A recent analysis of data from the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has revealed new evidence suggesting that collisions involving small nuclei with larger ones might produce tiny drops of quark-gluon plasma (QGP). This plasma, composed of free quarks and gluons, is believed to have filled the universe shortly after the Big Bang.
The RHIC regularly creates QGP by colliding gold ions. However, it was previously thought that smaller ion collisions wouldn't generate enough energy to form a QGP. Evidence from PHENIX suggests otherwise, showing particle flow patterns consistent with QGP formation in these smaller systems.
Recent findings published in Physical Review Letters provide direct evidence that particles in RHIC's small collision systems sometimes lose energy significantly. Gabor David, a PHENIX physicist and research professor at Stony Brook University, explained this phenomenon: "Those interactions lead to energy loss." He likened it to running through water instead of air.
Energetic collisions can also produce high-energy photons alongside quarks and gluons. These photons allow scientists to measure collision centrality and predict jet production. Axel Drees, Distinguished Professor at Stony Brook University, noted that more central collisions result in more interactions and thus more energetic jets.
Unlike quarks and gluons, photons escape the QGP without losing energy. Niveditha Ramasubramanian analyzed PHENIX's deuteron-gold collision data to extract direct photon signals. Her work resolved an unexplained increase in jets from peripheral collisions but revealed unexpected suppression in central collisions.
Ramasubramanian remarked on her analysis: "The initial motivation...was only to better understand the strange increase in energetic jets...which we did." Now a staff scientist at the French National Centre for Scientific Research, she added that the observed suppression was unexpected.