Search for heavy neutral leptons in top quark decays with same-sign, same-flavor dilepton final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

For decades, physicists have known that the universe is not quite complete as it stands. The standard model, the theory that describes the fundamental particles and forces of nature, works with stunning precision, yet it leaves a glaring gap: it cannot explain why neutrinos, those ghostly particles that stream through everything in the cosmos, have mass. The model originally assumed they were weightless, but experiments have confirmed they are not. This tiny discrepancy suggests that the standard model is merely a partial view of a deeper reality. To fill this gap, scientists have proposed the existence of heavy neutral leptons, massive cousins to the familiar neutrinos that could hold the key to understanding why the universe has matter at all. These hypothetical particles are predicted by theories that extend our current understanding, offering a mechanism to generate the small masses we observe in ordinary neutrinos while hiding their heavier counterparts from our view.

At the heart of this mystery is a specific type of heavy particle that behaves differently from the matter we know. While ordinary particles have distinct identities that prevent them from turning into their own mirror images, these heavy neutral leptons are theorized to be their own antiparticles. If such a particle exists, it would violate a fundamental rule of nature called lepton number conservation, a principle that usually keeps the count of certain particles balanced in every interaction. Finding evidence of this violation would be a monumental discovery, proving that nature allows processes that are strictly forbidden in our current textbook. It would confirm that these heavy particles exist and that they can transform in ways that ordinary matter cannot, potentially explaining the origin of the universe's matter-antimatter imbalance.

To hunt for these elusive particles, researchers at the Large Hadron Collider in Geneva turned their attention to the top quark, the heaviest known elementary particle. In the standard model, a top quark almost always decays into a bottom quark and a W boson, which then breaks down into lighter particles. However, if heavy neutral leptons exist, a top quark might occasionally take a different path, decaying into a bottom quark, a charged lepton like an electron or muon, and one of these heavy neutral leptons. The heavy particle would then travel a short distance before decaying itself, producing another charged lepton and a W boson. Because the heavy particle is its own antiparticle, the second lepton it produces would have the same electric charge as the first one. This creates a rare and distinctive signature: two electrons or two muons with the same charge, appearing together in a single collision event, accompanied by a spray of other particles.

The CMS collaboration, a team of thousands of scientists operating a massive detector at the collider, analyzed a vast collection of proton-proton collisions to find this specific pattern. They sift through data from 2016 to 2018, corresponding to an enormous amount of recorded information, looking for events where two particles of the same charge and type appear alongside jets of energy from broken-down quarks. The challenge is immense because the background noise of ordinary particle interactions is overwhelming. Most collisions produce pairs of particles with opposite charges, and finding two of the same charge usually indicates a measurement error or a rare, known process. To separate the potential signal from this noise, the team employed a sophisticated computer algorithm trained to recognize the subtle differences in energy and direction between a true heavy particle decay and a standard background event.

After applying their strict selection criteria and running the data through their analysis tools, the researchers found no evidence of the heavy neutral leptons they were seeking. The number of events they observed matched the predictions for ordinary background processes perfectly. There was no excess of same-charge lepton pairs that could be attributed to the decay of a top quark into a heavy neutral lepton. This null result is a significant finding in itself, as it allows the scientists to set strict boundaries on where these particles might be hiding. They determined that if these heavy particles exist and are produced in top quark decays, they must be extremely rare, occurring less than once in every few tens of thousands of top quark decays.

The study focused on heavy neutral leptons with masses between 20 and 100 gigaelectronvolts, a range that had not been thoroughly explored in this specific decay channel before. For particles in this mass range, the researchers calculated that the probability of a top quark decaying into a heavy neutral lepton is limited to a very small fraction. In the case of electrons, the decay happens less than one time in roughly 20,000 to 90,000 top quark decays, depending on the mass of the particle. For muons, the limit is even tighter, with the decay occurring less than once in roughly 40,000 to 270,000 decays. These limits are the most stringent constraints to date for this specific production method, effectively ruling out the existence of these particles in this mass range if they were produced at higher rates.

While the search did not uncover the heavy particles, it provided a crucial piece of the puzzle by narrowing the search space. The results tell us that if these heavy neutral leptons exist and interact with top quarks in the way the theory predicts, they must be even more elusive than previously thought. The data agrees with the standard model, suggesting that the heavy particles, if they exist, do not appear in top quark decays with the frequency required to explain the neutrino mass mystery through this specific mechanism alone. This forces theorists to reconsider the parameters of their models or look for these particles in other production channels. The work stands as a testament to the precision of modern particle physics, where the absence of a discovery is just as informative as a discovery itself, guiding the scientific community toward the next steps in understanding the fundamental building blocks of the universe.