The Higgs gives mass to matter, too

The Higgs field was recently found to give mass to leptons and quarks (matter particles) as well as the bosons (force particles). Lines in the diagram above indicate interactions between particles: The red lines are new.

Nearly 50 years before its discovery, the Higgs field was proposed as a way to explain why particles have mass. The Standard Model would be internally inconsistent if particles could have mass on their own (that is, as an intrinsic property like charge), but it would not be inconsistent to propose a new field that gives them an effective mass by interacting with them. That new field has come to be known as the Higgs field, and particles of this field are called Higgs bosons.

This story is well known, and it was told in many ways when the Higgs boson was discovered in 2012. What is less well known is that the problem of mass was not a single problem. The reason that force particles (such as W and Z bosons) cannot have intrinsic mass is different from the reason that matter particles (such as electrons and quarks) cannot have intrinsic mass. The effective mass of force particles and matter particles could come from different sources. There could be two Higgs fields, one that only interacts with and gives mass to force particles, the other to matter particles, or perhaps the mechanisms themselves could be completely different.

Many physicists expected that a single Higgs field would pull double duty and give mass to all the particles. This, however, was a hypothesis, based on the expectation that nature is simple and elegant.

As it turns out, nature seems to be simple and elegant. CMS scientists recently published a study of Higgs boson decays to matter particles, complementing its discovery, which was through its decays to force particles. The same Higgs field interacts with both types of particles in the expected way.

Specifically, the study focused on Higgs to tau pairs (tau is a heavy cousin of the electron) and Higgs to b quarks (the b quark is a heavy cousin of the quarks found in the protons and neutrons of an atom). Since this interaction is responsible for mass, it is stronger for more massive particles. Both of these decay products are hard to distinguish from backgrounds, especially the b quarks, so the statistical significance is weak (3.8 sigma, equivalent to a one in 14,000 chance that the combined observation is spurious). However, these decays and all the decays to force particles point back to a single Higgs boson. The basic principles of physics may yet be simple enough to fit on the front of a T-shirt.

Jim Pivarski

These U.S. graduate students contributed to this analysis.
These U.S. physicists participated in recent test beam studies at Fermilab that will help validate future upgrades to the CMS hadron calorimeter subsystem.