\subsubsection{Options for Far Detectors}

\paragraph{Totally Active Scintillating Detector (TASD)}

One option to the MIND for the Neutrino Factory detector is to build a totally active scintillator detector (TASD).
Although one could consider using TASD for the baseline Neutrino Factory with 25 GeV muon storage rings, TASD has
been primarily studied as an option for a lower energy machine (4--5 GeV).  For this scenario, the detector must have
excellent neutrino event detection efficiency down to interaction event energies of roughly 500 MeV.
Recently new ideas have made plausible large cost-effective detectors \cite{lenf1,lenf2}
for lower energy NF�s, and initial studies of the corresponding physics reach
suggest that an experiment with a 4--5 GeV Neutrino Factory would provide an impressive sensitivity
to the oscillation parameters. Furthermore, compared with higher energy facilities, a low-energy
NF would require a less expensive acceleration scheme and a cheaper storage ring that is easier to site with respect to
underground engineering issues.

\subparagraph{Overview}

As mentioned above, a Totally Active Scintillator Detector with a total mass is being investigated as a possible detector for a Neutrino Factory, both the high-energy (25 GeV stored $\mu$ beam) %\cite{Abe:2007bi} implementation and the low-energy option (4-5 GeV stored $\mu$ beam) \cite{lenf1,lenf2}.  This detector requires some new technologies and presents many challenges.  However, its performance is exceptional.   The TASD that has been studied for Neutrino Factory applications must, of course, be magnetized and has a segmentation that is approximately 10 times that of NO$\nu$A \cite{Paley:2008zza}. Magnetization of such a large volume ($>30,000$~m$^3$) is an additional technical challenge. 

The Neutrino Factory TASD being considered consists of plastic scintillator bars with a triangular cross-section arranged in planes which make x and y measurements.  Optimization of the cell cross section still needs further study, but for now we have chosen the MINER$\nu$A extrusion design \cite{McFarland:2006pz} .  The scintillator bars have a length of $15$~m and the triangular cross-section has a base of $3$~cm and a height of $1.5$~cm.  

Magnetizing the large detector volume presents a significant technical challenge for a Neutrino Factory TASD.  Conventional room temperature magnets are ruled out due to their prohibitive power consumption, and conventional superconducting magnets are too expensive, due to the cost of the enormous cryostats needed in a conventional superconducting magnet design. In order to eliminate 
the cryostat, we have investigated a concept based on the  superconducting transmission line (STL), the technology chosen for field excitation in MIND. The solenoid windings now consist of this superconducting cable which is confined 
in its own cryostat (Fig.~\ref{Fig:STL}).  Each solenoid ($10$ are required for the full detector) consists of $150$ turns and requires $7500$ m of STL. There is no large vacuum vessel and thus no large vacuum loads which make the cryostats for large conventional superconducting magnets so expensive. The 10 solenoids forming what we have called a ``Magnetic Cavern" is shown in Fig. ~\ref{fig:MagCav}.

\begin{figure}[ht]
\begin{center}
\includegraphics[width=0.7\textwidth,bb=0 0 858 616]{03-DetWG/figures/Mag_cav_sim.jpg}
\end{center}
\caption{Schematic of the Magnetic Cavern concept.}
\label{fig:MagCav}
\end{figure}

The Neutrino Factory TASD basic response has been simulated with GEANT4 version 8.1.  The GEANT4 model (Fig.~\ref{fig:tasd}) of the detector included each of the individual scintillator bars, but did not include edge effects on light collection, or the effects of a central wavelength shifting fiber. A uniform 0.5 Tesla magnetic field was simulated.

\begin{figure}[ht]
\begin{center}
\includegraphics[width=0.7\textwidth,bb=0 0 446 353]{03-DetWG/figures/TASD_sch.jpg}
\end{center}
\caption{Schematic of the Totally Active Scintillator Detector.}
\label{fig:tasd}
\end{figure}

 
Samples of isolated muons in the  momentum range between $100$~MeV$/c$ and $15$~GeV$/c$ were simulated to allow for the determination of the detector's momentum resolution and charge identification capabilities. The NUANCE \cite{Casper:2002sd} event generator was used to simulate 1 million $\nu_e$ and 1 million $\nu_\mu$ interactions. Events were generated in $50$ mono-energetic neutrino energy bins between $100$~MeV and $5$~GeV. The results that follow only have one thousand events processed through the full GEANT4 simulation and reconstruction.
 
The detector response was simulated assuming a light yield consistent with MINER$\nu$A measurements and current photo detector performance \cite{Paley:2008zza}. In addition, a 2 photo-electron energy resolution was added through Gaussian smearing. Since  a complete pattern recognition algorithm was beyond the scope of this initial study, the Monte Carlo information was used to aid in pattern recognition. All digitized hits from a given simulated particle where the reconstructed signal was above 0.5 photo electrons were collected. When using the isolated particles, hits in neighboring x and y planes were used to determine the 3 dimensional position of the particle. The position resolution was found to be approximately $4.5$~mm RMS with a central Gaussian of width $2.5$~mm. These space points were then passed to the RecPack Kalman track fitting package \cite{CerveraVillanueva:2004kt}. 
 
For each collection of points, the track fit was performed with an assumed positive and negative charge. The momentum resolution and charge misidentification rates were determined by studying the fitted track in each case which had the better $\chi^2$ per degree of freedom. Figure~\ref{fig:tasd-mom} shows the momentum resolution as a function of muon momentum. 

\begin{figure}[ht]
\begin{center}
\includegraphics[width=0.5\textwidth]{03-DetWG/figures/pres.eps}
\end{center}
\caption{TASD momentum resolution as a function of the muon momentum.}
\label{fig:tasd-mom}
\end{figure}

As a tracker, TASD achieves a resolution of better than $10\%$ over the momentum range studied. Figure~\ref{fig:Track} (left) shows the efficiency for reconstructing positive muons as a function of the initial muon momentum. The detector becomes fully efficient above $400$~MeV.  The charge mis-identification rate was determined by counting the rate at which the track fit with the incorrect charge had a better $\chi^2$ per degree of freedom than that with the correct charge. Figure~\ref{fig:Track} (right) shows the charge mis-identification rate as a function of the initial muon momentum.

\begin{figure}[ht]
\begin{center}
\includegraphics[width=0.49\textwidth]{03-DetWG/figures/fig5a.eps}
\includegraphics[width=0.49\textwidth]{03-DetWG/figures/fig5b.eps}
\end{center}
\caption{Left: Efficiency for reconstructing positive muons. Right: Muon charge mis-identification rate as a function of the initial muon momentum.}
\label{fig:Track}
\end{figure}


Based on these initial studies, TASD meets the required muon charge mis-ID rate criterion needed for physics at a Neutrino Factory and exhibits exceptional tracking and momentum resolution.  In order to fully understand the potential performance of TASD at a Neutrino Factory, a Monte Carlo study with full pattern recognition reconstruction and full neutrino event reconstruction must be performed, as it has been done for MIND.  This will allow for a detailed and quantitative understanding of backgrounds and thus present a realistic estimate of the detector's full potential at a Neutrino Factory.

%\begin{figure}[ht]
%\begin{center}
%\includegraphics[width=\textwidth]{03-DetWG/figures/fig5b.eps}
%\end{center}
%\caption{Muon charge mis-identification rate as a function of the initial muon momentum.}
%\label{fig:Track2}
%\end{figure}


\subparagraph{Performance}

In this section we will show the sensitivity to the neutrino oscillation parameters which can be obtained by a 20 kton Totally Active Scintillating Detector (TASD) used in conjunction with a low-energy neutrino factory. This setup was originally considered in Refs.~\cite{lenf1,lenf2} and later in Refs.~\cite{LENF,mythesis,mynsi}, in which full details can be found. As explained in the main text, we considered the possibility of the addition of the platinum channels ($\nu_{\mu}\rightarrow\nu_{e}$ and $\bar{\nu}_{\mu}\rightarrow\bar{\nu}_{e}$) in addition to the usual golden channels ($\nu_{e}\rightarrow\nu_{\mu}$ and $\bar{\nu}_{e}\rightarrow\bar{\nu}_{\mu}$). The main assumptions used to simulate the TASD are as follows: we assume an energy threshold of 0.5 GeV with a detection efficiency of $94\%$ above 1 GeV and $74\%$ below 1 GeV for $\nu_{\mu}$ and $\bar{\nu}_{\mu}$ with a background of $10^{-3}$. For $\nu_{e}$ and $\bar{\nu}_{e}$ we assume an efficiency of $47\%$ above 1 GeV and $37\%$ below 1 GeV with a background of $10^{-2}$. The main sources of background are assumed to arise from charge mis-identification and neutral current events. The energy resolution is assumed to be $10\%$ for all channels. A 100 kton liquid argon (LAr) detector was also simulated, with the assumptions used to simulate the optimistic performance being the same as for the TASD, so that the optimistic performance of a 100 kton LAr detector coincides roughly with that of a 100 kton TASD. 

In Fig.\ref{fig:exps} we show the performance of the 20 kton TASD to standard oscillation parameters when used with a low-energy neutrino factory with a single baseline of 1300 km and assuming a muon flux of $1.4\times10^{21}$ useful muon decays per year per polarity, running for 5 years per polarity. The $3\sigma$ $\theta_{13}$ discovery potential, CP discovery potential and sensitivity to the mass hierarchy as a function of $\sin^{2}2\theta_{13}$ in terms of the CP fraction are shown, for the LENF with a 20 kton TASD (red line), the LENF with a 100 kton LAr detector (blue band - the left-hand edge corresponds to the optimistic assumptions and the right-hand edge to the conservative assumptions), and other experiments - the HENF \cite{ISS} (black line), various $\beta$-beams (green \cite{betabeam}, orange \cite{gamma350_1,gamma350_2}, blue \cite{4ion} lines), T2HK \cite{ISS} (yellow line) and the wide-band beam \cite{WBB} (purple line). In these simulations we used the same oscillation parameters as in Ref.~\cite{baseline}, and have assumed a normal hierarchy.

It can be seen that a LENF with TASD has sensitivity to CP violation which is roughly equal to that of the pessimistic performance of the 100 kton LAr detector. The sensitivity to $\theta_{13}$ is also very good and again is roughly equal to that of the pessimistic LAr detector, whilst the sensitivity to the mass hierarchy is better than that of the pessimistic LAr detector. 

\begin{figure}[htp]
     \centering
     \subfigure[~$\theta_{13}$ discovery potential.]{
          \includegraphics[scale=0.5]{03-DetWG/figures/th13.eps}}
     \subfigure[~CP discovery potential.]{
          \includegraphics[scale=0.5]{03-DetWG/figures/cp.eps}}\\
     \subfigure[~Hierarchy sensitivity.]{
          \includegraphics[scale=0.5]{03-DetWG/figures/hier.eps}}
     \caption{$3\sigma$ allowed contours for the LENF with 20 kton TASD (red line) and 100 kton LAr detector (blue band), the HENF (black line), T2HK (yellow line), the wide-band beam (purple line) and three $\beta$-beams (green, orange, light blue lines) for a) $\theta_{13}$ discovery potential, b) CP discovery potential and c) hierarchy sensitivity. From Ref.~\cite{LENF}.}
\label{fig:exps}
\end{figure}


\paragraph{Liquid Argon Detectors}
\subparagraph*{Overview\\} 
%***Brief intro. praising the advantages of liquid argon: low threshold, bubble
%chamber recon etc
%(show latest pictures of events in ICARUS and from ArgoNeut)
% want to give some indication of performance for reconstructing electron CC
The Liquid Argon Time Projection Chamber (LArTPC) offers the possibility of
truly isotropic tracking and calorimetry with fine sampling at the level of a
few-percent of a radiation length. Pioneered by the ICARUS project~\cite{Amerio:2004ze},
extensive studies and R\&D projects have proven the potential of the  LArTPC
to provide a clean identification of $\nu_\mu,\nu_e$~charged current events 
with the efficient rejection of neutral current backgrounds, excellent energy
resolution and sensitivity over a wide energy band
including down to very low momentum thresholds.  
These properties make the LArTPC an interesting candidate for a large-scale
far detector for a Neutrino Factory project and a number of groups worldwide
are actively pursuing the LArTPC concept development. Fully contained neutrino interactions have recently been
reported by ICARUS illuminated by the CNGS beam (see Fig~\ref{Events}(left)) and from the
ARGONEUT collaboration situated in the NUMI beam at FNAL (see Fig~\ref{Events}(right)).   
\begin{figure}[htb]
\includegraphics[width=0.49\textwidth]{03-DetWG/figures/IcarusEvent1.eps} 
\includegraphics[width=0.49\textwidth]{03-DetWG/figures/ArgoNeutEvent2.eps} 
\caption{Neutrino interactions as seen in the (left) ICARUS (A. Guglielmi,
  Neutrino'10) and (right) ArgoNeut detectors (M. Soderberg, Neutrino'10) } 
\label{Events} 
\end{figure} 


The largest LArTPC constructed to date is the $2 \times 300$ ton ICARUS T600
detector which has a maximum drift length of $1.5$ m. 
 R\&D
programmes underway  in Europe, USA and Japan are focused on
the issues surrounding scaling up the 
LArTPC technology to the multi-kiloton scales demanded by a neutrino factory 
neutrino oscillation project in addition to  next generation projects for
particle astrophysics and proton decay.   

Several design concepts have emerged for large scale LArTPC detectors up to
$\sim 100$ kt scale: \cite{Rubbia:2009md,Bartoszek:2004si,Cline:2006st,Angeli:2009zza,Hasegawa-talkNeutrino2010,Fleming-talkNNN09}.
These designs achieve the
large mass scale by being made up either of a number of smaller identical
modules or by using a single volume based on Liquid Natural Gas (LNG)
cryogenic tank technology. 
Examples of projects from Europe, Japan and the US are shown in Figure~\ref{Concepts}.
GLACIER and the JPARC-to-Okinoshima project are both based on a single $100$
kton LNG tank with an electron drift distance of up to $20$m whereas the DUSEL
design achieves large mass scales by stacking 
$20$ kton modules thereby keeping the drift distance down to $2.5$ m. 
\begin{figure}[htb] 
\includegraphics[height=3.cm]{03-DetWG/figures/GLACIER.eps} 
\includegraphics[height=3.cm]{03-DetWG/figures/Japan100kt.eps} 
\includegraphics[height=4.cm]{03-DetWG/figures/DUSELV2.eps} 
\caption{(left) GLACIER, (middle) JPARC-to-Okinoshima 100 kt concept and
  (right) 20 kt detector for DUSEL. } 
\label{Concepts} 
\end{figure} 

With all of these large scale proposals, there are three main technical areas
where R\&D solutions need to be found:
\begin{itemize}
\item Engineering issues related to constructing large LAr volumes with very
  low thermal losses, preferably in an underground location and operation over
  many years in a
  safe and stable environment.
\item Drift lengths of  more than $10$ times the $1.5$ m ICARUS T600
  drift length will only be achievable if drift fields of $0.5-1$ kV/cm can be
  applied and if the level of electronegative impurities, which degrade the
  released ionisation charge, can be controlled at the level of a few tens of
  parts per trillion (ppt).  
\item Reconstructing the $\nu_e \rightarrow \nu_\mu$ and $\bar{\nu}_e
  \rightarrow \bar{\nu}_\mu$~`golden' oscillation channel of the Neutrino Factory
  requires the discrimination of the `oscillated'
  $\nu_\mu(\bar{\nu}_\mu)$~from the  $\bar{\nu}_\mu(\nu_\mu)$~created in the
  $\mu^-(\mu^+)$ decay. This can be achieved by identifying the charge of the
  muon from charged current interactions of the $\bar{\nu}_\mu,\nu_\mu$ 
but this in-turn demands that the entire LAr
  volume be magnetised.
% Whether a suitable technical solution can be found at
%  a reasonable cost is a key issue for large-scale LArTPC's.  
\end{itemize}


\subparagraph*{R\&D Activity\\}
In this section we summarise the status of R\&D activity in Liquid Argon technology.
\begin{itemize}

\item {\bf Readout and Electronics:}
The 2D reconstruction of charge in ICARUS modules is read out by planes that
cannot easily scale in size, due to their large capacitance.  An alternative proposal for single volume designs with
drift distances $>10$ m (e.g. GLACIER) is to operate in double phase liquid and gaseous Ar to amplify the
ionisation charge. A $3$ litre double phase TPC with thick GEM readout (macroscopic hole multipliers fabricated in standard PCBs) \cite{Badertscher-IEEE,Badertscher-Pisa} has
been successfully operated at CERN. More recent developments involve the production of a $X-Y$~2D projective
anode for a Japanese $250$ litre prototype, studies with bulk-micromegas \cite{Rubbia-NNN10} and imaging the
secondary scintillation light in a single phase LAr volume \cite{Lightfoot:2009zz}. 
ASIC developments using cold electronics (preamplifiers, digitisers and
multiplexers)  that work inside the cryostat are under development both in
Europe \cite{Centro-CERN} and the USA \cite{Fleming-talkNNN09}.

\item {\bf Liquid Argon Vessels:}
LAr storage vessels are based on a stainless steel vacuum
insulated dewar. One of the
large-scale designs \cite{Cline:2006st} scales this technology by
installing support structures within the LAr volume. Studies into the suitability of industrial Liquid Natural Gas (LNG) technology for large-scale LAr storage (up o $2\times 10^5$ m$^3$) \cite{Technodyne} have concluded that with only passive insulation, boil-off losses of only $0.04 \%$/day for a $100$ kton vessel are expected. A stainless
steel/Invar membrane is also being considered for the DUSEL $20$ kton design
\cite{Fleming-talkNNN09}.  Issues surrounding the feasibility of constructing and running
safely such detectors, particularly in underground sites has been studied as
part of the LAGUNA design study \cite{Rubbia:2010zz}. 

\item {\bf High Voltage Systems:}
The drift time of ionisation electrons over a distance of $\sim 10$ m is around
$10$ ms for a drift velocity of $1\, {\rm mm/\mu s}$. It is desirable for the drift velocity to be as high as possible, so drift fields in the region of a few kV/cm are needed (feedthroughs in the MV-range for 10~m). The ArDM experiment \cite{Kaufmann:2007zz} are generating the high voltages required inside the LAr volume via a submerged 
Cockcroft-Walton voltage multiplier. Drift fields of $\sim 1$ kV/cm have
already been demonstrated  over a  $1.2$ m drift distance \cite{Rubbia-Strasb}.

\item {\bf Liquid Argon Purity:}
Attaining and maintaining sufficient purity of LAr is a 
challenge given that electronegative impurities (mainly $O_2$) must be kept at a level of a
few tens of ppt in order to ensure electron lifetimes of the order of $10$ ms.
Proof-of-principle tests for purging with argon gas before
filling with LAr have been performed at
FNAL \cite{Jaskierny:2006sr}, KEK \cite{Maruyama-talkNNN09} and CERN \cite{Rubbia-Strasb} (a $6 \, m^3$~volume 
achieved $3$ ppm $O_2$ contamination after purging). 
A closed gaseous Ar purification stage (via cartridges) in addition to a LAr 
purification stage has been used in ICARUS. Options for purification cartridges  are being studied
as part of the ArDM project at FNAL \cite{Andrews:2009zza}. Direct measurements of long electron drifts are underway at CERN ($5$ m horizontal drift \cite{Cline-talkNNN09}) and at the  University of Bern
(ARGONTUBE: $5$m vertical drift). 

\item {\bf Magnetisation:} 
The magnetisation of a multi-kiloton LAr volume is one of the key issues 
surrounding the evaluation of a LAr TPC as a potential Neutrino Factory
detector. Small-scale prototype  LAr TPC's have been operated in
magnetic fields e.g. a $10$~lt, $150$~mm maximum drift length volume in a
field of $0.55$~T \cite{Badertscher:2005te}, with no significant degradation of the imaging
properties. Conceptual studies of magnetising a  multi-kiloton LAr volume
have concluded that conventional warm coils would dissipate MW's of heat
~\cite{Cline:2001pt}, so more recent ideas include immersing a
super-conducting solenoid directly into the LAr volume~\cite{Ereditato:2005yx}, or
using superconducting transmission lines ~\cite{Bross-talkNeutrino2010}. 
Studies are on-going regarding magnetisation issues or the potential of a `hybrid' detector consisting of a LAr TPC in close
proximity to a magnetised MIND-type module or measuring  $\nu/\bar{\nu}$~QE CC reactions with nuclei on a statistical basis 
\cite{Huber:2008yx} in the context of a low energy Neutrino Factory.
    

\item {\bf Algorithmic Reconstruction:}
With the increased interaction rates associated with neutrino beam
intensity upgrades, it is essential that automatic reconstruction algorithms for neutrino
interaction events in LAr are developed. Studies in Europe \cite{Barker} and via the LArSoft collaboration
in the USA \cite{Spitz-talkNPC} are actively developing algorithms to reconstruct
hit clusters, tracks, event vertex points and shower objects. The resulting
`toolbox' of algorithms will be used to validate the performance assumptions for
LAr detectors in common use for neutrino oscillation physics studies (see e.g. \cite{Barger:2007jq,FernandezMartinez:2010zza}).     
These tools will also be useful in estimating reliably the feasibility of non
magnetic field solutions to $\nu-\bar{\nu}$ separation in the golden channel
discussed above.


\item {\bf Future Roadmaps:}
The  R\&D programmes in Europe and Japan for future large-scale LAr detectors are closely coordinated. Work is ongoing in delivering the large electron multiplier (LEM) readout technologies, demonstration of long drift lengths (5 m), culminating with the construction of a $1$ kton device, which is widely accepted to be the largest possible 
affordable detector that will allow reliable extrapolations to $100$ kton. 
The roadmap in the USA includes the exploitation of the ArgoNeuT data in a neutrino beam, the construction of the 0.1 kton MicroBooNE detector and focusing on delivering a 20 kton detector for DUSEL. 
\end{itemize}

%\begin{figure} 
%\includegraphics[height=10.cm]{03-DetWG/figures/EuropeRoadmap.eps} 
%\caption{The R\&D roadmap for LArTPC development in Europe. } 
%\label{EuropeRoadmap} 
%\end{figure} 

%\begin{figure} 
%\center{
%\includegraphics[height=11.cm]{03-DetWG/figures/USRoadmap.eps} 
%\caption{The R\&D roadmap for LArTPC development in USA. } 
%\label{USRoadmap}} 
%\end{figure} 


\paragraph{Large-volume liquid scintillator detectors}

Large-volume liquid scintillator detectors (Borexino\cite{Alimonti:2008gc}, Kamland \cite{Suekane:2004ny}) are presently used for low-energy neutrino physics. Even larger scintillator detectors have been proposed, such as the 50 kton LENA \nocite{Wurm:2007cy,Oberauer:2006cd,Hochmuth:2006gz,MarrodanUndagoitia:2006qs,MarrodanUndagoitia:2006qn,MarrodanUndagoitia:2006rf,MarrodanUndagoitia:2006re,Undagoitia:2005uu,Autiero:2007zj,Wurm:2010ny,Lachenmaier:2010zz} \cite{Wurm:2007cy}--\cite{Lachenmaier:2010zz} detector or the 10 kton HanoHano\cite{Batygov:2008mr,Learned:2007zz} detector. Although these are totally active scintillator detectors (TASD), these single-volume detectors are conceptually different from segmented scintillator detectors.

The performance of a conventional large volume liquid scintillator detector for high-energy neutrinos (1--5 GeV) has been studied recently \cite{Learned:2009rv,Peltoniemi:2009xx,Smith:2010zzc,Moellenberg2010} and a more detailed study is under preparation \cite{LENA-GeV}. It was found that if the detector, particularly the read-out electronics, is well designed, one can reconstruct the simplest events by photon arrival times in phototubes. The lepton flavor identification was determined to be almost perfect and the energy resolution was found to be better than 5\%. Hence a large-volume liquid scintillator detectors can be used in conventional neutrino beam experiments \cite{Peltoniemi:2009hv} and high-energy beta beam \cite{Peltoniemi:2009zk} experiments, as well as in atmospheric neutrino experiments. If it can be magnetized, it could be also be considered for the detector at a Neutrino Factory \cite{Peltoniemi:2009zb}. 

\begin{figure}[tbhp]
\begin{center}
\includegraphics[width=0.6\textwidth,bb=0 0 1123 794]{03-DetWG/figures/cross_section.jpg}
\caption{A possible layout of the detector. This shows the transversal cross section. Photomultipliers (yellow circles) are located at all sides of the detector and cameras at the upper surface. In this case the vertical magnetic field is generated by longitudinal wires.}
\label{cross}
\end{center}
\end{figure}

Here we consider a magnetized single-volume liquid scintillator detector of approximately 100 kton. A phenylxylylethane (PXE) scintillator \cite{Back:2004zn, Quirk:2008zz, MarrodanUndagoitia:2009kq, Wurm:2010ad} doped with PPO (up tp 10 g PPO/1 kg PXE) is able to produce fast scintillation decay times, essential for high energy applications (such as the Neutrino Factory). Additionally, we may not need a separate tank and water shielding or buffer as in LENA, but can use rock walls with a suitable liner, thus reducing the cost and optimizing the detector volume.

The required shape and volume could be achieved with a horizontal cavern of 30 m diameter and a length of 150 m. The diameter is mostly limited by the transparency of the liquid, typical attenuation lengths being of the order of 15 m \cite{MarrodanUndagoitia:2009kq, Wurm:2010ad}. Previous studies done for LENA hint that at least 30 m width is accessible without excessive cost \cite{Nuijten2010}. 

It is assumed that photomultiplier tubes are installed in all sides of the detector volume, including the bottom, the surface and both ends. Photomultiplier tubes (PMT) designed to work in magnetic fields, possibly embedded in $\mu$-metal shields and aligned relative to the field with mirrors for light collection, will be required. As an alternative, semiconductor-based (or hybrid) photosensors are also under study. They may be faster and immune to magnetic fields, but may suffer from noise and are currently expensive.

Previous studies \cite{LENA-GeV} suggest that a 3\% photocathode coverage is sufficient for high-energy events. The devices should be able to record multiple photons reliably, with a time resolution in the $ns$ scale, to be achieved by 16 bit Flash ADCs. We may optionally install an array of one-photon capable cameras on the upper surface, when such technology is available and affordable (see e.g. \cite{Learned:2009rv}). The liquid-air interface can be used as a lens (objective), because of the high refractive index (1.6 - 1.8) of the oil used as the scintillator. To measure the very faint scintillation light we need single-photon capable pixelized photosensors. Potential devices exist based on a number of different technologies such as: ICCD (CCD with image intensifier), EMCCD (Electron multiplier CCD), SPADA (Single photon avalance diode array) or MCP (micro-channel plate) varieties with binary pixel readout. 

A severe problem with all semiconductor photodetectors is their very high noise rate. A typical noise rate is 100~kHz per pixel at room temperature. With cooling, we may expect 10 kHz per pixel at $0^\circ$ C, and no less than 1~kHz at the freezing point of the liquid, but somewhat lower at liquid nitrogen temperatures. Because of the noise, the camera readout must be externally triggered with the PMT coincidence. A local rolling memory for at least 100 ns, or the full readout time window, is required. The analysis of the PMT data determines the vertex and the end of the muon track with a precision of 20~cm. 

It is reasonable to assume 50\% quantum efficiency and 70\% geometric efficency. If we also assume 90\% transmission through the lens, this then gives a total photoefficiency of 30\%. Assuming a photocell composed of $64\times64$ pixels (4096 channels), a lens with a 15 cm diameter and an aperture of 0.30, we obtain a mean resolution of 10 cm at 15 m distance. With a lens diameter of 15 cm and 10\% coverage of the upper surface area, we will measure about 10 photons for a 10 cm pathlength along the trajectory of a muon (assuming a typical photon yield of $10^4$ photons/MeV and 20\% transparency at a distance of 15~m). 

The detector performance with PMT time signals was simulated with the prototype code ``Scinderella". The code simulates the passage of muons and scintillation light emission, individual hadrons and electromagentic showers and tries to make a fit to a test event using the simulated recorded data of the photosensors. For a more quantitative picture, a more detailed simulation applying GEANT4 and physical event generators should be used. Here we considered only the bending of muon tracks. Much more detail about the tracking capability is given elsewhere \cite{LENA-GeV}. 

It was found that the muon charge was reliably measured using the time profile information of the photosensors. The simulations indicate that a magnetic field strength between 0.02 T for ideal electronics and 0.05 T for current standard technology might be sufficient. Older technology designed only for low-energy events, like that used in Borexino \cite{Alimonti:2008gc}, is not sufficient. The decay time of the scintillator affected the performance. For high-energy events, the lower transparency of higher-doped scintillators is adequate, if we are not aiming for performance at low energies.

Using the additional camera setup reduces the 20 cm resolution achieved by the PMTs down to a statistical accuracy for an individual track to better than 5 cm. With this resolution, the charge of the muon can be identified for $B > 0.02$ T  and a 10 MeV energy resolution can be achieved. The camera setup, in particular, improves the capability to distinguish multiple tracks, such as charged pions and recoil protons. The identification of 10--80 cm long tracks is enhanced with the addition of the cameras when compared to a PMT only setup. The camera setup also makes recoil neutron tracks visible. The neutrons scatter from free protons, giving the proton recoil energies up to the half of the neutron energy, within a radius of $\sim 1$~m. Moreover, the neutron may spallate additional neutrons that are absorbed giving gammas of a few MeV. The combination of the camera and the PMT measurement may help to identify the absorption signals. 
 
The camera readout will improve the measurement of the enery of the hadron showers produced in multi-GeV neutrino interaction. Electrons and photons causing electromagnetic showers can be identified by observing the gap between the vertex and the shower, which is 42~cm on average. This is important for rejection of neutral current background events (and also for proton decay $p \to e^ + \pi^ 0$). 
 
Identifying a pion (from a muon) is important for neutral current background rejection. The pion decays with a mean life time of 26 ns at rest. The resulting muon (4 MeV for a decay at rest) has a pathlength of about 2 cm. In the case of decay in flight, the decay is recognized by an energy bump in the time profile and a sharp change in the direction of the path. To see the angle, we need a timing resolution better than 1 ns. Negative pions may be absorbed before decaying (giving photons).
	
We can summarise that a liquid scintillator detector setup with a magnetic field $B>0.02$~T, good PMTs and adequate electronics is sufficient to determine the muon charge. Combining PMT readout with a camera system gives additional tracking capability, with very good event identification and background rejection. Further studies to quantify these effects at a neutrino beam will be carrried out in the future.


