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Further search for the decay $K^+ \to π^+ ν\bar ν$ in the momentum region P < 195 MeV/c

E787 Collaboration

Abstract

We report the results of a search for the decay $K^+ \to π^+ ν\bar ν$ in the kinematic region with $π^+$ momentum $140 < P < 195$ MeV/c using the data collected by the E787 experiment at BNL. No events were observed. When combined with our previous search in this region, one candidate event with an expected background of $1.22 \pm 0.24$ events results in a 90% C.L. upper limit of $2.2 \times 10^{-9}$ on the branching ratio of $K^+ \to π^+ ν\bar ν$. We also report improved limits on the rates of $K^+ \to π^+ X^0$ and $K^+ \to π^+ X^1 X^2$ where $X^0, X^1, X^2$ are hypothetical, massless, long-lived neutral particles.

Further search for the decay $K^+ \to π^+ ν\bar ν$ in the momentum region P < 195 MeV/c

Abstract

We report the results of a search for the decay in the kinematic region with momentum MeV/c using the data collected by the E787 experiment at BNL. No events were observed. When combined with our previous search in this region, one candidate event with an expected background of events results in a 90% C.L. upper limit of on the branching ratio of . We also report improved limits on the rates of and where are hypothetical, massless, long-lived neutral particles.

Paper Structure

This paper contains 4 figures, 2 tables.

Figures (4)

  • Figure 1: Schematic side (a) and end (b) views of the upper half of the E787 detector. Č is the Čerenkov counter, BeO is the passive degrader, PbG is the lead-glass detector, B4 is an energy loss counter, MC is the microcollar detector, I and T are the inner and outer scintillation counters used in the trigger, RSSC are the Range Stack tracking chambers.
  • Figure 2: Kinetic energy (in MeV) versus range (in cm of plastic scintillator) distribution of events that remained after all cuts except that on momentum (top), and after the momentum cut (bottom). The triangles represent data from the 1996 run and the circles represent data from the 1997 run. Events from Monte Carlo simulation of $K^+ \to \pi^+ \nu \bar{\nu}$ are represented by the light dots. The group of events around 108 MeV was due to $K_{\pi2}$ decays and the events at higher energy were due to $K_{\mu2}$ decays. All events except for the one in the signal region (shown by the rectangular box) were eliminated by the $140 < P <195 {\rm ~MeV/c}$ cut on momentum.
  • Figure 3: The 90% C.L. upper limit for $B(K^+ \to \pi^+ X^0$) as a function of $M_{X^0}$, where $M_{X^0}$ is the mass of the recoiling system. The solid line is from this analysis. "Previous Result" is from pnn2_96. The limit for $M_{X^0}< 140 {\rm ~MeV/c^2}$ is derived from the result for Region 1 pnn1. The single event sensitivity as a function of $M_{X^0}$ is shown by the dotted line.
  • Figure :