\subsection{Introduction}

\subsubsection{Baseline description for the far detectors}

The IDS-NF baseline for the Neutrino Factory has been optimised as 
described in section \ref{Sect:PPEG}.
The optimum strategy to measure $\delta$, $\theta_{13}$, and the mass
hierarchy (the sign of $\Delta m^2_{13}$) includes having two
Magnetised Iron Neutrino Detectors (MIND), one with a fiducial mass of
100~kTon at $\sim 4\,000$~km and another with a fiducial mass 50~kTon
at $\sim 7\,500$~km.   
The latter has been termed the ``magic baseline'', since matter
effects cancel the effect of CP violation at this distance of
propagation through the Earth. 
The detector is optimised to carry out the detection of the ``golden
channel'' ($\nu_e\rightarrow \nu_\mu$) through the wrong-sign muon
signature. 
This strategy is more efficient for resolving degeneracies in the
neutrino-oscillation formulae and provides better sensitivity than,
for example, measuring the golden and the ``silver'' channel
($\nu_e\rightarrow \nu_\tau$) simultaneously.
   
The original ``golden channel at a Neutrino Factory'' analysis \cite{Cervera:2000kp} assumed a cylindrical geometry with a cross-sectional radius of 10~m, with iron plates 6~cm thick, scintillator planes 2~cm thick and a 1~T solenoidal field operating at a 50~GeV Neutrino Factory. The International Scoping Study (ISS) \cite{CerveraVillanueva:2008zz,Abe:2007bi} assumed a cuboidal geometry of $14\times 14$~m$^2$ with 4~cm thick iron and 1~cm thick scintillator and a 1~T dipole field, while operating at a 25 GeV Neutrino Factory. 

For the most recent studies we have adopted a baseline cuboidal geometry with a cross-sectional area of $15\times15$~m$^2$ and length of either 63~m or 125~m, depending on the mass of the detector. The thickness of each plane of iron is 3~cm, followed by two planes of scintillator, each with 1~cm thickness. The three planes form a module of thickness 5~cm. The lateral resolution requirement is 1~cm, which is provided by having co-extruded scintillator bars 15~m long and 3.5~cm wide, read out using optical fibres and silicon photo-multipliers (SiPMT). The magnetic field assumed for this baseline is a 1~T dipole field. Table \ref{tb:MIND_par} lists the key parameters of the two far detectors. The purpose of studying this geometry was to  perform a direct comparison with previous results and to determine the influence of a full reconstruction in conjunction with a modern likelihood analysis to eliminate background.

While we adopted this simplifying geometry and magnetic field to compare with previous simulations, we are aware that a 1~T dipole field is not practical from an engineering point of view. However, in section~\ref{sec:MIND_conceptual_design} we have now studied a more realistic octagonal geometry (14~m octagonal iron plates as shown in figure \ref{fig:MIND}), with a toroidal field between 1~T and 2.2~T over the whole fiducial area. These parameters can be achieved with a 100~kA/turn current traversing the centre of the MIND plates and are shown to be feasible to manufacture. This more realistic geometry has not been simulated yet but we do not expect that the performance will differ greatly from what has been achieved, and will be described in section~\ref{sec:MIND}.  
\begin{figure}
  \begin{center}
    \includegraphics[width=0.9\textwidth]%
      {03-DetWG/figures/100KT_Detector_closeup.eps}
  \end{center}
  \caption{
    Schematic drawing of the Magnetised Iron Neutrino Detector (MIND). 
  }
  \label{fig:MIND}
\end{figure}
\begin{table*}
  \caption{
    Baseline parameters for the Magnetised Iron Neutrino Detectors
    at a Neutrino Factory. MIND~1 refers to the detector at $\sim
    4\,000$~km and MIND~2 refers to the detector at $\sim 7\,500$~km. 
  }
  \begin{center}
    \begin{tabular}{|l|r|r|}
      \hline
      {\bf Parameter}                         & {\bf MIND 1} & {\bf MIND 2}\\
      \hline
      Distance (km)                           & 3000-5000 & 7000-8000     \\
      Fiducial mass (kT)                  & 100  & 50   \\
      Size iron plates (cm$^3$)               & 1500$\times$1500$\times$3 & 1500$\times$1500$\times$3 \\ 
      Length detector (m)                     & 125  & 62.5 \\
      Number iron plates               & 2500  & 1250  \\
      Dimensions scintillator bars (cm$^3$)        & 1500$\times$3.5$\times$1  & 1500$\times$3.5$\times$1   \\
      Number scintillator bars per plane      & 429 &  429 \\
      Total number of scintillator bars      & $2.14\times 10^6$ &  $1.07\times 10^6$ \\
      Total number of readout channels  & $4.28\times 10^6$ &  $2.14\times 10^6$   \\
      Photon detector & SiPMT & SiPMT \\
      Magnetic field (T)   & $>$~1 & $>$~1  \\
      \hline
    \end{tabular}
  \end{center}
  \label{tb:MIND_par}
\end{table*}

\subsubsection{Baseline description for the near detectors}
\label{sec:Baseline_near}

The baseline for the Neutrino Factory includes the need for one or
more near detectors. It is necessary to have one near detector for
each of the straight sections of the storage ring at each of the two
polarities, so four near detectors designed to carry out measurements
essential to the sensitivity of the oscillation-physics programme are
required.
The near-detector measurements that are essential for the neutrino
oscillation analysis are:
\begin{itemize}
  \item Determination of the neutrino flux through the measurement
        of neutrino-electron scattering;
  \item Measurement of the neutrino-beam properties that are
        required for the flux to be extrapolated with accuracy to the
        far detectors;
  \item Measurement of the charm production cross sections (charm
        production in far detectors is one of the principal
        backgrounds to the oscillation signal); and
  \item Measurement of the neutrino-nucleon deep inelastic,
        quasi-elastic, and resonant-scattering cross sections.
\end{itemize}

The intense neutrino beam delivered by the Neutrino Factory makes it
possible to carry out a unique neutrino-physics programme at the near
detectors. 
This programme includes fundamental electroweak and QCD physics, such
as measurements of parton distribution functions as a function of
$Q^2$ and Bjorken $x$, QCD sum rules, nuclear re-interaction effects,
strange particle production, a precise measurement of $\sin^2\theta_W$.
The near detector must also be capable of searching for new physics,
for example by detecting tau-leptons which are particularly sensitive
probes of non-standard interactions at source and at detection.
Tau neutrino detection is also important in the search for sterile
neutrinos.

Here we itemise the general design features of the near detector:
\begin{itemize} 
\item A detctor with micron-scale resolution for charm and tau
  identification (either a silicon vertex or an emulsion-based
  detector); 
\item A low-$Z$, high-resolution target for flux and
  $\nu_\mu$- and $\nu_e$-leptonic cross-section measurements (i.e., a
  scintillating-fibre tracker or a straw-tube tracker);  
\item A magnetic field for charged particle momentum measurement (with
  $\delta p/p \sim 1\%$ (for $p \sim 2-3$\,GeV);
\item A muon catcher for muon identification;
\item Electron identification capabilities;
\item Excellent energy resolution for flux extrapolation: this needs
  to be better than for the far detector, so the goal is to achieve
  $\delta E/E \sim 1\%$; and
\item A variety of nuclear targets to measure cross-sections in iron and
  as a function of the nuclear target mass number $A$. 
\end{itemize}
There are two options currently being considered: one which includes a
high resolution scintillating fibre tracker and the other includes a
transition-radiation straw-tube tracker. 
Both of these options will be studied to determine their capabilities.


