- Email: [email protected]
Catching some Zs: The solar neutrino problem, neutral current interactions, and the Sudbury Neutrino Observatory
Nuclear Instruments and Methods in Physics Research A 472 (2001) 348–353
Catching some Zs: The solar neutrino problem, neutral current interactions, and the Sudbury Neutrino Observatory T.D. Steiger1 Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, WA 98195-4290, USA
Abstract The Solar Neutrino Problem (SNP) rivals CP violation as one of the oldest problems in physics that lacks a compelling explanation. The Sudbury Neutrino Observatory (SNO)Fa heavy-water Cherenkov detectorFis the first solar neutrino experiment that has the ability to discriminate signals from different neutrino flavors. Since all three presently known flavors participate in Zo exchange, SNO is extremely sensitive to neutrino oscillations. A brief overview of the SNP is given, including a discussion of sterile neutrinos. The ability of SNO to discriminate between the various possible solutions to the SNP is also discussed. The SNO detector is described, including a description of the neutral-current detection methods, and the current status of the experiment is given. r 2001 Elsevier Science B.V. All rights reserved. PACS: 26.65.+t; 29.40.ka; 29.40.Cs Keywords: Solar neutrinos; Neutrino oscillations; Neutral current interactions
1. Introduction: the Solar Neutrino Problem The original goal of the first solar neutrino experiment was to test the hypothesis that energy production in the Sun is driven by nuclear reactions in the core [1]. Using the reaction 37 Cl(ne, e)37Ar, the Homestake experiment succeeded in inferring a copious flux of solar neutrinos. However, the flux implied by the observations was only about a third of the flux predicted by detailed calculations of the expected nuclear processes in the Sun [2]. This simple deficit
1
E-mail address: [email protected] (T.D. Steiger). On behalf of the SNO Collaboration
in the observed neutrino flux was the original manifestation of the Solar Neutrino Problem (SNP). Subsequent experiments have confirmed the solar neutrino deficit. The SAGE [3] and GALLEX [4] experiments are similar in style to Homestake, but they make use of the reaction 71 Ga(ne, e)71Ge, which has a lower Q value than the chlorine reaction and hence a lower energy threshold for detecting neutrinos. This allows the gallium experiments to detect neutrinos from the p–p reaction; the most abundant source of solar neutrinos. The Super-Kamiokande experiment [5] uses a water target and detects Cherenkov light from neutrino–electron scattering events. These
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 2 6 9 - 4
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events provide direction and energy information, which is unavailable in the chlorine and gallium experiments. The three types of solar neutrino experiments performed to data have different energy thresholds for neutrino detection: gallium B0.2 MeV, chlorine B0.8 MeV, and water Cherenkov B5 MeV. Hence, combining the observations of these experiments results in a coarse spectroscopy of the solar neutrino spectrum. The apparent result is that the observed deficit depends on neutrino energy. Using Fpp to represent the flux of lowenergy solar neutrinos (Eno0.4 MeV), F7Be for the flux at intermediate energies (0.4oEno5 MeV), and F8B for the high-energy flux (En>5 MeV), then a joint analysis of all of the solar neutrino data taken to date indicates Fpp BFSSM pp F 7 B0 Be
F 8 B0:5FSSM 8 B
ð1Þ
B
where the superscript SSM denotes the Standard Solar Model prediction for the flux [6]. This is the current formulation of the Solar Neutrino Problem; any candidate solution must not merely explain the deficit of solar neutrinos, but must also explain the observed energy dependence summarized above. A careful review of the relevant nuclear crosssections indicated that there were no serious problems with the Standard Solar Model inputs [7]. Furthermore, it can be shown that it is impossible to explain the experimental observations by a straightforward alteration to the Standard Solar Model itself [8]. Hence, some alteration to the physics of the neutrino seems to be required. A promising explanation for the SNP involves flavor oscillations in which the electron neutrinos generated in the Sun are converted to other flavors (that are more difficult to detect) before they arrive at the detectors on Earth. Solutions in which the oscillations take place in vacuum [9], and those that require interactions with solar matter to enhance the oscillations [10] remain viable. Conventional thinking dictates that oscillations require
349
massive neutrinos, and so are a signature for physics beyond the Standard Model. Recently, however, string theorists exploring the implications of extra spatial dimensions have proposed mechanisms by which even massless neutrinos may oscillate [11]. The implications that these different sorts of solutions have for the Sudbury Neutrino Observatory (SNO) will be explored in Section 3. 2. The Sudbury Neutrino Observatory The radiochemical chlorine and gallium experiments detect only electron neutrinos due to the reactions they utilize. Water Cherenkov detectors use electron scattering as a detection mechanism and so are sensitive to muon and tau neutrinos as well. However, this sensitivity is limited by the fact that nm and nt can only interact with electrons via Z exchange, whereas ne can interact via Z or W exchange. So the sensitivity of a water Cherenkov detector to nm and nt is suppressed by about a factor of 7. Furthermore, these interactions cannot be distinguished from ne interactions in practice. Thus, neutrino flavor oscillations pose a serious problem for all of the solar neutrino experiments to date, and may well explain the SNP. The Sudbury Neutrino Observatory consists of 1000 metric tons of heavy water (99.92% D2O) contained within a spherical acrylic vessel that is 12 m in diameter. The vessel is surrounded by a geodesic sphere that is 17.8 m in diameter and supports 9438 20-cm photomultipliers. All of this is immersed in 7000 metric tons of purified H2O and is located 2082 m underground (6000 m water equivalent) in the Creighton mine in Sudbury, Ontario. Since SNO uses D2O as a target for neutrino interactions, it is not limited to electron scattering as a detection mechanism. The three main interactions in SNO are ne þ d-p þ p þ e
ðCCÞ
nx þ d-p þ n þ nx ðNCÞ nx þ e -nx þe ðESÞ
ð2Þ
where xAfe; m; tg; CC refers to charged current (W exchange), NC refers to neutral current (Z exchange), and ES refers to electron scattering.
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The primary motivation for SNO is the comparison of the CC interaction, which is pure ne, with the NC interaction, which has equal sensitivity to ne, nm and nt. This comparison will allow SNO to discriminate between different neutrino flavors. The signature for an NC event is the presence of a free neutron in the D2O, and so detecting these neutrons is of the utmost importance to SNO. The methods SNO will employ to accomplish this are discussed in Section 4. The importance of the NC measurement implies strict radiopurity requirements and raises serious issues for detector construction. Both of the major naturally occurring radioisotope decay chains contain daughter nuclei that emit g-rays with Eg>2.2 MeV: 208Tl in the Th chain and 214Bi in the U chain. Such energetic g-rays can dissociate the deuterons in the heavy water producing neutrons that are indistinguishable from those due to neutrino interactions. Thus, all of the materials used in the construction of SNO must be free of U and Th contamination at levels in the ppb–ppt range, and the heavy water itself must be clean at the level of about 0.01 ppt. Mine dust has relatively high levels (ppm) of U and Th, so maintaining cleanliness during construction was a major concern. Looking at SNO from a civil engineering perspective, the mission was to build a 10-story building 2 km underground in a working nickel mine with less than 10 g of dust allowed. A comprehensive description of how this daunting mission was successfully accomplished is given in Ref. [13].
ports that oscillations have been observed [16,17]. The question now is not so much whether neutrinos oscillate, but exactly how do they oscillate? With its unique detection mechanisms SNO has the potential to provide the answer. The regions of neutrino-oscillation parameter space that are consistent with all existing solar neutrino measurements are shown in Fig. 1, with their now-familiar labels. SNO has significant resolving power to select a specific oscillation scenario based on three independent classes of observations: the shape of the CC energy spectrum, the time dependence of the signal rates, and a comparison of the NC and CC fluxes. For the vacuum oscillation scenarios SNO should observe a NC/CC ratio that is significantly different from 1. In addition, there should be little day–night asymmetry to the CC rate. In the specific case of VACL a distinctive distortion to the CC energy spectrum may be seen. Matter-enhanced oscillations into a noninteracting or ‘‘sterile’’ neutrino flavor can be identified by a NC/CC ratio of 1 coupled with a seasonal variation in the CC signal rate or a distortion of the shape of the CC spectrum. There should be little day–night asymmetry in the CC signal, but there may be such a effect in the NC signal.
3. The physics potential of SNO When SNO was proposed in 1984, neutrino oscillations were considered to be very speculative. The aim of SNO was simply to measure the ratio of the NC flux (due to ne, nm and nt) to the CC flux (pure ne). This ratio is a powerful and modelindependent ‘‘smoking gun’’ indicator for oscillations of solar neutrinos. Since that time, however, the situation has changed dramatically due to the theoretical discovery of the possibility of matter-enhanced (MSW) oscillations [14,15] and experimental re-
Fig. 1. The regions allowed by a global fit to all solar neutrino experiments are shown above (after Ref. [12]).
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T.D. Steiger / Nuclear Instruments and Methods in Physics Research A 472 (2001) 348–353
For MSW oscillations into active flavors, the NC/CC ratio will be significantly different from 1 and an appreciable day–night asymmetry is possible. Separating the LMA, SMA, and Low solutions is complicated, but SNO has some resolving power to do this through careful measurement of the CC spectral shape. Bahcall and co-workers have done theoretical studies that illustrate this point [18,19]. The sterile-neutrino solutions discussed above involve neutrinos that are normal except for their lack of weak interactions. An example of this sort of neutrino would be an everyday (massive) neutrino that acquires a right-handed helicity and hence does not interact with W or Z bosons. Recently, there has been theoretical interest in sterile neutrinos of a more exotic nature involving extra space–time dimensions [11,20,21]. The physics of extra dimensions presents a completely new way of thinking about the universe and requires many assumptions and seemingly well-explained phenomena to be re-visited. It is possible to concoct scenarios using extra dimensions that are consistent with all of the solutions in Fig. 1 and hence with all of the observations discussed above [22]. A detailed study of solarneutrino physics in the context of extra dimensions has not been carried out, so it is difficult to say whether SNO (or any experiment) can demonstrate a solution to the SNP based on extra dimensions. If SNO were to make observations that were incompatible with ALL of the solutions in Fig. 1 (after all systematic effects had been investigated), then this would provide strong motivation for such a study. The ranges of SNO observations expected for normal oscillation scenarios and experimental data from other measurements has been discussed in Refs. [18,19] using the approximate characteristics of the SNO detector and a restricted consideration of systematic effects. These ranges are shown in Table 1.
4. Neutral current detection in SNO The measurement of NC reactions via the detection of free neutrons in the heavy water is the key to the success of the SNO project. Due to
Table 1 Ranges of SNO observables that are consistent with standard oscillation scenarios [18]. Observations outside these ranges might provide motivation for a study of the effects of extra dimensions Observable
Oscillation range
No. oscillations
NC/CC Day–night asymmetry: 2((ND)/(N+D)) Shift in CC centroid: DT Shift in CC s: Ds
0.9–5.2 1–29
1 0
152–576 keV 29–199 keV
0 0
its importance several independent methods of determining the NC flux are being pursued. The first handle on nm and nt interactions in SNO will come from examining the ratio of the CC and ES fluxes. As with Super-Kamiokande, the ES interactions in SNO have a small component due to nm and nt. But SNO has the advantage of a pure CC channel for comparison, which allows the NC signal to be extracted as CC F ne ¼ : ES 0:86Fne þ 0:14ðFnm þ Fnt Þ
ð3Þ
Since SNO is small compared to Super-Kamiokande, the statistics on the ES flux will be poor. However, a more powerful determination can be made by comparing the SNO CC flux to the Super-Kamiokande ES flux [23]. The heavy water in SNO can be used to measure NC interactions by capturing the resultant neutrons on deuterium via the reaction 2H(n,g)3H with an efficiency of about 28%. The g-ray released has an energy of 6.25 MeV and can be detected by the phototubes via its interactions in the water. Furthermore, neutrons generated near the vessel will tend to escape into the acrylic before being captured. Hence, there will be a strong radial dependence to the NC signal that will aid in its extraction from other phototube signals. The main advantage of these techniques is that the necessary data are obtained ‘‘for free’’ simply by running SNO. The disadvantages are poor statistics and a lack of independence. The combined SNO/Super-Kamiokande result will be an
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important step, but it will not provide an independent verification of Super-Kamiokande’s observations. The NC detection with D2O is hampered by the fact that the energy of the g-ray is close to the energy threshold of the detector and the neutron capture probability is low. The systematic errors in the measurement are not independent from those in the CC measurement since the same phototube system is used for both. Also, it will be difficult to discriminate NC from CC events on an event-by-event basis. These measurements will be pursued due to their simplicity, but more robust techniques that require alterations to the SNO detector will provide the definitive data. Two systematically independent NC measurements will be performed, each of which requires a separate modification to the SNO detector. The first involves dissolving NaCl salt in the heavy water, the second requires deploying an array of 3 He proportional counters throughout the volume of the acrylic vessel. The addition of a metric ton of NaCl to the heavy water will allow neutrons to be detected via the reaction 35Cl(n,g)36Cl. The g cascade emitted in this case has a total energy of 8.6 MeV, which is well above threshold, and the capture efficiency is increased to B70%. The radial dependence is largely lost (due to the increased capture probability) but this is compensated by the richer event topology of the multi-photon events, which may allow some event-by-event discrimination. However, the measurement is still not systematically independent from the CC measurement. The other detection technique utilizes Neutral Current Detectors (NCDs), which are low-background 3He proportional counters containing an 85 : 15 mixture of 3He and CF4 [24]. Neutrons are detected via the reaction 3He(n,p)3H, which causes a proportional avalanche in the counters. The NCD array consists of 300 counters connected into 96 strings of 2–4 counters each. The 96 strings will be deployed on a 1-m-square grid throughout the heavy water. The detection efficiency of the array will be B37%. A schematic view of a single NCD string is shown in Fig. 2. The NCDs provide a detection channel for NC interactions that is completely independent from
Fig. 2. A schematic view of the 3He proportional counters.
the CC channel. They also provide clear event-byevent discrimination of NC and CC events. Furthermore, the NCDs can improve the measurement of the CC shape by removing the NC ‘‘background’’ from the phototube data. The disadvantages of NCDs are that they are technologically complicated, they occlude B15% of the light from CC events (this does not imply a 15% loss of events, since each event contains many photons), and they require the detector to be shut down for several months during installation. The SNO Collaboration is committed to pursuing all of the NC detection methods discussed above.
5. Current status of SNO SNO began operation in May 1999. The experiment will run in three distinct phases: pure D2O in Phase 1, D2O plus NaCl in Phase 2, and
T.D. Steiger / Nuclear Instruments and Methods in Physics Research A 472 (2001) 348–353 Table 2 Standard Solar Model interaction rates for SNO in Phase 2 (D2O plus NaCl). Assumptions: Standard fluxes from Ref. [25] and a 5 MeV threshold on the energy of the detected electron Reaction
Counts per year
CC NC ES
8395 2774 986
353
All early indications are they the Sudbury Neutrino Observatory will fulfill its promise and provide information that will be crucial to finally solving the Solar Neutrino Problem.
References
Table 3 Current SNO trigger rates Trigger type
Hardware threshold
Rate (Hz)
100 ns coincidence 20 ns coincidence Pulsed trigger Energy sum Pre-scaled trigger (1 : 1000)
18 PMT hits 18 PMT hits Zero threshold 150 pe 11 PMT hits
5–10 o1 5 5–7 {1
D2O plus NCDs in Phase 3. Phases 1 and 2 will each be approximately 1 year in duration, Phase 3 will continue as long as is practical. The experiment is currently in the midst of Phase 1, which began in November, 1999. The interaction rates predicted by the Standard Solar Model for Phase 2 are given in Table 2. SNO is currently running very smoothly with a total trigger rate of about 15 Hz at a hardware threshold of B2 MeV. A more detailed summary of the various trigger rates is given in Table 3. The average livetime for solar neutrino data is about 80%, and the livetime with a higher threshold (still suitable for supernova detection) is >95%. The loss rate for phototubes is about 0.6% per year, and for electronics channels it is about 0.1% per year (the latter are generally repairable). At SNO’s depth only about 3 muons per hour are detected. The preliminary results from SNO are very encouraging [26]. The ES events are seen to point back to the Sun, as expected, and the preliminary data are consistent with the predicted angular dependence of the CC events (roughly 1 13cos y:). The shape of the observed energy spectrum is consistent (within statistics) with the shape expected from 8B decay [27].
[1] R. Davis Jr., D.S. Harmer, K.C. Hoffman, Phys. Rev. Lett. 20 (1968) 1205. [2] B.T. Cleveland, T. Daily, R. Davis Jr., J.R. Distel, K. Lande, C.K. Lee, P.S. Wildenhain, J. Ullman, Astrophys. J. 496 (1998) 505. [3] J.N. Abdurashitov, et al., Phys. Rev. Lett. 83 (1999) 4686. [4] W. Hampel, et al., Phys. Lett. B 447 (1999) 127. [5] Y. Fukuda, et al., Phys. Rev. Lett. 82 (1999) 1810. [6] W.C. Haxton, Ann. Rev. Astron. Astrophys. 33 (1995) 459. [7] E.G. Adelberger, et al., Rev. Mod. Phys. 70 (1998) 1265. [8] K.M. Heeger, R.G.H. Robertson, Phys. Rev. Lett. 77 (1996) 3720. [9] S.P. Mikheyev, A.Y. Smirnov, Phys. Lett. B 429 (1998) 343. [10] G.L. Fogli, E. Lisi, D. Montanio, A. Palazzoi, Phys. Rev. D 62 (2000) 013002/1. [11] K.R. Dienes, E. Dudas, T. Gherghetta, Nucl. Phys. B B557 (1999) 25. [12] J.N. Bahcall, P.I. Krastev, A.Y. Smirnov, Phys. Lett. B 477 (2000) 401. [13] J. Boger, et al., Nucl. Instr. and Meth. A 449 (2000) 172. [14] S.P. Mikheev, A.Y. Smirnov, Sov. J. Nucl. Phys. 42 (1985) 913. [15] L. Wolfenstein, Phys. Rev. D 17 (1978) 2369. [16] E.D. Church, Nucl. Phys. A. 663–664 (2000) 799. [17] Y. Fukuda, et al., Phys. Rev. Lett. 82 (1999) 2644. [18] J.N. Bahcall, P.I. Krastev, A.Y. Smirnov, Phys. Rev. D 62 (2000). [19] J.N. Bahcall, P.I. Krastev, A.Y. Smirnov, Phys. Rev. D 63 (2001). [20] N. Arkani-Hamed, S. Dimopoulos, G. Dvali, J. MarchRussell, Neutrino masses from large extra dimensions, preprint hep-ph/9811448. [21] G.C. McLaughlin, J.N. Ng, Phys. Lett. B 470 (1999) 157. [22] K.R. Dienes, private communication. [23] F.L. Villante, G. Fiorentini, E. Lisi, Phys. Rev. D 59 (1999) 013006/1. [24] M.C. Browne, et al., IEEE Trans. Nucl. Sci. NS-46 (1999) 873. [25] J.N. Bahcall, S. Basu, M.H. Pinsonneault, Phys. Lett. B 433 (1998) 1. [26] A.B. McDonald, Nucl. Phys. B (Proc. Suppl.) 91 (2001) 21. [27] C.E. Ortiz, A. Garcia, R.A. Waltz, M. Bhattacharya, A.K. Komives, Phys. Rev. Lett. 85 (2000) 2909.
Catching some Zs: The solar neutrino problem, neutral current interactions, and the Sudbury Neutrino Observatory T.D. Steiger1 Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, WA 98195-4290, USA
Abstract The Solar Neutrino Problem (SNP) rivals CP violation as one of the oldest problems in physics that lacks a compelling explanation. The Sudbury Neutrino Observatory (SNO)Fa heavy-water Cherenkov detectorFis the first solar neutrino experiment that has the ability to discriminate signals from different neutrino flavors. Since all three presently known flavors participate in Zo exchange, SNO is extremely sensitive to neutrino oscillations. A brief overview of the SNP is given, including a discussion of sterile neutrinos. The ability of SNO to discriminate between the various possible solutions to the SNP is also discussed. The SNO detector is described, including a description of the neutral-current detection methods, and the current status of the experiment is given. r 2001 Elsevier Science B.V. All rights reserved. PACS: 26.65.+t; 29.40.ka; 29.40.Cs Keywords: Solar neutrinos; Neutrino oscillations; Neutral current interactions
1. Introduction: the Solar Neutrino Problem The original goal of the first solar neutrino experiment was to test the hypothesis that energy production in the Sun is driven by nuclear reactions in the core [1]. Using the reaction 37 Cl(ne, e)37Ar, the Homestake experiment succeeded in inferring a copious flux of solar neutrinos. However, the flux implied by the observations was only about a third of the flux predicted by detailed calculations of the expected nuclear processes in the Sun [2]. This simple deficit
1
E-mail address: [email protected] (T.D. Steiger). On behalf of the SNO Collaboration
in the observed neutrino flux was the original manifestation of the Solar Neutrino Problem (SNP). Subsequent experiments have confirmed the solar neutrino deficit. The SAGE [3] and GALLEX [4] experiments are similar in style to Homestake, but they make use of the reaction 71 Ga(ne, e)71Ge, which has a lower Q value than the chlorine reaction and hence a lower energy threshold for detecting neutrinos. This allows the gallium experiments to detect neutrinos from the p–p reaction; the most abundant source of solar neutrinos. The Super-Kamiokande experiment [5] uses a water target and detects Cherenkov light from neutrino–electron scattering events. These
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 2 6 9 - 4
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events provide direction and energy information, which is unavailable in the chlorine and gallium experiments. The three types of solar neutrino experiments performed to data have different energy thresholds for neutrino detection: gallium B0.2 MeV, chlorine B0.8 MeV, and water Cherenkov B5 MeV. Hence, combining the observations of these experiments results in a coarse spectroscopy of the solar neutrino spectrum. The apparent result is that the observed deficit depends on neutrino energy. Using Fpp to represent the flux of lowenergy solar neutrinos (Eno0.4 MeV), F7Be for the flux at intermediate energies (0.4oEno5 MeV), and F8B for the high-energy flux (En>5 MeV), then a joint analysis of all of the solar neutrino data taken to date indicates Fpp BFSSM pp F 7 B0 Be
F 8 B0:5FSSM 8 B
ð1Þ
B
where the superscript SSM denotes the Standard Solar Model prediction for the flux [6]. This is the current formulation of the Solar Neutrino Problem; any candidate solution must not merely explain the deficit of solar neutrinos, but must also explain the observed energy dependence summarized above. A careful review of the relevant nuclear crosssections indicated that there were no serious problems with the Standard Solar Model inputs [7]. Furthermore, it can be shown that it is impossible to explain the experimental observations by a straightforward alteration to the Standard Solar Model itself [8]. Hence, some alteration to the physics of the neutrino seems to be required. A promising explanation for the SNP involves flavor oscillations in which the electron neutrinos generated in the Sun are converted to other flavors (that are more difficult to detect) before they arrive at the detectors on Earth. Solutions in which the oscillations take place in vacuum [9], and those that require interactions with solar matter to enhance the oscillations [10] remain viable. Conventional thinking dictates that oscillations require
349
massive neutrinos, and so are a signature for physics beyond the Standard Model. Recently, however, string theorists exploring the implications of extra spatial dimensions have proposed mechanisms by which even massless neutrinos may oscillate [11]. The implications that these different sorts of solutions have for the Sudbury Neutrino Observatory (SNO) will be explored in Section 3. 2. The Sudbury Neutrino Observatory The radiochemical chlorine and gallium experiments detect only electron neutrinos due to the reactions they utilize. Water Cherenkov detectors use electron scattering as a detection mechanism and so are sensitive to muon and tau neutrinos as well. However, this sensitivity is limited by the fact that nm and nt can only interact with electrons via Z exchange, whereas ne can interact via Z or W exchange. So the sensitivity of a water Cherenkov detector to nm and nt is suppressed by about a factor of 7. Furthermore, these interactions cannot be distinguished from ne interactions in practice. Thus, neutrino flavor oscillations pose a serious problem for all of the solar neutrino experiments to date, and may well explain the SNP. The Sudbury Neutrino Observatory consists of 1000 metric tons of heavy water (99.92% D2O) contained within a spherical acrylic vessel that is 12 m in diameter. The vessel is surrounded by a geodesic sphere that is 17.8 m in diameter and supports 9438 20-cm photomultipliers. All of this is immersed in 7000 metric tons of purified H2O and is located 2082 m underground (6000 m water equivalent) in the Creighton mine in Sudbury, Ontario. Since SNO uses D2O as a target for neutrino interactions, it is not limited to electron scattering as a detection mechanism. The three main interactions in SNO are ne þ d-p þ p þ e
ðCCÞ
nx þ d-p þ n þ nx ðNCÞ nx þ e -nx þe ðESÞ
ð2Þ
where xAfe; m; tg; CC refers to charged current (W exchange), NC refers to neutral current (Z exchange), and ES refers to electron scattering.
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T.D. Steiger / Nuclear Instruments and Methods in Physics Research A 472 (2001) 348–353
The primary motivation for SNO is the comparison of the CC interaction, which is pure ne, with the NC interaction, which has equal sensitivity to ne, nm and nt. This comparison will allow SNO to discriminate between different neutrino flavors. The signature for an NC event is the presence of a free neutron in the D2O, and so detecting these neutrons is of the utmost importance to SNO. The methods SNO will employ to accomplish this are discussed in Section 4. The importance of the NC measurement implies strict radiopurity requirements and raises serious issues for detector construction. Both of the major naturally occurring radioisotope decay chains contain daughter nuclei that emit g-rays with Eg>2.2 MeV: 208Tl in the Th chain and 214Bi in the U chain. Such energetic g-rays can dissociate the deuterons in the heavy water producing neutrons that are indistinguishable from those due to neutrino interactions. Thus, all of the materials used in the construction of SNO must be free of U and Th contamination at levels in the ppb–ppt range, and the heavy water itself must be clean at the level of about 0.01 ppt. Mine dust has relatively high levels (ppm) of U and Th, so maintaining cleanliness during construction was a major concern. Looking at SNO from a civil engineering perspective, the mission was to build a 10-story building 2 km underground in a working nickel mine with less than 10 g of dust allowed. A comprehensive description of how this daunting mission was successfully accomplished is given in Ref. [13].
ports that oscillations have been observed [16,17]. The question now is not so much whether neutrinos oscillate, but exactly how do they oscillate? With its unique detection mechanisms SNO has the potential to provide the answer. The regions of neutrino-oscillation parameter space that are consistent with all existing solar neutrino measurements are shown in Fig. 1, with their now-familiar labels. SNO has significant resolving power to select a specific oscillation scenario based on three independent classes of observations: the shape of the CC energy spectrum, the time dependence of the signal rates, and a comparison of the NC and CC fluxes. For the vacuum oscillation scenarios SNO should observe a NC/CC ratio that is significantly different from 1. In addition, there should be little day–night asymmetry to the CC rate. In the specific case of VACL a distinctive distortion to the CC energy spectrum may be seen. Matter-enhanced oscillations into a noninteracting or ‘‘sterile’’ neutrino flavor can be identified by a NC/CC ratio of 1 coupled with a seasonal variation in the CC signal rate or a distortion of the shape of the CC spectrum. There should be little day–night asymmetry in the CC signal, but there may be such a effect in the NC signal.
3. The physics potential of SNO When SNO was proposed in 1984, neutrino oscillations were considered to be very speculative. The aim of SNO was simply to measure the ratio of the NC flux (due to ne, nm and nt) to the CC flux (pure ne). This ratio is a powerful and modelindependent ‘‘smoking gun’’ indicator for oscillations of solar neutrinos. Since that time, however, the situation has changed dramatically due to the theoretical discovery of the possibility of matter-enhanced (MSW) oscillations [14,15] and experimental re-
Fig. 1. The regions allowed by a global fit to all solar neutrino experiments are shown above (after Ref. [12]).
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T.D. Steiger / Nuclear Instruments and Methods in Physics Research A 472 (2001) 348–353
For MSW oscillations into active flavors, the NC/CC ratio will be significantly different from 1 and an appreciable day–night asymmetry is possible. Separating the LMA, SMA, and Low solutions is complicated, but SNO has some resolving power to do this through careful measurement of the CC spectral shape. Bahcall and co-workers have done theoretical studies that illustrate this point [18,19]. The sterile-neutrino solutions discussed above involve neutrinos that are normal except for their lack of weak interactions. An example of this sort of neutrino would be an everyday (massive) neutrino that acquires a right-handed helicity and hence does not interact with W or Z bosons. Recently, there has been theoretical interest in sterile neutrinos of a more exotic nature involving extra space–time dimensions [11,20,21]. The physics of extra dimensions presents a completely new way of thinking about the universe and requires many assumptions and seemingly well-explained phenomena to be re-visited. It is possible to concoct scenarios using extra dimensions that are consistent with all of the solutions in Fig. 1 and hence with all of the observations discussed above [22]. A detailed study of solarneutrino physics in the context of extra dimensions has not been carried out, so it is difficult to say whether SNO (or any experiment) can demonstrate a solution to the SNP based on extra dimensions. If SNO were to make observations that were incompatible with ALL of the solutions in Fig. 1 (after all systematic effects had been investigated), then this would provide strong motivation for such a study. The ranges of SNO observations expected for normal oscillation scenarios and experimental data from other measurements has been discussed in Refs. [18,19] using the approximate characteristics of the SNO detector and a restricted consideration of systematic effects. These ranges are shown in Table 1.
4. Neutral current detection in SNO The measurement of NC reactions via the detection of free neutrons in the heavy water is the key to the success of the SNO project. Due to
Table 1 Ranges of SNO observables that are consistent with standard oscillation scenarios [18]. Observations outside these ranges might provide motivation for a study of the effects of extra dimensions Observable
Oscillation range
No. oscillations
NC/CC Day–night asymmetry: 2((ND)/(N+D)) Shift in CC centroid: DT Shift in CC s: Ds
0.9–5.2 1–29
1 0
152–576 keV 29–199 keV
0 0
its importance several independent methods of determining the NC flux are being pursued. The first handle on nm and nt interactions in SNO will come from examining the ratio of the CC and ES fluxes. As with Super-Kamiokande, the ES interactions in SNO have a small component due to nm and nt. But SNO has the advantage of a pure CC channel for comparison, which allows the NC signal to be extracted as CC F ne ¼ : ES 0:86Fne þ 0:14ðFnm þ Fnt Þ
ð3Þ
Since SNO is small compared to Super-Kamiokande, the statistics on the ES flux will be poor. However, a more powerful determination can be made by comparing the SNO CC flux to the Super-Kamiokande ES flux [23]. The heavy water in SNO can be used to measure NC interactions by capturing the resultant neutrons on deuterium via the reaction 2H(n,g)3H with an efficiency of about 28%. The g-ray released has an energy of 6.25 MeV and can be detected by the phototubes via its interactions in the water. Furthermore, neutrons generated near the vessel will tend to escape into the acrylic before being captured. Hence, there will be a strong radial dependence to the NC signal that will aid in its extraction from other phototube signals. The main advantage of these techniques is that the necessary data are obtained ‘‘for free’’ simply by running SNO. The disadvantages are poor statistics and a lack of independence. The combined SNO/Super-Kamiokande result will be an
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important step, but it will not provide an independent verification of Super-Kamiokande’s observations. The NC detection with D2O is hampered by the fact that the energy of the g-ray is close to the energy threshold of the detector and the neutron capture probability is low. The systematic errors in the measurement are not independent from those in the CC measurement since the same phototube system is used for both. Also, it will be difficult to discriminate NC from CC events on an event-by-event basis. These measurements will be pursued due to their simplicity, but more robust techniques that require alterations to the SNO detector will provide the definitive data. Two systematically independent NC measurements will be performed, each of which requires a separate modification to the SNO detector. The first involves dissolving NaCl salt in the heavy water, the second requires deploying an array of 3 He proportional counters throughout the volume of the acrylic vessel. The addition of a metric ton of NaCl to the heavy water will allow neutrons to be detected via the reaction 35Cl(n,g)36Cl. The g cascade emitted in this case has a total energy of 8.6 MeV, which is well above threshold, and the capture efficiency is increased to B70%. The radial dependence is largely lost (due to the increased capture probability) but this is compensated by the richer event topology of the multi-photon events, which may allow some event-by-event discrimination. However, the measurement is still not systematically independent from the CC measurement. The other detection technique utilizes Neutral Current Detectors (NCDs), which are low-background 3He proportional counters containing an 85 : 15 mixture of 3He and CF4 [24]. Neutrons are detected via the reaction 3He(n,p)3H, which causes a proportional avalanche in the counters. The NCD array consists of 300 counters connected into 96 strings of 2–4 counters each. The 96 strings will be deployed on a 1-m-square grid throughout the heavy water. The detection efficiency of the array will be B37%. A schematic view of a single NCD string is shown in Fig. 2. The NCDs provide a detection channel for NC interactions that is completely independent from
Fig. 2. A schematic view of the 3He proportional counters.
the CC channel. They also provide clear event-byevent discrimination of NC and CC events. Furthermore, the NCDs can improve the measurement of the CC shape by removing the NC ‘‘background’’ from the phototube data. The disadvantages of NCDs are that they are technologically complicated, they occlude B15% of the light from CC events (this does not imply a 15% loss of events, since each event contains many photons), and they require the detector to be shut down for several months during installation. The SNO Collaboration is committed to pursuing all of the NC detection methods discussed above.
5. Current status of SNO SNO began operation in May 1999. The experiment will run in three distinct phases: pure D2O in Phase 1, D2O plus NaCl in Phase 2, and
T.D. Steiger / Nuclear Instruments and Methods in Physics Research A 472 (2001) 348–353 Table 2 Standard Solar Model interaction rates for SNO in Phase 2 (D2O plus NaCl). Assumptions: Standard fluxes from Ref. [25] and a 5 MeV threshold on the energy of the detected electron Reaction
Counts per year
CC NC ES
8395 2774 986
353
All early indications are they the Sudbury Neutrino Observatory will fulfill its promise and provide information that will be crucial to finally solving the Solar Neutrino Problem.
References
Table 3 Current SNO trigger rates Trigger type
Hardware threshold
Rate (Hz)
100 ns coincidence 20 ns coincidence Pulsed trigger Energy sum Pre-scaled trigger (1 : 1000)
18 PMT hits 18 PMT hits Zero threshold 150 pe 11 PMT hits
5–10 o1 5 5–7 {1
D2O plus NCDs in Phase 3. Phases 1 and 2 will each be approximately 1 year in duration, Phase 3 will continue as long as is practical. The experiment is currently in the midst of Phase 1, which began in November, 1999. The interaction rates predicted by the Standard Solar Model for Phase 2 are given in Table 2. SNO is currently running very smoothly with a total trigger rate of about 15 Hz at a hardware threshold of B2 MeV. A more detailed summary of the various trigger rates is given in Table 3. The average livetime for solar neutrino data is about 80%, and the livetime with a higher threshold (still suitable for supernova detection) is >95%. The loss rate for phototubes is about 0.6% per year, and for electronics channels it is about 0.1% per year (the latter are generally repairable). At SNO’s depth only about 3 muons per hour are detected. The preliminary results from SNO are very encouraging [26]. The ES events are seen to point back to the Sun, as expected, and the preliminary data are consistent with the predicted angular dependence of the CC events (roughly 1 13cos y:). The shape of the observed energy spectrum is consistent (within statistics) with the shape expected from 8B decay [27].
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