\section{Motivations} There is a steady 25 year long tradition of water \v{C}erenkov observatories having produced an incredibly rich harvest of seminal discoveries. The water \v{C}erenkov movement was started in the early 80's by the scientists searching for proton decay. It fulfilled indeed this purpose by extending the proton decay lifetimes a few orders of magnitude. Furthermore, water \v{C}erenkov's, through a serendipitous turn, as frequently happens in physics, have also inaugurated: \begin{itemize} \item particle astrophysics through the detection of the neutrinos coming from the explosion of the supernova 1987a by IMB and Kamioka, acknowledged by the Nobel prize for Koshiba \item the golden era of neutrino mass and oscillations by discovering hints for atmospheric neutrino oscillations while at the same time confirming earlier solar neutrino oscillation results. \end{itemize} The latest in the water \v{C}erenkov series, the well known Super-Kamiokande, has now given strong evidence for a maximal oscillation between \numu and $\nu_\tau$, and several projects with accelerators have been designed to check this result. The results of the K2K experiment confirm the oscillation, and other experiments (MINOS in the USA, OPERA and ICARUS at Gran Sasso) should refine most of the oscillation parameters by 2010. More recently, after the results from SNO and KamLAND, a solid proof for solar neutrino flavour oscillations governed by the so-called LMA solution has been established. We can no longer escape the fact that neutrinos have indeed a mass, although the absolute scale is not yet known. Furthermore, the large mixing angles of the two above-mentioned oscillations and their relative frequencies open the possibility to test CP violation in the neutrino sector if the third mixing angle, $\theta_{13}$, is not vanishingly small (we presently have only an upper limit at about 0.2 on $\sin^2(2\theta_{13})$, provided by the CHOOZ experiment). Such a violation could have far reaching consequences, since it is a crucial ingredient of leptogenesis, one of the presently preferred explanations for the matter dominance in our Universe. The ideal tool for these studies is thought to be the so-called neutrino factory, which would produce through muon decay intense neutrino beams aimed at magnetic detectors placed several thousand kilometers away from the neutrino source. However, such projects would probably not be launched unless one is sure that the mixing angle $\theta_{13}$, governing the oscillation between \numu and \nue at the higher frequency, is such that this oscillation is indeed observable. This is why physicists have considered the possibility of producing new conventional neutrino beams of unprecedented intensity, made possible by recent progress on the conception of proton drivers with a factor 10 increase in power (4 MW compared to the present 0.4 MW of the FNAL beam). While the present limit on $\sin^2(2\theta_{13})$ is around 0.2, these new neutrino ``superbeams'' would explore $\sin^2(2\theta_{13})$ down to $2\cdot 10^{-3}$ (i.e a factor 100 improvement on the \numu - \nue oscillation amplitude). European working groups have studied a neutrino factory at CERN for some years, based on a new proton driver of 4 MW, the SPL. Along the lines described above, a subgroup on neutrino oscillations has studied the potentialities of a neutrino superbeam produced by the SPL. The energy of produced neutrinos is around 300 MeV, so that the ideal distance to study \numu to \nue oscillations happens to be 130 km, that is exactly the distance between CERN and the existing Fr{\'e}jus laboratory. The present laboratory cannot house a detector of the size needed to study neutrino oscillations, which is around 1 million cubic meters. But the recent decision to dig a second gallery, parallel to the present tunnel, offers a unique opportunity to realize the needed extension for a reasonable price. Due to the schedule of the new gallery, a European project would be competitive only if the detector at Fr{\'e}jus reaches a sensitivity on $\sin^2(2\theta_{13})$ around $10^{-3}$, since other projects in Japan (T2K phase 1) and USA (NoVA) will have reached $10^{-2}$ by 2015. The working group has then decided to study directly a water \v{C}erenkov detector with a mass approaching 1 megaton, necessary to reach the needed sensitivity. This detector has been nicknamed MEMPHYS (for MEgaton Mass PHYSics). Its study has benefited from a similar study by our American colleagues, the so-called UNO detector with a total mass of 660 kilotons. Simulations have shown that the sensitivity on $\sin^2(2\theta_{13})$ at a level of $10^{-3}$ could indeed be fulfilled with MEMPHYS. %Of course, our This version of the project %is not the only one, as two competitors, since japanese and american physicists have their own project, with similar potentialities. But owing to a new idea recently proposed by Piero Zucchelli, the european project could have a unique characteristics which would make it very appealing. This idea is to send towards Fr{\'e}jus, together with the SPL superbeam, another kind of neutrino beam, called beta beam, made of \nue or \nubare produced by radioactive nuclei stored in an accumulation ring. CERN has a very good expertise on the production and acceleration of radioactive nuclei. Studies show that such beams would reach performances even better than those of the SPL on the oscillation between \nue and \numu, with a sensitivity on $\theta_{13}$ down to half a degree, with a factor four gain. But the main point is that both beams, if run simultaneously, would allow to study the violation of CP symmetry in a much more efficient and redundant way than when using only the SPL beam. This peculiarity, which would be a CERN exclusivity, would give a considerable bonus to our project concerning neutrino studies, since it could reach sensitivities on CP violation as good as those of a neutrino factory for $\sin^2(2\theta_{13})$ above $5\cdot 10^{-3}$. As mentioned in the beginning, such a detector will not only do the physics of neutrino oscillations, but would also address equally fundamental questions in particle physics and particle astrophysics. In particular, such a detector could reach a sensitivity around $10^{35}$ years on the proton lifetime, which is precisely the scale at which such decays are predicted by most supersymetric or higher dimension grand unified theories, thus giving the hope for a fundamental discovery. Such a detector would also bring a wealth of information on supernova explosions: it would detect more than $10^5$ neutrino interactions within a few seconds if such an explosion occurs in our galaxy, and would observe a statisticaly significant signal for explosions at distances up to 1 Mpc, and provide a supernova trigger to other astroparticle detectors (gravitational antennas and neutrino telescopes). For galactic supernova explosions, the huge available statistics would give access to a detailed description of the collapse mechanism and neutrino oscillation parameters. In addition, the huge mass of the detector could allow to detect for the first time the diffuse neutrinos from past SN explosions. The proposed detector is indeed a multipurpose detector addressing several issues of utmost importance.