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Register a new userSearch for the decay $K^+to pi^+ u bar u$ in the momentum region $P_pi < 195 {rm ~MeV/c}$
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E787 Collaboration
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We have searched for the decay $K^+ to pi^+ u bar u$ in the kinematic region with pion momentum below the $K^+ to pi^+ pi^0$ peak. One event was observed, consistent with the background estimate of $0.73pm 0.18$. This implies an upper limit on $B(K^+ to pi^+ u bar u)< 4.2times 10^{-9}$ (90% C.L.), consistent with the recently measured branching ratio of $(1.57^{+1.75}_{-0.82}) times 10^{-10}$, obtained using the standard model spectrum and the kinematic region above the $K^+ to pi^+ pi^0$ peak. The same data were used to search for $K^+ to pi^+ X^0$, where $X^0$ is a weakly interacting neutral particle or system of particles with $150 < M_{X^0} < 250 {rm ~MeV/c^2}$.
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We report the results of a search for the decay $K^+ to pi^+ u bar u$ in the kinematic region with $pi^+$ 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 $K^+ to pi^+ u bar u$. We also report improved limits on the rates of $K^+ to pi^+ X^0$ and $K^+ to pi^+ X^1 X^2$ where $X^0, X^1, X^2$ are hypothetical, massless, long-lived neutral particles.
An upper limit on the branching ratio for the decay $K^+ ! rightarrow ! pi^+ u overline{ u}$ is set at $2.4 times 10^{-9}$ at the 90% C.L. using pions in the kinematic region $214~{rm MeV}/c < P_pi < 231~{rm MeV}/c$. An upper limit of $5.2 times 10^{-10}$ is found on the branching ratio for decays $K^+ ! rightarrow ! pi^+ X^0$, where $X^0$ is any massless, weakly interacting neutral particle. Limits are also set for cases where $M_{X^0}>0$.
The current status of the experimental study of the rare kaon decay K^+ to pi^+ u bar u at Brookhaven National Laboratory (BNL) by the E787 and E949 collaborations is reported.
The NA62 experiment will begin taking data in 2015. Its primary purpose is a 10% measurement of the branching ratio of the ultrarare kaon decay $K^+ to pi^+ u bar{ u }$, using the decay in flight of kaons in an unseparated beam with momentum 75 GeV/c.The detector and analysis technique are described here.
Evidence of the $K^+tomu^+ ubar u u$ decay was searched for using E949 experimental data with an exposure of $1.70times 10^{12}$ stopped kaons. The data sample is dominated by the backgrond process $K^+tomu^+ u_mugamma$. An upper limit on the decay rate $Gamma(K^+tomu^+ ubar u u)< 2.4times 10^{-6}Gamma(K^+to all)$ at 90% confidence level was set assuming the Standard Model muon spectrum. The data are presented in such a way as to allow calculation of rates for any assumed $mu^+$ spectrum.
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