The Sudbury Neutrino Observatory (SNO) measures both the flux of the
electron-type neutrinos and the total flux of all active flavours of neutrinos
originating from the Sun. A model-independent test of neutrino flavour
transformation was performed by comparing these two measurements. In 2002, this
flavour transformation was definitively demonstrated. In this talk, results
from these measurements and the current status of the SNO detector are
presented.Comment: Proceedings of the International Europhysics Conference on High
  Energy Physics (EPS2003) 3 pages, 2 figures, 2 table

A. W. P. Poon

Sno Collaboration

English

arXiv

The Sudbury Neutrino Observatory (SNO) measures both the flux of the electron-type neutrinos and the total flux of all active flavours of neutrinos originating from the Sun. A model-independent test of neutrino flavour transformation was performed by comparing these two measurements. In 2002, this flavour transformation was definitively demonstrated. In this talk, results from these measurements and the current status of the SNO detector are presented.
         PACS: 26.65.+t Solar neutrinos – 14.60.Pq Neutrino mass and mixing – 95.85.Ry Neutrino, muon, pion, and other elementary particles; cosmic ray

A.W. Poon

EDP Sciences OAI-PMH repository (1.2.0)

Recent results from the sudbury neutrino observatory

Crossref

The Sudbury Neutrino Observatory (SNO) measures both the flux of the electron-type neutrinos and the total flux of all active flavours of neutrinos originating from the Sun. A model-independent test of neutrino flavour transformation was performed by comparing these two measurements. In 2002, this flavour transformation was definitively demonstrated. In this talk, results from these measurements and the current status of the SNO detector are presented. PACS: 26.65.+t Solar neutrinos – 14.60.Pq Neutrino mass and mixing – 95.85.Ry Neutrino, muon, pion, and other elementary particles; cosmic ray

Poon, AW

eScholarship - University of California

Lawrence Berkeley National LaboratoryLBL PublicationsTitleRecent results from the sudbury neutrino observatoryPermalinkhttps://escholarship.org/uc/item/1ft375thJournalEuropean Physical Journal C, 33(Suppl 1)ISSN1434-6044AuthorPoon, AWPublication Date2004-07-01DOI10.1140/epjcd/s2003-03-908-3 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaDigital Object Identifier (DOI) 10.1140/epjcd/s2003-03-908-3Eur Phys J C 33, s01, s823–s825 (2004) EPJ C directelectronic onlyRecent results from the Sudbury Neutrino ObservatoryA.W. Poon, for the SNO CollaborationaInstitute for Nuclear and Particle Astrophysics, Lawrence Berkeley National Laboratory, Berkeley, CA, USAReceived: 29 November 2003 / Accepted: 12 December 2003 /Published Online: 17 December 2003 – c© Springer-Verlag / Società Italiana di Fisica 2003Abstract. The Sudbury Neutrino Observatory (SNO) measures both the flux of the electron-type neutrinosand the total flux of all active flavours of neutrinos originating from the Sun. A model-independent testof neutrino flavour transformation was performed by comparing these two measurements. In 2002, thisflavour transformation was definitively demonstrated. In this talk, results from these measurements andthe current status of the SNO detector are presented.PACS. 26.65.+t Solar neutrinos – 14.60.Pq Neutrino mass and mixing – 95.85.Ry Neutrino, muon, pion,and other elementary particles; cosmic rays1 IntroductionFor over three decades, solar neutrino experiments [1]-[6]have been observing fewer neutrinos than what are pre-dicted by the detailed models (e.g. [7]) of the Sun. Theseexperiments probe different parts of the solar neutrino en-ergy spectrum, and show an energy dependence in themeasured solar neutrino flux. These observations can beexplained if the solar models are incomplete or neutri-nos undergo flavour transformation while in transit to theEarth. The Sudbury Neutrino Observatory, using 1000tonnes of 99.92% isotopically pure D2O as the neutrinotarget, was constructed to resolve this puzzle. Detailedphysical description of the SNO detector can be foundin [8].The SNO detector detects solar neutrinos through thefollowing channels:νe + d → p + p + e− (CC)νx + d → p + n + νx (NC)νx + e− → νx + e− (ES)The charged-current (CC) reaction on the deuteron is sen-sitive exclusively to νe, and the neutral-current (NC) re-action has equal sensitivity to all active neutrino flavours(νx; x = e, µ, τ). Elastic scattering (ES) on electron is alsosensitive to all active flavours, but with reduced sensitivityto νµ and ντ . SNO is currently the only experiment thatcan simultaneously observe the disappearance of νe’s andtheir appearance as another active species. By countingthe free neutron in the final state of the NC reaction, thea This work is supported by Canada: NSERC, IndustryCanada, NRC, Northern Ontario Heritage Fund, Inco, AECL,Ontario Power Generation, HPCVL, CFI; US: DoE; UK:PPARC. We thank the SNO technical staff for their strongcontributionstotal active 8B neutrino flux can be inferred for neutrinoswith energy above the 2.2-MeV kinematic threshold.In the first phase of the SNO experiment, the free neu-tron from the NC interaction was detected by observingthe 6.25-MeV γ ray following its capture on a deuteron.To enhance the neutron detection efficiency, 2 tonnes ofsalt was added in the second phase of the experiment. Inthis case, the free neutron is captured by 35Cl and a γ-raycascade with a total energy of 8.6 MeV is emitted. In thefinal phase of the experiment, 3He proportional counters,the “Neutral-Current Detectors” (NCD) [9], are deployedon a 1-m grid in the D2O volume. Free neutrons are cap-tured by 3He in this phase. Results [10,11,12] from thefirst phase of SNO experiment and its prospects in thesalt and NCD phases of the experimental program arepresented in this talk.2 Results from the pure D2O phaseThe data presented in this talk were recorded betweenNovember 2, 1999 and May 28, 2001. The target was pureD2O and over 450 million triggers were recorded. The cor-responding livetime was 306.4 days.Detector diagnostic triggers and instrumental back-ground events were first removed in the offline data pro-cessing. The latter reduction was accomplished by a set ofalgorithms that remove events that do not have the char-acteristics of Cherenkov light emission. For the events thatpassed the first offline reduction, the calibrated times andpositions of the hit photomultiplier tubes (PMTs) wereused to reconstruct the vertex position and the directionof the particle. The energy estimator then assigned an ef-fective kinetic energy Teff to each event based on thesereconstructed parameters and the number of hit PMTs.The energy estimator used calibration results such as lightattenuation as inputs, and was normalized to the detector824 A.W. Poon for the SNO Collaboration: Recent results from the Sudbury Neutrino ObservatoryTable 1. Correlation coefficients in solar neutrino signal ex-traction in the pure D2O phaseE constrained E unconstrainedNC, CC -0.520 -0.950CC, ES -0.162 -0.208ES, NC -0.105 -0.297response to a 16N source [13]. The linearity and spatial de-pendence of the energy estimator were checked by a 252Cffission source and a 19.8-MeV γ-ray source [14]. The neu-tron detection efficiency was calibrated with a 252Cf fissionsource.Radioactive decays of the daughters in the natural232Th and 238U chains were the dominant backgrounds inthe neutrino signal window. The levels of 232Th and 238Udaughters in the detector were determined by radiochemi-cal assays of the detector target [15] and by analyzing theCherenkov signals from their decays in the neutrino dataset. The radioactive backgrounds were found to be betterthan those specified in the design criteria of the detector.To test the null hypothesis of no neutrino flavourtransformation in solar neutrinos, the extended maxi-mum likelihood method was used to extract the CC, ESand neutron (i.e. NC+background) contributions in thecandidate data set. Background contributions were con-strained to the measured values. Data distributions in Teff ,the volume-weighted radial variable (R/RAV )3 and cos θwere simultaneously fitted to the probability density func-tions (PDFs) generated from simulations. RAV = 600 cmis the radius of the acrylic vessel, and θ is the anglebetween the reconstructed direction of the event and theinstantaneous direction from the Sun to the Earth. Assum-ing an undistorted 8B energy spectrum [16], the fluxes ofthe νe (φe) and non-νe (φµτ ) components were found tobe (in units of 106 cm−2 s−1):φe = 1.76+0.05−0.05(stat.)+0.09−0.09 (syst.)φµτ = 3.41+0.45−0.45(stat.)+0.48−0.45 (syst.)Figure 1 shows φµτ vs. φe deduced from the SNO data.Combining the statistical and systematic uncertainties inquadrature, φµτ is 5.3σ above zero, and provides strongevidence for flavour transformation. Removing the energyconstraint, the signal decomposition was repeated usingonly the cos θ and (R/RAV)3 distributions. The total fluxof active 8B neutrinos in the unconstrained fit was foundto be (in units of 106 cm−2 s−1)φNC = 6.42+1.57−1.57(stat.)+0.55−0.58 (syst.)The correlation coefficients from the energy constrainedand the unconstrained fits are shown in Table 1.3 Current status of the salt phaseNeutrino flavour transformation can be an energy de-pendent process (e.g. the MSW mechanism [17]). The0 1 2 3 4 5 6012345678)-1 s-2 cm6 (10eφ)-1 s-2 cm6 (10τµφ SNONCφSSMφSNOCCφSNOESφFig. 1. Flux of solar νµ or ντ vs. flux of νe deduced from thethree neutrino reactions in SNO. The diagonal bands show thetotal 8B flux as predicted by the SSM [7] (dashed lines) andthat measured with the NC reaction (solid band). The inter-cepts of these bands with the axes represent the ±1σ errors.The bands intersect at the fit values for φe and φµτ , indicat-ing that the combined flux results are consistent with flavourtransformationenergy constrained analysis in pure D2O made the ex-plicit assumption of an undistorted 8B neutrino spectrum.The unconstrained results suffered from the strong anti-correlation between the CC and NC channels. The addi-tion of 2 tonnes of NaCl to the D2O target was aimedat improving the separation between CC and NC withoutinvoking the energy constraint. Unlike the radiative cap-ture of a neutron on deuteron, neutron capture on 35Cltypically produces multiple γ rays while the CC and ESreactions produce single electrons. The greater isotropy ofthe Cherenkov light from neutron capture events relativeto CC and ES events allows good statistical separationof the event types. Simulations show that the statisti-cal uncertainties in the NC channel reduce significantlyin the unconstrained fit when Cherenkov light isotropy isincluded. This is shown in Table 2.There are a number of challenges to the analysis of thedata in the salt phase. A few of these challenges are de-scribed in the following. To minimize the possibility of in-troducing biases, a blind analysis procedure is being used.The data set used during the development of the analysisprocedures and the definition of parameters excluded anunknown fraction (<30%) of the final data set, includedan unknown admixture of muon-following neutron events,and included an unknown NC cross-section scaling factor.At the time of this talk, the blindness constraints have notbeen removed.After the addition of NaCl into the D2O target, theamount of detected Cherenkov light has dropped signif-icantly over a short period of time. Following this sharpdrop, there has been a gradual decline in the detector gain(Fig. 2). There are a number of contributors to the drop indetector gain. These include the degradation of the lightconcentrator surrounding each PMT, and the increase inthe concentration of polymer colloids and Manganese dueA.W. Poon for the SNO Collaboration: Recent results from the Sudbury Neutrino Observatory 825Table 2. Expected statistical uncertainties in the salt phaseFit Variables CC NC ESTeff , (R/RAV )3, cos θ 4.2% 6.3% 10%Teff , (R/RAV )3, cos θ, Isotropy 3.3% 4.6% 10%(R/RAV )3, cos θ, Isotropy 3.8% 5.3% 10%Fig. 2. Time evolution of the detector energy response to 16Ncalibration runs (6.13-MeV γ) taken at the center of the detec-tor. The number of fired PMTs, corrected for the variation ofthe number of online PMTs, is plotted against time. The endof the D2O phase is on May 28, 2001to the necessary modifications to the water purificationprocedures in the presence of NaCl. Preliminary studiesshow that the absolutely energy scale uncertainty in thesalt phase to be ∼1.1%. This is to be compared to 1.1%in the pure D2O phase.The SNO simulation package, which uses EGS4 [18]to simulate electromagnetic showers, predicted a moreisotropic light distribution than what is observed from theenergy and background calibration sources. It was subse-quently discovered that the electron scattering cross sec-tion in EGS4 neglects the Mott scattering terms, whichreduce the amount of large angle scattering. A correctionderived from a Monte Carlo treatment of multiple scat-tering of electrons has been added to the SNO simulationpackage. The agreement between the Monte Carlo predic-tion and the observed Cherenkov light isotropy improvedsignificantly after this correction is made.24Na is a non-negligible low energy background in theneutrino signal window. This radioisotope is produced inthe water circulation system by neutrons emitted fromthe rock wall in the underground laboratory. It can alsobe produced in-situ by calibration sources (e.g. the 252Cfsource). In order to understand the production and thedistribution of this background, a controlled neutron acti-vation has been carried out. The amplitude of this back-ground in the neutrino signal window is found to be small.At the time of this talk, the SNO collaboration is fi-nalizing the analysis procedures that are necessary for re-moving the data blindness constraints. The results of theenergy unconstrained NC measurement, along with its in-terpretation, will be reported in the near future.4 Future prospects and summaryThe SNO collaboration is now preparing for the NCDphase of its physics program. In this phase, the NC neu-trons are detected by the NCD array and the Cherenkovlight from the CC signals are detected by the PMTs. Thisseparation of the detection mechanism will significantlyreduce the statistical anti-correlation of the signals. In ad-dition, a measurement of the neutrino energy spectrum viathe CC reaction can be made with higher precision withthe reduction of NC signal in the Cherenkov light spec-trum.Once the data blindness constraint is removed in thesalt phase, the D2O target will be desalinated. All the3He counters have been constructed and are being charac-terized in the underground laboratory. There are intenseactivities in the final calibration of the detector in thesalt phase, preparing for the desalination process, prepar-ing for the NCD deployment, and integration of the NCDelectronics and data acquisition system.The SNO experiment has demonstrated conclusivelythat solar νe’s transform their flavour while in transit tothe Earth. A high precision measurement of the total ac-tive solar neutrino flux without any assumption of theenergy dependence of the neutrino flavour transformationprobability is near completion. In the next few months,the SNO physics program will enter its NCD phase. It isanticipated that the data from this NCD phase, along withdata from the previous phases, will significantly enhanceour understanding of the properties of neutrinos.References1. B.T. Cleveland et al.: Astrophys. J. 496, 505 (1998)2. Y. Fukuda et al.: Phys. Rev. Lett. 77, 1683 (1996)3. J.N. Abdurashitov et al.: J. Exp. Theor. Phys. 95, 181(2002)4. W. Hampel et al.: Phys. Lett. B 447, 127 (1999)5. M. Altmann et al.: Phys. Lett. B 490, 16 (2000)6. S. Fukuda et al.: Phys. Lett. B 539, 179 (2002)7. J.N. Bahcall, M.H. Pinsonneault, and S. Basu: Astrophys.J. 555, 990 (2001)8. The SNO Collaboration: Nucl. Instr. and Meth. A 449,172 (2000)9. M.C. Browne et al.: IEEE Trans. Nucl. Sci. 46, 873 (1999)10. Q.R. Ahmad et al. (the SNO Collaboration): Phys. Rev.Lett. 87, 071301 (2001)11. Q.R. Ahmad et al. (the SNO Collaboration): Phys. Rev.Lett. 89, 011301 (2002)12. Q.R. Ahmad et al. (the SNO Collaboration): Phys. Rev.Lett. 89, 011302 (2002)13. M.R. Dragowsky et al.: Nucl. Instr. Meth. A 481, 284(2002)14. A.W. Poon et al.: Nucl. Instr. and Meth. A 449, 172 (2000)15. T.C. Andersen et al.: Nucl. Instr. and Meth. A 501, 386(2003); T.C. Andersen et al.: Nucl. Instr. and Meth. A501, 399 (2003); I. Blevis et al.: arXiv:nucl-ex/0305022,submitted to Nucl. Instr. and Meth.16. C.E. Ortiz et al.: Phys. Rev. Lett. 85, 2909 (2000)17. S.P. Mikheyev and A.Y. Smirnov: Sov. J. Nucl. Phys. 42,913 (1985); L. Wolfenstein: Phys. Rev. D 17, 2369 (1978)18. W.R. Nelson et al.: SLAC report 265, 1 (1985)

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