Isospin Amplitudes in ${\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda }({\mathrm{\Sigma }}^0)$ and ${\mathrm{\Xi }}_b^0\to J/\psi {\mathrm{\Xi }}^0(\mathrm{\Lambda })$ 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 $|A_1({\mathrm{\Lambda }}_b^0\to J/\psi {\mathrm{\Sigma }}^0)/A_0({\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda })|$, 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.

Figure 1

Leading order Feynman diagrams for ${\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda }({\mathrm{\Sigma }}^0)$ and ${\mathrm{\Xi }}_b^0\to J/\psi {\mathrm{\Xi }}^0(\mathrm{\Lambda })$ decays.

Figure 2

Distribution of the $J/\psi \mathrm{\Lambda }$ mass for run 2 data. Error bars without data points indicate empty bins. Also shown is the projection of the joint fit to the data. The thick (blue) solid curve shows the total fit. For illustrative purposes, the ${\mathrm{\Lambda }}_b^0\to J/\psi {\mathrm{\Sigma }}^0$ signal component is artificially scaled to its measured upper limit. The shapes are identified in the legend.

Figure 3

Result of the hypothesis tests conducted using the $CLs$ method by varying $\mathcal{R}$ is shown. The observed $CLs$ distribution is shown by the round (black) points. The expected $CLs$ distribution (based on the background only hypothesis) is shown by the dashed line (black), with 1 and $2\sigma$ uncertainty bands depicted in dark shaded (green) and light shaded (yellow) bands. The observed and expected upper limits are obtained by seeing where the bands cross the $p$ value of 0.05 shown as the horizontal (red) line.