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 !

-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

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

photoproduction of  mesons off nuclei A   A at E  1.2 GeV (CB/TAPS@ELSA) in-medium  mass distribution (0) exp: D. Trnka (Giessen) theo: P. Mühlich et al., nucl-th/0310067 comparison to BUU calculation assuming m = m0 (1 -  /0); for  = 0.16 first experimental indication for a dropping  mass in the nuclear medium

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

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