https://doi.org/10.1140/epjc/s10052-020-8103-7
Regular Article - Theoretical Physics
Dark-sector physics in the search for the rare decays 
and 
1
INFN, Sezione di Trieste, Via Valerio 2, 34127, Trieste, Italy
2
Physics Department, University of Trieste, Strada Costiera 11, 34151, Trieste, Italy
3
NICPB, Rävala 10, 10143, Tallinn, Estonia
* e-mail: emidio.gabrielli@cern.ch
Received:
6
May
2020
Accepted:
31
May
2020
Published online:
13
June
2020
We compute the contribution of the decays 
and 
, where Q is a dark fermion of the dark sector, to the measured widths for the rare decays and . The recent experimental limit for 
from NA62 sets a new and very strict bound on the dark-sector parameters. A branching ratio for within the reach of the KOTO sensitivity is possible. The Grossman–Nir bound is weakened by the asymmetric effect of the different kinematic cuts enforced by the NA62 and KOTO experiments. This last feature holds true for all models where the decay into invisible states takes place through a light or massless intermediate state.
© The Author(s), 2020
