The top quark is very special. It’s the heaviest known elementary particle and therefore strongly interacts with the Higgs boson. The top quark’s interactions with other particles provide promising leads for searches for physics beyond the Standard Model. By taking accurate measurements of its properties using rare processes, physicists can explore new physics phenomena at the highest energies.
At the ongoing Rencontres de Moriond conference, the ATLAS collaboration at the Large Hadron Collider (LHC) announced the observation of one of these rare processes: the production of a single top quark in association with a photon through the electroweak interaction. With a statistical significance well above five standard deviations, the result represents the first observation of top-quark-photon production. This achievement was far from straightforward, as the search for this process was dominated by a large number of background collision events that mimic top-quark-photon production.
In their new analysis, the ATLAS researchers analysed the full LHC Run 2 data set, recorded by the detector between 2015 and 2018. They focused on collision events where the top quark decays via a W boson to an electron or a muon and a neutrino, and to a bottom quark. They further narrowed their search by seeking out a particular characteristic of top-quark-photon events: a “forward jet”, which is a spray of particles that is commonly produced and travels at small angles to the LHC’s proton beams.
To separate the top-quark-photon events from the background events, the ATLAS researchers used a neural network, which receives as input a number of variables or features, and finds the combination of those features that most accurately classifies a data event according to signal or background types.
The statistical significance of the ATLAS measurement of top-quark-photon production is 9.1 standard deviations – well above the 5 standard-deviation threshold required to claim observation of a process in particle physics. The expected significance, based on the Standard Model prediction, was 6.7 standard deviations.
This exciting measurement will allow physicists to look for hints of new interactions that might exist beyond the reach of the LHC. In particular, physicists can now use this process to infer information on new particles that could alter the top-quark-photon interaction. Further studies with new analysis techniques and a significantly larger data set from the upcoming Run 3 of the LHC promise an exciting road ahead.