1. 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

2. 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

3. 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      

4. 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

5. 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.

6. 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

7. 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