Isospin amplitudes in b-baryon decays

Ratios of isospin amplitudes in hadron decays are a useful probe of the interplay between weak and strong interactions, and allow searches for physics beyond the Standard Model. We present the first results on isospin amplitudes in $b$-baryon decays, using data corresponding to an integrated luminosity of $8.5 \mathrm{fb}^{-1}$, collected with the LHCb detector in $pp$ collisions at center of mass energies of 7, 8 and 13 TeV. The isospin amplitude ratio $|A_1(\Lambda_b \rightarrow J/\psi \Sigma^0)/A_0(\Lambda_b \rightarrow J/\psi \Lambda)|$, where the subscript on $A$ indicates the final-state isospin, is measured to be less than $1/21.8 $ at 95\% confidence level. This upper limit provides support for the light diquark structure in the $\Lambda_b$, and further confirms its SM status as an isosinglet. The Cabibbo suppressed $\Xi_b^0 \rightarrow J/\psi \Lambda$ decay is observed for the first time, allowing for the measurement $|A_0(\Xi_b^0 \rightarrow J/\psi \Lambda)/A_{1/2}(\Xi_b^0 \rightarrow J/\psi \Xi^0)| =0.37 \pm 0.06 \pm 0.02$, where the uncertainties are statistical and systematic, respectively. This measurement is consistent with the preservation of both isospin and SU(3) flavor symmetries.