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Workshop looks deep into the proton and QCD

20 July 2010

The DIS 2010 meeting in Florence covered a variety of collider results.

Résumé

Un atelier passe au crible les protons et la QCD

L’Atelier international sur la diffusion profondément inélastique et les sujets connexes était à l’origine un forum destiné à la discussion des résultats concernant la diffusion profondément inélastique en provenance du collisionneur électron–proton HERA. Il a cependant bien vite démontré sa capacité de réunir des théoriciens et des expérimentateurs pour discuter ensemble les résultats de toutes les expériences en collisionneur, s’agissant des dernières avancées des mesures de la structure du proton et de la dynamique QCD en général. Cette année, les toutes récentes mesures des propriétés inclusives des interactions proton–proton au LHC ont trouvé naturellement leur place dans la discussion lors du 18e atelier (DIS 2010), qui s’est tenu à Florence du 19 au 23 avril.

The International workshop on Deep Inelastic Scattering and Related Subjects began as a forum for discussing results on deep inelastic scattering (DIS) from the electron–proton collider, HERA. However, it has quickly become successful at bringing together theorists and experimentalists to discuss results from all collider experiments, both in terms of the latest developments in measurements of the proton structure and in QCD dynamics in general. This year the brand-new measurements of inclusive properties of proton–proton interactions at the LHC found a natural niche for discussion in the 18th workshop, DIS 2010, held in Florence on 19–23 April.

Volcanic disruptions

The cloud of volcanic ash present over most of Europe on the weekend before the workshop caused many flight cancellations and around 140 participants were unable to reach Florence in person; notably there were almost no participants from the UK or the US. On the other hand, more than 200 participants from mainland Europe embarked on long and often adventurous journeys to reach the conference site, a 16th-century cloister in the old part of the city. Owing to the late arrivals, the first day of plenary talks started a little later than planned – immediately with a coffee break, followed by an introduction from the director of INFN Florence, Pier Andrea Mandò.

The programme continued with a full agenda of plenary talks that set the scene and introduced a wealth of experimental results and recent developments in theory. Monica Turcato of Hamburg University and Katja Krueger of Heidelberg University presented the highlights from the ZEUS and H1 experiments at HERA. Horst Fischer of Albert-Ludwigs-Universität Freiburg reviewed the results on spin from all experiments. Thomas Gehrmann of Universität Zürich and Stefano Forte of Università di Milano reported on the recent progress in perturbative QCD. The session ended with the highlights from the ATLAS and CMS experiments at the LHC, with Thorsten Wengler of Manchester University and Ferenc Sikler of KFKI RMKI, Budapest, having the honour of showing the first published results on charged-particle spectra at 900 GeV and 2.36 TeV, as well as the first preliminary distributions at 7 TeV.

The opening day ended with a welcome cocktail, during which the conveners of the seven parallel sessions set a plan for installing EVO videoconferencing facilities to allow remote participation for those unable to get there, and reshuffled their programmes. Paul Laycock of Liverpool University was appointed convener of the Future of DIS working group “on the fly”, so relieving the organizers of a difficult situation.

The following two and a half days were dedicated to parallel sessions, which were held in the cloister’s painted rooms and library. The working groups covered a broad programme: parton densities; small-x, diffraction and vector mesons; QCD and final states; heavy flavours; electroweak physics and searches; spin physics; and the future of DIS.

The final two days of the conference began early, at 8.00 a.m., with plenary talks by US speakers over EVO. These included reports on: the rich physics of the CDF and DØ experiments at the Tevatron, by Massimo Casarsa and Qizhong Li of Fermilab; heavy-ion physics at RHIC, by Bill Christie of Brookhaven; and DIS results at Jefferson Lab, by Dave Gaskell. The plenary session on Friday had CERN’s Mike Lamont as a special guest, who reported on the status of the LHC accelerator and its performance. The conveners of the seven working groups summarized their sessions, splitting their reports into theoretical and experimental parts. Halina Abramowicz of Tel Aviv University concluded the workshop, pointing out how the different topics such as parton densities, low-x, diffraction, jets, heavy flavours and spin physics are all tools for improving understanding of the structure of the proton and its implications for the LHC.

Bright horizons

The combined results of ZEUS and H1 in neutral-current and charged-current cross-sections, used as input to fits of the parton distributions in the proton, have led to an incredible accuracy (1–2%), which allows a 5% uncertainty in the prediction of W and Z production at central rapidities at the LHC. The recent inclusion in the fits of combined data on charm reveals that the QCD evolution is sensitive to the treatment of heavy flavours and that the choice of the charm mass plays an important role in the predictions for the LHC. H1 and ZEUS are now focusing on the extension of the precision inclusive measurements to high/low photon virtualities, Q2, and high xBjorken. Also on the way is the completion of jet and heavy-flavour measurements based on all of the HERA statistics (0.5 fb–1 per experiment). Together, these will provide stringent tests of QCD at all Q2 and will further constrain the proton parton distributions.

Meanwhile, CDF and DØ now have 7 fb–1 each on tape and are sensitive to processes with cross-sections below 1 pb. Such a harvest provides a number of outstanding electroweak and QCD results: running αs has been measured at the highest pt ever, and the combined W mass measurement from the Tevatron is more precise than the direct measurements at LEP. The combined limit on the Standard Model Higgs lies in the range 163 <MHiggs <166 GeV at 95% confidence level. More results are on the horizon, with the 10 fb–1 expected by the end of 2011.

The newborn LHC experiments are performing well and are taking their first look at the particle spectra provided by nature at previously unexplored centre-of-mass energies. A few weeks after the first collisions, distributions at 7 TeV were already available. Figure 1 shows the multiplicity of charged particles as a function of the centre-of-mass energy from different measurements, including from ALICE at 7 TeV. Figure 2, where the average transverse momentum as a function of the charged-particle multiplicity of ATLAS data at 7 TeV is compared with various Monte Carlos (MCs), seems to point to the inadequacy of the models at this energy.

With increasing centre-of-mass energy, the momentum fraction of the partons can be small and the probability of multiparton interactions increases. Looking in detail at the event topology with the available LHC data is already informative: comparing the forward energy flow from minimum-bias events at different √s provides a new, independent constraint on the underlying event models. For example, figure 3 shows the ratio of energy flow measured by CMS at 7 TeV and 0.9 TeV as a function of the rapidity, compared with the Pythia MC.

Exclusive reactions – mainly at HERMES, Jefferson Lab and RHIC – allow the extraction of the generalized parton distributions. This was defined at the workshop as “a major new direction in hadron physics”, aimed at the 3D mapping of the proton and, more generally, of the nucleon.

In all, with results from Belle at KEK, BaBar at SLAC, COMPASS at CERN – as well as from Jefferson Lab, RHIC, the Tevatron, HERA and LHC experiments – QCD was seen at work over a range of studies from e+e to muon scattering and DIS to heavy ions and up to the energy frontier of LHC. In this stimulating contest, theory is preparing for present and future challenges with the first next-to-next-to-leading order (NNLO) calculations of precision observables and NNLO parton distributions. An example of the interplay between the precision of the available data and the theoretical predictions is given in figure 4, which shows a compilation of all of the αsmeasurements presented in the QCD session of the workshop.

The two main future projects, the LHeC electron–proton collider and the EIC electron–ion collider, were discussed extensively in the session on the future of DIS. The interest manifested by 350 or so registrations for the workshop promises a bright future for the field as well as for the DIS workshop series. The next workshop will be held at Jefferson Lab in April 2011 – a site in the US will be the ideal place to discuss future facilities.

• The workshop was organized by the University and INFN Florence, and by the University of Piemonte Orientale. We would like to thank the sponsors: INFN, DESY, CERN, Jefferson Laboratory, Brookhaven National Laboratory and CAEN Viareggio. Special thanks go to our co-organizers Giuseppe Barbagli, Dimitri Colferai and Massimiliano Grazzini, to all of the students and postdocs of our universities who helped out, and to the founder of the workshop series, Aharon Levy.

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