View of the open CMS detector, one of the four major experiments of the Large Hadron Collider. (Image: CERN)
The experiments at the Large Hadron Collider (LHC) detect rare events on a daily basis, but some are exceptionally rare, such as this latest result from the CMS collaboration. For the first time, the collaboration has observed the production of a single top quark along with a W and a Z boson, an extremely rare process that happens only once every trillion proton collisions. Finding this event in the LHC data is like searching for a needle in a haystack the size of an Olympic stadium.
The creation of a top quark, a W boson and a Z boson, known as tWZ production, opens up a new window for understanding the fundamental forces of nature. By closely studying tWZ production, physicists can investigate how the top quark interacts with the electroweak force, which is carried by the W and Z bosons. In addition, the top quark is the heaviest known fundamental particle, meaning that it has the strongest interaction with the Higgs field, so, studying the tWZ process could give us a deeper understanding of the Higgs mechanism. It could also point us to signs of new phenomena and physics beyond the Standard Model.
However, observing tWZ production is not easy. Not only is it one of the rarest Standard Model processes that can currently be observed at the LHC, but it is also very complex to analyse. This process looks very similar to another one known as ttZ production, in which a top and an anti-top quark are produced with a Z boson. This ttZ production happens about 7 times more often than tWZ production, which means that there is a lot of background noise that researchers must identify.
"Because of its rarity and its similarity with the ttZ process, observing tWZ production requires advanced analysis techniques involving state-of-the-art machine learning", says Alberto Belvedere, a researcher with the CMS collaboration at DESY. Using their machine learning algorithm, the researchers were able to separate the signal from tWZ production from the background data.
The CMS collaboration found that the rate of tWZ production was slightly higher than predicted by the theory. Future data and analysis will clarify whether this is just a statistical fluctuation or whether it could be the first hint of something beyond the known laws of physics.
"If there are unknown interactions or particles involved, the observed deviation between the measured rate [of tWZ production] and the prediction would rapidly become larger with increasing energies of the outgoing particles, an effect that is unique to the tWZ process", says Roman Kogler, a researcher with the CMS collaboration at DESY.
For now, the CMS collaboration has marked the first observation of a phenomenon so rare that it happens only once every trillion proton-proton collisions. This is yet another reminder of the LHC's ability to uncover nature's most elusive secrets.
