1. “Solar” Neutrino Oscillations (Dm2, q12)
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, 2004

2. Background Solar Neutrino Problem
The deficit of ne observed from the sun compared to solar model
Predictions was a longstanding puzzle, dating from Ray Davis’ first
37Cl experiment in the 1960’s.
Neutrino Oscillations
Experiments were primarily sensitive to ne, so flavor change from
ne to nm,t 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, 2004

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 q12;
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, 2004

4. MSW for World Leaders MSW effect modifies the ne survival probability
For production in matter with electron density ne :
Simple (and useful) limiting cases:
Below critical energy,
vacuum oscillations dominate
Above critical energy
matter effects dominate
Critical energy ~1.8 MeV for LMA, 8B
Goes as 1/Dm2
Bruce Berger
Aspen, February 3, 2004

5. The Sun as a Neutrino Source
Complicated energy spectrum
Different experiments sensitive
to different energy regions
Oscillations averaged out: sun
larger than oscillation length
(except for smallest Dm2)
Annual variation:
1/R2, baseline change
Day/night effects?
MSW effect in the earth
Different components produced
at different places in the sun
91% 7%
0.2%
0.008%
Bruce Berger
Aspen, February 3, 2004

6. 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
Aspen, February 3, 2004

7. Kamiokande / Super-K Water-Čerenkov detectors
Detect forward-scattered electrons from
neutrino-electron elastic scattering
nx + e–  nx + e–
Sensitive to nm,t as well as ne; s(nm,t)  0.15 s(ne)
Real-time detection of electron energy and direction
Only sensitive to most energetic 8B solar neutrinos
Flux result: SK/SSM =  0.015
Koshiba
Masatoshi
Bruce Berger
Aspen, February 3, 2004

8. Kamiokande / Super-K Directional information gives clear
evidence that neutrinos come from
the sun
7% seasonal variation
consistent with 1/R2
Day/night (zenith angle) effects small:
constrains earth MSW effect
ADN =  (2 )
N-D
N+D
Bruce Berger
Aspen, February 3, 2004

9. Allowed regions, pre-SNO, KamLAND
Four very different allowed
regions in Dm2, tan2q space
Many solutions at large or
maximal mixing
Why tan2q?
MSW effect is different for
m1<m2 and m1>m2 cases
tan2q>1 means m1>m2
Oscillation equations symmetric under
q  (p/2 - q), Dm2  -Dm2
Super-K zenith angle distribution
inconsistent with SMA
LMA
SMA
LOW
VAC
Bruce Berger
Aspen, February 3, 2004

10. 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)
Neutron detection done in three different ways:
Phase I (D2O phase) 2H + n  3H + g (6.25 MeV) 25%
add NaCl
Phase II (salt phase) 35Cl + n  36Cl + g (8.6 MeV) 83%
NaCl out, Neutral Current Detectors (NCD’s) in
Phase III (NCD phase) n detection via 3He 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, 2004

11. Sudbury Neutrino Observatory
NC, CC, ES rates all measured
NC sees full SSM flux!
Solar neutrino problem solved
5.3s appearance of nm,t in a ne beam
Ratio of CC to NC: strongly
constrains the mixing angle q12
CC/NC =   0.024
Day/night asymmetry (ne only):
ADN = 7.0  %
+ 1.3
- 1.2
Bruce Berger
Aspen, February 3, 2004

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, 2004

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
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
Aspen, February 3, 2004

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, 2004

15. Effects of Oscillations
Oscillations change both the
rate and energy spectrum
of detected events
Multiple reactors at different
baselines complicate the
signal
Reactor operation data is
critical
Example spectra (L.A.Winslow)
Top: Dm2=1.510-4, tan2q =0.41 (‘LMA II’)
Bottom: Dm2=0.710-4, tan2q =0.41 (‘LMA I’)
*top 4 reactors at full thermal power only
Bruce Berger
Aspen, February 3, 2004

16. Antineutrino Rate Analysis
Observed (145.1 days livetime)
No-oscillation expectation 86.8  5.6 (syst)
Background 1  1
(Nobs–NBG)/Nno-osc =  (stat)  (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
curve, shaded region:
global-fit solar LMA
Bruce Berger
Aspen, February 3, 2004

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, 2004

18. KamLAND parameter constraints
Shape analysis further
constrains LMA parameters
LMAI (lower)
LMAII (upper)
Constraints symmetric about
tan2q=1 due to absence of
MSW effects
KamLAND rate analysis
confirms LMA
rules out all other regions
Bruce Berger
Aspen, February 3, 2004

19. Global Fits Two different global fits General conclusions:
SNO Global Fits
Two different global fits
General conclusions:
Maximal mixing
ruled out at s
LMAII strongly disfavored
Best fit points:
tan2q  0.40
Dm2  6.5 x 10-5 (solar only)
 7.1 x 10-5 (w/KamLAND)
solar only
with KamLAND
Holanda & Smirnov Global Fits
Bruce Berger
Aspen, February 3, 2004

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 tan2q constraints
Improved day/night asymmetry can
better constrain Dm2 in solar-only fits
Holanda&Smirnov
Bruce Berger
Aspen, February 3, 2004

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. “4p” calibration system
KamLAND can provide the best Dm2 constraint and
a good tan2q constraint from reactor analysis
Monte Carlo study:
1000 sets of 500 events for each of:
‘LMA II’: Dm2=1.510-4, tan2q =0.41
‘LMA I’: Dm2=0.710-4, tan2q =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 Dm2 for
LMA I (4%) than LMA II (5%) (95% CL)
tan2q12 to  0.2 level (95% CL)
(without rate!)
Bruce Berger
Aspen, February 3, 2004

22. 7Be solar neutrino measurement?
Idea: directly detect solar 7Be 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.:
238U < 10­16 g/g < (3.5  0.5)10­18 g/g
232Th < 10­16 g/g < (5.2  0.8)10­17 g/g
40K < 10­18 g/g < 2.7  10­16 g/g
KamLAND proposal plots – not actual backgrounds!
Bruce Berger
Aspen, February 3, 2004

23. 7Be solar neutrino measurement?
KamLAND:
Backgrounds in the signal region currently
about 106 times too high
Working on purification methods to remove
85Kr (from nitrogen used in purification)
210Pb, 210Pb (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, 2004

24. 7Be solar neutrino measurement?
What do we learn from a 7Be neutrino flux measurement?
7Be line at 862 keV is well below the MSW transition,
at about 2.2 MeV, so vacuum effects dominate
Flux suppression just depends on q12, not sensitive to Dm2
SSM 7Be predictions are at the 10% level. This translates
into a larger uncertainty on q12 than current measurements.
Measuring the solar 7Be 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, 2004

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, 2004

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 ne flavor change
The oscillation parameters are in the LMA region
Mixing is non-maximal
“LMA I” is strongly preferred
The best measurement of tan2q will come from future SNO results
The best measurement of Dm2 will come from future
KamLAND reactor results
Solar and reactor (neutrino and antineutrino) results will independently
measure oscillation parameters
Measurement of solar 7Be 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, 2004