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, $O(\mathrm{GeV})$. There can be heavy-light neutrino mixing parametrized by $|B_{\ell N}{|}^2=10^{-5}$, which leads to the rare lepton-number-violating decay $W^{\pm }\to {\ell }_1^{\pm }{\ell }_2^{\pm }(q^{\prime }\bar{q}{)}^{\mp }$. 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 $A_{\mathrm{CP}}$ at the LHC. I consider three versions of the LHC -- HL-LHC, HE-LHC, FCC-hh -- and show that, for $5\mathrm{GeV}\le M_N\le 80\mathrm{GeV}$, small values of the CP asymmetry can be measured at $3\sigma$, in the range $1\%\le A_{\mathrm{CP}}\le 15\%$.

Comments: 6 pages, 2 figures, talk presented at BSM-2021. arXiv admin note: text overlap with arXiv:2011.03686