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Future (underground) Water Cherenkov Detectors
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 183–189 www.elsevier.com/locate/npbps
Future (underground) Water Cherenkov Detectors Masashi Yokoyamaa,b b Kavli
a Department of Physics, Graduate School of Science, University of Tokyo, Tokyo, Japan Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, Japan
Abstract Water Cherenkov detectors have been, and will continue, providing fascinating results in the fields of neutrino physics and astrophysics. Because its strong potential for a wide range of scientific subjects is well recognized, development of a large water Cherenkov detector has been carried out all over the world. The detector is well matured and basic technology to build the next generation, megaton-class detector is readily available. The physics potential and the status of technical development are reviewed. Keywords: water Cherenkov, neutrino, CP violation, mass hierarchy, photodetector,
1. Water Cherenkov Detectors Recent discovery of nonzero θ13 with accelerator [1, 2] and reactor [3, 4, 5] experiments has provided a strong motivation for the next generation neutrino oscillation experiments. The future we had been talking about until the last year has become now! Large water Cherenkov detector is one of prominent options for the next generation underground neutrino detector to be realized in near future. For more than 25 years, water Cherenkov detectors have been providing important results in the fields of neutrino physics and astrophysics. They include the observation of neutrinos from supernova [6, 7] and the discovery of neutrino oscillation with atmospheric [8] and solar [9, 10] neutrinos. In recent years, water Cherenkov detectors are used as the neutrino detector in long baseline neutrino experiments for precise measurements of neutrino mixing parameters [11, 12]. In addition, most stringent limits on nucleon decays are given by a water Cherenkov detector [13]. In a water Cherenkov detector, position, direction, and energy of relativistic charged particles are reconstructed by detecting the spatial and timing distributions of the Cherenkov photons with an array of single photon capable sensors. Ring imaging water Cherenkov detectors are also able to discriminate charged muons (μ± ) 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.04.009
from electrons, positrons, and gammas (e± / γ), as the later group of particles induce electromagnetic showers in the medium which modify the resulting ring patterns. They have high efficiency with 4π coverage for particles with energies more than 100 MeV, the energy region relevant for accelerator and atmospheric neutrino oscillation measurements as well as nucleon decay searches. For example, in T2K experiment 65% efficiency for νe signal is achieved while rejecting > 95% of π0 background [1]. On the other hand, water Cherenkov detectors have also good performance for detection of low energy neutrinos, down to a few MeV. Because the detector medium is pure water and the detector technology is simple, a huge detector can be constructed with relatively small cost. The detector technology is well matured, as is demonstrated by the Super-Kamiokande detector with 50 kton total volume running for more than 15 years. These features make water Cherenkov detectors an attractive option for the next generation underground neutrino detectors.
2. Physics Potential Water Cherenkov detectors have strong physics potential covering a wide range of subjects. Some
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νμ → νe signal νμ + ν¯ μ CC νe + ν¯ e CC NC
ν mode 3606 35 880 649
ν¯ mode 2339 23 878 678
600
Events/50MeV
Table 1: Expected number of events after νe appearance signal selection with the proposed Hyper-Kamiokande detector for sin2 2θ13 = 0.1, δ = 0, and normal mass hierarchy.
400
0
of examples are described below, taking the HyperKamiokande detector [14] as an example.
• CP violation in the lepton sector, • mass hierarchy (the sign of Δm223 ), and
400
Events/50MeV
One of the major motivations for building a large underground detector is the study of properties of neutrino through their flavor mixing. In the framework of standard three flavor mixing, neutrino flavor mixing can be described by three angles (θ12 , θ23 , θ13 ) and a complex phase (δ) in Pontecorvo-Maki-Nakagawa-Sakata matrix [15, 16, 17], and two mass-squared difference, Δm212 and Δm223 . Recent discovery of a ‘large’ value of the last mixing angle θ13 has opened a door to the remaining neutrino mixing parameters:
Total BG BG
200 0
2.1. Neutrino Oscillation
mode
200
1
Erec (GeV) mode
Total BG BG
100 00
2
1
Erec (GeV)
2
Figure 1: Reconstructed neutrino energy distributions for HyperKamiokande. Top: neutrino run (three years). Bottom: anti-neutrino run (seven years). Plots with δ = 0, 12 π, π, and 32 π are overlaid. sin2 2θ13 = 0.1 and normal mass hierarchy are assumed.
• octant of θ23 (θ23 < π/4 or θ23 > π/4). A water Cherenkov detector with a few hundred kiloton fiducial volume can address all of the above with a combination of accelerator and atmospheric neutrino measurements. 2.1.1. Long baseline experiment A long baseline (295 km) neutrino experiment with a neutrino beam from J-PARC [18] is proposed as a part of the Hyper-Kamiokande project. The main goal of the experiment is a search for CP violation in the neutrino oscillation by comparing muon- to electron-type oscillation probabilities for neutrino and antineutrino, P(νμ → νe ) and P(νμ → νe ). Table 1 shows the expected number of events after νe signal selection with HyperKamiokande, for sin2 2θ13 = 0.1, δ = 0, and normal mass hierarchy. A beam power of 750 kW and ten years running time in total (three years for neutrino mode and seven years for anti-neutrino mode) is assumed. For each of neutrino and anti-neutrino mode, O(1000) signal events are expected. Figure 1 shows the reconstructed
neutrino energy distributions for the case of δ = 0, 12 π, π, and 32 π. By using the number of events and the energy distributions, one can search for the CP asymmetry in neutrino oscillation. Figure 2 shows the sensitivity to CP violation as a function of the integrated beam power. The vertical axis shows the fraction of δ for which CP conserving case (sin δ = 0) is excluded with 3σ significance. Solid and dashed lines correspond to the case the mass hierarchy is known and unknown, respectively. The true mass hierarchy is normal in both cases. CP violation in neutrino oscillation can be established with a statistical significance of 3σ for 74%(54%) of the δ parameter space for sin2 2θ13 = 0.1, assuming that the mass hierarchy is known(not known). 2.1.2. Atmospheric neutrino observation Measurements of atmospheric neutrinos can provide additional and complementary information to the accelerator experiment. Enhancement of oscillation proba-
M. Yokoyama / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 183–189
sin2213= 0.1 / 0.03
80
0.3 0.25
60
13
0.2
40
0.15
20 0
Normal mass hierarchy Hyper-K 10 years 90% C.L.
sin2 2
100
185
0.1
0
1
2
3
4
5
6
7
8
7
• 10
9
10
Figure 2: Fraction of δ for which sin δ = 0 can be excluded with 3 σ as a function of the integrated beam power for J-PARC to HyperKamiokande. The ratio of neutrino and anti-neutrino mode is fixed to 3:7.
0.05 0
0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 sin2 23
Figure 4: Expected sensitivities for sin2 θ23 and sin2 2θ13 at 90% CL with ten years of Hyper-K atmospheric neutrino data. Stars in the contours represent the assumed true mixing angles. sin2 2θ23 = 0.99 (sin2 θ23 = 0.45 or 0.55) and normal mass hierarchy is assumed.
50 45
Normal mass hierarchy Hyper-K 10 years
40
sin2 23=0.6
35 2
30 sin2 23=0.5
25 20
sin2 23=0.4
15
2.2. Nucleon Decay
10 5 0
within the currently allowed range. Figure 4 shows the expected sensitivities for sin2 θ23 and sin2 θ13 , assuming sin2 2θ23 = 0.99 (sin2 θ23 = 0.45 or 0.55) and normal mass hierarchy. For sin2 2θ13 0.1, discrimination of the θ23 octant is possible.
0.06 0.08 0.1 2 0.12 0.14 0.16 0.18 sin 2
13
Figure 3: Significance for the mass hierarchy determination expected with ten years of atmospheric neutrino data with Hyper-Kamiokande.
bilities due to the MSW resonance inside the Earth occurs to either neutrino and anti-neutrino, depending on the mass hierarchy. The magnitude of the resonance depends on the octant of θ23 and the value of θ13 . Thus, atmospheric neutrinos can be used to extract those parameters. Because the sensitivity of atmospheric neutrino to the mass hierarchy and θ23 octant increases for larger value of θ13 , recent results are very encouraging for these measurements. Figure 3 shows the significance of the mass hierarchy determination expected with ten years of atmospheric neutrino data with Hyper-Kamiokande. Each line corresponds to the case with sin2 θ23 = 0.4, 0.5, and 0.6. For sin2 2θ13 0.1, the statistical significance will exceed 3σ after ten years of running, for any value of θ23
Although not directly connected to the neutrino physics, a search for nucleon (proton and/or bound neutron) decay is the only direct probe of the grand unified theories (GUTs) and has deep implications to the particle physics at very high energy. Next generation experiments will extend the sensitivity to nucleon decay searches in various decay modes by about one order of magnitude. 2.3. Astrophysics An underground water Cherenkov detector also functions as an astrophysical neutrino observatory. The targets of such observatory include (but not necessarily limited to) topics explained below. 2.3.1. Supernova core collapse neutrinos The sensitivity of the next generation detector to a distant supernova burst will reach to ∼Mpc. By measuring energy and timing distributions of neutrinos from a supernova burst, one can test models of supernova. Also, there is possibility that the mass hierarchy can be determined by studying the time profile of the supernova neutrino.
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2.3.2. Measurement of supernova relic (diffuse) neutrinos On average, one supernova explosion has been occurring every second somewhere in the universe. The neutrinos produced by all of the supernova explosions since the beginning of the universe must fill the present universe. By measuring the energy spectrum of supernova relic neutrinos, the history of heavy element synthesis can be studied. In order to suppress background and enhance the sensitivity, an option to add gadolinium into water is being studied [19].
Figure 5: Schematic view of the MEMPHYS detector [20].
2.3.3. Precise measurements of solar neutrinos With a large statistics sample of solar neutrinos, next generation water Cherenkov detectors can study interior of the Sun and measure possible time variation over short time periods. If the background and systematics can be controlled to the required level, MSW effect inside the Earth can be observed with day/night asymmetry. In addition, if a low energy threshold can be achieved, there is a possibility to see the spectrum upturn and test the framework of solar neutrino oscillation. 2.3.4. Indirect search of WIMP dark matter Neutrinos emitted by weakly interacting massive particles (WIMPs) annihilating in the Sun, Earth, and galactic halo can be detected using the upward-going muons observed in Hyper-K. Water Cherenkov detectors have unique sensitivity in the energy range of 10100 GeV. 3. Technical Development Because of the rich physics potential, development of large water Cherenkov detector has been carried out all over the world. There are three major efforts in three regions; Europe, US, and Japan. The status and prospects of technical development in three regions are summarized in this section. 3.1. MEMPHYS in Europe In Europe, a concept of megaton class water Cherenkov detector, MEMPHYS [20] (Fig. 5), has been studied in the framework of LAGUNA and LAGNALBNO programs. Underground laboratory at Fr´ejus located 130 km from CERN is considered as the candidate site. MEMPHYS consists of two cylindrical detector modules, each with 65 m in diameter and 103 m in height, giving a total fiducial volume of 530 kilotons. In order to realize a large water Cherenkov detector, development of photosensor and its readout system with
Figure 6: Left: Matrix of sixteen 8” Hamamatsu PMTs in PMm2 . Right: Backside of the matrix showing the watertight box for the electronics.
high performance and reduced cost is indispensable. An ASIC with high-speed discriminator, 12-bit ADC and TDC to readout a group of PMT is developed in an R&D program called PMm2 . The developed electronics and data acquisition system has been tested with 4 × 4 PMT matrix (Fig. 6). They are now being fully tested with the MEMPHYNO prototype (Fig. 7). The MEMPHYNO prototype consists of a tank of 2 × 2 × 2 m3 filled with water, a hodoscope made by four scintillator planes (two on top and two on bottom), and the PMT matrix in the water tank. The full functionality of the system will be tested with cosmic ray muons. 3.2. LBNE in United States In the United States, an intensive R&D was carried out over past few years by LBNE collaboration. The water Cherenkov detector was considered as one of two options for the far detector of LBNE experiment, proposed at Sanford Underground Laboratory in the Homestake gold mine in Lead, SD. Figure 8 shows a schematic view of the LBNE water Cherenkov detector. It has one large cavity of 65 m in
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Figure 9: Deck and balcony configuration and equipment on top for LBNE-WCD [21].
Figure 7: Photograph of MEMPHYNO prototype.
Figure 10: Linear PMT installation unit (PIU) concept deployed from deck [21].
Figure 8: Schematic view of the LBNE WC detector [21].
diameter and 81.3 m in height, with 200 kton fiducial volume. The detailed design developed by LBNE collaboration is summarized in a conceptual design report [21] which is publicly available. It covers all of necessary areas for realization of the detector and gives an excellent reference for any future water Cherenkov detector, although in LBNE the liquid argon TPC was chosen as the primary detector technology for the far detector. A few examples of R&D results are explained below. A conceptual design of water containment system, such as the vessel, liner, floor, and PMT installation has been developed. Figure 9 shows a schematic of the configuration of the deck assembly on top of the detector. PMT installation units on the wall are planned to be supported by cables and deployed from the deck (Fig. 10). This eliminates the need for additional liner penetration
into the wall. Another area of intense development was the photodetector and light collection system. In the reference design, 29,000 12-inch high-QE PMTs are assumed. Optical, electronic, and mechanical characteristics of PMTs are investigated. Many of PMT performance such as gain, noise, timing resolution, dark current, and response uniformity over photo cathode, are evaluated in test facilities. In order to increase the light collection efficiency and hence to reduce the number of PMTs necessary, light collectors are considered. Figure 11 shows two of light collector options studied in LBNE. An im-
Figure 11: Light collectors studied by LBNE: Left: Wavelength shifter plate. Right: Winston cone.
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Figure 12: Schematic view of the Hyper-Kamiokande detector [14].
plosion of PMT under high hydrostatic pressures may cause a catastrophic accident in a water Cherenkov detector. For a full understanding of the mechanical property of PMT and the dynamics of PMT implosion, real implosion tests of PMT have been carried out in a facility of the U.S. Navy. 3.3. Hyper-Kamiokande in Japan In Japan, the world-largest underground water Cherenkov detector, Super-Kamiokande (Super-K) has been running since 1996. As an upgrade of Super-Kamiokande, Hyper-Kamiokande (Hyper-K) with 560(990) kton fiducial(total) mass is proposed [14] (Fig. 12). In its baseline design, Hyper-K has two large cavities, each having a egg-shape cross section 48 meters wide, 54 meters tall, and 250 meters long. Each cavern will be optically separated by segmentation walls located every 49.5 m to form 5 (in total 10) compartments (Fig. 13), such that event triggering and event reconstruction can be performed in each compartment separately and independently. Because the compartment dimension of 50 m is comparable with that of Super-K (36 m) and is shorter than the typical light attenuation length in water achieved by the Super-K water filtration system, (>100 m @ 400 nm), detector performance of Hyper-K is expected to be basically the same as that of Super-K. The candidate site of Hyper-K is about 8 km south of Super-K and has the same off-axis angle and baseline from J-PARC neutrino beam as Super-K. For the excavation of large caverns, a baseline plan has been developed based on the geological survey and in-situ rock stress measurements. The baseline design of liner and PMT support is defined and summarized in a document. In order to further optimize the detector design in terms of cost, schedule and physics performance, alternative cavern shape and PMT support structure are under investigation. The construction of Hyper-K is estimated to take seven years from the start of construction.
Figure 13: Top: Cross section view of the Hyper-Kamiokande detector [14]. Bottom: Profile of Hyper-K detector showing PMT arrays and the support structure.
Figure 14: Test of 8-inch prototype HPD being tested in Kamioka.
As mentioned before, the photosensor is one of key components of a water Cherenkov detector. For HyperK, the same 20-inch PMT as Super-K is assumed in the baseline design. In order to achieve 20% photo cathode coverage, about 100,000 PMTs are necessary. In order to achieve less cost and better performance, development of 20-inch hybrid photodetector (HPD) has been started (Fig. 14). An HPD uses an avalanche photodiode instead of dynodes in a conventional PMT for multiplication of signal. Because of its simple structure, HPD is expected to be significantly less expensive than PMT. It also has higher single photon sensitivity and much better timing resolution than a conventional PMT. As the first step, 8-inch prototype will be tested in a 200-ton water tank in Kamioka. R&D in other important areas are also ongoing. Requirements for electronics and DAQ system in Hyper-K
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of θ13 has been observed, the next step in neutrino oscillation physics is clearly defined. A water Cherenkov detector is no doubt one of prominent options for the next generation experiment to be realized in near future. Acknowledgments The author would like to thank Thomas Patzak and Steve Kettell for providing information on MEMPHYS and LBNE-WCD. Figure 15: Design of the water and air supply system for the HyperKamiokande detector.
Figure 16: Tentative schedule of Hyper-K construction.
is similar to that in Super-K. In order to avoid difficulties to route massive cables and degradation of the signal, an option to put frontend electronics inside water tank near the photosensor is considered. Water purification system (Fig. 15) has been designed based on the experience with Super-K, XMASS and EGADS water systems. Detector calibration system is also being designed based on the experience of Super-K. Figure 16 shows a tentative schedule of Hyper-Kamiokande construction. 4. Summary Water Cherenkov detectors have been, and will continue, providing fascinating results in the fields of neutrino physics and astrophysics. Because its strong potential for a wide range of scientific subjects, development of a large water Cherenkov detector has been carried out all over the world. The detector is well matured and basic technology to build the next generation, megaton-class detector is readily available. Further R&D and optimization to enhance the physics capability and feasibility are ongoing. Now that a large value
References [1] T. Nakaya, in these proceedings; K. Abe et al. [T2K Collaboration], Phys. Rev. Lett. 107, 041801 (2011) [arXiv:1106.2822 [hep-ex]]. [2] R. Nichol, in these proceedings; P. Adamson et al. [MINOS Collaboration], Phys. Rev. Lett. 107, 181802 (2011) [arXiv:1108.0015 [hep-ex]]. [3] M. Ishitsuka, in these proceedings; Y. Abe et al. [DOUBLECHOOZ Collaboration], Phys. Rev. Lett. 108, 131801 (2012) [arXiv:1112.6353 [hep-ex]]. [4] D. Dwyer, in these proceedings; F. P. An et al. [DAYABAY Collaboration], Phys. Rev. Lett. 108, 171803 (2012) [arXiv:1203.1669 [hep-ex]]. [5] S. B. Kim, in these proceedings; J. K. Ahn et al. [RENO Collaboration], Phys. Rev. Lett. 108, 191802 (2012) [arXiv:1204.0626 [hep-ex]]. [6] K. Hirata et al. [KAMIOKANDE-II Collaboration], Phys. Rev. Lett. 58, 1490 (1987). [7] R. M. Bionta, G. Blewitt, C. B. Bratton, D. Casper, A. Ciocio, R. Claus, B. Cortez and M. Crouch et al., Phys. Rev. Lett. 58, 1494 (1987). [8] Y. Fukuda et al. [Super-Kamiokande Collaboration], Phys. Rev. Lett. 81, 1562 (1998) [hep-ex/9807003]. [9] S. Fukuda et al. [Super-Kamiokande Collaboration], Phys. Rev. Lett. 86, 5651 (2001) [hep-ex/0103032]. [10] Q. R. Ahmad et al. [SNO Collaboration], Phys. Rev. Lett. 89, 011301 (2002) [nucl-ex/0204008]. [11] M. H. Ahn et al. [K2K Collaboration], Phys. Rev. D 74, 072003 (2006) [hep-ex/0606032]. [12] K. Abe et al. [T2K Collaboration], Nucl. Instrum. Meth. A 659, 106 (2011) [arXiv:1106.1238 [physics.ins-det]]. [13] H. Nishino et al. [Super-Kamiokande Collaboration], Phys. Rev. D 85, 112001 (2012) [arXiv:1203.4030 [hep-ex]]. [14] K. Abe et al. [Hyper-Kamiokande Working Group], “Letter of Intent: The Hyper-Kamiokande Experiment — Detector Design and Physics Potential —,” arXiv:1109.3262 [hep-ex]. [15] B. Pontecorvo, Sov. Phys. JETP 6, 429 (1957) [Zh. Eksp. Teor. Fiz. 33, 549 (1957)]. [16] Z. Maki, M. Nakagawa and S. Sakata, Prog. Theor. Phys. 28, 870 (1962). [17] M. Kobayashi, in these proceedings. [18] T. Nakadaira, in these proceedings. [19] J. Beacom, in these proceedings; J. F. Beacom and M. R. Vagins, Phys. Rev. Lett. 93, 171101 (2004) [hep-ph/0309300]. [20] J. L. Borne et al., “The MEMPHYS project,” Nucl. Instrum. Meth. A 639, 287 (2011). [21] LBNE Collaboration, “The Long Baseline Neutrino Experiment (LBNE) Water Cherenkov Detector (WCD) Conceptual Design Report (CDR),” arXiv:1204.2295 [physics.ins-det].
Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 183–189 www.elsevier.com/locate/npbps
Future (underground) Water Cherenkov Detectors Masashi Yokoyamaa,b b Kavli
a Department of Physics, Graduate School of Science, University of Tokyo, Tokyo, Japan Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, Japan
Abstract Water Cherenkov detectors have been, and will continue, providing fascinating results in the fields of neutrino physics and astrophysics. Because its strong potential for a wide range of scientific subjects is well recognized, development of a large water Cherenkov detector has been carried out all over the world. The detector is well matured and basic technology to build the next generation, megaton-class detector is readily available. The physics potential and the status of technical development are reviewed. Keywords: water Cherenkov, neutrino, CP violation, mass hierarchy, photodetector,
1. Water Cherenkov Detectors Recent discovery of nonzero θ13 with accelerator [1, 2] and reactor [3, 4, 5] experiments has provided a strong motivation for the next generation neutrino oscillation experiments. The future we had been talking about until the last year has become now! Large water Cherenkov detector is one of prominent options for the next generation underground neutrino detector to be realized in near future. For more than 25 years, water Cherenkov detectors have been providing important results in the fields of neutrino physics and astrophysics. They include the observation of neutrinos from supernova [6, 7] and the discovery of neutrino oscillation with atmospheric [8] and solar [9, 10] neutrinos. In recent years, water Cherenkov detectors are used as the neutrino detector in long baseline neutrino experiments for precise measurements of neutrino mixing parameters [11, 12]. In addition, most stringent limits on nucleon decays are given by a water Cherenkov detector [13]. In a water Cherenkov detector, position, direction, and energy of relativistic charged particles are reconstructed by detecting the spatial and timing distributions of the Cherenkov photons with an array of single photon capable sensors. Ring imaging water Cherenkov detectors are also able to discriminate charged muons (μ± ) 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.04.009
from electrons, positrons, and gammas (e± / γ), as the later group of particles induce electromagnetic showers in the medium which modify the resulting ring patterns. They have high efficiency with 4π coverage for particles with energies more than 100 MeV, the energy region relevant for accelerator and atmospheric neutrino oscillation measurements as well as nucleon decay searches. For example, in T2K experiment 65% efficiency for νe signal is achieved while rejecting > 95% of π0 background [1]. On the other hand, water Cherenkov detectors have also good performance for detection of low energy neutrinos, down to a few MeV. Because the detector medium is pure water and the detector technology is simple, a huge detector can be constructed with relatively small cost. The detector technology is well matured, as is demonstrated by the Super-Kamiokande detector with 50 kton total volume running for more than 15 years. These features make water Cherenkov detectors an attractive option for the next generation underground neutrino detectors.
2. Physics Potential Water Cherenkov detectors have strong physics potential covering a wide range of subjects. Some
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νμ → νe signal νμ + ν¯ μ CC νe + ν¯ e CC NC
ν mode 3606 35 880 649
ν¯ mode 2339 23 878 678
600
Events/50MeV
Table 1: Expected number of events after νe appearance signal selection with the proposed Hyper-Kamiokande detector for sin2 2θ13 = 0.1, δ = 0, and normal mass hierarchy.
400
0
of examples are described below, taking the HyperKamiokande detector [14] as an example.
• CP violation in the lepton sector, • mass hierarchy (the sign of Δm223 ), and
400
Events/50MeV
One of the major motivations for building a large underground detector is the study of properties of neutrino through their flavor mixing. In the framework of standard three flavor mixing, neutrino flavor mixing can be described by three angles (θ12 , θ23 , θ13 ) and a complex phase (δ) in Pontecorvo-Maki-Nakagawa-Sakata matrix [15, 16, 17], and two mass-squared difference, Δm212 and Δm223 . Recent discovery of a ‘large’ value of the last mixing angle θ13 has opened a door to the remaining neutrino mixing parameters:
Total BG BG
200 0
2.1. Neutrino Oscillation
mode
200
1
Erec (GeV) mode
Total BG BG
100 00
2
1
Erec (GeV)
2
Figure 1: Reconstructed neutrino energy distributions for HyperKamiokande. Top: neutrino run (three years). Bottom: anti-neutrino run (seven years). Plots with δ = 0, 12 π, π, and 32 π are overlaid. sin2 2θ13 = 0.1 and normal mass hierarchy are assumed.
• octant of θ23 (θ23 < π/4 or θ23 > π/4). A water Cherenkov detector with a few hundred kiloton fiducial volume can address all of the above with a combination of accelerator and atmospheric neutrino measurements. 2.1.1. Long baseline experiment A long baseline (295 km) neutrino experiment with a neutrino beam from J-PARC [18] is proposed as a part of the Hyper-Kamiokande project. The main goal of the experiment is a search for CP violation in the neutrino oscillation by comparing muon- to electron-type oscillation probabilities for neutrino and antineutrino, P(νμ → νe ) and P(νμ → νe ). Table 1 shows the expected number of events after νe signal selection with HyperKamiokande, for sin2 2θ13 = 0.1, δ = 0, and normal mass hierarchy. A beam power of 750 kW and ten years running time in total (three years for neutrino mode and seven years for anti-neutrino mode) is assumed. For each of neutrino and anti-neutrino mode, O(1000) signal events are expected. Figure 1 shows the reconstructed
neutrino energy distributions for the case of δ = 0, 12 π, π, and 32 π. By using the number of events and the energy distributions, one can search for the CP asymmetry in neutrino oscillation. Figure 2 shows the sensitivity to CP violation as a function of the integrated beam power. The vertical axis shows the fraction of δ for which CP conserving case (sin δ = 0) is excluded with 3σ significance. Solid and dashed lines correspond to the case the mass hierarchy is known and unknown, respectively. The true mass hierarchy is normal in both cases. CP violation in neutrino oscillation can be established with a statistical significance of 3σ for 74%(54%) of the δ parameter space for sin2 2θ13 = 0.1, assuming that the mass hierarchy is known(not known). 2.1.2. Atmospheric neutrino observation Measurements of atmospheric neutrinos can provide additional and complementary information to the accelerator experiment. Enhancement of oscillation proba-
M. Yokoyama / Nuclear Physics B (Proc. Suppl.) 235–236 (2013) 183–189
sin2213= 0.1 / 0.03
80
0.3 0.25
60
13
0.2
40
0.15
20 0
Normal mass hierarchy Hyper-K 10 years 90% C.L.
sin2 2
100
185
0.1
0
1
2
3
4
5
6
7
8
7
• 10
9
10
Figure 2: Fraction of δ for which sin δ = 0 can be excluded with 3 σ as a function of the integrated beam power for J-PARC to HyperKamiokande. The ratio of neutrino and anti-neutrino mode is fixed to 3:7.
0.05 0
0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 sin2 23
Figure 4: Expected sensitivities for sin2 θ23 and sin2 2θ13 at 90% CL with ten years of Hyper-K atmospheric neutrino data. Stars in the contours represent the assumed true mixing angles. sin2 2θ23 = 0.99 (sin2 θ23 = 0.45 or 0.55) and normal mass hierarchy is assumed.
50 45
Normal mass hierarchy Hyper-K 10 years
40
sin2 23=0.6
35 2
30 sin2 23=0.5
25 20
sin2 23=0.4
15
2.2. Nucleon Decay
10 5 0
within the currently allowed range. Figure 4 shows the expected sensitivities for sin2 θ23 and sin2 θ13 , assuming sin2 2θ23 = 0.99 (sin2 θ23 = 0.45 or 0.55) and normal mass hierarchy. For sin2 2θ13 0.1, discrimination of the θ23 octant is possible.
0.06 0.08 0.1 2 0.12 0.14 0.16 0.18 sin 2
13
Figure 3: Significance for the mass hierarchy determination expected with ten years of atmospheric neutrino data with Hyper-Kamiokande.
bilities due to the MSW resonance inside the Earth occurs to either neutrino and anti-neutrino, depending on the mass hierarchy. The magnitude of the resonance depends on the octant of θ23 and the value of θ13 . Thus, atmospheric neutrinos can be used to extract those parameters. Because the sensitivity of atmospheric neutrino to the mass hierarchy and θ23 octant increases for larger value of θ13 , recent results are very encouraging for these measurements. Figure 3 shows the significance of the mass hierarchy determination expected with ten years of atmospheric neutrino data with Hyper-Kamiokande. Each line corresponds to the case with sin2 θ23 = 0.4, 0.5, and 0.6. For sin2 2θ13 0.1, the statistical significance will exceed 3σ after ten years of running, for any value of θ23
Although not directly connected to the neutrino physics, a search for nucleon (proton and/or bound neutron) decay is the only direct probe of the grand unified theories (GUTs) and has deep implications to the particle physics at very high energy. Next generation experiments will extend the sensitivity to nucleon decay searches in various decay modes by about one order of magnitude. 2.3. Astrophysics An underground water Cherenkov detector also functions as an astrophysical neutrino observatory. The targets of such observatory include (but not necessarily limited to) topics explained below. 2.3.1. Supernova core collapse neutrinos The sensitivity of the next generation detector to a distant supernova burst will reach to ∼Mpc. By measuring energy and timing distributions of neutrinos from a supernova burst, one can test models of supernova. Also, there is possibility that the mass hierarchy can be determined by studying the time profile of the supernova neutrino.
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2.3.2. Measurement of supernova relic (diffuse) neutrinos On average, one supernova explosion has been occurring every second somewhere in the universe. The neutrinos produced by all of the supernova explosions since the beginning of the universe must fill the present universe. By measuring the energy spectrum of supernova relic neutrinos, the history of heavy element synthesis can be studied. In order to suppress background and enhance the sensitivity, an option to add gadolinium into water is being studied [19].
Figure 5: Schematic view of the MEMPHYS detector [20].
2.3.3. Precise measurements of solar neutrinos With a large statistics sample of solar neutrinos, next generation water Cherenkov detectors can study interior of the Sun and measure possible time variation over short time periods. If the background and systematics can be controlled to the required level, MSW effect inside the Earth can be observed with day/night asymmetry. In addition, if a low energy threshold can be achieved, there is a possibility to see the spectrum upturn and test the framework of solar neutrino oscillation. 2.3.4. Indirect search of WIMP dark matter Neutrinos emitted by weakly interacting massive particles (WIMPs) annihilating in the Sun, Earth, and galactic halo can be detected using the upward-going muons observed in Hyper-K. Water Cherenkov detectors have unique sensitivity in the energy range of 10100 GeV. 3. Technical Development Because of the rich physics potential, development of large water Cherenkov detector has been carried out all over the world. There are three major efforts in three regions; Europe, US, and Japan. The status and prospects of technical development in three regions are summarized in this section. 3.1. MEMPHYS in Europe In Europe, a concept of megaton class water Cherenkov detector, MEMPHYS [20] (Fig. 5), has been studied in the framework of LAGUNA and LAGNALBNO programs. Underground laboratory at Fr´ejus located 130 km from CERN is considered as the candidate site. MEMPHYS consists of two cylindrical detector modules, each with 65 m in diameter and 103 m in height, giving a total fiducial volume of 530 kilotons. In order to realize a large water Cherenkov detector, development of photosensor and its readout system with
Figure 6: Left: Matrix of sixteen 8” Hamamatsu PMTs in PMm2 . Right: Backside of the matrix showing the watertight box for the electronics.
high performance and reduced cost is indispensable. An ASIC with high-speed discriminator, 12-bit ADC and TDC to readout a group of PMT is developed in an R&D program called PMm2 . The developed electronics and data acquisition system has been tested with 4 × 4 PMT matrix (Fig. 6). They are now being fully tested with the MEMPHYNO prototype (Fig. 7). The MEMPHYNO prototype consists of a tank of 2 × 2 × 2 m3 filled with water, a hodoscope made by four scintillator planes (two on top and two on bottom), and the PMT matrix in the water tank. The full functionality of the system will be tested with cosmic ray muons. 3.2. LBNE in United States In the United States, an intensive R&D was carried out over past few years by LBNE collaboration. The water Cherenkov detector was considered as one of two options for the far detector of LBNE experiment, proposed at Sanford Underground Laboratory in the Homestake gold mine in Lead, SD. Figure 8 shows a schematic view of the LBNE water Cherenkov detector. It has one large cavity of 65 m in
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Figure 9: Deck and balcony configuration and equipment on top for LBNE-WCD [21].
Figure 7: Photograph of MEMPHYNO prototype.
Figure 10: Linear PMT installation unit (PIU) concept deployed from deck [21].
Figure 8: Schematic view of the LBNE WC detector [21].
diameter and 81.3 m in height, with 200 kton fiducial volume. The detailed design developed by LBNE collaboration is summarized in a conceptual design report [21] which is publicly available. It covers all of necessary areas for realization of the detector and gives an excellent reference for any future water Cherenkov detector, although in LBNE the liquid argon TPC was chosen as the primary detector technology for the far detector. A few examples of R&D results are explained below. A conceptual design of water containment system, such as the vessel, liner, floor, and PMT installation has been developed. Figure 9 shows a schematic of the configuration of the deck assembly on top of the detector. PMT installation units on the wall are planned to be supported by cables and deployed from the deck (Fig. 10). This eliminates the need for additional liner penetration
into the wall. Another area of intense development was the photodetector and light collection system. In the reference design, 29,000 12-inch high-QE PMTs are assumed. Optical, electronic, and mechanical characteristics of PMTs are investigated. Many of PMT performance such as gain, noise, timing resolution, dark current, and response uniformity over photo cathode, are evaluated in test facilities. In order to increase the light collection efficiency and hence to reduce the number of PMTs necessary, light collectors are considered. Figure 11 shows two of light collector options studied in LBNE. An im-
Figure 11: Light collectors studied by LBNE: Left: Wavelength shifter plate. Right: Winston cone.
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Figure 12: Schematic view of the Hyper-Kamiokande detector [14].
plosion of PMT under high hydrostatic pressures may cause a catastrophic accident in a water Cherenkov detector. For a full understanding of the mechanical property of PMT and the dynamics of PMT implosion, real implosion tests of PMT have been carried out in a facility of the U.S. Navy. 3.3. Hyper-Kamiokande in Japan In Japan, the world-largest underground water Cherenkov detector, Super-Kamiokande (Super-K) has been running since 1996. As an upgrade of Super-Kamiokande, Hyper-Kamiokande (Hyper-K) with 560(990) kton fiducial(total) mass is proposed [14] (Fig. 12). In its baseline design, Hyper-K has two large cavities, each having a egg-shape cross section 48 meters wide, 54 meters tall, and 250 meters long. Each cavern will be optically separated by segmentation walls located every 49.5 m to form 5 (in total 10) compartments (Fig. 13), such that event triggering and event reconstruction can be performed in each compartment separately and independently. Because the compartment dimension of 50 m is comparable with that of Super-K (36 m) and is shorter than the typical light attenuation length in water achieved by the Super-K water filtration system, (>100 m @ 400 nm), detector performance of Hyper-K is expected to be basically the same as that of Super-K. The candidate site of Hyper-K is about 8 km south of Super-K and has the same off-axis angle and baseline from J-PARC neutrino beam as Super-K. For the excavation of large caverns, a baseline plan has been developed based on the geological survey and in-situ rock stress measurements. The baseline design of liner and PMT support is defined and summarized in a document. In order to further optimize the detector design in terms of cost, schedule and physics performance, alternative cavern shape and PMT support structure are under investigation. The construction of Hyper-K is estimated to take seven years from the start of construction.
Figure 13: Top: Cross section view of the Hyper-Kamiokande detector [14]. Bottom: Profile of Hyper-K detector showing PMT arrays and the support structure.
Figure 14: Test of 8-inch prototype HPD being tested in Kamioka.
As mentioned before, the photosensor is one of key components of a water Cherenkov detector. For HyperK, the same 20-inch PMT as Super-K is assumed in the baseline design. In order to achieve 20% photo cathode coverage, about 100,000 PMTs are necessary. In order to achieve less cost and better performance, development of 20-inch hybrid photodetector (HPD) has been started (Fig. 14). An HPD uses an avalanche photodiode instead of dynodes in a conventional PMT for multiplication of signal. Because of its simple structure, HPD is expected to be significantly less expensive than PMT. It also has higher single photon sensitivity and much better timing resolution than a conventional PMT. As the first step, 8-inch prototype will be tested in a 200-ton water tank in Kamioka. R&D in other important areas are also ongoing. Requirements for electronics and DAQ system in Hyper-K
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of θ13 has been observed, the next step in neutrino oscillation physics is clearly defined. A water Cherenkov detector is no doubt one of prominent options for the next generation experiment to be realized in near future. Acknowledgments The author would like to thank Thomas Patzak and Steve Kettell for providing information on MEMPHYS and LBNE-WCD. Figure 15: Design of the water and air supply system for the HyperKamiokande detector.
Figure 16: Tentative schedule of Hyper-K construction.
is similar to that in Super-K. In order to avoid difficulties to route massive cables and degradation of the signal, an option to put frontend electronics inside water tank near the photosensor is considered. Water purification system (Fig. 15) has been designed based on the experience with Super-K, XMASS and EGADS water systems. Detector calibration system is also being designed based on the experience of Super-K. Figure 16 shows a tentative schedule of Hyper-Kamiokande construction. 4. Summary Water Cherenkov detectors have been, and will continue, providing fascinating results in the fields of neutrino physics and astrophysics. Because its strong potential for a wide range of scientific subjects, development of a large water Cherenkov detector has been carried out all over the world. The detector is well matured and basic technology to build the next generation, megaton-class detector is readily available. Further R&D and optimization to enhance the physics capability and feasibility are ongoing. Now that a large value
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