Solar Neutrino Oscillations

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1 Solar Neutrino Oscillations ( m 2, θ 12 ) Background (aka where we were): Radiochemical experiments Kamiokande and Super-K Where we are: Recent results SNO and KamLAND Global picture Where we are going: Upcoming Results Future Measurements? Bruce Berger Aspen, February 3,

2 Background Solar Neutrino Problem The deficit of ν e observed from the sun compared to solar model Predictions was a longstanding puzzle, dating from Ray Davis first 37 Cl experiment in the 1960 s. Neutrino Oscillations Experiments were primarily sensitive to ν e, so flavor change from ν e to ν µ,τ could account for the difference MSW Effect (Mikheev-Smirnov-Wolfenstein) Forward scattering of neutrinos passing through matter is flavor-dependent: propagation through matter is different than in vacuum. Key element of understanding solar neutrino oscillations Bruce Berger Aspen, February 3,

3 Neutrino Oscillations MNSP (Maki-Nakagawa-Sakata-Pontecorvo) matrix in the lepton sector analogous to the CKM matrix in the quark sector Solar neutrino oscillations are dominated by θ 12 ; to good approximation we can neglect 1 3 mixing Electron neutrino survival probability is: If we can average over oscillations, this is just: Bruce Berger Aspen, February 3,

4 MSW for World Leaders MSW effect modifies the ν e survival probability For production in matter with electron density n e : Simple (and useful) limiting cases: Below critical energy, vacuum oscillations dominate Above critical energy matter effects dominate Critical energy ~1.8 MeV for LMA, 8 B Goes as 1/ m 2 Bruce Berger Aspen, February 3,

5 The Sun as a Neutrino Source Complicated energy spectrum Different experiments sensitive to different energy regions 91% 7% 0.2% Oscillations averaged out: sun larger than oscillation length (except for smallest m 2 ) 0.008% Annual variation: 1/R 2, baseline change Day/night effects? MSW effect in the earth Different components produced at different places in the sun Bruce Berger Aspen, February 3,

flux above threshold Results: Homestake (Cl): Φ Cl /SSM = 0.34 ± 0.03 SAGE+GALLEX/GNO: Φ Ga /SSM = 0.54 ± 0.

6 Radiochemical experiments ν e capture on select radioisotopes: Chlorine: ν e + 37 Cl e + 37 Ar Gallium: ν e + 71 Ga e + 71 Ge Detect decays of capture daughters > 814 kev > 233 kev Ray Davis Sensitive only to integrated ν e flux above threshold Results: Homestake (Cl): Φ Cl /SSM = 0.34 ± 0.03 SAGE+GALLEX/GNO: Φ Ga /SSM = 0.54 ± 0.03 (SSM is Standard Solar Model, BP00: Bahcall/Pinsonneault,Astrophys. J. 555, 990, 2001) Bruce Berger Aspen, February 3,

Water- erenkov detectors Kamiokande / Super-K Detect

scattering ν x + e ν x + e Koshiba Masatoshi Sensitive to ν µ,τ as

15 σ(ν e ) Real-time detection of electron energy and direction

7 Water- erenkov detectors Kamiokande / Super-K Detect forward-scattered electrons from neutrino-electron elastic scattering ν x + e ν x + e Koshiba Masatoshi Sensitive to ν µ,τ as well as ν e ; σ(ν µ,τ ) 0.15 σ(ν e ) Real-time detection of electron energy and direction Only sensitive to most energetic 8 B solar neutrinos Flux result: Φ SK /SSM = ± Bruce Berger Aspen, February 3,

8 Kamiokande / Super-K Directional information gives clear evidence that neutrinos come from the sun 7% seasonal variation consistent with 1/R 2 Day/night (zenith angle) effects small: constrains earth MSW effect N D A DN = ± (2 ) N+D Bruce Berger Aspen, February 3,

Allowed regions, pre-sno, KamLAND Four very different allowed regions in m 2, tan 2 θ space LMA Many solutions at large or maximal mixing SMA Why tan 2 θ?

9 Allowed regions, pre-sno, KamLAND Four very different allowed regions in m 2, tan 2 θ space LMA Many solutions at large or maximal mixing SMA Why tan 2 θ? MSW effect is different for m 1 <m 2 and m 1 >m 2 cases tan 2 θ>1 means m 1 >m 2 Oscillation equations symmetric under θ (π/2 θ), m 2 m 2 LOW VAC Super-K zenith angle distribution inconsistent with SMA Bruce Berger Aspen, February 3,

10 SNO Heavy-water- erenkov detector, 5 MeV threshold Three different ν detection modes: CC (charged current) ν e + D p + p + e ν e only NC (neutral current) ν x + D p + n + ν x all three flavors! ES (elastic scattering) ν x + e ν x + e (same as Super-K) Neutron detection done in three different ways: Phase I (D 2 O phase) 2 H + n 3 H + γ (6.25 MeV) 25% add NaCl Phase II (salt phase) 35 Cl + n 36 Cl + γ (8.6 MeV) 83% NaCl out, Neutral Current Detectors (NCD s) in Phase III (NCD phase) n detection via 3 He proportional counters 45% neutron detection efficiency, but much cleaner S/B Complicated analysis: the three signals are all backgrounds to each other Bruce Berger Aspen, February 3,

11 Sudbury Neutrino Observatory NC, CC, ES rates all measured NC sees full SSM flux! Solar neutrino problem solved 5.3σ appearance of ν µ,τ in a ν e beam Neutrino Flux ( 10 6 cm -2 sec -1 ) SSM Prediction (BPB 2000) Fraction of SSM Ratio of CC to NC: strongly constrains the mixing angle θ 12 CC/NC = ± ± Day/night asymmetry (ν e only): A DN = 7.0 ± 4.9 % 1.2 Bruce Berger Aspen, February 3, s -1 ) cm -2 6 (10 φ µτ SNO φ ES SK φ ES CC ES NC SNO φ CC SNO φ NC φ SSM φ (10 cm -2 s -1 e ) 0.0

12 SNO oscillation parameter constraints SNO 95% allowed regions overlaid with previous solar neutrino measurements SNO alone allows parts of all regions In global fits with the most recent SNO data, only the LMA region survives. Bruce Berger Aspen, February 3,

13 KamLAND Kamioka Liquid-Scintillator AntiNeutrino Detector Looking for disappearance of antineutrinos produced in nuclear reactors with LMA mixing parameters Baselines on the order of the oscillation length No significant MSW effects Detected ν e spectrum (no oscillations) Liquid scintillator calorimeter, sub-mev threshold Inverse β-decay: ν e + p e + + n Coincidence signal: prompt e + annihilation (E = E ν 0.8 MeV) delayed n capture (~190 µs) (E = 2.2 MeV) No directional information Reactor ν e spectrum Inverse β-decay cross-section Bruce Berger Aspen, February 3,

14 Antineutrinos from Japanese reactors Kashiwazaki Sum over ensemble of reactors 80% of flux from baselines km Variable flux! Takahama Ohi KamLAND Bruce Berger Aspen, February 3,

Oscillations change both the rate and energy spectrum of detected events Multiple reactors at different baselines complicate the signal Reactor operation data is critical Effects of Oscillations

15 Oscillations change both the rate and energy spectrum of detected events Multiple reactors at different baselines complicate the signal Reactor operation data is critical Effects of Oscillations Example spectra (L.A.Winslow) Top: m 2 = , tan 2 θ =0.41 ( LMA II ) Bottom: m 2 = , tan 2 θ =0.41 ( LMA I ) *top 4 reactors at full thermal power only Bruce Berger Aspen, February 3,

16 Observed 54 (145.1 days livetime) No-oscillation expectation 86.8 ± 5.6 (syst) Background 1 ± 1 (N obs N BG )/N no-osc = ± (stat) ± (syst) (statistics above on 54 events) Probability that 86.8 events would fluctuate down to 54 is < 0.05% Standard ν e propagation is ruled out at the 99.95% confidence level Antineutrino Rate Analysis curve, shaded region: global-fit solar LMA Bruce Berger Aspen, February 3,

17 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 Aspen, February 3,

18 KamLAND parameter constraints KamLAND rate analysis confirms LMA rules out all other regions Shape analysis further constrains LMA parameters LMAI (lower) LMAII (upper) Constraints symmetric about tan 2 θ=1 due to absence of MSW effects Bruce Berger Aspen, February 3,

Two different global fits General conclusions: Maximal mixing ruled out at 5.3-5.

19 Two different global fits General conclusions: Maximal mixing ruled out at σ LMAII strongly disfavored Best fit points: Global Fits SNO Global Fits solar only with KamLAND tan 2 θ 0.40 m x 10-5 (solar only) 7.1 x 10-5 (w/kamland) Holanda & Smirnov Global Fits Bruce Berger Aspen, February 3,

its measurements Better measurement of CC/NC ratio will improve tan 2 θ constraints Improved day/night

20 Future Measurements: SNO D2O phase complete, published Salt phase complete analysis of first half of data published analysis of full salt dataset in progress NCD s being installed now SNO will continue to improve its measurements Better measurement of CC/NC ratio will improve tan 2 θ constraints Improved day/night asymmetry can better constrain m 2 in solar-only fits Holanda&Smirnov Bruce Berger Aspen, February 3,

21 Future Measurements: KamLAND KamLAND continues to collect reactor neutrino data > 3x the first published dataset already Also working to understand our detector better e.g. 4π calibration system KamLAND can provide the best m 2 constraint and a good tan 2 θ constraint from reactor analysis Monte Carlo study: 1000 sets of 500 events for each of: LMA II : m 2 = , tan 2 θ =0.41 LMA I : m 2 = , tan 2 θ =0.41 Top 16 reactors, full thermal power, energy resolution smearing Fit for mixing parameters with shape-only analysis above 2.6 MeV No systematics included Clear separation of LMA I and LMA II Better fractional resolution on m 2 for LMA I (4%) than LMA II (5%) (95% CL) tan 2 θ 12 to ± 0.2 level (95% CL) (without rate!) Bruce Berger Aspen, February 3,

22 7 Be solar neutrino measurement? Idea: directly detect solar 7 Be neutrinos Measurement of single energy deposition from elastic scattering See a Compton edge in the data Low energy threshold Low radioactive backgrounds are required! E.g.: 238 U < g/g < (3.5 ± 0.5) g/g 232 Th < g/g < (5.2 ± 0.8) g/g 40 K < g/g < g/g KamLAND proposal plots not actual backgrounds! Bruce Berger Aspen, February 3,

23 7 Be solar neutrino measurement? KamLAND: Backgrounds in the signal region currently about 10 6 times too high Working on purification methods to remove 85 Kr (from nitrogen used in purification) 210 Pb, 210 Pb (from decay of radon that got into the system) Borexino: Has been on hold following a pseudocumene spill August 2002 Recent news: permission for limited use of water Hoping for vessel inflation this spring; water fill this summer; scintillator to follow? Bruce Berger Aspen, February 3,

24 7 Be solar neutrino measurement? What do we learn from a 7 Be neutrino flux measurement? 7 Be line at 862 kev is well below the MSW transition, at about 2.2 MeV, so vacuum effects dominate Flux suppression just depends on θ 12, not sensitive to m 2 SSM 7 Be predictions are at the ±10% level. This translates into a larger uncertainty on θ 12 than current measurements. Measuring the solar 7 Be neutrino flux will NOT improve our present knowledge of oscillation parameters unless SSM is improved (We ve learned a lot since these experiments were proposed) The measurement can improve the solar model, perhaps significantly Also serves as a crosscheck Bruce Berger Aspen, February 3,

25 Other Future Measurements? Detection of pp neutrinos? Flux predicted to ±1% Much higher flux, difficult to suppress backgrounds Several ideas under investigation: LENS, HERON, MOON Neutrino superbeams? Brookhaven-to-Homestake proposal includes a possible signal, but it s small Bruce Berger Aspen, February 3,

26 Conclusions The combined results of a number of experiments have given us a clear picture of solar neutrino oscillations The solar neutrino deficit is due to ν e flavor change The oscillation parameters are in the LMA region Mixing is non-maximal LMA I is strongly preferred The best measurement of tan 2 θ will come from future SNO results The best measurement of m 2 will come from future KamLAND reactor results Solar and reactor (neutrino and antineutrino) results will independently measure oscillation parameters Measurement of solar 7 Be neutrinos will not improve our knowledge of mixing parameters It will take ambitious future experiments to make further progress after SNO and KamLAND Bruce Berger Aspen, February 3,

KamLAND. Introduction Data Analysis First Results Implications Future

KamLAND. Introduction Data Analysis First Results Implications Future KamLAND Introduction Data Analysis First Results Implications Future Bruce Berger 1 Tohoku University, Sendai, Japan University of Alabama University of California at Berkeley/LBNL California Institute