Display of a WWW candidate event detected by the ATLAS experiment.
Today, the international ATLAS collaboration announced the first observation of the production of three W bosons (WWW production), an interesting physics process that has never been experimentally observed before. The Standard Model theory of particle physics predicts interactions between gauge bosons and these interactions contribute to the inclusive production of pairs and triplets of gauge bosons. The probability to produce three W bosons in one proton-proton collision is rare. With the data delivered by the Large Hadron Collider at a rate of 40 million collisions per second during the past ten years, ATLAS only found about 1600 WWW candidate events after all selection criteria were applied. Among them, only about 20% of events are expected to come from the WWW signal process while the rest 80% come from background processes mimicking the same detector signature as the signal. Due to its low production rate, it took 25 years for physicists to observe the triboson WWW process after the observation of the diboson WW process in 1996.
The University of Michigan ATLAS group has played important roles in this observation as well as several studies leading to this observation. Professor Junjie Zhu initiated the first study of WWW production using the semi-leptonic decay channel (WWW->lvlvjj where the two same-sign W bosons decay to leptons and the third W boson decays to two jets) back in 2014. Together with his students Lulu Liu and Ismet Siral and postdoc Jacob Searcy, they performed the first search using 20 fb-1 of data taken at 8 TeV. The other two collaborating institutes were Brookhaven National Laboratory and University of Science and Technology of China. The results they obtained were later combined with results from the fully-leptonic decay channel (WWW->lvlvlv where all three W bosons decay to leptons). Due to the limited statistics in data, the sensitivity was low. However, people were excited about this study, and the results were shown on the cover of the European Physical Journal C in March 2017.
Zhu and his student Ismet Siral and postdoc Siyuan Sun continued the study using 80 fb-1 of data taken at 13 TeV. This time they collaborated with colleagues from Harvard University, University of Science and Technology of China, and Argonne National Laboratory. Both semi-leptonic and fully-leptonic channels were analyzed. First evidence of the production of three W bosons was found with a significance of 3.2 standard deviations. The results were published in Physics Letter B in 2019 together with the WVZ search performed by scientists from University of Bonn. An article describing the results they obtained can be found on Phys.org.
Recently, Zhu and his student Wenhao Xu and postdoc Siyuan Sun analyzed 139 fb-1 of data taken at 13 TeV. This time they collaborated with colleagues from University of Oklahoma, Argonne National Laboratory, and INFN Gruppo Collegato di Udine and ICTP. Observation of WWW production was provided with a significance of 8.2 standard deviations. The WWW production rate was measured with a precision of 15%. The above image shows the event display for a WWW candidate. Two same-sign W bosons decayed into two electrons (the green bars), and the third W boson decayed into a muon (the red line). There are also three neutrinos in the final state and they are not detected, but shown in the detector as the missing transverse energy (the dashed white line). The conference note can be found here and a paper is under preparation.
Searcy, Sun, and Xu have been appointed as one of the primary contact persons for these three analyses (often one ATLAS analysis has two primary contact persons), and Zhu has been the internal paper editor for all three analyses. Siral graduated in May 2019 with a thesis focusing on the first evidence and Xu graduated in December 2020 with a thesis focusing on the first observation.
Zhu and his team plan to continue to study triboson processes with larger datasets collected by the ATLAS experiment in the future. In addition to WWW production, he is also working with Professor Bing Zhou and students Man Yuan, Zhichen Wang, and Shuzhou Zhang on the search for WVZ production (V=W, Z). Studies of these rare processes allow further tests of the gauge structure of the electroweak theory in the Standard Model and searches for signs of new physics.