1. In-medium properties of the omega meson from a measurement of
the excitation function in photoproduction
Boris Lemmer and Volker Metag
for the A2 collaboration
II. Physikalisches Institut
collaboration meeting,
Mainz, March 8-10, 2010

2. V. Bernard and U.-G. Meißner
model predictions for in-medium masses of mesons
V. Bernard and U.-G. Meißner
NPA 489 (1988) 647
NJL-model
mass degeneracy of chiral
partners expected at high
baryon densities
for rB:
1.) lowering of in-medium mass ) broadening of resonance
F. Klingl et al.
NPA 610 (1997) 297
NPA 650 (1999) 299
splitting of ω meson
strength due to coupling
to S11 and D13 resonances
P. Mühlich et al.,
NPA 780 (2006) 187

3. Experimental information on in-medium properties of mesons
1.) analysis of meson line shape on
a nuclear target in comparison to
a reference measurement on LH2
3 approaches:
M. Nanova
limited sensitivity; contributions
from high density suppressed ~1/2
2.) in-medium width from measurement
of the transparency ratio
M. Kotulla et al., PRL 100 (2008)
K. Gallmeister et al., Prog. Part. Nucl. Phys. 61 (2008) 283
3.) sensitivity to different in-medium modification
scenarios of the excitation function near the
photo-production threshold Ca

4. K+ and K- mesons in the nuclear medium
theoretical predictions:
D. B. Kaplan and A. E. Nelson PLB 175 (1986) 57
J. Schaffner-Bielich et al. Nucl. Phys. A 625 (1997) 325
free hadron:
hadron in medium
of baryon density :
concept of an
effective mass

5. Kaons in the nuclear medium
K+ and K--production in nucleon-nucleon and nucleus-nucleus collisions
KaoS-collaboration Ni + Ni  K+, K- + X
elementary reactions
energy above threshold [GeV]
MK/<Apart>
 [b]
energy above threshold [GeV]
for the same energy above threshold
 enhanced K--yield in nucleus-nucleus collisions
 evidence for K-mass modification in dense matter

6. Crystal Ball /TAPS set up in Mainz
Crystal Ball: 672 NaI modules
200 <  < 1600
TAPS: 384 BaF2 modules
10 <  < 200
photon beam
Ee= 1604 MeV
in this experiment: 700 MeV < E < 1400 MeV
data taken in April, June, and September 2008

7. 0; selection of prompt -rays
Time cuts: prompt coincidences TAPS  CB; TAPS  tagger; CB  tagger

8. Excitation function for  photoproduction near threshold
 + C   + X; 0

9. Excitation function for  photoproduction near threshold
 + Nb   + X; 0;

10. Comparison of measured excitation function with
GiBUU calculations
Boris Lemmer, Stefan Friedrich, Janus Weil
preliminary
data disfavour „broadening + mass shift“ scenario and
favour „Fermi motion + collisional broadening“ scenario

11. Summary and conclusions
information on in-medium properties of the  meson
difficult to extract from an analysis of the  lineshape;
contributions from higher densities suppressed by 1/2 ;
 sensitivity shifted to the surface region of the nucleus
in-medium broadening of the  meson to
deduced from measurement of the transparency ratio;
i.e. the  is broadened by a factor 16 relative to its vacuum width
excitation function for photo production near threshold sensitive to
different in-medium modifcation scenarios
 photoproduction cross sections on C, Nb deduced from data taken
at MAMIC for E= 900 – 1400 MeV
comparison of data with GiBUU simulations favours a broadening
but not an additional mass shift ot the  meson in a nuclear medium

12. Sensitivity to in-medium signal
In low density approximation:
Strength function parametrized by Breit Wigner shape 
for cases of strong in-medium broadening (, ) it is in principle difficult to
observe in-medium shape changes;
for details see: J. Lehr and U. Mosel, Phys. Rev.C 64 (2001)042202
S. Leupold, V. Metag, U. Mosel, arXiv:
1/
 1/2
for a large in-medium broadening of the meson
as observed for the  meson: (M. Kotulla et al., PRL 100 (2008) )
 contributions from higher densities suppressed by order of 1/2 

13. experimental approach:
hadron decay in the medium:
reconstruction of invariant mass from 4-momenta of decay products:
compare
with
listed in PDG
1.3 fm () ; 23 fm () ; 46 fm ()
ensure that decays occur in the medium:
 select shortlived mesons: decay length s= c
for  s 
cut on low meson momenta for  and  mesons
avoid distortion of 4-momentum vectors by final state interactions
 dilepton spectroscopy: r, w, f  e+e-
experimentally observed mass distribution = convolution of
spectral function with branching ratio into channel being studied 
F. Eichstaedt et al.,
Prog. Theo. Phys. Suppl. 168 (2007) 495

14. Information on in-medium properties of mesons
3 approaches:
M. Nanova
1.) analysis of meson line shape on a
nuclear target in comparison to
a reference measurement on LH2
limited sensitivity; contributions
from high density suppressed ~1/2
2.) in-medium width from
measurement of the
transparency ratio
P. Mühlich (PhD-thesis, Giessen 2006)
K. Gallmeister et al.
Prog. Part. Nucl. Phys. 61 (2008) 283 Ca
3.) sensitivity to different in-medium
modification scenarios of the
excitation function near the
photo-production threshold

15. Sensitivity to in-medium signal
In low density approximation:
Strength function parametrized by Breit Wigner shape 
for cases of strong in-medium broadening (, ) it is in principle difficult to
observe in-medium shape changes;
for details see: J. Lehr and U. Mosel, Phys. Rev.C 64 (2001)042202
S. Leupold, V. Metag, U. Mosel, arXiv:
1/
 1/2
for a large in-medium broadening of the meson
as observed for the  meson: (M. Kotulla et al., PRL 100 (2008) )
 contributions from higher densities suppressed by order of 1/2 

16. Enhanced  yield for dropping mass scenario
Measurement of ω excitation function
additional test for ω broadening and/or mass shift scenario
Gi-BUU simulations: P. Mühlich (PhD-thesis, Giessen 2006)
K. Gallmeister et al. Prog. Part. Nucl. Phys. 61 (2008) 283 Ca
Enhanced  yield for dropping mass scenario
below production threshold on free nucleon