STAR Physicists Track Sequential ‘Melting’ of Upsilons Findings provide evidence for 'deconfinement' and insight into the seething temperature of the hottest matter on Earth. Scientists using the Relativistic Heavy Ion Collider (RHIC) to study some of the hottest matter ever created in a labora
The data on upsilons add further evidence that the—are “deconfined,” or free from their ordinary existence locked inside other particles such as protons and neutrons. The findings will help scientists learn about the properties of the QGP, including its temperature.
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 Before we get to reforming, let’s talk about how these particles form. Charm and bottom quarks and antiquarks are created very early in the collisions—even before the QGP. At the instant of impact, when the kinetic energy of the colliding gold ions is deposited in a tiny space, it triggers the creation of many particles of matter and antimatter as energy transforms into mass through Einstein’s famous equation, E=mc.
The other advantage of upsilons is that, unlike J/psi particles, they come in three varieties: a tightly bound ground state and two different excited states where the quark-antiquark pairs are more loosely bound. The most tightly bound version should be hardest to pull apart and melt at a higher temperature.
The team looked at two decay “channels.” One decay path leads to electron-positron pairs, picked up by STAR’s electromagnetic calorimeter. The other decay path, to positive and negative muons, was tracked by STAR’s muon telescope detector.
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