Transcript Solar Neutrinos and the SNO Experiment

The
Ryan Martin,
Queen’s University, Kingston, ON,
Canada
8th January 2007- EPFL
The SNO Collaboration

Canada:


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:


Queen’s, Carleton, Guelph, Laurentian, University of British Columbia, TRIUMF
Oxford University
Portugal:

Lisbon Technical Institute
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

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
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)

Neutrino Oscillations



First proposed by Pontecorvo
Neutrinos are quantum states, flavour and energy
eigenbasis are different
The PMNS matrix:
e

Vacuum Oscillations (two flavours):
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
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

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
The INCO mine and the clean lab
The Heavy Water reactions

SNO is sensitive to three
different neutrino
reactions in Heavy Water:

Charged Current (CC):



Neutral Current (NC):



Only electron flavour
Strong Energy Correlation
All flavours
Neutron capture on D
releases gamma that
compton scatters electron
Elastic Scattering (ES):


Mostly electron flavour
Strong directional
sensitivity, low statistics
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
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 !
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
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
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)

Signal Extraction in Salt Phase



The signal extraction is performed with an extended maximum loglikelihood 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)
Cos(θsun)


Best handle on ES signal
Slight sensitivity to CC
β14 (Isotropy parameter)

NC signal is more isotropic and this observable
places the strongest constraint on it
Radial Distribution


Extracting external neutron backgrounds
Acrylic Vessel (AV) acts as a neutron sink on internal
neutrons
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!
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!)
The Neutral Current Detectors
(NCDs)
Neutron
Alpha
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
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
Results from QGF (used as a datacleaning cut)

When used as a 2Dcut:




76% of neutrons pass
16% of alphas pass
32% of WE pass
Signal/Background
improves by factor of
5
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

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
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

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 doublebeta decay isotopes (SNO++, kiloton
experiment!)

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!
The End!