Scientists Observe Single Top Quark Produced with W and Z Bosons for the First Time at CERN
For the first time in particle physics history, scientists at CERN’s Large Hadron Collider (LHC) have successfully observed one of the rarest processes predicted by the Standard Model: the simultaneous production of a single top quark, a W boson, and a Z boson. This extraordinary event, called tWZ production, has now been officially recorded by the CMS (Compact Muon Solenoid) Collaboration, marking another milestone in our quest to understand the fundamental forces that shape the universe.
This process is so rare that it happens only once in about a trillion proton–proton collisions. The discovery opens an entirely new window into how the top quark, the heaviest known fundamental particle, interacts with other elementary forces—particularly the electroweak force, which is carried by the W and Z bosons.
What Exactly Did Scientists Observe?
In the tWZ process, a single top quark is produced alongside a W boson and a Z boson. During the collision inside the LHC, two protons smash together at nearly the speed of light. From this violent burst of energy, new particles emerge—sometimes common ones like photons or pions, and sometimes, as in this case, incredibly rare combinations predicted by theory but almost impossible to see.
When the top quark forms, it quickly decays into a bottom quark (b quark) and another W boson. That b quark appears in the detector as a jet—a concentrated spray of particles represented visually as yellow cones in the CMS event display. The original W boson decays into a muon and an invisible neutrino, while the Z boson decays into two muons (shown as red lines in the detector image). By piecing together these decay patterns, researchers confirmed the event’s signature as tWZ production.
Why This Is Such a Big Deal
The top quark is a particularly special particle in the subatomic zoo. It’s roughly 180 times heavier than a proton, making it the most massive elementary particle ever discovered. Because of this, it has an unusually strong connection to the Higgs field, the quantum field that gives particles their mass. Studying how the top quark interacts with the W and Z bosons could help physicists test the boundaries of the Higgs mechanism and possibly uncover hints of new physics beyond the Standard Model.
The tWZ process is especially valuable because it’s a sensitive probe of electroweak interactions involving the top quark. If anything about this process deviates from what theory predicts, it could point toward unknown forces, new particles, or hidden dimensions that current physics cannot yet explain.
How Rare Is This Event?
Extremely rare. The CMS team describes finding this signal as “searching for a needle in a haystack the size of an Olympic stadium.” In every trillion proton–proton collisions at the LHC, you might only see this happen once.
Such rarity means the process is easily drowned out by other, much more common reactions. One of the biggest challenges was distinguishing tWZ from a similar process called ttZ production, where a top quark and an anti-top quark are created together along with a Z boson. The ttZ process occurs about seven times more frequently, making it a major background that can obscure the tWZ signal.
How Did They Detect It?
The observation relied on advanced analysis techniques, including machine learning algorithms designed to separate the true signal from the overwhelming background noise.
The researchers analyzed data collected during Run 2 and Run 3 of the LHC, using a total integrated luminosity of around 200 inverse femtobarns (fb⁻¹). That translates to an immense number of proton collisions recorded at energies of 13 TeV and 13.6 TeV.
Their analysis focused on events containing three or four charged leptons (electrons or muons), because those are the telltale signatures of W and Z boson decays. This multilepton strategy dramatically reduced contamination from other processes and increased the sensitivity of the search.
What the Results Showed
The CMS team measured the production cross section (essentially, the probability that the process will occur) for tWZ production:
- At 13 TeV: 248 ± 52 femtobarns
- At 13.6 TeV: 244 ± 74 femtobarns
These results come with a statistical significance of 5.8 standard deviations (σ)—well above the threshold of 5σ required in physics to claim an official discovery. That means the probability of the signal being a random fluctuation is less than one in a few million.
Interestingly, the observed rate is slightly higher than what theoretical models predict. The difference isn’t enough yet to declare new physics, but if future data show that the deviation grows at higher energies, it might signal the presence of undiscovered interactions or exotic particles.
Why Machine Learning Was Crucial
Because tWZ events are buried under vast amounts of background data, traditional analysis methods wouldn’t have been enough. Physicists at DESY and other research centers within the CMS collaboration trained sophisticated machine learning models to identify subtle patterns distinguishing tWZ events from look-alikes like ttZ.
The algorithm learned from millions of simulated collisions, refining its ability to tell signal from background. This is another example of how AI and data science are becoming central tools in modern physics, helping scientists make sense of the enormous datasets produced by experiments like the LHC.
The Broader Significance
This observation not only confirms another rare prediction of the Standard Model but also sets the stage for precision tests of the electroweak sector. The top quark’s role in these interactions could reveal whether the Higgs mechanism we know is complete—or whether there’s more to the story.
If future studies at higher energies show that the tWZ production rate keeps diverging from theoretical expectations, it would be a clear clue toward new physics. Some possibilities include anomalous couplings, composite particles, or even heavier, unseen bosons that subtly influence the outcome of these interactions.
For now, though, the discovery solidifies the Standard Model’s consistency, reminding us of just how powerful it still is—even as scientists continue to test its limits.
About the Top Quark and the LHC
The Large Hadron Collider is the world’s most powerful particle accelerator, located underground near Geneva, Switzerland. It consists of a 27-kilometer ring where protons are accelerated to nearly the speed of light and smashed together at enormous energies. Detectors like CMS and ATLAS then record the resulting particle debris.
The top quark itself was first discovered in 1995 at Fermilab’s Tevatron collider in the United States. It plays a central role in particle physics because of its immense mass and strong connection to the Higgs field. Every time physicists observe a new way the top quark interacts with other fundamental particles, they get a deeper glimpse into how the universe holds itself together at the smallest scales.
What’s Next
The CMS collaboration will continue collecting and analyzing data from the ongoing LHC runs. With future upgrades to the accelerator (the High-Luminosity LHC), physicists expect to gather even larger datasets, allowing them to measure tWZ production with greater precision.
If the slightly higher rate of tWZ events persists—and especially if it grows with energy—it could be one of the first indicators of physics beyond the Standard Model. For now, this achievement stands as proof of the LHC’s incredible capability to reveal the universe’s most elusive secrets.
Research Reference:
Observation of tWZ Production at the CMS Experiment (arXiv:2510.19080)
