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\title{Large underground, liquid based detectors for astro-particle physics in Europe: scientific case and prospects}
%

\author{First Author$^a$, Second Author$^b$\thanks{Corresponding
author.}~ and Third Author$^b$\\
\llap{$^a$}Name of Institute,\\
  Address, Country\\
\llap{$^b$}Name of Institute,\\
  Address, Country\\
  E-mail: \email{CorrespondingAuthor@email.com}}




\abstract{

This document reports on a series of experimental and theoretical studies conducted to 
assess the astro-particle physics potential of three future large-scale particle detectors 
proposed in Europe as next generation underground observatories. 

}%end of abstract

%\pacs{13.30.a,14.20.Dh,14.60.Pq,26.65.t+,29.40.Gx,29.40.Ka,29.40.Mc,95.55.Vj,95.85.Ry, 
%97.60.Bw}

%\submitto{Journal of Cosmology and Astroparticle Physics}

\keywords{Keyword1; Keyword2; Keyword3}

\begin{document}
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\bibliographystyle{Campagne}

\section{Physics motivation}
\label{sec:Phys-Intro}

Several outstanding physics goals could be achieved by the next generation of large underground observatories
in the domain of astro-particle and particle physics, neutrino astronomy and cosmology.
Proton decay \cite{Pati:1973rp}, in particular, is one of the most exciting prediction of Grand Unified Theories 
(for a review see \cite{Nath:2006ut}) aiming at the 
unification of fundamental forces in Nature. It remains today one of the most relevant open questions
of particle physics. Its discovery would certainly represent a fundamental milestone, contributing to clarifying our
understanding of the past and future evolution of the Universe.  


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Several experiments have been built and conducted to search for proton decay but they only yielded lower limits to the proton lifetime. 
The window between the predicted proton lifetime (in the simplest models typically below $10^{37} $ years) and that excluded 
 by experiments \cite{Kobayashi:2005pe}
($O$($10^{33}$) years, depending on the channel) is within reach, 
and the demand to fill the gap grows with the progress in other domains of particle physics, astro-particle physics and cosmology. 
To some extent, also a negative result from next generation high-sensitivity experiments
would be relevant to rule-out some of the
theoretical models based on SU(5) and SO(10) gauge symmetry or to further constrain the range of allowed parameters.
Identifying unambiguously proton decay and measuring its lifetime would set a firm scale for any Unified Theory, narrowing
the phase space for possible models and their parameters. This will be a mandatory step to go forward 
beyond the Standard Model of elementary particles and interactions.

Another important physics subject is the physics of 
%natural (A. Mirizzi 15may07)
astrophysical
neutrinos, as those from supernovae, from the Sun and from the interaction of primary cosmic-rays with the Earth's atmosphere. Neutrinos are above all important messengers from stars. 
Neutrino astronomy has a glorious although recent history, from the detection of solar neutrinos 
 \cite{Davis:1968cp,Hirata:1989zj,Anselmann:1992um,Abdurashitov:1994bc,Smy:2002rz,Aharmim:2005gt,Altmann:2005ix} 
to the observation of neutrinos from supernova explosion, \cite{Hirata:1987hu,Bionta:1987qt,Alekseev:1988gp},
acknowledged by the Nobel Prizes awarded to M. Koshiba and R. Davis.
These observations have given valuable information for a better understanding of the functioning
of stars and of the properties of neutrinos. However, much more information could be obtained if the energy spectra of
stellar neutrinos were known with higher accuracy.
Specific neutrino observations could give detailed information on the conditions of the production zone, 
whether in the Sun or in a supernova. 
A supernova explosion in our galaxy would be extremely important as the evolution mechanism of the collapsed star 
is still a puzzle for astrophysics.
An even more fascinating challenge would be observing neutrinos from extragalactic supernovae, either from identified sources 
or from a diffuse flux due to unidentified past supernova explosions.

Observing neutrinos produced in the atmosphere as cosmic-ray secondaries
\cite{Aglietta:1988be,Hirata:1988uy,Hirata:1992ku,Becker-Szendy:1992hq,Daum:1994bf,Allison:1999ms,Ashie:2005ik} 
gave the first compelling evidence
for neutrino oscillation \cite{Fukuda:1998mi,Kajita:2006cy}, a process that unambiguously points to the existence of new physics. 
While today the puzzle of missing atmospheric neutrinos can be considered solved, 
there remain challenges related to the sub-dominant oscillation phenomena. In particular, precise measurements of
atmospheric neutrinos with high statistics and small systematic errors \cite{TabarellideFatis:2002ni}
would help in resolving ambiguities and degeneracies that hamper the interpretation 
of other experiments, as those planned for future long baseline neutrino oscillation measurements.

Another example of outstanding open questions is that of the knowledge of the interior of the Earth.  
It may look hard to believe, but we know much better what happens inside the Sun than inside our own planet. 
There are very few messengers that can provide information, while a mere theory is not sufficient for building a credible model for the Earth. However, there is a new unexploited window to the Earth's interior,
by observing neutrinos produced in the radioactive decays of heavy elements in the matter. Until now, only the KamLAND 
experiment  \cite{Araki:2005qa} has been able to study these so-called geo-neutrinos opening the way to a completely new
field of research.  The small event rate, however,  does not allow to draw significant conclusions. 

The fascinating physics phenomena outlined above, in addition to other important subjects that we will address in the following,
could be investigated by a new generation of multipurpose 
experiments based on improved detection techniques. 
The envisioned detectors must necessarily be very massive (and consequently large) 
due to the smallness of the cross-sections and to the low rate of signal events,
and able to provide very low experimental background. 
The required signal to noise ratio can only be achieved in underground laboratories suitably shielded against cosmic-rays
and environmental radioactivity.
We can identify three different and, to large extent, complementary technologies capable to meet the challenge, based
on large scale use of liquids for building large-size, volume-instrumented detectors

\begin{itemize}
\item Water Cherenkov.
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;  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.

\item Liquid scintillator.
Experiments using a 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 to the Borexino
\cite{Back:2004zn} and KamLAND \cite{Araki:2004mb} experiments. 


\item 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 on such detectors has been gained within the ICARUS project \cite{Amerio:2004ze,Arneodo:2001tx}.
\end{itemize}

Three experiments are proposed to employ the above detection techniques: MEMPHYS \cite{deBellefon:2006vq} for WC,
LENA \cite{Oberauer:2005kw, Marrodan:2006} for liquid scintillator 
and GLACIER \cite{Rubbia:2004tz,Rubbia:2004yq,Ereditato:2004ru,Ereditato:2005ru,Ereditato:2005yx} for Liquid Argon. 
In this paper we report on the study of the physics potential of the experiments and identify features of complementarity 
amongst the three techniques. 

Needless to say, the availability of future neutrino beams from particle accelerators 
would provide an additional bonus to the above experiments.
Measuring oscillations with artificial neutrinos (of well known kinematical features)
with a sufficiently long baseline would allow to accurately determine the oscillation parameters
(in particular the mixing angle $\theta_{13}$ and the possible
CP violating phase in the mixing matrix). 
The envisaged detectors may then be used for observing neutrinos from the future Beta Beams and Super Beams
in the optimal energy range for each experiment. A common example 
%C Volpe 19/10/07 is a low-energy Beta Beam from CERN to MEMPHYS at Frejus, 130 km away 
is a Beta Beam from CERN to MEMPHYS at Frejus, 130 km away \cite{Campagne:2006yx}. 
High energy beams have been suggested \cite{Rubbia:2006pi}, 
favoring longer baselines of up to $O$(2000~km). 
%add C. Volpe review
An exhaustive review on the different Beta Beam scenario can be found in the reference \cite{Volpe:2006in}. 
The ultimate Neutrino Factory facility will require a magnetized detector to fully exploit the simultaneous availability of 
neutrinos and antineutrinos. This subject is however beyond the scope of the present study.

Finally, there is a possibility of (and the hope for) unexpected
discoveries. The history of physics has shown that 
several experiments have made their glory with discoveries in research fields that were outside the original goals of the experiments. 
Just to quote an example, we can mention the Kamiokande detector, mainly designed to search for proton decay
and actually contributing to the observation of atmospheric neutrino oscillations, to the clarification of the solar neutrino puzzle and 
to the first observation of supernova neutrinos \cite{Hirata:1987hu,Hirata:1988ad,Hirata:1989zj,Hirata:1988uy,
Fukuda:1998mi}.
All the three proposed experiments, thanks to their
outstanding boost in mass and performance, will certainly provide a significant potential for surprises and unexpected discoveries.

%
\acknowledgments
%\begin{acknowledgments}

We wish to warmly acknowledge support from all the various funding agencies.  We wish to thank the EU framework 6 project ILIAS for providing assistance particularly regarding underground site aspects (contract 8R113-CT-2004-506222).

%\end{acknowledgments}
\newpage
\section*{References}
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