Isospin amplitudes in ${\Lambda }_b^0\to J/\psi \Lambda ({\Sigma }^0)$ and ${\Xi }_b^0\to J/\psi {\Xi }^0(\Lambda )$ decays

Abstract

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

All figures and tables, along with machine-readable versions and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-039.html (LHCb public pages)

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• Observation of two new ${\Xi }_b^{-}$ baryon resonances
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• Quark model relations for b-baryon decay
• Estimates of the isospin-violating ${\Lambda }_b\to {\Sigma }^0\varphi ,{\Sigma }^{0}J/\psi$ decays and the $\Sigma -\Lambda$ mixing