Study of the decay K+ => pi+,nu,nubar in the momentum region 140<Ppi<199 MeV/c

TL;DR

<3-5 sentence high-level summary>The paper presents the final results from the E949 experiment on the rare kaon decay K^+ → π^+ ν ν̄ in the low-pion-momentum region, reporting three new candidate events atop a carefully estimated background, and combining with previous results to extract a branching ratio of (1.73^{+1.15}_{-1.05})×10^{-10}, consistent with the Standard Model. The analysis employs a blind procedure, data-driven background estimation via a bifurcation method, and a likelihood framework that partitions the signal region into cells with varying signal-to-background ratios, leveraging detailed detector information including an extensive photonic veto and precise tracking of stopped kaons. The results constrain possible new-physics contributions to flavor-changing neutral currents and provide a competitive, model-aware assessment of the πνν̄ final state, including explorations of scalar/tensor hypotheses and two-body exotic decays. The work highlights methodological advances in rare-decay searches—particularly in background suppression, acceptance calibration, and rigorous statistical treatment—that are essential for pushing sensitivity in stopped-kaon experiments.

Abstract

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay K+ => pi+,nu,nubar in the pion momentum region 140 < P_pi < 199 MeV/c in an exposure of 1.71e12 stopped kaons with an estimated total background of 0.93+-0.17(stat.)+0.32-0.24(syst.) events. This brings the total number of observed K+ => pi+,nu,nubar events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of (1.73+1.15-1.05)e-10. An interpretation of the results for alternative models of the decay K^ => pi+,nothing is also presented.

Figures (18)

• Figure 1: Momentum spectra of charged particles from $K^+$ decay in the rest frame. The values in parentheses represent the branching ratios of the decay modes ref:PDG. The hatched spectrum represents the $\pi^+$ spectrum from ${K}^+\to\pi^+\nu\bar{\nu}$ decay assuming the $V-A$ interaction. The densely hatched regions represent the $\pi\nu\bar{\nu}(1)$ and $\pi\nu\bar{\nu}(2)$ E949 signal regions.
• Figure 2: Schematic side (a) and end (b) views of the upper half of the E949 detector. An incoming $K^+$ is shown traversing the beam instrumentation, stopping in the target and decaying to $\pi^+\pi^0$. The outgoing charged pion and one photon from the $\pi^0\to\gamma\gamma$ decay are illustrated. Elements of the detector are described in the text.
• Figure 3: (Upper) Contribution of each photon veto detector in radiation lengths as a function of $\cos\theta$ for a photon emitted from the origin of the detector coordinate system. (Lower) Contribution of each photon veto element in the upstream and downstream regions within $18^\circ$ of the beam.
• Figure 4: Range in plastic scintillator vs. momentum for charged particles accepted by the $\pi\nu\bar{\nu}(1\!\!+\!\!2)$ trigger. The concentrations of events due to the two-body decays are labeled $K_{\pi2}$-peak and $K_{\mu2}$-peak. The decays $K^+\to\pi^0\mu^+\nu_\mu$ and $K^+\to\mu^+\nu_\mu\gamma$ contributed to the muon band. The pion band resulted from $K^+\to\pi^+\pi^0\gamma$ decays, $K_{\pi2}$ decays in which the $\pi^+$ scattered in the target or range stack and beam $\pi^+$ that scatter in the target. The boxes at low and high momentum represent the signal regions for this analysis and the previous $\pi\nu\bar{\nu}(1)$ analysis ref:pnn1PRD, respectively. This distribution represents 0.13% of the total kaon exposure.
• Figure 5: Schematic of the bifurcation method. The background level in region $A$ can be estimated from the number of events observed in regions $B$, $C$ and $D$ assuming CUT1 and CUT2 are uncorrelated. See text for details.