by Paul Kuijer & Marco van Leeuwen. Published: 25 November 2011

Last week the Hadron Collider Physics Symposium 2011 was held in Paris. Here two of the speakers, Paul Kuijer and Marco van Leeuwen, summarise their talks and experience of conference itself:

Paul Kuijer

My talk was an overview of soft physics for all LHC experiments which I have tried to explain here, as well as my overall impression of the conference. The Hadron Collider Physics (HCP) conference in Paris is an excellent opportunity to learn about the progress of all experiments using proton or nucleus beams. Of course, nowadays the results are mostly from the LHC experiments but, unlike more specialized conferences, there is ample opportunity to discuss different physics interests. The very successful start-up of the LHC during the past year has finally allowed the LHC experiments to start doing what they were designed for.

The lead-lead collisions are already being used to shed light on the properties of the Quark-Gluon-Plasma (QGP). Of course the Alice experiment was optimized for these measurements but ATLAS and CMS have proven to be strong competitors in this field also. The LHC experiments have not only confirmed the Relativistic Heavy Ion Collider (in Brookhaven) observation that a QGP can be formed in heavy-ion collisions but they have also started to look in more detail at its properties. The LHC is a great machine to study the QGP because it produces a much hotter and denser state that also lives longer and this makes it more easy to measure its properties. With the first year’s data, the size, temperature, density and life time of the QGP created was already estimated. In addition the viscosity is being investigated now. This is especially interesting because the viscosity is one of the few material properties of the QGP that can be calculated from the theory of the strong interactions (QCD). One of the methods, using a correspondence between the general theory of relativity and field theory (the AdS/CFT correspondence) allows us to determine a universal lower limit on the viscosity of the QGP. It appears that with the LHC we get very close to that limit and it will be interesting to see if this prediction holds.

The lead-lead collisions being provided now will increase the data sample for the more rare processes by an order of magnitude. This will allow ALICE to use many additional probes to study the QGP. The increase of the luminosity of the proton beams will boost the search for the HIGGS, SUSY and new physics and provide ALICE with a larger sample of rare signals and high multiplicity events to compare to the lead-lead events.

The p-p data and experiments also have many promises for the future. ATLAS and CMS are thoroughly searching for the HIGGS particle and signs of new physics beyond the standard model of particle physics. There are no signs of the HIGGS or SUSY so far and it appears more and more that, if it really exists, the HIGGS will be in one of the more difficult corners to explore. But of course ATLAS and CMS are well up to this task and with the data of 2012 and some time to analyze them during the LHC shutdown afterwards, the standard model HIGGS can probably not escape them any longer. Imagine the excitement of the student who will see the first small peak appearing in the plots, just in the place where it is most difficult to find! Of course, not finding the standard model HIGGS will be just as, or even more, interesting.

LHCb starts to see the asymmetry in the decay of charm particles. This has never been done before and it will certainly provide new insights. It is a tiny effect however so it will be a while before precise numbers will come out of the analysis. No other experiment is as capable of extracting this information as the LHCb experiment.

The HCP conference was exactly at the right time to summarize the results of the first large datasets at this highest ever energies. It is very interesting to compare different aspects of the strong interaction, be it with p-p collisions or lead-lead collisions and to look for the missing pieces of the standard model or even beyond the standard model. The excitement and expectation of new insights to be brought by the LHC must have been clear to every participant!

Marco van Leeuwen

Last week, at the Hadron Collider Physics Symposium in Paris, I had the pleasure to present some of the high-pt physics results from ALICE. The conference had a rather broad programme, with only two sessions about our main interest: heavy ion collisions and the Quark Gluon Plasma. There were a large number of other topics, such as the search for the Higgs and other new particles (Supersymmetric particles and even 'exotics'). My presentation was therefore aimed at a more general audience than I am normally used to.

High-pt physics in heavy ion collisions is about using high-energy quarks and gluons that are produced in the collisions to study the Quark Gluon Plasma (QGP). The high-energy quarks and gluons interact with the QGP and lose energy by radiating gluons. To test our understanding of the energy loss process, we performed a number of different measurements, which will be compared to model calculations that have already been compared with measurements from the Relativistic Heavy Ion Collider at lower collision energy. I showed a few of these comparisons in my presentation, but more work is needed to sort out which calculations are correct and which ones cannot explain the observed behaviour. Of particular interest is also the comparison of energy loss of light quarks and heavy quarks (charm and beauty). We have a first measurement of energy loss for charm quarks (and hadrons) and are currently working to also measure the beauty quark. All of these measurements were performed using the heavy ion collision data collected in November/December 2010. A new heavy ion run is ongoing and we expect to get about 10 times more collisions recorded this year, which will greatly improve the precision of the heavy flavour measurements.

It was also interesting to hear in the other sessions about the recent results on many other topics. There were presentations from the proton-anti-proton experiments at Tevatron and proton-proton collisions at the LHC. An impressive array of precision measurements has already been done at LHC, but so far, no new particles have been found. More collisions will be coming for these measurements (and searches) next year, and by the end of next year we will probably know whether the expected Higgs particle exists or not. There will also be another 'lead' run in 2012, where we will probably collide protons with lead nuclei, to measure effects in the wave function of lead nuclei which may contribute to some of the effects that we have observed in lead-lead collisions.

All in all, exciting times are ahead!