New results on Higgs bosons' decay into fermions

For a boson to be the Higgs boson, it has to be intimately related to the physical process it was hypothesized in 1964 to help understand. With new results published on June 22, physicists from CERN, the lab that runs the experiments that first discovered the Higgs boson, have found that to be true, further cementing the credibility of their theories as well as discovering more properties that could guide future experiments.

The Higgs boson is too short-lived to be spotted directly. Its lifetime is 10-22 seconds. In this period, it quickly decays into groups of lighter particles. The theory called the Standard Model of particle physics predicts how often the Higgs decays into which groups of particles. Broadly, the rate of this decay is guided by how strongly the Higgs couples to each particle, and such coupling gives rise to the particle’s mass (Note: the Higgs decays only into fundamental particles, not composite particles like protons and neutrons, because it gives mass only to fundamental particles).

The June 22 Letter in Nature Physics describes the champagne bottle boson‘s decay into fermions, the particles that make up all matter. By experimentally finding these rates, physicists accomplish two things. One, they assert the strength of whichever theory predicted these rates – the Standard Model, in this case (the Yukawa couplings, to be specific). Two, they establish that the Higgs boson does couple to fermions and gives them mass. The Letter draws its conclusions from experiments performed in 2011 and 2012.

The third generation of fermions

However, there is a limitation. Because the Higgs weighs 125 GeV, it could only have decayed into lighter fermions, not heavier ones. This means physicists have experimental proof for the Higgs giving mass to fermions lighter than itself; in this case, these are the so-called third generation fermions comprising the bottom quark and the tau lepton. Quarks are fundamental particles that come together to compose protons and neutrons. Leptons are some of the lightest of the matter particles, one common example of which is the electron.

In 2011, the Compact Muon Solenoid (CMS) experimental collaboration, which is the group of scientists that runs the CMS detector, had looked for Higgs bosons decaying into bottom quark-antiquark pairs. At this time, the Large Hadron Collider, which produces these particles by smashing protons together at high speeds, was operating at an energy of 7 TeV – i.e. each beam of protons coming into the collision had an energy of 7 TeV. The consequent results were published in January this year. The 2012 results concerned the search for Higgs bosons’ decays into tau lepton-antilepton pairs at 8 TeV. The pre-print paper submitted to arXiv is here (link to published paper).

The search for these particles is compounded by the fact that they aren’t just produced by the decaying Higgs boson but by a profusion of other Standard Model processes. The scientists at CERN use a combination of statistical techniques to single out which processes produced the particles of interest. They also use as many unique signatures as possible to narrow down their search. For example, the search for the bottom quark-antiquark pair of particles is reconstructed based on a Higgs boson being produced together with a W or a Z boson, whose decays have their own signatures.

The significance at which they report each decay process is in this table, picturized below.

Summary of results for the Higgs boson mass hypothesis of 125 GeV.
Summary of results for the Higgs boson mass hypothesis of 125 GeV.

The Letter, as you can see, is open access, as are all the papers linked to in it.