1. Neutrino Interaction measurement in K2K experiment (1kton water Cherenkov detector) Jun Kameda(ICRR) for K2K collaboration RCCN international workshop at Kashiwa Dec. 10 th, 2004

2. Contents 1.Introduction of K2K experiment and 1kton water Cherenkov neutrino detector. 2. Neutrino spectrum measurement 3. measurement of  0 production cross section

3. K2K Collaboration Canada France Italy Japan Korea Poland Russia Spain Swiss U.S.A.  12 country, 27 institute, ~160 members.

4. 1. K2K experiment ( Long baseline neutrino oscillation experiment from KEK to Kamioka KEK 12GeV PS ν beamline beam monitor Front Neutrino detectors Super-Kamiokande (Far Detector) 50kton water Cherenkov 250km Almost pure ν μ beam (~98%) Wide band = 1.3GeV The first long baseline neutrino oscillation experiment using accelerator. Purpose:   disappearance    x   e appearance (   e   interaction study at ~ 1GeV region

5. 1-1 Front neutrino detectors 1kton water Cherenkov detector SciBar detector （ from Oct.2004) Muon Chamber To Super-Kamiokande νbeam SciFi detector Purpose: Measure neutrino flux at KEK Measure neutrino energy spectrum Study neutrino interactions

6. 1-3. 1kton water Cherekov detector (1kton detector) 1000t cylindrical water tank 680 20inch PMTs with 70cm spacing (same as SK) the same detection mechanism, analysis algorithms and interaction MC as Super-K.  a smaller version of Super-Kamiokande (~1/50 volume) inside view

7. Typical Neutrino Event of 1kton detector p  Number of Cherenkov ring  number of particles Ring direction  particle direction Number of photons  momentum Ring pattern & opening angle  Particle ID ( e-like/  -like ) Reconstruction of the physics quantity: unrolled view Cherenkov light

8. 2. Measurement of the neutrino spectrum ν 200cm μ 1kton detector QE CC 1pi CC multi pi CC coh. pi NC θ μ Event Selection 25t fiducial volume Fully Contained (no escaping particles) 1-ring μ-like Quasi Elastic Enriched sample Fraction = ~60% ~ 1.1GeV

9. 2-1. Muon momentum & direction distribution QE CC 1pi CC multi pi CC coh. pi NC PμPμ deficit is observed in forward region θμθμ Data/MC In forward region, deficit was observed (~40% deficit < 10 degree) event/20 MeV/c

10. Problem of neutrino Interaction ? Charged Current single-  production (CC 1π ） Charged Current Coherent π production are most suspicious. Deficit was found in small  (small ｑ 2 ) region. as other detector (SciFi 、 SciBar) QE CC 1pi CC multi pi CC coh. pi NC θμθμ Reconstructed q2 （ assuming QE scatterng ）.

11. Suppression factor for 1pi production x q 2 /0.1 (q 2 <0.1GeV 2 region)  Agreement between data & MC become better for both case. W/o coherent pion production Apply phenomenological suppression factors from SciBar.  Data/MC

12. DATA 2.Neutrino spectrum fit Fit observed (, ) 2-dim distribution by weighted sum of MC templates. MC Quasi- elastic non-Q.E. Template is prepared for each Parent Neutrino Energy bin （ 8bin) 、 and Q.E.and non-Q.E. interactions fitting parameter f i (8 bin) : Weight for I’th energy bin(8bins) Rnqe/qe : Q.E./ nQE ratio ε ： systematic parameter α ： norm. factor

13. Analysis results for Toy MC data. w/ and w/o  cut.  No systematic biases are found. In the fitting, small angle region (  < 20deg.) is cut for 1kton detector. θμθμ cut f3f4f5 f6 f7f8f1f2 nqe/qe Effect of the  cut in the spectrum fit Treatment of the deficit in small  region in the spectrum fit e-scale

14. 2-3. Spectrum fit result f1 ( Eν<0.5GeV) 1.413 ± 0.416 f2 (0.5 <E<0.75) 1.136 ± 0.103 f3 (0.75<E<1.0 ) 1.097 ± 0.080 f4 (1.0 <E<1.5 ) 1.000 (fixed) f5 (1.5 <E<2.0 ) 0.856 ± 0.075 f6 (2.0 <E<2.5 ) 0.936 ± 0.172 f7 (2.5 <E<3.0 ) 0.776 ± 0.729 f8 (3.0 <E ) 1.000 (fixed) R(QE/nQE) 0.705 ± 0.113 E-scale 0.982 ± 0.005 We obtained the neutrino spectrum in 1kton detector. χ 2 ＝ 51.8 / 60 d.o.f. Parent neutrino energy distribution for FC 1Ring m-like events. Nominal MC After fitting For neutrino oscillation analysis, global fitting is carried out using 1kton,SciBar and SciFi detectors.

15.  0 event analysis Main background for    e search  0   can mimic an electron asymmetric decay Cherenkov ring overlapping     /   s separation single  0 events : good NC sample    Precise understandnig of  0 production rate 、  0 momentum/angle distribution is important for these analyses. Motivation: 00 00

16. Data set Period : 2000/Jan-Mar. 2001/Jan~Jul. (~ 3x10 19 pot) data MC Mass peak (MeV/c 2 ) data : 147.4 MC : 144.1 non-  0 BG very clean  0 sample Selection Criteria of  0 events 25t fiducial volume Fully-Contained number of rings = 2 both e-like PID M  : 85 ~ 215 MeV/c 2 FC 2ring  0 : 2496 events

17.  0 momentum & angular dist. (FC 2ring  0 sample) Shape of the distributions well agree between Data & MC. # of FC 2ring  0 events ( MC is normalized by area ) Forward FC 2ring  0 sample : large NC fraction ~ 87 % Backward

18. “ NC1  0 “ interaction  00 NC ? NC interaction accompanied with only one  0 and no other particle going out of a 16 O nucleus. (after nuclear re-scatterings) Generated  0 will be largely affected by nuclear effects in 16 O.  We measure  0 production cross section after nuclear effects. 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 “ NC1  0 “ ① ② FC 2ring  0 mom.

19. NC1  0 production in true 25t (after corrections) 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

20. N(  CC) as normalization : N(  CC) = N(FC  +PC) 25t obs x (1 – BG non- ) x purity x corrfid / eff = 5.65 ± 0.03 ± 0.26 x 10 4 stat. sys in 25 ton    FC single-ring  -like FC multi-ring  -like PC  CC enriched sample ++ = 0.065 ± 0.001 ± 0.007 stat. sys. at the K2K beam energy, ~1.3 GeV Then, cf.  (NC1  0 ) /  (  CC) = 0.064 from NEUT  (NC1  0 )  (  CC)

21. Summary We measured the neutrino spectrum by 1kton detector We observed a deficit in forward direction. Phenomenological suppression factor give a better agreement between data/ MC, but we cannot have strong conclusion. In the spectrum fit, low  region (<20deg) is cut. NC1  0 production rate and  0 momentum distribution is measured. We measured the cross section ratio  (NC1  0 )/  (  CC) at K2K neutrino beam energy.

22. Supplement

23. ① non-NC1  0 BG subtraction NC 1  0 : 71 % fraction in FC 2ring  0 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 % cross section 6.3 % M A in QE,1  (+-10%) 0.2 % QE (total cross section +-10%) <<1 % 1  (total cross section +-10%) 0.9 % coherent  (model dependence) 1.6 % DIS (model dependence) 5.1 % DIS (total cross section +-5%) 0.5 % CC/NC +-20% 3.2 %  nuclear rescattering1.6 % absorption prob. +-30% 1.5 % inelastic scattering prob. +-30% 0.7 %  0 from nucleon(or  interaction 2.3 % in water (2 nd interaction) total interaction prob. +-20% 2.3 % total: 6.9 % systematic error on BG subtraction

24. 1. Delivered protons on target Total Delivered POT Jun 1999 - Feb. 15, 2004 101.12x10 18 POT ( Jan. 16 - Feb. 15) 5.55x 10 18 POT POT/spill = ~ 5.4x10 12 Protons

25. 2. Muon Direction measured by muon monitor whole the K2K (I+II) run period Muon Direction was Stable within 1 mrad. +- 1mrad

26. 2-1. Muon momentum & direction distribution ν 200cm μ 1kton detector QE CC 1pi CC multi pi CC coh. pi NC θ μ PμPμ deficit is observed in forward region θμθμ Data/MC Event Selection 25t fiducial volume Fully Contained (no escaping particles) 1-ring μ-like （ QE enriched, fraction of Quasi elastic ～ 60% ）