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T2K neutrino experiment at JPARC Approved since 2003, first beam in April 2009. Priorities : 1. search for, and measurement of, e appearance sin 2 2 13 2. precise measurement of disappearance sin 2 2 23 and m 2 23 3. (Longer term) precise measurement of e, e Matter effects and CP violation

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T2K Scheme: Neutrino oscillations: measured neutrino events in SK prediction without oscillations (flux. ) -- there exist no measurement of particle production off carbon with 30 GeV protons -- the near detector neutrino flux is not identical to far detector neutrino flux (geometry) resulting uncertainties in the near/far ratio are thus not well known (up to 20%) -- there is no absolute secondary hadron flux measurement available on the T2K beam line cross-sections cannot be measured 30 (->50) GeV protons on 90 cm carbon target (disappearance) CC near/far ratio is *not* flat ,K,K p

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near/far ratio for backgrounds (esp. e ) are not constant either plots from Max Fechner’s thesis CC e e flux important for e appearance search: signal cross-section, backgrounds requires measurement of

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Systematic uncertainties due to the hadron production model F/N ratio difference among hadron production models: ~ 20% @ E 1GeV Syst. error due to F/N Goal of T2K Impossible to achieve T2K GOAL! It is difficult to evaluate the validity of the hadron production model !! The uncertainty is probably not less than the difference among several models inspired by similar data sets momentum flux G-FLUKA vs. MARS vs. FLUKA up to ~20% difference! e appearance disappearance e appearance disappearance Ratios of F/N ratios (sin 2 2 23 )~ 0.01, ( m 23 2 )<~ 3 10 5 eV 2 (sin 2 2 23 )~ 0.015 -0.03, ( m 23 2 )<~ 5-10 10 5 eV 2 MARS/G-FLUKA FLUKA/G-FLUKA

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T2K with and without NA49: I. 13 discovery: GOAL: search down to sin 2 13 ~0.008 (90%CL) (signal ~ 10 / 20 bkg, ~20% stat. error) NC 0 and CC e backgrounds will be predicted from the near detector Achieving the above goal requires N/F (all sources of errors) < 10% ==> requirement on N/F from hadro-production: <2-3% This can be achieved in NA49 with 10% precision on pion rates and K 0 K / ratio. NA49 has achieved 3% on both in the past. present status: -- no 30 GeV p-Carbon data -- interpolation between 12 GeV p-C (HARP, no K yet, no K 0 ) and 158 GeV (NA49!) required -- anomalous behaviour of K/pi ratio (3 to 7%) observed in nearby nuclei ==> errors on hadro-production are at least 20- 30% level and uncertain Without NA49, uncertainties in the particle production would yield the largest error on the background estimate to e search and preclude to achieve the aspired systematics. The sensitivity would be worsened by ~10-20%. This is as much an issue of reliability than of precision.

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T2K with and without NA49: II. Atmospheric oscillation parameters sin 2 2 23, m 2 23 Statistical accuracy: sin 2 2 23 ~0.01, ( m 2 23 ) = 3 10 -5 eV 2 (1%) (5yrs @0.75MW) statistics nonQE/QE (20%) Energy scale (4%) 10% on flux 10% on flux width flux slope (20%) In the presently favored region of m 2 23 the flux related errors are important wrt statistical accuracy. The 20-30% uncertainties on particle production would lead to up-to-20% errors on N/F ratio and would spoil the measurements, with syst. errors as large as sin 2 2 23 ~0.015-0.03, ( m 2 23 ) = 5-10 10 -5 eV 2 An NA49 measured precision of 10% per bin would lead to a N/F ratio error of 2-3% and render these small. Without NA49-T2K, ill-defined flux-errors would be worse than the statistical precision, and spoil the atmospheric parameters measts

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CONCLUSIONS For the first phase of T2K (5 yrs @0.75 MW on target): 10% measurements of hadron production ( K K 0 ) with NA49 ensure flux-related errors small wrt statistical errors: T2K stat: sin 2 2 23 ~0.01, ( m 2 23 ) = 3 10 -5 eV 2, sin 2 2 13 ~0.008 (90%CL) present: ( sin 2 2 23 ~0.04, ( m 2 23 ) = 2 10 -4 eV 2, sin 2 2 13 ~0.1 (90%CL) ) They are crucial to achieve the goals of the experiment with reliable systematic error estimates. Absolute neutrino cross section measurements with errors of 10% or better become possible. For the longer time scale (Hyper-KamiokaNDE or higher power) 3% measurements in NA49 should be achievable and will be important for precision meaasurements of transitions, ==> matter effects and CP violation

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