Event displays of four-top-quark production from ATLAS (left) and CMS (right). Event displays of four-top-quark production from ATLAS (left) and CMS (right). Picture: CERN
The ATLAS and CMS collaborations have observed a new way of producing top quarks. Top quarks, which are produced about once per 100 million collisions at the Large Hadron Collider LHC at CERN, can be created in many ways. Top quark-antiquark pairs are the most common, but particle physicists have also observed single top quarks that stand alone. Even four such quarks at once can be produced in a collision, where two top quarks and two top antiquarks are created. However, this does not happen often. For every collision that creates four top quarks, the scientists expect 4000 collisions to create the Higgs particles, which are also famously rare. At the Moriond particle physics conference currently taking place in Italy, the two collaborations have presented three new results on these extremely rare events.
The top quark's extremely high mass means it has a very close relationship to the Higgs boson. This means tour-top-quark production is not just interesting because it is rare, but also because of the influence of the Higgs boson can change the frequency of the creation of four quarks and how they behave. And the same is true for many hypothetical, undiscovered particles and forces, meaning the production of four top quarks provides hints to physics beyond the Standard Model.
“The search for events with four top quarks is a challenge, but it is worth it,” says Freya Blekman, Lead Scientist at DESY, who is also a professor at Universität Hamburg. Blekman has been studying the creation of four top quarks since the LHC started in 2010. She explains that new undiscovered particles or even an interaction between the top quark and the Higgs that doesn't match the calculations would change the frequency of the creation of four top quarks. “Besides, our four top quarks deliver spectacular events that provide experimentally exciting challenges.”
Each top quark decays into other particles, namely a W boson and a bottom quark, and each quark produces a distinctive jet of particles called a jet. The W boson, in turn, can produce either a charged lepton and a neutrino or two quark jets. Leptons are a class of particles to which the electron belongs. What this means for researchers is that the images of particle decays registered by the detector vary dramatically and, to identify four top quark events, can include anything from zero to four charged leptons, such as muons or electrons, and up to twelve jets. So finding four top quark events is a big challenge.
That is why all ATLAS and CMS analysis that look for four top quarks use machine learning algorithms to distinguish the four top quark events from the uninteresting “background” of other top quarks. The observation by ATLAS and CMS shown as a preliminary result at the Moriond conference in Italy are from re-analysis of previously published data from the LHC, with more advanced methods to reject backgrounds and in signatures with two, three or four leptons. But Blekman and her team worked on the most challenging final states with zero, one or two leptons, and to predict the behaviour of the overwhelming background from about 80 000 other top quarks for every collision with four top quarks, advances in experimental techniques and background assessment were needed to improve the understanding of these types of LHC events substantially. She mentions, “We managed to reach evidence of 3.9 standard deviations, or a chance of one in 10 000 that this is a statistical fluctuation, in one of the most technically demanding signatures studied at the LHC. This would not be possible if it were not for the machine learning experts that worked on making the analysis the best it could be.”
One such machine learning specialist, Vichayanun Wachirapusitanand from Chulalongkorn University in Thailand is co-supervised by Blekman. A member of the CMS collaboration working on his Ph.D., he went on to become first author of the recent publication of evidence of four top quark production. “Being a PhD student right at the centre of the action during this work is challenging for me. I am so grateful for all of our team working together and helping me throughout many years working on this project, and the three months I spent at DESY in Hamburg in 2022 really helped move my scientific work forward!” he says.
Interestingly, the number of collisions consistent with the production of four top quarks exceeds the prediction of the Standard Model when measured by both ATLAS and CMS, but is still fully consistent with it within the large measurement uncertainties. The accuracy of the analysis will improve significantly as more data is added, so the production of four top quarks is one of the exciting topics to be studied during the Run 3 of the LHC that started in 2022, as well as with the High Lumi LHC, which is expected to come on stream in the late 2020s and provide 20 times more data than now.
Further Information
CERN News
DESY-CMS publication (submitted 07 March 23)
