Measurement of Rare Kaon Decay K + → π + ν¯ν

In the Standard Model (SM) the transition K to πνν is a Flavor Changing Neutral Current (FCNC) process in which the first order weak decay is forbidden by the GIM mechanism but is allowed, though highly suppressed, in second order due to the differing masses of the up, charm and top quarks in the mediating loops. Theoretically, this decay is sensitive to the CKM matrix element Vtd, which is one of two CKM matrix elements containing CP violation information in the SM. A study of this decay can provide a constraint on the range of Vtd from the branching ratio of K → πνν̄. The SM predicts the branching ratio of this decay mode to be (0.72± 0.21)×10 from a fit using Bd − B̄d and Bs − B̄s mixing and other relevant SM parameters. The theoretical uncertainty in Br(K → πνν̄) is small (∼ 7%), and long distance contributions to this decay are found to be negligible . Therefore, a precise measurement can also serve as a probe for new physics beyond the SM. Most interestingly, this decay together with the neutral kaon decay KL → π νν̄ and the third most accurately measured CKM matrix element Vcb can also form a unitarity triangle in the ρ− η plane, thus providing an approach for understanding CP violation. The E787 experiment at Brookhaven National Laboratory found the first candidate event in 1995 data 4 and recently found the second candidate event in 1998 data . This talk will present the final E787 analysis result using data produced from about 6 × 10 charged kaons at the Alternating Gradient Synchrotron (AGS) accelerator.