Since the elusive Higgs boson was discovered in 2012, scientists have been poring over Large Hadron Collider (LHC) collision data for more evidence of interactions involving the particle. Now physicists of one LHC experiment have announced tantalizing evidence that the Higgs doesn’t only decay into other bosons; it also has the ability to decay into fermions — further proof that we are looking at a Standard Model Higgs boson and not something more exotic.
Theorized for decades, the hunt for the Higgs boson became the key reason why the LHC, the most powerful and expensive particle collider on the planet, was built. This historic physics quest came to a satisfactory conclusion when researchers overseeing experiments in two massive LHC detectors — the CMS (Compact Muon Solenoid) and ATLAS (A Toroidal LHC Apparatus) — revealed the discovery of a “new” boson that had a striking similarity to the theorized Higgs. Further experimental tests proved that this new boson was indeed the Higgs, a particle that mediates the Higgs field, a ubiquitous field that endows all matter with mass.
But until now, physicists have only seen one type of Higgs decay.
As particles (primarily protons) are accelerated close to the speed of light inside the LHC’s superconducting electromagnets and collided at unprecedented energies, physicists have glimpsed the extreme conditions that existed around the time of the Big Bang. By creating these mini-Big Bangs, energy can condense to form “free” particles that otherwise would not exist in nature. To track down any Higgs particles produced in these collisions, physicists need to investigate the particles produced from specific decays as outlined by Higgs theory.
The Higgs particle is highly unstable and decays very quickly. Until now, physicists have only detected one decay path — as the Higgs boson decays into other bosons. Bosons are the ‘force carriers’ of nature — e.g. the photon is a boson as it mediates the electromagnetic force; the W and Z bosons mediate the weak force (which is responsible for radioactive decay and nuclear fusion of atomic nuclei).
Now, for the first time, physicists have detected a regime where the Higgs also decays into fermions.
“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 Vincenzo Chiochia, from the University of Zurich’s Physics Institute. Chiochia is a member of the international team who analyzed the new data and one of the hundreds of scientists involved with The CMS Collaboration. This new research has been published in the journal Nature Physics.
Fermions are the subatomic building blocks of all matter — including quarks, leptons (such as electrons) and combinations of quarks (hadrons like protons and neutrons) — and, according to these new results, the Higgs boson can decay into the bottom quarks (a heavy type of quark) and tau particles (a kind of heavy electron).
“In July 2012, we knew we had discovered some sort of boson, and it looked a lot like it was a Higgs boson,” said Paul Padley of Rice University and member of the CMS Collaboration. “To firmly establish it’s the Standard Model Higgs boson, there are a number of checks we have to do. This paper represents one of these fundamental checks.”
This new line of evidence focuses on the detection of a particle ‘excess’ signal around the 125 gigaelectron volt (GeV) Higgs boson — the same “vanilla” Standard Model Higgs that was announced in 2012.
It does fall short of being a discovery, however. This new Higgs decay process has an experimental significance of 3.8 sigma, but the researchers expect that significance to rise to 4.4 sigma after further analysis. A ‘discovery’ will only be announced when this statistical significance rises to 5 sigma, a level that can only be attained after analysis of more post-collision decay particles in the LHC’s detectors.
This new line of Higgs decay measurements may exclude some hypotheses beyond the Standard Model, but CMS physicists see this as the beginning of a new journey in the hunt for exotic physics especially once the LHC recommences higher-energy collision experiments in 2015.
“We’re interested in understanding how the top quark (one of the Higgs’ decay products) fits into the Standard Model and whether, since it’s so heavy, it could have a special role in relating to the Higgs. Maybe there are hints of new physics that aren’t in the Standard Model,” said Karl Ecklund, also of Rice University. “We know the standard model is incomplete.”
Publication: Evidence for the direct decay of the 125 GeV Higgs boson to fermions, CMS Collaboration, Nature Physics, 2014. doi:10.1038/nphys3005