1 | \section{Motivations} |
---|
2 | |
---|
3 | There is a steady 25 year long tradition of |
---|
4 | water \v{C}erenkov observatories having produced an |
---|
5 | incredibly rich harvest of seminal discoveries. |
---|
6 | The water \v{C}erenkov movement was started in the early 80's |
---|
7 | by the scientists searching for proton decay. |
---|
8 | It fulfilled indeed this purpose by extending the proton decay |
---|
9 | lifetimes a few orders of magnitude. Furthermore, water \v{C}erenkov's, |
---|
10 | through a serendipitous turn, as frequently happens in physics, |
---|
11 | have also inaugurated: |
---|
12 | \begin{itemize} |
---|
13 | \item particle astrophysics through the detection of the neutrinos |
---|
14 | coming from the explosion of the supernova 1987a |
---|
15 | by IMB and Kamioka, acknowledged by the Nobel prize for Koshiba |
---|
16 | \item the golden era of neutrino mass and oscillations by discovering |
---|
17 | hints for atmospheric neutrino oscillations while at the same time |
---|
18 | confirming earlier solar neutrino oscillation results. |
---|
19 | \end{itemize} |
---|
20 | The latest in the water \v{C}erenkov series, |
---|
21 | the well known Super-Kamiokande, has now given strong evidence |
---|
22 | for a maximal oscillation between |
---|
23 | \numu and $\nu_\tau$, |
---|
24 | and several projects with accelerators have |
---|
25 | been designed |
---|
26 | to check this result. The results of the K2K |
---|
27 | experiment confirm the oscillation, and other experiments (MINOS in the USA, |
---|
28 | OPERA and ICARUS at Gran Sasso) should refine most of |
---|
29 | the oscillation parameters by 2010. |
---|
30 | |
---|
31 | More recently, after the results from SNO and KamLAND, a solid |
---|
32 | proof for solar neutrino flavour oscillations governed by the |
---|
33 | so-called LMA solution has been established. We can no longer |
---|
34 | escape the fact that neutrinos have indeed a mass, although the |
---|
35 | absolute scale is not yet known. Furthermore, the large mixing |
---|
36 | angles of the two above-mentioned oscillations and their relative |
---|
37 | frequencies open the possibility to test CP violation in the |
---|
38 | neutrino sector if the third mixing angle, $\theta_{13}$, is not |
---|
39 | vanishingly small (we presently have only an upper limit at about |
---|
40 | 0.2 on $\sin^2(2\theta_{13})$, provided by the CHOOZ experiment). Such a |
---|
41 | violation could have far reaching consequences, since it is |
---|
42 | a crucial ingredient of leptogenesis, one of the |
---|
43 | presently preferred explanations for the matter dominance in our |
---|
44 | Universe. |
---|
45 | |
---|
46 | The ideal tool for these studies is thought to be the so-called neutrino |
---|
47 | factory, which would produce through muon decay intense neutrino beams |
---|
48 | aimed at magnetic detectors placed several thousand kilometers away from |
---|
49 | the neutrino source. |
---|
50 | |
---|
51 | However, such projects would probably not be launched unless |
---|
52 | one is sure that the mixing angle $\theta_{13}$, governing the oscillation |
---|
53 | between \numu and \nue at the higher frequency, is such that this oscillation |
---|
54 | is indeed observable. |
---|
55 | This is why physicists have considered the possibility |
---|
56 | of producing new conventional neutrino beams of unprecedented intensity, made |
---|
57 | possible by recent progress on the conception of proton drivers with a factor |
---|
58 | 10 increase in power (4 MW compared to the present 0.4 MW of the FNAL beam). |
---|
59 | While the present limit on $\sin^2(2\theta_{13})$ is around 0.2, |
---|
60 | these new neutrino |
---|
61 | ``superbeams'' would explore $\sin^2(2\theta_{13})$ down to |
---|
62 | $2\cdot 10^{-3}$ (i.e a factor |
---|
63 | 100 improvement on the \numu - \nue oscillation amplitude). |
---|
64 | |
---|
65 | European working groups have studied a neutrino factory at CERN |
---|
66 | for some years, based on a new proton driver of 4 MW, the SPL. |
---|
67 | Along the lines described above, a subgroup on neutrino |
---|
68 | oscillations has studied the potentialities of a neutrino |
---|
69 | superbeam produced by the SPL. The energy of produced neutrinos is |
---|
70 | around 300 MeV, so that the ideal distance to study \numu to \nue |
---|
71 | oscillations happens to be 130 km, that is exactly the distance |
---|
72 | between CERN and the existing Fr{\'e}jus laboratory. The present |
---|
73 | laboratory cannot house a detector of the size needed to study |
---|
74 | neutrino oscillations, which is around 1 million cubic meters. But |
---|
75 | the recent decision to dig a second gallery, parallel to the |
---|
76 | present tunnel, offers a unique opportunity to realize the needed |
---|
77 | extension for a reasonable price. |
---|
78 | |
---|
79 | Due to the schedule of the new gallery, a European project would |
---|
80 | be competitive only if the detector at Fr{\'e}jus reaches a |
---|
81 | sensitivity on $\sin^2(2\theta_{13})$ around $10^{-3}$, since other projects in |
---|
82 | Japan (T2K phase 1) and USA (NoVA) will have reached $10^{-2}$ |
---|
83 | by 2015. The working group has then decided to study |
---|
84 | directly a water \v{C}erenkov detector with a mass approaching 1 megaton, |
---|
85 | necessary to reach the needed sensitivity. This detector has been |
---|
86 | nicknamed MEMPHYS (for MEgaton Mass PHYSics). Its study has benefited from a |
---|
87 | similar study by our American colleagues, the so-called UNO |
---|
88 | detector with a total mass of 660 kilotons. Simulations have shown |
---|
89 | that the sensitivity on $\sin^2(2\theta_{13})$ at a level of $10^{-3}$ |
---|
90 | could indeed be fulfilled with MEMPHYS. |
---|
91 | |
---|
92 | %Of course, our |
---|
93 | This version of the project |
---|
94 | %is not the only one, |
---|
95 | as two competitors, |
---|
96 | since japanese and |
---|
97 | american physicists have their own project, with similar |
---|
98 | potentialities. But owing to a new idea recently proposed |
---|
99 | by Piero Zucchelli, the european project could have a unique |
---|
100 | characteristics which would make it very appealing. This idea |
---|
101 | is to send towards Fr{\'e}jus, together with the SPL superbeam, |
---|
102 | another kind of neutrino beam, called beta beam, made of \nue or \nubare |
---|
103 | produced by radioactive nuclei stored in an accumulation ring. |
---|
104 | CERN has a very good expertise on the production and acceleration |
---|
105 | of radioactive nuclei. Studies show that such beams would reach |
---|
106 | performances even better than those of the SPL on the oscillation |
---|
107 | between \nue and \numu, with a sensitivity on $\theta_{13}$ down to half a |
---|
108 | degree, with a factor four gain. |
---|
109 | But the main point is that both beams, if |
---|
110 | run simultaneously, would allow to study the violation of CP |
---|
111 | symmetry in a much more efficient and redundant |
---|
112 | way than when using only the SPL |
---|
113 | beam. This peculiarity, which would be a CERN exclusivity, would |
---|
114 | give a considerable bonus to our project concerning neutrino |
---|
115 | studies, since it could reach sensitivities on CP violation as |
---|
116 | good as those of a neutrino factory for $\sin^2(2\theta_{13})$ above |
---|
117 | $5\cdot 10^{-3}$. |
---|
118 | |
---|
119 | As mentioned in the beginning, |
---|
120 | such a detector will not only do the physics of neutrino oscillations, |
---|
121 | but would also address equally fundamental questions in |
---|
122 | particle physics and particle astrophysics. |
---|
123 | |
---|
124 | In particular, such a detector could |
---|
125 | reach a sensitivity around $10^{35}$ years on the proton lifetime, which is |
---|
126 | precisely the scale at which such decays are predicted by most supersymetric |
---|
127 | or higher dimension grand unified theories, thus giving the hope |
---|
128 | for a fundamental discovery. |
---|
129 | |
---|
130 | Such a detector would also bring a wealth of information on supernova |
---|
131 | explosions: it would detect more than $10^5$ neutrino interactions |
---|
132 | within a few seconds if such an explosion occurs in our galaxy, and |
---|
133 | would observe a statisticaly significant signal for explosions at distances |
---|
134 | up to 1 Mpc, and provide a supernova trigger to other astroparticle detectors |
---|
135 | (gravitational antennas and neutrino telescopes). |
---|
136 | For galactic supernova explosions, the huge available |
---|
137 | statistics would give access to a detailed description of |
---|
138 | the collapse mechanism |
---|
139 | and neutrino oscillation parameters. In addition, the huge mass of the detector |
---|
140 | could allow to detect for the first time |
---|
141 | the diffuse neutrinos from past SN explosions. |
---|
142 | |
---|
143 | The proposed detector is indeed a multipurpose detector addressing several |
---|
144 | issues of utmost importance. |
---|