Accelerator Neutrino Oscillations Results and Prospects

1. Accelerator Neutrino Oscillations Results and Prospects III International Pontecorvo Neutrino Physics School 16-26 September, 2006 Koichiro Nishikawa Institute for Particle and Nuclear Studies KEK

2. The present observations are good at discovering a surprise (if it is a large effect) for which small scale (controlled) experiments do not have enough sensitivity. • Long baseline (100 – 108 km) size of earth, Sun size by luck • They are however not good at measuring underlying parameters very precisely. • Inherent uncertainties exist in calculation of various observables: • Fluxes of solar neutrinos on Earth • Nuclear reaction cross sections, chemical compositions, opacity, etc. • Fluxes of atmospheric neutrinos • Primary cosmic ray flux, nuclear interactions, etc. • Find model-independent observables • Solar neutrinos: • Comparison of NC and CC interactions • Spectral shape, day/night effect, etc • Atmospheric neutrinos • m/e ratio • Zenith angle distribution

3. Accelerator experiment • Neutrinos can be measured more than once • Relative change of spectrum • Effect of oscillation depend only on neutrino energy (fixed distance) • Beam energy can be chosen • Type of detector • Neutrino energy determination method can be chosen

4. Critical issues • Only the product F(Ei) x s(Ei) are measurable • Flux times cross section as a function of E • The P(na→nb) must be determined by minimizing the followings • s(E) poorly known at low-medium energy • Two measurements at different distances can reduce the the effect of ambiguities of cross sections • Fnear(En) , Ffar(En) different from 1/r2 unless decay at rest • Different spectrum due to finite decay length and acceptance at two distances – decay volume and distance • PID and En determination of observed events • background processes (eps. NC, etc.) different in near, far

5. Neutrino beams from accelerator with existing technologies Produce mesons by strong int. and let them decay in weak int. 1. Neutrinos from stopping p’s and m’s (LSND KARMEN) unique spectrum of nm, nm, ne no problem of Far/Near, cross section, energy determination 2. Neutrinos from in-flight decays • Wide Band Beam - sign selected by horn system but wide Dp band accepted, the highest intensity of nm (CHORUS, NOMAD,K2K, MiniBooNE, MINOS, CGSN…..) • Off-axis beam • Dichromatic beam-momentum selected by B and Q mangets • clean but the acceptance beam line limits n intensity

6. Decay at Rest (DAR) Inverse beta decay well known s Small intrinsic ne contamination few x 10-4 p- decay in flight contamination ?

7. 800MeV LINAC 1mA 600 msec width 10msec rep. Mineral oil (Cherenkov pattern) prompt e and g(2.2 MeV) p(n,g)d 800MeV Rapid cycling syn 200mA 200 nsec width 20msec rep. Gd loaded scintillator prompt e and g(7.8MeV) Gd(n,g) LSND/KARMEN Experiments • single measurement at one position • Ene+ from anti-ne + p→e++n • unique spectrum for anti-nm, nm, ne

8. Signal and Background

9. Gamma Ray Distribution

10. LSND Final Results

11. KARMEN Distributions

12. With NOMAD and reactor experiments

13. ‘Evidence’ of oscillations It is impossible to have only 3 neutrinos involved if all of the effects are the result of neutrino oscillations. Either some of the data are not due to oscillations, or there must be at least one undiscovered “sterile” neutrino or there must be CPT violation in the neutrino sector. or exotic processes nmne Dm2 (eV2) nmnt nenm,nt sin2 2q

14. MiniBooNE

16. Experimental issue • ‘MiniBooNE’ single detector • compare the results with MC only • signal = no muon, shower like events, not p0 • Backgrounds = NC p0 production, ne in the beam • PID e, m, p0 • Hadron production knowledge • p+production by 8 GeV proton →normalization and HE components to interact with NC p0 • K to give Ke3 decay (K→p+ e+ ne)

17. Total (NC+CC) CC Total CC quasi-elastic DIS CC single p NC single p0 A neutrino interaction model s/E (10-38cm2/GeV) En (GeV)

18. Approximate number of events and Background expected in MiniBooNE nmCharged Current, Quasi-elastic 500,000 events Intrinsic νe (from K&μ decay): 236 events Background π0 mis-ID: 294 events (Neutral Current Interaction) Otherνμmis-ID: 140 events Signal LSND-like nmne signal: 300 events ~10-3 of total neutrino events

19. Sensitivity to a Signal Signal Mis-ID Intrinsic νe Δm2 = 1 ev2 Δm2 = 0.4 ev2

21. HARP data on p, K NUANCE adjustment photon propagation in oil simulation PID e m seperation e-p0 seperation

28. Checking the reproducibility of s’s, detector sim.

30. ~10-3 of total neutrino events

32. Accelerator-based Long Baseline Neutrino Oscillation Experiments Long = distance>>decay region

33. Wide Band Beam • Maximum available neutrino intensity • Protons hit target • Pions (p) produced at wide range of angles • Magnetic horn to focus p • Rock shield range out m • n beam travels through earth to the experiment • m decay / p decay ~10-2 ,, Ke3→~1% ne contamination

34. Horn in K2K LE HE p+Al  p+ m+ +nm Need measurements of high energy (muon monitor) and low energy (neutrino events at near detector) secondary particle direction 200m

35. n pt~35MeV/c p Neutrino Beam • Typical characteristics • ne /nm ~ 0.01~0.001 (decay vol.) • lifetime of p/m ~ 0.01 • production cross sectionof K/p ~ 0.1and Ke3 ~0.01 • n divergence ~ 10mrad/E(GeV) • Horn focuses to about a few mrad • Far/near is not scale as 1/r2

36. Neutrino event vertex distribution at 300m from target LE 0.5<Em <1GeV HE 1<Em <2.5GeV Width HE-LE cm FWHM 2m/300m~ 6 mrad FWHM 4m/300m~ 10 mrad divergence is dominated by decay angle at these energies

37. Critical issues (reminder) • Only the product F(Ei) x s(Ei) are measurable • Flux times cross section as a function of E • The P(na→nb) must be determined by minimizing the followings • s(E) poorly known at low-medium energy • Two measurements at different distances can reduce the the effect of ambiguities of cross sections • Fnear(En) , Ffar(En) different from 1/r2 unless decay at rest • Different spectrum due to finite decay length and acceptance at two distances – decay volume and distance • PID and En determination of observed events • background processes (eps. NC, etc.) different in near,far

38. Critical issues-1 • s(E) poorly known at low-medium energy • Nuclear physics at GeV region • Pauli blocking • Nucleon Form factor • Final state interaction inside nucleus SciBooNE Minerva For several 100~1000km baseline

39. Quasi-elastic scattering cross-sections m- nm W p Cross-section (nm) 10-38cm2 magenta Old MC red new MC • Two form factors • MV fixed by e.m. (CVC) • Axial V form factor n s/En (10-38cm2/GeV) 1 10 100 GeV

40. Data on charged current processes • Not well known • Especially 2~3 GeV →SciBooNE →Minerva

41. 10-6 1.0 2.0 Neutrino spectrum and the far/near ratio (in K2K) n beam 300m 250km Far/Near Ratio beam MC w/ PION Monitor Angular acceptance (well collimated for HE) Finite decay volume length (shorter for HE, Near better accep. for MH ) En (GeV)

42. Accelerator NeutrinosPresent Status K2K (1999-2005Completed) MINOS (2005-) OPERA (2006-)

43. Brief history of K2K • 1995 • Proposed to study neutrino oscillation for atmospheric neutrinos anomaly. • 1999 • Started taking data. • 2000 • Detected the less 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 at the 4s level with both a deficit of nm and the distortion of the En spectrum. • 2004 Nov.6 • Terminated K2K due to horn trouble and high residual radiation level

44. K2K experiment ~1 event/2 days ~105 /2 days ~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 En

45. GPS SK Tspill TSK TOF=0.83msec Particle detection at 250km away Decay electron cut. 500msec 20MeV Deposited Energy No Activity in Outer Detector Event Vertex in Fiducial Volume More than 30MeV Deposited Energy 112 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)

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

47. Overall normalization error on Nsk for Nov99~ Errors KT: dominated by FV error SK: also. HARP~1 % 5.34%

48. Pion Monitor: pion distribution after horn Measure Momentum / Angle Dist. of π’s Just after Horn/Target +Well known π Decay Kinematics +Well Defined Decay Volume Geometry ⇒Predict νμ Energy Spectrum at Near Site Far Site Ring Image Gas Cherenkov Detector (Index of Refraction is Changeable) To Avoid Severe Proton Beam Background, νμ Energy Information above 1GeV is Available (β of 12GeV Proton ~ β of 2GeV π)

49. qp : : ….. w2 w1 w4 w3 pp : : Good agreement with old data. (Cho et.al.)Beam MC based on Cho et al.Errorassignment based on this measurements index of refraction : pp threshold position of ring : qp pp, qp gives two C-light peaks fit with S (wi • C-light)

50. Thin target data need assumption of secondary interaction in target Total cross section of p-Al Horn magnetic field ambiguity Proton beam profile