by Petrov Marian. Published: 15 September 2014

I first heard about CERN’s summer school from my university supervisor who has been to CERN as a summer student; like most of the professors of particle physics in my department. He said that it has been a great experience and gives the opportunity to attend lectures given by scientists who are all experts in their fields. Moreover, he pointed out the fact that one can always meet and interact with various people who work at CERN no matter which are their status and even if they have different research interests. One can always profit from this mosaic of scientists working in all different fields.

I recently graduated from the Royal Holloway, University of London with an MSc degree in Physics. My project at CERN is related to the study of two particle angular correlations in pp collisions. This is a new phenomenon which has not been properly studied using the newer data from the ALICE detector during the 2012 run because of the decrease of statistics due to pile up.

In PbPb collisions when the lead atoms collide they create a new state of matter, the so-called quark gluon plasma that is rapidly expanding. Because of this rapid expansion the emerging particles and jets are correlated and by determining this correlation one can make assumptions about the interactions that are taking place in the initial new state of matter. For example one may ask whether it behaves like a Fermi liquid (strongly interacting, non-perturbative) or more like a gas (weakly interacting, perturbative). Moreover these studies can justify the use of hydrodynamical relations to quantify the properties. To answer these questions one defines correlation functions, which are, sensitive to track parameters and the correlation function is plotted as a function of pseudorapidity and azimuthal separation one can observe a characteristic double ridge structure. This is predicted by QCD and matches theoretical predictions well, however pPb and pp collisions were investigated with the intention of seeing if this phenomenon can be seen again. It turns out that this is possible which is unexpected because the cross sections are much smaller in the latter two cases.

I work with the ALICE collaboration and therefore with the ALICE detector which has a certain coverage in pseudorapidity, most of the events which go into this study of proton proton collisions are within -1 to 1 in pseudorapidity coverage (the bigger the coverage the worse the statistics) and the task at hand is to use the newer data and try, if possible, to extend the range to -1.5 to 1.5 in pseudorapidity and to see if a ridge structure may form. In addition the aim is to improve the quality of the signal (and hence the statistics) by implementing tighter constraints on several track parameters  e.g. the number of clusters (charged particles going through the various silicon detectors) within a certain energy range. My supervisor has given me some of his code which I modified to try to reproduce histograms of the correlation functions with the new data and the new ranges. He has been very helpful and very patient at explaining the various bits of codes.

My experience at CERN has been very good, there is a lot of academic freedom here i.e. wide access to resources and papers as I was expecting. Fortunately my supervisor was able to find a nice desk for me to work at CERN. I fear that may become a problem in the future (shortage of work space) with more scientists converging at CERN. The lectures are engaging, one of them was given by an ALICE member who explained quite thoroughly the two particle correlations as my supervisor did as well, although some other lecturers went into too much detail that I could not follow unfortunately. I visited so far the ALICE and CMS detectors, participated in a few workshops and soon will go to see the ATLAS detector.  It is very good  to see them as I will be working as a PhD student in Oxford University's particle physics group in October and hence having knowledge of CERN as a summer student has been very helpful.

From my time as a summer student at CERN I will remember the nice conversations that I had with Jack Steinberger, the underground visits to the detectors and the chance to participate in the discussion sessions and ask the lecturers questions and maybe some of my trips to Geneva.