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Descrizione
The first observation of the $\Sigma^+ \to p \mu^+ \mu^-$ (very) rare decay at the LHCb experiment is presented.
This is a flavour changing neutral current sensitive to physics beyond the Standard Model, which could modify it's properties.
In particular the HyperCP experiment years ago presented an evidence of this decay with a hint of a possible unknown intermediate particle (HyperCP hypothesis). This was excluded by LHCb already in 2018.
A new measurement, exploiting an integrated luminosity of $5.4$ fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of $13$ TeV, has lead to an observation of this channel with a significance greater than $5$ standard deviations. Furthermore, by confirming the previous LHCb result, the HyperCp hypothesis has been rejected again.
A measurement of its integrated and differential branching fraction is ongoing. This would be the rarest baryon decay ever observed.
Finally prospects for additional observables, such as a CP violation measurement, will also be presented.
