Problem of plenty

The Higgs boson has finally been observed decaying into the particles it most often decays to – six years after it was discovered. The reason for this delay was noise.

The Higgs boson is a scalar boson: it has spin 0. To preserve the value of this spin quantum number, it decays into pairs of a fermion and an anti-fermion. Fermions have spin +1/2 and anti-fermions have spin -1/2; their combined spin is 0. Moreover, the Higgs boson is likelier to decay into a heavier fermion than a lighter fermion because the boson couples stronger with the former.

The heaviest fermion is the top quark, but it is too heavy itself for the Higgs to decay into. The next is the bottom quark – and theoretical calculations suggest the Higgs should decay into a bottom quark and anti-quark 60% of the time. The third is the tau particle, which weighs almost half as much as the bottom quark and to which the boson decays only 6% of the time.

Now, the problem is that hadron colliders – Large or not – also produce an abundance of bottom quarks through other mechanisms. The rules of quantum chromodynamics (QCD: the study of quarks and gluons) enable three in particular – flavour excitation, gluon-gluon fusion and gluon splitting – that produce lots of bottom quarks.

As a result, physicists couldn’t tell if bottom quarks detected during a collision at the LHC were from Higgs boson decays or due to QCD processes… until now. The physicists were able to isolate the bottom quarks produced by decaying Higgs bosons (from the data) as well as, according to an official blog post, compare the values of “other kinematic variables that show distinct differences between the signal and the various backgrounds”.

By combining calculations across all the data collected by the machine in 2015, 2016 and 2017, the results had a significance of 4.9σ – just shy of the 5σ threshold to claim a discovery. The significance does breach the threshold when other data filters are applied but more data-taking and analysis will be necessary before the official declaration comes through.

You learn something everyday.

Featured image: Event display for the H→bb decay analysis with the ATLAS detector. Credit: ATLAS Collaboration/CERN.