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\begin{document}
\title{JLC Central Tracker R\&D}

\author{Keisuke Fujii$^*$,\\
representing the JLC CDC Group}
\address{
$^*$IPNS, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
}

%\lefthead{LEFT head}
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\maketitle


\begin{abstract}
The current status of the JLC central tracker R\&D is reviewed,
stressing progress made since the Sitges meeting.
R\&D's future direction is also discussed, taking into
account possible changes in basic chamber parameters.
\end{abstract}

\section*{Introduction}

Let me now begin my talk with reminding you of
physics requirements on the central tracker performance\cite{Ref:jlc1}.
The most stringent requirement comes from
the recoil mass measurement for $e^+e^- \to Zh$
followed by $Z \to l^+l^-$.
In order to confirm the narrowness of the Higgs boson,
the recoil mass against the lepton pair from the
$Z$ decay should be measured to an accuracy only
limited by the natural beam energy spread.
This sets the following performance goal for the
momentum measurement:
$\sigma_{p_{l^\pm}}/p_{l^\pm} \lesssim 0.4~\%$ at
$p_{l^\pm} = 50~{\rm GeV}$.
Separation of $W$ and $Z$ through jet invariant mass measurements
demands a resolution comparable with
the gauge boson natural widths.
This necessitates good track-cluster matching:
$\sigma_Z \simeq 1~{\rm mm}$
and 2-track separation better than $2~{\rm mm}$.
Time stamping of tracks from different bunches will
be very important to suppress otherwise
overlapping mini-jet backgrounds,
which requires $\sigma_{T_0} \lesssim \mbox{bunch spacing (1.4/2.8 ns)}$.

These physics requirements should be translated into
chamber parameters and, consequently, R\&D items.
The current set
of CDC parameters:
$\sigma_{xy} = 85~\mu{\rm m}$,
a lever arm length of 185~cm
($= 230(\mbox{outer cylinder}) - 45(\mbox{support tube})$),
the number of sampling points $n=80$,
$B = 2$~T,
and a wire length of 460~cm 
(full lever arm above $45^\circ$),
satisfies the requirement from the recoil mass measurement.

The spatial resolution of $85~\mu{\rm m}$ indicates use of
a cool gas mixture and a mini-jet cell structure.
The required $z$ resolution of $1~{\rm mm}$
also suggests use of the mini-jet cell structure
with a stereo angle of $\tan\alpha_{stereo} \simeq 0.1$
through
$\sigma_z = \sigma_{xy}/\tan\alpha_{stereo}$.
The remaining physics demand on the time stamping
capability can then be translated into
the limit on the drift velocity through
$\sigma_{T_0} \simeq \sigma_{xy}/(v_{drift}\sqrt{n}) \lesssim 1.4~{\rm ns}$.
A drift velocity of $10~\mu{\rm m}/{\rm ns}$ satisfies the
requirement.
If we assume a jet cell with 5 sense wires and a maximum
drift length of $5~{\rm cm}$, there will be about 14k jet cells
and, correspondingly, about 100k wires total.

The chamber parameters described above lead us to the
following list of questions:
(1) Can we control gravitational and electrostatic sags for 4.6m-long wires?
(2) Can we achieve a spatial resolution of $85~\mu{\rm m}$
    everywhere in this big chamber?
(3) Can we achieve 2-track separation better than $2~{\rm mm}$?
(4) Can we stably operate 4.6m-long stereo cells?
(5) Is Lorentz angle small enough to allow straight cells?
(6) Is gas gain saturation observed for cool gas mixtures no problem?
(7) Is tension drop controllable for $Al$ wires?
(8) Is neutron background 
    ($\sim$3k hits/train for $B=2~$T\cite{Ref:sugimoto}) no problem?
    How big is the signal charge for a single neutron hit?
    Is there any dead time expected?
These questions naturally comprise our R\&D items.


\section*{What We Have Done Since Sitges Meeting}

Corresponding to these R\&D items, we set the following
publication plan:
(1) gravitational and electrostatic sags\cite{Ref:sag},
(2) cosmic ray tests of a 4.6m chamber\cite{Ref:cosm},
(3) 2-track separation (in preparation),
(4) designing of stereo-wire geometry\cite{Ref:stereo},
(5) Lorentz angle measurement\cite{Ref:lorentz},
(6) gas gain measurement\cite{Ref:gasgain},
(7) wire tension measurement,
(8) background studies, and
(9) general gas properties.
By the time of the Sitges meeting,
(1), (2), and (4) had been published.
Since then, (5) and (6) have been completed, while
(3) being intensively studied.
In what follows, I will thus quickly go through these 
three topics.

\subsection*{Gas Gain Measurements}

The gas gains for ${\rm CO}_2/\mbox{iso-}{\rm C}_4{\rm H}_{10}$
mixtures have been measured with a single wire proportional 
tube, primarily for supplementing the stereo-wire
geometry study\cite{Ref:stereo}.
The measurement revealed the easily saturating
nature of the mixtures:
already at a gas gain $\lesssim 10^{5}$,
a significant broadening of the pulse height peak started
and further high voltage increase
led us to the apparent onset of a new gas
multiplication mode as observed as a separate peak
that eventually dominated the spectrum.
This quick gas gain saturation can be attributed to
the small diffusion of the gas mixtures,
resulting in concentration of avalanche charge
on a small spot on the wire.
Further studies are necessary to clarify the implications
of this observation in terms of
local dead regions, 2-track separation,
and $dE/dx$ measurements.



\subsection*{Lorentz Angle Measurements}

The Lorentz angle, being one of the key parameters to constrain
the cell design, have been measured for our chamber gas
at different magnetic fields up to $1.5~{\rm T}$
with a newly developed precision measurement system
using 2 laser beams to produce primary
electrons and FADC's to read their signals
simultaneously\cite{Ref:lorentz}.
Final results from the measurement
are plotted in Fig.\ref{Fig:lorentz}-(a),
showing good agreements
with GARFIELD/MAGBOLTZ\cite{Ref:garfield} simulations.
Notice that for our gas mixture
the magnetic deflection coefficient
($\Psi$), as defined by $\tan\alpha = \Psi (B/E) v^0_D$,
is close to unity,
justifying the extrapolation of our measurement
to higher magnetic fields.
%
%%%%% fig. 4
\begin{figure}[htb]
\begin{minipage}[htb]{7cm}
\centerline{
\epsfig{file=figs/tanalpha.eps,width=7cm}}
\end{minipage}
\hfill
\begin{minipage}[htb]{7cm}
\centerline{
\epsfig{file=figs/psi.eps,width=7cm}}
\end{minipage}
\caption[Fig:lorentz]{\label{Fig:lorentz}\small
(a) Lorentz angle as a function of
electric field at different magnetic fields,
(b) magnetic deflection coefficient at $B = 1.5$~T
as a function of the electric field
for ${\rm CO}_2(90)/\mbox{iso-}{\rm C}_4{\rm H}_{10}(10)$.
Solid lines are GARFIELD/MAGBOLTZ simulations.
}
\end{figure}
%



\subsection*{2-Track Separation}

We have carried out 2-track separation experiments
for our small test chamber,
using $e^+e^-$ pairs produced by gamma conversions at
an $Al$ converter
(see Fig.~\ref{Fig:twotrack}-(a) for typical 
FADC signals for a single event).
Figure \ref{Fig:twotrack}-(b) shows
a preliminary result of 2-hit
separation efficiency for a single sense wire
as a function of 2-track distance,
in the case of normal incidence.
%%%%fig. 5
\begin{figure}[htb]
\begin{minipage}[htb]{7cm}
\centerline{
\epsfig{file=figs/twotrackevt.eps,width=6.5cm}}
\end{minipage}
\hfill
\begin{minipage}[htb]{7cm}
\centerline{
\epsfig{file=figs/twotrackeff.eps,width=6.5cm}}
\end{minipage}
\caption[Fig:twotrack]{\label{Fig:twotrack}\small
(a) FADC signals as a function of time (500~ns/division)
    for a typical 2-track event.
(b) 2-hit separation efficiency
as a function of 2-track distance.
}
\end{figure}
%
Although the preliminary result looks promising,
more studies are needed to investigate
possible space charge effects
due to the small diffusion nature of our
gas mixture.


\section*{Summary and Future Plan}

We have given affirmative answers
to most of the basic R\&D questions
we raised in the introduction:
in particular, we have demonstrated the possibility
to achieve an average spatial resolution of
$\sigma_{xy} \simeq 85~\mu{\rm m}$.
Still remaining are
those related to the gas gain saturation and space charge effects
inherent in the gas mixtures having small diffusion coefficients,
and the tension drop problem for $Al$ wires.
The space charge effects are being studied using
laser beams as well as charged particles, while
wire material studies are in progress.

As discussed in this meeting
by T.~Tauchi and Y.~Sugimoto\cite{Ref:tauchi,Ref:sugimoto},
there is a demand for a higher magnetic field
from beam-related background reduction point of view.
We are thus investigating possibilities to increase
the magnetic field from $2$~T to $3$~T.
If we could scale all the chamber parameters that have
the dimension of length by a factor of 2/3,
the chamber performance would stay unchanged.
This is, however, impracticable since
wire spacing, spatial resolution, 
and inner radius are difficult to scale, 
if not impossible.
The expected performance of a $3$~T design,
where these three parameters are kept unchanged,
is discussed by A.~Miyamoto\cite{Ref:miyamoto}
in this meeting.
In any case, any design change should be made
consistently to the whole detector system,
and should be justified by physics
simulation studies.
For this purpose, we started developing a full
detector simulator with GEANT4,
while brushing up the one based on GEANT3.

\vskip 0.5cm
\underline{ Acknowledgements}
\vskip 0.4cm

The author wishes to thank all the members of the JLC CDC group for useful
discussions and comments. 
The author would like to express his heartfelt gratitude to the
workshop organizers for their efforts to make it so comfortable and so
fruitful. 

\begin{references}
\bibitem{Ref:jlc1} JLC group, KEK report, 92-16 (1992).
\bibitem{Ref:sugimoto} Y.~Sugimoto, these proceedings.
\bibitem{Ref:sag} JLC CDC Group,
	{\it Nucl. Instr. Meth.} {\bf A383} (1996) 391.
\bibitem{Ref:cosm} JLC CDC Group, 
	{\it Nucl. Instr. Meth.} {\bf A441} (2000) 393.
\bibitem{Ref:stereo} JLC CDC Group,
	{\it Nucl. Instr. Meth.} {\bf A428} (1999) 403.
\bibitem{Ref:lorentz} JLC CDC Group,
	KEK Preprint 2000-113 (hep-ex/0012002),
	Submitted to {\it Nucl. Instr. Meth.} {\bf A}.
\bibitem{Ref:gasgain} JLC CDC Group, 
	{\it Nucl. Instr. Meth.} {\bf A447} (2000) 459.
\bibitem{Ref:garfield} R.~Veenhof, 
	Nucl.\ Instr. and Meth.\ {\bf A419} (1998) 726, \\ 
        S.F.~Biagi, {\it Nucl.\ Instr. and Meth.} {\bf A421} (1999) 234.
\bibitem{Ref:tauchi} T.~Tauchi, these proceedings.
\bibitem{Ref:miyamoto} A.~Miyamoto, these proceedings.
\end{references}
 
\end{document}

  
