Rare Lepton-Number-Violating $W$ Decays at the LHC: CP Violation

Abstract

Some models of leptogenesis involve a nearly-degenerate pair of heavy Majorana neutrinos $N_{1,2}$ whose masses can be small, $O({\rm GeV})$. There can be heavy-light neutrino mixing parametrized by $|B_{\ell N}|^2 = 10^{-5}$, which leads to the rare lepton-number-violating decay $W^\pm \to \ell_1^\pm \ell_2^\pm (q'{\bar q})^\mp$. With contributions to this decay from both $N_1$ and $N_2$, a CP-violating rate difference between the decay and its CP-conjugate can be generated. In this talk, I describe the prospects for measuring such a CP asymmetry $A_{\rm CP}$ at the LHC. I consider three versions of the LHC -- HL-LHC, HE-LHC, FCC-hh -- and show that, for $5~{\rm GeV} \le M_N \le 80~{\rm GeV}$, small values of the CP asymmetry can be measured at $3\sigma$, in the range $1\%\le A_{\rm CP} \le 15\%$.