Search for the Z boson decay to $\tau\tau\mu\mu$ in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper reports the first search for the rare Z boson decay to tau tau mu mu at the LHC, setting upper limits on its branching ratio and testing related effective-field-theory operators using CMS data.

Contribution

It presents the first experimental search for Z to tau tau mu mu decay and constrains related effective-field-theory operators at the LHC.

Findings

No significant excess observed over background.

Upper limit of 6.9 times the SM expectation on the decay ratio.

First constraints on six flavor-conserving four-lepton EFT operators.

Abstract

The first search for the Z boson decay to $\tau\tau\mu\mu$ at the CERN LHC is presented, based on data collected by the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb$^{-1}$. The data are compatible with the predicted background. For the first time, an upper limit at the 95% confidence level of 6.9 times the standard model expectation is placed on the ratio of the Z $\to$ $\tau\tau\mu\mu$ to Z $\to$ 4$\mu$ branching fractions. Limits are also placed on the six flavor-conserving four-lepton effective-field-theory operators involving two muons and two tau leptons, for the first time testing all such operators.