1. how is the mass of the nucleon generated?
mN = 938 MeV  mq  5 – 10 MeV
determined by interaction among partons
the role of chiral symmetry breaking
chiral symmetry =
fundamental symmetry of QCD for massless quarks
chiral symmetry broken
on hadron level
 600 MeV
-sym.
nature
the interaction among quarks has to become so strong that it overcomes their quantum mechanical resistance to localization (Wilczek)
mass split comparable to nucleon mass !

2. -mass in nuclei from photonuclear reactions
J.G.Messchendorp et al., Eur. Phys. J. A 11 (2001) 95 g w p0 p
gA   + X 
p0g
advantage:
• p0g large branching ratio (8 %)
• no -contribution (  0 : 7  10-4)
disadvantage:
• p0-rescattering

3. (including detector resolution and 2p0 background)
expected mass distribution for p, Nb
(including detector resolution and 2p0 background)
within 0.6 < Mp0g < 0.8 :

4. photoproduction of  mesons off nuclei
A   A at E  1.2 GeV
in-medium  mass distribution (0)
exp: D. Trnka (Giessen)
theo: P. Mühlich et al., nucl-th/
comparison to BUU calculation assuming
m = m0 (1 -  /0); for  = 0.16
first experimental indication for a dropping  mass in the nuclear medium

5.  photoproduction off different nuclear targets
indication for
in-medium mass
shift observed for all
nuclei studied

6. photoproduction of  mesons off nuclei at CB/TAPS@ELSA
momentum dependence of  signal
 mass modification only for p 0.5 GeV/c
first indication for an
in-medium modification
of the  meson mass !!
Higher statistics data will
provide momentum
dependence of  potential