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 $|A_1({\mathrm{\Lambda }}_b^0\to J/\psi {\mathrm{\Sigma }}^0)/A_0({\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda })|$, where the subscript on $A$ indicates the final-state isospin, is measured to be less than $1/21.8$ at 95% confidence level. The Cabibbo suppressed ${\mathrm{\Xi }}_b^0\to J/\psi \mathrm{\Lambda }$ decay is observed for the first time, allowing for the measurement $|A_0({\mathrm{\Xi }}_b^0\to J/\psi \mathrm{\Lambda })/A_{1/2}({\mathrm{\Xi }}_b^0\to J/\psi {\mathrm{\Xi }}^0)|=0.37\pm 0.06\pm 0.02$, where the uncertainties are statistical and systematic, respectively.

• Received 4 December 2019
• Revised 8 January 2020
• Accepted 14 February 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.111802

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