We report on a new measurement of the branching ratio B(KL → π 0 γγ) using the KTeV detector. We reconstruct 1982 events with an estimated background of 608, that results in B(KL → π 0 γγ) = (1.29 ± 0.03stat ± 0.05syst) × 10 −6. We also measure the parameter, aV , which characterizes the strength of vector meson exchange terms in this decay. We find aV = −0.31 ± 0.05stat ± 0.07syst. These results utilize the full KTeV data set collected from 1997 to 2000 and supersede earlier KTeV measurements of the branching ratio and aV .

The branching ratio of the rare decay 0 ! e e ÿ has been measured precisely, using the complete data set from the KTeV E799-II experiment at Fermilab. We observe 794 candidate 0 ! e e ÿ events using K L ! 3 0 as a source of tagged 0 s. The expected background is 52:7 11:2 events, predominantly from high e e ÿ mass 0 ! e e ÿ decays. We have measured B 0 ! e e ÿ ; m e e ÿ =m 0 2 > 0:95 6:44 0:25 stat 0:22 syst 10 ÿ8 , which is above the unitary bound from 0 ! and within the range of theoretical expectations from the standard model.

We present precise tests of CP and CPT symmetry based on the full dataset of K → ππ decays collected by the KTeV experiment at Fermi National Accelerator Laboratory during 1996, 1997, and 1999. This dataset contains 16 million K → π 0 π 0 and 69 million K → π + π − decays. We measure the direct CP violation parameter Re(ǫ−4 . We find the KL-KS mass difference ∆m = (5270 ± 12)×10 6 s −1 and the KS lifetime τS = (89.62 ± 0.05)×10 −12 s. We also measure several parameters that test CPT invariance. We find the difference between the phase of the indirect CP violation parameter, ǫ, and the superweak phase, φǫ − φSW = (0.40 ± 0.56)• . We measure the difference of the relative phases between the CP violating and CP conserving decay amplitudes for K → π + π − (φ+−) and for K → π 0 π 0 (φ00), ∆φ = (0.30 ± 0.35)• . From these phase measurements, we place a limit on the mass difference between K 0 and K 0 , ∆M < 4.8 × 10 −19 GeV/c 2 at 95% C.L. These results are consistent with those of other experiments, our own earlier measurements, and CPT symmetry.

Using the published KTeV samples of KL → π ± e ∓ ν and KL → π ± µ ∓ ν decays [1], we perform a reanalysis of the scalar and vector form factors based on the dispersive parameterization [2,3]. We obtain phase space integrals I e K = 0.15446 ± 0.00025 and I µ K = 0.10219 ± 0.00025. For the scalar form factor parameterization, the only free parameter is the normalized form factor value at the Callan-Treiman point (C); our best fit results in ln C = 0.1915 ± 0.0122. We also study the sensitivity of C to different parametrizations of the vector form factor. The results for the phase space integrals and C are then used to make tests of the Standard Model. Finally, we compare our results with lattice QCD calculations of FK /Fπ and f+(0).

We present a new determination of the parity of the neutral pion via the double Dalitz decay π 0 → e + e − e + e − . Our sample, which consists of 30 511 candidate decays, was collected from KL → π 0 π 0 π 0 decays in flight at the KTeV-E799 experiment at Fermi National Accelerator Laboratory. We confirm the negative π 0 parity, and place a limit on scalar contributions to the π 0 → e + e − e + e − decay amplitude of less than 3.3% assuming CPT conservation. The π 0 γ * γ * form factor is well described by a momentum-dependent model with a slope parameter fit to the final state phase space distribution. Additionally, we have measured the branching ratio of this mode to be B(π 0 → e + e − e + e − ) = (3.26 ± 0.18) × 10 −5 .PACS numbers: 14.40.Aq, 13.40.GpThe parity of the neutral pion has been determined indirectly by studying negative pions captured on deuterium [1,2]. The observed reactions imply that the π − is a pseudoscalar and that the parities of the π − and the π 0 are the same. It has long been known that the decay π 0 → γγ in principle offers a direct means of determining the π 0 parity through the polarizations of the photons [3,4]. Given that there are no available methods for measuring the polarization of a high-energy photon, this measurement has never been performed. However, it was soon noted that the double Dalitz decay π 0 → e + e − e + e − , which proceeds through an intermediate state with two virtual photons (see Fig. 1), is sensitive to the parity of the pion since the plane of a Dalitz pair is correlated with the polarization of the virtual photon [5,6]. This process was studied in a 1962 hydrogen bubble chamber experiment using stopping negative pion capture (π − p → nπ 0 ). That group observed 206 π 0 → e + e − e + e − events and reported that the observed distribution of the e + e − planes was consistent with a pseudoscalar pion and disfavored a scalar pion at the level of 3.6 standard deviations [7]; this experiment also produced a measurement of the branching ratio of this decay, which remains the most precise result to date. Using a sample of more than 30 000 π 0 → e + e − e + e − decays, we report new precise measurements of the properties of this decay. Our modeling of the decay includes for the first time a proper treatment of the exchange contribution to the matrix element, and consideration