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results of the ceres experiment show a significantly higher dilepton yield in the invariant mass region between about 200 and 600 mev compared to that from a hadronic cocktail based on known reaction channels . after adding in the  expected  secondary @xmath0 yield the observed yield is still underestimated by a hadronic cocktail by a sensitivity factor of about 3 . while there is also an attempt to explain these observations in terms of standard classical hadronic sources @xcite by exploiting their experimental uncertainties , most theoretical explanations invoke a change of the properties of the vector meson masses and widths .
some of these are based on classical collision broadening whereas others involve @xmath1 mixing as a precursor to chiral symmetry restoration @xcite . however , all these attempts have in common that in a first step they determine the in - medium properties in hot and dense equilibrium and then use these locally either in transport calculations or in more simplified expansion scenarios .
the latter procedure is obviously not a - priori correct for an ultrarelativistic heavy - ion collision which  at least in its early stages  proceeds far away from equilibrium .
it is worthwhile to realize that even in ultrarelativistic heavy - ion collisions with their high peak densities about 60 % of all dileptons are produced at densities below @xmath2 ( see fig .  7 in @xcite )
this is so because the observed yield inherently contains an integration over the whole reaction history and thus the late stages of the reaction with high pion densities contribute via @xmath3 a significant part to the total observed dilepton yield .
it is , therefore , intriguing to ask if other reactions involving more elementary probes on nuclei that proceed closer to equilibrium can be used to investigate the question of in - medium changes of hadronic properties . while the density in such reactions obviously is always below @xmath4 , this disadvantage may be overcome by the cleaner and more stable environment in which dilepton production proceeds in such reactions .    in this talk
i show that indeed pion- and photon - induced dilepton production on nuclei is as sensitive to possible in - medium changes as are heavy - ion reactions .
i will also  in the last part of this talk  show how in - medium changes of vector mesons are expected to show up not only in dilepton spectra , but also in photo - absorption processes .
such a connection , if it can be firmly established , clearly adds to the consistency of our picture of in - medium changes of hadrons .
details to all the points i am going to discuss in this talk can be found in a number of recent publications @xcite and in particular in the phd thesis of martin effenberger @xcite .
the calculations shown in this talk are all based on a new development in transport theory that allows one to also tranport broad resonances .
while former calculations ( for a review see @xcite ) had to employ simplifiying assumptions the calculations of effenberger et al.@xcite , for the first time , implemented a transport - theoretical treatment of broad mesons and treated the widths of the vector mesons consistently .
the theoretical breakthrough here was to transport not the phase - space density @xmath5 , but a spectral phase space density @xmath6 that contains also information about the spectral function of the resonance .    in @xcite
( see also @xcite ) we have discussed in detail that a consistent formulation of the scattering term , that takes the spectral function of the particles into account , puts automatically intrinsically broad , collision - broadened resonances ( such as , e.g.  the @xmath7 meson )  because of their frequent decay and reformation as they traverse the nucleus  back on their free spectral function when they leave the nucleus .
a problem arises , however , when particles with intrinsically sharp spectral functions , such as the nucleon or the @xmath8 meson , get collision broadened .
in such a case the repetitive decay and reformation is not frequent enough so that these particles would emerge from the collision still collision - broadened . in order to surpress this unphysical behavior we introduced an ad - hoc potential that drives all particles back on mass - shell when they leave the nucleus @xcite .
it is amusing that this potential , that we intuitively guessed in ref .
@xcite , can  for sharp resonances , where @xmath9  actually be derived from the kadanoff - baym equations @xcite .
the results presented here are based on this method ( for more details see @xcite ) .
the equation of motion for the spectral phase space distribution reads @xmath10 with the spectral phase - space - density @xmath11 the spectral function @xmath12 and the usual phase - space density @xmath13 . as stressed in particular by knoll
@xcite it is essential to maintain the consistency condition that the total width appearing in the spectral function is related to the loss - term @xmath14 in the transport equation @xmath15 since this condition presents a major consistency problem it has been fulfilled in the calculations reported here only for the vector mesons . in @xcite it was shown that the two - body collisional width does not broaden the nucleon resonances significantly .
for all further details i refer the reader to @xcite .
with the availability of a @xmath16 beam at gsi and the detectorsystem hades @xcite it will be possible to explore dilepton production in pion - induced reactions on nuclei .
we have , therefore , performed extensive studies of such reactions @xcite .
input to these calculations are cross sections predicted by the manley coupled channel analysis of @xmath17 data @xcite on which our analysis is based .
the transport calculations allow one to follow the collision history of all vector mesons produced . with realistic @xmath18 cross sections we obtain a collision width @xmath19 of up to 500 mev for the @xmath7 meson and 70 mev for the @xmath8 , both at normal nuclear matter density @xcite .
the overall sensitivity to these effects is about a factor 2 , which is close to the expected systematic uncertainty of these transport calculations .
however , this sensitivity can be enhanced by performing appropriate cuts on the dilepton cm momenta .
[ picut ] shows that with a cut that selects only slow vector mesons the sensitivity to various in - medium scenarios , in particular for the @xmath8 meson , is clearly strong enough to be seen experimentally .
pions have one possible disadvantage for such in - medium studies : they experience a strong initial - state interaction and thus a pion - beam illuminates only a part of the nucleus .
this is visible in fig.[pi - gamma ill ] which shows in the two lower figures the location of the first collision of the pion with the nucleons of the target ( left ) and all meson - baryon collisions ( right ) .
the upper two figures give the same information for a photon beam .
the left part shows clearly how the photon illuminates the whole nuclear volume and not just the surface .
subsequent collisions remember this initial state as shown in the right part of this figure .
thus , we expect that photon - induced experiments will be even more sensitive to in - medium changes of hadronic properties than experiments with pions .    that the theory , with the proper , consistent treatment of in - medium broadening describes the normal photonuclear processes very well is shown in a comparison with data for photo - pion , photo-2pion and photon - eta production data at energies up to 800 mev @xcite .
we are thus quite confident that we have these photonuclear processes well under control and can thus use these methods to predict and investigate the sensitivity of photonuclear dilepton production data to in - medium changes of vector meson properties .
photon - induced dilepton data have the problem that they necessarily contain a big contribution from the so - called bethe - heitler process in which the incoming photon radiates already a dilepton pair before any coupling to the hadrons takes place
. as is well known , however , this contribution can be surpressed by proper kinematical cuts @xcite , both for the incoherent and the coherent part @xcite .
[ gamdilsens ] then shows the sensitivity of the dilepton spectra to various in - medium changes of the vector mesons .
it is immediately apparent , that this sensitivity is just as large as for the ultrarelativistic heavy - ion collisions . in particular , again the @xmath8 properties
are reflected quite clearly in the ( still to be measured ) dilepton spectra . as in the case of pions
one can enrich the in - medium sensitivity by making a cut on the dilepton cm momentum .
all the in - medium effects on vector meson spectral functions are connected with a significant broadening and corresponding shift of strength to lower masses .
this has an interesting consequence for the decay of the n(1520 ) resonance that i have already pointed out in @xcite in 1997 . according to the particle data group listing , which is based on manley s analysis @xcite in this respect , this @xmath20 resonance decays on resonance with a probability of 15 - 25 % into @xmath21 and this , even though this decay is energetically closed if one works with the pole mass of the ( broad ) @xmath7 .
thus the decay can proceed only through the low - mass tails of the @xmath7-meson spectral function ; the rather large partial decay width in turn indicates a large coupling constant for the vertex @xmath22 .
if the @xmath7 meson s spectral function in nuclear matter indeed gains strength at low masses then the partial @xmath7 decay width of the @xmath23 resonance would increase dramatically .
this effect may already have been experimentally seen . the data for photoabsorption on nuclei exhibit a universal behavior that  beginning with rather small mass numbers @xmath24  scales with @xmath24 .
this universal cross section exhibits the @xmath25 resonance  though somewhat broadened  , but the second and third resonance regions around 1500 and 1650 mev , respectively , that are well seen in photoabsorption on the nucleon , have disappeared in nuclei @xcite .
fermi - motion alone is the more effective in smearing out resonance strength the higher the energy of the resonance is .
this alone explains the absence of the third resonance region @xcite .
the second resonance region , around 1500 mev , however , survives the effects of fermi - smearing .
we have shown in @xcite that this finds a natural explanation in the opening of the @xmath7-decay channel of the @xmath23 resonance . with @xmath7 strength moving in medium down to lower masses
the formerly nearly closed decay opens up dramatically thus leading to a very large ( several hundred mev ) total width of this resonance .
crucial for this argument is the high partial decay width of the @xmath20 resonance into the @xmath21 channel .
this has so far , not been directly verified by experiment ; the cited large partial decay width is based only on a theoretical coupled channel analysis @xcite .
although it is gratisfying to see that a similar result also emerges now from the cc calculations of friman , lutz and wolf @xcite , a direct experimental verification of this decay branch would necessitate a detailed partial - wave analysis of ( @xmath26 ) reactions on the nucleon which so far does not exist .
it is , however , encouraging , that the daphne collaboration @xcite and the taps collaboration @xcite , both at mami , have found experimental evidence for pion invariant mass spectra in such reactions that show clear deviations from phase - space .
in this talk i have shown that pion- and photon - induced reactions show a sensitivity to in - medium changes of hadrons that is as large as that observed in ultrarelativistic heavy ion collisions .
this result was originally unexpected because of the much lower baryon densities probed in such elementary reactions on nuclei ( @xmath27 ) , compared with those obtained in ultrarelativistic heavy - ion collisions ( @xmath28 ) .
it finds its explanation in the fact that all experimentally observed dilepton spectra contain an integration over the whole reaction history , which , in the case of heavy - ions , runs from @xmath29 to more than @xmath30 and then back down again to nearly @xmath31 while the pion- or photon - induced reaction on nuclei always proceeds close to @xmath4 , i.e.  much closer to equilibrium .    in my opinion , an unequivocal identification of the origins of changes of hadronic spectral functions observed in ultrarelativistic heavy - ion collisions
will not be possible as long as the more elementary reactions on nuclei have not been studied as well .
it is obvious that hadronic in - medium effects , if they are observed in such reactions at or below @xmath4 , have nothing to do with any transition to the qgp and even a possible connection with chiral symmetry restoration , a very popular keyword in this field , will be hard to establish ( of course , duality arguments can always be used to keep this connection alive ! ) .
what we can learn , however , are interaction rates of hadrons with baryons , also for unstable hadrons , and this alone will be an exciting prospect . experiments with hades using the pion beam at gsi and experiments with photon beams , starting now with taps at mami , will help us to clarify the issue of in - medium changes of hadrons .
for example , a close comparison of dilepton spectra obtained in pion - induced reactions with those obtained with photons should allow one to sample quite different density regions and thus in - medium effects , in particular for the @xmath8 meson .
this is illustrated without further words in fig .
[ spacedist ] .
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the sensitivity of dilepton production in elementary reactions of photons and pions with nuclei on in - medium changes of hadronic properties is studied . 
 it is shown that this sensitivity is comparable to that encountered in ultrarelativistic heavy - ion collisions . it is also shown how a significant broadening of the vector mesons affects other photonuclear processes .    * 
 hadronic in - medium effects * + * with elementary probes * + u. mosel + _ institut fuer theoretische physik , universitaet giessen + d-35392 giessen , germany + _