Scientists used the STAR detector at the Relativistic Heavy Ion Collider to track how upsilon particles dissociate in quark-gluon plasma. These upsilons are made of a bottom quark and antibottom quark held together by gluons with different binding energies: a tightly bound ground state , an intermediate variety , and the largest, most loosely bound state . Credit: Brookhaven National Laboratory
One way to confirm that collisions have created QGP is to look for evidence that the free quarks and gluons are interacting with other particles. Upsilons, short-lived particles made of a heavy quark-antiquark pair bound together, turn out to be ideal particles for this task. "If the quarks and gluons were still confined within individual protons and neutrons, they wouldn't be able to participate in the competing interactions that break up the quark-antiquark pairs," Tang said.Scientists have observed such suppression of other quark-antiquark particles in QGP—namely J/psi particles .
Left: Brookhaven Lab physicist Rongrong Ma adjusts a cable on the muon telescope detector while STAR co-spokesperson Lijuan Ruan looks on. Right: Ma and Ruan stand on the catwalk atop STAR where modules of the MTD surround STAR's house-sized central magnet. Credit: Brookhaven National Laboratory "We don't measure the upsilon directly; it decays almost instantly," Yang explained."Instead, we measure the decay 'daughters.'"