Is it the Higgs boson?

Editor’s note: Starting this year, Fermilab Today will bring you more results from research at the Energy, Intensity and Cosmic Frontiers. We will publish these results on Thursdays and Fridays, highlighting the full breadth of Fermilab’s experiments.

Particles have intrinsic spin and parity. Parity depends on what happens if you swap left for right, up for down and forward for backward. Positive parity means you won’t see a difference, while negative parity means you’ll see the opposite. The Higgs boson is predicted to have zero spin and positive parity. This analysis establishes the particle found in July 2012 most likely has positive parity, at least in the decay channel into two Z bosons.

On July 4, 2012, the CMS and ATLAS experiments announced the discovery of a new particle with a mass of 125 GeV. This particle was widely heralded in the press as the Higgs boson, but both experiments very carefully didn’t make that claim. Instead, both experiments used the language “a particle consistent with being a Higgs boson.”

So why were the experiments so cagey in their announcement? It’s very simple. The Higgs boson was predicted in 1964 to have very specific properties. It is a massive particle, neutral and containing no particles inside it. Getting a little more esoteric, the Higgs boson is also predicted to have zero quantum mechanical spin and positive parity. The latter has to do with what happens if you swap left with right, up with down and forward with backward.

So what do we know? Well, evidence suggested that the discovered boson decayed into pairs of fermions (bottom quarks and tau leptons, both with spin 1/2) and pairs of bosons (W and Z bosons and photons, with spin 1). From this simple observation, we can infer that the newly discovered particle was electrically neutral (a prediction of Higgs theory) and was a boson (another successful prediction). In addition, using what we know about the spin of the decay products and combining that with the rules of quantum mechanics, we also know from the particle’s decay into bosons that the spin of the parent had to be 0 or 2.

A spin 0 particle would support the Higgs hypothesis, while a spin 2 particle, not being predicted, might be even more interesting. On the other hand, the universe might be malicious, and the July 2012 announcement could be referring to not only one, but maybe two particles, with spins of 0 or 2 and with masses close enough to each other that we thought we were seeing just one particle when we might actually have been seeing two. Further, a particle decaying into fermions could have a spin of 1.

Thus the only way to be sure is to directly measure things like the spin and parity of the newly discovered particle(s?). To do that, we have to exploit things like the angular decay patterns. Using the decay chain of the newly discovered particle into pairs of Z bosons (which decay, in turn, into electron or muon pairs), we can explore the spin and parity of the new particle. While the data is not yet strong enough to distinguish between a spin 0 or a spin 2 particle, we can set a strong limit on the parity. The analysis strongly favors a positive parity over a negative parity. Since the Higgs boson is predicted to have positive parity, we can add one more bit of evidence to the case that the new particle is the Higgs boson. In addition, the mass of the new boson was measured very accurately, with a precision of about 0.5 percent. It is very important to measure the mass with extreme precision to guard against the possibility that several particles with nearly identical masses have fooled us into thinking only one was found.

This is not the last tale in the saga of the discovery (and verification of the discovery) of the Higgs boson, but it’s an important one. Others will be announced as they become available.

—Don Lincoln

These U.S. physicists contributed to this analysis.
These members of the Fermilab Electrical Engineering Department are working on the new front-end electronics that are critical for the CMS hadron calorimeter readout upgrade. These new electronics will provide both energy measurement and the time that the energy arrives at the chip. The timing information will help physicists sort out what is going on when 20 or more collisions occur simultaneously inside the CMS detector.