1. Queen’s University, Kingston, ON, Canada
The
Ryan Martin,
Queen’s University, Kingston, ON, Canada
8th January EPFL

2. The SNO Collaboration Canada: USA: UK: Portugal:
Queen’s, Carleton, Guelph, Laurentian, University of British Columbia, TRIUMF
USA:
University of Pennsylvania, Los Alamos National Lab, Lawrence Berkley National Lab, University of Washington, Brookhaven National Lab, University of Texas, University of Louisiana, Indiana University South Bend
UK:
Oxford University
Portugal:
Lisbon Technical Institute

3. Outline Solar Neutrinos The Solar Neutrino Problem
Neutrino Oscillations
The Sudbury Neutrino Observatory
Overview of the salt phase
The NCD phase
SNOLAB, SNO+ and the future

4. Solar Neutrinos
Neutrinos are created in the fusion reactions that power the Sun
SNO is sensitive to 8B neutrinos from the p-p reaction chain in the Sun (>7MeV)
pep neutrino flux has the smallest uncertainty

5. The Solar Neutrino Problem
Detection of solar neutrinos first proposed by Bahcall
Homestake experiment (Ray Davis) shows first signs of solar neutrino deficit
Until 2001, other experiments (SAGE, GALLEX) also see a solar neutrino deficit
Experimental evidence for the “solution” provided by Super Kamiokande in 1998 (atmospheric neutrino oscillations)

6. Neutrino Oscillations
First proposed by Pontecorvo
Neutrinos are quantum states, flavour and energy eigenbasis are different
The PMNS matrix:
Vacuum Oscillations (two flavours):

7. The Solar Survival Probability
The survival probability is energy dependent due to the MSW effect (yet to be observed experimentally)
SNO’s energy window not well positioned for observing MSW

8. The Situation before SNO
Long standing deficit of electron flavour neutrinos coming from the Sun
Need for an experiment that can measure the total flux of solar neutrinos and verify flavour-conversion
The energy spectrum of solar neutrinos is yet unmeasured

9. The SNO Detector Heavy Water (D2O) Cherenkov detector
2km underground (6000mwe) in active nickel mine
12m diameter Acrylic Vessel (AV)
9000 PMTs on 18m diameter geodesic structure (PSUP)
Surrounded by ultra-pure light water to shield from rock

10. The INCO mine and the clean lab

11. The Heavy Water reactions
SNO is sensitive to three different neutrino reactions in Heavy Water:
Charged Current (CC):
Only electron flavour
Strong Energy Correlation
Neutral Current (NC):
All flavours
Neutron capture on D releases gamma that compton scatters electron
Elastic Scattering (ES):
Mostly electron flavour
Strong directional sensitivity, low statistics

12. The Three Phases of SNO Phase I: Pure D20 Phase II: Salt (NaCl)
Measurement of all three reactions, but NC signal can only be extracted with “Energy Constrained” fit
Phase II: Salt (NaCl)
Neutron capture cross-section increased as well as energy released from capture (2.5 gammas on average)
The increase in isotropy of Cherenkov light from NC significantly increases the statistical separation between CC and NC (energy unconstrained)
Phase III: The Neutral Current Detectors
Designed to independently measure the NC flux
Addition of 40 3He proportional counters to count neutrons
Ended November 28th 2006 !

13. SNO Calibration About 20% of SNO time is devoted to calibrations
A manipulator system allows for various sources to be moved along x-y-z in the detector:
Laser Ball (optical and reconstruction)
16N (energy)-tagged gamma
252Cf (neutron detection efficiency)-fission

14. SNO Monte-Carlo The detector is fully modeled by Monte Carlo (SNOMAN)
The Monte Carlo is extensively tested with calibration data
Monte Carlo verification then allows for an accurate estimate of systematics

15. Basic Data Acquisition and Cleaning in Salt Phase
Triggered events are recorded (timing and position of PMTs that fired)
Low level data cleaning (instrumental background, pathological events)
Event reconstruction (position and direction of Cherenkov cone)
Observables calculated (Event energy)
High level data cleaning (fiducial volume, Cherenkov characteristics)

16. Signal Extraction in Salt Phase
The signal extraction is performed with an extended maximum log-likelihood fit
Probability Density Functions (pdfs) are generated for each observable and signal (by Monte Carlo)
Observable in salt phase:
Event direction
Isotropy
Radial Position
Energy
Signals and Backgrounds in salt phase:
NC, CC, ES (signals!)
External neutrons
Internal neutrons (indistinguishable from NC)

17. Cos(θsun)
Best handle on ES signal
Slight sensitivity to CC

18. β14 (Isotropy parameter)
NC signal is more isotropic and this observable places the strongest constraint on it

19. Radial Distribution Extracting external neutron backgrounds
Acrylic Vessel (AV) acts as a neutron sink on internal neutrons

20. Energy
Reconstructed energy of the event is based on the number of hit PMTs
Not constraining the CC energy shape allows one to measure it!

21. Results from Salt Phase
Total Flux
Energy Spectrum
Mixing Parameters:
-Δm2= (8 ± 0.5) x10-5 eV2
θ = (33.9 ± 2.3)°
(With KAMLAND data!)

22. The Neutral Current Detectors (NCDs)
Neutron
Alpha

23. NCD observables: Energy
ADC charge of NCD pulses is converted into energy spectrum (scaled from 210 Po peak)
An “energy fit” can be performed to extract neutron signal:
Do not know the background shape
Have to limit possible shapes under the neutron peak

24. QGF PSA
Pulse Shape Analysis (PSA): the idea to use pulse shapes to discriminate between neutrons and alphas
Queen’s Grid Fitter (QGF): a library of neutrons and alpha pulses is created from calibration and 4He data:
Data pulses are fit and the best neutron and best alpha chi-squared are determined
Currently, used as a cut (good neutron, bad alpha), before doing energy fit
Future (?), could be used as a pdf together with energy

25. Results from QGF (used as a data-cleaning cut)
When used as a 2D-cut:
76% of neutrons pass
16% of alphas pass
32% of WE pass
Signal/Background improves by factor of 5

26. The Future of SNO
After 7 years of successful data-taking, SNO is currently being dismantled
In the near future, publication of NCD results
In the long(er) term, combined analysis of the three phases
The NCDs are currently being “un-deployed”, in preparation for the Heavy Water extraction
SNO has demonstrated the INCO site to be a good candidate for future low background experiments

27. The SNO space is being expanded into a international low background facility for experiments on:
Direct Dark Matter Detection
Neutrino-less Double Beta Decay
Geo-Neutrinos
Low-Energy Solar Neutrinos

28. SNO+ The only thing that we don’t own is the heavy water!
Why not keep using everything else?!
SNO+: Filling the Acrylic Vessel with liquid scintillator
Can use the PMT and most of the electronics already in place

29. SNO+ Physics
Low energy solar neutrinos (pep), can test MSW effect on spectrum
Geo neutrinos (more events than KAMLAND)
Reactor neutrinos (medium baseline)
Could dope the scintillator with double-beta decay isotopes (SNO++, kiloton experiment!)

30. Summary
SNO has shown that the solar model prediction was correct after all
Strong constraints are now placed on the solar mixing angle
The MSW effect still remains to be observed (spectrum or day-night effect)
The techniques for maintaining a clean underground lab are now well developed
Bright future for the subterranean part of Sudbury!

31. The End!