Good things come in pairs

This figure illustrates the pair production of two massive particles, each of which decays into two jets of lighter particles.

It is often said that in quantum mechanics, anything that can happen will happen. The emphasis, however, should be on the “can happen.” Not all processes are possible. Whenever two particles collide, nature checks her rule book to see which collision products are allowed and then randomly chooses from the options. Physicists generally express these rules as conservation laws, quantities that must remain equal before and after a collision, such as the sum of all mass and energy in the particles. One could say that the goal of particle physics is to discover all of the conservation laws.

Some conserved quantities, such as mass-energy, can be any fractional number, while others seem to be limited to whole-number multiples of a fundamental constant. Angular momentum, surprisingly enough, seems to be an example of the latter: You can’t spin less than 5.27 × 10−35 joule·seconds. Conservation laws that are restricted to strict increments like this are called quantum numbers.

One consequence of conserved quantum numbers is that some particles can only be produced in pairs. If a conserved quantum number describing the colliding particles is zero, then the collision cannot result in a single particle whose quantum number is +1. A +1 particle must be accompanied by a −1 particle. This applies to the quantum numbers that we know (such as angular momentum, electric charge, strong and weak force charges) as well as possible new quantum numbers that we don’t yet know. Some particles might have gone undiscovered because they can only be produced in pairs.

In a recent paper, CMS physicists used this feature to search for new particles. Instead of looking for the pattern of debris that would be consistent with the decay of a single new massive particle, they searched for the decay of two particles, the +1 and the −1 of some quantum number. Although they don’t know the mass of the new particles, they can expect them both to have equal masses. Constraints like this help to eliminate backgrounds from known physics, which is important because almost all of the collisions result in known processes. Anything that can happen, will.

—Jim Pivarski

The physicists pictured above searched for pairs of new particles in a sample of about 550 trillion proton-proton collisions.