K2K NC p 0 production

1. K2K NC p0 production Shoei NAKAYAMA (ICRR, Univ. of Tokyo) for the K2K Collaboration July 28, 2004 @ NuFact04

2. Outline • Introduction • Motivation for p0 analyses • K2K neutrino beam, 1kt water Cherenkov detector • Selecting p0 events and data/MC comparison • FC 2ring p0 sample • data / MC comparison • Relative cross section measurement • efficiency corrected N(NC1p0) • taking ratio to N(nmCC) • Summary

3. Introduction

4. Motivation for p0 analyses • Main background to nm ne search p0  gg can mimic an electron • asymmetric decay • ring overlap • Usable to distinguish nm  nt from nm  ns in atmospheric nm oscillation single p0 events : good NC sample (see Nakayama’s talk @ICRC2003) g g g g nm nt nm ns no osc. nmCC NC no change attenuated need to know accurately p0 production cross section, p0 momentum and angular distribution

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. En (GeV) K2K neutrino beam • almost pure nm beam (~98%) • En ~ 1 GeV Aluminum target nm energy spectrum @ front detector site x1010 Front detector 200m decay tunnel p+ m+ + nm Nn / cm2 / 0.1GeV / 1020 pot p 12GeV PS

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 2m n beam 4m 25t fiducial volume

8. Selecting p0 events and data/MC comparison

9. Data set and selection criteria p0 events • single-event spill • Fully-Contained • number of rings = 2 • both e-like PID • Mgg : 85 ~ 215 MeV/c2 Observed data : 2000/1~3, 2001/1~7 (~ 3x1019 pot)

10. data MC data MC Nring and PID cuts Nring PID Preliminary Preliminary m-like e-like 1st ring m-like e-like 2nd ring ( MC is normalized by area ) hatch = NC where single p0 and no other mesons get out of 16O nucleus

11. data MC p0 mass distribution Preliminary ( MC is normalized by area ) the number of events after each cut Mass peak (MeV/c2) data : 147.4 ± 0.5 MC : 144.1 ± 0.3 Preliminary non-p0 BG FC 2ring p0 : 2496 events very clean p0 sample

12. (quasi-) elastic n N  ℓ N’ ( ℓ: lepton ) MA = 1.1 GeV/c2 resonant meson production n N  ℓ N’ p (h, K) based on Rein&Sehgal MA = 1.1 GeV/c2 coherent p production n16O  ℓ16O p Rein&Sehgal J.Marteau et al., NIM A451(2000) cross section rescaled by ~0.7 Neutrino interaction MC ( “NEUT” version4.5 ) n interaction vertex nuclear effects • p and p rescattering in 16O nucleus • absorption • inelastic scattering • charge exchange • Pauli blocking • Fermi motion • nuclear potential • deep inelastic scattering • n N  ℓ N’ ppp… • GRV94 + JETSET • A.Bodek et al., hep-ex/0203009 rescaled by q2/(q2+0.188)

13. The fraction of each interaction channel CC multi p production 4.1 % CC resonant meson production 8.8 % NC resonant meson production 60.4 % NC elastic scattering 5.0 % NC multi p production 10.8 % FC 2ring p0 sample NC coherent p0 production 10.4 % large NC fraction ~ 87 %

14. data MC p0 momentum distribution 100 MeV/c bin 25 MeV/c bin Preliminary Preliminary stat. error only # of FC 2ring p0 events # of FC 2ring p0 events ( MC is normalized by area ) The observed data is reproduced fairly well by our neutrino MC.

15. data MC p0 production angle distribution 10 bins 40 bins Preliminary Preliminary backward forward # of FC 2ring p0 events # of FC 2ring p0 events stat. error only ( MC is normalized by area ) Agreement between data and neutrino MC is good.

16. Relative cross section measurements(NC1p0) / s(nmCC)

17. What can be measured ? p0’s from 16O are largely modified by nuclear effects. Probability of each p0-16O interaction in NEUT MC  We measure the p0 production cross section after nuclear effects.

18. “ NC1p0 “ definition n NC only one p0 and no other mesons get out of a 16O nucleus via NC interactions. (after nuclear re-scatterings) ? p0 BG interactions CC NC NC or CC invisible m n, invisible m Recoil p p0 p0 p0 invisible p+ p0 w/ invisible m p0 w/ invisible p,m p0 produced outside nuclei

19. NC1p0 estimation sys. true/rec = 1.03 ± 0.02 rec  true fiducial correction non-NC1p0 subtraction FC 2ring p0 mom. NFC2Rp0obs x rpure x corrfid NNC1p0 = eff Preliminary detection and reconstruction efficiency raw NC1p0 mom.

20. non-NC1p0 BG subtraction fraction NC1p0fraction curve estimated by neutrino MC NC 1p071 % CC w/ invisible p, m 3 % CC w/ invisible m 6 % Preliminary m m BG p0 p0 p NC1p0 NC w/ invisible p 7 % p0 produced outside the nuclei 10 % inner : stat. outer : stat.+sys. n nor m p0 p0 p

21. systematic error on BG subtraction total: 6.9 % • n cross section 6.3 % • MA(QE) 1.11.0 && MA(1p) 1.11.0 0.2 % • QE cross section +-10% <<1 % • 1p cross section +-10% 0.9 % • DIS cross section +-5% 0.5 % • w/o Bodek reweighting(DIS) 5.1 % • w/o Marteau reweighting(coherent p) 1.6 % • CC/NC +-20% 3.2 % • nuclear rescattering 1.6 % absorption probability +-30% 1.5 % inelastic scattering probability +-30% 0.7 % p0 from nucleon(or p) interaction 2.3 % in water (2nd interaction) total interaction probability +-20% 2.3 % estimated by reweighting estimated from different MC sets generated with varied cross section

22. efficiency correction FADC peak cut (effectively 1000p.e. cut) p0 detection efficiency curve estimated by MC FC cut Preliminary Preliminary Nring = 2 1st ring is e-like 2nd ring is e-like invariant mass cut systematic errors for overall efficiency ring counting 5.4 % PID 3.9 % escale(+-3%) 0.3 % total 6.8 % inner : stat. outer : stat.+sys. overall efficiency : ~46%

23. NC1p0 production in true 25t (eff corrected) Preliminary data (inner: stat, outer: stat+sys) MC true (inner: MC stat, outer: MC stat + sys error on shape from our MC model uncertainties) # of NC1p0 interactions stat. sys. N(NC1p0) : 3.69 ± 0.07 ± 0.37 x 103 Preliminary in 25 ton normalized by the number of all events in 25t fiducial

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

25. N(nmCC) estimation N(nmCC) = N(FCm+PC)25tobs x (1 – BGnon-n) 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 104 BG: NC inefficiency: multi-ring PID FADC cut Nobs FC1Rm 22612 FCmRm 12386 PC 15228 in 25 ton Preliminary Preliminary systematic error on N(nmCC) eff energy scale 1.2 % PID 1 % (1/eff) x rpure E spec. 0.5 % cross section 1.1 % corrfid fiducial volume 4 %  total 4.6 %

26. Results Preliminary stat. sys. s(NC1p0) / s(nmCC) = 0.065 ± 0.001 ± 0.007 at the K2K beam energy, <En>~1.3 GeV cf. s(NC1p0) / s(nmCC) = 0.064 from NEUT cf. s(nmCC) ~ 1.1 x 10-38 cm2 / nucleon from NEUT (K2K beam spectrum averaged) <En> ~ 1.3 GeV

27. Summary • 2496 FC 2ring p0 events are observed. • Our neutrino MC reproduces data quite well. (p0 momentum, p0 direction) • Efficiency corrected number of NC1p0 interactions and their momentum distribution are measured. • s(NC1p0) / s(nmCC) = 0.065 ± 0.001 ± 0.007 at the K2K beam energy.