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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. CB@MAMI collaboration meeting, Mainz, March 8-10, 2010. V. Bernard and U.-G. Meißner - PowerPoint PPT Presentation
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1
Boris Lemmer and Volker Metagfor the A2 collaboration
II. Physikalisches Institut
CB@MAMI collaboration meeting,Mainz, March 8-10, 2010
In-medium properties of the omega meson from a measurement of
the excitation function in photoproduction
2
model predictions for in-medium masses of mesons
V. Bernard and U.-G. MeißnerNPA 489 (1988) 647
NJL-model
mass degeneracy of chiralpartners expected at high baryon densities
for rB:
1.) lowering of in-medium mass 2.) 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
XωNγ
XωAγA σA
σT
M. Kotulla et al., PRL 100 (2008) 192302
MeV130)ρρ(Γ 0ω
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
theoretical predictions:
D. B. Kaplan and A. E. Nelson PLB 175 (1986) 57J. Schaffner-Bielich et al. Nucl. Phys. A 625 (1997) 325
K+ and K- mesons in the nuclear medium
free hadron:
hadron in medium of baryon density :
22eff
220
pρm
ρVpmρE
220 pmE
concept of aneffective mass
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energy above threshold [GeV]
[
b]
energy above threshold [GeV]
MK/<
Ap
art>
K10
1K
K10
1K
KK
KK
K+ and K--production in nucleon-nucleon and nucleus-nucleus collisions
Kaons in the nuclear medium
KaoS-collaboration Ni + Ni K+, K- + X
elementary reactions
enhanced K--yield in nucleus-nucleus collisions evidence for K-mass modification in dense matter
for the same energy above threshold
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Crystal Ball /TAPS set up in Mainz
Crystal Ball: 672 NaI modules200 < < 1600
TAPS: 384 BaF2 modules10 < < 200
photon beam
Ee= 1604 MeVin this experiment: 700 MeV < E < 1400 MeV
data taken in April, June, and September 2008
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0; selection of prompt -rays Time cuts: prompt coincidences TAPS CB; TAPS tagger; CB tagger
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Excitation function for photoproduction near threshold
+ C + X; 0
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Excitation function for photoproduction near threshold + Nb + X; 0;
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Comparison of measured excitation function with GiBUU calculations
Boris Lemmer, Stefan Friedrich, Janus Weil
data disfavour „broadening + mass shift“ scenario andfavour „Fermi motion + collisional broadening“ scenario
preli
mina
ry
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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
MeV130)ρρ(Γ 0ω
• 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
medvactot ΓΓΓ
In low density approximation: 0
0medmed ρrρ
ρΓrρΓ
tot
statefinalV
2tot
222V
2
tot
tot
statefinalV
Γ
Γ
Γμmμ
ΓμΓ
ΓμA
μdσd
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:0907.2388
1/1/
1/2
for a large in-medium broadening of the meson
as observed for the meson: (M. Kotulla et al., PRL 100 (2008) 192302)
contributions from higher densities suppressed by order of 1/2
0tot Γ16Γ ρΓΓ medtot
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experimental approach:
hadron decay in the medium: 21XXH
221H pppρ,μ
• reconstruction of invariant mass from 4-momenta of decay products:
• compare 0 pρ,μH
with Hm listed in PDG
• avoid distortion of 4-momentum vectors by final state interactions dilepton spectroscopy: e+e-
• experimentally observed mass distribution = convolution of spectral function with branching ratio into channel being studied
μd
σd21
XXH μΓ
ΓμA
tot
XXH21
F. Eichstaedt et al.,Prog. Theo. Phys. Suppl. 168 (2007) 495
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
1mcp
βγ
14
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
2.) in-medium width from measurement of the transparency ratio
XωNγ
XωAγA σA
σT
3 approaches:
P. Mühlich (PhD-thesis, Giessen 2006)K. Gallmeister et al. Prog. Part. Nucl. Phys. 61 (2008) 283
CaMeV140)ρρ(Γ 0ω
3.) sensitivity to different in-medium modification scenarios of the excitation function near the photo-production threshold
M. Nanova
limited sensitivity; contributions from high density suppressed ~1/2
15
Sensitivity to in-medium signal
medvactot ΓΓΓ
In low density approximation: 0
0medmed ρrρ
ρΓrρΓ
tot
statefinalV
2tot
222V
2
tot
tot
statefinalV
Γ
Γ
Γμmμ
ΓμΓ
ΓμA
μdσd
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:0907.2388
1/1/
1/2
for a large in-medium broadening of the meson
as observed for the meson: (M. Kotulla et al., PRL 100 (2008) 192302)
contributions from higher densities suppressed by order of 1/2
0tot Γ16Γ ρΓΓ medtot
16
Measurement of ω excitation function
Enhanced yield for dropping mass scenario below production threshold on free nucleon
• 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
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