New Physics Implications of Recent Search for $K_L\to {\pi }^0\nu \overline{\nu }$ at KOTO

The KOTO experiment recently reported four candidate events in the signal region of $K_L\to {\pi }^0\nu \overline{\nu }$ search, where the standard model only expects $0.10\pm 0.02$ events. If confirmed, this requires physics beyond the standard model to enhance the signal. We examine various new physics interpretations of the result including these: (1) heavy new physics boosting the standard model signal, (2) reinterpretation of “$\nu \overline{\nu }$” as a new light long-lived particle, or (3) reinterpretation of the whole signal as the production of a new light long-lived particle at the fixed target. We study the above explanations in the context of a generalized new physics Grossman-Nir bound coming from the $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ decay, bounded by data from the E949 and the NA62 experiments.

Figure 1

The recent result of KOTO events [6] (NA62 result [7]), Eq. (1) [Eq. (2)], is represented by the green (blue) band. The red ellipses show our simultaneous fits to both. The GN bound with (without) interference with the SM is shown by the solid (dashed) blue line. The red dots are the best fit points on those lines. Only statistical uncertainties are taken into account.

Figure 2

Left: branching ratio of $K_L\to {\pi }^0\nu \overline{\nu }$ or ${\pi }^{0}X$ that can accommodate the KOTO events. The dotted blue (solid gray) line corresponds to the central value of $K_L\to {\pi }^{0}X({\pi }^0\nu \overline{\nu })$ interpretation, with blue shaded band (dashed horizontal lines) for two-sided 68% confidence interval. An uncertainty of Monte Carlo statistics is less than 10% thus omitted here. The dashed (dotted) vertical line corresponds to $m_X=180(280)\mathrm{MeV}$, and its left-hand side is compatible with the observed events (the signal region). Right: the new particle has finite lifetime considering the GN bound and $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ search, and the allowed parameter space for two body decay $K_L\to {\pi }^{0}X$ in mass and lifetime of $X$ is shown. The $K^{+}\to {\pi }^{+}X$ bound is translated to $K_L$ bound assuming a saturation of the GN bound. The purple (blue) shaded region is constrained by NA62 [7] at 95% CL (E949 [9] at 90% CL). Too short lifetime leads to $\mathcal{B}(K_L\to {\pi }^{0}X)>1\%$, which is inconsistent with untagged $K_L$ branching ratio [20]. The $\mathcal{B}(K_L\to {\pi }^{0}X)={10}^{-4}$, ${10}^{-6}$, and ${10}^{-8}$ are indicated on the plot. The green shaded region is constrained from KTEV search for $K_L\to {\pi }^0\gamma \gamma$ assuming $\mathcal{B}(X\to \gamma \gamma )=1$ [21].