arXiv subjects
Publications and source records attributed to M. G. Ryskin.
The addition of forward proton detectors to LHC experiments will significantly enlarge the potential for studying New Physics. A topical example is Higgs production by the central exclusive diffractive process, pp -> p+H+p. We discuss the exclusive production of Higgs bosons in both the SM and MSSM. Special attention is paid to the backgrounds to the H -> bbbar signal.
The present status of the heavy-quark production theory is critically reviewed in the first contribution. The second contribution summarises the present heavy flavour data from HERA and gives an outlook of what can be expected from HERA-II. The potential of the LHC experiments for charm and beauty physics is reviewed in the 3rd contribution. Then the relevance of saturation and small-x effects to heavy quark production at HERA and at the LHC are discussed. The non-perturbative aspects of heavy-quark fragmentation and their relevance to HERA and LHC are discussed in the next contribution. Finally, a comparison of different theoretical predictions for HERA and LHC based on different approaches is presented.
We quantify the central inclusive background contributions to exclusive Higgs production at the LHC arising from double-Pomeron-exchange processes. We consider the H --> bb(bar) signal. We study processes mediated by the fusion of two `hard' Pomerons, and also by the fusion of `soft' Pomerons. The latter background is found to be very small, and the former is found to be less than the exclusive signal.
We calculate the nucleon parameters in isospin symmetric and asymmetric nuclear matter using the QCD sum rules. The higher moments of the nucleon structure functions are included. The complete set of the nucleon expectation values of the four-quark operators is employed. We analyze the role of the lowest order radiative corrections beyond the logarithmic approximation.
We analyse the latest H1 large rapidity gap data to obtain diffractive parton distributions, using a procedure based on perturbative QCD, and compare them with distributions obtained from the simplified Regge factorisation type of analysis. The diffractive parton densities and structure functions are made publically available.
We tested several choices of the in-medium value of the Bjorken scaling variable assuming the nucleon structure function in nucleus to be the same as that of free nucleon. The results unambiguously show that it is different.
The triple-Pomeron coupling lies at the heart of the predictions for high energy diffractive processes. We explain why the existing determinations, which use single-particle inclusive hadronic data a+b -> a+Y, underestimate the value of the bare coupling, due to the neglect of soft rescattering which populates the rapidity gaps. We describe how data for the process gamma+p -> J/psi +Y can be used to give a much more reliable estimate of the bare coupling. We use the existing, fragmentary, J/psi data from HERA to show that the triple-Pomeron coupling is probably about 3 times larger than the previous determinations. We emphasize the importance of an explicit measurement of the mass spectrum of the Y system which accompanies J/psi production at HERA. The consequences for ultra high energy cosmic ray showers are mentioned.
We consider the production and decay of multiquark systems in the framework of string models where the hadron structure is determined by valence quarks together with string junctions. We show that the low mass multiquark resonances can be very narrow.
We propose a new variable, R_j, in order to identify exclusive double-diffractive high E_T dijet production. The variable R_j is calculated using the transverse energy E_T and pseudorapidity of the jet with the largest E_T. For a purely exclusive event the value of R_j -> 1, if we were to neglect hadronisaton and the detector resolution effects. To illustrate the expected R_j-distribution we also compute exclusive three-jet production; and, moreover, include jet smearing effects. By studying the predictions as a function of the size of the rapidity interval, delta(eta), which allows for additional gluon radiation, one can probe the QCD radiation effects which are responsible for the Sudakov suppression of the exclusive amplitude. In this way we may check, and improve, the formalism used to predict the cross sections of exclusive double-diffractive Higgs boson (and/or other New Physics) production.
Central exclusive Higgs boson production, pp -> p + H + p, at the LHC and s-channel resonant Higgs production in the photon-collider option of the ILC can provide a very important contribution to the comprehensive study of the Higgs sector. Especially attractive is the b bbar Higgs decay mode, which for certain MSSM scenarios may become the discovery channel in exclusive Higgs production at the LHC and the Photon Collider (PC). Strongly suppressed and controllable backgrounds is an obvious requirement for the success of these exclusive measurements. One of the main sources of background comes from additional gluon radiation which leads to a three-jet b bbar g final state. We perform an explicit calculation of the subprocesses gg -> q qbar g and gamma gamma -> q qbar g, where the incoming particles are required to be in a J_z=0 state and the two gluons form a colour singlet, and investigate the salient properties of these potentially important background processes.
In principle, leading neutrons produced in photoproduction and deep-inelastic scattering at HERA have the potential to determine the pion structure function, the neutron absorptive cross section and the form of the pion flux. To explore this potential we compare theoretical predictions for the x_L and p_t spectra of leading neutrons, and the Q^2 dependence of the cross section, with the existing ZEUS data.
It is shown that the observation of the spectra of leading neutrons from proton beams can be a good probe of absorptive and migration effects. We quantify how these effects modify the Reggeized pion-exchange description of the measurements of leading neutrons at HERA. We are able to obtain a satisfactory description of all the features of these data. We also briefly discuss the corresponding data for leading baryons produced in hadron-hadron collisions.
We review recent issues concerning exclusive Higgs production at the LHC, and related processes at the Tevatron and HERA.
We discuss the contribution of so-called semi-enhanced hard rescattering corrections to central exclusive diffractive Higgs production, pp -> p+H+p, at the LHC. We present arguments to show that these corrections are small. We confirm these expectations by considering HERA data for leading neutron production.
QCD sum rules for the nucleon are considered in complex q^2 plane with inclusion of the radiative corrections of the order alpha_s. It is shown that the radiative corrections affect mainly the residue lambda^2 of the nucleon pole. Their influence on the value of the nucleon mass is much smaller. Following the ideas of Ioffe and Zyablyuk we expand the analysis to complex values of q^2. This provides a more stable solution. Varying the weights of the contributions of different dimensions by changing the value of the angle in the complex plane we find the value of the six-quark condensate which insures the best consistency of the right hand sides and left hand sides of the sum rules. The corresponding value of the six-quark condensate appears to be only about 10% smaller then the one, provided by the factorization approximation. The value of the four-quark condensate also appears to be close to the one, corresponding to the factorization assumption. The role of the gluon condensate and its possible values are discussed.
We present a detailed study of the central exclusive production of the Standard Model Higgs Boson in the WW decay channel at the LHC. We include estimates of the experimental acceptance, including that of the proposed proton tagging detectors at 220m and 420m around either ATLAS and / or CMS, and the level 1 trigger acceptances. We give first estimates of the photon-photon and glue-glue background processes in the semi-leptonic and fully-leptonic decay channels. We find that there will be a detectable signal for Higgs masses between 140 GeV and 200 GeV, and that the backgrounds should be controllable.
The asymptotic collinear factorisation theorem, which holds for diffractive deep-inelastic scattering, has important modifications in the sub-asymptotic HERA regime. We use perturbative QCD to quantify these modifications. The diffractive parton distributions are shown to satisfy an inhomogeneous evolution equation. We emphasise that it is necessary to include both the gluonic and sea-quark t-channel components of the perturbative Pomeron. The corresponding Pomeron-to-parton splitting functions are derived in the Appendix.
We study the effect of absorptive corrections due to parton recombination on the parton distributions of the proton. A more precise version of the GLRMQ equations, which account for non-linear corrections to DGLAP evolution, is derived. An analysis of HERA F_2 data shows that the small-x gluon distribution is enhanced at low scales when the absorptive effects are included, such that a negative gluon distribution at 1 GeV is no longer required.