For a few millionths of a second, shortly after the Big Bang, the universe was filled with an astonishingly hot, dense soup made of all kinds of particles moving at near light speed. This mixture was dominated by quarks – fundamental bits of matter – and by gluons, carriers of the strong force that normally “glue” quarks together into familiar protons and neutrons and other species. In those first evanescent moments of extreme temperature, however, quarks and gluons were bound only weakly, free to move on their own in what’s called a quark-gluon plasma.
To recreate conditions similar to those of the very early universe, powerful accelerators make head-on collisions between massive ions, such as gold or lead nuclei. In these heavy-ion collisions the hundreds of protons and neutrons in two such nuclei smash into one another at energies of upwards of a few trillion electronvolts each. This forms a miniscule fireball in which everything “melts” into a quark-gluon plasma.
The fireball instantly cools, and the individual quarks and gluons (collectively called partons) recombine into a blizzard of ordinary matter that speeds away in all directions. The debris contains particles such as pions and kaons, which are made of a quark and an antiquark; protons and neutrons, made of three quarks; and even copious antiprotons and antineutrons, which may combine to form the nuclei of antiatoms as heavy as helium. Much can be learned by studying the distribution and energy of this debris. An early discovery was that the quark-gluon plasma behaves more like a perfect fluid with small viscosity than like a gas, as many researchers had expected.
One type of debris is rare but particularly instructive. In an initial heavy-ion collision, pairs of quarks or gluons may slam directly into each other and scatter back-to-back – a spurt of energy that quickly condenses to a jet of pions, kaons, and other particles. First observed in accelerator-based experiments in the early 1980s, jets are fundamental to quantum chromodynamics, the theory that explains how quarks and gluons can combine depending on their different “colours” (a quantum property that has nothing to do with visible colours).
In heavy-ion collisions, the first evidence for jets was seen in 2003 in the STAR and PHENIX experiments at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) in the US. These jets showed a remarkable difference from those in simpler collisions, however. In the most striking measurement, STAR observed that one of the two back-to-back jets was invariably “quenched,” sometimes weakened and sometimes completely extinguished. The further a jet has to push through the dense fireball of a heavy-ion collision – 30 to 50 times as dense as an ordinary nucleus – the more energy it loses.
Jets are “hard probes”, by nature strongly interacting but moving so fast and with so much energy that they are often not completely absorbed by the surrounding quarks and gluons in the quark-gluon plasma. The degree of jet quenching – a figure that emerges in data from millions of collision events – plus the jets' orientation, directionality, composition, and how they transfer energy and momentum to the medium, reveal what’s inside the fireball and thus the properties of the quark-gluon plasma.
Recently the ALICE, ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) have confirmed the phenomenon of jet quenching in heavy-ion collisions. The much greater collision 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. Theoretical understanding of these measurements is challenging, however, and is one of the most important problems in quantum chromodynamics today.