoproduction on a polarized target is calculated in the leading log approximation of pQCD using the hadron-parton duality hypothesis. The spin-spin asymmetry is expressed in terms of the spin dependent gluon and quark structure functions in the small $x$ region. Therefore the $γ^*+p\to ρ+p$ reaction provides a promising tool to study the spin dependent gluon distribution $ΔG(x,q^2)$.

hep-ph↗
arXiv1997

Dijet hadroproduction with rapidity gaps and QCD double logarithmic effects

We show that the hadroproduction of a pair of jets with large transverse energy in the central region bounded by rapidity gaps is an ideal process to see important double logarithmic QCD suppression effects. We compute the cross sections for both exclusive and inclusive double-diffractive dijet production at Tevatron and LHC energies for a range of rapidity configurations.

hep-ph↗
arXiv1997

$σ_L/σ_T$ in the $ρ^0$ - meson diffractive electroproproduction

The background due to the direct diffractive dissociation of the photon into the $π^+π^-$-pair to the "elastic" diffractive $ρ^0$-meson production in electron-proton collisions is calculated. At large $Q^2$ the interference between resonant and non-resonant $π^+ π^-$ production changes the $σ^L/σ^T$ ratio with the mass of the $2π$ (i.e. $ρ^0$-meson) state.

hep-ph↗
arXiv1997

The rapidity gap Higgs signal at LHC

We compare the WW and pomeron-pomeron fusion mechanisms for the double-diffractive production of a Higgs boson. We determine the suppression of the 'rapidity gap' pomeron-pomeron fusion events due to QCD radiative effects. In particular we use leading log techniques to estimate the cross sections for both exclusive and inclusive double-diffractive Higgs production at LHC energies. The same approach can be applied to the double-diffractive central production of large E_T dijets. These two processes provide one of the most justified applications of various aspects of leading logarithm QCD techniques.

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