\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.
