CMS Collaboration Makes Historic Discovery of Single Top Quark

The CMS collaboration at CERN has achieved a significant milestone by observing the production of a single top quark alongside a W boson and a Z boson. This groundbreaking event, noted for its extreme rarity, occurs only once in approximately every trillion proton collisions at the Large Hadron Collider (LHC). The findings were announced on November 3, 2025, and have been made available on the arXiv preprint server.

Understanding the Rare tWZ Production

This specific process, referred to as tWZ production, is pivotal for advancing our understanding of fundamental forces. The top quark, being the heaviest known fundamental particle, has a significant interaction with the Higgs field. By investigating tWZ production, physicists can explore how the top quark engages with the electroweak force, which is mediated by the W and Z bosons. This could potentially unveil new insights into the Higgs mechanism and highlight signs of phenomena beyond the Standard Model of particle physics.

Observing tWZ production is not straightforward. It ranks among the rarest events that can be measured at the LHC and involves complex analysis. The process bears similarities to ttZ production, where a top quark and an anti-top quark are produced alongside a Z boson. Notably, ttZ production occurs approximately seven times more frequently than tWZ production, leading to considerable background noise that researchers must differentiate from their signal.

“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,” explained Alberto Belvedere, a researcher with the CMS collaboration at DESY. Utilizing a machine learning algorithm, the team successfully isolated the signal from tWZ production amidst the surrounding data.

Implications of the Findings

The CMS collaboration’s analysis revealed that the rate of tWZ production was slightly higher than theoretical predictions. Researchers are now tasked with determining whether this discrepancy is merely a statistical fluctuation or an indication of new physical interactions. “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,” noted Roman Kogler, another researcher with the CMS collaboration.

This historic observation emphasizes the LHC’s capability to unveil some of nature’s most elusive secrets. The study not only showcases the potential for groundbreaking discoveries in particle physics but also reinforces the ongoing commitment of CERN and its collaborations to push the boundaries of our understanding of the universe.

For further details, refer to the paper titled “Observation of tWZ production at the CMS experiment,” available on arXiv.