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%  Author: Giovanni Signorelli
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%  Title : The MEG experiment: status and prospects    
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%  WG    : WG4
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\documentclass[10pt]{iopart}
\usepackage{graphicx}

\def\meg{\relax\ifmmode{{\mu^+ \rightarrow  \mathrm{e}^+ + \gamma}}%
               \else${\mu^+ \rightarrow  \mathrm{e}^+ + \gamma}\ $\fi}%

\begin{document}
\title[The MEG experiment at PSI]{The MEG experiment at PSI: 
status and prospects}
\author{Giovanni Signorelli\footnote{On behalf of the MEG collaboration}}
\address{Scuola Normale Superiore, Piazza dei Cavalieri 7, 
         56126 Pisa, Italy, and\\
         INFN Sezione di Pisa, Via Livornese 1291, San~Piero a Grado, 
         Pisa, Italy }
\ead{Giovanni.Signorelli@pi.infn.it}

\begin{abstract}
The present status of the MEG experiment, an experiment to search for
the \meg decay at PSI, is described. 
The experiment will utilize an innovative liquid Xenon 
calorimeter in which only the information provided by the scintillation
light is used for the photon energy reconstruction. A major emphasis is given
to the recently measured  transparency characteristics of the liquid Xenon.
\end{abstract}
%\submitto{\jpg}
%
\section{Introduction}
Lepton Flavor Violation (LFV) is  generically  allowed in any viable 
extension of the Standard Model (SM). In particular, Super-Symmetric
Grand-Unified theories predict rates for LFV processes, such as \meg,
not far from the present experimental limit~\cite{BHS}. For this 
specific process the present experimental bound is set by the MEGA
experiment~\cite{MEGA} to $BR(\meg)<1.2 \cdot 10^{-11}$, while the 
SUSY-GUT predictions lie in the range $10^{-12} \div 10^{-14}$.  
A search of the \meg\ decay to such a level of 
accuracy could either lead to a 
discovery of new physics or pose serious constraints on the parameters
of Super-Symmetric theories. For this reason an experiment 
which will run at the Paul Scherrer Institut (PSI, Switzerland) is being  
designed by a Swiss-Japanese-Italian-Russian collaboration to reach 
a sensitivity for the \meg\ branching ratio in the range 
$10^{-13} \div 10^{-14}$. The experiment is planned to start in 2004 and
be completed in 2007, before the operation of LHC experiments. 

In the following sections we will review the present status of the 
experiment, with a particular emphasis on the liquid Xenon calorimeter
research and development, using a large prototype, which 
demonstrated that the transparency of liquid Xenon to 
self-scintillation light (absorption length $\lambda_{\rm abs} > 1$~m) 
makes the sensitivity goal achievable.

\section{The MEG Detector}
The MEG experiment will be built at PSI, the laboratory
hosting the highest-intensity continuous muon beam in the world
($3 \cdot 10^{8}~\mu^+/$~sec). 
The signature of a \meg\ event is given by a coincidence
of a 52.8~MeV photon with a 52.8~MeV positron in space, time
and direction. The background is 
dominated by positrons in the high energy part of the Michel spectrum
%$\mu^+ \to e^+ \nu \bar\nu$ 
accidentally overlapping with high energy photons from the 
radiative muon decay $\mu^+ \to e^+ \nu \bar\nu \gamma$~~or from 
positron annihilation-in-flight.

A complete description of the MEG detector, together with a schematic 
drawing, can be found elsewhere~\cite{donato,nproposal}. 
Here a brief overview of various sub-detectors is given.
%A schematic of the MEG detector is shown in figure~\ref{fig:megdetector}.
Positive muons coming from the $\pi E5$ beam line will be brought 
to stop on a thin target where they undergo spontaneous decay.
%\begin{figure}
%\includegraphics[width=\columnwidth]{megdetector.eps}
%\caption{\label{fig:megdetector}Schematic drawing of the MEG detector.}
%\end{figure}
Positron trajectories will be measured by 17 radial drift chambers filled
with a (50:50) Helium-Ethane mixture placed in a inhomogeneous magnetic 
field. The field is 1.25~T at the center of the detector,
decreases along the magnet axis, and is shaped
such in a way as to provide two important features: 
the radius of the positron
trajectories depends on its {\em absolute} momentum and not on its transverse
component, and the positrons are quickly swept away after the 
first turn to avoid low longitudinal momentum positrons to spiralize 
and saturate the drift chambers. 
%The radial coordinate will be obtained
%by charge division while the longitudinal one will be determined exploiting
%the charge distributions on the cathode patterns on the kapton foils (Vernier
%patterns). 
The timing of the positrons will be measured by two 
segmented plastic scintillator 
timing counters, one at each side of the target, with a 
resolution of 100~ps FWHM.
Photons will traverse the thin superconducting coil of the magnet 
and convert in the liquid
Xenon calorimeter. This detector is made of a $\sim 0.8$~m$^3$ volume
of liquid
Xenon read on all sides by more than 800 photomultipliers sensitive 
in the UV, to match the $175$~nm wavelength 
of the Xenon scintillation light. Only
scintillation light will be used for the photon energy measurement. Liquid 
Xenon was chosen for its high light yield (comparable to NaI), high 
atomic number and fast response ($\tau \simeq 45$~ns)~\cite{Doke}.
The trigger will be given by a signal from the timing counters coincident, 
in time and direction, with a high energy photon in the calorimeter. 
All the signals will be digitized at a high frequency (2~GHz) in order
to reduce the event pile-up as much as possible. 
 
\section{Detector Performance}
The sensitivity of the MEG experiment crucially depends on the 
capability to distinguish a background event from the \meg\ signal. 
This is directly related to each sub-detector resolution. The most 
critical and innovative device is the liquid Xenon detector whose
energy
resolution is expected to be of few percents FWHM at 52.8~MeV. 
The major uncertainty concerning the liquid 
Xenon is its transparency to scintillation 
light: a loss in the number of photons could crucially spoil the 
predicted resolution. 
For this reason, and to study the procedures of handling the gas
at cryogenic temperatures, a liquid Xenon detector prototype (``large
prototype'') was built, and its test at KEK, Japan is 
the main issue in which the collaboration is currently putting 
its effort. The large prototpe has a 
$40 \times 40 \times 50$~cm$^3$ Xenon active volume 
read by 228 PMTs, equipped with
$^{241}$Am $\alpha$-sources and blue LEDs for calibration. Three pairs of 
trigger counters can be placed outside the volume to study the response
of the detector to cosmic rays. Handling of liquid Xenon and calibration 
procedures for the PMTs have been successfully developed and shown to be
appropriate for the final detector in order to reach a 
percent-level energy resolution, though 
major problems have been discovered regarding the Xenon purity level.

Comparison with the absorption spectra of O$_2$ and H$_2$O showed that 
even a ppm of these contaminants in the Xenon can give rise to 
a tremendous absorption of UV light. This was the reason of the %we obtained 
very short absorption lengths obtained at the beginning of our tests 
($\lambda_{\rm abs} \simeq 7$~cm). The presence of water at ppm level was 
also confirmed with a mass-spectrometric analysis. 
The absorption length was measured comparing the light seen by PMTs 
at different distances from the $\alpha$-sources, and using cosmic rays. 
To overcome the contamination problem a complete 
circulation and purification scheme was introduced 
{\em during} detector operation. Liquid Xenon was continuously evaporated 
and pumped, in gaseous phase, through an Oxisorb 
%\footnote{Oxisorb is a trademark of Messer Griesheim, Inc.} 
cartridge and a heated gas getter purifier.
%
\begin{figure}
\begin{center}
\mbox{
\includegraphics[width=0.47\linewidth]{lxemc.eps}
}
\mbox{
\includegraphics[width=0.47\linewidth]{lxegxe.eps}
}
\caption{Comparison of $\alpha$-source
data in liquid Xenon to Monte Carlo simulation (left) and
to $\alpha$-source data in cold gaseous Xenon (right) for three different 
runs: before the purification (circles), 
after two weeks of purification (squares), and
after one month of purification (stars).}
\label{fig:alpha}
\end{center}
\end{figure}
%
An increase of both light yield and absorption length was measured 
(see figure~\ref{fig:alpha}), the 
purification time being limited by the pumping speed 
(5~cc/min of liquid Xenon). After four weeks of purification %we measured
a $\lambda_{\rm abs}> 100$~cm was measured with $\alpha$-sources
and the result was  confirmed with cosmic ray data. 
To our knowledge, such a low absorption in
liquid Xenon was never measured before. The sensitivity to higher absorption
lengths is limited by the larger side of the prototype (50~cm).

\section{Conclusion}
Much progress was made in the design and construction of the MEG
detector for a \meg\ search experiment at PSI. In particular the 
liquid Xenon transparency has been demonstrated to 
be at the  
level needed for the proposed experiment 
in order to reach a sensitivity of 
$5 \cdot 10^{-14}$ (and consequently a 90\% CL upper limit of $\sim 10^{-13}$
in case of no candidate observed)~\cite{nproposal}.

\section*{References}
\begin{thebibliography}{99}

\bibitem{BHS}
Barbieri~R and Hall~L~J,
%``Signals for supersymmetric unification,''
{\it Phys.\ Lett.}\ B{\bf 338}, 212 (1994)
%[arXiv:hep-ph/9408406].

\nonum
Barbieri~R~, Hall~L~J and Strumia~A,
%``Violations of lepton flavor and CP in supersymmetric unified theories,''
{\it Nucl.\ Phys.\ } B{\bf 445}, 219 (1995)
%[arXiv:hep-ph/9501334].

%MEGA1%
\bibitem{MEGA}
Brooks~M~L \etal  [MEGA Collaboration],
%``New limit for the family-number non-conserving decay \meg'',
{\it Phys.\ Rev.\ Lett.}\  {\bf 83}, 1521 (1999)
%[arXiv:hep-ex/9905013].

%Previous reports
\bibitem{donato}
Nicol\`o D, NUFACT01 proceedings, to appear on {\it Nucl.\ Instr.\ Meth.}~A
({\it proc.\ suppl.})
\nonum Mihara S, NUFACT01 proceedings, ibid.


\bibitem{nproposal}
Baldini A and Mori T (spokepersons) \etal
``The MEG experiment: search for the \meg\ decay at PSI'' (2002)
available at the web-site: {\tt http://meg.psi.ch}


%liquid Xenon
\bibitem{Doke}
Doke T and Masuda K,
%``Present Status Of Liquid Rare Gas Scintillation Detectors And Their New  Application To Gamma-Ray Calorimeters,''
{\it Nucl.\ Instrum.\ Meth.}\  A{\bf 420}, 62 (1999)

\end{thebibliography}

\end{document}







