1. K2K NC  0 production Shoei NAKAYAMA (ICRR, Univ. of Tokyo) for the K2K Collaboration July 28, 2004 @ NuFact04

2. Outline Introduction –Motivation for  0 analyses –K2K neutrino beam, 1kt water Cherenkov detector Selecting  0 events and data/MC comparison –FC 2ring  0 sample –data / MC comparison Relative cross section measurement –efficiency corrected N(NC1  0 ) –taking ratio to N(  CC) Summary

3. Introduction

4. Motivation for  0 analyses Main background to    e search  0   can mimic an electron asymmetric decay ring overlap Usable to distinguish    from   s in atmospheric  oscillation single  0 events : good NC sample (see Nakayama’s talk @ICRC2003)          s  CC NC no change attenuated need to know accurately  0 production cross section,  0 momentum and angular distribution no osc.

5. K2K experiment JAPAN: High Energy Accelerator Research Organization (KEK) / Institute for Cosmic Ray Research (ICRR), Univ. of Tokyo / Kobe University / Kyoto University / Niigata University / Okayama University / Tokyo University of Science / Tohoku University KOREA: Chonnam National University / Dongshin University / Korea University / Seoul National University U.S.A.: Boston University / University of California, Irvine / University of Hawaii, Manoa / Massachusetts Institute of Technology / State University of New York at Stony Brook / University of Washington at Seattle POLAND: Warsaw University / Solton Institute Since 2002 JAPAN: Hiroshima University / Osaka University CANADA: TRIUMF / University of British Columbia EUROPE: Rome / Saclay / Barcelona / Valencia / Geneva RUSSIA: INR-Moscow

6. K2K neutrino beam E (GeV) almost pure  beam (~98%) E ~ 1 GeV N / cm 2 / 0.1GeV / 10 20 pot  energy spectrum @ front detector site x10 10 12GeV PS p Aluminum target Front detector 200m decay tunnel  +   + + 

7. 1kt water Cherenkov detector 1000t cylindrical water tank a smaller version of Super-Kamiokande (~1/50 volume) 680 20inch PMTs with 70cm spacing (same as SK) the same detection mechanism, analysis algorithms and interaction MC as SK beam 25t fiducial volume 2m 4m

8. Selecting  0 events and data/MC comparison

9. Data set and selection criteria Observed data : 2000/1~3, 2001/1~7 (~ 3x10 19 pot)  0 events single-event spill Fully-Contained number of rings = 2 both e-like PID M  : 85 ~ 215 MeV/c 2

10. Nring and PID cuts Nring PID e-like  -like e-like  -like 1 st ring 2 nd ring hatch = NC where single  0 and no other mesons get out of 16 O nucleus data MC data MC ( MC is normalized by area ) Preliminary

11.  0 mass distribution very clean  0 sample ( MC is normalized by area ) data MC FC 2ring  0 : 2496 events the number of events after each cut non-  0 BG Mass peak (MeV/c 2 ) data : 147.4 ± 0.5 MC : 144.1 ± 0.3 Preliminary

12. Neutrino interaction MC ( “NEUT” version4.5 ) (quasi-) elastic – N  ℓ N’ ( ℓ : lepton ) – M A = 1.1 GeV/c 2 resonant meson production – N  ℓ N’  ( , K) – based on Rein&Sehgal – M A = 1.1 GeV/c 2 coherent  production – 16 O  ℓ 16 O  –Rein&Sehgal –J.Marteau et al., NIM A451(2000) cross section rescaled by ~0.7  and p rescattering in 16 O nucleus – absorption – inelastic scattering – charge exchange Pauli blocking Fermi motion nuclear potential deep inelastic scattering – N  ℓ N’  … – GRV94 + JETSET – A.Bodek et al., hep-ex/0203009 rescaled by q 2 /(q 2 +0.188) interaction vertex nuclear effects

13. The fraction of each interaction channel NC resonant meson production 60.4 % NC coherent  0 production 10.4 % NC multi  production 10.8 % NC elastic scattering 5.0 % CC resonant meson production 8.8 % CC multi  production 4.1 % large NC fraction ~ 87 % FC 2ring  0 sample

14.  0 momentum distribution 100 MeV/c bin 25 MeV/c bin stat. error only data MC The observed data is reproduced fairly well by our neutrino MC. # of FC 2ring  0 events Preliminary ( MC is normalized by area )

15.  0 production angle distribution 10 bins 40 bins stat. error only data MC Agreement between data and neutrino MC is good. backwardforward Preliminary # of FC 2ring  0 events ( MC is normalized by area ) Preliminary

16. Relative cross section measurement  (NC1  0 ) /  (  CC)

17. What can be measured ?  0 ’s from 16 O are largely modified by nuclear effects. Probability of each  0 - 16 O interaction in NEUT MC  We measure the  0 production cross section after nuclear effects.

18. “ NC1  0 “ definition  00 NC ? only one  0 and no other mesons get out of a 16 O nucleus via NC interactions. (after nuclear re-scatterings) 00  invisible  CC 00, invisible  NC or CC ++  invisible 00 NC Recoil p BG interactions  0 w/ invisible  0 w/ invisible ,  0 produced outside nuclei

19. NC1  0 estimation N FC2R  0 obs x r pure x corr fid N NC1  0 = eff non-NC1  0 subtraction rec  true fiducial correction detection and reconstruction efficiency FC 2ring  0 mom. raw NC1  0 mom. true/rec = 1.03 ± 0.02 sys. Preliminary

20. non-NC1  0 BG subtraction NC1  0 fraction curve estimated by neutrino MC NC1  0 BG inner : stat. outer : stat.+sys. NC 1  0 71 % fraction 00  00  00    00  or  CC w/ invisible  6 % CC w/ invisible ,  3 % NC w/ invisible  7 %  0 produced outside the nuclei 10 % Preliminary

21. systematic error on BG subtraction cross section6.3 % MA(QE) 1.1  1.0 && MA(1  ) 1.1  1.00.2 % QE cross section +-10%<<1 % 1  cross section +-10%0.9 % DIS cross section +-5%0.5 % w/o Bodek reweighting(DIS)5.1 % w/o Marteau reweighting(coherent  )1.6 % CC/NC +-20%3.2 %  nuclear rescattering1.6 % absorption probability +-30%1.5 % inelastic scattering probability +-30%0.7 %  0 from nucleon(or  interaction2.3 % in water (2 nd interaction) total interaction probability +-20%2.3 % estimated from different MC sets generated with varied cross section total: 6.9 % estimated by reweighting

22. efficiency correction  0 detection efficiency curve estimated by MC inner : stat. outer : stat.+sys. overall efficiency : ~46% FADC peak cut (effectively 1000p.e. cut) FC cut Nring = 2 1 st ring is e-like 2 nd ring is e-like invariant mass cut systematic errors for overall efficiency ring counting5.4 % PID3.9 % escale(+-3%)0.3 % total6.8 % Preliminary

23. NC1  0 production in true 25t (eff corrected) N(NC1  0 ) : 3.69 ± 0.07 ± 0.37 x 10 3 stat. sys. data (inner: stat, outer: stat+sys) MC true (inner: MC stat, outer: MC stat + sys error on shape from our MC model uncertainties) normalized by the number of all events in 25t fiducial in 25 ton # of NC1  0 interactions Preliminary

24.  CC interactions as a normalization  CC enriched sample : FC single-ring  -like + FC multi-ring  -like + PC    FC single-ring  -likeFC multi-ring  -like PC High  CC fraction, High efficiency  CC fraction 96.5 % 91.2 % 98.5 %

25. N(  CC) estimation N(  CC) = N(FC  +PC) 25t obs x (1 – BG non- ) x purity x corrfid / eff = 50226 x (1 – 0.015) x 0.960 x 1.02 / 0.854 stat. sys. = 5.65 ± 0.03 ± 0.26 x 10 4 BG: NCinefficiency: multi-ring PID FADC cut effenergy scale1.2 % PID1 % (1/eff) x r pure E spec.0.5 % cross section1.1 % corr fid fiducial volume4 %  total 4.6 % N obs FC1R  22612 FCmR  12386 PC 15228 systematic error on N(  CC) in 25 ton Preliminary

26. Results  (NC1  0 ) /  (  CC) = 0.065 ± 0.001 ± 0.007 cf.  (NC1  0 ) /  (  CC) = 0.064 from NEUT cf.  (  CC) ~ 1.1 x 10 -38 cm 2 / nucleon from NEUT (K2K beam spectrum averaged) stat. sys. ~ 1.3 GeV at the K2K beam energy, ~1.3 GeV Preliminary

27. Summary 2496 FC 2ring  0 events are observed. Our neutrino MC reproduces data quite well. (  0 momentum,  0 direction) Efficiency corrected number of NC1  0 interactions and their momentum distribution are measured.  (NC1  0 ) /  (  CC) = 0.065 ± 0.001 ± 0.007 at the K2K beam energy.