1. The XXII International Conference on Neutrino Physics and Astrophysics in Santa Fe, New Mexico, June 13-19, 2006 The T2K 2KM Water Cherenkov Detector M. Fechner (CEA-DAPNIA), N. Tanimoto (Duke University), for the T2K experiment 2KM working group T2K is a next generation long-baseline oscillation experiment that will start in 2009, using an intense µ  beam produced at J-PARC (Japan). The far detector is Super-Kamiokande (SK), the 50 kt water Cherenkov detector located 295 km from J-PARC ; the beam will be 2.5 ◦ off-axis to the detector, with a peak neutrino energy of 700 MeV. The goals of T2K are the precise measurement of ∆m² 23 and  23 from µ disappearance, and the search for e appearance, which would bring new information on  13 the last remaining unknown mixing angle. A 1kton water Cherenkov detector located 2 km away from the neutrino source has many advantages: ● Same target as SK ● Same algorithms / techniques for event reconstruction ● Same fluxes 2 km away from the the target there is about one interaction per accelerator spill and ~200,000 charged current interactions/100 tons/year. Signal events are oscillated e CC events. We select single-ring, e-like events, that pass extra e/    separation cuts. There are 3 types of background : Intrinsic beam e contamination (irreducible) Mis-identified NC   events Mis-identified   CC events Very similar distributions at SK and 2KM ! To show the effectivness of the 2KM we can do a Simple scaling without any corrections: Use just spherical attenuation and ratio of fiducial masses N sk = N 2km (M sk /M 2km )(L 2km /L SK ) 2 ( sk / 2km ) The systematics that remain are : - extrapolation/efficiency differences (~ 5%) - energy scale (2.1% at each detector) - fiducial volume (~4% total) Then, Using the 2KM we can predict what we will see at SK Assumed to be 1 Total systematic error is 8.0%. There is excellent agreement between SK and 2KM over a wide energy range! This is conservative we know how to reduce the total error to ~5%. 295km Super Kamiokande Tokyo Nagoy a Osak a Liquid Argon Water Cherenkov MRD  beam  beam The 2KM detector consists of three parts : Liquid Argon detector Water Cherenkov detector Muon range detector 2km detector site Unoscillated neutrino flux spectra at 280 m, 2 km and 295 km. The fluxes have been normalized to each other. SK (295km) 2km 280m SK (295km) 2km 280m  e E (GeV ) A detector located 2 km away from the T2K neutrino source will see the same neutrino flux spectrum as Super-Kamiokande and measure the background of electron neutrino appearance with the same nuclear target and the same detector technology as Super-Kamiokande. T2K Overview 2KM Water Cherenkov Detector 2KM WC Events Look Like SK Backgrounds for e Appearance Extrapolation From 2KM to SK SK (20” PMTs) 2km 8” PMTs The 2KM WC detector has been optimized to respond like SK. A full GEANT4 simulation was developed and tuned to K2K-1kton data. To match SK performance, a fine pixelisation with ~5600 8” PMT is needed A Simulated   in the 2KM(left) and SK(right) J-PARC = Japan Proton Accelerator Research Complex 2KM Flux is Almost the Same as SK Background BG: NC  0 Expected Signal Measuring e Appearance Electrons Signal: E (GeV ) J-Parc 5660 PMTs 11146 PMTs Most Serious BG The TOTAL BG error should be ~< 10%. To maximize potential we should control the extrapolation errors to <~ 5%. Signal+Background Total Background Background from  We should measure BG in beam before oscillations.