arXiv subjects
Publications and source records attributed to M. G. Ryskin.
We present an evolution equation which simultaneously sums the leading BFKL and DGLAP logarithms for the integrated gluon distribution in terms of a single variable, namely the emission angle of the gluon. This form of evolution is appropriate for Monte Carlo simulations of events of high energy pp (and p\bar{p}) interactions, particularly where small x events are sampled.
The partonic transverse momentum, k_t distribution plays a crucial role in driving high-energy hadron interactions. If k_t is limited we have old fashioned Regge Theory. If k_t increases with energy the interaction may be described by perturbative QCD. We use BFKL diffusion in ln k_t, supplemented by a stronger absorption of low k_t partons, to estimate the growth of the mean transverse momenta with energy. This growth reveals itself in the distribution of secondaries produced at the collider energies. We present a simple, BFKL-based, model to demonstrate the possible size of the effect. Moreover, we propose a way to evaluate experimentally the shape of the parton transverse momenta distribution by studying the spectra of the (D or B) mesons which contain one heavy quark.
We show how it is possible to rewrite the BFKL equation for the unintegrated gluon distribution, in terms of integrated gluons, similar to that used in DGLAP. We add to our equation the next-to-leading log terms which provide exact energy-momentum conservation and account for the kinematic constraint in real gluon emission. In this way the equation includes the major part of the higher-order corrections to BFKL evolution. We discuss the possibility to obtain a unified BFKL-DGLAP evolution equation relevant to processes at the LHC where both log(1/x) and logQ^2 are large simultaneously.
We present a study of the central exclusive production of light meson pairs, concentrating on the region of lower invariant masses of the central system and/or meson transverse momentum, where perturbative QCD cannot be reliably applied. We describe in detail a phenomenological model, using the tools of Regge theory, that may be applied with some success in this regime, and we present the new, publicly available, Dime Monte Carlo (MC) implementation of this for $ππ$, $KK$ and $ρρ$ production. The MC includes a fully differential treatment of the survival factor, which in general depends on all kinematic variables, as well as allowing for the so far reasonably unconstrained model parameters to be set by the user. We present predictions for the Tevatron and LHC and show how future measurements may further test this Regge--based approach, as well as the soft hadronic model required to calculate the survival factor, in particular in the presence of tagged protons.
We emphasize the role of the x dependence of the quark condensate <|\bar q(0)q(x)|> originated by the zero fermion modes in the instanton vacuum. The effect is not negligible since the inverse size of the instanton 1/ρ~600MeV is comparable with the value of Borel mass M~1GeV. Using as an example the sum rules for the nucleon we show that this "non-locality" may strongly affected the conventional results obtained under the <|\bar q(0)q(x)|>=<|\bar q(0)q(0)|> approximation.
We suggest a somewhat indirect method for estimation of the nonlocal scalar quark condensate. The approach is based on analysis of the polarization operator of nucleon current in instanton medium.
We show how the photon input parton distribution function (PDF) may be calculated with good accuracy, and used in an extended DGLAP global parton analysis in which the photon is treated as an additional point-like parton. The uncertainty of the input photon PDF is relatively small, since the major part of the distribution (which is produced by the coherent emission of the photon from a proton that remains intact) is well known. We present the expected photon PDFs, and compare the predictions with ZEUS data for isolated photon electroproduction at negative rapidities.
Inclusive charged hadron cross sections, $dσ/dη$, and the mean transverse momenta, $ $, are considered within the two component model, which combines the power-like and the exponential terms in $p_T$. The observed dependences of the spectra shape on energy and the event multiplicity qualitatively agree with that expected from the Regge theory with the perturbative QCD pomeron. Finally, the dependences observed are used to make predictions on the mean transverse momenta, $ $ as function of multiplicity at LHC-energies, which are tested on available experimental data.
The cross section for exclusive psi(2S) ultraperipheral production at the LHC is calculated using gluon parametrisations extracted from exclusive J/psi measurements performed at HERA and the LHC. Predictions are given at leading and next-to-leading order for pp centre-of-mass energies of 7, 8 and 14 TeV, assuming the non-relativistic approximation for the psi(2S) wave function.
We study the behaviour of elastic and diffractive proton dissociation cross sections at high energy. First, we describe what would be expected to be observed at the LHC based on conventional Regge theory. We emphasize the tension between these expectations and the recent LHC measurements, and we discuss the possibilty to modify the classic Reggeon Field Theory in a physically-motivated way so as to accommodate the tendencies observed at the LHC. As a result, we show that we are able to achieve a `global' description of the wide variety of high energy elastic and diffractive data that are presently available, particularly from the LHC experiments. The model is based on only one pomeron pole, but includes multi-pomeron interactions and, significantly, includes the transverse momentum dependence of intermediate partons as a function of their rapidity, which provides the rapidity dependence of the multi-pomeron vertices. We give predictions for diffractive observables at LHC, and higher, energies.
We present a `global' description of the wide variety of high energy elastic and diffractive data that are presently available, particularly from the LHC experiments. The model is based on only one pomeron pole, but includes multi-pomeron interactions and, significantly, includes the transverse momentum dependence of intermediate partons as a function of their rapidity, which provides the rapidity dependence of the multi-pomeron vertices. We give predictions for diffractive observables at LHC energies.
We trace the dependence of the nucleon mass and of the effective size of the nucleon on the characteristics of the instanton medium, which are the average instanton size $ρ$ and the distance between the instantons in instanton medium R. We consider several scenario for variation of the values of $ρ$ and R.
We introduce the physical factorisation scheme, which is necessary to describe observables which are `not completely inclusive'. We derive the formulae for NLO DGLAP evolution in this scheme, and also for the `rotation' of the conventional MSbar PDFs into the physical representation. Unlike, the MSbar prescription, where, for example, the gluon PDF at NLO obtains an admixture of the quark-singlet PDF, and vice-versa, the physical approach does not mix parton PDFs of different types. That is, the physical approach retains the precise quantum numbers of each PDF. The NLO corrections to DGLAP evolution in the physical scheme are less than those in the MSbar case, indicating a better convergence of the perturbative series.
We show that the conventional prescription used for DGLAP parton evolution at NLO has an inconsistent treatment of the contribution from the infrared (IR) region. We illustrate the problem by studying the simple example of QED evolution, treating the electron and photon as partons. The deficiency is not present in a physical approach which removes the IR divergency and allows calculation in the normal 4-dimensional space.
We present the results of recent work on the central exclusive production of meson pairs. We concentrate on the case of flavour-singlet pseudoscalar mesons eta, eta' and show that the central exclusive process, as modelled using a recent novel application of the 'hard exclusive' perturbative formalism, is potentially highly sensitive to the size of the gluon content of these states. We also discuss the exclusive production of meson pairs in the lower mass region, and present the first results of the new Dime Monte Carlo.
We show that to correctly describe the effects of the heavy-quark mass, m_h, in DGLAP evolution, it is necessary to work in the so-called `physical' scheme. In this way, we automatically obtain a smooth transition through the heavy-quark thresholds. Moreover, we show that to obtain NLO accuracy, it is sufficient to account for the heavy-quark mass, m_h, just in the LO (one-loop) splitting function. The use of the MS-bar factorisation scheme is not appropriate, since at NLO we deal with a mixture of quarks and gluon (that is, the mass of the heavy parton is not well-defined). The formulae for the explicit m_h dependence of the splitting functions, and for alpha_s, are presented.
We present the results of a recent novel application of the `hard exclusive' perturbative formalism to the process gg --> MMbar at large angles, where M (Mbar) is a light meson (anti-meson). As well as discussing the important theoretical features of the relevant leading-order gg --> qqbar(gg) qqbar(gg) 6-parton amplitudes, we also comment on their phenomenological implications. In particular, we consider the central exclusive production of meson pairs at comparatively large transverse momentum k_t, which is expected to proceed via this mechanism. We show that this leads to various non-trivial predictions for a range of exclusive processes, and that the cross sections for the eta' and eta mesons display significant sensitivity to any valence gg component of the meson wavefunctions.
New, more precise data on diffractive J/psi photoproduction and on exclusive J/psi production in the process pp -> p + J/psi + p at the LHC allow a better constraint on the low-x gluon distribution in the domain of relatively low scales Q^2 ~ 2-6 GeV^2. We account for the `skewed' effect and for the real part of the amplitude, as well as for the absorptive corrections in the case of the exclusive process pp -> p + J/psi + p. The predictions for exclusive J/psi production at \sqrt{s}=8 and 14 TeV and for exclusive Upsilon(1S) production at 7, 8 and 14 TeV are also given. We present results at leading and next-to-leading order.