Physics

Evidence Found For The Higgs Boson Direct Decay Into Fermions

June 23, 2014 | by Stephen Luntz

Photo credit: CERN. This modelling of the decay of the Higgs boson had electrons among the particles produced in the decay, but until now the observed products were limited to other bosons

Fermions have been observed at CERN that appear to result from the direct decay of the Higgs boson. This would be the first time the Higgs has been seen decaying directly to fermions, rather than other bosons. The finding is in keeping with the standard model of particle physics, giving physicists a little more confidence that the model they have been using for decades actually does describe reality.

Bosons are particles where more than one can occupy the same quantum state, including not only the Higgs, but also the particles that carry forces. Fermions, on the other hand include most of the particles more familiar to non-physicists, such as electrons, protons and neutrons. Two fermions in the same place must have different properties, such as spin.

The Higgs boson is so short lived that we cannot observe it directly, only observe its presence through the products to which it decays. Consequently, these products are of vital importance if we are to understand the Higgs, and its place in the forces of nature. 

One of the predictions drawn from the standard model of particle physics is that fermions should interact with the Higgs field in proportion to their mass. The Compact Muon Solenoid (CMS) collaboration, one of the two teams that worked in parallel to detect the Higgs, have reanalyzed the observations that led to the announcement of the Higgs and reported in Nature Physics that this is what they have observed. This is important because it is in keeping with the central idea of the Higgs field, which is that it gives other particles mass.

The CMS team report bottom quarks, one of the easiest subatomic particles to identify, and tau leptons, both fermions, in the Higgs decay. The nature of these particles are important because the paper notes, “The current measurements mainly constrain the couplings to the up-type top quark.”

"This prediction was confirmed," says Professor Vincenzo Chiochia of the University of Zurich; "a strong indication that the particle discovered in 2012 actually behaves like the Higgs particle proposed in the theory."

"This is a major step forwards," explains Chiochia . "We now know that the Higgs particle can decay into both bosons and fermions, which means we can exclude certain theories predicting that the Higgs particle does not couple to fermions."

The standard model of particle physics provides only a starting point to understanding the laws of nature. There are several competing extensions to the model, and confirmation of it opens up room to test these out. In some of these extensions there is not one Higgs boson, but several different ones with varying properties.

“What we are trying to do is establish whether this particle is really consistent with the Higgs boson, the particle we predict in our Standard Model, and not one of many Higgs bosons, or an imposter that looks like it but has a different origin,” says CMS team member Dr Markus Klute of MIT.

Despite Chiochia's confidence the significance of the observations was 3.8 standard deviations, short of the 5 sigma level used by particle physicists to classify an observation as confirmed, rather than potentially the result of other interactions or random fluctuations. Further research is planned to try to reach five standard deviations when the Large Hadron Collider restarts collisions in 2015.