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M. G. Ryskin

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

285 records · Page 16Linked to original sources
arXiv1997

Consistent Treatment of Charm Evolution in Deep Inelastic Scattering

We present a formulation which allows heavy quark (c, b, ...) mass effects to be explicitly incorporated in both the coefficient functions and the splitting functions in the parton evolution equations. We obtain a consistent procedure for evolution through the threshold regions for c anti-c and b anti-b production in deep inelastic scattering, which allows the prediction of the charm and bottom quark densities. We use the new formulation to perform a next-to-leading order global parton analysis of deep inelastic and related hard scattering data. We find that the optimum fit has alpha_S(M_Z^2) = 0.118. We give predictions for the charm components of the proton structure functions F_2 and F_L as functions of x and Q^2 and, in particular, find that F_2^c is in good agreement with the existing measurements. We examine the Q^2 range of validity of the photon-gluon fusion model for c anti-c electroproduction. We emphasize the value of a precision measurement of the charm component F_2^c at HERA.

hep-ph↗
arXiv1997

Role of the non-resonant background in the $ρ^0$-meson diffractive electro- and photoproproduction

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. The amplitude for the background process $γp \to π^+ π^- p$ is proportional to the $π$-meson - proton cross section. Therefore, describing the HERA data, we can estimate $σ(πp)$ at energy $s_{πp}\sim (2 - 3) \times 10^{3}$ GeV$^2$ that is considerably higher the existing data. At large $Q^2$ the interference between resonant and non-resonant $π^+ π^-$ production leads also to the more slow increase of the $σ^L/σ^T$ ratio with the mass of the $2π$ (i.e. $ρ^0$-meson) state.

hep-ph↗
arXiv1997

Diffractive electroproduction of rho meson excitations

We use perturbative QCD and parton-hadron duality to estimate the cross sections for the diffractive electroproduction of rhoprime(1-) and rho(3-) resonances which occur in the 1.3-1.8 GeV mass interval. We present the cross sections and the ratios sigma_L/sigma_T as a function of Q^2. We compare the predictions with those for the diffractive electroproduction of rho mesons. We show how such diffractive electroproduction measurements at HERA can probe features of the perturbative QCD 'Pomeron'.

hep-ph↗
arXiv1996

The Space-Time Picture of the Wee Partons and the AGK Cutting Rules in Perturbative QCD

We discuss the space-time picture of scattering amplitudes with single and double ladder exchange in perturbative QCD. Particular emphasis is given to the Abramovsky Gribov Kanchelli (AGK) rules which describe the relative contributions of the diffractive dissociation and processes with other multiplicities to the elastic scattering amplitude. Inside the Pomeron Pomeron interaction vertices we find a new matrix which describes the transition from one s-cut to another and which goes beyond the original the AGK rules.

hep-ph↗
arXiv1996

The QCD description of diffractive rho meson electroproduction

We critically review the QCD predictions for the cross sections sigma_L and sigma_T for diffractive rho meson electroproduction in longitudinally and transversely polarised states in the HERA energy region. We show that both perturbative and non-perturbative approaches which involve convolution with the rho meson wave function predict values of sigma_T which fall-off too quickly with increasing Q^2, in comparison with the data. We present a perturbative QCD model based on the open production of light qqbar pairs and parton-hadron duality, which describes all features of the data for rho electroproduction at high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of sigma_L/sigma_T. We find that precise measurements of the latter can give valuable information on the Q^2 behaviour of the gluon distribution at small x.

hep-ph↗
arXiv1996

Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections $σ^{L,T}$ for the diffractive production of open charm $(c\overline{c})$ from longitudinally and transversely polarised photons (of virtuality $Q^2$) incident at high energy $(\sqrt{s})$ on a proton target. We study both the $Q^2$ and $M^2$ dependence of the cross sections, where $M$ is the invariant mass of the $c\overline{c}$ pair. Surprisingly, the result for $σ^T$, as well as for $σ^L$, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the {\it square} of the gluon density $g (x, K^2)$, and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for $x = (Q^2 + M^2)/s$ and scale $K^2 = (m_c^2 + \langle k_T^2 \rangle) (1 + Q^2/M^2)$ where the average quark transverse momentum squared $\langle k_T^2 \rangle \sim m_c^2$. As compared to diffractive $J/ψ$ production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the $J/ψ$ wave function. We estimate the fraction of diffractive events that arise from $c\overline{c}$ production.

hep-ph↗
arXiv1996

Flavor Singlet Contribution to the Structure Function $g_1$ at Small-x

The singlet contribution to the $g_1(x,Q^2)$ structure function is calculated in the double-logarithmic approximation of perturbative QCD in the region $x \ll 1$. Double logarithmic contributions of the type $(α_s \ln ^2 (1/x))^k$ which are not included in the GLAP evolution equations are shown to give a power-like rise at small-x which is much stronger than the extrapolation of the GLAP expressions. The dominant contribution is due to the gluons which, in contrast to the unpolarized case, mix with the fermions also in the region $x \ll 1$. The two main reasons why the small-x behavior of the double logarithmic approximation is so much stronger than the usual GLAP evolution are: the larger kinematical region of integration (in particular, no ordering in transverse momentum) and the contributions from non-ladder diagrams.

hep-ph↗
arXiv1996

Diffractive $J/ψ$ photoproduction as a probe of the gluon density

We use perturbative QCD, beyond the leading $\ln Q^2$ approximation, to show how measurements of diffractive $J/ψ$ production at HERA can provide a sensitive probe of the gluon density of the proton at small values of Bjorken $x$. We estimate both the effect of the relativistic motion of the $c$ and $\overline{c}$ within the $J/ψ$ and of the rescattering of the $c\overline{c}$ quark pair on the proton. We find that the available data for diffractive $J/ψ$ photoproduction can discriminate between the gluon distributions of the most recent sets of partons.

hep-ph↗
arXiv1995

Heavy Quark Production in Parton Model and in QCD

The cross sections of heavy quark production in $pp$ collision and for their photo- and electroproduction are calculated in the framework of QCD. The virtual nature of the interacting gluons as well as their transverse motion and different polarizations are taken into account. The obtained cross sections exhibit more rapid growth with the initial energy than the parton model predictions and the $p_T$ distributions of produced heavy quarks are more smooth.

hep-ph↗
arXiv1995

NON-SINGLET STRUCTURE FUNCTIONS AT SMALL X

The small x behaviour of the non-singlet structure function is studied within the double logarithmic approximation (DLA) of perturbative QCD. Since there is neither $k_T$ nor $θ$ ordering in the ladder Feynman graphs, the predicted non-singlet quark densities for the HERA kinematical range ($x\sim 10^{-3}$) exceed the values calculated from the small-$x$ approximation of the conventional Altarelli-Parisi evolution by a factor up to ten.

hep-ph↗
arXiv1995

Diffractive Vector Meson Production at Large Momentum Transfer

The diffractive process $γp\to V + X$ (where $V$ is a vector meson and $X$ results from the dissociation of the proton) is studied. In particular, we consider the region of large momentum transfer (i.e. $|t|\ggΛ_{QCD}^2$) and large centre-of-mass (CM) energy, $s$. The asymptotic $(s \to \infty, s/|t|\gg 1$) behaviour is derived from the BFKL equation and compared to that which is obtained in the Born approximation (two-gluon exchange). We also calculate the corrections to the Born graphs by iterating the BFKL kernel numerically. Improved convergence of the BFKL series is found by summing the logarithms which occur when an exchanged gluon goes nearly on shell. Importantly, we find evidence that the asymptotic solution to the BFKL equation is inappropriate over most of the HERA range and we provide more realistic predictions for the cross section. The predicted cross section is not too small and can be measured at HERA, up to momentum transfers $|t| \sim 10$GeV$^2$.

hep-ph↗
arXiv1995

How does the BFKL-Pomeron couple to quarks ?

We investigate the coupling of the BFKL-Pomeron to quarks and to colorless states. Special emphasis is given to the effective quark-quark scattering amplitude of Mueller and Tang.

hep-ph↗
arXiv1993

On the Parton Interpretation of Quark Fragmentation into Hadrons with Different Spins

We calculate the spin density matrix of the hadron $h$ created via quark fragmentation in the process $e^-e^+ \to q\bar q \to h + X$. In the case of $h=Λ$ the experimental data could possibly elucidate the problem of $s$-quark contribution to the spin of the $Λ$-hyperon, to be compared with the case of the proton (``spin crisis''). Generally we find that for hadrons with spin 1/2 the parton description, using only probabilities, works much better than for vector particles, since in the former case the non diagonal matrix elements of the hadron spin density matrix are suppressed by a small factor $p_T/(z\sqrt s)$, where $p_T$ is the hadron transverse momentum inside the jet.

hep-ph↗