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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

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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

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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

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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

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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

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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

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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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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