Measurement of the branching fraction of the $Λ_b^0\to J/ψΛ$ decay and isospin asymmetry of $B\to J/ψK$ decays

Abstract

This paper describes a measurement of the $Λ_b^0\to J/ψΛ$ branching fraction using data collected with the LHCb experiment in proton-proton collisions from 2016 to 2018. The dataset corresponds to an integrated luminosity of 5.4$\,\text{fb}^{-1}$. The branching fraction is determined relative to that of $B^0\to J/ψK^0_\text{S}$ decays, $\frac{\mathcal{B}(Λ_b^0\to J/ψΛ)}{\mathcal{B}(B^0\to J/ψK^0_\text{S}} = 0.750 \pm 0.005 \pm 0.022 \pm 0.005 \pm 0.062\,,$ yielding $\mathcal{B}(Λ_b^0\to J/ψΛ) = (3.34 \pm 0.02 \pm 0.10 \pm 0.08 \pm 0.28)\times 10^{-4}$, where the first uncertainty is statistical, the second systematic, the third due to external inputs on branching fractions and the fourth due to the ratio of $Λ_b^0$ baryon and $B^0$ meson hadronisation fractions. In addition, the isospin asymmetry between the rates of $B^0\to J/ψK^0_\text{S}$ and $B^+\to J/ψK^+$ decays is measured to be $A_{\rm I} = -0.0135 \pm 0.0004 \pm 0.0133$, where the first uncertainty is statistical and the second systematic.

Figures (9)

• Figure 1: Mass distributions of selected (top) ${{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^+}\xspace$, (middle) ${{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^0_{\mathrm{S}}}\xspace$ and (bottom) ${{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{\Lambda\xspace}\xspace$ candidates obtained by combining the data in the $p_{\mathrm{T}}$ range [4, 20]$\text{\,Ge V\!/}c$. The line and shaded regions indicate the result of the fits and are obtained by summing the results of the fits to the individual $p_{\mathrm{T}}$ bins.
• Figure 2: Efficiency ratio between ${{\Lambda\xspace}\xspace^0_{\mathrm{b}\xspace}\xspace}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{\Lambda\xspace}\xspace$ and ${{\mathrm{B}\xspace}\xspace^0}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^0_{\mathrm{S}}}\xspace$ decays determined from corrected simulation samples in intervals of $\mathrm{b}\xspace$-hadron transverse momentum for the (left) long-track and (right) downstream-track samples.
• Figure 3: Isospin asymmetry between ${{\mathrm{B}\xspace}\xspace^0}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^0}\xspace$ and ${{{\mathrm{B}\xspace}\xspace^+}\xspace}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^+}\xspace$ decays in bins of $\mathrm{B}\xspace$-meson $p_{\mathrm{T}}$ for the long and downstream-track samples averaged. The inner (outer) bars indicate the statistical (total) uncertainty on the data points. The green (blue) band indicates the average isospin asymmetry across the $p_{\mathrm{T}}$ bins and its statistical (total) uncertainty.
• Figure 4: Ratio of yields between ${{\Lambda\xspace}\xspace^0_{\mathrm{b}\xspace}\xspace}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{\Lambda\xspace}\xspace$ and ${{\mathrm{B}\xspace}\xspace^0}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^0_{\mathrm{S}}}\xspace$ decays in bins of $\mathrm{b}\xspace$-hadron $p_{\mathrm{T}}$ for the (left) long-track and (right) downstream-track samples. The dashed line shows the result of the fit described in the text. The inner and outer shaded regions correspond to 68% and 95% confidence intervals, respectively. These regions include contributions from the statistical uncertainty on the event yields and uncertainties on the external parameters entering the branching-fraction ratio fit.
• Figure 5: Reconstructed ${{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^+}\xspace$ mass distribution of selected ${{{\mathrm{B}\xspace}\xspace^+}\xspace}\xspace\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace{{\mathrm{K}\xspace}\xspace^+}\xspace$ candidates in bins of ${{\mathrm{B}\xspace}\xspace^+}\xspace$ meson $p_{\mathrm{T}}$. The line and shaded regions show the result of the fit described in the text.