Why is the Higgs boson so light? That’s one of the questions that has been bothering particle physicists since the famous particle was discovered in 2012. This is because the theory of how the particle interacts with the most massive of all observed elementary particles, the top quark, involves corrections at a fundamental (quantum) level that could result in a Higgs mass much larger than the measured value of 125 GeV. How large? Perhaps as much as sixteen orders of magnitude larger than the measured Higgs mass. Since the Higgs mass is so light, this suggests more particles could exist that cancel the quantum corrections from the top quark (and other heavy particles).
In a paper posted online and submitted to the journal Physical Review Letters, the ATLAS collaboration reports results of a combination of searches for a new particle – dubbed a vector-like top quark – that could help keep the Higgs boson light.
Various proposals attempt to cancel out the large quantum corrections to the Higgs boson mass. Many of them involve vector-like top quarks, which are hypothetical particles not predicted by the Standard Model of particle physics. Unlike the Standard Model top quark, which always decays to a bottom quark and a W boson, vector-like top quarks would decay in one of three different ways, if they decayed to Standard Model particles. Specifically, a vector-like top quark would decay to a bottom quark and a W boson, or to a Z boson and a top quark, or still to a Higgs boson and a top quark.
To maximise the chances of finding vector-like top quarks, the ATLAS collaboration conducted several different types of search using data from proton–proton collisions collected at the Large Hadron Collider (LHC) in 2015 and 2016 at an energy of 13 TeV; each individual search is sensitive to a particular set of particle decays. They then combined the results to increase the sensitivity to vector-like top quarks, yet found no sign of them.
Despite this, their analysis allowed them to expand the reach of individual searches and place the most stringent lower bounds on the mass of vector-like top quarks to date. The analysis excludes vector-like top quarks with masses below about 1300 GeV for any combination of the three top-quark decays into Standard Model particles. The previous best lower limit from an individual search was 1190 GeV.
It will now get more challenging: for masses heavier than 1300 GeV a single vector-like top quark is created more often than a pair. But with a wealth of data coming from the LHC, the search continues.