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Understanding Neutrino Oscillation at K2K Experiment: Latest Results and Analysis

Discover the latest findings from the K2K experiment, exploring neutrino oscillation phenomena with detailed analysis and measurements from near detectors. Gain insights into the observed neutrino energy spectrum, interaction rates, and implications for particle physics research.

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Understanding Neutrino Oscillation at K2K Experiment: Latest Results and Analysis

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  1. March 6th, 2005 @XLth Rencontres de Moriond New K2K Results C. Mariani (INFN Rome) for K2K collaboration

  2. K2K Collaboration 250km 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 U.S.A.: Duke University CANADA: TRIUMF / University of British Columbia ITALY: Rome FRANCE: Saclay SPAIN: Barcelona / Valencia SWITZERLAND: Geneva RUSSIA: INR-Moscow

  3. 1995 Proposed to study neutrino oscillation for atmospheric neutrinos anomaly. 1999 Started taking data. 2000 Detected smaller number of neutrinos than the expectation at a distance of 250 km. Disfavored null oscillation at the 2s level. 2002 Observed indications of neutrino oscillation. The probability of null oscillation is less than 1%. 2004 Confirm neutrino oscillation with both a deficit of nm and the distortion of the En spectrum. 1. Introduction and history of K2K

  4. 2. K2K experiment ~1 event/2days ~1011nm/2.2sec (/10m10m) ~106nm/2.2sec (/40m40m) nm 12GeV protons nt SK p+ m+ Target+Horn 100m ~250km 200m decay pipe p monitor Near n detectors (ND) m monitor (monitor the beam center) • Signal of n oscillation at K2K • Reduction of nm events • Distortion of nm energy spectrum

  5. ~1GeV neutrino beam by a dual horn system with 250kA. Neutrino beam and the directional control The beam direction monitored by muons X center ≤1 mrad Y center <1mrad ~5 years

  6. Accumulated POT (Protons On Target) 10.5x1019 POT, 8.9×1019 POTfor Analysis Accumulated POT (×1018) K2K-II K2K-I K2K-II K2K-I protons/pulse (×1012) Oct-Nov 04 Jan 03

  7. n beam 250km Neutrino spectrum and the far/near ratio Far/Near Ratio nm energy spectrum @ K2K near detector beam MC w/ PION Monitor 10-6 1.0 2.0 En(GeV) En (GeV)

  8. GPS SK Tspill TSK TOF=0.83msec SK Events Decay electron cut. 500msec 20MeV Deposited Energy No Activity in Outer Detector Event Vertex in Fiducial Volume More than 30MeV Deposited Energy 107 events Analysis Time Window 5msec -0.2<TSK-Tspill-TOF<1.3msec (BG: 1.6 events within 500ms 2.4×10-3events in 1.5ms) TDIFF. (ms)

  9. 3. Analysis Overview KEK Observation #n, pm and qm Measurement F(En), n int. n interaction MC Far/Near Ratio (beam MC with p mon.) SK Observation #n and En rec. Expectation #n and En rec. (sin22q, Dm2)

  10. CC quasi elastic (CCQE) CC (resonant) single p(CC-1p) DIS NC Total (NC+CC) CC Total CC quasi-elastic DIS CC single p NC single p0 NEUT: K2K Neutrino interaction MC s/E (10-38cm2/GeV) En (GeV)

  11. 1KT Water Cherenkov Detector (1KT) Scintillating-fiber/Water sandwich Detector (SciFi) Lead Glass calorimeter (LG) before 2002 Scintillators Bar Detector (SciBar) after 2003 Muon Range Detector (MRD) 4. Near detector measurements LG calorimeter Muon range detector

  12. obs SK N=107 N =150.9 +12 -10 4.1 1KT Flux measurement • The same detector technology as Super-K. • Sensitive to low energy neutrinos. Far/Near Ratio (by MC)~1×10-6 M: Fiducial mass MSK=22,500Kton, MKT=25ton e: efficiency eSK-I(II)=77.0(78.2)%, eKT=74.5% exp SK

  13. Full Active Fine-Grained detector (target: CH). Sensitive to a low momentum track. Identify CCQE events and other interactions (non-QE) separately. DATA CC QE CC 1p CC coherent-p CC multi-p 4.2 SciBar neutrino interaction study CCQE Candidate 2 track events CCQE n p non-QE m Dqp =qobs -qCCQE p m 25 Dqp (degree)

  14. Neutrino energy reconstruction

  15. K2K observed forward m deficit. A source is non-QE events. For CC-1p, Suppression of ~q2/0.1[GeV2] at q2<0.1[GeV2] may exist. (0.1GeV2 value comes from fitting 2 track nonQE sample in SciBar) For CC-coherent p, The coherent p may not exist. We do not identify which process causes the effect. The MC CC-1p (coherent p) model is corrected phenomenologically. Oscillation analysis is insensitive to the choice. DATA CC 1p CC coherent-p A hint of K2K forward m deficit. Preliminary q2rec q2rec(GeV/c)2 (Data-MC)/MC SciBar non-QE Events q2rec(GeV/c)2

  16. 1KT Fully Contained 1 ring m (FC1Rm) sample. SciBar 1 track, 2 track QE (Dqp≤25), 2 track nQE (Dqp>25)where one track ism. SciFi 1 track, 2 track QE (Dqp≤25), 2 track nQE (Dqp>30) where one track ism. After applying the low q2 suppression of nQE observed in SciBar, the angular distributions of all other samples are reasonably reproduced. 4.3 Near Detector Spectrum Measurements

  17. 1600 800 0 pm (MeV/c) 1KT: m momentum and angular distributions.with measured spectrum flux measurement low q2 corr. 1Kt m-like sample Quasi elastic Single pion 20 0 90 qm (deg.)

  18. (pm,qm) for 1track, 2trackQE and 2track nQE samples F(En), nQE/QE Fitting parameters F(En),nQE/QE ratio Detector uncertainties on the energy scale and the track counting efficiency. The change of track counting efficiency by nuclear effect uncertainties; proton re-scattering and p interactions in a nucleus … Strategy Measure F(En) in the more relevant region of qm20 for 1KT and qm10 for SciFi and SciBar. Apply a low q2 correction factor to the CC-1p model (or coherent p). Measure nQE/QE ratio for the entire qm range. 4.4 Near Detectors combined measurements

  19. En QE (MC) nQE(MC) MC templates 0-0.5 GeV KT data qm (MeV/c) 0.5-0.75GeV 0.75-1.0GeV 1.0-1.5GeV Pm (MeV/c) • n flux FKEK(En) (8 bins) • n interaction (nQE/QE)

  20. c2=638.1 for 609 d.o.f F1 ( En < 500) = 0.78  0.36 F2 ( 500 En < 750) = 1.01  0.09 F3 ( 750 En <1000) = 1.12  0.07 F4 (1000 En <1500) = 1.00 F5 (1500 En <2000) = 0.90  0.04 F6 (2000 En <2500) = 1.07  0.06 F7 (2500 En <3000) = 1.33  0.17 F8 (3000 En ) = 1.04  0.18 nQE/QE = 1.02  0.10 The nQE/QE error of 10% is assigned based on the sensitivity of thefitted nonQE/QE value by varying the fit criteria. q>10(20 ) cut: nQE/QE=0.95 0.04 standard(CC-1p low q2 corr.): nQE/QE=1.02 0.03 No coherent: p=nQE/QE=1.06 0.03 Flux measurements F(En) at KEK preliminary En

  21. SciFi (K2K-IIa with measured spectrum) qm 1trk Pm 1trk flux measurement Pm 2trk QE qm 2trk QE Pm 2trk non-QE qm 2trk non-QE 10 0 2 (GeV/c) 0 40 (degree)

  22. SciBar (with measured flux) qm 1trk Pm 1trk flux measurement Pm 2trk QE qm 2trk QE qm 2trk nQE Pm 2trk nQE 10

  23. 5. Super-K oscillation analysis • Total Number of events • Enrec spectrum shape of FC-1ring-m events • Systematic error term f x : Systematic error parameters Normalization, Flux, and nQE/QE ratio are in fx Near Detector measurements, Pion Monitor constraint, beam MC estimation, and Super-K systematic uncertainties.

  24. K2K-SK events (56) for Enrec *Updated from the previous analysis

  25. KS probability=0.08% #SK Events Toy MC Expected shape (No Oscillation) Enrec[GeV] CC-QE assumption 107 150.9 V: Nuclear potential

  26. Best fit values. sin22q = 1.51 Dm2 [eV2] = 2.1910-3 Best fit values in the physical region. sin22q = 1.00 Dm2 [eV2] = 2.7910-3 DlogL=0.75 6. Results A toy MC Dm2 12.6% 2.79 sin22q=1.51 can occur due to a a statistical fluctuation with 12.6%. sin22q 1.51 1.00

  27. Data are consistent with the oscillation. • NSKobs=107 • NSKexp (best fit)=150.9 preliminary Dm2[eV2] Best Fit KS prob.=36% sin22q Enrec[GeV] Based on DlnL

  28. Dm2<(1.87~3.58)×10-3 eV2 at sin22q=1.0 (90% C.L.) Log Likelihood difference from the minimum. DlnL DlnL - 68% - 90% - 99% - 68% - 90% - 99% sin22q Dm2[eV2]

  29. nm disappearance versus En shape distortion En shape NSK (#nm) Dm2[eV2] Dm2[eV2] sin22q sin22q Both disappearance of nm and the distortion of En spectrum have the consistent result.

  30. K2K-I vs K2K-II

  31. Null oscillation probability The null oscillation probabilities are calculated based on DlnL. *: The value is changed from the previous one. Disappearance of nm and distortion of the energy spectrum as expected in neutrino oscillation. K2K confirmsneutrino oscillation discovered in Super-K atmospheric neutrinos.

  32. With 8.91019 POT, K2K has confirmed neutrino oscillations at 4.0s (hep-ex/0411038). Disappearance of nm 3.0s Distortion of En spectrum 2.6s 8. Summary Dm2[eV2] K2K new results 0.006 - 68% - 90% - 99% preliminary 0.004 0.002 0.0 0.2 0.4 0.6 0.8 1.0 sin22q

  33. Backup slide

  34. nonQE value depends on fitting condition. Small angle cut: 0.979 CC1p re-weighting: 1.056 No coherent p: 1.093 CC1p re-weighting factor +/– 0.03 (1s) (SciBar only fit): +/– 4% In total, ~10% uncertainty exists. Fitting error is ~5%  add 10% to nonQE error. Additional Error on nonQE 8% Official value 3.5%

  35. 7. Other Physics in K2K (K2K-I data only) nm+H2ONC1p0 nmne search 1KT 90%CL limit 90%CL sensitivity Dm2[eV2] not NC1p0 Mgg(MeV) sin22qme =0.0650.0010.007 PRL 93 (2004) 051801 =0.064 (our MC)

  36. CC quasi elastic (CCQE) Smith and Moniz with MA=1.1GeV CC (resonance) single p(CC-1p) Rein and Sehgal’s with MA=1.1GeV DIS GRV94 + JETSET with Bodek and Yang correction. CC coherent p Rein&Sehgal with the cross section rescale by J. Marteau NC + Nuclear Effects Total (NC+CC) CC Total CC quasi-elastic DIS CC single p NC single p0 NEUT: K2K Neutrino interaction MC s/E (10-38cm2/GeV) En (GeV)

  37. Overall normalization error on Nsk for Nov99~ Errors Take errors not considered in matrix Central Value 76.05evts KT: dominated by FV error SK: also. 5.34%

  38. Null oscillation DlnL preliminary The null oscillation probabilities are calculated based on DlnL. *: The value is changed from the previous one. Disappearance of nm and distortion of the energy spectrum as expected in neutrino oscillation. K2K confirmedneutrino oscillation discovered in Super-K atmospheric neutrinos.

  39. The detector position 295m 294m-1% MC difference between KT and SK KT; MA(QE)=1.1 s (NCel)KT=1.1×s(NCel)SK SK; MA(QE)=1.0  Efficiency change! -1% NSKexp Change ~2% The change of NSKexp in K2K-I (Bugs) 294m 295m

  40. CC-1p suppression versus coherent p

  41. Systematic Bias without the MC correction. ND (SciBar) measurement DATA; MC w/ CC-1p suppression MC template; Default MC Oscillation Results MC w/o low q2 correction sin22q=1.00 Dm2=2.65 ×10-3 eV2 Prob.(null oscillation)=0.0049% Corrected MC for low q2 sin22q=1.00 Dm2=2.73×10-3eV2 Prob.(null oscillation)=0.011% Toy MC systematic bias There is a small bias in nQE/QE and the low energy flux. nQE/QE flux

  42. Oscillation result with a default MC Without low q2 MC correction The result w/o low q2 MC correction gives the better (biased) measurement due to the more low energy flux and the smaller nQE/QE.

  43. Best fit values. sin22q =1.08, 1.51 Dm2 [eV2] =2.7310-3, 2.1910-3 Best fit values in the physical region. sin22q =1.00, 1.00 Dm2 [eV2] =2.8610-3 , 2.7910-3 DlogL=0.02, 0.75 KSK2K-I & K2K-II =77% K2K-I vs K2K-II

  44. Enrec for K2K-I and K2K-II K2K-I K2K-II

  45. K2K-I vs K2K-II

  46. 1KT sys error summary and the change. SciBar sys. error …

  47. Systematic error Fiducial: updated +-4.0% (<- 4.0%) : Smy’s analysis Escale : updated +-0.3% FADC (scale) updated +-0.8% FADC(cut position) +-1.5% (New) : Shaomin’s analysis Background +-0.5% Multi-event +-0.7% Event rate(New) +-2.0% Profile (New) +-0.3% KT origin +-4.9% (Prev. 4.4%) SK efficiency +-3.0% CT norm. +-0.3 % for K2K-II ±0.6% for K2K-I Far/Near +5.6 -7.3% 1KT sys. error

  48. SciBar Systematic Error (2track/1track) • Detector oriented • Vertex Matching Efficiency +2.7% -1.0% • Threshold Effect (+-15%) +0.7% • Xtalk Effect (2%/4%/6%) +1.1% -1.1% • Finding Efficiency +0.9% -4.3% • Total (Detector) +3.1% -4.5% • Nuclear Effect • Proton Re-scattering +2.9% -2.6% • Pion Absorption +1.7% -1.7% • Pion inelastic scattering +2.3% -2.9% • Total (Nuclear) +4.1% -4.2% • Total +5.1% -6.2%

  49. SciBar Systematic Error (2track QE/2track nQE) • Detector oriented • Angle resolution +1.0% • X-talk Effect +2.2% -2.9% • Momentum scale (+-2.7%) +1.5% -4.3% • Total (Detector) +2.8% -5.2% • Nuclear Effect • Proton Re-scattering +2.9% -2.8% • Pion Absorption -5.4% • Pion inelastic scattering +0.3% -4.7% • Total (Nuclear Effect) +2.9% -7.7% • Total +4.0% -9.3%

  50. Track finding Efficiency 2nd Track Efficiency (MC) MC true 2nd Track (NHITX/Y≧3Hit) Detected! (= 1. Could Find 2nd Track 2. Rec.track overlap with more than 1 true Hit) 1.0 Overall Eff. 59.0% >20Hit 85.3% >40Hit 91.1% 0.8 0.6 Finding Efficiency 0.4 0.2 (This can be improved!) 0 20 40 60 80 100 120 140 (NHIT) Evaluated DATA/MC w/ Eye scan.

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