a r X i v :n u c l -e x /0205006v 1  14 M a y  2002
FIRST RESULTS FROM THE SUDBURY NEUTRINO OBSER V ATORY
G.A.McGREGOR a
Department of Physics,Denys Wilkinson Building,Keble Road,Oxford OX13RH,U.K.
The Sudbury Neutrino Observatory (SNO)is a water imaging ˇCerenkov detector.Utilising
a 1kilotonne ultra-pure D 2O target,it is the first experiment to have equal sensitivity to all flavours of active neutrinos.This allows a solar-model independent test of the neutrino oscillation hypothesis to be made.Solar neutrinos from the decay of 8B have been detected at SNO by the charged-current (CC)interaction on the deuteron and by the elastic scattering (ES)of electrons.While the CC interaction is sensitive exclusively to νe ,the ES interaction has a small sensitivity to νµand ντ.In this paper,the recent solar neutrino results from the SNO experiment are presented.The measured ES interaction rate is found to be consistent with the high precision ES measurement from the Super-Kamiokande experiment.The νe flux deduced from the CC interaction rate in SNO differs from the Super-Kamiokande ES measurement by 3.3σ.This is evidence of an active neutrino component,in addition to
νe ,in the solar neutrino flux.These results also allow the first experimental determination of the active 8B neutrino flux from the Sun,and this is found to be in good agreement with solar model predictions.
1Introduction
Over the past 30years,solar neutrino experiments 1,2,3,4,5,6have measured fewer neutrinos than are predicted by models of the Sun.7,8A comparison of the predicted and observed solar neutrino fluxes for these experiments are shown in table 1.These observations can be explained if the solar models are incomplete or neutrinos undergo a flavour changing process while in transit to
Table1:Summary of solar neutrino observations at different solar neutrino detectors. Experiment SSM Flux7
2.56±0.16(stat.)±0.16(sys.)SNU
SAGE2128+9−7SNU
77.5±6.2(stat.)+4.3
−4.7(sys.)SNU
65.8+10.2
−9.6(stat.)+3.4
−3.6
(sys.)SNU
Kamiokande5  5.05(1+0.20
−0.16)×106cm−2s−1
Super-Kamiokande6  5.05(1+0.20
−0.16
)×106cm−2s−1
the Earth,the most accepted of which is neutrino oscillations.This puzzle is known as the solar neutrino problem.
2The Sudbury Neutrino Observatory
2.1The SNO Detector
SNO9is an imaging waterˇCerenkov detector located at a depth of2092m(6010m of water equivalent)in the INCO,Ltd.Creighton mine near Sudbury,Ontario.The detector,shown in figure1,is situated in a large barrel shaped cavity22m in diameter and34m in height.The 1kilotonne ultra-pure D2O target is contained within a transparent acrylic vessel(AV)12m in diameter and5.5cm thick.A17.8m diameter geodesic sphere(PSUP-photomultiplier support structure)surrounds the AV and supports9456inward looking and91outward looking20cm photomultiplier tubes(PMTs).The PSUP is supported by steel ropes attached to the deck.The remaining volume isfilled with ultra-pure H2O which acts as a cosmic ray veto and as a shield from naturally occurring radioactivity in both the construction materials and the surrounding rock.The light water also supports the D2O and AV with the remaining weight supported by 10Vectran rope loops.
A physics event trigger is generated in the detector when18or more PMTs exceed a threshold of∼0.25photo-electrons within a coincidence time window of93ns.The trigger reaches100% efficiency when the PMT multiplicity is≥23.The instantaneous trigger rate is about15-20Hz, of which6-8Hz are physics triggers and the rest are diagnostic triggers.
2.2Neutrino Interactions in SNO
By utilising a D2O target,the SNO detector is capable of simultaneously measuring theflux of electron type neutrinos and the totalflux of all active neutrinos from8B decay in the Sun through the following interactions:
νe+d→p+p+e−(CC)
νx+d→νx+p+n(NC)
νx+e−→νx+e−(ES)
The charged-current(CC)interaction on the deuteron is sensitive exclusively toνe,and the neutral-current(NC)interaction has equal sensitivity to all active neutrinoflavours(νx,x=e,µ,τ). Elastic scattering(ES)on the electron is also sensitive to all activeflavours,but has enhanced sensitivity toνe.
Figure1:A cross-sectional view of the SNO detector.
3Results from SNO
The results presented here are the recent results from the SNO collaboration.10Full details of the analysis will not be presented here;readers are encouraged to consult the original paper. The results
are from data recorded between Nov.2,1999and Jan.15,2001,corresponding to 240.95days of live time.The neutrinofluxes deduced from the CC and ES interactions at SNO are:
ΦCC SNO =1.75±0.07(stat.)+0.12
−0.11
(sys.)±0.05(theor.)×106cm−2s−1
ΦES SNO=2.39±0.34(stat.)+0.16
−0.14
(sys.)×106cm−2s−1
where the theoretical uncertainty is the CC cross section uncertainty.11The difference between
ΦCC SNO andΦES SNO is0.64±0.40×106cm−2s−1,or1.6σ.The ratio ofΦCC
SNO
to the predicted8B
solar neutrinoflux given by the BPB01solar model7is0.347±0.029where all the uncertainties are added in quadrature.
The Super-Kamiokande6experiment has made a high precision measurement of the8B solar neutrinoflux deduced from the ES interaction:
ΦES SK=2.32±0.03(stat.)+0.08
−0.07
(sys.)×106cm−2s−1
The measurementsΦES SNO andΦES SK are consistent.Assuming that the systematic errors are
Table2:Systematic uncertainties onfluxes.
Error source ES error
(per cent)
-5.2,+6.1
±0.5
±0.5
±3.1
±0.7
±0.5
-0.8,+0.0
-0.2,+0.0
-
0.2,+0.0
0.0
±0.1
-0.6,+0.7
±0.1
0.0 Experimental uncertainty-5.7,+6.8
3.0
Solar Model-16,+20
normally distributed,the difference betweenΦCC
SNO andΦES SK is0.57±0.17×106cm−2s−1,or3.3σ.
The probability thatΦCC
SNO
is a≥3.3σdownwardfluctuation is0.04%.
The CC energy spectrum was also extracted from the data and no evidence for spectral distortions was found.
3.1Systematic Uncertainties
The systematic uncertainties in the SNO results are shown in table2.The dominant uncer-tainties are the energy scale and the reconstruction accuracy.The reconstruction accuracy was determined using a triggered16N6.13MeVγ-ray source.12Figure2shows some of the results of such a study.When the source was operated at low rate,the reconstruction accuracy was observed to become worse.This was found to be because the PMT calibration characteristics were dependent on the readout history of the PMT,compounded in non-central16N calibration runs by a readout rate gradient across the detector.This was addressed by the HCA calibra-tion13which allowed the reconstruction accuracy of neutrino events to be correctly estimated at∼3%(rather than∼10%).
3.2Total Active8B Neutrino Flux
Remembering that SNO’s CC measurement is only sensitive to electron neutrinos,whereas Super-Kamiokande’s ES measurement has a weak sensitivity to all activeflavours,one can deduce the total8B solar neutrinoflux.Stated explicitly,the experimental sensitivities to neutrinoflavours are:
seifertΦCC
SNO
=Φe;ΦES SK=ǫΦµτ
whereΦµτis the combinedνµandντflux andǫ=1/6.481.These equations can be solved forΦe andΦµτ.This is shown graphically infigure3.
−800
−600−400
−2000200400600800
Source z−Position (cm)
−20−15−10−505101520M e a n  S h i f t  (f i t −s r c ) i n  z −P o s i t i o n  (c m )
Figure 2:The shift in the reconstructed position of the 16N source as a function of source position.The HCA calibration corrects the inward shift seen in the low rate 16
N data.SNOMAN is the SNO Monte Carlo package.
φ(νe ) (106
cm -2s -1
)
φ(νµτ) (106
c m -2s -1
)
φ(νe ) (relative to BPB01)
φ(νµτ) (r e l a t i v e  t o  B P B 01)
1
Figure 3:The flux of 8B solar neutrinos which are µor τflavour vs.the flux of electron neutrinos as deduced from the SNO and Super-Kamiokande results.The diagonal bands show the total 8B flux as predicted by the BPB01(dashed lines)and that derived from the SNO and Super-Kamiokande results (solid lines).The intercepts
of these bands with the axes represent the ±1σerrors.
Figure4:Left:Comparison of the data and Monte Carlo NHITS distributions.The sharply falling component at lower NHITS is from24Na decays,and the higher NHIT bump is from neutron capture on35Cl.Right:The
decay of the activated24Na in the D2O.
The preferred value of the total active neutrino8Bflux is:
ΦTOT
SNO+SK
=5.44±0.99×106cm−2s−1
which is in good agreement with the standard solar model prediction:
ΦTOT BPB01=5.05+1.01
−0.81
×106cm−2s−1
This is thefirst determination of the total activeflux of8B neutrinos generated by the Sun.
4The NaCl Phase of the SNO Experiment
The deployment of NaCl to enhance the NC capability of the SNO detector began on May28, 2001.The presence of NaCl in the D2O causes the free neutron,produced by the NC interaction, to be captured by35Cl.This produces an excited state of36Cl which decays to its ground state via a cascade ofγ-rays with a total energy of∼8.6MeV.The neutron detection efficiency is significantly enhanced,and the high multiplicity of theγ-ray cascade allows statistical separation from CC events based on the PMT hit pattern.
4.1The24Na Calibration Source
The addition of NaCl to the D2O presented the opportunity to deploy24Na as a containerless source.This is desirable for two reasons:24Naβγdecays are similar to theβγdecays of208Tl and214Bi;and a containerless source avoids the difficulties in modeling complex sources.
Activating23Na in the D2O was achieved by using the‘super-hot’thorium source,which produces2.0×107±5%2.614MeVγ-rays per minute(producing neutrons from deuteron pho-todisintegration).Figure4shows the results of such a deployment.Comparing the detector response from the24Na calibration source to Monte Carlo predictions gives confidence in,and allows systematic uncertainties to be assigned to,techniques designed to monitor the208Tl and 214Bi levels within the D2O.

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