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We present a study of the central exclusive production (CEP) of meson pairs 1 , M M , at sufficiently high invariant mass that a perturbative QCD formalism is applicable. Within this framework, M M production proceeds via the gg → M M hard scattering sub-process, which can be calculated within the hard exclusive formalism. We present explicit calculations for the gg → M M helicity amplitudes for different meson states and, using these, show results for meson pair CEP in the perturbative regime. a KRYSTHAL collaboration b speaker

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Motivated by the recent experimental observation of exclusive χ c events at the Tevatron, we revisit earlier studies of central exclusive scalar χ c0 meson production, before generalising the existing formalism to include χ c1 and χ c2 mesons. Although χ c0 production was previously assumed to be dominant, we find that the χ c1 and χ c2 rates for the experimentally considered χ c → J/ψγ → µ + µ − γ decay process are in fact comparable to the χ c0 rate. We have developed a new Monte Carlo event generator, SuperCHIC, which models the central exclusive production of the three χ c states via this decay chain, and have explored possible ways of distinguishing them, given that their mass differences are not resolvable within the current experimental set-up. Although we find that the severity of current experimental cuts appears to preclude this, the acceptance does not change crucially between the three states and so our conclusions regarding the overall rates remain unchanged. This therefore raises the interesting possibility that exclusive χ c1 and χ c2 production has already been observed at the Tevatron.2 In the χc case the agreement becomes especially striking after taking into account the revised value of the total χc0 width which has been reduced by a factor 1.4 [37] as compared to the value in the Review of Particle Properties (2002) used in [5].3 CHIC is a publicly available Monte Carlo implementation of the χc0 analysis of Ref. [5]. 4 This applies not only to χc(1P ) states, but also to possible higher excitations χc(nP ). 5 S enh is largely independent of the χ spin assignment. Note also that the relative number of events where the forward protons dissociate is larger for χc1 and χc2 than for χc0.

We discuss the qualitative features of the recent data on multiparticle production observed at the LHC. The tolerable agreement with Monte Carlos based on LO DGLAP evolution indicates that there is no qualitative difference between 'hard' and 'soft' interactions; and that a perturbative QCD approach may be extended into the soft domain. However, in order to describe the data, these Monte Carlos need an additional infrared cutoff k min with a value k min ∼ 2 − 3 GeV which is not small, and which increases with collider energy. Here we explain the physical origin of the large k min . Using an alternative model which matches the 'soft' high-energy hadron interactions smoothly on to perturbative QCD at small x, we demonstrate that this effective cutoff k min is actually due to the strong absorption of low k t partons. The model embodies the main features of the BFKL approach, including the diffusion in transverse momenta, lnk t , and an intercept consistent with resummed next-to-leading log corrections. Moreover, the model uses a two-channel eikonal framework, and includes the contributions from the multi-Pomeron exchange diagrams, both non-enhanced and enhanced. The values of a small number of physically-motivated parameters are chosen to reproduce the available total, elastic and proton dissociation cross section (pre-LHC) data. Predictions are made for the LHC, and the relevance to ultra-high-energy cosmic rays is briefly discussed. The low x inclusive integrated gluon PDF, and the diffractive gluon PDF, are calculated in this framework, using the parameters which describe the high-energy pp and pp 'soft' data. Comparison with the PDFs obtained from the global parton analyses of deep inelastic and related hard scattering data, and from diffractive deep inelastic data looks encouraging.

We calculate the Coulomb effects on the cross section for e + e − → W + W − taking into account the instability of the W bosons. We carefully explain the consequences of instability throughout the energy range which will be accessible at LEP2. We present a formula which allows these effects to be easily implemented.----- †

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.

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 unitarity.Recently the TOTEM collaboration at the LHC has published the results of the first measurements at √ s=13 TeV of the pp total cross section σ tot = 110.6 ± 3.4 mb [1] and of the ratio of the real-to-imaginary parts of the forward pp-amplitude, ρ =Re/Im= 0.10 ± 0.01 [2]. Since the latter value appears to be sufficiently smaller than that predicted by the conventional COMPETE parametrization (ρ = 0.13 − 0.14) [3], it may indicate either a slower increase of the total cross section at higher energies or a possible contribution of the odd-signature amplitude. (Note that within the COMPETE parametrization the odd-signature term is described by secondary Reggeons and dies out with energy.) Note that a C-odd amplitude, which arises from the so-called Odderon, and which depends weakly on energy, is expected in perturbative QCD 1 , see in particular [4,5,6] and for reviews e.g. [7,8]. However the naive estimates show that its contribution is rather small; say, ∆ρ Odd ∼ 1mb/σ tot < ∼ 0.01 [9] at the LHC energies.On the other hand, it is possible to introduce the Odderon phenomenologically as an object which does not violate first principles and the axiomatic theorems. In fact it was stated in [10] that the new TOTEM result is a definitive confirmation of the experimental discovery of the Odderon in its maximal form.