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تسجيل مستخدم جديدSearch 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 previou
s 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 r
ate $Gamma(K^+tomu^+ ubar u u)< 2.4times 10^{-6}Gamma(K^+to all)$ 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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