Inna Kucher (Ecole Polytechnique)

Jets as a probe of the Quark-Gluon Plasma

Abstract: Shortly after the Big Bang, the universe existed as a dense state of deconfined quarks and gluons lasting only a few millionths of a second before cooling to form protons and neu- trons. To study this today, particle accelerators are used to recreate these conditions of the early universe. Accelerators perform head-on collisions between massive ions, resulting in hundreds of protons and neutrons smashing into one another at energies upwards of a few trillion electronvolts. The resulting fireball "melts" particles into the deconfined state of the early universe, a Quark-Gluon Plasma (QGP). The fireball instantly cools and the individual quarks and gluons recombine into ordinary matter particles that speeds away in all direc- tions. The resulting debris contains pions, kaons, protons, neutrons, and corresponding anti-particles, providing a suite of probes used to learn about the QGP.

The subject of this seminar is jets as probe of the QGP. Jet measurements in heavy-ions begins with STAR and PHENIX experiments at Brookhaven National Laboratory’s Relativis- tic Heavy Ion Collider (RHIC), where one of the jets in back-to-back di-jet decays came out "quenched"—weakened—or, at times, completely suppressed. The degree of jet quench- ing, the jet orientations, composition, and how they transfer energy and momentum to the medium, all reveal a great deal about the properties of the quark-gluon plasma.

The ALICE, ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) all ob- served the phenomenon of jet quenching in heavy-ion collisions. The much greater colli- sion energies at the LHC push measurements to much higher jet energies than are accessi- ble at RHIC, allowing new and more detailed characterization of the quark-gluon plasma. The most important results on jet modification from the LHC are discussed in detail.