Search for an Axion-Like Particle in $B\rightarrow K^{(*)} a (\rightarrowγγ)$ Decays at Belle

TL;DR

The study probes axion-like particles (ALPs) in $B\rightarrow K^{(*)} a$ with $a\rightarrow \gamma\gamma$ using a 711 fb$^{-1}$ Belle data sample at the $\Upsilon(4S)$, covering $m_a$ from 0.16 to 4.50 GeV/$c^2$ (0.16–4.20 GeV/$c^2$ for the $K^{*}$ modes). It employs multivariate backgrounds suppression, data-driven calibration, and unbinned likelihood fits to extract potential signals, including the possibility of displaced decays. No significant excess is found; 90% CL upper limits on the ALP–W coupling $g_{aW}$ are set as a function of $m_a$, spanning $3\times10^{-6}$ to $3\times10^{-5}$ GeV$^{-1}$ and improving previous constraints by about a factor of two. The results constrain electroweak-coupled ALP scenarios and demonstrate Belle’s continued sensitivity to light, long-lived pseudoscalars, with implications for complementary searches such as BaBar and NA62.

Abstract

We report a search for an axion-like particle $a$ in $B\rightarrow K^{(*)} a (\rightarrowγγ)$ decays using data collected with the Belle detector at the KEKB asymmetric energy electron-positron collider. The search is based on a $711 \mathrm{fb^{-1}}$ data sample collected at the $Υ4S$ resonance energy, corresponding to a sample of $772\times10^6$ $Υ4S$ events. In this study, we search for the decay of the axion-like particle into a pair of photons, $a \rightarrow γγ$. We scan the two-photon invariant mass in the range $0.16\ \mathrm{GeV/}c^2-4.50\ \mathrm{GeV}/c^2$ for the $K$ modes and $0.16\ \mathrm{GeV/}c^2-4.20\ \mathrm{GeV}/c^2$ for the $K^{*}$ modes. No significant signal is observed in any of the modes, and 90\% confidence level upper limits are established on the coupling to the $W$ boson, $g_aW$, as a function of $a$ mass. The limits range from $3 \times 10^{-6} \mathrm{GeV}^{-1}$ to $3 \times 10^{-5} \mathrm{GeV}^{-1}$, improving the current constraints on $g_aW$ by a factor of two over the most stringent previous experimental results.

Figures (15)

• Figure 1: The Feynman diagram for a $B\xspace\rightarrow\xspace K\xspace^{(*)}{a}\xspace(\rightarrow\xspace\gamma\gamma\xspace)$ decay.
• Figure 2: Diphoton invariant mass distribution of ALP candidates in $B^+\xspace\xspace\rightarrow\xspace K^+\xspace{a}\xspace$ decay, overlaid with simulated background contributions from $e^+e^-\xspace\rightarrow\xspace q\overline q\xspace$ (blue vertical hatched), $e^+e^-\xspace \rightarrow\xspace 焇{(4S)}\xspace \rightarrow\xspace B^+\xspace { B^-\xspace}\xspace$ (red cross-hatched), and $e^+e^-\xspace \rightarrow\xspace 焇{(4S)}\xspace \rightarrow\xspace B^0\xspace { \overline{ B}{}\xspace^0\xspace}\xspace$ (green diagonal hatched) normalized to the experimental data luminosity, with all weights applied.
• Figure 3: Extracted signal yield ($N_{\rm Sig}$) (top) and the significance level ($Z_{\rm Sig}$) (bottom) for four kaon modes. The grey bands are the excluded regions corresponding to the $\pi^0\xspace,\eta$ and $\eta^\prime$ mass regions.
• Figure 4: 90% CL upper limits on the coupling $g_{{a}\xspace W}\xspace$ as a function of the ALP mass obtained with the CLs method with simultaneous fit to the four kaon modes. The green and yellow bands are the $\pm 1$ and $\pm 2$ standard deviation ranges, respectively, for the expected upper limits in the background only model. The red bands are the excluded $\pi^0\xspace,\eta$ and $\eta^\prime$ mass regions. The vertical dashed line indicates the nominal $\eta_c$ mass. Systematic uncertainties are included in the figure.
• Figure 5: The 90% CL upper limits on the coupling $g_{{a}\xspace W}\xspace$ from a simultaneous fit to the four $B\xspace\rightarrow\xspace K\xspace^{(*)}{a}\xspace$ modes as a function of the ALP mass, compared with existing constraints BaBar:2021ichPhysRevLett.118.111802NA62:2021zjwGoudzovski:2022vbt.