The recent discovery of a large CP violating asymmetry in KL-->pi+pi-e+e- mode has prompted us to seach for the associated KL-->pi 0 pi 0 e+e- decay mode in the KTeV-E799 experiment at Fermilab. In 2.7 x 10(11) K(L) decays, one candidate event has been observed with an expected background of 0.3 event, resulting in an upper limit for the KL-->pi 0 pi 0 e+e- branching ratio of 6.6 x 10(-9) at the 90% C.L.

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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.

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The debate over the possible role of strong, low-barrier hydrogen bonds in stabilizing reaction intermediates at enzyme active sites has taken place in the absence of an awareness of the upper limits to the strengths of low-barrier hydrogen bonds involving amino acid side chains. Hydrogen bonds exhibit their maximal strengths in isolation, i.e., in the gas phase. In this work, we measured the ionic hydrogen bond strengths of three enzymatically relevant model systems in the gas phase using anion photoelectron spectroscopy; we calibrated these against the hydrogen bond strength of HF2(-), measured using the same technique, and we compared our results with other gas-phase experimental data. The model systems studied here, the formate-formic acid, acetate-acetic acid, and imidazolide-imidazole anionic complexes, all exhibit very strong hydrogen bonds, whose strengths compare favorably with that of the hydrogen bifluoride anion, the strongest known hydrogen bond. The hydrogen bond strengths of these gas-phase complexes are stronger than those typically estimated as being required to stabilize enzymatic intermediates. If there were to be enzyme active site environments that can facilitate the retention of a significant fraction of the strengths of these isolated (gas-phase), hydrogen bonded couples, then low-barrier hydrogen bonding interactions might well play important roles in enzymatic catalysis.