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Results from the Salt Phase of SNO

The Sudbury Neutrino Observatory (SNO) has recently completed an analysis of data from the salt phase of the experiment, in which NaCl was added to the heavy-water neutrino target to enhance sensitivity to solar neutrinos. Results from the 391-day salt data set are summarized, including the measured solar neutrino fluxes, the electron energy spectrum from charged current interactions, and the day-night neutrino flux asymmetries. Constraints on neutrino mixing parameters including the new measurements are also given.


Introduction
The Sudbury Neutrino Observatory (SNO) is a heavy water Cherenkov detector located 2092 m underground near Sudbury, Ontario.
The charged current reaction is sensitive only to electron neutrinos, while the neutral current reaction is sensitive to any active flavor, ν x , x = e, µ, τ . The elastic scattering reaction has some sensitivity to non-electron flavors, but is primarily sensitive to electron neutrinos. The NC reaction in SNO allows a measurement of the total flux of all active 8 B solar neutrinos, while a comparison of the neutrino fluxes measured through the NC and CC reactions tests for solar neutrino flavor change.
Cherenkov radiation from the electrons produced in the CC and ES reactions is detected in an array of approximately 9500 photomultiplier tubes (PMTs). The neutrons liberated in the NC reaction must be detected through a secondary capture reaction. The SNO experiment was designed to run in three phases, incorporating three distinct neutron capture signatures to ensure a robust NC measurement. In each phase, the neutron response features, systematic uncertainties, background characteristics, and analysis techniques differ. In the first phase of the experiment, neutrons were detected using the 6.25 MeV gamma signature from neutron capture on deuterium. Results from the first phase confirmed solar model predictions of the 8 B flux, demonstrated solar neutrino flavor change, and contributed to determination of the underlying neutrino oscillation parameters. 3 4 5 In the second phase of the experiment, ultra-pure salt (NaCl) was dissolved in the D 2 O, to a concentration of (0.196 ± 0.002)% by weight. The higher cross section for neutron capture on 35 Cl and the higher energy (8.6 MeV) released in the reaction improve SNO's efficiency for detecting NC neutrons. In addition, the multiple gammas produced in the 35 Cl capture reaction result in a more isotropic distribution of Cherenkov light relative to the distribution of light in single-electron events. Using the isotropy of light produced in a neutrino event, the CC and NC signals can be statistically separated without requiring constraints on the solar neutrino energy spectrum. First neutrino flux results for 254 days of salt data were published in 2004. 6 Results for the 391-day salt data set have recently been reported 2 and are summarized in this article.
Finally, the third phase of the SNO experiment began in the fall of 2004, following the deployment of 36 strings of 3 He proportional counters and 4 strings of 4 He proportional counters inside the heavy water volume. Neutron capture in the 3 He counters allows an event-by-event determination of the NC rate, completely decoupled from the PMT detection of the CC and ES signals.

Data Analysis in the Salt Phase
The effect of the added salt on SNO's neutron detection efficiency is illustrated in figure 1(a), which shows the neutron capture efficiency for a 252 Cf calibration source as a function of the radial position of the source in the detector volume. Accounting for the fiducial volume and energy cuts used in the solar neutrino analysis, the neutron capture efficiency in the salt phase is 40.7%, compared to 14.4% in the pure-D 2 O phase. The higher energy of neutron capture events in the salt phase also boosts the NC signal further above the analysis energy threshold. A Monte Carlo comparison of the energy spectrum for neutrons in the salt and pure-D 2 O phases is shown in figure 1(b).
A fiducial volume cut of 550 cm and an analysis energy threshold of T ef f > 5.5 MeV are used to reduce the number of background events in the final salt event sample, where T ef f is the effective electron kinetic energy for the event, T ef f = E − 0.511 MeV. The final data set consists of 4722 events recorded over 391.4 days of detector live time between July 2001 and August 2003. This data set contains some remaining backgrounds as well as neutrino interaction events. The dominant background in SNO is neutrons from the photodisintegration of deuterium by gammas with energies above 2.2 MeV. Radioactive contaminants from the U and Th decay chains are the primary source of these gammas, which can originate within the D 2 O or in the acrylic vessel and external regions of the detector. The total contribution of neutron backgrounds from inside the D 2 O region is determined, using a variety of in-situ and ex-situ techniques, to be 125.1 +37. 3 −32.0 events. Additionally, an estimated 3.2 +4.6 −4.4 events in the salt data set are gamma rays from the products of atmospheric neutrino interactions. These "internal" neutron and gamma backgrounds are held fixed in the statistical signal extraction process that is used to determine the CC, NC, and ES reaction rates. Neutrons and gamma rays produced outside the D 2 O region can also propagate into the fiducial volume. Neutron backgrounds due to these "external" sources will have a characteristic radial profile that can be used to distinguish them from signal neutrons.
To extract the numbers of CC, NC, ES, and external neutron (EN) events in the data set, event distributions in several variables are fit to characteristic signal distributions derived from Monte Carlo calculations. The event variables used are the effective electron kinetic energy T ef f , the radius of the reconstructed event vertex ρ = (r/600.5cm) 3 , the cosine of the angle between the reconstructed event direction and a vector from the sun cos θ ⊙ , and a parameter characterizing the light isotropy in the event, β 14 . The β 14 parameter is based on Legendre polynomials in terms of the angle between each pair of hit PMTs relative to the event vertex. Events with a more isotropic distribution of light have smaller values of β 14 .
Detailed Monte Carlo simulations are used to generate probability density functions (PDFs) characterizing signal distributions in these variables. These PDFs are shown in figure 2. In a statistical fit, the isotropy distributions provide a powerful tool for distinguishing neutrons from CC and ES events, the direction distributions help to identify the ES contribution, and the radial distributions separate the external neutron background.
The energy distribution shown for the CC signal assumes an undistorted 8 B neutrino spectrum. To make measurements that are independent of model assumptions about the energy spectrum, we can extract the CC and ES signals separately in each energy bin. In such an "energy unconstrained" analysis, the extracted CC spectrum can be used to test models of neutrino oscillation. This model-independent signal extraction is possible because of the power of the β 14 distribution for separating event classes in salt. In the D 2 O phase, the CC and ES spectra had to be constrained to break statistical correlations between the signals.

Systematic Uncertainties
Systematic uncertainties in detector response are evaluated through comparisons of Monte Carlo simulations and calibration data. The primary calibration sources used to study systematic uncertainties are a 16 N 6.13 MeV gamma-ray source and a 252 Cf fission neutron source. The 16 N source is used to study energy response, event reconstruction performance, and detector stability over time. The 252 Cf source is used to evaluate neutron response characteristics. Systematic uncertainties are propagated by perturbing the PDFs according to the estimated 1σ variation in each response parameter, and then repeating the signal extraction process.
The dominant systematic uncertainties on the CC and NC extracted fluxes in the energyunconstrained analysis are due to uncertainty in the β 14 parameter. Uncertainties of less than a percent in the mean isotropy values translate to uncertainties of around 4% in the CC and NC fluxes. The energy scale uncertainty in the salt phase is estimated to be 1.15%, which contributes an uncertainty of around 3.5% in the NC flux, but has a smaller effect on the CC and ES fluxes. An uncertainty of 1% in radial reconstruction accuracy is also one of the larger contributions to the overall systematic error, resulting in a ∼ 3% uncertainty in each flux. The ES flux uncertainty is dominated by a 5% systematic uncertainty due to uncertainty in angular resolution. A much more detailed discussion of calibrations and systematic uncertainties can be found in the recent salt phase publication. 2

Solar Neutrino Flux Results from an Energy-Unconstrained Analysis
From an energy-unconstrained extended maximum likelihood fit to extract the contributions of each signal to the data set, the number of NC events is 2010 ± 85, the number of CC events is 2176 ± 78, and the number of ES events is 279 ± 26. The external neutron background is 128 ± 42 events. Accounting for acceptance factors and detector live time, we can convert these extracted event numbers into equivalent fluxes of 8 The flavor composition of 8 B solar neutrinos detected by SNO is summarized in figure 3,

Charged Current Spectrum and Day-Night Asymmetries
The favored model for explaining solar neutrino flavor change invokes matter-enhanced neutrino oscillation in the solar interior, through the so-called Mikheyev-Smirnov-Wolfenstein (MSW) effect. In addition to predicting solar neutrino flavor change, the MSW effect has two other predictions that are potentially testable in SNO. Matter effects in the sun could distort the energy spectrum of solar neutrinos, and additional matter effects in the earth could affect the flavor composition of neutrinos that pass through terrestrial material before reaching a detector. The energy-unconstrained analysis of SNO's salt phase data allows an extraction of the CC electron energy spectrum, which can be used to test for spectrum distortions. Matter effects in the earth can be tested by looking for day-night asymmetries in the rate of charged current interactions. For both the spectrum and day-night analyses, a number of differential systematic uncertainties were evaluated. Calibration sources were used to study energy-dependent systematic effects. In-situ techniques using low-energy backgrounds and secondary products from cosmic-ray muon interactions were used to study diurnal variations for the day-night analysis.
The extracted CC energy spectrum is shown in figure 4, with statistical uncertainties. Systematic uncertainties are shown with respect to the prediction assuming an undistorted 8 B solar neutrino spectrum. The measured spectrum is consistent with no distortions, and is also consistent with the spectrum predicted by the best-fit MSW model, corresponding to the so-called Large Mixing Angle (LMA) region of the neutrino oscillation parameter space.
To search for day-night effects, we construct asymmetry parameters in terms of the measured day and night fluxes, All asymmetries are consistent with zero. The extracted day and night CC spectra from this analysis can be used to construct the CC asymmetry as a function of energy, shown in figure 5. Neutrino oscillation parameters in the LMA region predict very small day-night asymmetries, and a comparison to the LMA predicted asymmetry is shown in the figure.
Since an asymmetry in the NC rate is not expected for standard neutrino oscillations, a statistically significant NC asymmetry would be evidence for sterile neutrinos or exotic physics. Within the standard neutrino oscillation picture, we can constrain the neutral current asymmetry to be zero to reduce the statistical correlations in the extraction of the day-night asymmetries. With this additional constraint, the asymmetries become A CC = −0.037 ± 0.063(stat.) ± 0.032(syst.) Repeating this analysis with the assumption of an undistorted 8 B neutrino energy spectrum further reduces the uncertainties, and allows the salt phase charged-current asymmetry to be combined with the energy-constrained asymmetry results from the first phase of SNO. The combined day-night asymmetry from both phases is A salt+D 2 O = 0.037 ± 0.040(stat.+syst.). The day-night asymmetry results from SNO are consistent with no asymmetry, and also with the best-fit MSW model, which predicts an asymmetry of ∼ 3.5%.

MSW Parameter Constraints
The salt phase results for the fluxes, spectra, and day-night asymmetries can be combined with SNO's previous results and the results of other solar neutrino experiments to produce constraints on the fundamental neutrino parameters in the MSW model. Use of the isotropy parameter in the salt phase allows NC and CC events to be statistically separated without any assumptions about the underlying neutrino energy spectrum. The new flux results confirm and improve previous results, demonstrate solar neutrino flavor change, and contribute to evidence for solar neutrino oscillations. Salt phase results also include the first presentation of the extracted charged current electron energy spectrum with statistical and systematic uncertainties fully evaluated. This spectrum is consistent with the spectrum predicted for an undistorted 8 B neutrino spectrum, and is also consistent with the spectrum predicted by the best-fit MSW model. Day-night asymmetries have been constructed to test for possible MSW effects in the earth. Measured asymmetries are consistent with zero, and also with the predictions for the best-fit MSW model. A global analysis of the salt phase results along with other solar and reactor neutrino measurements has been performed, giving best-fit values of ∆m 2 = (8.0 +0. 6 −0.4 ) × 10 −5 eV 2 ,tan 2 θ = 0.45 +0.09 −0.07 . More details as well as additional results can be found in the recent salt phase publication. 2