Paricle Flow in Heavy-Ion Collisions with CMS

Final Report Summary - PF4CMSHI (Paricle Flow in Heavy-Ion Collisions with CMS)

The project PF4CMSHI was devoted to the study of heavy-ion collisions at the Large Hadron Collider (LHC) using the Compact Muon Solenoid (CMS) experiment. The LHC has large experiments at four interaction points. Most of the running time is dedicated to proton collisions, however for about one month per year, heavy ions are collided. The goal is primarily to study a state of matter known as the Quark-Gluon Plasma (QGP) which is created in such collisions, and which existed in the early universe. One of the experiments, ALICE, is primarily dedicated to the study of heavy-ion collisions. However, for an interesting class of observables, the so-called hard probes, the scientific opportunities are in some cases greater at the "general purpose" experiments, CMS and ATLAS.

One observable for which CMS is particularly well-suited are high pT jets, which are a standard tool in high-energy physics. The direct reconstruction of jets in heavy-ion collisions became a clean observable with the arrival of the LHC. Shortly after the first collisions of lead ions (Pb-Pb) were recorded, a press release from CERN announced the first direct observation of "jet quenching" from the ATLAS and CMS collaborations [1]. Jet quenching is the suppression of high pT jets due to loss of energy of hard-scattered partons as they traverse the QGP. Such measurements provide a powerful window into behavior of nuclear matter in its deconfined phase. The researcher was the contact person for the first publication from CMS on the phenomenon, which remains among the top-cited CMS papers. He adapted the CMS particle-flow algorithm for use heavy-ion collisions, which has since become the standard methodology for jet reconstruction. Particle flow attempts to move as close as possible to the particle level, enhancing the performance of jet reconstruction.

The aim of the project was to capitalize on the researchers expertise and further develop jet reconstruction techniques in heavy ions. The foremost accomplishment in this regards was the first measurement of jets from bottom quarks (b-jets) ever performed in heavy ions. Such a measurement is interesting because many theoretical models predict a flavor dependence of the jet quenching effect. While identification of b-jets ("b-tagging") is an essential methodology in elementary particle physics, it had so far not been achieved in heavy-ion collisions. The researcher presented preliminary results at Quark Matter 2012 in August of 2012, the most important conference in heavy-ion physics. He was selected as the runner-up for the best talk at the conference. Further results were presented at the Hard Probes conference in November 2013. The paper is about to be submitted to Physical Review Letters. A public analysis summary may be found here [3].

A very important aspect of the project was strengthening heavy ion effort in CMS (CMS-HI) in Europe. Currently only one institute in France participates in CMS-HI, the Laboratoire Leprince-Ringuet, where this project was conducted. In Europe, only a handful of physicists from Hungary and CERN are involved. The entire group consists of about 100 physicists, mostly in the United States. Despite the small number of scientists CMS has had a large impact on the field in many ways both competitive to and complementary with the ALICE experiment, which had been invaluable to the field. This project, along with two other Marie Curie grants (one currently under negotiation) as well as an ERC grant has served as a springboard for European involvement in CMS-HI. The European investment has not gone unnoticed at the national level. The researcher has been awarded a permanent position in the CNRS to stay a LLR and is continuing participation in CMS-HI. We will continue to seek support at the national level and hope to initiate European partnerships to expand the scope of European participation in the CMS-HI program.

[1] http://press.web.cern.ch/press-releases/2010/11/lhc-experiments-bring-new-insight-primordial-universe
[2] Phys.Rev. C84 (2011) 024906
[3] https://cds.cern.ch/record/1472721/

Final Report Summary - PF4CMSHI (Paricle Flow in Heavy-Ion Collisions with CMS)

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