|A jet of water sprayed through water loses energy and changes shape, as illustrated by this Jacuzzi jet. CMS scientists studied a similar phenomenon in an exotic liquid of quarks and gluons.|
Despite the complexity of particle colliders and the instrumentation needed to analyze their results, the ultimate aim of most particle physics experiments is to understand something simple. At a fundamental level, most natural phenomena turn out to be simple in profound ways. By contrast, our macroscopic world is teeming with complexity: A bucket of water is by far more complex than an electron. The exact way that water sloshes, curdles in turbulent flow and pinches into droplets when it splashes would be difficult to simulate on the world’s biggest supercomputers, even though the basic interactions between individual atoms are pretty well understood.
One part of the quantum world has this kind of complexity, however: the strong force that binds quarks. Unlike the electromagnetic force between atoms, the particles that make up the strong force are themselves attracted via the strong force, which begets more strong force. Physicists call them gluons because they make such a sticky mess. Like the bucket of water, the strong force is notoriously difficult to calculate because some of its properties are emergent — they arise from the interplay of many interactions.
One of these emergent properties is the fact that a lone quark flying away from a collision creates gluons, which create quarks, which create gluons, and becomes a jet of particles flying in roughly the same direction. Another is that if you get enough quarks in a small space (by colliding heavy nuclei), they undergo a phase transition into a new kind of liquid ruled by strong force interactions. Recently, scientists discovered that jets are eaten by the liquid: They are absorbed into the droplet and sometimes disappear entirely.
To get a more complete picture of this phenomenon, scientists have used the CMS experiment to study an in-between case, jets that are partially but not completely absorbed by the strong-force liquid. Like a hose sprayed through water, this results in misshapen jets. The angles among particles that make up the jet are noticeably wider than usual, and the exact amount of broadening tells us a little more about the nature of this new state of matter.
|The U.S. physicists pictured above made major contributions to this first-ever jet shape analysis in heavy ion collisions.|