New Physics Implications of Recent Search for KL→π0νν¯

Abstract

The KOTO experiment recently reported four candidate events in the signal region of KL → π 0 νν search, where the standard model only expects 0.10 ± 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: (1) heavy new physics boosting the standard model signal, (2) reinterpretation of "νν" 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 + → π + νν decay, bounded by data from the E949 and the NA62 experiments.

Breaking the Grossman-Nir bound in kaon decays

“…For definiteness, we takeĉ a to be the central value of the SM prediction [24] and suppose thatĉ b stems from O LNC dim-9 in Eq. (39) and has the formĉ b = F K F π Be iθ /Λ 5 where F K(π) is the kaon (pion) decay constant [4], B = − qq /(3F 2 π ) ≈ 2.8 GeV where qq is the quark [11] of recent KOTO results [1,5] and to the latest NA62 limit [6], and the red dot labeled NP corresponds to (Λ, θ) = (39 GeV, −π/4) in this model.…”

“…(41) on the Λ-θ plane. We depict the predictions of this toy scenario for Λ ∈ [1, 60] GeV and θ ∈ [−π, π] with the green region on the right panel and compare them to an interpretation [11] of the recent KOTO results [1,5] as well as to the latest NA62 limit [6]. Clearly this model has parameter space which can explain the anomaly in the new preliminary KOTO data [1] but the NP scale has to be of order tens of GeV, as the left plot indicates, this will be discussed in detail in our upcoming publication.…”

“…Here we adopt the framework of effective field theory below the electroweak scale, where all the effective operators respect the SM residual gauge symmetry U(1) em × SU(3) color and contain only 1 Several different scenarios involving new particles contributing to K → π plus missing energy to break the GN bound have been studied recently [10][11][12][13][14][15][16][17].…”

Light gauge boson interpretation for (g − 2)μ and the KL→ π0 + (invisible) anomaly at the J-PARC KOTO experiment

“…We set our upper bound by requiring Γ(D + → π + X) < Γ D + ,total − Γ D + ,K 0 using the inclusive value of the branching ratios, to avoid a significant modification of the total width of D + meson. 4 At m X m π 0 , it gives only a weak upper bound g X ∼ < 1.31 × 10 −2 , and thus not sensitive to KOTO and (g − 2) µ preferred region.…”

“…The required branching ratio of K L → π 0 νν for the three candidate events is [4] Br(K L → π 0 νν) KOTO = 2.1 +2.0 −1.1 × 10 −9 , (1.1)…”