in-medium properties of mesons - experimental results compared to theoretical predictions

Transcription

1 in-medium properties of mesons - experimental results compared to theoretical predictions Volker Metag II. Physikalisches Institut Outline: theoretical predictions for medium modifications of mesons exp. approaches and results on the real part of the ω, η - nucleus potential exp. approaches and results on the imaginary part of the ω, η - nucleus potential search for meson-nucleus bound states summary & outlook *funded by the DFG within SFB/TR16 Symposium on the occasion of Wolfram Weise s 7th. birthday Munich, April 14th, 216 1

2 hadron masses how does the intrinsic structure of hadrons change in a dense nuclear medium?? 2

3 hadron masses how does the intrinsic structure of hadrons change in a dense nuclear medium?? W. Weise NPA 61 (1996) 35c mesons = excitations of the QCD vacuum QCD vacuum: complicated structure characterized by condensates in the nuclear medium: condensates are changed change of the hadronic excitation energy spectrum 2

4 hadron masses how does the intrinsic structure of hadrons change in a dense nuclear medium?? W. Weise NPA 61 (1996) 35c mesons = excitations of the QCD vacuum QCD vacuum: complicated structure characterized by condensates in the nuclear medium: condensates are changed change of the hadronic excitation energy spectrum V. Bernard and U.-G. Meißner, NPA 489 (1988) 647 G.E.Brown and M. Rho, PRL 66 (1991) 272 m? m < qq >? < qq >.8( ) widespread theoretical and experimental activities to search for in-medium modifications of hadrons T.Hatsuda and S. Lee, PRC 46 (1992) R34 m V m V =(1 );.18 2

5 symmetry breaking in the hadronic sector MeV/c 2 nonet of pseudoscalar mesons η M=958 MeV/c 2 M=548 MeV/c2 K M=498 MeV/c 2 π M=14 MeV/c

6 symmetry breaking in the hadronic sector MeV/c 2 nonet of pseudoscalar mesons η M=958 MeV/c 2 M=548 MeV/c2 K M=498 MeV/c π M=14 MeV/c 2 V. Bernard, R.L. Jaffe, U.-G. Meissner, NPB 38 (1988) 753 S. Klimt, M. Lutz, U. Vogel, W Weise, NPA 516 (199) 429 W. Weise, NBI Copenhagen, 1993 mass as a result of symmetry breaking symmetry breaking 3

7 symmetry breaking in the hadronic sector MeV/c 2 nonet of pseudoscalar mesons η M=958 MeV/c 2 M=548 MeV/c2 K M=498 MeV/c π M=14 MeV/c 2 V. Bernard, R.L. Jaffe, U.-G. Meissner, NPB 38 (1988) 753 S. Klimt, M. Lutz, U. Vogel, W Weise, NPA 516 (199) 429 W. Weise, NBI Copenhagen, 1993 mass as a result of symmetry breaking partial restoration of chiral symmetry predicted to occur in a nucleus impact on meson masses?? symmetry breaking partial symmetry restoration 3

8 model predictions for the in-medium mass of the η meson NJL-model NJL-model linear σ model V. Bernard and U.-G. Meissner, Phys. Rev.D 38 (1988) 1551 H. Nagahiro, M. Takizawa and S. Hirenzaki, Phys. Rev. C 74 (26) 4523 S. Sakai and D. Jido PRC 88 (213) 6496 SU(3) SU(2) Δm η (ρ) -8 MeV almost no dependence of η mass on density Δm η (ρ) -15 MeV Δm η (ρ) +2 MeV QMC-model S. Bass and A. Thomas, PLB 634 (26) 368 Δm η (ρ) -4 MeV for θ ηη = -2 4

9 model predictions for in-medium mass/width of the ω meson QCD sum rules T. Hatsuda and S. Lee Phys. Rev. C 46 (1992) R34 QMC-model K. Saito, K. Tushima, A. W. Thomas PRC 55 (1997) 2637 M. Lutz et al., NPA 76 (22) 437 ω QCD sum rule approach: drop of ρ, ω mass by about 15% at ρ=ρ ρ m ω (ρ) = m(1-α ) for ρ<ρ ρ decrease of ω-mass by 15% at ρ=ρ splitting into ω-like and N*N -1 mode due to coupling to nucleon resonances ω N* ω ρ" ρ" +... N -1 5

10 model predictions for in-medium mass/width of the ω, Φ meson F. Klingl et al., NPA 61 (1997) 297; NPA 65 (1999) 299 ω P. Mühlich et al., NPA 78 (26) 187 ω P. Gubler, W. Weise PLB 751( 215) 396 Φ with increasing nuclear density lowering of in-medium mass broadening of resonance spectral function for ω at rest: almost no mass shift; strong in-medium broadening Δm/m < 2% asymmetric broadening Γ(ρ) 45 MeV 6

11 model predictions for in-medium mass/width of the ω, Φ meson F. Klingl et al., NPA 61 (1997) 297; NPA 65 (1999) 299 ω P. Mühlich et al., NPA 78 (26) 187 ω P. Gubler, W. Weise PLB 751( 215) 396 Φ with increasing nuclear density lowering of in-medium mass broadening of resonance spectral function for ω at rest: almost no mass shift; strong in-medium broadening Δm/m < 2% asymmetric broadening Γ(ρ) 45 MeV experimental task: search for { mass shift? broadening? } structures? of hadronic spectral functions 6

12 From theoretical predictions to experimental observables 7

13 From theoretical predictions to experimental observables calculations of meson spectral functions assume: infinitely extended nuclear matter in equilibrium at ρ, T = const.; meson at rest in nuclear medium theoretical predictions 7

14 From theoretical predictions to experimental observables calculations of meson spectral functions assume: infinitely extended nuclear matter in equilibrium at ρ, T = const.; meson at rest in nuclear medium theoretical predictions experimental observables 7

15 From theoretical predictions to experimental observables calculations of meson spectral functions assume: infinitely extended nuclear matter in equilibrium at ρ, T = const.; meson at rest in nuclear medium transport calculations (GiBUU, HSD, UrQMD,...) are needed for comparison with experiment!!! theoretical predictions transport calculations experimental observables initial state effects: absorption of incoming beam particles non equilibrium effects: varying density and temperature absorption and regeneration of mesons fraction of decays outside of the nuclear environment final state interactions: distortion of momenta of decay products 7

16 sensitivity of ω line shape measurement to in-medium modifications of the ω meson J. Weil, U. Mosel, V. Metag, PLB 723 (213) 12 ω GiBUU ω e + e - br: ω π γ br: 8.3% only 2-3 % of the ω decays occur within the nuclear medium; < d > = βγcτ 17 fm a density dependent mass shift is smeared out due to the nuclear density profile 8

17 sensitivity of ω line shape measurement to in-medium modifications of the ω meson J. Weil, U. Mosel, V. Metag, PLB 723 (213) 12 ω GiBUU ω e + e - br: ω π γ br: 8.3% only 2-3 % of the ω decays occur within the nuclear medium; < d > = βγcτ 17 fm a density dependent mass shift is smeared out due to the nuclear density profile GiBUU a density dependent mass shift is smeared out due to the in-medium collisional broadening of the ω-signal: Γ(ρ) MeV ω due to π absorption (π -FSI) ω π γ decays in the center of the nucleus are suppressed; only ω π γ decays in the surface region can be reconstructed 8

18 meson-nucleus optical potential U(r) =V (r)+iw (r) 9

19 meson-nucleus optical potential U(r) =V (r)+iw (r) V (r) = m( ) (r) real part in-medium mass modification 9

20 meson-nucleus optical potential U(r) =V (r)+iw (r) V (r) = m( ) (r) (r) W (r) = /2 = 1 2 ~c (r) inel real part imaginary part in-medium mass modification lifetime shortened in-medium width inelastic cross section 9

21 meson-nucleus optical potential U(r) =V (r)+iw (r) V (r) = m( ) (r) (r) W (r) = /2 = 1 2 ~c (r) inel real part imaginary part in-medium mass modification lifetime shortened in-medium width inelastic cross section mass and lifetime (width) may be changed in the medium 9

22 experimental approaches to determine the meson-nucleus optical potential U(r) =V (r)+iw (r) real part V (r) = m( ) (r) line shape analysis excitation function momentum distribution meson-nucleus bound states T A = A! X A N! X D. Cabrera et al., NPA 733 (24)13 1

23 experimental approaches to determine the meson-nucleus optical potential U(r) =V (r)+iw (r) real part V (r) = m( ) (r) imaginary part (r) W (r) = /2 = 1 2 ~c (r) inel line shape analysis excitation function momentum distribution meson-nucleus bound states transparency ratio measurement T A = T A = A A! A! X A N! X N! X D. Cabrera et al., NPA 733 (24)13 1

24 CBELSA/TAPS Experiment E γ Eγ= = GeV photon beam Forward Plug MiniTAPS 216 BaF2 )] 2 [1/(4 MeV/c 1 4 ω π γ 3γ C 2 m=792.5±.4 MeV/c 2 σ=25.8±.3 MeV/c σm counts 3% m Crystal Barrel 132 CsI solid target: 12 C and 93 Nb 4π photon detector: ideally suited for identification of multi-photon final states ω π γ 3γ BR 8.2% η π π η 6γ BR 8.5% )] 2 [1/(6 MeV/c N π γ N π π η [MeV/c ] η π π η 6γ Nb m=957.6±.5 MeV/c 2 σ=11.8±.3 MeV/c σm 3177 counts m M π γ 1% [MeV/c ] M π π η 2 11

25 High acceptance Dielectron spectrometer beams from SIS18: protons, nuclei, pions 2. GeV < s < 3.2 GeV Side View START FW spectrometer with good particle identification and high invariant mass resolution: 2% at ρ/ω versatile detector for rare probes: dielectrons e + e - strangeness: Λ, K ±, Σ, Ξ, Φ 12

26 The real part of the meson-nucleus optical potential 13

27 p + p, Nb 3.5 GeV pee > 8 MeV/c dilepton invariant mass spectra G. Agakishiev et al., Phys. Lett. B 715 (212) 34 shape of mee spectrum in p+nb identical to reference spectrum in p+p 14

28 dilepton invariant mass spectra p + p, Nb 3.5 GeV G. Agakishiev et al., Phys. Lett. B 715 (212) 34 pee > 8 MeV/c pee< 8 MeV/c pee< 8 MeV/c shape of mee spectrum in p+nb identical to reference spectrum in p+p strong e + e - excess yield below ω peak attributed to ρ-like channels; no hint for change in ω line shape; strong ω absorption N * N ρ γ * e + e - 14

29 comparison to GiBUU simulations J. Weil, H. van Hees, U. Mosel; EPJA 48 (212) 111 comparison to different in-medium modification scenarios G. Agakishiev et al., Phys. Lett. B 715 (212) 34 HADES data p+nb at 3.5 GeV ρ-contribution ω-contribution invariant mass me+e- [GeV] difficult to distinguish between different in-medium scenarios: difficult to disentangle ρ, ω contributions and to extract individual in-medium properties 15

30 ] 2 counts / [9MeV/c line shape analysis: ω line shape from ω π γ in photo-nuclear reaction comparison with GiBUU calculations for different in-medium scenarios x 1 γ Nb ω + X scenarios: no mod. coll. broad. shift + broad. shift only E γ =9-13 MeV M. Thiel et al., EPJA 49 (213) [MeV/c ] m π γ advantage: no ρ π γ ω π γ br: 8.3% sensitivity limited by 5 effects: 1) mass resolution σ 3%; only mass shifts 3% observable 2) only 3% of all ω π γ decays occur within the Nb nucleus 3) ω decays occur over a wide range of densities, thereby smearing out any density-dependent signal 4) ω π γ signal smeared out and reduced due to large in-medium width (Γmed 16 Γvac ) 5.) due to π absorption (π -FSI) ω π γ decays in the center of the nucleus are suppressed 16

31 The real part of the ω-nucleus potential ω π γ sensitive to nuclear density at production point J. Weil, U. Mosel and V. Metag, PLB 723 (213 ) 12 measurement of the excitation function of the meson in case of dropping mass - higher meson yield for given s because of increased phase space due to lowering of the production threshold cross section enhancement π γ excitation function E γ thr 17

32 The real part of the ω-nucleus potential ω π γ sensitive to nuclear density at production point J. Weil, U. Mosel and V. Metag, PLB 723 (213 ) 12 measurement of the excitation function of the meson in case of dropping mass - higher meson yield for given s because of increased phase space due to lowering of the production threshold cross section enhancement π γ excitation function momentum distribution of the meson: in case of dropping mass - when leaving the nucleus hadron has to become on-shell; mass generated at the expense of kinetic energy downward shift of momentum distribution π γ momentum distribution γ+ 93 Nb π γ+x E γ = GeV E γ thr 17

33 σ/a [µb] Carbon excitation function for ω photoproduction off C comparison with GiBUU calculation MAMI V. Metag et al., PPNP, 67 (212) 53 M. Thiel et al., EPJA 49 (213) 132 excitation function momentum distribution E γ thr CB/TAPS@MAMI CBELSA/TAPS GiBUU collisional broad. and mass shift V = MeV V = -2 MeV V = -4 MeV V = -55 MeV V = -94 MeV V = -125 MeV [GeV] E γ vacuum collisional broadening(cb) CB+mass shift (-16%) mass shift (-16%) V(ρ=ρ) = -(42±17(stat)±2(syst)) MeV data not consistent with strong mass shift scenario (Δm/m -16%) 18

34 σ η [µb] ELSA C data σ tot σ diff Excitation function and momentum distribution for η photoproduction off C E γ thr V(ρ=ρ ) = MeV V(ρ=ρ ) = -25 MeV V(ρ=ρ ) = -5 MeV V(ρ=ρ ) = -75 MeV V(ρ=ρ ) = -1 MeV V(ρ=ρ ) = -15 MeV σ η N =11 mb data: M. Nanova et al., PLB 727 (213) 417 calc.: E. Paryev, J. Phys. G 4 (213) 2521 dσ η /dp η [µb/gev/c] C data σ η N =11 mb V(ρ=ρ ) = MeV E γ =15-22 MeV V(ρ=ρ ) = -25 MeV V(ρ=ρ ) = -5 MeV V(ρ=ρ ) = -75 MeV V(ρ=ρ ) = -1 MeV V(ρ=ρ ) = -15 MeV E γ [MeV] V η (ρ=ρ) = -(4±6) MeV p η [GeV/c ] V η (p η 1.1 GeV/c;ρ=ρ) = -(32±11) MeV data disfavour strong mass shifts 19

35 Excitation function and momentum distribution for η photoproduction off Nb ELSA calc.: E. Paryev, priv. communication [µb] σ η' > Nb σ tot σ diff µb/gev/c] Nb data = GeV E γ 1 1 σ inel η'- N = 11 mb P R E L I M I N A R Y η' thr E γ V(ρ=ρ V(ρ=ρ V(ρ=ρ V(ρ=ρ V(ρ=ρ V(ρ=ρ ) = MeV [GeV] V η (ρ=ρ) = -(47±15) MeV ) = - 25 MeV ) = - 5 MeV ) = - 75 MeV ) = -1 MeV ) = -15 MeV E γ /dp η [' dσ η' < 1 1 P R P R E L E I L M I M I N I N A A R R Y Y V(ρ=ρ ) = MeV V(ρ=ρ ) = -25 MeV V(ρ=ρ ) = -5 MeV V(ρ=ρ ) = -75 MeV V(ρ=ρ ) = -1 MeV V(ρ=ρ ) = -15 MeV [GeV/c] P η' V η (p η 1. GeV/c;ρ=ρ) = -(4±15) MeV data disfavour strong mass shifts 2

36 Compilation of results for the real part of the ω- and η -nucleus optical potential ω η excitation function excitation function mom. distribution C Nb peak E kin weighted average V C [MeV] V ωa (ρ=ρ) = -(29±19(stat)±2(syst))MeV [MeV] V η A (ρ=ρ) = V η'a -(39±5(stat)±15(syst)) MeV 21

37 The imaginary part of the meson-nucleus optical potential: momentum dependence 22

38 /A [µb/(gev/c)] ω dσ/dp momentum differential cross section for ω, η produced off C, Nb ω E γ = GeV η γ C,Nb ωx C Nb P R E L I M I N A R Y /A [µb/(gev/c)] η' dσ/dp γ C,Nb η X C Nb P R E L I M I N A R Y [MeV/c] momentum differential cross sections p ω [MeV/c] p η' m TNb/C (pm) = 12 σ γnb mx (pm) 93 σ γc mx (pm) 23

39 momentum dependence of transparency ratio for ω, η m ω 93 σ η γc mx (pm) M. Kotulla et al., PRL 1 (28) M. Kotulla et al., PRL 114 (215) TNb/C (pm) = 12 σ γnb mx(pm) M. Nanova et al., PLB 71 (212) 6 ω TNb/C.4-.6 η TNb/C.7-.8 absorption of η mesons much weaker than for ω mesons!! 24

40 momentum dependence of imaginary potential for ω, η Glauber model: high energy Eikonal approximation ω η m m m TNb/C (pm) Γ (ρ=ρ) (pm) = - 2 Im U (pm) ) [MeV] ω -(Im U M. Kotulla et al., PRL 1 (28) M. Kotulla et al., PRL 114 (215) PRL 114 (215) this experiment PRELIMINARY ) [MeV] η' -(Im U M. Nanova et al., PLB 71 (212) 6 PLB 71 (212) 6 this experiment PRELIMINARY extrapolation to production threshold: ω s ω - s thr [MeV] Im U(ρ=ρ,p ω =) = (3±1) MeV s η' - s thr [MeV] η Im U(ρ=ρ,p η =) = (7±3) MeV extension to high moment allows for dispersion relation analysis, providing link between real and imaginary part of potential 25

41 compilation of results for real and imaginary part of the ω, η -nucleus optical potential ω η UωA(ρ=ρ)= -((29±19(stat)±2(syst) + i(3±1)) MeV imaginary part [MeV] ω Uη A(ρ=ρ)= -((39±5(stat)±15(syst) + i(7±3)) MeV V. Metag Hyp.Int. 234 (215) η' Im U Re U ; ω not a good candidate to search for meson-nucleus bound states! potential depth [MeV] Re U >> Im U ; η promising candidate to search for mesic states first (indirect) observation of in-medium mass shift of η at ρ=ρ and T= in good agreement with QMC model predictions (S. Bass et al., PLB 634 (26) 368) 26

42 search for ω-mesic states E. Marco and W. Weise, PLB 52 (21) 59 excitation energy spectrum of 11 B dω [nb/mev/sr] σ π γ /de kin 2 d S. Friedrich et al., PLB 736 (214) C(γ,p)ω X 1 Carbon ω π γ [MeV] E π γ ω-mesic states quasifree ω-mesic states quasifree intensity in bound state region consistent with tail due to large imaginary part 27

43 search for η -mesic states in hadronic reactions 12 C(p,d)η 11 C K. Itahashi et al., PETP 128 (212) 61 H. Nagahiro et al., PRC 87 (213) 4521 particle identification by time-of-flight 2.5 GeV protons 12 C target missing mass spectrometry: Δmm =2.5 MeV/c 2 S2 aerogel Cerenkov GeV/c deuterons (protons) MWDC S4 p/d separation by aerogel Cerenkov TOF S2-S4 (diff. 2 ns) plastic scintillators aerogel Cerenkov 28

44 search for η -mesic states in hadronic reactions 12 C(p,d)η 11 C K. Itahashi et al., PETP 128 (212) 61 H. Nagahiro et al., PRC 87 (213) 4521 particle identification by time-of-flight no structures in bound state region observed; deep potentials with V > 1 MeV excluded 2.5 GeV protons 12 C target missing mass spectrometry: Δmm =2.5 MeV/c 2 p/d separation by aerogel Cerenkov TOF S2-S4 (diff. 2 ns) S2 aerogel Cerenkov plastic scintillators GeV/c deuterons (protons) MWDC aerogel Cerenkov S4 28

45 outlook: search for η -mesic states in photo-nuclear reactions 12 C(γ,p) η GeV p η γ formation and decay of η -mesic state Δp/p 1-2 % BGO-OD ideally suited for exclusive measurement approved proposal: ELSA/3-212-BGO 29

46 outlook: search for η -mesic states in photo-nuclear reactions 12 C(γ,p) η GeV p η N γ η formation and decay of η -mesic state Δp/p 1-2 % BGO-OD ideally suited for exclusive measurement approved proposal: ELSA/3-212-BGO 29

47 outlook: search for η -mesic states in photo-nuclear reactions C(γ,p) η GeV p η N C(γ,p) η GeV γ η formation and decay of η -mesic state Δp/p 1-2 % BGO-OD ideally suited for exclusive measurement approved proposal: ELSA/3-212-BGO 29

48 Summary & Outlook how does the intrinsic structure of hadrons change in a dense nuclear medium?? 3

49 Summary & Outlook how does the intrinsic structure of hadrons change in a dense nuclear medium?? meson properties do change in a strongly interacting medium!! all mesons are broadened; their lifetime is shortened through inelastic collisions Γ ω (ρ=ρ; p=) 6 MeV; Γ η (ρ=ρ; p=) 15 MeV; large mass modifications Δm > 1 MeV (as predicted by some calculations) have not been observed for the η meson an in-medium mass drop of Δm (ρ=ρ) - 4 MeV has been determined in-medium effects described within meson-nucleus optical potential the η meson is a good candidate for forming meson-nucleus bound states since Im U << Re U search for η mesic states ongoing 3

50 real part of ω-nucleus potential from ω kinetic energy ELSA γ 1 θp 11 the higher the attraction the lower the kinetic energy of the ω meson H. Nagahiro, priv. com. ω E γ = GeV p dω [nb/mev/sr] σ π γ /de kin d , W ) (V - (156,7) MeV - (1,7) MeV - ( 5,7) MeV - (,7) MeV - (-2,7) MeV - (-5,7) MeV [MeV] E π γ 31

51 real part of ω-nucleus potential from ω kinetic energy ELSA γ 1 θp 11 the higher the attraction the lower the kinetic energy of the ω meson ω E γ = GeV p dω [nb/mev/sr] σ π γ /de kin 2 d H. Nagahiro, priv. com., W ) (V - (156,7) MeV - (1,7) MeV - ( 5,7) MeV - (,7) MeV - (-2,7) MeV - (-5,7) MeV [MeV] E π γ Wω (ρ=ρ)= -Γ/2 = -(7±1) MeV d [nb/mev/sr] /de kin 2 d Carbon S. Friedrich, PhD thesis (Univ. Giessen) Ekin=(6.5±7)MeV [MeV] E peak position [MeV] d) PRELIMINARY potential depth [MeV] V ω (ρ=ρ) = -(15±35) MeV 31