\subsection{Double beta decay}
\label{ch:cha41}
The most promising way to distinguish between Dirac and \mas is \nbb (\obb{})
\be
(Z,A) \ra (Z+2,A) + 2 e^-  \quad (\Delta L =2)
\ee
only possible if 
\neus are massive \majo particles.
The measured quantity is called effective \majo \neu mass \ema and given by
\be
\label{eq:ema}\ema = \mid \sum_i U_{ei}^2 \eta_i m_i \mid
\ee
with the relative CP-phases $\eta_i = \pm 1$, $U_{ei}$ as the mixing
matrix elements and
$m_i$ as the
corresponding mass eigenvalues. 
From the experimental point, the evidence for \obb is a peak in the sum energy
spectrum of the electrons 
at the
Q-value of the involved transition. 
The best limit is coming from the Heidelberg-Moscow \expe resulting in a 
bound of \cite{bau99}
(Fig.\ref{pic:heimo}) 
\be
\label{eq:thalb}
\ton > 5.7 \cdot 10^{25} y \ra \ema < 0.2 eV \quad (90 \% CL)
\ee 
having a sensitivity of $\ton > 1.6 \cdot 10^{25} y$. 
Eq.(\ref{eq:ema}) has to be modified in case of heavy \neus ($m_{\nu}
\gsim $1 MeV). For such heavy \neus the mass can no longer be neglected in the
\neu propagator resulting in an A-dependent
contribution
\be
\ema =  \mid \sum_{i=1,light}^N U^2_{ei} m_i + \sum_{h=1,heavy}^M F (m_h,A) 
U^2_{eh} m_h  \mid
\ee
By comparing these limits for isotopes with different atomic mass, 
interesting limits
on the mixing angles and \ntau parameters for an MeV \ntau 
can be obtained \cite{hal83,zub97}.
\begin{figure}[bht]
\begin{center}
\epsfig{file=heimopeak.eps,width=7cm,height=5cm}
\caption{Observed sum energy spectrum of the electrons around the expected \obb{} line position
obtained by the
Heidelberg-Moscow experiment. No signal peak is seen. The two
different spectra correspond to data sets with (black) and 
without (grey) pulse
shape discrimination.}
\label{pic:heimo}
\end{center}
\end{figure} 
\paragraph{Future}
Several upgrades are planned to improve the existing half-life limits, only three are mentioned here, for
details see \cite{zub98}.
The next to come is NEMO-3, a giant TPC using double beta emitters up to 10 kg in form of thin foils,
which should start operation in 2000.
Even more ambitious would be the usage of
large amounts of materials (in the order of several hundred kg to tons)
like enriched \xehsd added to
scintillators
\cite{rag94}, 750 kg $TeO_2$ in form of cryogenic bolometers (CUORE) \cite{fio98} or a
huge cryostat containing several hundred detectors of
enriched \gess with a total mass of 1 ton (GENIUS) \cite{kla98}.  

\documentstyle[12pt,a41,epsfig]{article}
%A useful Journal macro
\def\Journal#1#2#3#4{{#1} {\bf #2}, #3 (#4)}
\def\bi#1{#1}
% Some useful journal names
\def\NCA{\em Nuovo Cimento}
\def\NIM{\em Nucl. Instrum. Methods}
\def\NIMA{{\em Nucl. Instrum. Methods} A}
\def\NIMB{{\em Nucl. Instrum. Methods} B}
\def\NPB{{\em Nucl. Phys.} B}
\def\NPBP{{\em Nucl. Phys.} B (Proc. Suppl.)}
\def\NPA{{\em Nucl. Phys.} A}
\def\PLB{{\em Phys. Lett.}  B}
\def\PRL{\em Phys. Rev. Lett.}
\def\PRD{{\em Phys. Rev.} D}
\def\PRC{{\em Phys. Rev.} C}
\def\PRA{{\em Phys. Rev.} A}
\def\PR{{\em Phys. Rev. }}
\def\PRP{{\em Phys. Rep. }}
\def\EPC{{\em Europ. Phys. J.} C}
\def\ZPC{{\em Z. Phys.} C}
\def\ZPA{{\em Z. Phys.} A}
\def\ZP{{\em Z. Phys. }}
\def\PS{{\em Physics Scripta }}
\def\PAN{\em Phys. Atom. Nucl.}
\def\ARAA{{\em Ann. Rev. Astr. Astroph.}}
\def\ARNP{{\em Ann. Rev. Nucl. Part. Phys.}}
\def\GCA{\em Geochim. Cosmochim Acta}
\def\PNPP{\em Prog. Nucl. Part. Phys.}
\def\PTP{\em Prog. Theo. Phys.}
\def\PTPS{\em Prog. Theo. Phys. Suppl.}
\def\APJ{\em ApJ}
\def\AA{\em Astron. Astroph.}
\def\AAS{\em Astron. Astroph. Suppl. Ser.}
\def\RPP{\em Rep. Prog. Phys.}
\def\RMP{\em Rev. Mod. Phys.}
\def\ASP{\em Astroparticle Physics}
\def\EPL{\em Europhys. Lett.}
\def\IJMP{\em Int. Journal of Modern Physics}
\def\JPG{\em Journal of Physics G}
\def\NAT{\em Nature}
\def\SCI{\em Science}
\def\JETP{\em JETP Lett.}
\def\ANYAS{\em Ann. NY. Acad. Sci}
\def\BASI{\em Bull. Astr. Soc. India}
\def\BAPS{\em Bull. Am. Phys. Soc.}
\def\SJNP{\em Sov. Journal of Nucl. Phys.}
\def\ZETZ{\em Zh. Eksp Teor. Fiz.}
\def\ra{\rightarrow}
\def\be{\begin{equation}}
\def\ee{\end{equation}}
\def\bea{\begin{eqnarray}}
\def\eea{\end{eqnarray}}
\newcommand{\pola}{\stackrel{\rightarrow}{\Rightarrow}}
\newcommand{\polb}{\stackrel{\rightarrow}{\Leftarrow}}
\newcommand{\polc}{\stackrel{\rightarrow}{\Uparrow}}
\newcommand{\pold}{\stackrel{\rightarrow}{\Downarrow}}
%\newcommand{\gsim}{\stackrel{\lower.7ex\hbox{$>$}{\lower.7ex\hbox{$\sim$}}}
%\newcommand{\lsim}{\stackrel{\lower.7ex\hbox{$<$}{\lower.7ex\hbox{$\sim$}}}
\def\st{\scriptstyle}
\def\sst{\scriptscriptstyle}
\def\mco{\multicolumn}
\newcommand{\gsim}{\mbox{$\stackrel{>}{\sim}$ }}
\newcommand{\lsim}{\mbox{$\stackrel{<}{\sim}$ }}
\newcommand{\expe}{experiment }
\newcommand{\expek}{experiment, }
\newcommand{\tran}{transition }
\newcommand{\sens}{sensitivity }
\newcommand{\exps}{experiments }
\newcommand{\expsp}{experiments. }
\newcommand{\gut}{grand unified theory }
\newcommand{\bb}{double beta decay }
\newcommand{\eos}{equation of state }
\newcommand{\obb}{0\mbox{$\nu\beta\beta$ decay}}
\newcommand{\zbb}{2\mbox{$\nu\beta\beta$ decay} }
\newcommand{\nbb}{neutrinoless double beta decay }
\newcommand{\majo}{Majorana }
\newcommand{\ma}{Majorana neutrino }
\newcommand{\mas}{Majorana neutrinos }
\newcommand{\lsnd}{LSND }
\newcommand{\gno}{GNO }
\newcommand{\ica}{ICARUS }
\newcommand{\SNO}{Sudbury Neutrino Observatory }
\newcommand{\uee}{\mbox{$U^2_{e1}$} }
\newcommand{\uez}{\mbox{$U^2_{e2}$} }
\newcommand{\ued}{\mbox{$U^2_{e3}$} }
\newcommand{\mn}{massive neutrinos}
\newcommand{\nbar}{\mbox{${\bar \nu}_{\mu} - \bar {\nu_e}$} }
\newcommand{\nmune}{\mbox{$\nu_{\mu} - \nu_e$} }
\newcommand{\dm}{dark matter }
\newcommand{\ugm}{upward going muons }
\newcommand{\nosz}{neutrino oscillations }
\newcommand{\noszp}{neutrino oscillations. }
\newcommand{\osz}{oscillation }
\newcommand{\oszsp}{oscillations. }
\newcommand{\oszs}{oscillations }
\newcommand{\acc}{accelerator }
\newcommand{\com}{component }
\newcommand{\hdm}{hot dark matter }
\newcommand{\lbls}{long baseline experiments }
\newcommand{\vev}{vacuum expectation value }
\newcommand{\atm}{atmospheric neutrinos }
\newcommand{\ssm}{see-saw-mechanism }
\newcommand{\sn}{supernova }
\newcommand{\sna}{SN 1987A }
\newcommand{\sne}{supernovae }
\newcommand{\crs}{cosmic rays }
\newcommand{\agn}{AGN }
\newcommand{\enu}{\mbox{$E_{\nu}$} }
\newcommand{\tnu}{\mbox{$T_{\nu}$} }
\newcommand{\tg}{\mbox{$T_{\gamma}$} }
\newcommand{\msun}{M_{\odot} }
\newcommand{\pmts}{photomultipliers }
\newcommand{\adn}{almost degenerated neutrinos }
\newcommand{\delm}{\mbox{$\Delta m^2$} }
\newcommand{\nul}{\mbox{$\nu_L$} }
\newcommand{\nulb}{\mbox{$\bar{\nu}_L$} }
\newcommand{\nur}{\mbox{$\nu_R$} }
\newcommand{\nurb}{\mbox{$\bar{\nu}_R$} }
\newcommand{\nui}{\mbox{$\nu_i$} }
\newcommand{\nua}{\mbox{$\nu_{\alpha}$} }
\newcommand{\nuei}{\mbox{$\nu_1$} }
\newcommand{\nuzw}{\mbox{$\nu_2$} }
\newcommand{\nulc}{\mbox{$(\nu_L)^C$} }
\newcommand{\nurc}{\mbox{$(\nu_R)^C$} }
\newcommand{\nr}{\mbox{$N_R$} }
\newcommand{\me}{\mbox{$m_{\nu_e}$} }
\newcommand{\mbe}{\mbox{$m_{\bar{\nu}_e}$} }
\newcommand{\mmu}{\mbox{$m_{\nu_\mu}$} }
\newcommand{\mtau}{\mbox{$m_{\nu_\tau}$} }
\newcommand{\bnel}{\mbox{$\bar{\nu}_e$} }
\newcommand{\bnmu}{\mbox{$\bar{\nu}_\mu$} }
\newcommand{\bntau}{\mbox{$\bar{\nu}_\tau$} }
\newcommand{\nel}{\mbox{$\nu_e$} }
\newcommand{\nmu}{\mbox{$\nu_\mu$} }
\newcommand{\ntau}{\mbox{$\nu_\tau$} }
\newcommand{\sint}{\mbox{$sin^2 2\theta$} }
\newcommand{\sk}{Super-Kamiokande }
\newcommand{\munu}{\mbox{$\mu_{\nu}$} }
\newcommand{\mub}{\mbox{$\mu_B$} }
\newcommand{\ton}{\mbox{$T_{1/2}^{0\nu}$} }
\newcommand{\tzn}{\mbox{$T_{1/2}^{2\nu}$} }
\newcommand{\snp}{solar neutrino problem }
\newcommand{\mamo}{magnetic moment}
\newcommand{\lh}{left-handed }
\newcommand{\rh}{right-handed }
\newcommand{\neu}{neutrino }
\newcommand{\neus}{neutrinos}
\newcommand{\neusph}{neutrinosphere }
\newcommand{\neusp}{neutrinos.}
\newcommand{\sit}{\mbox{$sin \theta$}}
\newcommand{\siqt}{\mbox{$sin^2 \theta$}}
\newcommand{\cost}{\mbox{$cos \theta$}}
\newcommand{\coqt}{\mbox{$cos^2 \theta$}}
\newcommand{\ema}{\mbox{$\langle m_{\nu_e} \rangle$ }}
\newcommand{\mm}{\mbox{$m_{\nu_\mu}$}}
\newcommand{\mt}{\mbox{$m_{\nu_\tau}$}}
\newcommand{\gess}{\mbox{$^{76}Ge$ }}
\newcommand{\rnzhz}{\mbox{$^{222}Rn$ }}
\newcommand{\moeh}{\mbox{$^{100}Mo$ }}
\newcommand{\ndhf}{\mbox{$^{150}Nd$ }}
\newcommand{\caav}{\mbox{$^{48}Ca$ }}
\newcommand{\gdhs}{\mbox{$^{160}Gd$ }}
\newcommand{\hohds}{\mbox{$^{163}Ho$ }}
\newcommand{\rehsa}{\mbox{$^{187}Re$ }}
\newcommand{\tehaz}{\mbox{$^{128}Te$ }}
\newcommand{\tehd}{\mbox{$^{130}Te$ }}
\newcommand{\cdhsz}{\mbox{$^{116}Cd$ }}
\newcommand{\xehsd}{\mbox{$^{136}Xe$ }}
\newcommand{\xehed}{\mbox{$^{131}Xe$ }}
\newcommand{\cshed}{\mbox{$^{131}Cs$ }}
\newcommand{\cshsd}{\mbox{$^{137}Cs$ }}
\newcommand{\clsd}{\mbox{$^{37}Cl$ }}
\newcommand{\arsd}{\mbox{$^{36}Ar$ }}
\newcommand{\arad}{\mbox{$^{38}Ar$ }}
\newcommand{\arsid}{\mbox{$^{37}Ar$ }}
\newcommand{\arv}{\mbox{$^{40}Ar$ }}
\newcommand{\kav}{\mbox{$^{40}K$ }}
\newcommand{\gaes}{\mbox{$^{71}Ga$ }}
\newcommand{\cref}{\mbox{$^{51}Cr$ }}
\newcommand{\seza}{\mbox{$^{82}Se$ }}
\newcommand{\zsn}{\mbox{$^{96}Zr$ }}
\newcommand{\csz}{\mbox{$^{12}C$ }}
\newcommand{\sfz}{\mbox{$^{15}O$ }}
\newcommand{\sfs}{\mbox{$^{16}O$ }}
\newcommand{\sdz}{\mbox{$^{13}N$ }}
\newcommand{\sszw}{\mbox{$^{12}N$ }}
\newcommand{\ssz}{\mbox{$^{16}N$ }}
\newcommand{\gees}{\mbox{$^{71}Ge$ }}
\newcommand{\ihsz}{\mbox{$^{127}I$ }}
\newcommand{\yhss}{\mbox{$^{176}Yb$ }}
\newcommand{\hev}{\mbox{$^4He$ }}
\newcommand{\hed}{\mbox{$^3He$ }}
\newcommand{\bes}{\mbox{$^7Be$ }}
\newcommand{\lis}{\mbox{$^7Li$ }}
\newcommand{\ba}{\mbox{$^8B$ }}
\newcommand{\cms}{\mbox{$cm^{-2}s^{-1}$ }}
\newcommand{\zhf}{\mbox{$/10^5 m^2 y$ }}


\begin{document}
\sloppy
\begin{center}
%{\Large \bf Neutrinos}
\section*{Terrestrial neutrino mass searches}
\medskip
{\it K. Zuber$^a$}\\
%$^a$ Dept. of physics, University of California, San Diego (GEORGE PLEASE EDIT THIS !!!)\\
$^a$ Lehrstuhl f\"ur Exp. Physik IV, Universit\"at Dortmund, 44221 Dortmund
\end{center}
\setcounter{section}{1}
\subsection{Introduction}
Neutrinos play a fundamental role in several fields of physics from cosmology down to
particle physics. Even more, the observation of a non-vanishing rest mass of neutrinos would
have a big impact on our present model of particle physics and might guide towards grand
unified
theories. Currently three evidences exist showing effects of massive neutrinos: the deficit in
solar neutrinos, the zenith angle dependence of atmospheric neutrinos and the excess events
observed by LSND. These effects are explained with the help of neutrino
oscillations, thus depending on \delm{} = $m_2^2 - m_1^2$, where $m_1,m_2$ are the \neu{} mass
eigenvalues and therefore are not absolute mass measurements. 
For a recent review on the physics of massive neutrinos see \cite{zub98}. 
\input massnel.tex
\input massmu.tex
\input masstau.tex
\input bb.tex
\input mamo.tex
\input ref.tex
\end{document}


\subsection{Mass measurement of the muon \neu{}}
The way to obtain limits on \mmu is given by the two-body decay of
the $\pi^+$.
%For pion decay at rest the \neu mass is determined by
%\be
%\mmu^2 = m_{\pi^+}^2 +  m_{\mu^+}^2 - 2 m_{\pi^+} (p_{\mu^+}^2 +
%m_{\mu^+}^2)^{(1/2)}
%\ee
A precise measurement of the muon momentum $p_{\mu}$ and
knowledge of $m_{\mu}$
and
$m_{\pi}$ is required. 
These measurement was done at the PSI resulting in a limit of \cite{ass96}
\be
\mmu^2 = (-0.016 \pm 0.023) MeV^2 \quad \ra \quad \mmu < 170 keV (90
\%CL)
\ee
%Investigations of pionic atoms
%lead to two values of $m_\pi = 139.56782 \pm 0.00037$ MeV and $m_\pi =
%139.56995 \pm 0.00035$ MeV respectively \cite{jec94}, but a recent independent
%measurement supports
%the higher value by measuring  $m_\pi = 139.57071 \pm 0.00053$ MeV \cite{len98}.\\
A new idea looking for pion decay in flight using the g-2 storage ring at BNL has been proposed
recently \cite{cus99}. Because the g-2 ring would act as a high resolution spectrometer an
exploration
of \mmu down to 8 keV seems possible. Such a bound would have some far reaching consequences:
First of all it would be the largest step on any neutrino mass improvement within the last 20
years (Fig.\ref{pic:pdg}). Secondly it would bring any magnetic moment calculated within the 
standard model and associated with \nmu down to a
level of vanishing
astrophysical importance. Furthermore it would once and for all exclude that a possible 17 keV
mass eigenstate is the dominant contribution of \nmu . Possibly the largest impact is on
astrophysical topics. All bounds on \neu properties derived from stellar evolution are typically
valid for \neu masses below about 10 keV, so they would then apply for \nmu as well. For example, plasma
processes like $\gamma \ra \nu \bar{\nu}$ would contribute to stellar energy losses and significantly prohibit
helium ignition, unless the neutrino has a magnetic moment smaller than $\mu_{\nu} < 3 \cdot 10^{-12} \mu_B$
\cite{raf99} much more stringent than laboratory bounds. 
\begin{figure}
\begin{center}
\epsfig{file=neu.eps,width=8cm,height=6cm}
\caption{Evolution of neutrino mass limits over the last 15 years using the Particle Data Group values.
Extrapolated values are given for 2000 and 2002. Electron neutrino limits are given for $\beta$-decay (black 
diamonds) and SN
1987A (green diamonds), for \nmu{} as triangles and \ntau as squares. As can be seen, the proposed measurement
of $m_{\nmu}$ at
the g-2 \expe{} would result in the largest factor obtained. The mass scale corresponds to eV (\nel), keV
(\nmu) and MeV (\ntau) respectively.}
\label{pic:pdg}
\end{center}
\end{figure}

\subsection{Mass measurements of the electron neutrino}
The classical way to determine the mass of $\bar{\nel}$ (which is identical to $m_{\nu_e}$
assuming CPT invariance) is the
investigation of the
electron spectrum in beta decay.
% Neutrino masses should manifest itself in a reduction of
%the
%maximal energy available for the observed electrons. The 
%corresponding Kurie-plot ends 
%vertical at Q - $m_{\nu}c^2$, where Q is the transition energy.
A finite \neu mass will reduce the phase space and leads to a 
change of the shape
of the electron spectra.
In case several mass
eigenstates contribute, the total electron spectrum is given by a 
superposition
of the individual
contributions
\be
N(E) \propto F(E,Z) \cdot p \cdot E \cdot (Q-E) \cdot \sum^3_{i=1} 
\sqrt{(Q-E)^2 - m_i^2}
\mid U_{ei}^2
\mid 
\ee
where F(E,Z) is the Fermi-function, $m_i$ are the mass eigenvalues, $U_{ei}^2$ are
the mixing matrix elements connecting weak and mass eigenstates and $E,p$ are energy and momentum
of the emitted electron. The different involved $m_i$ produce kinks 
in the Kurie-plot 
where the size of the kinks is a measure
for the corresponding mixing
angle. This was discussed in connection with the now ruled out 17 keV
- \neu . A new sensitive search for kinks in the region 4-30 keV using
$^{63}$Ni
was done recently resulting in an overall upper limit of $ U_{e2}^2 < 10^{-3}$ \cite{hol99}.\\
Searches for an eV-\neu are done near the endpoint region of isotopes with low Q - values.
The preferred isotope under study is tritium, with an endpoint energy of about 18.6 keV.
By extracting o \neu mass limit out of their data, most \exps done in the past end up with
negative $m_{\nu}^2$ fit values,
which need
not to have a common origin. 
\begin{figure}
\begin{center}
\begin{tabular}{cc}
\epsfig{file=combine_q3_q4_q5.eps,width=8cm,height=6cm} &
\epsfig{file=re187spec.eps,width=7cm,height=5cm}
\end{tabular}
\caption{left: Mainz 1998 electron spectrum near the endpoint of tritium decay. The
signal/background ratio is increased by a factor 
of 10 in comparison with the
1994 data. The Q-value of 18.574 keV is marking to the center of mass of the rotation-vibration 
excitations of the
molecular ground state of the daughter ion $^3HeT^+$. right: \rehsa $\beta$-spectrum obtained with a
cryogenic bolometer by the Genoa group. Calibration peaks can also be seen.}
\label{pic:mainz}
\end{center}
\end{figure}
For a detailed discussion of the \exps see \cite{hol92,ott95}.
While until
1990 mostly magnetic spectrometers were used
for the measurements, the new \exps in Mainz and Troitzk use electrostatic retarding
spectrometers \cite{lob85,pic92}. 
Fig.\ref{pic:mainz} shows the
present electron spectrum near the
endpoint as obtained with the Mainz spectrometer. 
The current obtained limits are 2.8 eV (95 \% CL) ($m_{\nu}^2 = - 3.7 \pm 5.3 (stat.) \pm 2.1 (sys.) eV^2$)
\cite{wei99} and 2.5 eV (95 \% CL)
($m_{\nu}^2 = - 1.9 \pm 3.4 (stat.) \pm 2.2 (sys.) eV^2$)
\cite{lob99} respectively. The final sensitivity should be around 2 eV.\\
Beside this, the Troitzk \expe observed excess counts in the
region of interest,
which can be described by a monoenergetic line a few eV below the endpoint. 
Even more, a semiannual modulation of the line position is observed \cite{lob99}. Clearly
further 
measurements are needed to investigate this effect. Considerations of building a new larger scale
version of such a spectrometer exist, to probe neutrino masses down below 1 eV.\\
A complementary strategy is followed by using cryogenic microcalorimeters. Because these
\exps measure the total energy released,
final state effects are not important. This method allows the investigation
of the $\beta$-decay of \rehsa, which has the lowest Q-value of all $\beta$-emitters (Q=2.67
keV). Furthermore the associated half-life measurement would be quite important, because
the \rehsa - $^{187}$Os pair is a well known cosmochronometer and a more precise half -
life measurement would sharpen the dating of events in the early universe like the formation of 
the solar system.
Cryogenic bolometers were build in form of metallic Re as well as AgReO$_4$ crystals and
$\beta$ - spectra (Fig.\ref{pic:mainz}) were measured \cite{gat99} \cite{ale99}, but at present 
the experiments
are not
giving any limits on \neu masses. Investigations to use this kind of technique also for
calorimetric measurements on tritium \cite{dep99} and on $^{163}$Ho \cite{meu98} are currently done.
Measuring accurately
branching ratios of atomic transitions or the internal bremsstrahlung spectrum in $^{163}$Ho
is interesting because this would result directly in a limit on \me{}.
\begin{thebibliography}{99}
\bibitem{zub98} K. Zuber, \Journal{\PRP}{305}{295}{1998}
\bibitem{hol99} E. Holzschuh et al.,\Journal{\PLB}{451}{247}{1999}
\bibitem{hol92} E. Holzschuh, \Journal{\RPP}{55}{1035}{1992}
\bibitem{ott95} E.W. Otten, \Journal{\NPBP}{38}{26}{1995}
\bibitem{lob85} V.M. Lobashev et al., \Journal{\NIMA}{240}{305}{1985}
\bibitem{pic92} A. Picard et al., \Journal{\NIMB}{63}{345}{1992}
\bibitem{wei99} C. Weinheimer et al., \Journal{\PLB}{460}{219}{1999}
\bibitem{lob99} V.M. Lobashev et al., \Journal{\PLB}{460}{227}{1999}
\bibitem{gat99} F. Gatti et al., \Journal{Nature}{397}{137}{1999}
\bibitem{ale99} A. Alessandrello et al., \Journal{\PLB}{457}{253}{1999}
\bibitem{dep99} D. Deptuck, private communication
\bibitem{meu98} P. Meunier, \Journal{\NPBP}{66}{207}{1998}
\bibitem{ass96} K. Assamagan et al., \Journal{\PRD}{53}{6065}{1996}
\bibitem{cus99} P. Cushman , K. Jungmann, private communication
\bibitem{raf99} G. Raffelt, Prep.  to appear in ARNPS Vol.49
\bibitem{pas97} L. Passalacqua, \Journal{\NPBP}{55C}{435}{1997}
\bibitem{bar98} R. Barate et al., \Journal{\EPC}{2}{395}{1998}
\bibitem{bau99} L. Baudis et al., \Journal{\PRL}{83}{41}{1999}
\bibitem{hal83} A.Halprin, S.T.Petcov, S.P.Rosen, \Journal{\PLB}{125}{335}{1983}
\bibitem{zub97} K. Zuber, \Journal{\PRD}{56}{1816}{1997}
\bibitem{rag94} R.S. Raghavan, \Journal{\PRL}{72}{1411}{1994}
\bibitem{fio98} E. Fiorini, private communication
\bibitem{kla98} H.V. Klapdor-Kleingrothaus, J. Hellmig, M. Hirsch, 
\Journal{\JPG}{24}{483}{1998}
\bibitem{lee77} B.W. Lee, R.E. Shrock, \Journal{\PRD}{16}{1444}{1977}
\bibitem{mar77} W.J. Marciano, A.I. Sanda, \Journal{\PLB}{67}{303}{1977}
\bibitem{pal92}P.B. Pal, \Journal{\IJMP}{A7}{5387}{1992}
\bibitem{kra90} D. Krakauer et al., \Journal{\PLB}{252}{171}{1990}
\bibitem{abe87} K. Abe et al., \Journal{\PRL}{58}{636}{1987}
\bibitem{kim88} C.S. Kim, W.J. Marciano, \Journal{\PRD}{37}{1368}{1988}
\bibitem{ams97}C. Amsler et al., \Journal{\NIMA}{396}{115}{1997}
\bibitem{bed97} A.G. Beda et al., \Journal{\PAN}{61}{66}{1998}
\bibitem{bar96} I. Barabanov et al., \Journal{\ASP}{5}{159}{1996}
\end{thebibliography}

