The Higgs boson was detected using its decay into bosons but scientists from the CMS experiment at the Large Hadron Collider have found evidence for the direct decay of the Higgs boson into fermions.

If the Higgs particle can decay into both bosons and fermions, we can exclude certain theories predicting that the Higgs particle does not couple to fermions. As a group of elementary particles, fermions form the matter while bosons act as force carriers between fermions.

According to the standard model of particle physics, the interaction strength between the fermions and the Higgs field must be proportional to their mass. "This prediction was confirmed," explains Professor Vincenzo Chiochia from the University of Zurich's Physics Institute, "a strong indication that the particle discovered in 2012 actually behaves like the Higgs particle proposed in the theory."





Combined data analysis





The researchers analyzed the data gathered at the LHC between 2011 and 2012, combining the Higgs decays into bottom quarks and tau leptons, both of which belong to the fermion particle group. The results reveal 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 five sigma.





How the Higgs decay modes were measured





Three different processes were studied, whereby the UZH researchers analyzed the Higgs decay into taus. Because the Higgs particle is extremely short-lived, it cannot be detected directly, but rather only via its decay products.

The bottom quarks and taus, however, have a long enough lifetime to be measured directly in the CMS experiment's pixel detector.