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Résumé |
Could a new interaction exist in nature ? Beyond weak,
electromagnetic and strong interactions within the standard model, it
is natural to enquire about an additional one, associated with an
extra-$U(1)$ gauge group. The corresponding boson, called $U$ forty
years ago, may be (very) light, and (very) weakly coupled, its
properties depending on $m_U$, the size of its coupling, and
associated current (which further depends on the Brout-Englert-Higgs sector). Its vector part is a combination of electromagnetic with $B$ and $L$ (or $B-L$) currents. Axial couplings may also be present, which may make the spin-1 $U$ boson interact much like a spin-0 axion-like particle.\\
We discuss, in connection with the underlying theory, some of the effects of such a new boson, including its
production in $e^+ \,e^-$ annihilations and beam dump experiments, $\psi$, $Y$
and $K$ decays, parity-violation effects in atomic physics, anomalous
magnetic moments of charged leptons, non-standard neutrino
interactions, a possible ''protophobic'' behaviour suppressing $\pi^0
\to \gamma \, U$ decays (related with a tentative interpretation of
the Atomki anomaly in some nuclear reactions). The $U$ may also serve as a mediator to a new dark sector, allowing for thermally-produced light dark matter particles.
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