Some discoveries happen instantly while others happen in stages, keeping the celebrations going as the truth unravels itself bit by bit. The latter is the case with the discovery of the Higgs particle.
The latest piece of evidence to fall in place is the decay of Higgs boson to fermions. The CMS collaboration, in a paper published online in Nature Physics, declares the “evidence for direct decay of the 125 GeV Higgs boson into fermions.” Similar observations have also been reported by ATLAS experiment in a note put up on their website.
This discovery complements the earlier experiments in which the Higgs was seen to decay into bosons such as pairs of W, Z and photons.
The paper describes the CMS collaboration’s analysis of some events from their data in which the Higgs boson decays into tau-lepton pairs; they have also seen events where the Higgs decays into b quark and anti-b quark pairs.
“These measurements find evidence for Higgs coupling [by decay] to fermions. In the Standard Model, the Higgs also gives mass to the fermions in addition to the weak gauge bosons.
A consequence of this is that the [strength of the] Higgs coupling to the fermions is proportional to its mass, which the measurements are confirming,” says Shrihari Gopalakrishna, particle physicist at the Institute of Mathematical Sciences, Chennai.
The Standard Model is a longstanding theory that describes electromagnetic, weak and strong interactions by invoking an internal symmetry called the gauge symmetry. Related to this symmetry are the gauge particles, which mediate the corresponding interaction.
The photon is an example of a gauge particle: it mediates the electromagnetic interaction. Its masslessness is responsible for the long range of the electromagnetic force. On the other hand, the gauge particles mediating the weak interactions need to be massive, since the corresponding force has a short range. It was in this context that the Higgs mechanism was postulated in 1964 to give the weak gauge bosons their mass. The associated Higgs particle would decay into massive W and Z bosons which are the gauge particles for weak interactions.
This was exactly the nature of the interactions observed in late 2012, following which it was declared that the Higgs had been discovered. However, in the Standard Model, the Higgs is also responsible for giving mass to the matter particles, which are fermions. The latest results from CMS and ATLAS are preliminary evidence for this, as tau-leptons are fermions, unlike the gauge particles, which are bosons.
At a statistical significance of 3.8 sigma, which the CMS paper in Nature Physics reports, the process of Higgs going to tau-anti-tau is pretty good evidence, and further work may be needed to ensure the same for the b-anti-b processes.
One reason it has taken so long to find these events is that b–anti-b pairs are produced in large numbers through other interactions, as well.
A sensitive experiment and ensuing analysis were needed to see the signal over the dominating background noise.
“This [result] is close to the prediction of the Standard Model and puts a constraint on physics beyond the Standard Model, but it does not rule out physics beyond the Standard Model. This is a milestone for experimentalists,” says V. Ravindran, particle physicist from the Institute of Mathematical Sciences, Chennai.
Interestingly, the observed Higgs particle has a mass less than what is predicted by the Standard Model. Dr. Gopalakrishna says: “The Standard Model fails to explain why the Higgs boson mass is light enough to have been observed at the Large Hadron Collider. Many proposals for physics beyond the Standard Model have been made in order to explain this shortcoming. So far, the LHC has not found any direct or indirect evidence for the presence of such new physics, which makes us wonder if we are missing some important piece of the puzzle. The upcoming higher energy run of the LHC to begin early next year should tell us more.”