First observation of K{L}-->pi{+/-}e{-/+}nue{+}e{-}.

Area of Science:

• Particle Physics
• High Energy Physics
• Quantum Field Theory

Background:

• Kaon decays provide crucial tests for the Standard Model of particle physics.
• Understanding rare kaon decays, such as K{L}-->pi{+/-}e{-/+}nue{+}e{-}, is essential for probing fundamental interactions.

Purpose of the Study:

• To report the first experimental observation and branching fraction measurement of the K{L}-->pi{+/-}e{-/+}nue{+}e{-} decay.
• To compare measured kinematical distributions with theoretical predictions from chiral perturbation theory (ChPT).

Main Methods:

• Analysis of a large dataset of K{L}-->pi{+/-}e{-/+}nue{+}e{-} events.
• Determination of the branching fraction and differential decay rates.
• Comparison of experimental results with theoretical calculations in ChPT at O(p{4}) and leading order.

Main Results:

• The branching fraction B(K{L}-->pi{+/-}e{-/+}nue{+}e{-} with M_{e{+}e{-}}>5 MeV/c{2}, E_{e{+}e{-}}^{*}>30 MeV) was measured to be (1.285+/-0.041)x10{-5}.
• The ratio Gamma(K{e3ee}M{e{+}e{-}}>5 MeV/c{2})/Gamma(K{e3}) was found to be [4.57+/-0.04(stat)+/-0.14(syst)]x10{-5}.
• Measured kinematical distributions agree with ChPT O(p{4}) predictions but disagree with leading-order ChPT.

Conclusions:

• The results provide the first experimental evidence for the K{L}-->pi{+/-}e{-/+}nue{+}e{-} decay.
• The agreement with ChPT O(p{4}) validates theoretical predictions for this decay mode.
• Discrepancies with leading-order ChPT highlight the importance of higher-order corrections in understanding kaon decays.