Search for Baryon-Number Violating ${\mathrm{\Xi }}_b^0$ Oscillations

A search for baryon-number violating ${\mathrm{\Xi }}_b^0$ oscillations is performed with a sample of $pp$ collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The baryon number at the moment of production is identified by requiring that the ${\mathrm{\Xi }}_b^0$ come from the decay of a resonance ${\mathrm{\Xi }}_b^{*-}\to {\mathrm{\Xi }}_b^0{\pi }^{-}$ or ${\mathrm{\Xi }}_b^{\prime -}\to {\mathrm{\Xi }}_b^0{\pi }^{-}$, and the baryon number at the moment of decay is identified from the final state using the decays ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{\pi }^{-},{\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+}$. No evidence of baryon-number violation is found, and an upper limit at the 95% confidence level is set on the oscillation rate of $\omega <0.08{\mathrm{ps}}^{-1}$, where $\omega$ is the associated angular frequency.

Figure 1

(a) A non-BNV quark diagram for a ${\mathrm{\Xi }}_b^{\prime ,*-}\to {\mathrm{\Xi }}_b^0{\pi }^{-}$ strong decay followed by a ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{\pi }^{-}$ weak decay. (b) The corresponding BNV diagram with a ${\mathrm{\Xi }}_b^0$ to ${\overline{\mathrm{\Xi }}}_b^0$ oscillation followed by a decay to the final state ${\overline{\mathrm{\Xi }}}_c^{-}{\pi }^{+}$.

Figure 2

Spectra of the mass difference $\delta m\equiv m({\mathrm{\Xi }}_b^0\pi )-m({\mathrm{\Xi }}_b^0)-m_{\pi }$ in the data after the full selection, for the OS sample (black points with error bars) and SS decays (red, hatched histogram). The blue curve is a fit to the OS data. The ${\mathrm{\Xi }}_b^{\prime -}$ and ${\mathrm{\Xi }}_b^{*-}$ peaks are at $\delta m\approx 3.7$ and $24\mathrm{MeV}/c^2$, and the $\delta m$ resolution at these points is approximately 0.2 and $0.5\mathrm{MeV}/c^2$, respectively; the ${\mathrm{\Xi }}_b^{*-}$ also has a non-negligible natural width of $\mathrm{\Gamma }\approx 1.7\mathrm{MeV}$ [23]. Inset: Detail of the region $2.0<\delta m<5.5\mathrm{MeV}/c^2$.

Figure 3

Distribution of $\delta m$ vs decay time. The OS data are shown as grey points and the SS data as larger red triangles. The vertical lines indicate the decay time bins. The horizontal lines are intended to guide the eye and indicate the ${\mathrm{\Xi }}_b^{\prime -}$ and ${\mathrm{\Xi }}_b^{*-}$ regions. Because of selection requirements, few candidates are present at short decay times.

Figure 4

For illustration, plots of the decay time distribution of signal candidates. In these plots, the two resonance regions are combined, and background is subtracted with a simplified statistical procedure that, unlike the likelihood described in the text, allows negative SS yields. (Upper) The OS and SS data. (Lower) Detail of the SS data, along with the expected SS yields under various hypotheses for the angular frequency $\omega$ of the oscillations.