Search for charged-lepton-flavour violating $e\mu tq$ interactions at $\sqrt{s}=13$ TeV with the ATLAS detector

In the universe as we understand it, particles called leptons come in three distinct families: electrons, muons, and taus. For decades, the fundamental laws of physics have held that these families are strictly separate. An electron should never spontaneously turn into a muon, just as a muon should never become an electron. This rule of "flavor conservation" is a cornerstone of the Standard Model, the best theory we have for how the smallest building blocks of nature interact. However, we know this rule is not absolute for all particles. Neutrinos, the ghostly cousins of these charged leptons, are known to change their flavor as they travel, a discovery that earned a Nobel Prize and proved that nature is more fluid than our simplest equations suggest. This raises a tantalizing question for physicists: if neutrinos can break the rules, could the heavier, charged versions of these particles do the same? Finding a single instance where an electron and a muon swap identities in a high-energy collision would be a smoking gun for new physics, pointing to forces or particles that exist beyond our current understanding.

To hunt for this rare transformation, scientists at the Large Hadron Collider in Switzerland turned their attention to the heaviest known elementary particle: the top quark. This particle is so massive that it decays almost instantly, vanishing in a fraction of a second after it is created. The ATLAS experiment, a massive detector the size of a cathedral that surrounds the collision point, has been recording billions of these collisions. In a recent analysis, the team sifted through data corresponding to 140 inverse femtobarns of collisions, a volume of information that represents the entire dataset collected by the detector during its second major run. They were looking for a very specific, unusual signature: a top quark that, instead of decaying into the usual mix of particles, splits into three charged particles—an electron, a muon, and a jet of other particles—while a second top quark in the same event behaves normally. This specific combination of an electron and a muon appearing together from a single top quark would be a clear violation of the flavor conservation rule.

The researchers approached this search by looking at two different ways such a violation could occur. The first scenario involves a single top quark being created directly alongside the lepton pair, while the second involves a pair of top quarks being created, where one of them decays in this forbidden way. Because the signal they are hunting for is expected to be incredibly faint, buried beneath a mountain of ordinary background events, the team used sophisticated computer algorithms to distinguish the signal from the noise. These algorithms acted like a highly trained filter, learning to recognize the subtle patterns in the energy and direction of the particles that would indicate a top quark had behaved strangely. They focused on events containing exactly three charged particles—electrons or muons—and at least one jet of debris, carefully removing any events that looked like common background processes, such as the decay of a Z boson, which can mimic the signal.

After running their analysis, the team found no evidence of the charged-lepton-flavor violation they were seeking. The number of events they observed matched the predictions of the Standard Model perfectly, with no strange excesses appearing in the data. This null result is, in its own way, a significant discovery. It allows the scientists to place strict limits on how often this forbidden decay could possibly happen. They calculated that if such a process does occur, it must be incredibly rare, happening less than once in every few hundred million top quark decays. Specifically, they determined that the probability of a top quark decaying into an electron, a muon, and an up-quark is less than six in a billion, while the probability of it decaying into an electron, a muon, and a charm-quark is less than three in ten million.

These limits are translated into constraints on the theoretical "strength" of the new forces that would be required to make this violation happen. The researchers found that any new physics capable of causing this effect must be extremely weak or exist at energy scales far beyond what the current collider can reach. The study effectively rules out a wide range of theoretical models that predicted these violations would be common enough to be seen with the current amount of data. While the search did not find the new particle or force they hoped for, it has successfully narrowed the map of where that new physics might be hiding. The absence of a signal confirms that the Standard Model remains robust in this domain, but it also sets the stage for future experiments with even more powerful machines to continue the hunt for the cracks in nature's rules.