Neutrino Oscillations: Experimental Results + Future Measurements Introduction “Solar” Neutrino Oscillations D(m12)2, q12 “Atmospheric” Neutrino Oscillations D(m23)2, q23 LSND-type Oscillations? q13, dCP: Future Experiments Bruce Berger PHENO 2004 – March 26, 2004

(Brief) Introduction The well-established oscillations (solar, atmospheric) can be described by three active, massive neutrinos and the “MNSP” (Maki-Nakagawa- Sakata-Pontecorvo) matrix, analogous to the CKM matrix: Mass hierarchy not completely known Both solar and atmospheric oscillations are well approximated by two-flavor mixing because q13 is small, D(m23)2  D(m12)2 Two-flavor survival probability, e.g. solar ne Bruce Berger PHENO 2004 – March 26, 2004

Solar Neutrino Oscillations The sun as a neutrino source: Complicated energy spectrum MSW Effect: propagation through matter modifies the ne survival probability 91% 7% 0.2% 0.008% Below critical energy, vacuum oscillations dominate Above critical energy matter effects dominate Critical energy depends on Dm2, ~1.8 MeV for LMA Spectral distortion near critical energy Bruce Berger PHENO 2004 – March 26, 2004

Radiochemical Experiments ne capture on select radioisotopes: Chlorine: ne + 37Cl  e– + 37Ar > 814 keV Gallium: ne + 71Ga  e– + 71Ge > 233 keV Detect decays of capture daughters Sensitive only to integrated ne flux above threshold Results: Homestake (Cl): Cl/SSM = 0.34  0.03 SAGE+GALLEX/GNO: Ga/SSM = 0.54  0.03 (SSM is “Standard Solar Model”, BP00: Bahcall/Pinsonneault,Astrophys. J. 555, 990, 2001) Ray Davis Bruce Berger PHENO 2004 – March 26, 2004

Kamiokande/Super-K Water-Čerenkov, detects forward-scattered electrons from neutrino-electron elastic scattering nx + e–  nx + e– Only sensitive to most energetic 8B solar neutrinos Real-time detection of electron energy and direction Flux result: SK/SSM = 0.465  0.015 Constant suppression: no time variation or energy distortion detected Koshiba Masatoshi Bruce Berger PHENO 2004 – March 26, 2004

SNO Heavy-water-Čerenkov detector, 5 MeV threshold Three different n detection modes: CC (charged current) ne + D  p + p + e– ne only NC (neutral current) nx + D  p + n + nx all three flavors! ES (elastic scattering) nx + e–  nx + e– (same as Super-K) Full SSM flux seen 5.3s appearance of nm,t in a ne beam Ratio of CC to NC: strongly constrains the mixing angle q12 CC/NC = 0.306  0.026  0.024 SNO also sees constant suppression Bruce Berger PHENO 2004 – March 26, 2004

q12, D(m12)2 from solar data SNO 95% allowed regions overlaid with previous solar neutrino measurements In global fits with the most recent SNO data, only the LMA region survives. Maximal mixing ruled out at 5.4s Bruce Berger PHENO 2004 – March 26, 2004

KamLAND Studies “solar”-type LMA oscillations in reactor antineutrinos Reactor baselines on the order of the oscillation length Source is a time-varying ensemble of reactors Liquid scintillator calorimeter, sub-MeV threshold Inverse b-decay: ne + p ® e+ + n Coincidence signal: prompt e+ annihilation (E = En  0.8 MeV) delayed n capture (~190 ms) (E = 2.2 MeV) No directional information Detected ne spectrum (no oscillations) Inverse b-decay cross-section Reactor ne spectrum Bruce Berger PHENO 2004 – March 26, 2004

Antineutrino Rate Analysis Observed 54 (145.1 days livetime) No-oscillation expectation 86.8  5.6 (syst) Background 1  1 (Nobs–NBG)/Nno-osc = 0.611  0.085 (stat)  0.041 (syst) (statistics above on 54 events) Probability that 86.8 events would fluctuate down to 54 is < 0.05% “Standard” ne propagation is ruled out at the 99.95% confidence level Neutrinos and antineutrinos both see effects consistent with the same mixing parameters curve, shaded region: global-fit solar LMA Bruce Berger PHENO 2004 – March 26, 2004

Rate + Shape Analysis Fit prompt (positron) energy spectrum above 2.6 MeV with full reactor information (power, fuel, flux), 2-flavor mixing Energy spectrum shape provides additional constraints on oscillation parameters Bruce Berger PHENO 2004 – March 26, 2004

KamLAND mixing parameter constraints Shape analysis further constrains LMA parameters LMAI (lower) LMAII (upper) Best-fit values of mixing parameters are in the same region for both neutrinos and antineutrinos > test of CPT Constraints symmetric about tan2q=1 due to absence of MSW effects Under CPT: KamLAND rate analysis confirms LMA rules out all other regions Bruce Berger PHENO 2004 – March 26, 2004

Future SNO: Improved CC/NC measurement to further constrain tan2q Holanda&Smirnov SNO: Improved CC/NC measurement to further constrain tan2q Improved day/night ratio better constrains Dm2 in solar-only fits Possible to see MSW distortions? KamLAND: Better measurements with more data Distinguish LMAI vs. LMAII Observe shape distortion, oscillation? > published data are consistent with constant supression Scatter of 500-event MC datasets generated with common mixing parameters Bruce Berger PHENO 2004 – March 26, 2004

7Be solar neutrino measurement? Borexino Players: Borexino, KamLAND Goal is a direct measurement of the solar 7Be neutrino flux Tough measurement: single ES event need very low background to statistically extract the signal SSM 7Be prediction is at the ~10% level > This measurement is not expected to improve the determination of oscillation parameters > Measurement can improve the SSM Still an important check: 7Be neutrino energy is below the MSW transition Bruce Berger PHENO 2004 – March 26, 2004

pp neutrinos? The pp solar neutrino flux is by far the largest (>90%) and best-determined (~1%) Measurement of pp neutrinos could give the best solar neutrino oscillation parameter constraints Even though the rates are high, backgrounds are the key issue R&D efforts on a variety of approaches M.Nakahata Bruce Berger PHENO 2004 – March 26, 2004

Atmospheric neutrino oscillation Cosmic-ray showers in the earth’s atmosphere produce nm, ne In an underground detector, atmospheric n’s from different directions have different baselines Detect neutrinos through charged-current interactions in the detector: ne  e, nm  m The q23 oscillation causes a deficit of upward-going nm Best atmospheric oscillation measurement is from Super-K, though multiple other experiments have seen the same effect (Kamiokande, IMB, MACRO, Soudan-II, …) Bruce Berger PHENO 2004 – March 26, 2004

Super-K atmospheric measurement Most recent analysis presented at NOON 2004 (C.Saji) Super-K is working on a final analysis of the “SK-I” dataset: improved analysis, MC, flux predictions, calibrations, cross-sections, … SK-II is taking more data. Results improve as (data)1/2 Bruce Berger PHENO 2004 – March 26, 2004

K2K “Atmospheric” oscillations have also been detected with accelerator neutrinos: K2K (KEK-to-Super-K): 250 km baseline Data collection ongoing, new near detector Bruce Berger PHENO 2004 – March 26, 2004

Super-K L/E analysis New, very interesting preliminary analysis of Super-K data presented at NOON 2004 by M.Ishitsuka Analyze data vs. L/E instead of zenith angle Uses subset of events with good L/E resolution Dip at first oscillation minimum, later maxima/minima smeared out Oscillation preferred to other explanations of deficit Better Dm2 resolution than standard zenith-angle analysis (Note added May 26, 2004: preprint hep-ex/0404034 now available) Bruce Berger PHENO 2004 – March 26, 2004

MINOS Long-baseline accelerator neutrino experiment sending nm’s from the NuMI beam at Fermilab past Madison to the Soudan mine Detect neutrinos through CC interaction in steel+scintillator detector with B-field Tunable beam energies Beam scheduled for April 2005 Already taking atmospheric neutrino data > Can distinguish nm from nm, test CPT Bruce Berger PHENO 2004 – March 26, 2004

MINOS MINOS limits compared to current Super-K values Better constraints for higher Dm2 – oscillation minimum at higher energy Bruce Berger PHENO 2004 – March 26, 2004

nt appearance Goal: observe nt appearance in nm beam to confirm that the osciallation is to nt CERN to Gran Sasso: CNGS beam optimized for nt appearance – high enough energy Detectors: ICARUS: liquid Argon TPC OPERA: lead emulsion ICARUS cryostat Bruce Berger PHENO 2004 – March 26, 2004

Future Improved “atmospheric” parameter measurements from: MINOS Continued Super-K operation > Is mixing really maximal? Confirmation of oscillation from Super-K L/E analysis CPT test: MINOS comparison of nm, nm Direct observation of nt appearance Bruce Berger PHENO 2004 – March 26, 2004

LSND? LSND reported a significant excess of ne in a nm beam (3.8s) So far, this result has not been reproduced by any other experiment Some parameter space is still left… Bruce Berger PHENO 2004 – March 26, 2004

LSND? LSND can’t fit into the MNSP picture along with the solar and atmospheric oscillations as the third oscillation: the mass differences don’t add up If LSND is right, we need another oscillation somehow… CPT violation? > Consistent results for neutrinos, antineutrinos in solar sector > Two different mass differences in atmospheric sector strongly disfavored by Super-K Additional sterile neutrinos? > 3+1 (one sterile) disfavored > 3+2, etc. possible If LSND is correct, things get very interesting > Fully testing LSND is an experimental priority Bruce Berger PHENO 2004 – March 26, 2004

MiniBooNE MiniBooNE is a short-baseline accelerator neutrino experiment running at Fermilab that can definitely confirm or rule out LSND See Sam Zeller’s talk! two-ring event Bruce Berger PHENO 2004 – March 26, 2004

q13, dCP In the MNSP picture, we should have a third oscillation, characterized by q13, D(m13)2  D(m23)2 How big is q13? Experiment: Currently only have limits Theory: No firm predictions > Some models suggest q13 should be within the range of the next generation of experiments > Global three-flavor fits favor q13 just below current limits… If q13 is small, why? Symmetry? If q13 is large enough, we can also try to measure CP violation in the lepton sector, characterized in MNSP by dCP > CP violation could be large: can it explain the matter/antimatter asymmetry through leptogenesis? Bruce Berger PHENO 2004 – March 26, 2004

Reactor q13 limits The best limits on q13 come from short (~1km) baseline reactor neutrino measurements that saw no flux deficit. High-precision measurement: 2.7% systematic error at CHOOZ: Bruce Berger PHENO 2004 – March 26, 2004

Reactor measurement of q13 Goal is to try to measure the subdominant q13 oscillation in ne disappearance Measurement requires control of systematics to the 1% level: How do we do better than CHOOZ? Use two detectors to cancel flux uncertainties The disappearance effect depends only on q13: no matter effects, ambiguities from dCP detector 1 detector 2 Bruce Berger PHENO 2004 – March 26, 2004

Reactor measurement of q13 Double-CHOOZ: approved Fast, cheap approach sin22q13 down to 0.03 Options in the US? Braidwood (Illinois) > flat site, vertical access Diablo Canyon (California) > horizontal access, more overburden Need reactor company agreement… Other sites: Daya Bay (China), Kashiwazaki (Japan), Krasnoyarsk (Russia), Angra dos Reis (Brazil) Braidwood Diablo Canyon Bruce Berger PHENO 2004 – March 26, 2004

Sensitivity to q13 Bruce Berger PHENO 2004 – March 26, 2004

Off-axis accelerator q13 With the right accelerator beam, we can measure q13 , dCP in nm  ne appearance Rate depends on multiple unknown parameters: q13, dCP, and matter effects (hierarchy) Bruce Berger PHENO 2004 – March 26, 2004

Off-axis accelerator q13 A single measurement doesn’t unambiguously measure q13 Parameter degeneracies can be resolved by: Running in both neutrino and antineutrino modes Running at different baselines: multiple experiments are complementary sin22q13 ~sind ~cosd Bruce Berger PHENO 2004 – March 26, 2004

Off-axis accelerator q13 Two experiments (not a complete list of proposals!): T2K: Tokai-to-Super-K high priority in Japan Proposed NuMI off-axis experiment Soudan Bruce Berger PHENO 2004 – March 26, 2004

Longer term: neutrino superbeams Example: Brookhaven-to-Homestake A wideband, high-flux, on-axis beam allows parameter measurement by observing multiple oscillation peaks Potential for very good measurements Improved detectors, beams necessary Bruce Berger PHENO 2004 – March 26, 2004

Conclusions Neutrino Oscillations is a rich experimental field, with a variety of sources (sun, atmosphere, reactors, accelerators) and detection techniques Two different oscillation effects are well-established > Experiments ongoing to improve the parameter measurements And verify that the oscillation interpretation is correct LSND signal: MiniBooNE is key > If LSND signal is real, things get very interesting Big goals: > Measure q13 oscillation > Measure CP violation in neutrino sector Other questions: > Which hierarchy? > Absolute mass scale? > Are neutrino’s Dirac or Majorana? Non-oscillation experiments are also a big part of the field: neutrinoless 2b decay, cosmological limits, etc. Bruce Berger PHENO 2004 – March 26, 2004