Hunting for forbidden decays of the Z boson

3 August 2021 | By

The particles that make up matter can do all sorts of amazing things, like turn into other particles or annihilate with their anti-particles. But there are also things that they aren’t allowed to do, like create or destroy electric charge. For example, a photon or Z boson can decay into an electron and an anti-electron (i.e. a positron), but not into two electrons. These rules give physicists clues about the basic laws of the Universe.

Sometimes, physicists notice that particles seem to obey a rule, but they don’t understand why. For example, Z bosons have been seen turning into an electron and a positron, or a muon and an anti-muon, but never into an electron and an anti-muon, or a muon and a positron. Such processes wouldn’t create or destroy electric charge, nor would they break any other known rule. Do they truly never happen, or are they just very rare? If they never happen, there could be a rule in the Universe that forbids electrons converting into muons or vice-versa.

Physicists know that cousins of these particles – the mysterious neutrinos – can make this kind of switch, a mechanism called “lepton flavour violation”. Electron neutrinos have been spotted turning into muon neutrinos and vice versa, a discovery that helped physicists understand long-standing mysteries about the neutrinos produced in the Sun. It also made them question whether electrons and muons could also make this kind of change.

The ATLAS Collaboration has set the most powerful limits on flavour-violating Z boson decays to electrons and muons.

Figure 1: Distribution of the invariant mass of the Z→eμ candidates (x-axis), for data (points) and expected backgrounds (lines). A final total fit is shown in blue, while the Z →ττ component is in green, the Z→μμ component is in black and the pink curve represents all remaining background contributions. A hypothetical Z→eμ signal, whose branching fraction is scaled to 20 times the observed upper limit, is shown in red for illustration purposes. The lower panel shows the ratio of observed data to the expected background yields. (Image: ATLAS Collaboration/CERN)

In a new study, the ATLAS Collaboration looked for Z bosons decaying into an electron and an anti-muon, or into a muon and a positron. But finding a single example of this unusual combination of particles is not enough to claim this decay has been seen, as other processes may leave a similar signature in the ATLAS detector. For example, if the Z decays into tau-leptons, it may produce an electron and muon along with several other particles.

To isolate the signal they are looking for, physicists calculated the mass of the particle that produced the electron and muon, which would tend to cluster around the mass of the Z boson if the searched-for decay did in fact happen. Physicists used a machine learning technique – a Gradient Boosted Decision Tree – to distinguish between signal and background events and improve the sensitivity of their search.

The new analysis examines the full ATLAS dataset collected during Run 2 of the LHC (2015–2018). In these several years of collision data, physicists saw many events with an electron and a muon, as shown in Figure 1, but there is no evidence for a cluster of events near the mass of the Z boson, which is about 91 GeV.

The ATLAS Collaboration cannot rule out that the Z boson can decay into an electron and a muon, but they can make statistical statements about how rare it has to be to evade detection. They have set the most powerful limit on this process yet: if the Z boson decays to an electron and a muon, it does so less than 3.2 times in 10 million opportunities. This gives theorists more clues about whether or not this rule against electrons and muons switching into each other is a basic rule of the Universe.