A search for even heavier light

A photon, a particle of light, has no mass. A Z boson has most of the same properties as a photon, but it is very massive. Even heavier photon-like particles could exist, but a new result from CMS sets the lower limit of their mass to at least 12.5 times heavier than Z bosons.

You may have heard that when matter and antimatter collide, they annihilate into pure energy. That is, when a negatively charged electron encounters its positively charged twin, the positron, the two form a new particle with no charge and twice the energy. The reverse is true as well: a particle of pure energy can decay into an electron and a positron, or a muon and an antimuon, or any other pair of opposites. The lightest particle of pure energy is the photon – light itself – but heavier variants have been discovered in colliders. The heaviest is the Z boson. There are many reasons to believe that more are waiting to be found.

CMS recently presented results on a search for a new particle, dubbed Z’ (pronounced Z prime), which would decay into an electron and a positron or a muon and an antimuon. If any such particle exists, it would have to be more than twelve and a half times heavier than the Z boson.

Though CMS physicists were searching for a deceptively simple signal – two oppositely charged particles – this result is an impressive demonstration of the precision of the CMS detector. When a super-heavy Z’ decays, its mass becomes the energy of motion of the two charged particles, making them the fastest electrons and muons ever produced in a laboratory. As these electrons and muons zip through the detectors, it is difficult to get a precise measurement of their energy. But despite the difficulties, the CMS detector performed beautifully, yielding the most definitive test to date.

There are as many ways to interpret this result as there are theories that predict new particles of pure energy. Some are related to the unification of all the forces in the Standard Model, others are inspired by superstrings. Still others are related to warped extra dimensions. As the size of the LHC dataset and collision energy increase, so will sensitivity to even heavier light. The next result could be a discovery – we are in uncharted territory.

— Jim Pivarski

These early-career physicists contributed to the search for Z’, which required input from physicists from across the globe.

Small misalignments of the muon detectors can result in large mismeasurements of muon energy. This group of U.S. physicists helped to ensure that the positions of the muon detectors were precisely known.