Abstract
Meson decays offer a good opportunity to probe new physics. The rare kaon decay $K^+ \rightarrow \pi^+ \nu\bar{\nu}$ is one of the cleanest of them and, for this reason, is rather sensitive to new physics, in particular, vector mediators. NA62 collaboration, running a fixed-target experiment at CERN, recently reported an unprecedented sensitivity to this decay, namely a branching fraction of $BR(K^+ \rightarrow \pi^+ \nu\bar{\nu}) = (11^{+4.0}_{-3.5})\times 10^{-11}$ at 68\% C.L. Vector mediators that couple to neutrinos may yield a sizeable contribution to this decay. Motivated by the new measurement, we interpret this result in the context of a concrete $Z^\prime$ model, and put our findings into perspective with the correlated $K_L \rightarrow \pi^0 \nu \bar{\nu}$ decay measured by KOTO collaboration, current, and future colliders, namely the High-Luminosity and High-Energy LHC.
Highlights
Mesons have played a key role in the understanding of properties of elementary particles
We compare our results with the last limits from KOTO [55] and, NA62 Run1 (2016 þ 2017 þ 2018) [11,12,56]
From the left-panel of Fig. 3 one can conclude that the KOTO experiment excludes at most Z0 masses around
Summary
Mesons have played a key role in the understanding of properties of elementary particles. The introduction of strangeness along with isospin lead us to the eight-fold way, based on the SU(3) flavor symmetry. These theoretical insights have contributed to the discovery of quantum chromodynamics as we know today. Another good example is the famous θ − τ puzzle. Two different decays were found for charged strange mesons known at the time as θþ and τþ. The decay modes had different parities, but the particles were shown to have the same mass and lifetime.
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