The Blois Workshop comes to CERN

The 13th International Conference on Elastic and Diffractive Scattering – the “Blois Workshop” – dates back to 1985, when the first meeting was held in the picturesque, old French town of Blois, famous for the 14th-century Royal Château de Blois. The conference series continues to focus on progress towards understanding the physics of hadronic interactions at high energy. A major strength of the meetings is the way in which they facilitate detailed discussion between theorists and experimentalists, thereby motivating new ways of formulating theoretical approaches and confronting them with experimental measurements – past, present and future.

More than 100 participants from 18 countries attended the latest meeting in the series, held at CERN on 29 June – 3 July 2009. The relatively informal manner of the 70 talks encouraged discussion. Appropriately, given the imminent start-up of the LHC, the following topics featured prominently: the total proton–proton (pp) cross-section; elastic pp scattering; inelastic diffractive scattering in electron–proton (ep), pp and heavy-ion collisions; central exclusive production; photon-induced processes; forward physics and low-x QCD; and cosmic-ray physics.

Theoretical developments

On the theoretical side, important aspects of soft diffraction were nicely introduced by Alexei Kaidalov of the Institute of Theoretical and Experimental Physics (ITEP) in Moscow, who emphasized factorization effects and unitarization in the framework of Reggeon calculus. Although everyone anticipates that the total pp cross-section will continue to rise with increasing energy – following the pioneering prediction of H Cheng and T T Wu in 1970 – a number of contributions made distinct predictions for its value at LHC energies – typically ranging between 90 mb and 140 mb, with surprising predictions as high as 250 mb. Several other features of elastic scattering at LHC energies were also considered within the framework of different models that were successful at lower energies. André Martin of CERN, with his long-established theoretical rigour, reported on a new limit for the inelastic cross-section.

The central production of various exclusive final states with one or two “leading protons” – Higgs production at the LHC, in particular – was also a source of much debate. This subject challenges different approaches in QCD, notably the “gluon ladder”, and how these approaches relate to the long-standing theoretical construct, the Pomeron. Douglas Ross of Southampton University presented an interesting treatment of the Balitsky–Fadin–Kuraev–Lipatov (BFKL) kernel of such a ladder, based on the extraction of the low-x gluon distribution in experiments at the HERA ep collider. The issue of the “rapidity-gap survival probability” as an explanation for substantial factorization-breaking in inelastic diffraction in hadron–hadron collisions (as opposed to ep collisions) continues to challenge theory and is important when developing models for central Higgs production. Mark Strikman of Penn State University presented a notable proposal of a new sum rule.

The workshop devoted a full day to contributions dealing with the physics of QCD at various extremes, such as at the lowest parton fractional momenta (low-x QCD) and at the highest densities achievable, e.g. in heavy-ion collisions. Emil Avsar and Tuomas Lappi of CEA/Saclay and Francesco Hautmann of Oxford University reviewed the physics of gluon saturation and possible modifications of the standard QCD evolution equations at tiny values of Bjorken-x. A second topic, summarized by Raphael Granier de Cassagnac of the Laboratoire Leprince-Ringuet, Gines Martinez of SUBATECH, and Jean-Yves Ollitrault of Saclay, concerned studies of the collective behaviour of a multiparton system in a hot, dense state such as a quark–gluon plasma. Various other talks covered the latest experimental and theoretical developments in each of these two active research areas of the strong interaction, all with prospects at the LHC very much in mind.

Experimental highlights

Presentations on experimental developments highlighted the challenge of diffractive physics and the way that it relies on a particularly close symbiosis of measurement and theory. The phenomenology of elastic pp scattering, based on long-standing measurements at the Intersecting Storage Rings at CERN and later experiments at CERN and Fermilab, continues within either “classic Regge” or “geometrical” approaches. The latter is now beginning to produce a “transverse” view of the proton’s structure, as Richard Luddy of Connecticut University explained. Such understanding will be testable in the near future in deep-exclusive lepton-scattering experiments, for example in COMPASS at CERN where, as Oleg Selyugin of JINR described, such measurements may be interpreted in terms of generalized parton distributions.

As at all meetings since EDS returned to Blois in 1995, there were reports from the experiments at HERA on the status of the deep-inelastic structure of the diffractive interaction, this time by Henri Kowalski of DESY and Alexander Proskuryakov of Moscow State University. The impressive precision of the data reveals beautiful features that demonstrate the quark and gluon components of the t-channel (i.e. the leading) exchange mechanism. Put differently, the data are sensitive to the parton structure of the proton’s diffractive interaction. Results on the scale dependence of these leading exchanges, measured at HERA in exclusive meson production, now provide precise data with which QCD theory has to be reconciled, as Pierre Marage of the Université Libre de Bruxelles explained.

The main experimental highlights came, arguably, from the CDF experiment at Fermilab’s Tevatron with the measurements of the central exclusive two-photon production (pp → ppγγ) and di-jet production (pp → pp+2 jets), presented by James Pinfold of Alberta University, Christina Mesropian and Konstantin Goulianos of Rockefeller University and Michael Albrow from Fermilab. Both processes are important as precursors for the exclusive Higgs search at the LHC; the agreement of the predictions, made prior to the measurements, with the data is an important milestone in the preparation for exclusive Higgs hunting – appropriately christened “Higgs with no mess” by the experimentalists concerned.

A session dedicated to ultrahigh-energy cosmic-ray observations underlined their complementarity to collider measurements in view of understanding hadronic interactions, as Jörg Hörandel of Radboud University, Nijmegen, explained. Alessia Tricomi of INFN/Catania University pointed out that, in particular, forward experiments can contribute valuable data to the development of models of air showers.

With its first data, the LHC will already provide new measurements that are crucial to this active field.

Other topics at the meeting included photon-induced processes from the BaBar and Belle experiments, with reviews of relevant heavy-ion results from RHIC at Brookhaven and prospects for the LHC. Looking further into the future, Paul Newman of Birmingham University reported on possibilities for ep and electron–ion interactions at an LHeC.

Given the venue of EDS ’09, perhaps the most appropriate session was the one that was concerned with new experiments. Taking advantage of the presence of the unique breadth of expertise present at an EDS meeting, a panel discussion took place between representatives of theory and experiments, moderated by Karsten Eggert of Case Western Reserve University and CERN. It provided the opportunity to exchange ideas about which measurements to carry out first at the LHC, how to create synergies between different experiments and about future upgrade possibilities for the forward proton detectors. Several new ideas for possible measurements at the LHC were proposed and discussed. With its first data, the LHC will already provide new measurements that are crucial to this active field. The meeting ended with a strong sense of anticipation, given the imminent diffractive data at a new energy scale from the first run of the LHC.