arXiv subjects

M. G. Ryskin

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

At least 73 records · Page 4Linked to original sources

arXiv2016

Exclusive $J/ψ$ process tamed to probe the low $x$ gluon

We address the question as to whether data for J/ψmesons produced exclusively in the forward direction at the LHC can be used in global parton analyses (based on collinear factorization) to pin down the low x gluon PDF. We show that it may be possible to overcome the problems that (i) the process is described by a skewed or Generalized Parton Distribution (GPD), (ii) it is very sensitive to the choice of factorization scale and (iii) there is bad LO, NLO,... perturbative stability to the predictions. However, we start by briefly explaining how the alternative k_T factorization approach has been used to describe the process.

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arXiv2016

Exclusive J/psi and Upsilon photoproduction and the low x gluon

We discuss the potential to constrain the small-$x$ PDFs using the exclusive production of heavy vector mesons. The calculation of $J/ψ$ and $Υ$ photoproduction at NLO in collinear factorisation is described. The different behaviour of the NLO corrections for $J/ψ$ and $Υ$ is highlighted and we outline what might be expected from the inclusion of these processes in a PDF fit.

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arXiv2016

Sudakov effects in photon-initiated processes

We consider the effect of the Sudakov factor in photon-initiated processes, corresponding to the no branching probability for the initial-state photon. We demonstrate how such a factor follows simply from the solution of the DGLAP equation for the photon PDF, and is therefore included automatically by this. We use this result to argue that the appropriate scale for the QED coupling $α$ associated with an initial-state photon is not the virtuality of the photon, but rather the factorization scale at which the photon PDF is evaluated, and therefore that the use of the on-shell renormalization scheme is not appropriate for such processes. We also discuss exclusive photon-initiated processes, and demonstrate that no explicit Sudakov factor is required in this case.

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arXiv2016

The photon PDF in events with rapidity gaps

We consider photon-initiated events with large rapidity gaps in proton-proton collisions, where one or both protons may break up. We formulate a modified photon PDF that accounts for the specific experimental rapidity gap veto, and demonstrate how the soft survival probability for these gaps may be implemented consistently. Finally, we present some phenomenological results for the two-photon induced production of lepton and $W$ boson pairs.

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arXiv2016

The production of a diphoton resonance via photon-photon fusion

Motivated by the recent LHC observation of an excess of diphoton events around an invariant mass of 750 GeV, we discuss the possibility that this is due to the decay of a new scalar or pseudoscalar resonance dominantly produced via photon-photon fusion. We present a precise calculation of the corresponding photon-photon luminosity in the inclusive and exclusive scenarios, and demonstrate that the theoretical uncertainties associated with these are small. In the inclusive channel, we show how simple cuts on the final state may help to isolate the photon-photon induced cross section from any gluon-gluon or vector boson fusion induced contribution. In the exclusive case, that is where both protons remain intact after the collision, we present a precise cross section evaluation and show how this mode is sensitive to the parity of the object, as well as potential $CP$-violating effects. We also comment on the case of heavy-ion collisions and consider the production of new heavy colourless fermions, which may couple to such a resonance.

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arXiv2016

Multiparticle production: an old-fashioned view

We consider the dynamics of high energy multiparticle production and discuss how the space-time picture of inelastic interaction may reveal itself in identical particles Bose-Einstein correlations.

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arXiv2016

Advantages of exclusive γγproduction to probe high mass systems

We recall that the exclusive production of high mass objects via γγfusion at the LHC is not strongly suppressed in comparison with inclusive γγfusion. Therefore it may be promising to study new objects produced by the γγsubprocess in experiments with exclusive kinematics. We list the main advantages of exclusive experiments. We discuss the special advantage of observing $γγ\to X \to γZ$ exclusive events.

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arXiv2016

Exclusive physics at the LHC with SuperChic 2

We present a range of physics results for central exclusive production processes at the LHC, using the new SuperChic 2 Monte Carlo event generator. This includes significant theoretical improvements and updates, most importantly a fully differential treatment of the soft survival factor, as well as a greater number of generated processes. We provide an overview of the latest theoretical framework, and consider in detail a selection of final states, namely exclusive 2 and 3 jets, photoproduced vector mesons, two-photon initiated muon and $W$ boson pairs and heavy $χ_{c,b}$ quarkonia.

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arXiv2016

Two scales in Bose-Einstein correlations

We argue that the secondaries produced in high energy hadron collisions are emitted by small size sources distributed over a much larger area in impact parameter space occupied by the interaction amplitude. That is, Bose-Einstein correlation of two emitted identical particles should be described by a `two-radii' parametrization ansatz. We discuss the expected energy, charged multiplicity and transverse momentum of the pair (that is, $\sqrt{s},~N_{\rm ch}, k_t$) behaviour of both the small and large size components.

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arXiv2016

Inclusive prompt $χ_{c,b}(1^{++})$ production at the LHC

We study the prompt production of the $χ_c(1^+)$ and $χ_b(1^+)$ mesons at high energies. Unlike $χ(0^+,2^+)$ production, $χ(1^+)$ mesons cannot be created at LO via the fusion of two on-mass-shell gluons, that is $gg\toχ_{c,b}(1^+)$ are not allowed. However, the available experimental data show that the cross sections for $χ_c(1^+)$ and $χ_c(2^+)$ are comparable. We therefore investigate four other $χ(1^+)$ production mechanisms: namely, (i) the standard NLO process $gg\toχ_{c,b}(1^+)+g$, (ii) via gluon virtuality, (iii) via gluon reggeization and, finally, (iv) the possibility to form $χ_{c,b}(1^+)$ by the fusion of three gluons, where one extra gluon comes from another parton cascade, as in the Double Parton Scattering processes.

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arXiv2015

Exclusive $J/ψ$ and $Υ$ photoproduction and the low $x$ gluon

We study exclusive vector meson photoproduction, $γp \to V + p$ with $V=J/ψ$ or $Υ$, at NLO in collinear factorisation, in order to examine what may be learnt about the gluon distribution at very low $x$. We examine the factorisation scale dependence of the predictions. We argue that, using knowledge of the NLO corrections, terms enhanced by a large $\ln(1/ξ)$ can be reabsorbed in the LO part by a choice of the factorisation scale. (In these exclusive processes $ξ$ takes the role of Bjorken-$x$.) Then, the scale dependence coming from the remaining NLO contributions has no $\ln(1/ξ)$ enhancements. As a result, we find that predictions for the amplitude of $Υ$ production are stable to within about $\pm 15\%$. This will allow data for the exclusive process $p p \to pΥp$ at the LHC, particularly from LHCb, to be included in global parton analyses to constrain the gluon PDF down to $x\sim 10^{-5}$. Moreover, the study of exclusive $J/ψ$ photoproduction indicates that the gluon density found in the recent global PDF analyses is too small at low $x$ and low scales.

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arXiv2015

Nucleon QCD sum rules in instanton medium

We try to find the grounds for the standard nucleon QCD sum rules, based on more detailed description of QCD vacuum. We calculate the polarization operator of the nucleon current in the instanton medium. The medium (QCD vacuum) is assumed to be a composition of the small-size instantons and some long-wave gluon fluctuations. We solve the corresponding QCD sum rules equations and demonstrate that there is a solution with the value of the nucleon mass close to the physical one if the fraction of the small size instantons contribution w_s \approx 2/3.

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arXiv2015

Bose-Einstein correlation to measure the size of event of different types

Bose-Einstein correlations of identical hadrons produced in high- energy pp collisions at the LHC is a good instrument to probe the size of the domain which emits these hadrons in different classes of events. This provides an additional information on the dynamics of multiparticle production. In particular this way we may measure the radius of the colour tube/string which create the secondary pions.

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arXiv2015

Central exclusive production within the Durham model: a review

We review recent results within the Durham model of central exclusive production. We discuss the theoretical aspects of this approach and consider the phenomenological implications in a variety of processes, comparing to existing collider data and addressing the possibilities for the future.

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arXiv2015

Exclusive production of double $J/ψ$ mesons in hadronic collisions

We present the first calculation of exclusive double $J/ψ$ production in hadronic collisions. We analyse in detail the form of the Born-level $gg \to J/ψJ/ψ$ amplitudes within the non-relativistic quarkonium approximation and discuss the implications of this for the central exclusive production channel, within the `Durham' perturbative model. In addition we show that this direct single parton scattering contribution is expected to be strongly dominant in the exclusive case. We present predictions for the LHC and show that the expected cross sections are in reasonable agreement with the LHCb Run-I measurement of exclusive double $J/ψ$ production, with the measured invariant mass distribution described well by the theory. Motivated by this encouraging result we present predictions for observables that may be measured in LHC Run-II, and estimate the size of the expected cross sections in the $ψ(2S)$ and $χ_{c}$ cases.

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arXiv2014

t dependence of the slope of the high energy elastic pp cross section

We consider the main factors which cause the variation of the value of the local slope of the elastic pp cross section B(t)=d[\ln(dσ_el(pp)/dt]/dt with t. Namely, we discuss the role of the pion-loop insertion in the pomeron trajectory, the t-dependence of the pomeron-nucleon coupling and the role of the eikonalization of the proton-proton amplitude in both the one- and two-channel eikonal models.

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arXiv2014

On the loop approximation in nucleon QCD sum rules

There was a general believe that the nucleon QCD sum rules which include only the quark loops and thus contain only the condensates of dimension d=3 and d=4 have only a trivial solution. We demonstrate that there is also a nontrivial solution. We show that it can be treated as the lowest order approximation to the solution which includes the higher terms of the Operator Product Expansion. Inclusion of the radiative corrections improves the convergence of the series.

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arXiv2014

Lessons from LHC elastic and diffractive data

In the light of LHC data, we discuss the global description of all high energy elastic and diffractive data, using a one-pomeron model, but including multi-pomeron interactions. The LHC data indicate the need of a $k_t(s)$ behaviour, where $k_t$ is the gluon transverse momentum along the partonic ladder structure which describes the pomeron. We also discuss tensions in the data, as well as the $t$ dependence of the slope of $dσ_{el}/dt$ in the small $t$ domain.

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