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%======================================================================-
\title{The IceCube Neutrino Telescope}

\author{Shigeru Yoshida for the IceCube collaboration
\address[Chiba]{Department of Physics, Faculty of Science, 
        Chiba University, \\ 
        Chiba 263-8522, Japan}%
        \thanks{The full author
        list is appeared in~\cite{IceCubeReco}.}
}
%======================================================================-
       


\begin{document}


%======================================================================-
\begin{abstract}
         The IceCube Neutrino Telescope, a huge Neutrino Telescope
with 1 cubic km instrumented volume, starts construction
in 2004. The project status and the expected sensitivity 
and performance for detecting high energy cosmic neutrinos are reported. 
%Preliminary results based on AMANDA experience without taking into account 
%IceCube's new digital technology indicate  a sensitivity to a diffuse muon
%neutrino energy flux of $10^{-8}$ GeV/cm$^2$ sr sec.
%Proposed models for GRB emission should be observable in less than one year. 
The capability of EHE neutrino detection is also briefly mentioned.
\vspace{1pc}
\end{abstract}

%======================================================================-
% typeset front matter (including abstract)
\maketitle



%======================================================================-
\section{Introduction: IceCube Detector}

It has been discussed that many of the proposed astrophysical models 
of ultra-high energy (UHE) neutrino production would require 
a scale of a cubic kilometer
target volume to secure their detection. 
The IceCube detector~\cite{IceCube} has been designed
and will be constructed following this philosophy using
the deep ice at the South Pole as an interaction target.
%It is the result of further technological development of the
%basic ideas of the AMANDA telescope~\cite{AMANDA}, which has demonstrated the deep
%ice at the South Pole indeed provides an excellent interaction target 
%for UHE neutrino detection,
%which validated the idea of having a much larger neutrino telescope
%in Antarctica. It will be realized as the IceCube detector~\cite{IceCube}.

The instrumented volume where Cherenkov light from neutrino-induced
charged leptons is detected contains an array of 4800 PMTs
each enclosed in a transparent pressure sphere to comprise a
Digital Optical module (DOM). Eighty strings are regularly spaced by
125 m over an area of approximately one square kilometer with DOMs
at depths of 1.4 to 2.4 km below the ice surface. Each string,
containing 60 DOMs spaced by 17m, will be deployed into a hole drilled with
pressurized hot water. A complimentary air shower array on the ice surface
called IceTop is also built for providing the background-veto capability
as well as study of the cosmic ray origin in the knee region ($\sim 10^{16}$ eV).

%=======================================================================
\begin{figure}[thb]
  \begin{center}
    \includegraphics[width=0.3\textwidth,clip=true]{DOM.ps}
  \end{center}
 \vspace{-2.5pc}
  \caption{Schematic view of the IceCube Digital Optical Module.}
\label{fig:DOM}
 \vspace{-1.5pc}
\end{figure}
%=======================================================================

The IceCube DOM is shown in Fig.~\ref{fig:DOM}.
It contains a 10 inch diameter PMT
supported by coupling gel, a signal processing electronics
board, an LED flasher board for calibration, and the PMT base with high voltage
supplier, all of which are housed in the spherical pressure glass.
%It contains its own processor, memory flash file system, and real-time
%operating system. Its new digital technology enables it to schedule
%background communication while acquiring data, to invoke all calibration
%functions under software control, and most notably,
%to digitize the PMT pulse and store the full waveform information. 

%=======================================================================
\begin{figure}[thb]
  \begin{center}
    \includegraphics[width=0.35\textwidth,clip=true]{sf0086.eps}
  \end{center}
  \vspace{-2.5pc}
  \caption{The relative PMT sensitivity as a function of distance
from the photocathode center. The scanned data sliced by
the horizontal axis with 146.25$^{\circ}$ from the dynode orientation
is plotted here. The spikes are due to the deadspace of the shield
of the geomagnetic field and not the real response of a PMT.}
\label{fig:PMTscan}
 \vspace{-1.5pc}
\end{figure}
%=======================================================================

Our PMT choice is the HAMAMATSU R7081 with 10 dynodes
which has exhibited excellent charge resolution and low noise.
The operation gain is planned to be $\sim 5\times 10^7$.
%with dynamic range of $\sim 200$ photo-electrons per 15 ns.
The PMT response has also been investigated in some detail
such as two dimensional relative sensitivity
on the photocathode surface. An example is shown
in Fig.~\ref{fig:PMTscan} where the sensitivity is
plotted as a function of distance from the cathode center.
These results are all implemented in the IceCube detector
Monte Carlo simulation we are now developing.
The overall noise for each individual DOM is 
targeted to be 500 Hz which can be
realized due to the sterile and low temperature environment
of the deep ice and an effort to use low activity materials. 



%======================================================================-
\section{The Basic Performance and Sensitivity to Astrophysical $\nu_{\mu}$}

The backgrounds for searches for extraterrestrial neutrinos
come from atmospheric muons and neutrinos produced
by decay of mesons generated from cosmic ray (CR)
interactions in the atmosphere.
CR-interaction induced background events can be identified
by the fact that they result mainly in down-going tracks
in the instrumented volume of ice. 
%Our search for cosmic neutrinos will be performed, therefore,
%by excluding the vast backgrounds caused by atmospheric muons
%(and eventually atmospheric neutrinos also), which requires extensive simulations.

The discrimination of the astrophysical neutrino signals
from the background is based on the geometrical parameters
obtained by the various reconstruction algorithms, the reduced
likelihood of the reconstruction, number of PMT channels
receiving an unscattered Cherenkov photon, the track length
and so on. The details are found in~\cite{IceCubeReco}. The initial
results indicated that the atmospheric muon background 
can be reduced by more than factor of $\sim 10^6$. 
%The atmospheric
%neutrino background remains significant, but
%applying a channel multiplicity cut can exclude
%the background completely since the energy spectrum
%of astrophysical neutrinos ($\sim E^{-2}$) is much harder
%than that of the background neutrinos ($\sim E^{-3.7}$). 
The effective
area for upward moving neutrino-induced UHE muons is 1.2 km$^2$
at 1 PeV. The pointing resolution is better than 1.0$^{\circ}$.
%
%=======================================================================
%\begin{figure}[htb]
%  \begin{center}
%   \includegraphics[width=0.45\textwidth,clip=true]{diffuse.int.eps}
%  \end{center}
%  \vspace{-2.5pc}
%  \caption{Expected sensitivity of the IceCube detector.The various
%neutrino production models in AGNs/GRBs are shown 
%for comparison~\cite{Man01,Stecker96,Waxman99}.
%The {\it mrf} implies the ``Model Rejection Factor'' which is described
%in~\cite{IceCubeReco} for details.}
%\label{fig:expected_sensitivity}
%\end{figure}
%=======================================================================
%
%The sensitivity obtained for a diffuse neutrino flux
%is summarized in Fig.~\ref{fig:expected_sensitivity}.
As with the sensitivity, we found~\cite{IceCubeReco}
diffuse flux of $E_{\nu}^2 dN_{\nu}/dE_{\nu} = 1\times 10^{-8}$ cm$^{-2}$ sec$^{-1}$ sr$^{-1}$ GeV
is detectable in three years of observation. 
An angular search cone of $1^{\circ}$ in one year of observation would reach a sensitivity of
$E_{\nu}^2 dN_{\nu}/dE_{\nu} = 5.5\times 10^{-8}$ cm$^{-2}$ sec$^{-1}$ GeV
for point sources emissions. The best order-of-magnitude estimate from the W-B model
on the GRB UHE neutrino emissions is about 1 event per 100 GRBs~\cite{WB}.



%=======================================================================
\section{Some Remarks on the IceCube EHE Neutrino Sensitivity}

%=====================================================
\begin{figure}
  \begin{center}
\includegraphics[width=.45\textwidth,clip=true]{atmMuon_angle_10PeV}
  \end{center}
  \vspace{-2.5pc}
\caption{\label{fig:atmMuon_angle_10PeV}
Dependence of the muon and tau fluxes originated
in the cosmogenic neutrinos at the IceCube depth 
on the cosine of zenith angle.
The integral flux above 10 PeV is plotted in linear scale.
The atmospheric muon fluxes estimated by the two different 
methods~\cite{yoshida03} are also shown by the solid and dashed curves.}
 \vspace{-1.5pc}
\end{figure}
%=====================================================

%=====================================================
\begin{figure*}[hbt]
  \begin{center}
\includegraphics[width=.8\textwidth,clip=true]{iceCube_sensitivity}
  \end{center}
  \vspace{-2.5pc}
\caption{\label{fig:iceCube_sensitivity}
The IceCube sensitivities on the EHE neutrino fluxes. 90 \% C.L.
limits by a 1km$^2$ detection area with 10 years observation 
are drawn. The left panel shows the case of $\nu_{\mu}$ and
the right panel shows the $\nu_{\tau}$ case. 
Labels refer to GZK (\cite{yoshida93} for the lower curve,
\cite{kalashev02} for the upper curve), TD~\cite{SLBY},
and  Z-burst~\cite{yoshida98}.}
 \vspace{-0.5pc}
\end{figure*}
%=====================================================

It has been argued that
underground neutrino telescopes being operated and/or planned
to be built are capable of detecting Extremely-high Energy 
(EHE:$10^{17}\sim 10^{20}$ eV) neutrinos~\cite{halzen01}.
EHE neutrinos during the propagation do not penetrate
the earth but are involved in
charged/neutral current interactions
that generate charged leptons and hadronic showers 
because their cross sections are expected to be
enhanced in the ultra-high energy regime. 
%The relevant transportation equations were solved and
%the energy spectra of the neutrinos and the secondary produced
%leptons were calculated~\cite{syoshidaICRC}. 
%It has been found that
%the intensity strongly depends on the nadir angle.
Fig.~\ref{fig:atmMuon_angle_10PeV} shows dependences 
of the secondary muon and tau fluxes on the zenith angles.
The initial primary cosmogenic neutrino fluxes
are taken from Ref.~\cite{yoshida93} in the GZK 
cosmogenic neutrino model~\cite{berezinsky69}.
Strong attenuation by the earth can be seen but
the fluxes are more or less stable in 
the region of the ``downward'' events
where $\cos\theta\geq 0$. 
Intensity of the downward going muons and taus are
larger than the upward ones by an order of magnitude.
%Secondary muons and taus form a potentially
%detectable downgoing bulk with intensity of $\sim$ three
%order of magnitude lower than the neutrino fluxes.
One can also see that the muon background attenuates
faster than the neutrino-induced EHE muons and taus,
and there is a window where the signals dominate the muon
background. The detail discussions are found in Ref.~\cite{yoshida03}.
The event rate is found to be 0.27 $(\mu+\tau)$/km$^2$ year 
for the cosmogenic neutrino
fluxes with the moderate source evolution. Note that
the downward event rate is 0.25 /km$^2$ year and dominates
in the overall rate.

The IceCube sensitivity on EHE neutrinos can be evaluated
by the secondary produced $\mu$'s and $\tau$'s 
event rate per energy decade $dN/dLogE$.
Fig.~\ref{fig:iceCube_sensitivity}
shows the resultant sensitivity
with 1 km$^2$ detection area. The appropriate EHE downgoing event
detection criteria
was found to be energy deposit greater than 10 PeV in
1 km$^3$ volume of ice~\cite{yoshida03}. 
The various model predictions
are also shown for comparison. 
The topological defects (TD) scenario would be severely constrained
while the cosmogenic neutrino possibilities would not be ruled out
by absence of signals. 

%=======================================================================
\section{Project Status}

The project has been approved by the U.S. National Science Board
and the N.S.F. is the major source of the project budget.
Significant contributions for the cost of the detector are also 
being made by Germany, Sweden, and Belgium. The grants for
the man power support and the detector calibration efforts
has been approved by the Japanese agencies.
Full construction
will start in FY 2004 and take 6 years to complete. 
In this year, defined by project year 2,
production and testing of $\sim$ 100 DOMs are scheduled
in addition to shipping the new hot water drill to the South Pole.
The detector verification, related software to
utilize fully the DOM waveform information, and the efforts
to expand the energy range (below TeV and above EeV) are
under way.


%===========================================================================
\begin{thebibliography}{9}
%\bibitem{AMANDA} E.~Andres, {\it et al.}, Astropart.~Phys. 13 (2000) 1.

\bibitem{IceCube} See http://icecube.wisc.edu/

\bibitem{IceCubeReco} 
IceCube Collaboration, J.~Ahrens, {\it et al.}, 
astro-ph/0305196, accepted for publication in Astropart.~Phys.

%\bibitem{Man01} K.~Mannheim, R.~J.~Protheroe, J.~P.~Rachen, 
%Phys. Rev. D 63 (2001) 023003

%\bibitem{Stecker96} 
%F.~W.~Stecker, M.~H.~Salomon, Space Sci. Rev. 75 (1996) 341.

\bibitem{WB} E.~Waxman, J.~Bahcall, Phys. Rev. D 59 (1999) 023002.

\bibitem{halzen01} J.~Alvarez-Mu\~{n}iz and F.~Halzen,
Phys.~Rev.~D  63 (2001) 037302.

%\bibitem{syoshidaICRC}
%S.~Yoshida, Proc. 28th ICRC (Tsukuba, Japan) HE.2.4 (2003) 1487.

\bibitem{yoshida93}
S.~Yoshida and M.~Teshima, Prog.~Theor.~Phys.  89 (1993) 833.

\bibitem{berezinsky69}
V.~S.~Beresinsky and G.~T.~Zatsepin, Phys.~Lett. 28B (1969) 423.

\bibitem{yoshida03}
S.~Yoshida, R.~Ishibashi, H.~Miyamoto, 
astro-ph/0312078, to be submitted to Phys.~Rev.~D (2003).

\bibitem{kalashev02} O.~E.~Kalashev, V.~A.~Kuzimin, D.~V.~Semikoz,
and G.~Sigl, Phys.~Rev.~D 66 (2002) 063004.

\bibitem{SLBY} G.~Sigl, S.~Lee, P.~Bhattacharjee, and S.~Yoshida,
Phys.~Rev.~D 59 (1999) 043504.

\bibitem{yoshida98} 
S.~Yoshida, G.~Sigl, and S.~Lee, Phys.~Rev.~Lett. 81 (1998) 5505.

\end{thebibliography}

\end{document}
