Studying medium modifications of mesons in elementary reactions

1. Studying medium modifications of mesons in elementary reactions Volker Metag II. Physikalisches Institut, Universität Gießen, Germany Lecture given at Berkeley School „Medium Properties, Chiral Symmetry and Astrophysical Phenomena“ May 21-25, 2007, Berkeley, USA

2. Energy/Mass Distribution in the Universe accelerated expansion of universe  dark energy (homogen. distributed) gravitational lensing  dark matter stars  visible (baryonic) matter

3. elementary particles electromagnetic interaction u c t g e- e- photon (g) e- g d s b leptons quarks e- strong interaction ne nm nt Z0 q q gluon (g) e µ t W± q q weak interaction I II III three generations of matter u e- W- e d The Standard Model interactions force carriers

4. masses of quarks and leptons 105 104 103 102 10 1 e 10-1 10-2 10-3 10-4 10-5 10-6 M l,q [MeV/c2] Leptons Quarks t b t c s m nt d u nm • masses of elementary particles • (quarks, leptons) generated • by interaction with Higgs-field • search for Higgs-particle (LHC) ne

5. Baryons (qqq) Mesons (qq) pseudoscalar mesons:J = 0-, +(ud), 0(uu-dd)/ ,-(du) proton: (uud) J = ½+, neutron: (udd) J = ½+, vector mesons: J = 1-, +(ud), 0(uu-dd)/ , -(du) (uu-dd)/ , (ss) hadrons: strongly interacting composite particles

6. atom 10-10 m atomic nucleus 10-14 m nucleon 10-15 m M   mi M » mi M   mi binding energy effect  10-8 binding energy effect  10-3 the mass of composite systems nucleon: mass not determined by sum of constituent masses m = E/c2; „mass without mass“ (Wilczek) mass given by energy stored in motion of quarks and by energy in colour gluon fields

7. mN = 938 MeV  mq  5 – 10 MeV the role of chiral symmetry breaking • chiral symmetry = fundamental symmetry of • QCD for massless quarks • chiral symmetry broken • on hadron level The Nobel Prize in Physics 2004 1535 1520 „for the discovery of asymptotic freedom in the theory of the strong interaction" 1260 600 ≈ 290 ≈ 600 1232 ≈ 490 ≈ 470 938 135 770 Gross Politzer Wilczek the interaction among quarks has to become so strong that it overcomes their quantum mechanical resistance to localization (Wilczek) chiral symmetry scalar mesons vector mesons nucleon how is the mass of the nucleon generated? mass split comparable to hadron masses !

8. magnetisation M ferro magnetic para magnetic TCurie phase transition: ferromagnetism  paramagnetism restoration of full rotational symmetry ferromagnetic: rotational symmetry about 1 axis paramagnetic: full rotational symmetry temperature T

9. chiral condensate as function of baryon density B and temperature T C. Ratti, M. Thaler, W. Weise, PRD73 (2006) 014019 is not an observable!! + higher order terms hadronic spectral function: , . p - beams SIS18 heavy ion reactions: A+AV+X mV(>>0;T>>0) SPS elementary reaction: ,   V+X mV(=0;T=0) SIS300 /(FAIR) RHIC LHC QCD sum rules: provide link between hadronic observables and condensates

10. hadrons = excitations of the QCD vacuum Mass [GeV] G.E.Brown and M. Rho, PRL 66 (1991) 2720 T.Hatsuda and S. Lee, PRC 46 (1992) R34 hadron masses • 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

11. K. Saito, K. Tushima, and A.W. Thomas PRC 55 (1997) 2637 Quark-meson coupling model (QMC) - meson F. Klingl et al. NPA 610 (1997) 297 NPA 650 (1999) 299  - meson for rB: 1.) lowering of in-medium mass 2.) broadening of resonance decrease of  -mass by  15% at normal nuclear matter density model predictions for in-medium masses of mesons

12. R. Rapp and J. Wambach, EPJA 6 (1999) 415 P. Mühlich, priv. com. broadening and strength below vacuum pole mass  spectral function (structure due to coupling to S11,P13 resonances) Model predictions for spectral functions of r and w mesons

13. Motivation for studying in-medium modifications of hadrons: Test concepts for hadron mass generation by comparing predictions based on these concepts with experimental observations how hadron properties are changed in a strongly interacting environment. possible in-medium modifications of hadrons: • in-medium mass shift • (partial restoration of chiral symmetry, meson-baryon coupling) • in-medium broadening of hadron resonances • (meson-baryon coupling, collisional broadening) • hadron-nucleus bound states • (meson-nucleus attractive potential)

14. experimental approach: reconstruction of invariant mass from 4-momenta of decay products: dilepton spectroscopy: r, w, f e+e- essentialadvantage:no final state interactions !! from heavy-ion collisions from elementary reactions advantage: well controlled conditions: important for theoretical interpretation no time dependence of baryon density: B B(t);T=0; advantage: sizable effects due to high densities and temperatures disadvantage: any signal represents an integration over the full space-time history of the heavy-ion collision with strong variations in densities and temperatures  lectures by J. Stroth, G. Usai disadvantage: small medium effects since   0 and T=0 Information on medium modifications of mesons

15.  A  e+e- + X JLAB-CLAS: G7 p (12 GeV) A ,  +X KEK-E325: E = 0.6-3.8 GeV All targets CB e+e- invariant mass spectra from photon and proton induced reactions ,  -peaks observed; additional yield above combinatorial background assigned to   e+e-

16.  A  e+e- + X JLAB-CLAS: G7 p (12 GeV) A ,  +X KEK-E325: w/o shift • shifted in mass: no broadening!!  slightly broadened; no mass shift M. Naruki et al., PRL 96 (2006) 092301 e+e- invariant mass spectra from photon and proton induced reactions No consistent picture!!  lecture by Chaden Djalali

17. mass shift of  - meson for low recoil momenta in Cu:  - meson in the nuclear medium KEK-E325: p (12 GeV) A ,  +X;  e+e- R.Muto et al., PRL 98 (2007) 042501

18. J.G.Messchendorp et al., Eur. Phys. J. A 11 (2001) 95 gA   + X p g p0g g  w p0 fm simulation:  Nb   +X g g -mass in nuclei from photonuclear reactions advantage: • p0g large branching ratio (8 %) • no -contribution (  0 : 7  10-4) disadvantage: • p0-rescattering

19. quasi-monochromatic photon beam via electron bremsstrahlungs tagging linearly and circularly polarized ELSA@BONN DFG TR-16

20. photon beam TAPS 528 BaF2 Crystal Barrel 1290 CsI SciFi Detector Crystal Barrel and TAPS

21. p0gg h´ p0 p0 h 6g h p0 p0 p0  6g ct = 1 •103 fm ct = 2.5 •107 fm ct = 1.5 •105 fm Line Shape for long-lived Mesons Strategy of the experiment • compare line shape for vacuum (LH2 data) with nuclear target data if deviation  in-medium modification • background subtracted line shapes of long-living mesons no deviation in the line shape observed for long-living mesons!

22. D. Trnka et al, PRL 94 (2005) 192303 First Observation of w mass modification in the medium background subtracted w signal for different 3-momenta • w line shape off Nb (nuclear medium) compared to w lineshape off LH2 (vacuum) • enhancement on low mass side of w signal for small momenta • sensitivity to low 3-momenta important for w studies (pw < 500 MeV) decays outside and inside the nucleus decays outside the nucleus

23. cut on po g momentum to enhance in-medium decays: ppg < 500 MeV • new data analysis (fully inclusive event class: correlated po and g): better statistics • new background analysis: event class: correlated po and g in one event mixing one po and one g from different events New Analysis of CBELSA/TAPS Data • mixed event background subtraction: shape model independent fully inclusive 0 signal for 12C target

24. new  signal off 12C • p < 500 MeV • background shape model independent • comparison  signal off 12C (medium) signal off LH2 (vacuum) response from simulation (vacuum) • clear deviation in the line shape observed Mixed Event Background subtracted: w signal fully inclusive 0 signal for 12C target lowering of w mass!

25. decomposition • line shape of vacuum contribution taken from LH2 experiment • shape of in-medium contribution taken from BUU simulation using linear scaling m = m0(1 - a/0) • systematic study of medium and vacuum part under way (a~16% fits) In-medium and Vacuum Line Shape P. Mühlich and U. Mosel, NPA (2006) C: in-medium: 35%

26. after subtraction of mixed event background inclusive 0 -invariant mass spectrum decomposition in vacuum and in-medium decays consistent with  0  -signal on Nb

27. access to in-medium  width in-medium  width proportional to  absorption: (,|p|)  vabs normalization to C!! transparency ratio: = 47 MeV = 34 MeV 93 MeV 94 MeV 180 MeV P. Mühlich and U. Mosel NPA 773 (2006) 156 M. Kaskulov, E.Hernandez and E. Oset EPJ A 31 (2007) 245

28. access to in-medium  width in-medium  width proportional to  absorption: (,|p|)  vabs comparison to data (D.Trnka et al.) (0,<|p|>750 MeV/c)  95 MeV  gets broadened in the medium by a factor 10!! normalization to C!! transparency ratio: = 47 MeV = 34 MeV 93 MeV 94 MeV 180 MeV

29. Overall picture M.Naruki et al., PRL 96 (2006) R. Muto et al., PRL 98 (2007) Ch. Djalali et al., priv. comm. R. Arnaldi et al., PRL 96 (2006) D. Trnka et al, PRL 94 (2005) D. Adamova et al., subm. to PLB CBELSA/TAPS reaches the lowest 3-momenta high sensitivity to w decays inside the nucleus

30. The population of meson-nucleus bound states in recoil-free kinematics  A attraction strong enough to allow for  bound states?? forward going nucleon takes over photon momentum magic incident energies : E  930 MeV : E  2750 MeV

31. E. Marco and W. Weise, PLB 502 (2001) 59 quasifree quasifree -mesic states T. Nagahiro et al. N. Phys. A 761 (2005) 92 attractive potential repulsive potential signature for -mesic states no intensity for negativeenergies theoretically predicted signatures for  bound states

32. comparison of carbon and LH2 data 1200 MeV < E < 2200 MeV: small proton angles: 60 < p< 100 normalisation on quasi-elastic peak Carbon LH2 excess yield relative to LH2 reference spectrum for negative energies (bound state regime) evidence for 11B ?? D. Trnka priv. communication preliminary!!!

33. Current experiment at ELSA: search for -mesic states Aerogel Cerenkov TAPS Crystal Barrel SFB/TR16

34. -mass in nuclei:   e+e- M. Effenberger et al. PRC 60 (1999) 044614 W. Schön et al.Act. Phys. Pol. B 27 (96) 2959 expected signal: -A at p = 1.3 GeV/c -pbound  nat rest  ne+e- “at rest”: | pw |  | pF | exp: detect e+e- HADES  lecture by Joachim Stroth

35. summary • evidence for dropping  mass in the nuclear medium: • in-medium properties of the  meson: • in-medium  width (=0, <|p|>750 MeV/c)  95 MeV •  in-medium broadening by factor 10! • evidence for  mesic nuclei? • improved experiment ongoing • in-medium properties of the  meson from • ,p A  + X  e+e- +X (HADES) • Further high resolution experiments (KEK, CLAS, NA60) needed to obtain an overall consistent picture of in-medium • properties of light vector mesons (, , )

36. CBELSA/TAPS collaboration