Precision smashing

Analyzing jets from a particle collision is something like analyzing the aftermath of the collision of two pocket-watches. CMS scientists recently analyzed collisions in which hundreds of particles seemed to be channeled into six streams.

It is sometimes said that if particle physicists wanted to figure out how an expensive Swiss watch works, they would smash it and deduce its structure from the cogs, springs and glass that fly apart. To understand the inner life of protons, this is exactly what they do—smash two of them together and analyze the aftermath. It is not as ridiculous as it sounds. The basic laws of energy and momentum conservation make a precision science of this violent practice.

Energy and momentum are both quantities that describe the inertia of an object or a system of particles. Energy is the sum of motion in all directions, while momentum is the net motion, depending on direction. These quantities are useful because they are constants—even the messiest explosion maintains a constant momentum because leftward-going particles exactly balance rightward-going particles. The energy of an explosion is also conserved, as long as the potential energy that caused the explosion is counted. Among subatomic decays, this initial energy is the mass of the decaying particle, according to E = mc2 (energy equals mass times a large constant).

To analyze a collision, physicists add up the observed energy and momentum and solve both equations to deduce the masses of the particles that decayed before they could be observed directly. In a recent paper, CMS scientists applied an extreme version of this technique. They looked at events in which hundreds of visible particles seemed to be channeled in six streams, known as jets. Assuming that each jet was caused by a cascade of decays, starting from a single quark, they applied energy and momentum conservation to find out if triplets of quarks descended from an as-yet unknown particle.

Although the search turned up negative (no new particles yet), the experimenters’ ability to deal with so many jets is impressive. There are 10 different ways to identify two triplets among six quarks. If all combinations are considered, the nine wrong ones would drown out the right one, weakening the sensitivity of the analysis. These scientists noticed that wrong combinations usually produce more energetic quarks than right ones, and used this correlation to sharpen the precision of their search. Not bad for smashing watches.

—Jim Pivarski

The Rutgers physicists pictured above were all major contributors to this analysis.
The Fermilab physicists above provided invaluable help with Monte Carlo generators, which are computer simulations of theoretical expectations.