arXiv subjects

M. G. Ryskin

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

At least 55 records · Page 3Linked to original sources

arXiv2018

Elastic and diffractive scattering at the LHC

Inspired by the new TOTEM data on elastic $pp$ scattering at 13 TeV, we study the possibility to describe all the diffractive collider data ($σ_{\rm tot}, dσ_{\rm el}/dt$, $ρ\equiv$Re$A$/Im$A$, \sd) in a wide interval of energy (0.0625 to 13 TeV) in the framework of a two-channel eikonal model. We show that a satisfactory description can be achieved without an odd-signature (Odderon) exchange contribution. We consider the possible role of the QCD Odderon which may improve the description of $ρ$ and discuss the importance of the odd-signature term if the amplitude were to exceed the black disc limit.

hep-ph↗

arXiv2018

Open charm production and low x gluons

We compare the rapidity, $y$, and the beam energy, $\sqrt{s}$, behaviours of the cross section of the data for $D$ meson production in the forward direction that were measured by the LHCb collaboration. We describe the observed cross sections using NLO perturbative QCD, and choose the optimal factorization scale for the LO contribution which provides the resummation of the large double logarithms. We emphasize the inconsistency observed in the $y$ and $\sqrt{s}$ behaviours of the $D$ meson cross sections. The $y$ behaviour indicates a very {\it flat} $x$ dependence of the gluon PDF in the unexplored low $x$ region around $x\sim 10^{-5}$. However, to describe the $\sqrt{s}$ dependence of the data we need a steeper gluon PDF with decreasing $x$. Moreover, an even steeper behaviour is needed to provide an extrapolation which matches on to the well known gluons found in the global PDF analyses for $x\sim 10^{-3}$. The possible role of non-perturbative effects is briefly discussed.

hep-ph↗

arXiv2018

Black disk, maximal Odderon and unitarity

We argue that the so-called maximal Odderon contribution breaks the `black disk' behaviour of the asymptotic amplitude, since the cross section of the events with Large Rapidity Gaps grows faster than the total cross section. That is the `maximal Odderon' is not consistent with the unitarity.

hep-ph↗

arXiv2018

Multiple interactions and rapidity gap survival

Observations of rare processes containing large rapidity gaps at high energy colliders may be exceptionally informative. However the cross sections of these events are small in comparison with that for the inclusive processes since there is a large probability that the gaps may be filled by secondary particles arising from additional soft interactions or from gluon radiation. Here we review the calculations of the probability that the gaps survive population by particles from these effects for a wide range of different processes.

hep-ph↗

arXiv2018

Elastic proton-proton scattering at 13 TeV

The predictions of a model which was tuned in 2013 to describe the elastic and diffractive $pp$- and/or $p\bar p$-data at collider energies up to 7 TeV are compared with the new 13 TeV TOTEM results. The possibility of an odd-signature Odderon exchange contribution is discussed.

hep-ph↗

arXiv2017

Many body effects in nuclear matter QCD sum rules

We calculate the single-particle nucleon characteristics in symmetric nuclear matter with inclusion of the 3N and 4N interactions. We calculated the contribution of the 3N interactions earlier, now we add that of the 4N ones. The contribution of the 4N forces to nucleon self energies is expressed in terms of the nonlocal scalar condensate (d=3) and of the configurations of the vector-scalar and the scalar-scalar quark condensates (d=6) in which two diquark operators act on two different nucleons of the matter.These four-quark condensates are obtained in the model-independent way. The density dependence of the nucleon effective mass, of the vector self energy and of the single-particle potential energy are obtained. We traced the dependence of the nucleon characteristics on the actual value of the pion-nucleon sigma term. We obtained also the nucleon characteristics in terms of the quasifree nucleons, with the noninteracting nucleons surrounded by their pion clouds as the starting point. This approach leads to strict hierarchy of the many body forces.

nucl-th↗

arXiv2017

Photon-Photon Collisions with SuperChic

The SuperChic Monte Carlo generator provides a common platform for QCD-mediated, photoproduction and photon-induced Central Exclusive Production (CEP), with a fully differential treatment of soft survival effects. In these proceedings we summarise the processes generated, before discussing in more detail those due to photon-photon collisions, paying special attention to the correct treatment of the survival factor. We briefly consider the light-by-light scattering process as an example, before discussing planned extensions and refinements for the generator.

hep-ph↗

arXiv2017

Total $π^+ p$ cross section extracted from the leading neutron spectra at the LHC

We use the very forward neutron energy spectra measured by the Large Hadron Collider forward (LHCf) experiment at 7 TeV to extract the $π^+ p$ total cross section at centre-of-mass energies in the range 2.3$-$3.5 TeV. To do this we have to first isolate the $π$-exchange pole in forward neutron production in $pp$ collisions, by evaluating other possible contributions. Namely, those from $ρ$ and $a_2$ exchange, from both eikonal and enhanced screening effects, from migration, from neutron production by $Δ$-isobar decay and from diffractive nucleon excitations. We discuss the possible theoretical uncertainties due to the fact that the data do not exactly reach the $π$ pole. We choose the kinematical domain where the pion contribution dominates and demonstrate the role of the different corrections which could affect the final result.

hep-ph↗

arXiv2017

Low x gluons determined by open charm production

We use the LHCb data on the forward production of $D$ mesons at 7, 13 and 5 TeV to make a direct determination of the gluon distribution, $xg$, at NLO in the $10^{-5} \lesssim x \lesssim 10^{-4}$ region. We first use a simple parametrization of the gluon density in each of the four transverse momentum intervals of the detected $D$ mesons. Second, we use a double log parametrization to make a combined fit to all the LHCb $D$ meson data. The values found for $xg$ in the above $x$ domain are of the similar magnitude (or a bit larger) than the central values of extrapolations of the gluons obtained by the global PDF analyses into this small $x$ region. However, in contrast, we find $xg$ has a weak dependence on $x$.

hep-ph↗

arXiv2017

$χ_c$ decays and the gluon content of the $η'$, $η$ mesons

The large mass of the $η'$ meson indicates that a sizeable gluon component is present in the meson wave function. However, the $χ_{c0}$ and $χ_{c2}$ decays to $η'$ mesons, which proceed via a purely gluonic intermediate state and we would therefore naïvely expect to be enhanced by such a component, are in fact relatively suppressed. We argue that this apparent contradiction may be resolved by a proper treatment of interference effects in the decay. In particular, by accounting for the destructive interference between the quark and gluon components of the $η'$ distribution function, in combination with a model for strange quark mass effects, we demonstrate that the observed $χ_{c(0,2)}\toη(')η(')$ branching ratios can be reproduced for a reasonable gluon component of the $η'$, $η$ mesons.

hep-ph↗

arXiv2017

Optimal choice of factorization scales for the description of jet production at the LHC

To obtain more precise parton distribution functions (PDFs) it is important to include data on inclusive high transverse energy jet production in the global parton analyses. These data have high statistics and the NNLO terms in the perturbative QCD (pQCD) description are now available. Our aim is to reduce the uncertainty in the comparison of the jet data with pQCD. To ensure the best convergence of the pQCD series it is important to choose the appropriate factorization scales, $μ_F$. We show that it is possible to absorb and resum in the incoming PDFs and fragmentation function ($D$) an essential part of the higher $α_s$ order corrections by determining the `optimal' values of $μ_F$. We emphasize that it is necessary to optimize different factorization scales for the various factors in the cross section: indeed, both of the PDFs, and also the fragmentation function, have their own optimal scale. We show how the values of these scales can be calculated for the LO (NLO) part of the pQCD prediction of the cross section based on the theoretically known NLO (NNLO) corrections. After these scales are fixed at their optimal values, the residual factorization scale dependence is much reduced.

hep-ph↗

arXiv2017

Scale dependence of open $c\bar{c}$ and $b\bar{b}$ production in the low $x$ region

The `optimal' factorization scale $μ_0$ is calculated for open heavy quark production. We find that the optimal value is $μ_F=μ_0\simeq 0.85\sqrt{p^2_T+m_Q^2} $; a choice which allows us to resum the double-logarithmic, $(α_s\lnμ^2_F\ln(1/x))^n$ corrections (enhanced at LHC energies by large values of $\ln(1/x)$) and to move them into the incoming parton distributions, PDF$(x,μ_0^2)$. Besides this result for the single inclusive cross section (corresponding to an observed heavy quark of transverse momentum $p_T$), we also determined the scale for processes where the acoplanarity can be measured; that is, events where the azimuthal angle between the quark and the antiquark may be determined experimentally. Moreover, we discuss the important role played by the $2\to 2$ subprocesses, $gg\to Q\bar{Q}$ at NLO and higher orders. In summary, we achieve a better stability of the QCD calculations, so that the data on $c\bar{c}$ and $b\bar{b}$ production can be used to further constrain the gluons in the small $x$, relatively low scale, domain, where the uncertainties of the global analyses are large at present.

hep-ph↗

arXiv2017

Can invisible objects be `seen' via forward proton detectors at the LHC?

We discuss the possibility of identifying invisible objects (e.g. dark matter) via missing mass in Central Exclusive Processes; that is, in events where only forward protons are detected at the LHC. We show that the signal must have a cross section greater than 0.25 fb in order that it can be detected. We estimate the huge background caused by soft proton dissociation and evaluate the requirements of the detector `veto' system needed to sufficiently suppress these events. In addition, we discuss the related process leading to the possible identification of the particles from the so-called `compressed-mass Beyond the Standard Model (BSM)' scenarios. In this case the background is not so severe.

hep-ph↗

arXiv2017

Exclusive $J/ψ$ production at the LHC in the $k_T$ factorization approach

The recent LHCb data for exclusive $J/ψ$ peripheral production at 13 TeV motivate an improved `NLO' analysis to estimate the gluon distribution at low $x$ in which we re-calculate the rapidity gap survival factors and use a more precise expression for the photon flux. We comment on the difference between the $k_T$ and collinear factorization approaches.

hep-ph↗

arXiv2016

Three nucleon forces in nuclear matter in the QCD sum rules

We calculate the single-particle nucleon characteristics in symmetric nuclear matter with inclusion of the 3N interactions. The contribution of the 3N forces to the nucleon self energies are expressed in terms of the nonlocal scalar condensate (d=3) and of the configuration of the two four-quark condensates (d=6) in which two diquark operators act on two different nucleons of the matter. The most important part of the contribution of the four-quark condensate is calculated in a model-independent way. We employed a relativistic quark model of nucleon for calculation of the rest part. The density dependence of the vector and scalar nucleon self energies and of the single-particle potential energy are obtained.

nucl-th↗

arXiv2016

Jet activity as a probe of high-mass resonance production

We explore the method of using the measured jet activity associated with a high mass resonance state to determine the corresponding production modes. To demonstrate the potential of the approach, we consider the case of a resonance of mass $M_R$ decaying to a diphoton final state. We perform a Monte Carlo study, considering three mass points $M_R=0.75,\,1.5\,,2.5$ TeV, and show that the $γγ$, $WW$, $gg$ and light and heavy $q\overline{q}$ initiated cases lead to distinct predictions for the jet multiplicity distributions. As an example, we apply this result to the ATLAS search for resonances in diphoton events, using the 2015 data set of $3.2\,{\rm fb}^{-1}$ at $\sqrt{s}=13$ TeV. Taking the spin-0 selection, we demonstrate that a dominantly $gg$-initiated signal hypothesis is mildly disfavoured, while the $γγ$ and light quark cases give good descriptions within the limited statistics, and a dominantly $WW$-initiated hypothesis is found to be in strong tension with the data. We also comment on the $b\overline{b}$ initial state, which can already be constrained by the measured $b$-jet multiplicity. Finally, we present expected exclusion limits with integrated luminosity, and demonstrate that with just a few 10's of ${\rm fb}^{-1}$ we can expect to constrain the production modes of such a resonance.

hep-ph↗

arXiv2016

The exclusive $J/ψ$ process at the LHC tamed to probe the low $x$ gluon

The perturbative QCD expansion for $J/ψ$ photoproduction appears to be unstable: the NLO correction is large (and of opposite sign) to the LO contribution. Moreover, the predictions are very sensitive to the choice of factorization and renormalization scales. Here we show that perturbative stability is greatly improved by imposing a $`Q_0$ cut' on the NLO coefficient functions; a cut which is required to avoid double counting. $Q_0$ is the input scale used in the parton DGLAP evolution. This result opens the possibility of high precision exclusive $J/ψ$ data in the forward direction at the LHC being able to determine the low $x$ gluon distribution at low scales.

hep-ph↗

arXiv2016

Photon-initiated processes at high mass

We consider the influence of photon-initiated processes on high-mass particle production. We discuss in detail the photon PDF at relatively high parton $x$, relevant to such processes, and evaluate its uncertainties. In particular we show that, as the dominant contribution to the input photon distribution is due to coherent photon emission, at phenomenologically relevant scales the photon PDF is already well determined in this region, with the corresponding uncertainties under good control. We then demonstrate the implications of this result for the example processes of high-mass lepton and $W$ boson pair production at the LHC and FCC. While for the former process the photon-initiated contribution is expected to be small, in the latter case we find that it is potentially significant, in particular at larger masses.

hep-ph↗