Search Results (36 total)

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay K⁺→π⁺νν̅ in the pion momentum region 140<P_π<199  MeV/c in an exposure of 1.71×10¹² stopped kaons with an estimated total background of 0.93±0.17(stat)_-0.24^+0.32(syst) events. This brings the total number of observed K⁺→π⁺νν̅ events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of B(K⁺→π⁺νν̅ )=(1.73_-1.05^+1.15)×10^-10. An interpretation of the results for alternative models of the decay K⁺→π⁺+nothing is also presented.

Three events for the decay K⁺→π⁺νν̅ have been observed in the pion momentum region below the K⁺→π⁺π⁰ peak, 140<P_π<199  MeV/c, with an estimated background of 0.93±0.17(stat.)_-0.24^+0.32(syst.) events. Combining this observation with previously reported results yields a branching ratio of B(K⁺→π⁺νν̅ )=(1.73_-1.05^+1.15)×10^-10 consistent with the standard model prediction.

Experiment E949 at Brookhaven National Laboratory studied the rare decay K⁺→π⁺νν̅ and other processes with an exposure of 1.77×10¹² K⁺’s. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of 0.30±0.03 events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio B(K⁺→π⁺νν̅ )=(1.47_-0.89^+1.30)×10^-10, consistent with the standard model prediction of (0.74±0.20)×10^-10. This is a more detailed report of results previously published [V. V. Anisimovsky , Phys. Rev. Lett. 93, 031801 (2004)].

A sample of kinematically identified K⁺→π⁺π⁰ decays obtained with the E949 detector was used to search for the helicity-suppressed decay π⁰→νν̅ resulting in an upper limit of 2.7×10^-7 at 90% confidence level. The upper limit is also applicable to π⁰ decays into unknown weakly-interacting particles.

We report the results of a search for the decay K⁺→π⁺νν̅ 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±0.24 events results in a 90% C.L. upper limit of 2.2×10^-9 on the branching ratio of K⁺→π⁺νν̅ . We also report improved limits on the rates of K⁺→π⁺X⁰ and K⁺→π⁺X¹X² where X⁰,X¹,X² are hypothetical, massless, long-lived neutral particles.

An additional event near the upper kinematic limit for K⁺→π⁺νν̅ has been observed by experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is B(K⁺→π⁺νν̅ )=(1.47_-0.89^+1.30)×10^-10 based on three events observed in the pion momentum region 211<P<229   MeV/c. At the measured central value of the branching ratio, the additional event had a signal-to-background ratio of 0.9.

We have performed a search for the angular-momentum forbidden decay K⁺→π⁺γ with the E787 detector at BNL. No events were observed in the π⁺ kinematic region around 227 MeV/c. An upper limit on the branching ratio for the decay is determined to be 3.6×10^-7 at 90% confidence level.

Additional evidence for the rare kaon decay K⁺→π⁺νν̅ has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, 211<P<229 MeV/c, bringing the total for the combined data set to two. Including all data taken, the backgrounds were estimated to contribute 0.15±0.05 events. The branching ratio is B(K⁺→π⁺νν̅ )  =  1.57_-0.82^+1.75×10^-10.

The first search for the decay K⁺→π⁺π⁰νν̅ has been performed with the E787 detector at BNL. Based on zero events observed in the kinematical search region defined by 90 MeV/c <P_π⁺<188 MeV/c and 135 MeV<E_π⁰<180 MeV, an upper limit B(K⁺→π⁺π⁰νν̅ )<4.3×10^-5 at 90% confidence level is established.

We have performed a measurement of the K⁺→π⁺π⁰γ decay and have observed 2×10⁴ events. The best fit to the decay spectrum gives a branching ratio for direct photon emission of (4.7±0.8±0.3)×10^-6 in the π⁺ kinetic energy region of 55 to 90 MeV and requires no component due to interference with inner bremsstrahlung.

We report the first measurement of a structure-dependent component in the decay K⁺→μ⁺ν_μγ. Using the kinematic region where the muon kinetic energy is greater than 137 MeV and the photon energy is greater than 90 MeV, we find that the absolute value of the sum of the vector and axial-vector form factors is |F_V+F_A|  =  0.165±0.007±0.011. This corresponds to a branching ratio of B(SD⁺)  =  (1.33±0.12±0.18)×10^-5. We also set the limit -0.04<F_V-F_A<0.24 at 90% C.L.

A search for additional evidence for the rare kaon decay K⁺→π⁺νν̅ has been made with a new data set comparable in sensitivity to the previous exposure that produced a single event. No new events were found in the pion momentum region examined, 211<P<229 MeV/c. Including a reanalysis of the original data set, the backgrounds were estimated to contribute 0.08±0.02 events. Based on one observed event, the new branching ratio is B(K⁺→π⁺νν̅ )  =  1.5_-1.2^+3.4×10^-10.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

An upper limit on the branching ratio for the decay K⁺→e⁺νμ⁺μ^- is set at 5.0×10^-7 at a 90% confidence level, consistent with predictions from chiral perturbation theory.

We have observed the rare decay K⁺→π⁺μ⁺μ^- and measured the branching ratio γ(K⁺→π⁺μ⁺μ^-)/γ(K⁺→all)  =  [5.0±0.4(stat)±0.7(syst)±0.6(th)]×10^-8. We compare this result with predictions from chiral perturbation theory and estimates based on the decay K⁺→π⁺e⁺e^-.

The first observation of the decay K⁺→π⁺γγ is reported. A total of 31 events was observed with an estimated background of 5.1±3.3 events in the π⁺ momentum range from 100 to 180 MeV/c. The corresponding partial branching ratio, B(K⁺→π⁺γγ,100 MeV/c<P_π⁺<180 MeV/c), is [6.0±1.5(stat)±0.7(syst)]×10^-7. No K⁺→π⁺γγ decay was observed in the π⁺ momentum region greater than 215 MeV/c. The observed π⁺ momentum spectrum is compared with the predictions of chiral perturbation theory.

An event consistent with the signature expected for the rare kaon decay K⁺→π⁺νν̅ has been observed. In the pion momentum region examined, 211<P<230 MeV/c, the backgrounds are estimated to contribute 0.08±0.03 events. If the event is due to K⁺→π⁺νν̅ , the branching ratio is 4.2_-3.5^+9.7×10^-10.

An upper limit on the branching ratio for the decay K⁺→π⁺νν̅ is set at 2.4×10^-9 at the 90% C.L. using pions in the kinematic region 214 MeV/c<P_π<231 MeV/c. An upper limit of 5.2×10^-10 is found on the branching ratio for decays K⁺→π⁺X⁰, where X⁰ is any massless, weakly interacting neutral particle. Limits are also set for cases where M_X⁰>0.

A search for the decays K⁺→π⁺νν̅ and K⁺→π⁺X⁰, where X⁰ is any weakly interacting neutral particle or system of neutral particles with 150<M_X⁰<250 MeV/c², was performed in the pion kinematic region below the K_π2 (K⁺→π⁺π⁰) peak. An upper limit on B(K⁺→π⁺νν̅ ) is set at 1.7 × 10^-8 (90% C.L.) and B(K⁺→π⁺X⁰) is restricted to the 10^-7 level or lower. Limits are also set for the production of K⁺→π⁺νν̅ through scalar or tensor interactions at 1.8 × 10^-8 and 1.0 × 10^-8 (90% C.L.), respectively.

An upper limit on the branching ratio for the decay K⁺→π⁺νν̅ is set at 7.5 × 10^-9 (90% C.L.) using the kinematic region above the K_π2 (K⁺→π⁺π⁰) peak. When combined with our result from below the K_π2 peak, we obtain the limit 5.2 × 10^-9. An upper limit of 1.7 × 10^-9 is also found on the branching ratio for decays of the form K⁺→π⁺X⁰ where X⁰ is any massless, weakly interacting neutral particle. Limits are also set for cases where M_X⁰>0.

A search for the decay π⁰→γX, where X is any long-lived weakly interacting neutral vector particle with mass smaller than the neutral pion pass, was performed using neutral pions tagged by K⁺→π⁺π⁰. A 90%-C.L. upper limit for the branching ratio of the two-body decay B(π⁰→γX)<5×10^-4 is set. Limits are also set for three-body decays π⁰→γXX’.

From a retroactive data analysis, we derive the first upper bound on a |ΔL|=2 hyperon decay: B(Ξ^-→pμ^-μ^-)<3.7×10^-4 (90% C.L.). We also comment on the decay Σ^-→pμ^-μ^- and |ΔL|=2 decays of charmed and b baryons, in particular, Ξ_c⁺→Ξ^-μ^-μ^-. Finally, rough upper limits are given on B(Σ^-→pμ^-e^-) and B(Ξ^-→pμ^-e^-).

From a retroactive data analysis, we obtain the first upper bound on the ‖ΔL‖=2 kaon decay K⁺→π^-μ⁺μ⁺: B(K⁺ →π^-μ⁺μ⁺)<1.5×10^-4. We also discuss K⁺→π^-μ⁺e⁺ and ‖ΔL‖=2 decays of D and B mesons. We suggest experiments which can improve our limit and discuss the connection with direct searches for heavy neutrinos in K_μ2⁺ decay.

An experimental upper limit on the branching ratio for the decay π⁰→νν¯ is set at 8.3×10^-7 (90% C.L.). This decay is forbidden if neutrinos are purely left handed. The limit also applies to any decays of the π⁰ to weakly interacting neutrals.