SNO and future solar neutrino experiments

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67 www.elsevier.com/locate/npbps

SNO and future solar neutrino experiments A. B. McDonald a a

Queen’s University, Kingston, Ontario, Canada, for the SNO Collaboration

Abstract The SNO Collaboration has completed a combined analysis of 8B solar neutrino data from all three phases of the project. The combined analysis resulted in a total flux of active neutrino flavors from 8B decays in the sun of 6 2 1 . A three-flavor neutrino oscillation analysis combining the SNO results 5.25 r 0.16(stat.)00..11 13 ( syst.) u 10 cm s with results of all other solar neutrino experiments and the KamLAND experiment yielded 2 5 2 2 0.030 , and sin 2 T13 (2.511..58 ) u 102 . The results of the SNO analysis will 'm21 (7.4100..21 19 ) u 10 eV , tan T 12 0.446 0.029 be presented, along with a discussion of future projects that will seek to study lower energy neutrinos to improve on our knowledge of the properties of neutrinos and the sun. Keywords: Solar neutrinos, neutrino properties, solar models

1. Initial solar neutrino measurements Neutrinos from the sun were a puzzling but promising physics topic starting with the pioneering calculations of Bahcall and measurements of Ray Davis and his collaborators in the 1960’s [1]. Figure 1 shows the spectrum of electron neutrinos from the sun, exhibiting the range of energies and fluxes [2]. A number of experiments measured these fluxes using reactions that were exclusively sensitive to electron neutrinos (inverse beta reaction on Cl, Ge [3,4]) or predominantly sensitive to electron neutrinos (elastic scattering from electrons in water [5]) and observed smaller fluxes than calculated by factors of two to three. This discrepancy between experiment and theoretical calculations came to be known as the Solar Neutrino Problem and was discussed as possibly arising from incomplete calculations of solar fluxes or from electron neutrinos changing their type before

reaching the detectors. The different thresholds for the detectors as listed in the caption of figure 1 made it difficult to distinguish clearly between these two possibilities because the solar model calculations were required for the comparison of these experiments. 2. Sudbury Neutrino Observatory (SNO) The Sudbury Neutrino Observatory was designed to use the deuterium in 1000 tonnes of heavy water to measure two reactions, one sensitive exclusively to electron neutrinos (Charged Current: CC) and one equally sensitive to all active neutrino types (Neutral Current: NC). With these two simultaneous measurements it is then possible to determine what fraction of the neutrinos reaching the detector have survived as electron neutrinos, thereby determining if any have changed their type, as well as providing an independent measure of the initial flux of solar neutrinos.

———

Corresponding author. Tel.: +1-613-541-1405; fax: +1-613-533-6813; e-mail: [email protected]. 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.03.012

62

A.B. McDonald / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67

Results from the first phase of SNO showed clearly that about 2/3 of the electron neutrinos from 8B decay in the sun changed to other active types before reaching the earth. [6] A previous comparison for the results for the CC reaction with the ES reaction measured by the Superkamiokande experiment [7] showed a 3.3 V effect. [8] Normalized to the integrated rates above the detector kinetic energy threshold of 5 MeV, the flux of 8B neutrinos measured with each reaction in SNO, assuming the standard spectrum shape [6] is (all fluxes are presented in units of 106 cm-2s-1): ) CC 1.76 00..06 05 ( stat .) r 0.09( syst.) Fig. 1. Fluxes of neutrinos from the sun [2]. Thresholds for detection by experiments are Homestake: 0.867 MeV[1]; SAGE,Gallex,GNO: 0.250 MeV [3,4]; Kamiokande, SuperKamiokande: 4-7 MeV [5]; SNO Charged Current: 5-7 MeV, SNO Neutral Current: 2.2 MeV.

The reactions used for the detection of solar neutrinos in SNO are:

Q e  d o p  p  e  (CC) v x  d o p  n  Q x ( NC) Q x  e  o Q x  e  (ES) The charged current (CC) reaction is sensitive only to electron neutrinos, while the neutral current (NC) reaction is sensitive to all active neutrino flavors (x = e,PW) above the energy threshold of 2.2 MeV. The elastic scattering (ES) reaction on electrons is sensitive to all flavors as well, but with reduced sensitivity to QP and QW. The Cherenkov light produced by the electrons in the final state is used to observe the CC and ES reactions. The NC reaction is observed through the detection of the neutron in the final state of the reaction using three different techniques for the separate phases of the experiment. For phase 1 of the experiment, with pure heavy water, the neutrons captured on deuterium and produced 6.25 MeV gammas. For phase 2, about two tons of NaCl was added to the heavy water, resulting in capture on Cl with higher energy gammas and discrimination of CC and NC on a statistical basis via pattern recognition. In phase 3, an array of 3He-filled detectors was installed in pure heavy water. The neutron signals for the NC reaction from these detectors are totally independent of the light generated by events from the CC reaction, thereby breaking any covariance between the two types of events.

) ES

24 2.39 00..23 ( stat.) r 0.12( syst.)

) NC

44 46 5.09 00..43 ( stat.) 00..43 ( syst.)

A simple change of variables resolves the data directly into electron and PW components: )e )PW

1.76 r 0.05( stat.) r 0.09( syst.) 48 3.41 r 0.45( stat.) 00..45 ( syst.)

assuming the standard 8B shape for the neutrino spectrum. Combining the statistical and systematic .66 which is uncertainties in quadrature, ) P W 3.41.0064 5.3 V above zero, providing strong evidence for flavor transformation consistent with neutrino oscillations [6]. The total flux of 8B neutrinos was found to be consistent with solar model calculations. Phase 2 and phase 3 provided techniques for the separate measurement of the NC flux giving considerably improved accuracy for the total flux of 8B neutrinos and the ratio of CC and NC measured fluxes. The ratio of fluxes measured with the CC and NC reactions was determined to be [9] )CC / ) NC 0.304 r 0.026(stat ) r 0.024(syst) . For large angle MSW oscillations [10, 11] in the sun, this ratio can be directly related to the oscillation parameters: ) CC / ) NC | sin 2 T12 . The SNO result can be used to infer that the observed value of T12 deviates from maximal mixing by more than 5.4 V>@  A massive combined analysis of data from the three phases of the SNO experiment has been completed [12], providing the best accuracy for flux measurements and neutrino oscillation parameters associated with solar neutrinos. The combined analysis resulted in a total flux of active neutrino flavors from

A.B. McDonald / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67 6 2 1 B decays of 5.25 r 0.16(stat.) 00..11 . 13 ( syst.) u 10 cm s This result was consistent with but more precise than both the BPS09(GS), 5.88 r 0.65 u 106 cm 2 s 1 , and BPS09(AGSS09), 4.85 r 0.58 u 106 cm 2 s 1 , 8B solar model predictions [13] for two different metal abundances.

8

A two-flavor neutrino oscillation analysis yielded 2 5 eV2, tan 2 T12 0.446 00..030 . A 'm21 (5.6 00..19 14 ) x10 029 three-flavor neutrino oscillation analysis combining this result with results of all other solar neutrino experiments and the KamLAND experiment yielded 2 5 2 2 0.030 , and 'm21 (7.4100..21 19 ) u 10 eV , tan T 12 0.446 0.029

sin 2 T13 (2.511..58 ) u 102 . This implied an upper bound of sin 2 T13  0.053 at the 95% confidence level. The survival probability for electron neutrinos was parameterized by a polynomial as a function of energy, including day-night asymmetry. Figure 2 shows the survival probability for electron neutrinos as a function of energy, along with survival probabilities derived from other experiments. The Borexino experiment has recently reported a measurement of the pep neutrino flux at 1.4 MeV corresponding to a survival probability of 0.64 r 0.17 [14]. They also set an upper limit on the CNO flux at 1.5 times the Standard Solar Model [15] flux.

Fig. 2. Various solar Qe survival probability measurements compared to the LMA prediction for 8B neutrinos. The dashed line is the best fit Large Mixing Angle solution for 8B neutrinos and the gray shaded band is the 1V uncertainty. The circle is the result of the Borexino measurement [14] for 7Be neutrinos and the square is an average of measurements of pp neutrinos with small contributions from higher energy reactions removed (see [12])

The SNO collaboration has also searched for periodicities in the solar neutrino data. No significant

63

effect has been seen for a day/night asymmetry and limits have been set as a function of energy for the CC reaction[12]. A variation in the total flux with a period of 1 yr is observed, consistent with modulation of the 8 B neutrino flux by the Earth’s orbital eccentricity. No other significant sinusoidal periodicities are found with periods between 1 d and 10 years. [16]. A limit of 22 x 103 cm-2/s (90% C. L.) has been set on the flux from the hep reaction in the sun [17], which is consistent with solar model predictions of about 8 x 10 3 cm-2/s. 3. Physics objectives for future measurements

3.1. Solar Physics Major objectives for future measurements are more accurate measurements of pp, pep, CNO and lower energy 8B neutrinos. As discussed by Aldo Serenelli at this conference [18] measurements of 8B and CNO neutrino fluxes can be combined to provide discrimination between the several values of heavy element abundances that are presently uncertain. 3.2. Neutrino Physics Future measurements of pp, pep and 8B electron neutrino survival probabilities, to improve the accuracy of the data in figure 2, can be used to verify the validity of the Large Mixing Angle (LMA) solution for oscillation of solar electron neutrinos with matter interactions in the sun and to search for other more unusual physics that might appear as subdominant effects. This region of energy is an interesting one for detailed testing of matter effects as they change significantly between pp neutrino energies where the oscillation effects are essentially vacuum oscillations and 8B neutrino energies where neutrino oscillations are strongly dominated by LMA matter interactions in the sun. Other physics effects can also show up in this energy region as pointed out by several authors. De Holanda and Smirnov [19] have pointed out that sterile neutrinos with a certain range of oscillation parameters could result in a dip in the survival probability in this region. Friedland, Lunardini and Pena-Garay [20] have shown that non-standard neutrino interactions could also make significant perturbations in the shape of the survival probability curves. Barger, Huber and Marfatia [21] have suggested that significant perturbations could arise from effects associated with mass-varying neutrinos. In addition to solar physics

64

A.B. McDonald / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67

objectives, future solar neutrino measurements will be designed to determine electron survival probabilities as accurately as possible to search for effects that may be indicative of such new physics phenomena. 4. Future Solar Neutrino Experiments In addition to ongoing measurements of solar neutrinos such as Borexino and SAGE, Table 1 lists future solar neutrino measurements that are under construction or have been proposed and indicates the principal physics objectives for each of the measurements. 4.1. SNO+, KamLAND The SNO+ collaboration is converting the former SNO detector for operation with liquid scintillator (linear alkyl benzene: LAB plus fluor) with first operation scheduled for 2013 [22]. The principal initial objective will be measurements of neutrino-less double beta decay with dissolved Nd or possibly other isotopes. However, the first six months or so of operation will be with pure scintillator to study backgrounds and assess the potential for solar neutrino measurements following the double beta decay measurements. Assuming that radioactive backgrounds levels can be maintained at low levels as has been attained by experiments such as BOREXINO, the SNO+ detector can provide accurate measurements of pep and CNO neutrinos, aided significantly by the greater depth of the SNOLAB site that strongly reduces the background from 11C decay induced by cosmic rays. The goals are r 5 % uncertainty for a measurement of pep and r 10 % uncertainty for the measurement of CNO flux after 3 years of operation. The pep measurements will be aimed at studies of neutrino physics effects distorting the electron neutrino survival probability as a function of energy. The electron neutrino flux from the pep reaction is very well defined by solar models as it is strongly correlated with the pp reaction which is the principal reaction defining the overall solar luminosity. Therefore an accurate measurement of the pep neutrino flux can define the survival probability accurately at about 1.4 MeV neutrino energy. It will also be an objective to study the survival probability of 8B neutrinos as a function of energy down to a threshold for the detection of the scattered

electron from the ES process of about 2 MeV. The measurements of the CNO flux will be aimed at providing constraints on solar models as outlined above. In order to use LAB it is necessary to hold down the SNO+ central acrylic vessel against the buoyancy of the outside water and so a rope net has been added to the vessel and attached to the floor of the cavity using cleanly drilled holes with anchor bolts. Extensive new scintillator purification systems are being constructed. Otherwise, the majority of the detector can be reused with updating to the trigger system and a new data acquisition system. The objective is for a substantial reduction in radon levels in the central vessel and so a sealed rather that a flow-through system will be used with a radon-free cover gas. The KamLAND-Zen collaboration [23] is presently concentrating on measurements of double beta decay in Xe with an internal balloon and has postponed potential measurements of solar neutrinos until after the completion of the double beta decay measurements. It also has the capability to use ~ 1 kT of liquid scintillator for such studies. 4.2. XMASS, CLEAN Liquid noble gas detectors have attractive properties for the real-time detection of pp solar neutrinos. The purity of the detector material, coupled with excellent scintillation light output can provide clean signals for pp solar neutrinos if a detector with sufficient mass (10’s of tons) can be used. The XMASS [24] detector has been in operation with 835 kg of liquid Xe for double beta decay measurements since 2010. Future plans include a 20 ton detector for pp solar neutrino measurements. Liquid Xe has the advantage of excellent self shielding against local external radioactive backgrounds. The CLEAN collaboration [25] has proposed the creation of a liquid neon detector of about 50 tons total mass for the detection of pp solar neutrinos at a rate of about 1 event per day per ton for a 50 keV threshold. Liquid Ne has the advantage of very low natural inherent radioactivity and can be highly purified. The Mini-CLEAN detector is presently being built with about 500 kg of liquid Ar or Neon. The principal objective is the measurement of Dark Matter particles, but the detector will provide valuable information for

A.B. McDonald / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67

the development of a future larger scale detector for solar neutrino detection. 4.3. LENS, MOON, IPNOS The LENS experiment [26], pioneered by the late Ragu Raghavan, is designed to detect solar neutrinos via the charged current electron neutrino capture on 115 In. The capture of an electron neutrino on 115In yields an electron and a 115Sn nucleus in an excited state. This excited state decays with mean lifetime of 4.76 ms by emission of a 115 keV gamma ray followed by a 497 keV gamma ray, giving a triple coincidence signature. The threshold energy for neutrino capture on 115In is 115 keV. The energy of the electron emitted from the neutrino capture relates directly to the energy of the capture neutrino. The principal challenge for pp neutrino detection in LENS is the background originating from 115In beta decay which is at a rate of 2.5 MHz compared to an expected pp neutrino rate of 400 per year. In order to reduce background in a 10 ton LENS to an acceptable level, the detector must be highly segmented (cellular). The detector design is being tested in the microLENS experiment, currently being constructed .

Fig. 3. Simulation of the solar neutrino spectrum as it could be measured in LENS. Upper half shows the expected counts in a 10 ton detector after 5 years of operation. The lower half shows expected contributions from various solar neutrino fluxes.

By the use of the charged current reaction, the experiment would be sensitive only to electron neutrinos and could provide a measure of the pp, 7Be, CNO, and pep neutrinos as shown in figure 3. The electron neutrino survival probability could be obtained by comparison with fluxes calculated by solar models. However, if sufficiently accurate measurements exist via elastic scattering from electrons in other detectors, the survival probability

65

could be obtained independently from solar models via the small part of the elastic scattering cross section sensitive to all active neutrino types through neutral current interactions. An accurate determination of the pp neutrino survival probability could provide an improvement in the determination of the T mixing angle. The MOON collaboration [27] plans to use the inverse beta reaction on 100Mo, with a threshold of 168 keV to study pp and 7Be solar neutrinos. The solar neutrino signal can be selected by requiring delayed coincidence with the successive beta decay of 100Tc, and thus natural and cosmogenic backgrounds are reduced substantially. Small prototype detectors with high granularity are in operation for R&D with the objective of eventually producing a full scale detector on the order of three tons. The IPNOS collaboration is also pursuing the study of solar neutrinos using InP detectors and is working on prototype detectors [28]. 4.4. MEGAPROJECTS Very large scale projects using water or liquid argon aimed principally at other physics objectives such as long baseline neutrino oscillations could be capable of observing 8B solar neutrinos with very high rates. Examples under planning are HyperK, MEMPHYS, LBNE and GLACIER. The LENA project [29] with about 50 kton of liquid scintillator could provide measurements of 8B, 7Be, pep, and CNO neutrinos assuming strong control on radioactivity in the detector. The rates would be hundreds to thousands per day. An interesting coincidence process for 13C detection of 8B neutrinos [30] could provide sensitivity at rates of about 10 to 20 per kt per year, with little background, down to the threshold of 2.2 MeV. 5. Summary Solar neutrino measurements have provided a very accurate perspective on the sun’s inner regions and have tested solar models in great detail. They have also provided definitive information on neutrino properties and flavour change processes. Future measurements show considerable promise for the continuation of this process both for solar physics and neutrino physics. A number of projects are in construction for operation

66

A.B. McDonald / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67

Table 1 Future Solar Neutrino Experiments (Beyond those already in operation) Physics

Detection Medium

Status

pep/CNO via ES SNO+

780 tonnes Liquid Scintillator

Kamland-2

~ 1 kT Liquid Scintillator

2014 short run, then after EE After Kamland-Zen

pp via ES XMASS

20 tons Liquid Xe

835 kg since 2010 for EE

CLEAN

50 tons Liquid Ne

MiniCLEAN (500 kg) 2013

LENS

10 tons 115In

PLENS under development

MOON

3 tons 100Mo

R&D in progress

IPNOS

115

R&D in progress

7

pp, Be via CC

In

Megaprojects HyperK, MEMPHYS

Megaton Water Cerenkov

Planning

LBNE, GLACIER

50 to 100 kTon Liquid Ar

Planning

LENA

50 kTon Liquid Scintillator

Planning

planning during the next few years or planning for the longer term. References [1] R. Davis, Jr., Phys. Rev. Lett. l2, 302 (1964); J.N. Bahcall, Phys. Rev. Lett. 17 (1966) 398. [2] John N. Bahcall, Aldo M. Serenelli, and Sarbani Basu), Astrophysical J, 621 (2004) L85 astro-ph/0412440. [3] B.T. Cleveland, R Davis et al., Ap. J. 496, 505 (1998); K. Lande and P. Wildenhain, Nucl. Phys. B (Proc. Suppl.) 118 (2003) 49. [4] J. N. Abdurashitov et al. (SAGE Collaboration), Phys. Rev. C 80 (2009) 015807; M. Altmann et al. (GNO Collaboration), Physics Letters B 616, 174 (2005); [5] Y. Fukuda et al. (Kamiokande Collaboration), Phys. Rev. Lett. 77, 1683 (1996). [6] Q. R. Ahmad et al. (SNO Collaboration), Phys. Rev. Lett. 89, 011301 (2002); Phys. Rev. Lett. 89, 011302 (2002). [7] S. Fukuda et al. (SuperKamiokande Collaboration), Phys. Rev. Lett. 86, 5651 (2001). [8] Q. R. Ahmad et al. (SNO Collaboration), Phys. Rev. Lett. 87, 071301 (2001). [9] Q. R. Ahmed et al. (SNO Collaboration), Phys.Rev.Lett. 92 (2004) 181301. [10] Z. Maki, N. Nakagawa, and S. Sakata, Prog. Theor. Phys. 28 (1962) 870; V. Gribov and B. Pontecorvo, Phys. Lett. B28 (1969) 493.

[11] L. Wolfenstein, Phys. Rev. D 17 (1978) 2369; S.P. Mikheyev and A. Smirnov, Nuovo Cimento C 9 (1986) 17. [12] B. Aharmim et al. (SNO Collaboration, arXiv:1109.0763v1 (2011) [13] A. M. Serenelli, S. Basu, J. W. Ferguson, and M. Asplund, Astrophys. J. Lett. 705 (2009) L123. [14] Borexino Collaboration, arXiv:1104.1816v1 (2011); arXiv 1110.3230 (2011) [15] A.M. Serenelli, W.C. Haxton and C. Pena-Garay, arXiv:1104.1639. [16] SNO Collaboration: Physical Review, D72 (2005) 052010. [17] SNO Collaboration, Astrophysical Journal 653, 1545 (2006); C. Howard, SNO thesis, 2011, to be published [18] A. M. Serenelli, this conference and arXiv:1104.1639. [19] P. C. de Holanda and A. Yu. Smirnov Phys.Rev. D69 (2004) 113002. [20] A. Friedland, C. Lunardni, C. Pena-Garay, Phys.Lett.B594 (2004) 347. [21] V. Barger, P. Huber, D. Marfatia, Phys.Rev.Lett. 95 (2005) 211802. [22] M. C. Chen (SNO+ collaboration), 34th International Conference on High Energy Physics (ICHEP 2008), Philadelphia, Pennsylvania, arXiv:0810.3694 [hep-ex] [23] KamLAND-Zen Collaboration, Phys.Rev. C86 (2012) 021601 [24] XMASS collaboration: http://www-sk.icrr.utokyo.ac.jp/xmass/index-e.html [25] D.N. McKinsey and K.J. Coakley, Astropart. Phys. 22 (2005) 355; M.G. Boulay, A. Hime and J. Lidgard, arXiv:nuclex/0410025 (2004); D.N. McKinsey and J.M. Doyle, J. Low Temp. Phys. 118 (2000) 153; A Hime. arXiv:1110.1005

A.B. McDonald / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 61–67

[26] LENS Collaboration: R. S. Raghavan, Phys. Rev. Lett., 37 (1976) 259; arXiv:0705.2769v1. [27] H. Ejiri (MOON Collaboration), Progress Particle Nucl. Phys. 64 (2010) 249; K. Fushimi et al., J. phys. Conference Series, 203 0120064 [28] IPNOS collaboration: Y. Fukuda, T. Izawa, Y. Koshio, S. Moriyama, T. Namba, M. Shiozawav, Nucl. Inst. Meth. A 623 (2010) 460. [29] LENA collaboration, arXiv:1104.5620 [30] A. Ianni, D. Montanino, F. Villante, Journal of Physics: Conference Series 39 (2006) 272–274

67