Search for $\boldsymbol{B\to hν\barν}$ decays with semileptonic tagging at Belle

Abstract: We present the results of a search for the rare decays $B\to h\nu\overline{\nu}$, where $h$ stands for $K^+,:K^0_{\mathrm{S}},:K^{\ast +},:K^{\ast 0},:\pi^+,:\pi^0,:\rho^+$ and $\rho^{0}$. The results are obtained with $772\times10^{6}$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB $e^+ e^-$ collider. We reconstruct one $B$ meson in a semileptonic decay and require a single $h$ meson but nothing else on the signal side. We observe no significant signal and set upper limits on the branching fractions. The limits set on the $B\to K^0_{\mathrm{S}}\nu\overline{\nu}$, $B^0\to K^{*0}\nu\overline{\nu}$, $B\to \pi^+\nu\overline{\nu}$, $B^0\to\pi^0\nu\overline{\nu}$, $B^+\to\rho^+\nu\overline{\nu}$, and $B^0\to\rho^0\nu\overline{\nu}$ channels are the world's most stringent.

Search for Rare $B \rightarrow h \nu \overline{\nu}$ Decays at Belle

The paper presents an extensive analysis of rare decay modes of $B$ mesons, specifically looking at $B \rightarrow h \nu \overline{\nu}$, where $h$ can be a variety of charmless states such as $K^+$, $K^0$, $K^{\ast +}$, $K^{\ast 0}$, $\pi^+$, $\pi^0$, $\rho^+$, and $\rho^0$. These decays are theoretically intriguing due to their rare occurrence in the standard model, primarily proceeding through either penguin or box diagrams, making them sensitive probes of new physics beyond the standard model (BSM).

Methodology

The team utilized the Belle detector at the KEKB collider to analyze $772 \times 10^6$ $B\overline{B}$ pairs. The search employed semileptonic tagging to reconstruct one $B$ meson, enhancing the branching fraction sensitivity compared to previous methods that used hadronic tagging. The semileptonic tagging offers a statistically independent and more efficient sample by reconstructing $B \rightarrow D^{(\ast)}l\nu_l$ for both $B^+$ and $B^0$ mesons.

The invisible nature of the neutrinos in the final state required sophisticated techniques to isolate the signal, including neural network-based event selection optimized across various kinematic variables. The extra energy, $E_{\mathrm{ECL}}$, in the Belle detector played a crucial role in rejecting background events.

Results and Discussion

Despite the sophisticated methods employed, the analysis did not yield significant signals for any of the channels investigated. The upper limits set on the branching fractions represent the most stringent constraints to date:

• $B \rightarrow K^+ \nu \overline{\nu}$: $< 1.9 \times 10^{-5}$
• $B \rightarrow K^0 \nu \overline{\nu}$: $< 1.3 \times 10^{-5}$
• $B \rightarrow K^{\ast +} \nu \overline{\nu}$: $< 6.1 \times 10^{-5}$
• $B \rightarrow K^{\ast 0} \nu \overline{\nu}$: $< 1.8 \times 10^{-5}$
• $B \rightarrow \pi^+ \nu \overline{\nu}$: $< 1.4 \times 10^{-5}$
• $B \rightarrow \pi^0 \nu \overline{\nu}$: $< 0.9 \times 10^{-5}$
• $B \rightarrow \rho^+ \nu \overline{\nu}$: $< 3.0 \times 10^{-5}$
• $B \rightarrow \rho^0 \nu \overline{\nu}$: $< 4.0 \times 10^{-5}$

The significance of the highest potential signal, $B \rightarrow K^{\ast +} \nu \overline{\nu}$, reached only $2.3\sigma$, emphasizing the challenge of detecting these rare events.

Implications and Future Work

The results contribute critical data points for testing various BSM theories, especially those offering explanations for discrepancies observed in related decay channels, such as deviations in angular observables in $B^0 \rightarrow K^{\ast 0} \mu^+ \mu^-$ decays. Given that decays involving $B \rightarrow K^{(\ast)} \nu \overline{\nu}$ are theoretically clean, primarily mediated by a $Z$ boson, they offer a strategic target for ongoing and future experimental and theoretical studies.

Future experiments will benefit from further improving sensitivity in detecting these rare decay processes either through advanced detector technology or by exploring complementary decay modes that maintain the theoretical benefits but allow enhanced resolution capabilities. As new physics models evolve, these decays will undoubtedly continue to be integral to shaping our understanding of fundamental particle interactions.