Large Apparatus studying Grand Unification and Neutrino Astrophysics
There are fundamental questions in particle and astro-particle physics that can only be answered with next-generation very large volume underground observatories searching for rare events. LAGUNA is a new collaboration of 26 institutes across Europe aiming to design such an experiment. Sheffield is founder member of LAGUNA with interests in several areas:
The proton, one of the main building blocks of matter, is known to be an extremely stable particle, yet many models predict that it might not live forever. A positive detection of proton decay would represent the most generic and directly verifiable consequence of the unification of the fundamental forces of Nature. Thought by many to be as important as the search for the Higgs boson or the existence of supersymmetric particles, the discovery of proton decay would have a tremendous impact on our understanding of Nature at the highest energies (in an energy domain in the range of 1016 GeV, to be compared with the domain of energy of 103 GeV explored by the highest energy collider ever built, the Large Hadron Collider LHC at CERN), yielding otherwise inaccessible information on the structure of matter at extremely small scales. LAGUNA will allow exploring otherwise unreachable domains at the highest energies.
The neutrino is unique among the fundamental particles in that it has no conserved quantum numbers except, perhaps, a global lepton number. The recent discovery that the neutrino changes type, or flavour, as it travels through space, a phenomenon referred to as neutrino oscillations, implies that neutrinos have a tiny, but non-zero mass, that lepton flavour is not conserved, and that the Standard Model of particle physics is incomplete. Neutrinos can travel very large distances in space and traverse dense zones of the Universe, since they only very weakly interact with matter, and provide therefore unique information on their sources. LAGUNA will allow for unprecedented measurements of fundamental neutrino properties, providing us with new and deep insights into their sources, notably the Sun, the core-collapse supernovae and the Earth itself.
These fields of research are at the forefront of astro-particle and particle physics and are the subject of intense investigation worldwide. Europe is currently leading deep underground science with its currently four long-running laboratories, including BOULBY, and two emerging deep underground laboratories. The LAGUNA consortium includes the highest-level expertise in Europe for the task of designing the necessary experiment and infrastructure for aims above.
A rich field of investigations for very large detectors
The successful detection of neutrinos from the supernova SN-1987A by the Kamiokande experiment (Japan) and confirmed by the IMB experiment (USA) has opened the field of neutrino astronomy. It was recognized with the Nobel Prize in 2002. Actually, it opened a 20-year long tradition of incredibly rich physics with large underground detectors, the largest one being the 22.5 kton Super-Kamiokande detector in Japan. These detectors have achieved a certain number of technical break-throughs and have achieved fundamental results like the solution of the solar neutrino puzzle and the understanding of the physics of the Sun, the discovery of non-vanishing neutrino masses by atmospheric neutrinos, and the confirmation of these results by detecting reactor neutrinos and accelerator neutrinos in the KamLAND experiment and Super-Kamiokande, respectively. Limits on the flux of supernovae relic neutrinos have been set. The searches for the finite lifetime of protons, a direct test of the Grand Unification of the fundamental elementary interactions (strong, electromagnetic and weak forces), have been pushed towards limits in the range of a few 1033 years. Last but not least, KamLAND has announced first evidence of so-called geo-neutrinos, emitted by radioactive elements within the Earth, opening the way to new methods of investigation of the Earths interior. In a next step, the neutrino flavor oscillation mixing matrix is going to be further studied with an intense accelerator neutrino beam from the newly built J-PARC accelerator complex in Japan to Super-Kamiokande (T2K experiment), complementing the efforts at Fermilab in USA and at the CERN-Gran Sasso in Europe.
Further advances in low energy neutrino astronomy and neutrino astro-particle physics beyond those listed above, as well direct investigation of Grand Unification (GU) of fundamental interactions require the construction of next-generation very large volume underground observatories. With complementary techniques, facilities on the mass scale of 50 kton to 500 kton could dramatically increase the potential of past and present underground detectors.
Large underground infrastructure is needed
There is currently no infrastructure in the world able to host instruments of this size, although many European national underground laboratories with high-level technical expertise are currently operated with forefront smaller-scale underground experiments. Very large underground laboratories are being considered in Japan in the context of the Hyper-Kamiokande project and in the USA as part of the DUSEL process. A pan-European research infrastructure able to host a new generation underground instruments with total volumes in the range of 100 000 m3 up to 1 000 000 m3 would provide new and unique scientific opportunities and very likely lead to fundamental discoveries in the field of particle and astro-particle physics, attracting interest from scientists worldwide.
LAGUNA focuses on the study of feasibility and design of such a new infrastructure in Europe and on the scrutiny of the technical requirements as called from the next generation large-scale underground observatories. LAGUNA intends to explore different detector technologies currently being investigated by various European research institutes, and different potential underground sites in order to identify the scientifically and technical most appropriate and cost-effective strategy for future large-scale underground detectors in Europe.
The science of LAGUNA
Investigating the proton lifetime up to 1035 years will provide a very stringent, perhaps ultimate, test of the Grand Unification hypothesis. After the optical observation of supernovae (SN) by mankind during the last centuries and the SN1987A neutrino detection, the next observable event with neutrinos will occur with high probability in the next decade and with near certainty in the next 30 years. In this context, neutrinos might shed more light on the SN explosion mechanisms than optical light. Meanwhile the background flux of neutrinos from relic supernovae can be observed. The study of neutrinos properties has shown the first indication of physics beyond the Standard Model of Elementary Particles. New discoveries, like CP-violation in the leptonic sector, are expected in this field. High-energy accelerators like the CERN Large Hadron Collider (LHC) or the planned International Linear Collider (ILC) will not be able to answer the above fundamental questions about Nature.
Large Underground Experiment Designs
Several conceptual ideas for next-generation very-massive, multi-purpose underground detectors have emerged worldwide and in Europe over the last years. All the designs consist of large volumes of liquid observed by detectors, which are arranged on the inner surfaces of the vessels. The liquid simultaneously acts as the target and as the detecting medium. The first one relies on the concept of Super-Kamiokande and uses water (MEMPHYS R&D project), the second builds on the initial experience with ICARUS and uses Liquid Argon (GLACIER R&D project), the third extrapolates experience gained in reactor experiments and BOREXINO and uses liquid scintillator (LENA R&D project).
R&D projects being discussed in Europe as possible next generation very large volume underground detectors: MEMPHYS, LENA and GLACIER
The three technologies of LAGUNA
In LAGUNA we are evaluating these three technologies that are believed to be well adapted for the physics goals under consideration; all are based on the large scale use of liquid as the active component in the detector:
- Water Cerenkov Imaging: As the cheapest available (active) target material, water is the only liquid that is realistic for extremely large detectors, up to several hundreds or thousands of ktons. Water Cerenkov detectors have sufficiently good resolution in energy, position and angle. The technology is well proven, as previously used for the IMB, Kamiokande and Super-Kamiokande experiments.
- Liquid scintillator: Experiments using liquid scintillator as active target provide high-energy resolution and offer low-energy threshold. They are particularly attractive for low energy particle detection, as for example solar neutrinos and geo-neutrinos. Also liquid scintillator detectors feature a well established technology, already successfully applied at relatively large scale in the Borexino and KamLAND experiments.
- Liquid Argon Time Projection Chambers (LAr TPC): This detection technology has among the three the best performance in identifying the topology of interactions and decays of particles, thanks to the bubble-chamber like imaging performance. Liquid Argon TPCs are very versatile and work well with a wide particle energy range. Experience with such detectors has been gained within the ICARUS project.
The three technologies have in common similar requirements for their design, installation and operation in the future underground facilities. They have similar (high) discovery potential and exhibit some interesting elements of complementarity. In addition, the three proposed solutions are backed by rather large and active European communities.
Capability of the LAGUNA technologies
From a practical point of view, the most straightforward liquid is water, where the detection is based on the Cherenkov light emission by the final state particles. This faint light is detected by a very large number of photomultipliers positioned on the surface of the container. The technology was pioneered by the IMB and Kamiokande experiments (USA and Japan, respectively) and successfully extended to Super-Kamiokande during many years of operation. Super-Kamiokande, the largest Water Cherenkov detector ever built, has a fiducial mass of 22.5 kton observed by about 11 000 large-size photomultipliers. The possibility of building a water Cherenkov detector with a fiducial mass of about 500 kton observed by about 200 000 photomultipliers is currently being investigated by different groups around the world, and for different underground sites. While water is a cheap medium, the size of such detectors is limited by the cost of excavation and of the photomultipliers. The MEMPHYS project is being discussed for deployment in an extended Frejus laboratory (France/Italy). In the US, the UNO detector is being proposed for a future underground facility in North America. In Japan Hyper-Kamiokande will provide an extension of Super-Kamiokande by a factor 20, using a new cavern to be excavated near Super-Kamiokande. Hyper-Kamiokande will also serve as the far detector for the second phase of the T2K experiment which is presently going to direct a neutrino beam from J-PARC to the Kamiokande site. Water-Cerenkov detectors are most efficient for neutrino interactions with a single Cherenkov ring and are therefore in practice ideally matched for neutrino energies below 1 GeV. They have also a high sensitivity for proton decays with two isolated Cherenkov rings like for example the channel p → e+ πo.
Inside view of the Superkamiokande detector in Japan. The large volume of water is seen by 11000 photodetectors for a total sensitive mass of 22.5 ktons.
A second possibility is the liquid Argon Time Projection Chamber pioneered and developed under European leadership over many years of R&D in the ICARUS program. This technology is able to image the rare events with the quality of the bubble-chambers, which are famous for having led to important discoveries in particle physics in the 1970s. However, compared to a bubble-chamber, the liquid Argon TPC is fully electronic and can be in principle be extrapolated to very large masses, possibly beyond many tens of kilotons. The Liquefied Natural Gas (LNG) technology developed by the petrochemical industry has proven that the safe storage of very large volumes of cryogen is possible. The ionization charge produced by charged particles when they traverse the medium and the associated scintillation light can be independently readout and provide a tracking-calorimetry detector. Thanks to their imaging capability, liquid Argon detectors can provide improved sensitivity for the proton decay channels where backgrounds are serious in the Water Cherenkov detectors, such as the channel p → K+. GLACIER is a European design for a new generation liquid Argon TPC, eventually scalable up to at least 100 kton, only limited by the cost of liquid Argon and of the needed cryogenic power. In this context, dedicated R&D for the extrapolation of the liquid Argon TPC to very large scales is been pursued in Europe. Interest in the technology has recently also grown in the USA in the context of a second generation long-baseline experiment at Fermilab.
A third possibility is a very large liquid scintillator volume observed by photomultipliers. The scintillator technology is based on the pioneering developments within the BOREXINO and DoubleCHOOZ experiments. The light yield of a scintillator is much larger than that of water Cherenkov detector, resulting in a much better energy resolution and lower detection threshold. A high efficiency can be achieved in the search for the proton decay via p → K+, as the Kaon and its decay products can be observed directly. In addition to the detection of Supernova neutrinos and the diffuse Supernova neutrino background, the very low threshold allows to measure different contributions to the solar neutrino spectrum at high statistics. Moreover, due to the delayed coincidence signal of electron antineutrinos liquid scintillator is the only proposed technology able to detect geoneutrinos. LENA is a European proposal for such a detector in the range of 50-100 ktons. Already with 50 kt the detector will provide interesting physic results. This mass can be however enlarged as the main costs are just due to the price of the liquid scintillator and of the photomultipliers. There is a growing interest in the liquid scintillator technique in the USA and Canada, as the upgrade of the SNO experiment will rely on this technology as well as the proposed deep ocean geoneutrino observatory HanoHano.
View of the BOREXINO detector at the LNGS underground laboratory.
The three mentioned detector types represent a variety of complementary aspects. MEMPHYS would collect the largest statistics, GLACIER would have the best pattern recognition, LENA would have the lowest energy threshold. MEMPHYS and LENA are superior in anti-neutrino detection while GLACIER is best in neutrino detection. Neutrinos and anti-neutrinos together provide the full information to study supernovae. MEMPHYS has complementary sensitivity to LENA and GLACIER on proton decay flavour signatures.
Physics potential of the three types of instruments considered
0.5 x 1035
1.1 x 1035
0.4 x 1035
1.0 x 1035
0.2 x 1035
|SN at 10 kpc
2.5 x 10 (νe)
3.0 x 104
1.0 x 103 (e)
9.0 x 103 (anti νe)
3.0 x 103
5.0 x 103 (p)
6.0 x 102 (p)
2.0 x 105 (anti νe)
1.0 x 103 (e)
events (5 years)
# events, 1 year
|8B ES: 4.5 x 104
Abs: 1.6 x 105
|7Be: 2.0 x 106
pep: 7.7 x 104
CNO: 7.6 x 104
8B(CC): 3.6 x 102
8B(NC): 5 x 103
|8B ES: 1.1 x 105|
# events, 1 year
|1.1 x 104||TBD||4.0 x 104|
# events, 1 year
|Below threshold||1.5 x 103||Below threshold|
Prefeasibility study for Boulby mine
The Boulby mine has over 1000 km of tunnels excavated over the last 40 years and the potential for expansion is excellent and there is already interest from the mine operators Cleveland Potash Ltd (CPL). Whereas excavations in the salt seam are limited to 8 m wide by 5 m high new labs as large as those seen in existing hard rock locations are possible elsewhere at that site. In its current form LENA appears to be viable, and based on the cavity geometry permitted by the surrounding rock, GLACIER and potentially MEMPHYS could also be adapted to fit. The concept of a new underground science facility is strongly supported by CPL. LAGUNA includes strategic exploration of viability and a full appraisal of the feasibility of establishing a full laboratory with all associated services required at Boulby.
|1. ETH Zurich||Swiss Federal Institute of Technology Zurich||Switzerland|
|2. U-Bern||University of Bern||Switzerland|
|3. U-Jyväskylä||University of Jyväskylä||Finland|
|4. U-Oulu||University of Oulu||Finland|
|5. Rockplan||Kalliosuunnittelu Oy Rockplan Ltd||Finland|
|6. CEA/ DSM/ DAPNIA||Commissariat à l'Energie Atomique /Direction des Sciences de la Matière||France|
|7. IN2P3||Institut National de Physique Nucléaire et de Physique des Particules (CNRS/IN2P3)||France|
|8. MPG||Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V.||Germany|
|9. TUM||Technische Universität München||Germany|
|10. U-Hamburg||Universität Hamburg||Germany|
|11. INFN||Istituto Nazionale Fisica Nucleare||Italy|
|12. DITS||Dipartimento di Ingegneria Idraulica Trasporti e Strade [DITS] dell'Universita' La Sapienza di Roma||Italy|
|13. AGT||AGT Ingegneria Srl, Perugia||Italy|
|14. IFJ PAN||H.Niewodniczanski Institute of Nuclear Physics of the Polish Academy of Sciences, Krakow||Poland|
|15. IPJ||A.Sołtan Institute for Nuclear Studies||Poland|
|16. US||University of Silesia||Poland|
|17. UWr||Wroclaw University||Poland|
|18. KGHM CUPRUM||KGHM CUPRUM Ltd Research and Development Centre||Poland|
|19. IGSMiE PAN||Mineral and Energy Economy Research Institute of the Polish Academy of Sciences||Poland|
|20. LSC||Laboratorio Subterraneo de Canfranc||Spain|
|21. UGR||University of Granada||Spain|
|22. UDUR||University of Durham||United Kingdom|
|23. U-Sheffield||The University of Sheffield||United Kingdom|
|24. Technodyne||Technodyne International Ltd||United Kingdom|
|25. ETL||Electron Tubes||United Kingdom|
|26. U-Aarhus||University of Aarhus||Denmark|
Thanks to all in the LAGUNA consortium for input to this report.
Liquid Argon R&D
Liquid argon is becoming recognised as a unique and versatile detector technology for particle physics including proton decay. The Sheffield group is actively pursuing an R&D programme in LAr technology for applications in three possible future areas:
- Neutrino Factory LAr Detector - UKNF
- T2K LAr Detector - T2K2
- Proton Decay and Dark Matter LAr Detector - LAGUNA-ArDM
Liquid argon basics
Noble liquid detectors using Xenon or Argon can efficiently act as targets for rare event detection. Xenon or Argon provide a high event rate because of their high density and high atomic number and large target masses are readily conceivable. They have high scintillation and ionization yields because of their low ionization potentials. Both scintillation and ionization are measurable and can be used to very effectively discriminate between particles and produce tracking information in real time.
Liquid argon for proton decay and LAGUNA
Liquid argon has significant advantages for the proposed LAGUNA design. In 2004 a pre-study on the feasibility of a large underground liquid argon storage tank was mandated by ETH Zurich to Technodyne Ltd. in the UK and resulted in a conceptual design. The Sheffield group is participating in Liquid Argon detector development for proton decay through the GLACIER and ArDM programmes at CERN and Zurich.
Conceptual design of large underground Argon tank by Technodyne Ltd.
One of the fundamental parameter in order to scale the size of a liquid Argon TPC is the maximal possible drift path. Tests are underway at Bern on this using a 5 m long detector column. When searching for electron-neutrino appearance there will be both an irreducible intrinsic electron-neutrino background and a background due to event misidentification. In a next generation experiment aiming at maximum sensitivity to CP-violation or matter effects in neutrino oscillations, one should aim at reducing the backgrounds from event misidentification as much as possible in order to profit at most from the increased statistics. Eventually, the limiting factor will be the knowledge of the intrinsic electron-neutrino background so other sources of backgrounds should be suppressed below this contamination, which is generally at the level of the percent in the region of the oscillation maximum. This is not the case in the present T2K and proposed NoVA where a ratio electron-neutrino to neutral current (misidentified π0’s) is achieved at the cost of efficiency (ε ~40% for T2K, ~20% for NoVA). The liquid Argon TPC imaging should offer optimal conditions to reconstruct with very high efficiency the electron appearance signal in the energy region of interest in the GeV range, while considerably suppressing the NC background consisting of misidentified π0s.