A search for feebly-interacting particles in $B$ decays with missing energy at Belle

Abstract

We present a search for an invisible hidden-sector particle $ X_{\rm{inv}}$, produced in $B^0 \to \bar{D}^0 X_{\rm{inv}}$ and $B^\pm\to h X_{\rm{inv}}$ decays, where $h = π^\pm$, $K^\pm$, $D_{s}^\pm$, $p^\pm$. The search is performed using $e^+ e^-$ collision data recorded with the Belle detector, corresponding to 711~fb$^{-1}$. No significant signal is observed. We set 90\% confidence level upper limits ranging between $10^{-4}$ and $10^{-6}$ on the branching fraction $\mathcal{B}(B \to h X_{\rm{inv}})$ as a function of $m_{ X_{\rm{inv}}}$. Corresponding limits are set on $\mathcal{B}(B \to hX)\times\mathcal{B}(X\toγγ)$ for lifetimes $cτ_X$ between $10~μ$m and 10 m. Many of these limits are the first direct constraints on their respective decays. Our results provide the most stringent exclusion limits to date on the branching fractions for all search channels, and exclude previously unexplored regions of parameter space relevant to several new physics models.

Figures (9)

• Figure 1: Momentum distributions of the $K^+\xspace$ and $D^0\xspace$ in the $B_{\rm{sig}}\xspace$-frame for the two control channels, $B^+\xspace\xspace\rightarrow\xspace{J / \psi }\xspace K^+\xspace$ (top) and $B^0\xspace\rightarrow\xspace\bar{ D}{}\xspace^0\xspace\pi^0\xspace$ (bottom), respectively, overlaid with the expected distributions from simulation.
• Figure 2: Upper limits on the branching fraction $\mathcal{B}(B\rightarrow\xspace hX_{\rm{inv}}\xspace)$ at 90% CL as a function of $m_{X_{\rm{inv}}\xspace}$ are shown in black. Replacing the invisible decay assumption with $X\xspace\rightarrow\xspace\gamma\gamma$ yields the colored limits, shown for several orders of magnitude of $c\tau_X\xspace$. Mass regions vetoed for known SM particles $\pi^0\xspace$, $K^{(*)0}$, $D^{(*)0}$, $\eta_c$, $\chi_{c1}$, and $\psi(2S)$ are shaded gray. Not all veto regions apply to every channel.
• Figure 3: Excluded regions for $m_{X_{\rm{inv}}\xspace}$ up to 5.1 ${\mathrm{\,Ge V\!/}c^2}$ at 90% CL shown for: (a) the coupling $g_{aW}$ in the invisible ALP scenario from Ref. Izaguirre_2017, and (b) the mixing angle $\theta$ for the dark scalar in the missing energy scenario from Ref. PhysRevD.101.095006. Exclusions from BABAR Lees_2013BabarMono, NA62 NA62:2020xlgNA62:2020pwi, E949 BNL-E949:2009dza, and CMS CMS:2018uag are overlaid for comparison.
• Figure 4: Limits on the coupling strength $g_{aW}$ assuming the ALP decays via $a\xspace\rightarrow\xspace\gamma\gamma$, including the constraints from other experiments presented in Ref. Izaguirre_2017, with the addition of more recent constraints that appear in Ref. Sungjin25.
• Figure 5: 90% confidence level exclusion regions on the ALP-quark couplings described in Ref. zhang2023belle.