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\title{Status of the MINOS experiment}

\author{Ruben Saakyan\address{University College London, Gower Street, 
 London WC1E 6BT, United Kingdom}, for the MINOS Collaboration}
       
\begin{document}

\begin{abstract}
The MINOS long-baseline neutrino oscillation experiment is presently under
construction at the Soudan mine and the Fermi National 
Accelerator Laboratory. We report on the current status of the experiment, 
the projected schedule and its physics capabilities.
\vspace{1pc}
\end{abstract}

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\maketitle

\section{Introduction}
Growing evidence for neutrino oscillations being responsible for
the observed solar and atmospheric neutrino anomalies \cite{Fuk98,Fuk01} has shown
the necessity of second generation experiments using a well-controlled 
and understood neutrino source. The MINOS (Main Injector Neutrino Oscillation
Search) experiment \cite{TDR98} is a long baseline neutrino oscillation 
experiment utilizing 
a neutrino beam produced at FNAL and directed at a 5.4 kt detector (Far 
Detector) located 731 km away in the Soudan mine, northern Minnesota. 
A smaller 980-ton detector
(Near Detector) will measure the neutrino beam at the Fermilab
site where no effect from neutrino mixing is expected. Comparison between the 
two detectors will allow the measurement of the oscillation parameters with a high accuracy 
providing their true values are within the region indicated by 
Super-Kamiokande \cite{SK01}.

\section{Neutrino beam}
The Main Injector is a high intensity proton synchrotron which has 
a multiple purpose of producing the antiprotons for the Tevatron,
preaccelerating protons and antiprotons before their injection into the Tevatron and providing an intense beam for fixed target experiments. 
The NuMI (Neutrinos at Main Injector) 
neutrino beam is obtained from the decay of pions and kaons produced in
the interactions of 120 GeV protons with a carbon target. The anticipated
proton flux is $4 \times 10^{13}$ protons per pulse with a repetition rate
of 1.9 sec. Single turn extraction will be used with a pulse length of 
10 \nolinebreak $\mu$s. 

The secondary mesons will be focused by two parabolic horns and allowed
to decay in a 675 m long evacuated decay pipe. Combining this with the 50 m 
distance from target to start of the decay pipe gives a 725 m long decay 
region for the beamline. The distance between the two horns can be changed 
by moving the second horn with respect to the other. By adjusting the horn 
positions and currents one can select three different neutrino energy 
regimes: low ($<E_{\nu}> \sim 3 GeV$), medium ($<E_{\nu}> \sim 6 GeV$) and
high ($<E_{\nu}> \sim 15 GeV$). The anticipated neutrino event spectra at 
the far detector site for the three energy configurations are shown in Fig. 1.
The possibility to optimize the beam energy during the course of the 
experiment provides the flexibility to react to the initial 
results. In order to have optimal sensitivity in the low part of the 
atmospheric neutrino allowed region preferred by Super-Kamiokande 
data \cite{SK01} the collaboration have decided to start taking data with the 
low-energy beam configuration. Two years of data taking in this configuration
would yield about 5,000 beam-related charge-current (CC) neutrino events in the
Far Detector in the absence of oscillations. The statistics would be much 
higher in the high-energy beam configuration: about 30,000 $\nu_{\mu}$ CC 
events are expected in this case. 

An alternative method of changing the beam energy is currently being 
investigated. It is proposed to implement a flexible arrangement where the 
target can be remotely moved in the beamline direction to change the beam 
energy and the horns remain fixed \cite{NuMIB783}. In addition to having 
the possibility of a continuously-variable beam energy, the movement of the 
target can be performed more quickly, thus reducing the downtime 
significantly. 

Substantial progress has been made in the NuMI beamline construction. The
excavation of the proton carrier tunnel and target hall are completed. 
Most of the decay tunnel and some portion of the Near Detector hall are also
excavated.
The NuMI beam turn on is expected in autumn 2004.

\begin{figure}[ht]
\begin{center}
\includegraphics[height=6cm]{/home/saakyan/meetings/kektc5/proceedings/beamspectra.eps}
\end{center}
\caption{Neutrino interaction spectra in the far detector predicted for
three different focusing conditions corresponding to high, medium and low energy beam.}
\end{figure}

\section{MINOS Detectors}

MINOS design includes two detectors: the Far Detector and the Near Detector.
One can obtain an unbiased pattern of oscillations comparing the 
Near/Far neutrino rates
and energy spectra . The detectors
have been designed to be as similar as possible.
They are iron/scintillator sampling calorimeters with identical longitudinal
and transverse granularity and an average toroidal magnetic 
field of about 1.3 T. The 
2.54 cm thick iron plates are interleaved with scintillator planes composed of
4.1 cm wide, 1.0 cm thick strips. The strip orientation differs by 90 degrees
in successive planes so as to provide X and Y coordinate measurements, Z being
the number of the plane. The scintillator is commercially extruded polystyrene. A thin TiO$_{2}$ loaded layer has been coextruded on the outside to provide
a reflective surface layer and a protective layer for handling. Each strip 
has a groove about 2 mm deep and 1.3 mm wide located in the centre of one 
wide side. A green wavelength-shifting fiber, 1.2 mm in diameter, is glued
into the groove. The light produced in the scintillator is trapped into the
fiber, shifted in frequency and delivered to the photomultipliers. Although
the principal design of both detectors is very similar there are differences
reflecting the fact that experimental conditions at the near and far site are
not identical. 

\subsection{Far Detector}

The MINOS Far Detector will be located in a newly excavated cavern in the Soudan 
mine 690 m below the surface. The detector is composed of 486 iron/scintillator
octagonal modules giving a total mass of 5.4 kt. The detector will be split 
up into two independent supermodules, each one toroidally magnetized by a  
current carrying coil threaded through a hole through the center of the
super-module. A sketch of one supermodule is shown in Fig. 2. The scintillator
strips in the Far Detector are read out from both ends. Clear fibers are used
to transport the light from the green fibers to 16 pixel Hamamatsu M16 
photomultiplier tubes. Each pixel is 4$\times$4 $mm^{2}$ 
and will be coupled to 
8 fibers from 8 different scintillator strips from the same plane but spaced 
about 1 m apart. Such multiplexing provides a substantial reduction in cost of
photomultipliers and readout electronics. The set of strips connected to a 
given pixel is different at the two ends allowing one to ``de-multiplex'' the 
event using the resulting pattern difference, i. e. to identify which strips
were responsible for a particular event. 

\begin{figure}[ht]
\begin{center}
\includegraphics[height=6cm]{/home/saakyan/meetings/kektc5/minosdetectorrendering.eps}
\end{center}
\caption{Sketch of the MINOS Far Detector supermodule}
\end{figure}

A detailed GEANT-based Monte-Carlo simulation of the MINOS detector gives
an energy resolution $\sigma/E$ of approximately 23\%/$\sqrt{E}$ for 
electromagnetic showers and 55\%/$\sqrt{E}$ for hadronic showers. The magnetic
field allows measurement of the muon momentum by curvature (14\% resolution
for a 10 GeV muon) as well as by range for stopping muons ($\sim 6\%$).

Civil construction and outfitting of the Soudan mine is complete and the 
detector installation has begun. At the time of writing 100 planes have been
installed and commissioned which is $\sim$ 41\% of the first supermodule
($\sim$ 1.1 kt). Routine installation of the Far Detector continues at a rate of 
5--7 planes/week, faster that it was anticipated in the original schedule. 
First cosmic ray muons have been already observed in the installed planes
which are also used to debug calibration and DAQ procedures. The installation
of the first supermodule will be completed by September 2002. During the 
construction of the second supermodule the first supermodule will be used to 
acquire the cosmic ray and atmospheric neutrino data. The whole detector 
installation will be completed by September 2003.

MINOS will be the first large underground neutrino experiment using a magnetic 
field and atmospheric neutrino and upward going muons 
studies can greatly benefit from that. Even though the statistical 
significance will be below that of Super-Kamiokande, MINOS will be able to 
measure the charge of the muons as well as the energy of throughgoing and
partially contained events. 

\subsection{Near Detector} 

The Near Detector is functionally identical to the Far Detector although
its physical design has some different features due to the differences
in characteristics of the neutrino beam at the two sites such as, for instance, 
much higher rates and a slightly lower average energy of the neutrino beam 
at the near site compared to the far site due to the effect of beam divergence. 
The detector has the same composition and modularity as the Far Detector
and is composed of 282 ``squashed'' octagon plates giving a mass of 980 t.
The detector is divided into two parts: 120-plane calorimeter consisting of
``veto'', ``target'' and ``hadron spectrometer'' regions and  162-plane 
muon spectrometer (Fig. 3). In the calorimeter section 4/5 of the plates are 
partially instrumented around the beam fiducial region; every fifth plane 
is fully instrumented. The planes in the muon spectrometer section are fully 
instrumented but only every fifth plane. One sided optical readout with no 
optical summing will be used. The fibers will be read out by Hamamatsu
M64 tubes (64 2$\times$2 $mm^{2}$ pixels).
The average magnetic field seen by a muon will be practically identical in 
both Near and Far Detectors. The Near Detector will be used to study carefully
the beam, to measure  $\nu_{e}$ contamination and to understand the beam
at the far detector site which will substantially reduce the systematic 
uncertainty of the prediction. Excavation of the Near Detector hall is 
underway. 
About half of the steel required for the Near Detector has been delivered. 
Installation of the detector is expected to start in October 2003 
and will take about 8 months.

\begin{figure}[ht]
\begin{center}
\includegraphics[height=6cm]{/home/saakyan/meetings/kektc5/newnear2.eps}
\end{center}
\caption{Sketch of the MINOS Near Detector}
\end{figure}

\section{Calibration}

The two MINOS detectors must have a relative energy calibration of 2\%
in order to reduce systematic errors to less than the expected
statistical errors on the oscillation parameters. This will be accomplished
in several stages. Any nonlinearity in the readout as well as any short-term
changes in PMT gain will be measured by injecting light from pulsed 
LEDs into the readout chain \cite{NIMLI99}.  Cosmic ray muons will be used to perform the
strip-to-strip calibration and the relative calibration between the Near
and Far Detectors. Finally, a comparison of the energy deposition by muons 
with that from electromagnetic and hadronic showers will be carried out 
with a smaller detector (the Calibration Detector) which is a replica of the 
MINOS detectors but with a more manageable size. The Calibration Detector
was built, commissioned and exposed in 2001 
to the T11 test beam at CERN with far
detector readout electronics. The analysis of this data is currently underway. 
The calibration program for the next three years includes the exposure 
of the detector equipped with far and near detector electronics
to hadrons, electrons and muons of known energies at the T7, T9 and T11 
testbeam areas at CERN \cite{NuMIL681}. 

\section{Physics Goals and Sensitivity}

Large statistics and well controlled systematics give MINOS powerful
physics tools to study neutrino oscillation phenomena. The experiment
will address the following issues:

\begin{itemize}

\item [-] {The neutrino oscillation hypothesis with 
$\nu_{\mu}\rightarrow\nu_{\tau}$ being the dominant mode. In this case
MINOS should see the disappearance in the $\nu_{\mu}$ charge current (CC) 
sample without loss of neutral current (NC) events  and without 
appearance in the $\nu_{e}$ CC sample.}

\item [-] {Precise measurement (at the roughly 10\% level) of the oscillation
parameters $\Delta m^{2}$ and $sin^{2}2\theta$ for the dominant  
$\nu_{\mu}\rightarrow\nu_{\tau}$ mode. The primary measurement for this is 
 the comparison of rates and spectra of the CC events in the Far and Near 
Detectors.}


\item [-] {Measurement or improvement of the limit on the
$\nu_{\mu}\rightarrow\nu_{sterile}$ oscillation mode. This can be done
by comparing NC/CC ratio with what is predicted for the
$\nu_{\mu}\rightarrow\nu_{\tau}$ oscillation mode.}


\item [-] {Measurement or improvement of the limit on the 
$\nu_{\mu}\rightarrow\nu_{e}$ oscillation mode. The $\nu_{e}$ CC appearance
can be detected using topological criteria for electromagnetic shower 
identification.}


\item [-] {Studying possible contributions from more exotic hypotheses
such as neutrino decay \cite{NeDec}, extra dimensions \cite{Extra} or
decoherence \cite{Deco}.}

\end{itemize}

Below we shall elaborate on some of these studies in more detail and estimate 
potential sensitivities that could be achieved in a 10 kt-yr run which
corresponds to a run of approximately 2 years' duration. 

\underline{$\nu_{\mu}\rightarrow\nu_{\tau}$.} 
The most robust oscillation measurement MINOS will offer is a determination of 
any change in the ratio of the numbers of NC versus CC events between the 
Near and Far Detectors -- $\frac{(NC/CC)_{near}}{(NC/CC)_{far}}$.
This test is relatively systematic free as the expected NC/CC ratio in the 
absence of oscillations is fairly constant over the entire range of
neutrino energies. The uncertainties due to Monte Carlo based acceptance 
and absolute neutrino flux calculations essentially cancel,
thus reducing systematics significantly. A simple and robust mechanism to 
discriminate between NC and CC $\nu_{\mu}$ events is a cut based on event length.
More sophisticated methods including pattern recognition of the muon track 
and artificial neural networks to identify event types are being studied.

The recent Super-Kamiokande results from 1289 days of contained events and
up-going muon events give 90\% C.L. allowed parameter regions of
$sin^{2}2\theta > 0.88$ and 
$1.6 \times 10^{-3} < \Delta m^{2} < 4.0 \times 10^{-3} eV^{2}$ \cite{SK01}. 
According to the results of the search for charged current $\nu_{\tau}$ events,
the Super-K data is consistent with $\nu_{\tau}$ appearance at roughly 
the two sigma level. This led the MINOS collaboration to 
the decision to use the low 
energy beam configuration for the initial run. 

\begin{figure}[ht]
\begin{center}
\includegraphics[height=6cm]{/home/saakyan/meetings/kektc5/senscc_le.eps}
\end{center}
\caption{Expected $\Delta m^{2}$ and $sin^{2}2\theta$ sensitivity for 
a 10 kt-yr exposure with the low energy beam. The Super-K 90\% 
allowed region is also shown, shaded}
\end{figure}


Neutrino oscillations will
deplete the  $\nu_{\mu}$ flux at certain energies. A measurement of 
the $\nu_{\mu}$ energy spectrum will therefore yield the mixing parameters
directly. Fig.4 shows the sensitivity of the CC energy test to the oscillation
mixing parameters for three different $\Delta m^{2}$ values:
0.002, 0.0035 and 0.005 $eV^{2}$. The parameter region allowed by 
Super-K data is also shown. In the absence of oscillations the measurement
of the $\nu_{\mu}$ CC spectrum will provide the most stringent limit 
on $\Delta m^{2}$ of $< 7 \times 10^{-4} eV^{2}$ at the maximum 
mixing angle at 90\% C.L. as shown in Fig. 5. Both plots assume 
a 10kt-yr exposure and the low energy beam option.

\begin{figure}[ht]
\begin{center}
\includegraphics[height=6cm]{/home/saakyan/meetings/kektc5/cclim.eps}
\end{center}
\caption{Expected 90\% confidence limits from the CC energy distributions
in MINOS for a 10 kt-yr exposure with the low energy beam. The Super-K 90\%
allowed region is also shown, shaded}
\end{figure}

\underline{$\nu_{\mu}\rightarrow\nu_{sterile}$.}
Although the Super-Kamiokande analysis shows that pure 
$\nu_{\mu}\rightarrow\nu_{sterile}$ oscillation is disfavoured at
99\% C.L. MINOS can improve current limits with the NC/CC ratio measurement.
In \cite{Ster} it was shown that more stringent limits can be obtained
for three different oscillation scenarios: $\nu_{\mu}\rightarrow\nu_{\tau}$,
50\% $\nu_{\mu}\rightarrow\nu_{\tau}$ + 50\% $\nu_{\mu}\rightarrow\nu_{sterile}$
and $\nu_{\mu}\rightarrow\nu_{sterile}$.

\underline{$\nu_{\mu}\rightarrow\nu_{e}$.}
$\nu_{e}$ interactions can be identified by the presence of an
electromagnetic shower. The MINOS tracking calorimeters should be able
to identify such a shower using its typical characteristics such as shower 
compactness in both longitudinal and transverse dimensions and a large deposit
of energy near the vertex. One background is the $\nu_{e}$ component in the 
beam ($\sim 0.6\%$). A somewhat more serious background is generated by 
the NC events with a topology similar to a $\nu_{e}$ interaction. Both these
backgrounds can be measured with the Near Detector and the corresponding
uncertainty will be mainly due to statistical fluctuations in their predicted
number in the Far Detector. The simulations show that MINOS can offer a modest 
improvement over the best current limit by CHOOZ \cite{CHOOZ} (Fig. 6). 

\begin{figure}[ht]
\begin{center}
\includegraphics[height=6cm]{/home/saakyan/meetings/kektc5/nuelim.eps}
\end{center}
\caption{MINOS $\nu_{\mu}\rightarrow\nu_{e}$ sensitivity for a 10 kt-yr
exposure. The region excluded by CHOOZ and predicted KamLAND limit are
also shown}
\end{figure}

Should the rate be measurable, MINOS would be able, thanks to the magnetic
field, to measure the sign of $\Delta m^{2}$ by performing the same measurement
with $\bar{\nu}_{\mu}$ using the fact that matter effects contribute with 
opposite sign for a neutrino and an antineutrino beam.

\section{Conclusion}
Large scale construction of the MINOS experiment is underway. The installation
and commissioning of the detectors in the Soudan mine and FNAL will be 
completed by summer 2004. We expect the beam to turn on in late 2004. 
In two years of data taking MINOS will be able not only to confirm the oscillation
hypothesis but to provide also the first ever high precision measurement of the
neutrino mixing parameters.

\section*{Acknowledgements}
Funding for MINOS has been provided by the U.S. Department of Energy
and by the U.K. Particle Physics and Astronomy Research Council.

\begin{thebibliography}{9}
\bibitem{Fuk98} Y. Fukuda {\it et al., Phys. Rev. Lett.} {\bf 81} (1998) 1562.
\bibitem{Fuk01} Y. Fukuda {\it et al., Phys. Rev. Lett.} {\bf 86} (2001) 5656.

\bibitem{TDR98} MINOS Technical Design Report, Fermilab, 1998.

\bibitem{SK01} T. Toshito for the Super-Kamiokande Collaboration, hep-ex/0105023.

\bibitem{NuMIB783} M. Kostin {\it et al.}, NuMI-B-783 (2001).
  
\bibitem{NIMLI99} B. Anderson {\it et al., Nucl. Instr. and Meth.} {\bf A423} (1999) 320-327. 

\bibitem{NuMIL681} J. Thomas {\it et al.}, NuMI-L-681 (2000).

\bibitem{NeDec} S. Pakvasa, hep-ph/0004077.

\bibitem{Extra} R. N. Mohapatra {\it et al.}, hep-ph/0006278;
R. Barberi, P. Creminelli and A. Strumia, hep-ph/0002199.

\bibitem{Deco} E. Lisi, A. Marrone and D. Montanino, {\it Phys. Rev. Lett.}
{\bf 85} (2000) 1166.

\bibitem{Ster} D. Petyt, NuMI-L-691 (2000).

\bibitem{CHOOZ} M. Apollonio {\it et al., Phys. Lett.} {\bf B420} (1998) 397
and {\it Phys. Lett.} {\bf B466} (1999) 415.

\end{thebibliography}


\end{document}




