Evidence for the direct decay of the Higgs boson into fermions

Researchers at CERN have found evidence for the direct decay of the Higgs boson into fermions. This comes two years after the discovery of the particle and indicates that it behaves in the way the standard model of particle physics predicts.

The Higgs boson is associated with a mechanism first put forward in 1964 by Robert Brout, François Englert and Peter Higgs to account for the different ranges of two fundamental forces of nature. This mechanism is postulated to give rise to the masses of all the fundamental particles. In order to test that idea fully, it is necessary to measure the direct decay of the Higgs boson into all kinds of particles.

Writing in the journal Nature Physics, the scientists explained that although “properties of the new boson are consistent with those of the long-sought agent responsible for electroweak symmetry breaking … [an] important open question is whether the new particle also couples to fermions, and in particular to down-type fermions”. As a group of elementary particles, fermions form the matter while bosons act as force carriers between fermions.

Until now, the Higgs particle could only be detected through its decay into bosons. But measurements from both CERN’s CMS and ATLAS experiments have reported evidence that the Higgs boson decays directly to fermions at a rate consistent with that predicted by the standard model of particle physics, the theory that accounts for the fundamental particles of visible matter and the interactions that work between them, giving structure to matter.

According to the model, the interaction strength between the fermions and the Higgs field must be proportional to their mass. As noted by Professor Vincenzo Chiochia from the University of Zurich, whose group was involved in analysing the data, “This prediction was confirmed; a strong indication that the particle discovered in 2012 actually behaves like the Higgs particle proposed in the theory.”

The researchers combined the Higgs decays into bottom quarks and tau leptons, both of which belong to the fermion particle group. Measurement from the CMS experiment’s pixel detector revealed that an accumulation of these decays comes about at a Higgs particle mass near 125 gigaelectron volts (GeV) and with a significance of 3.8 sigma. This means that the probability of the background alone fluctuating up by this amount or more is about one in 14,000. In particle physics, a discovery is deemed confirmed from a significance of 5 sigma.

“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,” said Professor Chiochia.

More information will be available as results are announced at the International Conference on High Energy Physics (ICHEP), which starts in Valencia, Spain, on 2 July.