PowerPoint-Pr

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Hadrons in the nuclear medium - recent
experimental results
Volker Metag II. Physikalisches
Institut Universität Giessen Germany
5th. International Conference on Perspectives in
Hadron Physics Trieste, Italy, May 22-26, 2006
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Motivation

• QCD-vacuum complicated structure
• characterized by condensates

• in the nuclear medium
• condensates are changed
• change of the hadronic excitation
• energy spectrum

• widespread experimental activities to search for
  
• in-medium modifications of hadrons

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model predictions for in-medium masses of mesons
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Model predictions for spectral functions of r and
w mesons
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Model predictions for spectral functions of r and
w mesons
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experimental approach dilepton spectroscopy r,
w, f ? ee-
essential advantage no final state interactions
!!

• CLAS (Jlab) C. Tur et al., ?A ??, ?, ? X

• HADES (GSI) planned experiment ?- p ? ? n on
  bound proton

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?-mass in nuclei from photonuclear reactions
advantage p0g large branching ratio (8 )
no ?-contribution (? ? ?0? 7 ? 10-4)
disadvantage p0-rescattering
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Expected ? in-medium signal
rescattering of pions in nuclei predominantly
proceeds through ?(1232) excitation scattered
pions have Ekin?150 MeV
no distortion by pion rescattering expected in
mass range of interest
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comparison of meson masses and lineshapes for LH2
and nuclear targets
?0
?
??
No change of mass and lineshape for longlived
mesons (?0, ?, ??) decaying outside nuclei
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inclusive ???0? signal for LH2 and Nb target
D. Trnka et al., PRL 94 (2005)192303
difference in line shape of ? signal for proton
and nuclear target
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decomposition of ? signal into in-medium and
vacuum decay contributions
Nb in-medium 45
C in-medium 40
lineshape of vacuum contribution taken from LH2
experiment
shape of in-medium contribution taken from BUU
simulation (P. Mühlich and U. Mosel, NPA
(2006)), assuming m? m0(1 - 0.16 ?/?0)
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access to in-medium ? width
in-medium ? width proportional to ? absorption ?
? ?v?abs
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access to in-medium ? width
in-medium ? width proportional to ? absorption ?
? ?v?abs
normalization to C!!
transparency ratio
? 19 MeV
? 34 MeV
37 MeV
94 MeV
74 MeV
Comparison to data (D.Trnka et al.) ?(?0) ? 45
95 MeV
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momentum dependence of ? signal (Nb-target)

• mass modification only for p?? 0.5 GeV/c

determination of momentum dependence of ? -
nucleus potential requires finer momentum bins
? improved 2nd. generation experiment
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?-, ?-meson-nucleus potential
K. Saito, K. Tsushima, A.W. Thomas, hep-ph/0506314
predictions within the quark meson coupling model
(QMC)
? E(1s) -39 MeV ? 29 MeV ? E(1s) -100
MeV ? 31 MeV
? E(1s) -56 MeV ? 33 MeV ? E(1s) -118
MeV ? 33 MeV
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correlation between ? momentum and proton angle
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comparison of data on LH2 and C
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Summary and outlook
major step forward towards understanding the
origin of hadron masses

• first evidence for ? mesic 11B

• remaining open questions
• ? momentum dependence of ?-nucleus potential?
• ? structure in ? strength function?
• ? confirm existence of ? mesic states in
  heavier nuclei

• higher statistics needed !!
• improved experiments planned at MAMI and ELSA

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CBELSA/TAPS collaboration
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