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Strategies for Future Accelerator Neutrino Physics
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222 www.elsevier.com/locate/npbps
Strategies for Future Accelerator Neutrino Physics Vittorio Palladino University Federico II and INFN Napoli, Italy
Abstract Accelerator neutrino (ν) physics has come back to the forefront, with the discovery of ν transitions, our first and unique window beyond the standard model. The experimental program to provide a complete map of the ν mixing matrix, including its far reaching CP violation sector, and test its unitarity constraints is likely to extend over several future decades, as it has been for quark mixing. So far, conventional ν beams based on pion (π) decay have been used and more are already being planned, at higher power (superbeams), in Japan and the US, in conjunction with larger or novel detectors. Superbeams have limited potential, however. Novel very intense beams of ν parents, longer lived than π’s, accelerated and then coasted in a decay storage ring replacing the π decay tunnel, promise the ultimate reach. R&D for muon decay ring (ν factory) and ion decay ring (betabeam) experiments is thus a decisive task today. Keywords: Neutrino, Neutrino Oscillations, Neutrino beams, Neutrino Detectors, Neutrino R&D
1. Introduction This talk is an introduction to those following it in this morning session, that will in turn introduce the few most promising future projects under study. Somewhat European in perspective, it will try to keep an international breath. In the summer of 2006, the CERN Council Strategy Document suggested that Europe should be ”in position to define the optimal accelerator ν program · · · in around 2012”. To match this challenge, the EUROnu Design Study (DS) is preparing design reports for three possible future ν beam facilities (a superbeam, betabeam and νfactory) and the LAGUNA DS is preparing feasibility reports of underground sites capable to host new, very large far ν detectors. On the basis of these studies and of the international context, the NEu2012 Network is preparing to propose a strategy road map for the next revision of the CERN Council Strategy in 2012 or so. A Eu ν strategy must inevitably be a global strategy. Internationally, Japan is defining [1] its plans for the future of JPARC, where large US and Eu teams are active. In the US, around Fermilab, plans are being made for a new conventional ν beam [2] from the Main Injector 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.035
while, simultaneously, the US muon (μ) accelerator program [3] is supporting the International Design Study of a ν-Factory (IDS-NF) at least as much as European and other partners. A long term strategy is needed. Quark mixing first emerged in 1953, with the discovery of strangeness, almost 60 years ago. The CKM mixing matrix has been the object of intense studies since and we are today still building strangeness, charm and beauty factories. The study of the ν PNMS mixing matrix just began and has a long way to go [4]. Its 3 by 3 nature has still to be established. Its CP violating (CPV) phase has to be measured. Its unitarity and invariance properties (CP, T, CPT) have to be mapped. Many of these answers can and will only come from high energy neutrinos. Novel, superior ν beams appear to be, eventually, inevitably needed. Anticipating it, the natural conclusion of this talk is that the accelerator ν communities face today two simultaneous tasks: 1) do the experiments we can do or propose today, progressing as much as possible with conventional ν beams and the ν detectors presently possible. Larger
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V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
detector mass and beam power, in this order of priority today, are the weapons of this next attack. 2) prototype the ultimate experiments, i.e. design, prototype and build the third possible weapon, novel ν beams, betabeam [5] or/and ν-factories [3], storage and decay rings instead of a decay tunnel, capable of more ν per unit time and unit beam power. Such a long term program, second generation conventional (super)beams first and ultimate neutrino production rings later, can be sustained only if the indispensable R&D effort that it implies is strengthened today and kept at adequate levels in the years ahead. 2. The study of neutrino transitions It is one of the frontier subjects of particle physics today. At this conference, the highlights are the first appearance ντ candidate in OPERA [7] and first ν event in ICARUS [8]. About 50% of the sessions had talks on these studies or on studies ancillary to them. The independent existence of two ν flavor transitions is today very solidly established [4]. They both indicate a large flavor mixing and an oscillatory modulation in L/E, respectively with wavelength of 500 Km/GeV (atmospheric) and 15000 Km/GeV (solar). The favored explanation today of all available data is that: 1) the known 3 ν flavor eigenstates νe , νμ , ντ are infact each a linear mix of three ν mass eigenstates ν1 , ν2 , ν3 of mass m1 , m2 , m3 2) a 3 by 3 complex matrix governs the strength of the flavor mixing. It contains four physical quantities, three mixing angles θ12 , θ23 , θ13 and one CP violating phase δ. The two detected transitions are then part of a larger set of phenomena still to be observed. The measurements of the solar and atmospheric wavelengths respectively imply a best value of 7.6 10−5 eV 2 for the squared mass difference δm212 and a common value of 2.4 10−3 eV 2 , 30 times greater, for the other two differences, δm213 and δm223 . The measured mixing strength is large for both transitions, very much unlike quark transitions. The solar mixing sin2 θ12 turns out to be about 1/3, while the atmospheric mixing sin2 θ23 is even larger, intriguingly close to 1/2, its maximal value. Atmospheric transitions, that behave much as if they were mostly νμ → ντ , fit well terrestrial experiments aiming a ν beam at a massive far ν detector. These experiments can be tightly controlled and precise, we expect now from them the measurement of the three decisive physical quantities that are still unknown. The third mixing parameter sin2 θ13 has not yet been measured. It is not large, an experimental upper bound
of about 3% follows from the failure to detect, so far, νμ → νe transition over the flight path (baseline) of the atmospheric transitions. Detection of that transition or of its inverse transition νe → νμ is essential to establish the 3 by 3 nature of ν mixing. The subdominant νμ ↔ νe transition is also the one where the largest effects from the CP violating phase δ are expected. Its experimental signature would be an asymmetry between ν and ν transition rates and/or an asymmetry between direct and inverse transition rates (time reversal). This subdominant direct or inverse channel is expected to provide also the third and last piece of information still missing, the sign of δm2atm , that is the sign of δm213 and δm223 that determines whether m3 is much larger (normal νe mass hierarchy) or much smaller (inverted hierarchy) than m1 and m2 . An additional asymmetry between ν and ν transition rates on top of the one due to a non zero CPV phase must exist, due to the different interaction of ν and ν with electrons when propagating through matter over large distances. The sign of this asymmetry can tell us what is the correct hierarchy. If νμ ↔ νe promises the largest rewards, all nine possible transitions among ν flavors should be studied. The history of quark mixing shows us that unitarity and invariance properties (CP, T, CPT) of all rows of the ν mixing matrix will ultimately have to be precisely mapped. In practice, the three possible transitions of νe and the three of νμ are accessible to measurements, while transitions of ντ presently appear unfortunately out of experimental reach. Conventional beams have so far provided us with the best tool to study νμ transitions over atmospheric baselines. The limitation of their low rates can be mitigated by assembling larger far detector masses and using higher power proton drivers (superbeams). Up to a point, however. The rate limitation, their irreducible contamination of νe ’s and other experimental complications have lead us to consider novel ν beams. The novel idea common to these novel ν beams is storing and coasting intense beams of longer lived ν parents, beta emitting ions (betabeam) or/and muons (νfactories), accelerated to high energies, inside storage and decay rings that replace the traditional π decay tunnel. In the case of betabeams, long lived beta emitting ions produce pure νe or νe beams for thorough studies of νe transitions. One can envisage to study all the experimentally accessible transitions by a combination of superbeam and betabeam experiments. In the case of ν-factories, the ν parents are muons. μ+ beams produce simultaneous beams of νμ and νe , while
V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
μ− beams do the opposite. As for betabeam, the most rewarding ”golden” channel will be νe → νμ , but νfactories promise eventually the best map of all accessible transitions in a single unique well controlled experimental setup. The price to pay is magnetization of large far detector volumes, necessary to separate experimentally the ν and ν interaction samples. 3. Current generation accelerator studies of neutrino transitions To date, K2K and MINOS [6] have confirmed the original atmospheric result. OPERA may be soon confirming νμ → ντ dominance. Emphasis is therefore now shifting to the subdominant νμ → νe transitions, still using conventional beam coupled with far detectors with good detection efficiency and low background for single electrons. One new experiment exploiting a new conventional beam, T2K [9], is already taking data at JPARC. It still uses the 22 kton fiducial volume of SuperKamioka, 295 Km away. Present beam power (100 Kwatts) already exceeds that available from the K2K beam but it is expected to be steadily improving up to 700 Kwatt. It will be also the first neutrino beam pointing at the far detector at a small off axis angle. This provides a better defined and lower neutrino energy that matches better the 500 Km/GeV wavelength. A second experiment, NOνA [10], using off-axis the Fermilab existing MuMI conventional beam, is in advanced state of preparation. It is assembling a 20 Ktons totally active scintillator detector (TASD) in a new off axis far (800 Km) detector location in Minnesota and will be soon taking data with an increasing fraction of the total detector. Plans do exist to make the beam power available to NOνA increasingly larger than what NuMI has so far being delivering to MINOS, up to 700 KW or so. By 2018 or so, these two experiments will either discover the νμ → νe transition and measure a non zero third mixing parameter sin2 (2θ13 ), thus proving the 3 by 3 nature of the mixing matrix, or will impose on it a 1% or slightly lower upper limit on it. Figure 1 illustrates the 3 σ discovery potential of the third mixing parameter expected from these two accelerator experiments and a number of reactor experiments that are also in preparation.
219
ploiting conventional beams. The key experimental factor for further progress is sizeably larger detector mass, hundreds of KiloTons (KT) of instrumented water or equivalent, and larger, MegaWatts (MW) scale, proton beam power. They must be inevitably the weapons of further attack. Both are quite a challenge. Assembling larger detector mass demands new larger underground sites. Instrumented Water Cerenkov tanks much larger than SuperKamiokande will be a highly non trivial extrapolation. The Liquid Argon technique, that has larger efficiency and lower backgrounds, is only approaching maturity and we do not know yet how and when tanks really larger than ICARUS will become important contributors. It is somewhat obvious but still important, in this context, to remind the relevance of these detectors for the equally fundamental measurement of nucleon lifetime. An upgraded T2K program is being planned, and a few possible candidate sites for new water tanks up to one Megaton are being envisaged. Baselines up to slightly above 1000 Km are being considered. Higher proton beam power is also a clear objective, the new T2K beam was designed to evolve to withstand up to 4 MW. In the last few years, however, concern has emerged about the possibility to go beyond 1.7 MW proton beam power with the JPARC synchrotron. An entirely new NuMI beam, pointing to the new unprecedentedly large DUSEL underground detector location is next flagship project at Fermilab. An essential feature is its longer (1300 Km) baseline. It would be upgradable to operate at 2 or more MW, thanks to a new proton driver, the X Project that is at the heart of Fermilab future strategies. The Deep Underground Science and Engineering Laboratory at Homestake, S. Dakota (DUSEL) lab is now established as the site and for a 300 KTs water tank and/or an equivalent mass of Li-Ar. The new NuMI project plans to go back to a wide band beam so to use (facing the novel challenge that are implied) also the distorsion of the νμ energy spectrum to measure the mixing parameters. Less defined plans do exist in Europe too, where, just as in all regions, the relative merits of large water tanks (MEMPHYS) and of smaller, but more efficient, Li-Ar (GLACIER) tanks are being assessed. 5. Longer term prospects for neutrino transitions
4. Next generation of studies of neutrino transitions Beyond T2K and NOνA, plans are being made to start probing the ν mass hierarchy and CP violation, still ex-
The physics reach expected by a few variants of all mentioned future ν beam facilities, including the novel neutrino facilities that will be further detailed later, are
220
V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
ĐŽŶǀĞŶƚŝŽŶĂů ďĞĂŵƐ ͗WŚĂƐĞ /
sin22θ θ13 discovery at 3σ σ CL NF μ beams
β beams
limits of π beams rate beam+detect bkgnds π beams
s13eiδ coupling
EUROnu physics WP
Figure 1: Expected 3 σ discovery potential of the third mixing parameter from T2K, NOνA and a number of reactor experiments that are also in preparation, for the normal ν mass hierarchy (NH).
summarized in the next three figures 2, 3, 4. Produced by the joint physics subgroups of IDS-NF and EUROnu, these figures were included in a recent report [11] from the Neutrino Panel of the CERN Scientific Policy Committee (SPC). Figure 2 shows the 3 σ discovery potential of sin2 (2θ13 ). It depends on the fraction of values of the CPV phase that can be simultaneously accessed. The limits of pion decay beams, due to their low rates and their irreducible νe content, are evident. Figure 3 shows the 3 σ discovery potential of the sign of δm2atm , ie of the correct ν mass hierarchy. A long baseline (LBL) is essential here, modest baseline superbeam experiments have only limited or no reach. Figure 4 shows the 3 σ discovery potential of a CP violating phase δ. The SPC Panel concluded that it is unrealistic to expect a high intensity ν source of any kind in Europe before 2020. On that time scale, Europe should rather pay close attention to the mentioned superbeam programs in Japan and USA. To be competitive in the 2020’s, Europe should concentrate on the R&D for a new intense source, ν factory or betabeam, decay ring facilities capable to bring about neutrino beams of substantially higher neutrino flux and free of wrong flavor backgrounds. An outstanding proton driver must inevitably be the motor of such a scientific program. At CERN a 4 MW (or more) high power superconducting proton linac (HP SPL) has been and is the subject of extensive and today now rather advanced study. It is the natural continuation of the so called LINAC-4, presently under construction. It promises even more potential than the present design of the Project X proton linac at Fermilab that in turn appears quite superior to the potential of the JPARC up-
Figure 2: Expected 3 σ discovery potential of the third mixing parameter of a few variants of all the future ν beam facilities mentioned in the text.
grade program. ν factory or betabeam are two different, possibly complementary, longer term approaches. Betabeam neutrinos are of lower energy and should be coupled with detectors hundreds of Km away. Neutrino factory neutrinos are of higher energies and are best exploited by detectors thousands of Km away. Design reports for both facilities are being prepared for 2012 by the EUROnu design study, that, in the neutrino factory case, operates within the larger international context of the IDS. Design will include the engineering work necessary to evaluate safety infrastructures and, last but not least, costs. We will soon have more realistic ideas of the feasibility of these options and of the global effort implied by their realization.
6. Betabeams Details will be plentiful in E. Wildner’s talk. There are many variants of betabeam envisaged, only two are presently seriously being studied. Very large Water Cerenkov and Li-argon detectors are again the natural partners of such betabeams. The so called baseline betabeam envisages to produce ν (ν) from 6 He (18 Ne) radioactive ions accelerated to γ = 100. Neutrinos have a few hundred MeV of energy, suited to CERN-Frejus like baselines. This is the most mature and solid option that has now been object of several years of studies and can now be seen as almost established. The second variant being studied envisages to produce ν (ν) from higher Q longer lived 8 Be (8 B) radioactive ions still accelerated to γ = 100, with 1 GeV or so
V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
221
sign Δm σ CL Δ 2atm discovery at 3σ
NuFact’s LBL π beam
Modest BL π and β
EUROnu physics WP
Figure 3: Expected 3 σ discovery potential of the sign of δm2atm , ie of the correct ν mass hierarchy, of a few variants of all the future ν beam facilities mentioned in the text.
energy suited to CERN-Gran Sasso like baselines. EUROnu is its first exploratory study. The physics potential of other very very promising betabeam options has been pointed out. It has been shown that higher γ can be extremely rewarding. But the additional technical challenges involved will not yet be seriously studied. 7. Neutrino Factories Again, details will be plentiful in K. Long’s talk. A well defined baseline neutrino factory design has been taking shape over more than 10 years. First feasibility studies in the US at the turn of the century have been followed by an International Scoping Study first and now the International Design Study (IDS). That includes the large magnetic detector indispensable to exploit the factory. The concept of a 50-100 Kton magnetized iron neutrino detector (MIND) has been developed, a large version of MINOS or, if one prefers, a very large version of the glorious CDHS detector. The European contributions have been important, in particular those of the UK component. It is a pleasure to recognize, however, the driving push coming from
the US, first via the NFMCC1 , then also via the MTF2 at Fermilab where a dedicated Muon Test Area has been built and operates now. NFMCC anf MTF have recently merged in the US Muon Accelerator Program (MAP), even more solidly based and better funded. A muon based view of Fermilab future is the real drive behind it, a neutrino factory being a possible step on the way to a muon collider that could really be the long term future of the Fermilab laboratory. 8. Conclusions The long term accelerator neutrino program outlined, second generation conventional (super)beams first and ultimate neutrino production rings later, can be sustained only by a strengthened, very enthusiastic and well supported R&D program. The several R&D demonstration projects in progress are largely international, in particular for the ν factory, and Europe should definitely do more in this sector. We will not be ready, otherwise, when superior neutrino beams will be needed. 1 Neutrino 2 Muon
Factory and Muon Collider Collaboration Task Force
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V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
WsƐĞŶƐŝƚŝǀŝƚŝĞƐ ͙
ĂƐ ŝŶƚŚĞZĞƉŽƌƚŽĨ ƚŚĞ^Wν ƉĂŶĞů ƚŽ ŽƵŶĐŝů ϭϲͬϯͬϭϬ
NB
IDS ←ISS ←BENE
Figure 4: Expected 3 σ discovery potential of a non zero CP violating phase δ of a few variants of all the future ν beam facilities mentioned in the text. It includes the caption adopted for the SPC Panel report, detailing the meaning of each line.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
K. Sakashita’s contribution to these Proceedings. R. Svoboda’s contribution to these Proceedings. K. Long’s contribution to these Proceedings. S. Parke’s contribution to these Proceedings. E. Wildner’s contribution to these Proceedings. B. Vahle’s contribution to these Proceedings. O. Sato’s contribution to these Proceedings. A. Guglielmi’s contribution to these Proceedings. T. Kobayashi’s contribution to these Proceedings. K. Heller’s contribution to these Proceedings. CERN Yellow Report 2010-003, ISSN 0007-8328, ISBN 97892-9083-354-3
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222 www.elsevier.com/locate/npbps
Strategies for Future Accelerator Neutrino Physics Vittorio Palladino University Federico II and INFN Napoli, Italy
Abstract Accelerator neutrino (ν) physics has come back to the forefront, with the discovery of ν transitions, our first and unique window beyond the standard model. The experimental program to provide a complete map of the ν mixing matrix, including its far reaching CP violation sector, and test its unitarity constraints is likely to extend over several future decades, as it has been for quark mixing. So far, conventional ν beams based on pion (π) decay have been used and more are already being planned, at higher power (superbeams), in Japan and the US, in conjunction with larger or novel detectors. Superbeams have limited potential, however. Novel very intense beams of ν parents, longer lived than π’s, accelerated and then coasted in a decay storage ring replacing the π decay tunnel, promise the ultimate reach. R&D for muon decay ring (ν factory) and ion decay ring (betabeam) experiments is thus a decisive task today. Keywords: Neutrino, Neutrino Oscillations, Neutrino beams, Neutrino Detectors, Neutrino R&D
1. Introduction This talk is an introduction to those following it in this morning session, that will in turn introduce the few most promising future projects under study. Somewhat European in perspective, it will try to keep an international breath. In the summer of 2006, the CERN Council Strategy Document suggested that Europe should be ”in position to define the optimal accelerator ν program · · · in around 2012”. To match this challenge, the EUROnu Design Study (DS) is preparing design reports for three possible future ν beam facilities (a superbeam, betabeam and νfactory) and the LAGUNA DS is preparing feasibility reports of underground sites capable to host new, very large far ν detectors. On the basis of these studies and of the international context, the NEu2012 Network is preparing to propose a strategy road map for the next revision of the CERN Council Strategy in 2012 or so. A Eu ν strategy must inevitably be a global strategy. Internationally, Japan is defining [1] its plans for the future of JPARC, where large US and Eu teams are active. In the US, around Fermilab, plans are being made for a new conventional ν beam [2] from the Main Injector 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.035
while, simultaneously, the US muon (μ) accelerator program [3] is supporting the International Design Study of a ν-Factory (IDS-NF) at least as much as European and other partners. A long term strategy is needed. Quark mixing first emerged in 1953, with the discovery of strangeness, almost 60 years ago. The CKM mixing matrix has been the object of intense studies since and we are today still building strangeness, charm and beauty factories. The study of the ν PNMS mixing matrix just began and has a long way to go [4]. Its 3 by 3 nature has still to be established. Its CP violating (CPV) phase has to be measured. Its unitarity and invariance properties (CP, T, CPT) have to be mapped. Many of these answers can and will only come from high energy neutrinos. Novel, superior ν beams appear to be, eventually, inevitably needed. Anticipating it, the natural conclusion of this talk is that the accelerator ν communities face today two simultaneous tasks: 1) do the experiments we can do or propose today, progressing as much as possible with conventional ν beams and the ν detectors presently possible. Larger
218
V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
detector mass and beam power, in this order of priority today, are the weapons of this next attack. 2) prototype the ultimate experiments, i.e. design, prototype and build the third possible weapon, novel ν beams, betabeam [5] or/and ν-factories [3], storage and decay rings instead of a decay tunnel, capable of more ν per unit time and unit beam power. Such a long term program, second generation conventional (super)beams first and ultimate neutrino production rings later, can be sustained only if the indispensable R&D effort that it implies is strengthened today and kept at adequate levels in the years ahead. 2. The study of neutrino transitions It is one of the frontier subjects of particle physics today. At this conference, the highlights are the first appearance ντ candidate in OPERA [7] and first ν event in ICARUS [8]. About 50% of the sessions had talks on these studies or on studies ancillary to them. The independent existence of two ν flavor transitions is today very solidly established [4]. They both indicate a large flavor mixing and an oscillatory modulation in L/E, respectively with wavelength of 500 Km/GeV (atmospheric) and 15000 Km/GeV (solar). The favored explanation today of all available data is that: 1) the known 3 ν flavor eigenstates νe , νμ , ντ are infact each a linear mix of three ν mass eigenstates ν1 , ν2 , ν3 of mass m1 , m2 , m3 2) a 3 by 3 complex matrix governs the strength of the flavor mixing. It contains four physical quantities, three mixing angles θ12 , θ23 , θ13 and one CP violating phase δ. The two detected transitions are then part of a larger set of phenomena still to be observed. The measurements of the solar and atmospheric wavelengths respectively imply a best value of 7.6 10−5 eV 2 for the squared mass difference δm212 and a common value of 2.4 10−3 eV 2 , 30 times greater, for the other two differences, δm213 and δm223 . The measured mixing strength is large for both transitions, very much unlike quark transitions. The solar mixing sin2 θ12 turns out to be about 1/3, while the atmospheric mixing sin2 θ23 is even larger, intriguingly close to 1/2, its maximal value. Atmospheric transitions, that behave much as if they were mostly νμ → ντ , fit well terrestrial experiments aiming a ν beam at a massive far ν detector. These experiments can be tightly controlled and precise, we expect now from them the measurement of the three decisive physical quantities that are still unknown. The third mixing parameter sin2 θ13 has not yet been measured. It is not large, an experimental upper bound
of about 3% follows from the failure to detect, so far, νμ → νe transition over the flight path (baseline) of the atmospheric transitions. Detection of that transition or of its inverse transition νe → νμ is essential to establish the 3 by 3 nature of ν mixing. The subdominant νμ ↔ νe transition is also the one where the largest effects from the CP violating phase δ are expected. Its experimental signature would be an asymmetry between ν and ν transition rates and/or an asymmetry between direct and inverse transition rates (time reversal). This subdominant direct or inverse channel is expected to provide also the third and last piece of information still missing, the sign of δm2atm , that is the sign of δm213 and δm223 that determines whether m3 is much larger (normal νe mass hierarchy) or much smaller (inverted hierarchy) than m1 and m2 . An additional asymmetry between ν and ν transition rates on top of the one due to a non zero CPV phase must exist, due to the different interaction of ν and ν with electrons when propagating through matter over large distances. The sign of this asymmetry can tell us what is the correct hierarchy. If νμ ↔ νe promises the largest rewards, all nine possible transitions among ν flavors should be studied. The history of quark mixing shows us that unitarity and invariance properties (CP, T, CPT) of all rows of the ν mixing matrix will ultimately have to be precisely mapped. In practice, the three possible transitions of νe and the three of νμ are accessible to measurements, while transitions of ντ presently appear unfortunately out of experimental reach. Conventional beams have so far provided us with the best tool to study νμ transitions over atmospheric baselines. The limitation of their low rates can be mitigated by assembling larger far detector masses and using higher power proton drivers (superbeams). Up to a point, however. The rate limitation, their irreducible contamination of νe ’s and other experimental complications have lead us to consider novel ν beams. The novel idea common to these novel ν beams is storing and coasting intense beams of longer lived ν parents, beta emitting ions (betabeam) or/and muons (νfactories), accelerated to high energies, inside storage and decay rings that replace the traditional π decay tunnel. In the case of betabeams, long lived beta emitting ions produce pure νe or νe beams for thorough studies of νe transitions. One can envisage to study all the experimentally accessible transitions by a combination of superbeam and betabeam experiments. In the case of ν-factories, the ν parents are muons. μ+ beams produce simultaneous beams of νμ and νe , while
V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
μ− beams do the opposite. As for betabeam, the most rewarding ”golden” channel will be νe → νμ , but νfactories promise eventually the best map of all accessible transitions in a single unique well controlled experimental setup. The price to pay is magnetization of large far detector volumes, necessary to separate experimentally the ν and ν interaction samples. 3. Current generation accelerator studies of neutrino transitions To date, K2K and MINOS [6] have confirmed the original atmospheric result. OPERA may be soon confirming νμ → ντ dominance. Emphasis is therefore now shifting to the subdominant νμ → νe transitions, still using conventional beam coupled with far detectors with good detection efficiency and low background for single electrons. One new experiment exploiting a new conventional beam, T2K [9], is already taking data at JPARC. It still uses the 22 kton fiducial volume of SuperKamioka, 295 Km away. Present beam power (100 Kwatts) already exceeds that available from the K2K beam but it is expected to be steadily improving up to 700 Kwatt. It will be also the first neutrino beam pointing at the far detector at a small off axis angle. This provides a better defined and lower neutrino energy that matches better the 500 Km/GeV wavelength. A second experiment, NOνA [10], using off-axis the Fermilab existing MuMI conventional beam, is in advanced state of preparation. It is assembling a 20 Ktons totally active scintillator detector (TASD) in a new off axis far (800 Km) detector location in Minnesota and will be soon taking data with an increasing fraction of the total detector. Plans do exist to make the beam power available to NOνA increasingly larger than what NuMI has so far being delivering to MINOS, up to 700 KW or so. By 2018 or so, these two experiments will either discover the νμ → νe transition and measure a non zero third mixing parameter sin2 (2θ13 ), thus proving the 3 by 3 nature of the mixing matrix, or will impose on it a 1% or slightly lower upper limit on it. Figure 1 illustrates the 3 σ discovery potential of the third mixing parameter expected from these two accelerator experiments and a number of reactor experiments that are also in preparation.
219
ploiting conventional beams. The key experimental factor for further progress is sizeably larger detector mass, hundreds of KiloTons (KT) of instrumented water or equivalent, and larger, MegaWatts (MW) scale, proton beam power. They must be inevitably the weapons of further attack. Both are quite a challenge. Assembling larger detector mass demands new larger underground sites. Instrumented Water Cerenkov tanks much larger than SuperKamiokande will be a highly non trivial extrapolation. The Liquid Argon technique, that has larger efficiency and lower backgrounds, is only approaching maturity and we do not know yet how and when tanks really larger than ICARUS will become important contributors. It is somewhat obvious but still important, in this context, to remind the relevance of these detectors for the equally fundamental measurement of nucleon lifetime. An upgraded T2K program is being planned, and a few possible candidate sites for new water tanks up to one Megaton are being envisaged. Baselines up to slightly above 1000 Km are being considered. Higher proton beam power is also a clear objective, the new T2K beam was designed to evolve to withstand up to 4 MW. In the last few years, however, concern has emerged about the possibility to go beyond 1.7 MW proton beam power with the JPARC synchrotron. An entirely new NuMI beam, pointing to the new unprecedentedly large DUSEL underground detector location is next flagship project at Fermilab. An essential feature is its longer (1300 Km) baseline. It would be upgradable to operate at 2 or more MW, thanks to a new proton driver, the X Project that is at the heart of Fermilab future strategies. The Deep Underground Science and Engineering Laboratory at Homestake, S. Dakota (DUSEL) lab is now established as the site and for a 300 KTs water tank and/or an equivalent mass of Li-Ar. The new NuMI project plans to go back to a wide band beam so to use (facing the novel challenge that are implied) also the distorsion of the νμ energy spectrum to measure the mixing parameters. Less defined plans do exist in Europe too, where, just as in all regions, the relative merits of large water tanks (MEMPHYS) and of smaller, but more efficient, Li-Ar (GLACIER) tanks are being assessed. 5. Longer term prospects for neutrino transitions
4. Next generation of studies of neutrino transitions Beyond T2K and NOνA, plans are being made to start probing the ν mass hierarchy and CP violation, still ex-
The physics reach expected by a few variants of all mentioned future ν beam facilities, including the novel neutrino facilities that will be further detailed later, are
220
V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222
ĐŽŶǀĞŶƚŝŽŶĂů ďĞĂŵƐ ͗WŚĂƐĞ /
sin22θ θ13 discovery at 3σ σ CL NF μ beams
β beams
limits of π beams rate beam+detect bkgnds π beams
s13eiδ coupling
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Figure 1: Expected 3 σ discovery potential of the third mixing parameter from T2K, NOνA and a number of reactor experiments that are also in preparation, for the normal ν mass hierarchy (NH).
summarized in the next three figures 2, 3, 4. Produced by the joint physics subgroups of IDS-NF and EUROnu, these figures were included in a recent report [11] from the Neutrino Panel of the CERN Scientific Policy Committee (SPC). Figure 2 shows the 3 σ discovery potential of sin2 (2θ13 ). It depends on the fraction of values of the CPV phase that can be simultaneously accessed. The limits of pion decay beams, due to their low rates and their irreducible νe content, are evident. Figure 3 shows the 3 σ discovery potential of the sign of δm2atm , ie of the correct ν mass hierarchy. A long baseline (LBL) is essential here, modest baseline superbeam experiments have only limited or no reach. Figure 4 shows the 3 σ discovery potential of a CP violating phase δ. The SPC Panel concluded that it is unrealistic to expect a high intensity ν source of any kind in Europe before 2020. On that time scale, Europe should rather pay close attention to the mentioned superbeam programs in Japan and USA. To be competitive in the 2020’s, Europe should concentrate on the R&D for a new intense source, ν factory or betabeam, decay ring facilities capable to bring about neutrino beams of substantially higher neutrino flux and free of wrong flavor backgrounds. An outstanding proton driver must inevitably be the motor of such a scientific program. At CERN a 4 MW (or more) high power superconducting proton linac (HP SPL) has been and is the subject of extensive and today now rather advanced study. It is the natural continuation of the so called LINAC-4, presently under construction. It promises even more potential than the present design of the Project X proton linac at Fermilab that in turn appears quite superior to the potential of the JPARC up-
Figure 2: Expected 3 σ discovery potential of the third mixing parameter of a few variants of all the future ν beam facilities mentioned in the text.
grade program. ν factory or betabeam are two different, possibly complementary, longer term approaches. Betabeam neutrinos are of lower energy and should be coupled with detectors hundreds of Km away. Neutrino factory neutrinos are of higher energies and are best exploited by detectors thousands of Km away. Design reports for both facilities are being prepared for 2012 by the EUROnu design study, that, in the neutrino factory case, operates within the larger international context of the IDS. Design will include the engineering work necessary to evaluate safety infrastructures and, last but not least, costs. We will soon have more realistic ideas of the feasibility of these options and of the global effort implied by their realization.
6. Betabeams Details will be plentiful in E. Wildner’s talk. There are many variants of betabeam envisaged, only two are presently seriously being studied. Very large Water Cerenkov and Li-argon detectors are again the natural partners of such betabeams. The so called baseline betabeam envisages to produce ν (ν) from 6 He (18 Ne) radioactive ions accelerated to γ = 100. Neutrinos have a few hundred MeV of energy, suited to CERN-Frejus like baselines. This is the most mature and solid option that has now been object of several years of studies and can now be seen as almost established. The second variant being studied envisages to produce ν (ν) from higher Q longer lived 8 Be (8 B) radioactive ions still accelerated to γ = 100, with 1 GeV or so
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sign Δm σ CL Δ 2atm discovery at 3σ
NuFact’s LBL π beam
Modest BL π and β
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Figure 3: Expected 3 σ discovery potential of the sign of δm2atm , ie of the correct ν mass hierarchy, of a few variants of all the future ν beam facilities mentioned in the text.
energy suited to CERN-Gran Sasso like baselines. EUROnu is its first exploratory study. The physics potential of other very very promising betabeam options has been pointed out. It has been shown that higher γ can be extremely rewarding. But the additional technical challenges involved will not yet be seriously studied. 7. Neutrino Factories Again, details will be plentiful in K. Long’s talk. A well defined baseline neutrino factory design has been taking shape over more than 10 years. First feasibility studies in the US at the turn of the century have been followed by an International Scoping Study first and now the International Design Study (IDS). That includes the large magnetic detector indispensable to exploit the factory. The concept of a 50-100 Kton magnetized iron neutrino detector (MIND) has been developed, a large version of MINOS or, if one prefers, a very large version of the glorious CDHS detector. The European contributions have been important, in particular those of the UK component. It is a pleasure to recognize, however, the driving push coming from
the US, first via the NFMCC1 , then also via the MTF2 at Fermilab where a dedicated Muon Test Area has been built and operates now. NFMCC anf MTF have recently merged in the US Muon Accelerator Program (MAP), even more solidly based and better funded. A muon based view of Fermilab future is the real drive behind it, a neutrino factory being a possible step on the way to a muon collider that could really be the long term future of the Fermilab laboratory. 8. Conclusions The long term accelerator neutrino program outlined, second generation conventional (super)beams first and ultimate neutrino production rings later, can be sustained only by a strengthened, very enthusiastic and well supported R&D program. The several R&D demonstration projects in progress are largely international, in particular for the ν factory, and Europe should definitely do more in this sector. We will not be ready, otherwise, when superior neutrino beams will be needed. 1 Neutrino 2 Muon
Factory and Muon Collider Collaboration Task Force
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Figure 4: Expected 3 σ discovery potential of a non zero CP violating phase δ of a few variants of all the future ν beam facilities mentioned in the text. It includes the caption adopted for the SPC Panel report, detailing the meaning of each line.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
K. Sakashita’s contribution to these Proceedings. R. Svoboda’s contribution to these Proceedings. K. Long’s contribution to these Proceedings. S. Parke’s contribution to these Proceedings. E. Wildner’s contribution to these Proceedings. B. Vahle’s contribution to these Proceedings. O. Sato’s contribution to these Proceedings. A. Guglielmi’s contribution to these Proceedings. T. Kobayashi’s contribution to these Proceedings. K. Heller’s contribution to these Proceedings. CERN Yellow Report 2010-003, ISSN 0007-8328, ISBN 97892-9083-354-3