SUPPLEMENTS ELSEVIER Nuclear Physics B (Proc. Suppl.) 110 (2002) 308-310

Solar Neutrino Results from the Sudbury Neutrino Observatory

Ian T. Lawsona*

BDepartment of Physics, University of Guelph, Guelph, Canada NlG 2Wl

Solar neutrinos from the decay of ‘B have been detected at the Sudbury Neutrino Observatory (SNO) via the charged-current (CC) reaction on deuterium and the elastic scattering (ES) of electrons. The CC reaction is sensitive exclusively to electron neutrinos while the ES reaction also has a small sensitivity to muon and tau neutrinos. The flux of electron neutrinos from ‘B decays measured by the CC reaction and ES reaction, assuming no flavour transformation, will be presented. These flux measurements provide evidence that there is a non-electron flavour active neutrino component in the solar flux. The total flux of active ‘B neutrinos will be presented and shown to be in good agreement with predictions of solar models.

For more than 30 years, solar neutrino exper- iments [l-6] have observed fewer neutrinos than are expected by the models of the Sun [7,8]. This deficit can occur if the Sun’s electron neutrinos (ve’s) change type while traveling to Earth. The Sudbury Neutrino Observatory (SNO) measures the flux of 8B neutrinos through the reactions:

u, + d + p+p+e (CC> vz + d + p+n+v,

h+e --f v, + e g

The charged-current (CC) reaction is sensitive ex- clusively to v, and the neutral-current (NC) re- action has equal sensitivity to all active neutrino flavours (z = e,p,T). The elastic scattering (ES) reaction is also sensitive to all active flavours, but with a reduced sensitivity to V~ and v,.

Comparing the solar neutrino flux inferred from the reaction rates of these channels under the no- oscillation premise provides evidence for flavour- changing neutrino oscillations. If Y,‘S from the Sun transform into another active flavour, then the so- lar neutrino flux deduced from the CC reaction rate (a”(~=)) is less than the ES or NC reaction rate.

This paper reports results [9] from the first meas- urement of the solar ‘B neutrino flux using CC and ES reactions. The measured QES(v,) is consistent with the ES measurement by Super-Kamiokande [6]. However, the measured acc(v,) at SNO is smaller and is therefore inconsistent with the null hypothesis of a pure v, part in the solar neutrino flux. This indicates that v,‘s emitted by the Sun have changed into the other active neutrino species.

*for the SNO Collaboration.

The SNO [lo] detector is an imaging water Cerenkov detector located in the INCO Creighton mine, near Sudbury, ON, Canada. It contains 1000 tonnes of DzO inside a 12 m diameter spherical acrylic vessel (AV) which is surrounded by a shield of Hz0 contained in a 34 m high barrel-shaped cavity of diameter 22 m. A stainless steel structure 17.8 m in diameter supports 9456 20-cm photomul- tiplier tubes (PMTs) with light concentrators.

The data reported in this paper were recorded from Nov. 2, 1999 to Jan. 15, 2001 and give a live time of 240.95 days. The data were divided into two sets. One set contained -70% of the data and set the data analysis procedures. The remainder of the data was used for a blind test of the statistical bias in the analysis after the analysis procedures were finalized. Analyses of the data sets using the same procedures showed no statistical differences.

Calibration of the PMT time and charge ped- estals, slopes, offsets, charge vs. time dependen- cies and second order dependencies are performed using electronic pulsers and pulsed light sources. Optical calibrations used a pulsed laser light. The absolute energy scale and errors are measured with a triggered 16N source. The resulting Monte Carlo predictions of detector response are tested using a 252Cf neutron source, which provides a distribu- tion of 6.25 MeV T-rays from neutron capture and a 3H(p,r)4He source providing 19.8 MeV y-rays.

The analysis began by removing the instrumen- tal backgrounds. Electrical pickup can make false PMT hits and electrical discharges in the PMTs or insulating materials may produce false light. These events appear different from cerenkov light and

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I. YY Lawson /Nuclear Physics B (Proc. SuppI.) 110 (2002) 308-310

are removed using cuts based on the PMT posi-

tions, PMT time and charge, event-to-event time correlations and veto PMTs. For events passing

this stage, the calibrated times and positions of the fired PMTs are used to reconstruct the vertex position and direction of the particle. The recon-

struction accuracy and resolution is measured us- ing Compton electrons from a 16N source and the energy and source variation of reconstruction are

checked with a 8Li p source. The vertex resolution is 16 cm and the angular resolution is 26.7’.

An effective kinetic energy T,ff is assigned to

each event based upon its position, direction and the number of hit PMTs within the prompt (un- scattered) photon peak. For an electron of total

energy I?,, the derived energy response can be pa- rameterized by a Gaussian:

where Eee = Tee +m,, and the energy resolution is

given by o.(K) = ( 0.4620 + 0.547Oa + 0.0087223,)

MeV. The systematic uncertainty on this absolute energy calibration is f1.4%.

Further instrumental background is removed us- ing reconstruction figures of merit, PMT time res- iduals and the average opening angle between two hit PMTs, measured from the reconstructed ver- tex. These cuts test the theory whereby the event has the features of single electron Cerenkov light. The effects of these and the rest of the instrumental background cuts are measured using the 16N and aLi sources dep lo y ed in the detector. The volume- weighted signal loss is measured to be 1.4+:::% and the residual background within the DzO is < 0.2%.

Above the threshold Tee 2 6.75 MeV the neu- trino signals include CC in the D20, ES in the DsO and HzO, the residual tail of neutron capture events and high energy y-rays from radioactivity. The data show a clear neutrino signal within the DzO. The fiducial volume is limited to 550 cm to reduce backgrounds and minimize systematic er- rors from optics and reconstruction near the AV.

Backgrounds from radioactivity are measured by radio-assays of U and Th decay chain products.

Low energy radioactivity backgrounds are removed by the high threshold imposed, as are most neutron capture events. The total contribution from all radioactivity in the detector is found to be < 0.2% for low energy backgrounds and < 0.8% for high

Figure 1. Distributions of (a) cos& and (b) ex- tracted kinetic energy spectrum for CC events. The Monte Carlo for an undistorted ‘B spectrum are the histograms. The ratio of the data to the expected ki- netic energy distribution is shown in (c).

energy backgrounds. The final data set has 1169 events for T,R 2 6.75

MeV and R < 550 cm. A maximum likelihood fit is used to extract the CC, ES and neutron component in the data set. Distributions in (R/RAv)~, Tetr and cos 80 are fit to probability density functions made from Monte Carlo assuming no flavour changes and the ‘B spectrum from Ortiz [ll]. cos 00 is the

angle between the reconstructed direction of the event and the direction from the Sun to the Earth (see Fig. l(a)). The forward peak arises from the strong directionality in the ES reaction. The cos O. distribution for the CC reaction, before detector response, is expected to be 1 0.34ocose~ [12]. The extraction yields 975.4 f 39.7 CC events,

106.1 f 15.2 ES events and 87.5 f 24.7 neutron

events. The errors given are statistical only; the systematic errors are shown in Table 1.

The *B neutrino flux is determined from nor- malising the observed integrated event rate above the energy threshold. Assuming the 8B spectrum from [ll], the flux deduced from the CC and ES reactions are:

310 1.7: Lawson /Nuclear Physics B (Proc. Suppl.) 110 (2002) 308-310

Table 1 Systematic uncertainties on the fluxes. Error source CC error ES error

(percent) (percent) Energy scale 5.2, +6.1 3.5, +5.4 Energy resolution f0.5 f0.3 Energy scale non-linearity f0.5 f0.4 Vertex accuracy f3.1 f3.3 Vertex resolution f0.7 f0.4 Angular resolution f0.5 f2.2 High energy y’s 0.8, +O.O 1.9, +o.o Low energy background 0.2, +o.o 0.2, fO.0 Instrumental background 0.2, +o.o 0.6, +O.O Trigger efficiency 0.0 0.0 Live time fO.l fO.l Cut acceptance 0.6, +0.7 0.6, +0.7 Earth orbit eccentricity fO.l fO.l 170 ‘80

, 0.0 0.0

@$go(ve) = 1.75 f O.O7(stat.)‘o,;::(sys.) f O.OB(theor.)

x106cm 2s ’ @boo = 2.39 f 034(stat.)+0,:~~(sys.) x 106cm 2s ‘,

where the theoretical error is the CC cross section error [13]. Radiative corrections to the CC cross section have not been applied, but are expected to decrease the CC flux [14]. The difference between @ggo(~.) and Oreo is 0.64 f 0.40 x 10scm 2s i,

or 1.60. The ratio of @ggo(~=) to the predicted 8B solar neutrino flux [7] is 0.347 f 0.029, where all uncertainties are added in quadrature.

The Super-Kamiokande experiment has mea sured the ES flux very precisely: chgg(v,) = 2.32 f

O.O3(stat.)+~;~(sys.) x 106cm 2s ‘. @%,(~z) and chcg(:(y,) are consistent with each other. Assum- ing that the systematic uncertainties are normally distributed, the difference between ipEg and @ggo(~,) is 0.57 f 0.17 x lo6 cm 2s ‘, or 3.3u.

The CC energy spectrum is extracted from the data by repeating the signal extraction with the CC energy spectral constraint removed (see Fig. 1 (b) and (c)). Th ere is no evidence for spectral dis- tortion under the no-oscillation hypothesis.

Using a%(~~) and (PscNco(~.), one can infer the flux of the non-electron flavour active neutrino ip(vc17) to be &jg = +(ve) + 0.154cP(+,). This

is shown in Fig. 2, in which a(~,,~) is shown vs. a(~~). The two data bands are the lo measure- ments of @go and @gg, and the error ellipses are 68%, 95% and 99% joint probability contours for (h(v,) and a(~,,~). The best fit to a(~,,~) is 3.69 f 1.13 x lo6 cm 2s ‘.

The total 8B flux derived from SNO and Super-

NV,) (relative to BPBOI) 0 0.2 0.4 0.6 0.8 1 1.2

8

0

0 1 2 3 4 5 6

$(v c ) (10” al&‘)

Figure 2. Flux of non-electron flavour active sB solar neutrinos (a(+)) versus ip$o and ih$g.

Kamiokande is shown as the diagonal band in

Fig. 2. The agreement with the standard solar model prediction is good. The total flux of active sB neutrinos is ‘P(v=) = 5.44 f 0.99 x lo6 cm 2s ‘.

In summary, the results reported here are the

first direct indication of a non-electron flavour com-

ponent in the solar neutrino flux, and the first ex-

perimental determination of the total flux of the

sB solar neutrinos.

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