Doctoral Dissertation

30.11.2018: Jet shape modification in heavy-ion collisions measured in the ALICE experiment (Vargyas)


30.11.2018 12:00 — 14:00

Location: Ylistonrinne , FYS1
Release: 30.11.2018: Jet shape modification in heavy-ion collisions measured in the ALICE experiment
The recent doctoral thesis in physics focuses on the study of the hottest matter ever created in laboratory, the quark-gluon plasma and a detector upgrade that would enhance the future research.

For a few millionth of a second after the Big Bang, the Universe was so hot that the quarks and gluons were not bound inside the nucleons but instead could move relatively freely, forming a new state of matter, called the quark-gluon plasma (QGP). A few microseconds later, the Universe cooled down and quarks formed protons and neutrons, the building blocks of matter. In his doctoral thesis M.Sc. Márton Vargyas from the University of Jyväskylä has explored the properties of the QGP as it can be recreated in heavy-ion collisions, in laboratories like the CERN's Large Hadron Collider (LHC). Analyzing the data can thus shed light on how the matter behaves in conditions that prevailed upon the early Universe.

- As the QGP is so short-lived, that it cannot be studied by external probes, we have to rely on probes generated by itself. A widely used candidate is a jet, explains Vargyas.

Basic research for physics

The jet's seed is a very energetic parton (a quark or a gluon), created after the collision. Free partons do not exist in nature. Instead, the observed final state particles are always some bound states of partons, like for example nucleons. Final state particles are created in a process, where the original parton radiates several new partons that then form the final particles. As the momentum of the original parton is large, the radiated particles are also around the same axis, forming a collimated spray of particles, called a jet. In most of heavy-ion collisions these jets have to traverse the QGP to reach our detectors. In proton-proton collisions, however, there is no QGP formed. Comparing results from heavy-ion collisions with that of proton-proton collisions thus provides a useful technique for the study of the QGP. It was found that the jets in Pb-Pb collisions are narrower than jets in proton-proton collisions, in the high-momentum regime of the study. One knows that quark initiated jets are narrower as compared to gluon jets. Hence one possible explanation to the observed narrowing might be that gluons are absorbed to the medium with higher probability.

- My contribution was the development of the analysis software that extracted the observables from the raw ALICE data, and the interpretation of the results, says Vargyas.

This was basic research, with no foreseeable practical applications. The other part of the thesis, however, is more hardware -oriented. The ALICE experiment's Time Projection Chamber detector will be upgraded during the Long Shutdown 2 (2019–2020) of the LHC, part of which is the replacement of its Multi-Wire Proportional Chamber, related to data readout, with Gas Electron Multiplier foils.

 - I developed the Quality Assurance software and analyzed the foils produced at CERN. The most interesting part of this work was the correlation study between the foils' hole parameters and their gain, tells Vargyas.

The thesis work has been carried out in collaboration with the Helsinki Institute of Physics at the University of Helsinki, Finland, and at CERN's LHC's ALICE experiment. The research has been funded by the University of Jyväskylä, the Väisälä Foundation and the Helsinki Institute of Physics. 

Márton Vargyas defends his doctoral dissertation in physics “Jet shape modification in Pb-Pb collisions at sqrt(sNN) = 2.76 TeV using two-particle correlations'' on the 30th of November, 2018 at noon 12:00 in lecture hall FYS1. The opponent is Professor Roy Lacey (Stonybrook University) and Custos is Professor Jan Rak (University of Jyväskylä). The doctoral dissertation is held in English.

Márton Vargyas obtained his Master of Science degree in physics in Eötvös Loránd University, Budapest, Hungary, in 2013. The research of the PhD was supervised by Prof. Jan Rak and Dr. Sami Räsänen. Márton has spent a year at CERN.

Additional information:

The dissertation is published in the Department of Physics, University of Jyväskylä Research Report series with the number 126. ISBN 978-951-39-7609-5 (print), ISBN 978-951-39-7610-1 (online). Link to thesis:

More information

Márton Vargyas

Doctoral Student

Department of Physics