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all confirmed experimental data acquired to date indicate the conservation of lepton numbers .
this fact can be described by several different empirical laws @xcite , some of which follow additive and some obey multiplicative , parity - like , schemes .
experiments have given no indication yet for favouring any of them .
the standard model states for every lepton flavour a separate additively conserved quantum number
. however , such lepton numbers have no status , unless their conservation can be associated with a local gauge invariance @xcite .
mixings between different generations are well known in the quark sector and the cabbibo - kobayashi - maskawa matrix @xcite relates the weak quark eigenstates with their mass eigenstates .
a familiar example are the @xmath2-@xmath3 oscillations . at present
we are left puzzled why leptons do not show any similar mixing .
recent experimental hints for neutrino oscillations , which have a potential for changing this situation , are not covered here ( see e.g. @xcite ) .
many extensions to the standard model have been proposed and are presently discussed which try to explain further some of its not well understood features like e.g. parity violation in weak interaction or particle mass spectra .
they are put by hand into this remarkable theoretical framework which appears to serve as an extremely robust description of all confirmed particle physics .
lepton flavor violation ( lfv ) appears naturally in such models which include left - right - symmetry , supersymmetry , technicolor , grand unification , string theories , compositeness , and many others .
they continue to stimulate experimental searches in a large range of energies .
highest sensitivity has generally been reached in dedicated search experiments particularly on kaons ( @xmath4 ) and muons ( @xmath0 ) ( table [ lnv_limits ] ) , where also a high discovery potential for new physics exists @xcite , as well as in non accelerator experiments searching for neutrinoless double @xmath1-decay .
the decays of heavier objects created in high energy collisions , however , can be observed less accurately .
the progress in the @xmath4 and @xmath0 ( see sec .
[ kaons ] and [ muons ] ) field is indicated in fig.[history ] which shows more than 10 decades of improvement since the first experiments in the late 1940 s .
the highest recent gain in sensitivity is for muonium ( m=@xmath5 ) to antimuonium ( @xmath6=@xmath7 ) conversion due to a new , yet unused signature ( see sec .
[ mmbar ] ) .
it was noted already in the early 60ies that , e.g. the process @xmath10 is closely related to muonium - antimuonium conversion @xcite .
indeed such scattering experiments were carried out at the princeton - stanford storage rings at stanford yielding the at the time best limit on the coupling constant @xmath11 @xcite .
today , similar proposals have been made for scattering of high energy @xmath12 on @xmath12 , @xmath12 on @xmath13 , @xmath14 on @xmath14 and @xmath14 on @xmath15 @xcite .
they were mainly discussed in connection with bileponic gauge bosons .
even a lower limit for the cross section of the process @xmath16 was found , provided the sum of the light neutrino masses exceeds @xmath17 ev @xcite .
pronounced resonances have been predicted particularly for such experiments at the next linear collider or the high energy end of a muon collider .
+ although lepton flavour conservation remains a mystery and searches for its violation were not blessed with a successful observation yet , both the theoretical and experimental work in this connection have led to a deeper understanding of particle interactions .
one particular value of the experiments are their continuos contributions towards guiding theoretical developments by excluding various speculative models .
zeldovitch , dan .
ssr * 86 * , 505 ( 1952 ) b. pontecorvo , sov.phys.-jetp * 37 * , 1751 ( 1959 ) and sov .
jetp * 6 * , 381 ( 1958 ) n. cabbibo and r. gatto , phys.rev.lett .
* 5 * , 114 ( 1960 ) ; n. cabbibo , nuovo cim . * 19*,612 ( 1961 ) e.j .
konopinski and h.m .
mahmoud , phys.rev.*92 * , 1045 ( 1953 ) g. feinberg and s. weinberg , phys.rev.lett .
* 6 * , 381 ( 1961 ) a.  halprin and a.  masiero , phys.rev.d*48 * , 2987 ( 1993 ) m. kobayashi and t. maskawa , prog .
. phys . * 49 * , 652 ( 1973 ) j. stone , this volume r.n .
mohapatra , prog.part.nucl.phys . * 31 * , 39 ( 1993 ) o. adriani et al .
phys . lett .
b * 316 * , 427 ( 1993 ) ; l. bugge et al .
, in : proc .
europhysics conference on high - energy physics , brussels , j. lemonne et al .
( eds . ) , world scientific , singapore ( 1996 ) ; p. abreu et al .
, z. phys .
c * 73 * , 243 ( 1997 ) a. freyberger et al .
, phys.rev.lett.*76 * , 3065 ( 1996 ) r. ammar et al . , phys.rev.d * 49 * , 5701 ( 1994 ) k. edwards et al .
, phys.rev.d * 55 * , 3919 ( 1997 ) w. molzon , jhf98 workshop , tsukuba , ( 1998 ) ; see also ref .
@xcite t. kirk , jhf98 workshop , tsukuba ( 1998 ) j. belz , proc .
intersections between particle and nuclear physics , 6th conf , t.w .
donnelly ( ed . ) , aip press , new york , p.763 ( 1997 ) ; r. ray , jhf98 workshop , tsukuba ( 1998 ) m. zeller , priv .
com.;see also ref .
@xcite and s. eilerts , loc .
@xcite , p. 779
( 1997 ) k. eitel , doctoral thesis , university of karlsruhe ( 1995 ) w. bertl et al .
, nucl.phys . b*260 * , 1 ( 1985 ) m.d .
cooper et al . , loc .
@xcite , p. 34
( 1997 ) s. eggli et al .
, publication in preparation ( 1998 ) j. kaulard et al .
, submitted for publication ( 1998 ) v. meyer et al . ,
@xcite , p. 429
( 1997 ) h.v .
klapdor - kleingrothaus and m. hirsch , z.phys.a*359 * , 361 ( 1997 ) ; h.v .
klapdor - kleingrothaus , proc . beyond the desert conference , institute of physics publishing , bristol ,
p.485 ( 1998 ) v.m .
lobashev , in : proc .
neutrino 96 , world scientific , singapore(1997 ) d. bliss
, phys.rev.d * 57*,5903 ( 1998 ) s.h .
kettell , hep - ex/9801016 ( 1998 ) and references therein t.s .
kosmas , g.k .
leontaris , j.d .
vergados , prog.part.nucl.phys . * 33 * , 397 ( 1994 ) r. barbieri , l. hall and a. strumia , nucl.phys .
b*445 * , 219 ( 1995 ) j.p .
miller et al , loc .
@xcite , p. 792
( 1997 ) u. chattopadhyay and p. nath , phys.rev . d*53 * , 1648 ( 1996 ) t. bolton et al .
rev d*38 * , 2077 ( 1988 ) a. v.d .
schaaf et al . , letter of intent to psi , r98 - 05.0 ( 1998 ) f. cuypers and s. davidson , eur.phys.j . *
c2 * , 503 ( 1998 ) m. raidal and a. santamaria , hep - ph/9710389 ( 1997 ) w. molzon et al .
, proposal to bnl e-940 ( 1997 ) v.w .
hughes and g. zu putlitz , in : _ quantum electrodynamics _ , world scientific , singapore , t. kinoshita ( ed . ) , p. 822
( 1990 ) k. jungmann , in : _ atomic physics 14 _ ( new york : aip press ) , d. wineland et al .
( ed . ) , p. 102
( 1994 ) a.  halprin , phys.rev.lett . * 48 * , 1313 ( 1982 ) p. herczeg and r.n .
mohapatra , phys.rev.lett . * 69 * , 2475 ( 1992 ) w.s .
hou and g.g .
wong , phys.rev .
d*53 * 1537 ( 1996 ) r.n .
mohapatra , z.phys .
c*56 * , s117 ( 1992 ) h. fujii et al .
, phys.rev .
d * 49 * 559 ( 1994 ) r. abela et al .
, phys.rev.lett . *
77 * 1951 ( 1996 ) b.e .
matthias et al . , phys.rev.lett . *
66 * , 2716 ( 1991 ) l. willmann and k. jungmann , lecture notes in physics , vol .
499 , ( 1997 ) l. chatterjee et al .
rev . d*46 * , 46 ( 1992 ) k. horrikawa and k. sasaki , phys .
rev . d*53 * , 560 ( 1996 ) g.g .
wong and w.s .
hou , phys.lett.b*357 * , 145 ( 1995 ) v.a .
gordeev et al , jetp lett .
* 59 * , 589 ( 1994 ) g.g .
wong and w.s .
hou , phys.rev.d*50 * , r2962 ( 1994 ) p. frampton , phys.rev.lett*69 * , 1889 ( 1994 ) ; see also : hep - ph/97112821 ( 1997 ) p. frampton and s. harada , hep - ph/9711448 ( 1997 ) ) p. frampton , priv .
( 1998 ) p. herczeg , conference  beyond the desert 97  , castle ringberg ( 1997 ) c. athanassopoulos et al .
, phys.rev .
c**54 , 2685 ( 1996 ) ; see also : nucl - ex/9709006 r.b .
palmer and j.c .
gallardo , physics/9802002 ( 1998 ) ; r.b .
palmer , physics/9802005 ( 1998 ) y. kuno , priv . com .
( 1998 ) s. glashow , phys.rev.lett .
* 6 * , 196 ( 1961 ) w.c .
barber et al , phys.rev.lett .
* 22 * , 902 ( 1969 ) p. frampton , phys.rev .
d * 45 * , 4240 ( 1992 ) w.s .
hou , nucl .
b*51a * , 40 ( 1996 ) m. raidal , phys.rev.d*57 * , 2013 ( 1998 )
dedicated experiments searching for lepton flavour violation can be performed very sensitively using k - decays and @xmath0-decays as well as neutrinoless double @xmath1-decay and muonium to antimuonium conversion . 
 although there is no confirmed signal reported yet , stringent limits for parameters in speculative extensions to the standard model can be set . 
 some models could recently be ruled out .