New research conducted by physicists from CERN’s Compact Muon Solenoid experiment at the Large Hadron Collider has confirmed that the Higgs boson decays to fermions, as predicted by the Standard Model of particle physics.
Higgs boson decays to four muons: lines denote other particles, and energy deposited is shown in blue. Image credit: CMS.
In 2012, CERN’s CMS and ATLAS experiments reported the discovery of a new boson with a mass near 125 GeV and properties compatible with those expected for the Higgs boson – a fundamental particle first proposed in 1964.
In the Standard Model, the Higgs boson is a spin-zero particle predicted to arise from the Higgs field which is responsible for electroweak symmetry breaking. As such, the Higgs boson couples directly to the W and Z bosons, and indirectly to photons.
To date, physicists have confirmed that the Higgs boson decays to either γγ, WW, or ZZ boson pairs, as predicted by the theory.
But the Standard Model also predicts that the boson decays to fermions.
“In nature, there are two types of particles: fermions and bosons. Fermions, quarks and leptons make up all the matter around us. Bosons are responsible for mediating interaction between the elementary particles,” explained Dr Ketino Kaadze of Kansas State University, a co-author of the paper published in the journal Nature Physics.
Using data collected at the Large Hadron Collider in 2011 and 2012, Dr Kaadze and his colleagues have demonstrated that the Higgs boson decays to bottom quarks and tau leptons, both of which belong to the fermion particle group.
“This also was predicted in 1964 but not observed until after the Higgs boson was identified in 2012,” Dr Kaadze added.
“This prediction was confirmed – a strong indication that the particle discovered in 2012 actually behaves like the Higgs particle proposed in the theory,” said co-author Prof Vincenzo Chiochia of the University of Zurich’s Physics Institute.
He added: “this is a major step forward. We now know that the Higgs boson can decay to both bosons and fermions, which means we can exclude certain theories predicting that the Higgs particle does not couple to fermions.”
“We think that the boson is responsible for the generation of mass of fundamental particles,” Dr Kaadze said.
“For example, the electrons acquire their mass by interacting with the Higgs boson. As electrons are not massless, they form stable orbits around nuclei, thus allowing the formation of electrically neutral matter from which the Earth and all of us are made,” he said.
“Even slight changes of the masses of fundamental particles around us would change the Universe very drastically, and the Higgs boson is the centerpiece that ties it all together.”
The Higgs boson was the last key component needed to confirm the Standard Model: a low-energy theory that explains the workings of the Universe at the smallest length scales.
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CMS Collaboration. Evidence for the direct decay of the 125 GeV Higgs boson to fermions. Nature Physics, published online June 22, 2014; doi: 10.1038/nphys3005
