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\centerline{\Large \bf 
Application for transnational access to the CERN testbeam}
\smallskip
\centerline{\Large \bf 
for high granularity shower studies}


\bigskip
\centerline{\Large 
P.D.~Dauncey$^1$ (Coordinator),
M.~Stanitzki$^2$,
N.K.~Watson$^3$,
J.A.~Wilson$^3$}

\bigskip
\centerline{\Large
$^1$ Imperial College London,}
%\smallskip
\centerline{\Large 
$^2$ Rutherford Appleton Laboratory,}
%\smallskip
\centerline{\Large 
$^3$ University of Birmingham}

\bigskip
\centerline{\Large \bf August 2010}

\section{Scientific background}
The SPiDeR (\underline{S}ilicon \underline{Pi}xel \underline{De}tector
\underline{R}\&D) collaboration~\cite{SPIDERWEB}
is investigating
monolithic CMOS pixel sensors for future collider detectors, 
in particular high
energy lepton colliders (ILC and CLIC).
The collaboration is studying both fundamental technologies for future
detector sensors and also novel applications of CMOS sensors. These could
be applied both to tracking and calorimetry. In particular, the
collaboration is currently studying the feasibility of a digital
electromagnetic calorimeter (DECAL).
The DECAL would be a new approach to electromagnetic   
calorimetry where the initial electron
or photon energy is estimated by counting particles rather than from the
deposited energy.
This work is being undertaken in association with
the CALICE collaboration~\cite{CALICEWEB} and we include members of
both the ILD and SiD collaborations.


\section{Electromagnetic shower density}
The DECAL concept uses a silicon-tungsten sampling calorimeter,
where the silicon detector pixels are such that the probability of 
more than one shower particle passing through any pixel is small.
This means that the number of pixel hits gives a direct estimate of the 
number of particles, which is itself proportional to the total shower
energy. This removes the Landau contribution to
the energy resolution which occurs in more standard 
sampling calorimeters with analogue readout,
and so should give better resolution.

Simulations indicate that
the major uncertainty in the predicted performance of a DECAL 
is in
the particle densities in the core of the showers. The 
detector pixel
size must be small compared with the average distance between
particles in order to correctly count the particles in the core.
However,
there are no systematic measurements of electromagnetic
showers with highly granular ($\sim$ 50\,$\mu$m) detectors
so the predictions from shower simulations must be verified
experimentally
to be confident of the predicted performance of a DECAL.

The aim of this beam test is to perform measurements of the
core densities in electromagnetic showers from around 10 to 100\,GeV.
This dramatically extends the range of measurements from those
done by us at DESY in March 2010, which only covered 1 to 5\,GeV.
First results from the DESY data were shown at the ICHEP conference
this summer~\cite{TPAC}.

\section{Beam test programme}
The programme will consist of measuring the core density in
electromagnetic showers across the range of electron beam energies between
10 and 100\,GeV and with
a range of thicknesses of tungsten converter between 1 and 10
radiation lengths. We foresee around 40 different energy/converter
combinations and will require at least 100k events per combination
for a significant measurement. In addition, runs with an out-of-time
trigger to record backgrounds, noise and event pileup effects will
be needed. It is foreseen that a total of around 6M
events will be needed to be taken within the one week.

The actual measurements will be taken using the EUDET telescope
directly. Tungsten converter plates will be installed between
the two arms of the telescope. The front three planes will then
define the incoming electron track, giving a well-defined projected
position and direction for the centre of the shower in the tungsten. 
The back three telescope
planes will record the shower emerging from the chosen thickness of
the converter. The pattern and density of the hits measured will give
unprecedented information on the showers. The pixel size of the
EUDET telescope sensors, $\sim$ 18\,$\mu$m, is more than sufficient
for the granularity assumed for a DECAL and so should easily be
able to separate the particles in the core.

To obtain 6M events in one week
requires an averaged event rate of at least 10Hz. With a spill duty
cycle of 17\,sec per minute, this implies a required
in-spill rate of 35Hz, which should
be easily achievable. If a higher
rate is possible, then more combinations and/or more data per 
combination can be taken.

Except for the modifications for out-of-time triggers, which are
currently under discussion, the measurement only depends on the
EUDET telescope. This has been running for several months at
CERN this year and so no major technical issues are foreseen.


\section{Financial request}
CERN has granted us one week of beam time from Monday 20 to Monday 27 
September.
We request travel
support for five people to come to CERN for this run, some of whom
will arrive on September 19.
(One other person will be involved but will be funded from other
sources.)

For each of the five people, we will need a flight from the UK (approx
200 euros)
as well as accomodation (80 euros/night) and subsistance 
(40 euros/day) for the duration of their stays. These visits will
vary between five and eight days and total around 30 days for all five 
people. We will also need a rental car (200 euros) for the whole period
to get to and from the beam test area.
Hence, we estimate the total cost to be around 4800 euros.

Note, there are no equipment transport costs; the tungsten sheets
we will use are small and can be carried out in hand luggage by one 
of the visitors.

\begin{thebibliography}{99}
\bibitem{SPIDERWEB}
\url{https://www.spider.ac.uk}.

\bibitem{CALICEWEB}
\url{https://twiki.cern.ch/twiki/bin/view/CALICE/WebHome}.

\bibitem{TPAC}
P.~D.~Dauncey, on behalf of the SPiDeR Collaboration,
``Performance of CMOS sensors for a digital electromagnetic calorimeter'',
ICHEP2010 International Conference on High Energy Physics, Paris,
July 22-28 2010, and proceedings thereof.

\end{thebibliography}

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