Improved Branching Ratio Measurement for the DecayKL0→μ+<

Abstract

We report results from Experiment 871, performed at the BNL AGS, of a measurement of the branching ratio K(0)(L)-->&mgr;(+)&mgr;(-) with respect to the CP-violating mode K(0)(L)-->pi(+)pi(-). This experiment detected over 6200 candidate &mgr;(+)&mgr;(-) events, a factor of 6 more than that seen in all previous measurements combined. The resulting branching ratio gamma(K(0)(L)-->&mgr;(+)&mgr;(-))/gamma(K(0)(L)-->pi(+)pi(-)) = (3. 474+/-0.057)x10(-6) leads to a branching fraction B(K(0)(L)-->&mgr;(+)&mgr;(-)) = (7.18+/-0.17)x10(-9), which is consistent with the current world average, and reduces the uncertainty in this decay mode by a factor of 3.

Effects of mixing with quark singlets

“…As only the imaginary parts of the CKM products enter in the expression for Br(K L → π 0 νν), it is possible to have this decay rate much larger while keeping Br(K + → π + νν) in agreement with experiment. However, there is a model-independent limit Br(K L → π 0 νν) Br(K + → π + νν) ≤ 4.376 (78) that holds provided lepton flavour is conserved [97]. We will find later that in our models this bound can be saturated, leading to a increase in Br(K L → π 0 νν) of an order of magnitude, even keeping Br(K + → π + νν) at its SM value.…”

“…Although theoretically this is a clean calculation, the extraction from actual experimental data is very difficult. The branching ratio Br(K L → µ + µ − ) = 7.18 ± 0.17 × 10 −9 [78], can be decomposed in a dispersive part [Re A] 2 and an absorptive part [Im A] 2 . The imaginary part can be calculated from Br(K L → γγ) and amounts to (7.07 ± 0.18) × 10 −9 [5], which almost saturates the total rate.…”

Kaon decays in the standard model

“…The corresponding branching ratio for the muon case is BR(K L → µ + µ − ) = (6.84±0.11)×10 −9 (Ambrose et al, 2000). At O(p 6 ) in the chiral expansion, the K L → γγ amplitude is given by the π 0 , η, η exchange mechanism shown in Fig.…”

Bs,d→ell+ell−andKL→ell+ell−in SUSY models with non-minimal sources of flavour mixing

“…This expectation cannot be directly confronted with experimental data because of the long-distance amplitude generated by the two-photon intermediate state. Indeed the precise experimental measurement of B SM (K L → µ + µ − ) [27] turns out to be completely saturated by the absorptive two-photon contribution, signaling a cancellation between shortdistance and dispersive long-distance amplitudes. Taking into account the estimates of the dispersive long-distance amplitude in Refs.…”