Speaker: Inna Kucher (Ecole Polytechnique)
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 directions. 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 Relativistic 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 quenching, 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 observed 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 accessible 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.