In a breakthrough, researchers at CERN have found the first evidence for the direct decay of the Higgs boson into fermions — a strong indication that the particle discovered in 2012 is the Higgs boson.
The findings confirm that the bosons decay to fermions — a group of particles that includes all leptons and quarks — as predicted by the Standard Model of particle physics.
“This is an enormous breakthrough,” said Markus Klute, an assistant professor of physics at Massachusetts Institute of Technology (MIT).
“Now we know that particles like electrons get their mass by coupling to the Higgs field, which is really exciting,” said Klute.
In July 2012 researchers from the ATLAS and Compact Muon Solenoid (CMS) experiments at the European Organisation for Nuclear Research (CERN), said they had observed a new particle in the mass region of 125 to 126 gigaelectronvolts (GeV).
Preliminary studies showed the new particle’s properties were consistent with those predicted for the Higgs boson by the Standard Model, but much more work was needed to confirm.
Researchers wanted to clarify whether there was a single Higgs or many different Higgs particles, as predicted by various extensions of the Standard Model, Klute said.
“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,” he said.
Previous analysis of the data produced by experiments at CERN’s Large Hadron Collider, in Switzerland, has shown that like the Higgs boson of the Standard Model, the new particles have no spin, and rapidly decay by splitting into pairs of photons, W bosons, or Z bosons. But it remained uncertain whether they could also decay to fermion pairs, Klute said.
Now the team from the CMS Collaboration has demonstrated that the bosons also decay to fermions in a way that is consistent with the Standard Model Higgs.
“We have now established the main characteristics of this new particle, in its coupling to fermions and to bosons, and its spin—parity structure; all of these things are consistent with the Standard Model,” Klute said.
To determine whether the particles could decay to fermions, the researchers fired protons at each other in a 6—metre—diameter solenoid and used specialised detectors to determine which particles were produced in the resulting collisions.
The researchers were hunting for particles called tau leptons, which have a mass of around 1.7 GeV, making them around 3,500 times heavier than their little sibling, the electron.
They were able to confirm the presence of decay to tau leptons with a confidence level of 3.8 standard deviations — a one in 10,000 chance that the signal they saw would have appeared if there were no Higgs particles.
The study was published in the journal Nature Physics.