Model-Independent Evidence for $J/\psi p$ Contributions to ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ Decays

The data sample of ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ decays acquired with the LHCb detector from 7 and 8 TeV $pp$ collisions, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, is inspected for the presence of $J/\psi p$ or $J/\psi K^{-}$ contributions with minimal assumptions about $K^{-}p$ contributions. It is demonstrated at more than nine standard deviations that ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ decays cannot be described with $K^{-}p$ contributions alone, and that $J/\psi p$ contributions play a dominant role in this incompatibility. These model-independent results support the previously obtained model-dependent evidence for $P_c^{+}\to J/\psi p$ charmonium-pentaquark states in the same data sample.

Figure 1

Excitations of the $\mathrm{\Lambda }$ baryon. States predicted in Ref. [8] are shown as short horizontal bars (black) and experimentally well-established ${\mathrm{\Lambda }}^{*}$ states are shown as green boxes covering the mass ranges from $M_0-{\mathrm{\Gamma }}_0$ to $M_0+{\mathrm{\Gamma }}_0$. The $m_{Kp}$ mass range probed in ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ decays is shown by long horizontal lines (blue). The $l_{\max }(m_{Kp})$ filter is shown as a stepped line (red). All contributions from ${\mathrm{\Lambda }}^{*}$ states with $J^P$ values to the left of the red line are accepted by the filter. The filter works well also for the excitations of the $\mathrm{\Sigma }$ baryon [8, 12] (not shown).

Figure 2

Efficiency-corrected and background-subtracted $m_{Kp}$ distribution of the data (black points with error bars), with $\mathcal{F}(m_{Kp}|H_0)$ superimposed (solid blue line). $\mathcal{F}(m_{Kp}|H_0)$ fits the data by construction.

Figure 3

Legendre moments of $\cos {\theta }_{{\mathrm{\Lambda }}^{*}}$ as a function of $m_{Kp}$ in the data. Regions excluded by the $l\le l_{\max }(m_{Kp})$ filter are shaded.

Figure 4

Efficiency-corrected and background-subtracted $m_{J/\psi p}$ distribution of the data (black points with error bars), with $\mathcal{F}(m_{J/\psi p}|H_0)$ (solid blue line) and $\mathcal{F}(m_{J/\psi p}|H_1)$ (dashed black line) superimposed.

Figure 5

Distributions of $\mathrm{\Delta }(-2\ln L)$ in the model-independent pseudoexperiments corresponding to $H_0$ (red falling hatched) compared to the distributions for pseudoexperiments generated from various amplitude models and, in the inset, to the bifurcated Gaussian fit function (solid line) and the value obtained for the data (vertical bar).

Figure 6

Efficiency-corrected and background-subtracted $m_{J/\psi K}$ distribution of the data (black points with error bars), with $\mathcal{F}(m_{J/\psi K}|H_0)$ (solid blue line) and $\mathcal{F}(m_{J/\psi K}|H_1)$ (dashed black line) superimposed.