19.6.2019 The Hunt for hot nuclear matter (Snellman)
In particle physics hot nuclear matter usually means quark gluon plasma (QGP). It is a matter so hot that quarks and gluons are no longer confined to nucleons, i.e. protons and neutrons, but move freely within the plasma. To turn ordinary matter into quark gluon plasma requires temperatures of about 2000 billion kelvin. These high temperatures can be reached in high energy collisions between atomic nuclei in laboratories, for example at the hadron collider (LHC).
Tomas Snellman studies particle jets in collisions between protons and lead nuclei, which have been measured at CERN at the ALICE experiment of the large hadron collider (LHC).
An important goal in the measurements performed at ALICE was to find out if the characteristics of a proton-lead collision can be explained using only the properties of cold nuclear matter. Cold nuclear matter is simply used to refer to the ordinary state of atomic nuclei, which is cold by the standards of particle physics.
“In the field it has been established that quark gluon plasma in created in lead-lead collisions at LHC. The interesting question is whether this can happen also in proton-lead collisions”, Snellman says.
By the scales in particle physics research atomic nuclei are "large". Thus the ball of colliding matter in a collision between two heavy nuclei is large enough to turn into quark gluon plasma. On the other hand, a single proton is so small, that it was deemed unlikely that QGP would be created.
“However some proton-lead collisions have shown indications of the creation of QGP. It remains unknown what actually happens in proton-lead collisions.”
“In my research I studied whether jets from either the average proton-lead collision or from an exceptionally active collision differ from jets observed in proton-proton collisions. Especially changes in the active collisions could provide clear proof of the creation of QGP. However within the current experimental capabilities no proof could be found”, Snellman explains.
"Thus the question of QGP in proton-lead collision remains an open one. Certain measurements support the creation of QGP, but especially measurements based on particle jets, like this thesis, see no signs. As the potential QGP droplet would be small in proton-lead collisions, the signals would be weak. This explains a part of the discrepancy, but not all of it. A solution would require a better theoretical understanding of the underlying phenomena, but also on the experimental side we need better control of the biases affecting our measurements so that even a weak signal could be detected", concludes Snellman.
M.Sc. Tomas Snellman defends his doctoral dissertation in Physics "Jet transverse momentum distributions from reconstructed jets in p-Pb collisions at √s_(NN ) = 5.02 TeV" on 19th of June at 12:00 fysiikan laitoksen luentosalissa FYS1. Opponent Professor Jamie Nagle (University of Colorado, USA) and Custos Dr. Dong Jo Kim (Ģֱ). The doctoral dissertation is held in English.
The dissertation is published at JYU Disserations series, 99, 2019. ISBN 978-951-39-7800-6 (PDF) URN:ISBN:978-951-39-7800-6 ISSN 2489-9003
Link to publication:
More information:
M. Sc. Tomas Snellman, tomas.snellman@cern.ch
Communications officer Tanja Heikkinen, tanja.s.heikkinen@jyu.fi, tel. + 358 50 581 8351