Measurements of meson mass at J-PARC K. Ozawa (KEK) (KEK) Contents: Physics motivation Current results J-PARC Summary

Origin of Hadron mass 2011/11/30 Zimanyi school 2011, K. Ozawa 95% of the (visible) mass is dynamically generated by the strong interaction. This mechanism is actively studied both theoretically and experimentally. Current quark masses generated by spontaneous symmetry breaking (Higgs field) Constituent quark masses should be generated by QCD dynamical effects 2

Naïve Theory 2011/11/30 Zimanyi school 2011, K. Ozawa 3 High Temperature High Density Quark – antiquark pairs make a condensate and give a potential. Chiral symmetry is breaking, spontaneously. Chiral symmetry exists. Mass ~ 0 (Higgs only) Vacuum contains quark antiquark condensates. So called “QCD vacuum”. q q Vacuum When T and  is going down,  as a Nambu-Goldstone boson.

Experimental approach? 2011/11/30 Zimanyi school 2011, K. Ozawa 4 When chiral symmetry is restored, mass of chiral partner should be degenerated. m will decrease in finite /T matter. “QCD vacuum”, i.e. quark condensates can be changed in finite density or temperature. Then, chiral symmetry will be restored (partially). “QCD vacuum”, i.e. quark condensates can be changed in finite density or temperature. Then, chiral symmetry will be restored (partially). Vacuum   0 T  0 In finite  Chiral properties can be studied at finite density and temperature. However, measurements of chiral partner is very difficult. We measure mass modification of narrow resonance. However, measurements of chiral partner is very difficult. We measure mass modification of narrow resonance. mass  (J P = 1 - ) a 1 (J P = 1 + )  mm m = 0 Degenerate Best Observable…

Mass shift and Condensate 2011/11/30 Zimanyi school 2011, K. Ozawa 5 In fact, Mass modification of a vector meson can be connected to quark condensates. q q Vacuum QCD sum rule Average of Imaginary part of ( 2 ) vector meson spectral function Average of Imaginary part of ( 2 ) vector meson spectral function T.Hatsuda and S.H. Lee, PRC 46 (1992) R34 Prediction Spectrum mVmV Theoretical Assumption Measurements of mass related information in hot/dense matter is essential! Example:

2011/11/30 Zimanyi school 2011, K. Ozawa Predicted “ spectra ” Predicted mass spectral function of vector mesons () in hot and/or dense matter. –Lowering of in-medium mass –Broadening of resonance R. Rapp and J. Wambach, EPJA 6 (1999) 415  - meson 6 P. Muehlich et al., Nucl. Phys. A 780 (2006) 187  - meson In addition, several models predict mass spectra of mesons and it can be compared with experimental results directly.

CURRENT EXPERIMENTS 2011/11/30 Zimanyi school 2011, K. Ozawa 7

Generate hot/dense media heavy ion reactions: A+A  V+X m V (  >>  0 ;T>>0) SPS LHC RHIC /11/30 8 Zimanyi school 2011, K. Ozawa Measurements of Vector Meson mass spectra in hot/dense medium will provide QCD medium information. Leptonic (e + e -,  +  - ) decays are suitable, since lepton doesn’t have final state interaction. Hot matter experiments

SPS-CERES results 2011/11/30 Zimanyi school 2011, K. Ozawa 9 D. Miskowiec, QM05 talk Existing of Mass modification is established. PLB663, 43 (2008)

NA60 SPS 10 PRL 96, (2006) 2011/11/30 10 Zimanyi school 2011, K. Ozawa [van Hees+R. Rapp ‘06] Next, Let’s go to RHIC! Spectrum is well reproduced with collisional broadening. Muon pair invariant mass in Pb-Pb at s NN =19.6 GeV

RHIC RHIC 2011/11/30 Zimanyi school 2011, K. Ozawa 11 Black Line –Baseline calculations Colored lines –Several models Low mass M>0.4GeV/c 2 : –some calculations OK M<0.4GeV/c 2 : not reproduced –Mass modification –Thermal Radiation Excess from known hadronic sources is also observed. However, it looks different. Excess from known hadronic sources is also observed. However, it looks different. No concluding remarks at this moment. New data with New detector (HBD) can answer it. Electron pair invariant mass in Au-Au at s NN =200 GeV PRC81(2010)

New detector! 2011/11/30 Zimanyi school 2011, K. Ozawa 12 signal electron Cherenkov blobs partner positron needed for rejection e+e+ e-e-  pair opening angle ~ 1 m Constructed and installed by Weizmann and Stony Brook group. ~20 p.e. few p.e. Hadron Single electron Great performance!

Then, Nucleus! elementary reaction: , p,   V+X m V (  =  0 ;T=0) , . p - beams J-PARC CLAS At Nuclear Density /11/30 13 Zimanyi school 2011, K. Ozawa Stable system Saturated density Experiments, CBELSA/TAPS (Japan) CLAS J-Lab Cold matter experiments

Two Experimental methods –Direct measurements of mass spectra Emitted Proton Neutron p  Nucleon Hole Target Decay Meson –Meson bound state spectroscopy 2011/11/30 Zimanyi school 2011, K. Ozawa 14

Results with Bound states K. Suzuki et al., Phys. Rev. Let., 92(2004)  bound state is observed in Sn(d, 3 He) pion transfer reaction. Reduction of the chiral order parameter, f*  () 2 /f  2 =0.64 at the normal nuclear density,  =  0 is indicated. 2011/11/30 Zimanyi school 2011, K. Ozawa  bound state Y. Umemoto et al., Phys. Rev. C62(2004)

Direct Measurements of Mass KEK-PS E325: 12 GeV proton induced. p+A  , ,  + X Electron decays are detected. 2011/11/3016 Zimanyi school 2011, K. Ozawa

E325 Spectrometer 2011/11/3017 Zimanyi school 2011, K. Ozawa

E325 Results I m  = m 0 (1 -   /  0 ) for  = /11/30 Zimanyi school 2011, K. Ozawa 18 Induce 12 GeV protons to Carbon and Cupper target, generate vector mesons, and detect e+e- decays with large acceptance spectrometer. Cu e+e-e+e- e+e-e+e-  /  The excess over the known hadronic sources on the low mass side of  peak has been observed. M. Naruki et al., PRL 96 (2006) KEK E325,  e + e -

Note: CLAS J-Lab 2011/11/30 Zimanyi school 2011, K. Ozawa 19 Induce photons to Liquid dueterium, Carbon, Titanium and Iron targets, generate vector mesons, and detect e+e- decays with large acceptance spectrometer. m  = m 0 (1 -   /  0 ) for  = 0.02 ± 0.02 No peak shift of  Only broadening is observed   /  R. Nasseripour et al., PRL 99 (2007)

2011/11/30 Zimanyi school 2011, K. Ozawa E325 Result II:   e + e - Cu  <1.25 (Slow) Invariant mass spectrum for slow  mesons of Cu target shows a excess at low mass side of . Measured distribution contains both modified and un-modified mass spectra. So, modified mass spectrum is shown as a tail. 20 First measurement of  meson mass spectral modification in QCD matter. R. Muto et al., PRL 98(2007) Excess!!

 <1.25 (Slow) 1.25<  < <  (Fast) 2011/11/30 Zimanyi school 2011, K. Ozawa Mass modification is seen only at heavy nuclei and slowly moving  Mass Shift: m  = m 0 (1 -   /  0 ) for  = Target/Momentum dep.

NEW J-PARC 2011/11/30 Zimanyi school 2011, K. Ozawa 22

2011/11/30 Zimanyi school 2011, K. Ozawa Performance of the 50-GeV PS Beam Energy ： 50 ＧｅＶ (Currently, 30GeV) Repetition: 3.4 ~ 5-6s Flat Top Width ： 0.7 ~ 2-3s Beam Intensity:3.3x10 14 ppp, 15A (2×10 14 ppp, 9A) E Linac = 400MeV (180MeV) Beam Power: 750kW (270kW) Numbers in parentheses are ones for the Phase 1. 23

2011/11/30 Zimanyi school 2011, K. Ozawa Linac J-PARC Cascaded Accelerator Complex: 3GeV Rapid Cycling (25Hz) Synchrotron 50GeV Synchrotron Materials and Life Science Facility Hadron Hall (Slow Extracted Beams) Neutrino Beamline to Super-Kamiokande 24 Hadron Hall

2011/11/30 Zimanyi school 2011, K. Ozawa Hadron Hall NP-HALL 56m(L)×60m(W) Upgrade of E325 Large statistics 25 Stopped  for Clear mass modification

Exp1: Upgrade of KEK-E325 Large acceptance (x5 for pair ) Cope with high intensity beam and high rate (x10) Good mass resolution ~ 5 MeV/c 2 Good electron ID capability 2011/11/30 Zimanyi school 2011, K. Ozawa times higher statistics!!

What can be achieved? 2011/11/30 Zimanyi school 2011, K. Ozawa 27 Pb  Modified  [GeV/c 2 ]  from Proton Invariant mass in medium         p dep. High resolution Kinematic dependence

Detector components 2011/11/30 Zimanyi school 2011, K. Ozawa 28 Tracker ~Position resolution 100μm High Rate(5kHz/mm 2 ) Small radiation length (~0.1% per 1 chamber) Electron identification Large acceptance High pion 90% e-eff. Gas Cherenkov EMCal

R&D Items 2011/11/30 Zimanyi school 2011, K. Ozawa 29 Develop 1 detector unit and make 26 units. ① GEM foil ③ Hadron Blind detector Gas Cherenkov for electron-ID ② GEM Tracker Ionization (Drift gap) + Multiplication (GEM) High rate capability + 2D strip readout CsI + GEM photo-cathode 50cm gas(CF 4 ) radiator ~ 32 p.e. expected CF4 also for multiplication in GEM

Exp 2: stopped  meson 2011/11/30 Zimanyi school 2011, K. Ozawa 30       n   A   + N+X  00 Beam momentum is ~ 1.8 GeV/c. As a result of KEK-E325, 9% mass decreasing (70 MeV/c 2 ) can be expected. Generate  meson using beam. Emitted neutron is detected at 0. Decay of  meson is detected. If  momentum is chosen carefully, momentum transfer will be ~ 0.  momentum [GeV/c]  momentum [GeV/c] 0

2011/11/3031 Zimanyi school 2011, K. Ozawa Experimental setup  - p   1.8 GeV/c  0   0    Target: Carbon 6cm Small radiation loss Clear calculation of  bound state Also, Ca, Nb, LH 2 Neutron Detector Flight length 7m 60cm x 60 cm (~2°) Gamma Detector Good resolution 75% of 4 Beam Neutron Gamma Detector

Detectors 2011/11/30 Zimanyi school 2011, K. Ozawa 32 Timing resolution Timing resolution of 80 ps is achieved (for charged particle). It corresponds to mass resolution of 22 MeV/c 2. Neutron Efficiency Iron plate (1cm t) is placed. Efficiency is evaluated using a hadron transport code, FLUKA. Neutron efficiency of 25% can be achieved. Neutron DetectorEM calorimeter CsI EMCalorimeter Existing detector + upgrade （ D.V. Dementyev et al., Nucl. Instrum. Meth. A440(2000), 151 ） 912 mass resolution of 18 MeV/c 2 can be achieved.

Expected results 2011/11/30 Zimanyi school 2011, K. Ozawa 33 H. Nagahiro et al, Calculation for 12 C( , n) 11 B  Final spectrum is evaluated based on a theoretical calculation and simulation results. Expected Invariant mass spectrum Stopped  is selected by forward neutron Generation of  is based on the above theoretical calculation. Detector resolution is taken into account. Yield estimation is based on 100 shifts using 10 7 beam. Estimated width in nucleus is taken into account.

Exp 3: Study of Baryon sector 2011/11/30 Zimanyi school 2011, K. Ozawa  bound state N*(1535) K-K s-wave resonance (Chiral Unitary model) Chiral partner of nucleon (Chiral Doublet model)  – N is strongly coupled with N* How to study N* experimentally?  in nucleus makes N* and hole Generate slowly moving  in nucleus LOI by K. Itahashi et. al Calc. by H. Nagahiro 34

Experiment for  2011/11/30 Zimanyi school 2011, K. Ozawa LOI by K. Itahashi et. al Calc. by H. Nagahiro, D. Jido, S. Hirenzaki et. al Forward neutron is detected. missing mass distribution is measured. In addition, measurements of invariant mass of N* decay Simulation 35

Exp 4:  bound state? 2011/11/30 Zimanyi school 2011, K. Ozawa Cu  <1.25 (Slow) Bound? s s u u d K+ Λ Φ p u d s u s Detection: p -> K +  Generation: pp ->  36

Exp 5: J-PARC 2011/11/30 Zimanyi school 2011, K. Ozawa 37 It’s under discussion with Prof. T. Csorgo. Please join us and come to Japan! It’s under discussion with Prof. T. Csorgo. Please join us and come to Japan!

Summary According to the theory, Hadron mass is generated as a results of spontaneous breaking of chiral symmetry. Many experimental efforts are underway to investigate this mechanism. Some results are already reported. New experiments for obtaining further physics information are proposed. –Explore large kinematics region –Measurements with stopped mesons 2011/11/30 Zimanyi school 2011, K. Ozawa 38

Back up

Chiral symmetry 2011/11/30 Zimanyi school 2011, K. Ozawa 40 The lagrangian does not change under the transformation below. This symmetry is called as Chiral symmetry. V(q) q Symmetric in rotation Gluon Divide with chirality Neglect (if m ~0) quarkmass

Breaking of symmetry 2011/11/30 Zimanyi school 2011, K. Ozawa 41 Potential is symmetric and Ground state (vacuum) is at symmetric position Potential is still symmetric, however, Ground state (vacuum) is at non-symmetric position This phenomenon is called spontaneous symmetry breaking When the potential is like V() =  4, V ~ self energy of ground state ~ mass If the interaction generate additional potential automatically, V’() = -a* 2

Theoretical efforts Nambu-Jona-Lasino model –Nambu and Jona-Lasino, 1961 –Vogl and Wise, 1991 –Hatsuda and Kunihiro, 1994 Chiral Perturbation theory –Weinberg 1979 QCD sum rule –Shifman et al., 1979 –Hatsuda and Lee, 1992 Lattice QCD –Wilson, 1974 Empirical models –Potential model (De Rujula et al., 1975), Bag model (Chdos et al., 1974) In addition, Collisional broadening, nuclear mean field … 2011/11/30 Zimanyi school 2011, K. Ozawa 42 ‘ T.Hatsuda and S. Lee, PRC 46 (1992) R34 Vector meson mass Connect hadron properties and chiral properties using QCD and/or phenomenology. G.E.Brown and M. Rho, PRL 66 (1991) 2720

RHIC&PHENIX 2011/11/30 Zimanyi school 2011, K. Ozawa 43

Not only mass spectra, K. Suzuki et al., Phys. Rev. Let., 92(2004)  bound state is observed in Sn(d, 3 He) pion transfer reaction. Reduction of the chiral order parameter, f*  () 2 /f  2 =0.64 at the normal nuclear density ( =  0 ) is indicated. 2011/11/30 Zimanyi school 2011, K. Ozawa  bound state Y. Umemoto et al., Phys. Rev. C62(2004) New exp. will be done at RIKEN D. Jido et al., arXiv: Jido-san et al. shows that - nucleus scattering length is directly connected to quark condensate in the medium.

Mass spectra measurements Following four experiments, –TAGX INS-Japan –CBELSA/TAPS experiments –CLAS g7 J-Lab-USA Use heavy and light nucleus and extract mass modification 45 Obtained spectra is combination of two spectra, such as decayed in nucleus and free space. Expected mass spectra 2011/11/3045 Zimanyi school 2011, K. Ozawa Note

46 Zimanyi school 2011, K. Ozawa INS-ES TAGX experiment EE STT model Present work Previous work MeV MeV 672±31 MeV MeV 730 MeV 743±17 MeV Eγ~ GeV, sub/near-threshold ρ 0 production PRL80(1998)241,PRC60:025203,1999.: mass reduced in invariant mass spectra of 3He(γ, ρ 0 )X,ρ 0 --> π+π− Phys.Lett.B528:65-72,2002: introduced cos analysis to quantify the strength of rho like excitation Phys.Rev.C68:065202,2003. In-medium 0 spectral function study via the H-2, He-3, C-12 (+ -) reaction. 2011/11/30 Try many models, and channels Δ, N*, 3π,…

Background is not an issue Combinatorial background is evaluated by a mixed event method. Form of the background is determined by acceptance and reliable. We should be careful on normalization. 2011/11/30 Zimanyi school 2011, K. Ozawa 47 Absolute normalization using like- sign pairs CLAS KEK Normalized using mass region above . There is enough statistics The problem: Each experiment can’t apply another method.

2011/11/30 Zimanyi school 2011, K. Ozawa 48 1g/c m 2 C Cu Experimentalists face to reality - E325 simulation- e+e- KEK-E325 Target materialbeam (p/spill) Interaction length(%) radiation length(%) C 0.64x %0.4% Cu X40.05% X40.5% X4

Condensates and Spectrum 2011/11/30 Zimanyi school 2011, K. Ozawa 49 Unfortunately, quark condensates is not an observable. We can link condensates and vector meson spectrum. How to access quark condensate experimentally? q q Vacuum QCD sum rule Average of Imaginary part of ( 2 ) vector meson spectral function Average of Imaginary part of ( 2 ) vector meson spectral function T.Hatsuda and S. Lee, PRC 46 (1992) R34 Prediction Spectrum mVmV The relation is established by Prof. Lee and Prof. Hatsuda. Assume

Next step 2011/11/30 Zimanyi school 2011, K. Ozawa 50 Then, calculate quark condensate using QCD sum rule. Experimental requirements 1. High statics 2. Clear initial condition Average of Imaginary part of ( 2 ) Assumed Spectrum Evaluate quark condensate directly. Not comparison btw predictions and measurements. Replace by average of measured spectra p

GEM foils Dry etching method is developed in Japan. –Hole shape is improved and cylindrical hole GEM has better Gain stability. –Thicker GEM foils is generated. 2011/11/30 Zimanyi school 2011, K. Ozawa 51 wet wet etching dry dry etching Hole shape cylindrical A hole with cylindrical shape double-conical A hole with double-conical shape Wet Dry 100m 1 foil 50m 3 foils Applied Voltage per 50 m [V] Thicker GEM foil Stability of GEM gain

GEM Tracker 2011/11/3052 Zimanyi school 2011, K. Ozawa Gas: Ar/CO 2 2D readout: kapton t=25m (Cu: t=4m both side) 290 m KEK σ~ 105 μm Residual [mm]

HBD: in the beginning… 2011/11/30 Zimanyi school 2011, K. Ozawa 53 1~2 photo electrons Too small… dE/dx (Blind ON) dE/dx + Light (Blind OFF) Compare Measured Charge w/wo Cherenkov light blind Charge [A.U.] This part of evaporated CsI looks gone!! Low Q.E.?

Beam test results 2011/11/30 Zimanyi school 2011, K. Ozawa 54 GEM Configuration 2mm 11mm Drift gap 11mm, 500V/cm Amplifing part 50 μm GEM×3 X i – X SSD [mm] Charge division Response Function Weighted mean of Strip Charge σ~ 105 μm Residual [mm] Position Resolution:  pos = Charge spread /  N eff Worse resolution for tilted track  pos =  Require good 30 Charge Spread

③ Hadron Blind Detector 紫外域に感度を持つ CsI 光電面 –Cherenkov 光検出に最適 –GEM 上面に CsI 光電面を蒸着 –100 m GEM を用いる Radiator ガス : CF 4 –High UV 2011/11/30 55 CsI 光電面による Cherenkov 光検出器 MESH 2 mm 1.5 mm 3.25 mm 5.25 mm LCP ( 100um ) CsI pad Cerenkov photon Ionization (40) Photoelectron (32) 50um GEM 50um GEM 32 x 800 –  Threshold 4 GeV/c GEM ３層を電子増幅に使用 –Gain ~ CsI GEM Important for e/ Pad 読み出しで位置情報も Reverse Bias to suppress ionization!! Ref. NIM A523, 345, 2004 Electron CF 4 Radiator By K. Aoki Zimanyi school 2011, K. Ozawa

56 J-PARC 2011/11/30 Zimanyi school 2011, K. Ozawa Cerenkov blob,  ~34mm Beam – 600 MeV positron Defined are 1cm x 1cm

Contradiction? Difference is significant What can cause the difference? –Different production process –Peak shift caused by phase space effects in pA? Need spectral function of  without nuclear matter effects Note: similar momentum range E325 can go lower slightly 2011/11/30 Zimanyi school 2011, K. Ozawa 57 We need to have a new experiment to investigate the problem. CLAS KEK R.S. Hayano and T. Hatsuda, Ann. Rev.

Results from CBELSA/TAPS disadvantage:  0 -rescattering advantage:  0  large branching ratio (8 %) no  -contribution (    0  : 7  )       p  A   + X  00 TAPS,    0  with +A 2011/11/30 Zimanyi school 2011, K. Ozawa 58 m  = m 0 (1 -   /  0 ) for  = 0.13 No sensitivities for mass modification arXiv: V. Metag at Hawaii JPS-DNP meeting

TAPS results II 2011/11/30 Zimanyi school 2011, K. Ozawa 59 M. Kotulla et al, PRL 100 (2008) E  = 900 – 1400 MeV Large  width in nuclei due to -N interaction. 60 MeV/c 2 even at stopped . In-medium decays Essential: Focus on Small momentum Issue: Yield estimation of decays

Branching Ratio 60 M. Kotulla et al, PRL 100 (2008) MeV/c 2 even at stopped . 2011/11/30 Zimanyi school 2011, K. Ozawa

Decay Yield Evaluation Based on measured crosssection of  - p   n for backward  (G. Penner and U. Mosel, nucl-th/ , J. Keyne et al., Phys. Rev. D 14, 28 (1976)) Production cross section 0.02 mb/sr s = 2.0 GeV 0.17 mb/sr s = 2.0 GeV Beam intensity 10 7 / spill, 6 sec spill length Neutron Detector acceptance  = 2°(60 cm x 60 7m) Gamma Detector acceptance 90% for  Radiation loss in target 11% Survival probability in final state interaction 60% Beam Time 100 shifts Branching Ratio 1.3 % 8.9 % 2011/11/30 Zimanyi school 2011, K. Ozawa 61 H. Nagahiro et al calculation based on the cross section and known nuclear effects. Assumed potential is consistent with  absorption in nucleus. Interact w nuclei No interact Total Large width ~ 60 MeV/c 2

Neutron Measurement Timing resolution Beam test is done at Tohoku test line Timing resolution of 80 ps is achieved (for charged particle). It corresponds to mass resolution of 22 MeV/c /11/3062 Zimanyi school 2011, K. Ozawa Neutron Efficiency Iron plate (1cm t) is placed to increase neutron efficiency. Efficiency is evaluated using a hadron transport code, FLUKA. Neutron efficiency of 25% can be achieved. Bound region We can not see a clear bound peak. At this moment, there is no beam line at J-PARC to have enough TOF length and beam energy

Gamma detector CsI EMCalorimeter T-violation’s one is assumed. （ D.V. Dementyev et al., Nucl. Instrum. Meth. A440(2000), 151 ） 2011/11/3063 Zimanyi school 2011, K. Ozawa Assumed Energy Resolution Obtained  meson spectra for stopped K decays Muon holes should be filled by additional crystals. Acceptance for  is evaluated as 90%. Fast simulation is tuned to reproduce existing data.

Simulation 1.Assume base resolution as shown in the previous slide 2.Apply additional smearing to match the existing data. –Depend on crystal position 2011/11/30 Zimanyi school 2011, K. Ozawa 64 Tail due to holes w. Fiducial cut  data  data  Sim.  Sim. wo. Fiducial Simulation Tune w. Fiducial Simulation results for  Fiducial Cut for  meson Direct  from  decay hits away from holes (red crystal)

Final Spectrum 2011/11/30 Zimanyi school 2011, K. Ozawa 65 Including Background: Main background is 2  decays and 1 missing Bound region One can select bound region as Energy of  < E 0, which is measured by the forward neutron counter. Invariant Mass spectrum for the bound region Strong kin. effects

Expected results 2011/11/30 Zimanyi school 2011, K. Ozawa 66 H. Nagahiro et al Generation of  Decay of (Invariant Mass) Large abs. No int. Large abs. Large int.

“Mass” correlation? 2011/11/30 Zimanyi school 2011, K. Ozawa 67 Neutron energy spectrum Interact w. nuclei and Mass modification No interact No mass mod. Smearing Correlation to invariant mass reconstructed by    decay) Expected Missing mass spectrum generation) Non-correlation?Same mass?

“Mass” Correlation 2011/11/30 Zimanyi school 2011, K. Ozawa 68 Invariant Mass VS Missing Energy Non-correlated model Correlated model Correlation analysis will useful for reducing kinematical effects.

simulation Issue: Final state interaction 2011/11/30 Zimanyi school 2011, K. Ozawa 69 no distortion by pion rescattering expected in mass range of interest. P. Mühlich, PhD-thesis, Giessen, 2006 GiBUU simulation assuming dropping in-medium ω mass Open symbol Scattered  ω J. G. Messchendorp et al., Eur. Phys. J. A 11 (2001) 95 Re-scattering of  is governed by  dynamics Our decayed  has T of 256 MeV Similar kinematics range However, 0.13/( ) = 0.37 is missing.