Lepton flavor violation: the search for mismatched Higgs boson decays at CMS

The Standard Model allows for the Higgs boson to decay to identically flavored pairs of leptons, such as electrons and muons, but not to mixed pairings of lepton flavors. Evidence of the latter would be a sign of new physics.

The Standard Model allows for the Higgs boson to decay to identically flavored pairs of leptons, such as electrons and muons, but not to mixed pairings of lepton flavors. Evidence of the latter would be a sign of new physics.

For a moment, forget everything you know about twins and imagine you were told “the only way two siblings could have been born on the same day is if they were identical twins.” You would go about life assuming that the only twins in the world were siblings who were the same age and looked exactly alike. Of course, in reality, there are fraternal twins, and the first time you encountered a pair of nonidentical siblings born on the same day, you’d have to assume that your initial information was at least incomplete. Physicists are trying to test a principle of the Standard Model by looking for a particle version of fraternal twins, or lepton flavor violation.

These U.S. scientists contributed to this result.

These U.S. scientists contributed to this result.

The fundamental particles known as fermions that make up ordinary matter all seem to come in multiple flavors or generations. For example, the electron has a heavier cousin called the muon. Apart from its mass, a muon behaves much the same way as an electron in terms of having similar properties and interacting with the same forces. One key exception is the flavor itself, a quantity unique to a given flavor of particle. For example, an electron has an “electron number” of +1 while its antiparticle, the positron, has a corresponding number of -1. Muons, on the other hand, have an electron number of 0 but have corresponding “muon numbers.” The Standard Model requires that certain types of interactions, say the decay of a Higgs boson, always conserve lepton flavor. This means that a Higgs boson can decay into an electron and a positron (which would sum to an electron flavor of zero) or a muon and an antimuon (again, a muon flavor sum of zero), but not to an electron and an antimuon, the latter being an example of lepton flavor violation. In short, the Standard Model requires that identical twins of particles emerge from Higgs boson decays and expressly forbids fraternal twins.

Thus, observing decays of Higgs boson into fraternal twins of lepton pairs, say an electron and a muon, would be a strong sign of physics beyond the Standard Model. CMS physicists searched for evidence of such decays, specifically for Higgs boson decays to electron-muon and electron-tau lepton pairs. The search, performed in the dataset accumulated by CMS in 2012 and reported in a paper submitted to Physics Letters B, yielded no evidence of either type of decay. The results did place the tightest bounds yet on the possible rates of such decays and allowed physicists to place constraints on some models of physics beyond the Standard Model.