For the first time, scientists from the CMS experiment on the Large Hadron Collider (LHC) at CERN have succeeded in finding evidence for the direct decay of the Higgs boson into fermions. Previously, the Higgs particle could only be detected through its decay into bosons. "This is a major step forwards," explains Professor Vincenzo Chiochia from the University of Zurich's Physics Institute, whose group was involved in analyzing the data. "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." 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," says Chiochia; "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.
Measuring the Higgs decay modes
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.
The University of Zurich and the Large Hadron Collider
The University of Zurich is actively involved in the LHC at CERN with five experimental research groups: The groups headed by professors Florencia Canelli, Vincenzo Chiochia and Ben Kilminster conduct research with the CMS detector, Professors Ulrich Straumann's and Nicola Serra's groups with the LHCb detector. For the analysis and interpretation of the data, they are supported by the groups under professors Thomas Gehrmann, Stefano Pozzorini, Gino Isidori and PD Dr. Massimiliano Grazzini.
The CMS detector at CERN
The CMS detector measures the energy and impulse of photons, electrons, muons and other charged particles with high precision. Different measuring instruments are arranged in tiers inside the 12,500-ton detector. 179 institutions worldwide are involved in the construction and operation of the CMS detector. The Swiss institutions are the University of Zurich, ETH Zurich and the Paul Scherrer Institute, which jointly developed and constructed the CMS pixel detector.
- The CMS Collaboration. Evidence for the direct decay of the 125 GeV Higgs boson to fermions. Nature Physics, 2014 DOI: 10.1038/nphys3005
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