: Construction of the first stage of the Pierre Auger Observatory has begun. The aim of the Observatory is to collect unprecedented information about cosmic rays above 10(18) eV. The first phase of the project, the construction and operation of a prototype system, known as the engineering array, has now been completed. It has allowed all of the sub-systems that will be used in the full instrument to be tested under field conditions. In this paper, the properties and performance of these sub-systems are described and their success illustrated with descriptions of some of the events recorded thus far. (C) 2003 Elsevier B.V

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 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.

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.

An upper limit of 16% (at 95% c.l.) is derived for the photon fraction in cosmic rays with energies greater than 10 19 eV, based on observations of the depth of shower maximum performed with the hybrid detector of the Pierre Auger Observatory. This is the first such limit on photons obtained by observing the fluorescence light profile of air showers. This upper limit confirms and improves on previous results from the Haverah Park and AGASA surface arrays. Additional data recorded with the Auger surface detectors for a subset of the event sample support the conclusion that a photon origin of the observed events is not favored.