A. Kibayashi

Indications of neutrino oscillation in a 250 km long-baseline experiment.

The K2K experiment observes indications of neutrino oscillation: a reduction of nu(mu) flux together with a distortion of the energy spectrum. Fifty-six beam neutrino events are observed in Super-Kamiokande (SK), 250 km from the neutrino production point, with an expectation of 80.1(+6.2)(-5.4). Twenty-nine one ring mu-like events are used to reconstruct the neutrino energy spectrum, which is better matched to the expected spectrum with neutrino oscillation than without. The probability that the observed flux at SK is explained by statistical fluctuation without neutrino oscillation is less than 1%.

Observation of Large CP Violation and Evidence for Direct CP Violation in B0→π+π- Decays

We report the first observation of CP violation in B0-->pi(+)pi(-) decays based on 152x10(6) gamma (4S)-->BB decays collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. We reconstruct a B0-->pi(+)pi(-) CP eigenstate and identify the flavor of the accompanying B meson from its decay products. From the distribution of the time intervals between the two B meson decay points, we obtain A(pipi)=+0.58+/-0.15(stat)+/-0.07(syst) and S(pipi)=-1.00+/-0.21(stat)+/-0.07(syst). We rule out the CP-conserving case, A(pipi)=S(pipi)=0, at a level of 5.2 standard deviations. We also find evidence for direct CP violation with a significance at or greater than 3.2 standard deviations for any S(pipi) value.

Level-1 silicon vertex detector trigger at Belle

We present a newly developed fast trigger system that uses the silicon vertex detector (SVD) for the Belle experiment at the KEKB energy-asymmetric e/sup +/e/sup -/ collider. It is designed to issue the trigger with a latency of 1.85 /spl mu/s for the Belle central trigger system (GDL). The primary purpose of the SVD trigger is to reduce beam-induced background events by requiring tracks that come from the interaction point. The system receives trigger signals from VA1TA front-end integrated circuits, which read out signals on the silicon-strips. SVD tracks are constructed with them in combination with the trigger information of the central drift chamber (CDC), which is located outside the SVD. Performance of the SVD trigger is evaluated with collision data.

The Belle L1.5 trigger

During the summer shutdown of 2003 the new silicon vertex detector SVD2 was installed in Belle at the KEK-B factory in Tsukuba, Japan and started to take data in October 2003. It provides important improvements in the tracking capabilities and adds new trigger functionality. In addition a new hardware trigger level 1.5 was designed and installed that takes advantage of the digitized SVD hit data. The improvement in the Belle trigger system is important to deal with the increasing luminosity and higher beam currents of the KEK-B collider. The design of the level 1.5 trigger system is reviewed and results from the data taking are presented.