ATLAS Paper on Higgs to Tau Coupling

The ATLAS experiment at the LHC has released a lengthy paper presenting results from an analysis looking for Higgs bosons decaying to tau leptons (H→ττ). This decay can occur due to the Yukawa coupling (a 3-particle vertex, so no virtual particles are needed). The Yukawa coupling combined with the non-zero vacuum expectation value of the Higgs generates the mass of the tau. Similar couplings are expected to exist for the other fermions. Because of the relationship between the mass and the coupling, the Standard Model provides a prediction for the coupling: m_\tau / vev. Thus, the heavier the fermion, the larger the decay width and higher the branching fraction.

The tau channel is a pretty difficult channel to analyze in Higgs searches. Taus can’t be seen directly. Only their decay products (jets or other leptons) are seen, which makes it much harder to reconstruct things like the momentum. These particles are found in many other processes as well: direct production of leptons and jets, decay products of other particles like W and Z bosons, and even other decays of the Higgs. Furthermore, Z→ττ presents in irreducible background since it has the same final state. So, in order to make this measurement, these backgrounds must be accurately modeled.

The final result shows a value consistent with the Standard Model prediction. The statistical strength of the result isn’t quite high enough to call this a true measurement of the Higgs to tau Yukawa coupling, but it is enough to present the measurement as evidence of a nonzero coupling. With more data (and also an ever more thorough understanding of the background physics and detector), the analysis of this channel should continue to get stronger.