In the first measurement, scientists counted the charged particles that were produced from a few thousand of the most central lead-ion collisions—those where the lead nuclei hit each other head-on. The result showed that about 18,000 particles are produced from collisions of lead ions, which is about 2.2 times more particles than produced in similar collisions of gold ions at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider.
With the LHC’s lead-ion collisions taking place at more than 13 times the energy of RHIC’s gold-ion collisions, predicting a big increase in the number of particles produced would seem to be a no-brainer. Surprisingly, however, the opposite was true. The majority of theories predicted a number lower than that measured by ALICE, because of a strange property of the world of quarks and gluons, the fundamental particles that make up a lead nucleus.