Last week, the ATLAS experiment at the Large Hadron Collider in Switzerland, showed evidence for the first time that a Higgs boson decays into a pair of tau particles. It is one of the crucial results that has followed on from the discovery of the Higgs boson.
But what makes this result so important?
On July 4, 2012, two experiments (ATLAS and CMS) at CERN announced the discovery of a new boson particle.
Later in the same year, the new particle was confirmed as the Higgs boson. Ever since, scientists have been working to pin down the properties of the particle.
The Higgs particle is a very unstable. Once it’s produced, it disintegrates (or decays) immediately into other light stable particles. Scientists hoped to find both the rate of its disintegration, and the particles it disintegrates into.
Breaking it down
All of nature is made up of bosons and fermions. The difference between the two types of particles is the way they spin, and that bosons are “gregarious” while fermions are “solitary”. Bosons include, among other things, photons, gluons, W and Z particles. Fermions include leptons and quarks.
At the time of discovery, the Higgs boson was seen to disintegrate into a pair of bosons; that is, into a pair of photons and a pair of W or Z particles.
The Standard Model of Particle Physics predicts that Higgs can decay into fermions in addition to bosons. But the decay rates can be different.
The scientists' job is to confirm or to rule out the possibility of the Higgs boson’s decay into fermions. Any deviation from the prediction would give rise to something new, something never observed before.
Scientists were investigating specifically whether the Higgs decayed into tau particles; these are a type of fermion and the heavy cousin of electrons. A tau’s mass is a few thousand times higher than that of an electron.
The preliminary results of the ATLAS collaboration at CERN show clear evidence of such decay. The decay rates are consistent with the Standard Model predictions.
The first run of the Large Hadron Collider finished earlier this year and the second run will start in 2015. The collider will operate at higher energies and produce several time more collisions and hence more Higgs bosons. The additional data from these runs will shed more light on these initial findings.
It will be important to study other possible fermionic decays when the collider is restarted. For example, scientists will be looking at whether Higgs decays into a pair of muons, another type of fermion lighter than tau but heavier than an electron.
In addition to important Higgs studies, the ATLAS experiment is searching for new or rare phenomena in nature. These include super-symmetric partners, evidence of a dark matter candidate and the existence of extra-dimensions of space.