Determination of the η -nucleus optical potential

Transcription

1 Determination of the η -nucleus optical potential M. Nanova II. Physikalisches Institut for the CBELSA/TAPS Collaboration Outline: motivation experimental approaches for determining the η -nucleus optical potential - transparency ratio measurement - excitation function of the η -meson - momentum distribution - η -N bound states summary & outlook *funded by the DFG within SFB/TR6 Electromagnetic Probes of Strongly Interacting Matter, ECT* Trento, May , Trento, Italy

2 nonet of pseudoscalar mesons motivation M=958 MeV/c 2 η M=548 MeV/c 2 K M=498 MeV/c 2 π M=4 MeV/c 2 MeV/c D. Jido et al., NPA (22) The NJL Model masses as a result of symmetry breaking 2

3 spectral function of the η meson model predictions - based on NJL V. Bernard und U.-G. Meiβner, Phys. Rev. D 38 (988) 55 H. Nagahiro, M. Takizawa and S. Hirenzaki, Phys. Rev. C 74 (26) 4523 SU(3) SU(2) the mass of the η meson is almost independent of density some experimental results large medium effect could be seen even at normal nuclear density η p scattering length: aη p ~. fm - COSY (P. Moskal et al., PLB 482 (2) 356) Δmη -2 MeV - RHIC (Csörgo et al., PRL 5 (2) 823) 3

4 theoretical evaluation of the η -nucleus optical potential prediction of η - 2 C bound states and their widths for different η -meson nucleus potentials D. Jido et al., PRC 85 (22) 322 Binding Energy [MeV] V [MeV] V,W [MeV] s p s p s p d s p d (-,-2) (-5,-2) (-2,-2) (-,-5) experimental determination of the η -nucleus optical potential? 4

5 Experimental approaches to determine the meson-nucleus optical potential meson-nucleus optical potential U(r) =V (r)+iw (r) meson mass shift V (r) = m( ) line shape analysis: direct determination of Δm (r) meson absorption (r) W (r) = /2 = 2 ~c (r) inel Transparency ratio measurement excitation function: provides information about the depth of V(r) T A = A! X A N! X meson momentum distribution: provides information about the depth of V(r) experimental observable to extract the in-medium width of the meson meson-nucleus-bound states: direct determination of Ebin (Δm) 5

6 Crystal Experiment photoproduction of η meson beamtime 23 beamtime 29 photon beam Eγ= GeV Eγ=.7-3. GeV photon beam Forward Plug MiniTAPS Crystal Barrel Crystal Barrel TAPS solid target: 2 C, 4 Ca, 93 Nb and 28 Pb solid target: 2 C 4π photon detector: ideally suited for identification of multiphoton final states η π π η 6γ BR 8.% 6

7 C counts / MeV/c 2 counts / MeV/c C M = (956. ±.9) MeV/c 2 = (8.8 ±.) MeV 384 counts 9 2 M( ) [MeV/c 2 ] Nb Transparency ratio measurement of η with CBELSA/TAPS Eγ = 5-22 MeV η π π η 6γ BR: 8.% photoproduction of η meson off 2 C, 4 Ca, 93 Nb and 28 Pb M = (955.6 ±.) MeV/c 2 = (8.9 ±.6) MeV 385 counts 9 2 M( ) [MeV/c 2 ] counts / MeV/c 2 counts / MeV/c Ca M = (955.3 ±.6) MeV/c 2 = (9. ±.9) MeV 28 counts 9 2 M( ) [MeV/c 2 ] Pb M = (956.4 ±.6) MeV/c 2 = (9. ±.6) MeV 5 counts 9 2 M( ) [MeV/c 2 ] T A M. Nanova et al., PLB 7 (22) T C A = 2 normalized to carbon exp data ( )= MeV ( )=5 MeV ( )=2 MeV ( )=25 MeV ( )=3 MeV ( )=35 MeV ( )=4 MeV A! X A C! X E =.7 GeV 2 at low density approximation: ( ) = ( ) ( ) = Im ( ) v inel ; E Γη (< pη >.5 GeV/c ) 5-25 MeV; ρ=.7 fm -3 ; ση inel 3 - mb W(ρ=ρ)= -Γ/2 = - ( ) MeV A 7

8 momentum distribution of the η - transparency ratio M. Nanova et al.,plb 7 (22) 6 T A Ca Nb Pb p [MeV/c] inel [mb] [MeV] p [MeV/c] curve:e. Oset, A. Ramos, PLB 74 (2) 334 ( ) = inel = p ( ) = ( ) where E < > < inel > TA - no strong variation with momentum indicating no significant two-step processes W(ρ=ρ)= -Γ/2 = - MeV 8

9 Determination of the real part of the η -nucleus potential J. Weil, U. Mosel and V. Metag, arxiv: (PLB ) - 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 - measurement of the 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 E. Paryev, J. Phys. G: Nucl. Part. Phys. 4 (23) 252 based on γp η p and γn η n exp. data σ γc->η X [µb].. γ 2 C->η X Coll. Broadening (σ η N =8 mb) Coll. Broadening + Mass Shift (-5%) E thr N σ γnb->η X [µb]. γ 93 Nb->η X Coll. Broadening (σ η N =8 mb) Coll. Broadening + Mass Shift (-5%) E thr N dσ γc->η X /dp η [µb/(gev/c)] γ 2 C->η X E γ = GeV Coll. Broadening (σ η N =8 mb) Coll. Broadening + Mass Shift (-5%) dσ γnb->η X /dp η [µb/(gev/c)] γ 93 Nb->η X E γ = GeV Coll. Broadening (σ η N =8 mb) Coll. Broadening + Mass Shift (-5%) E E γ [GeV] E γ [GeV] p η [GeV/c] p η [GeV/c] 9

10 experimental data on η photoproduction off proton and deuteron CBELSA/TAPS Collaboration elementary cross sections: γp η p V. Crede et al.,prc 8 (29) 5522 γn η n I. Jaegle et. al.,epja 47 (2) CLAS data, M. Williams et al., PRC 8 (29) 4523

11 experimental data on η photoproduction off 2 C Eγ = MeV η π π η 6γ BR: 8.% d /d(cos ) [µb] cos( ) cm

12 [µb] - η photoproduction off C comparison of CBELSA/TAPS data with calculations by E. Paryev, J. Phys. G: Nucl. Part. Phys. 4 (23) 252 and priv. communication excitation function C data (29) tot diff E thr N ) = MeV ) = -5 MeV ) = - MeV ) = -5 MeV P R E L I M I N A R Y decay mode: η π π η d /dp [µb/gev/c] - C data (29) momentum distribution E =5-22 MeV ) = MeV ) = -5 MeV ) = - MeV ) = -5 MeV P R E L I M I N A R Y E [MeV] calculations normalized to data in Eγ = 2-25 MeV p [GeV/c ] calc. normalized to data in pη =.5-.8 GeV/c strong mass shift not consistent with data (deep real potential excluded?) 2

13 estimation of the of η -nucleus potential depth exp. data and the 3 scenarios divided by the calculation for scenario V(ρ=ρ)= MeV ratio excitation function P R E L I M I N A R Y C data (29) ) = MeV ) = - MeV ) = -5 MeV ) = -5 MeV ratio momentum distribution E =5-22 MeV C data (29) ) = MeV ) = -5 MeV ) = - MeV ) = -5 MeV P R E L I M I N A R Y E [MeV] high sensitivity to different scenarios at threshold.5.5 p [GeV/c ] data favour V(ρ=ρ) -5 MeV attractive! 3

14 sensitivity to different scenarios [µb] Carbon data tot diff 2 V = MeV V = -25 MeV; V = -5 MeV V = - MeV; V = -5 MeV 25-4 MeV below threshold - ) = MeV ) = -5 MeV ) = - MeV ) = -5 MeV E thr N E [MeV] d /d(cos ) [µb] MeV 2-22 MeV cos( ) cm at threshold above threshold high sensitivity to different scenarios at threshold again: data favour V(ρ=ρ) -5 MeV 4

15 estimation of the of η -nucleus potential depth d /dp [µb/gev/c] - momentum distribution C data (29) E =5-22 MeV ) = MeV ) = -5 MeV ) = - MeV ) = -5 MeV average momentum [GeV/c].5..5 exp. data model calculations p [GeV/c ] potential depth [MeV] V(ρ=ρ) = -5±2 MeV W(ρ=ρ) = - MeV V >> W! 5

16 population of η -mesic states in photo induced reactions γ η p forward going proton takes up momentum of incoming photon beam, leaving meson almost at rest captured by nucleus in case of an attractive interaction attractive: V(r) < mesic state; mass drop H. Nagahiro et al., Nucl. Phys. A 76 (25) 92 repulsive: V(r) > two ways of measuring excitation energy of mesic nucleus: ) inclusive measurement missing mass spectroscopy: measure spectrum of forward going proton 2) exclusive measurement measure kinetic energy of decay products of mesic state + forward going p 6

17 spectroscopy of η -mesic state: inclusive measurement missing mass measurement of (p,d) reaction with K. Itahashi et al., Prog. Theo. Phys. 28(22) 6 2 C(p,d) η 2.5 GeV H. Nagahiro et al., PRC 87 (23) 452 target 7

18 decay of the η -mesic state: final states measure kinetic energy of decay products of mesic state decay of η - mesic states: quasi-free production: η N M B η N N N η escape H. Nagahiro et al., PRC 87 (23) C(p,d) η 2.5 GeV η -bound state expected to decay predominantly by emission of lighter mesons: η N ηn (E. Oset and A. Ramos, PLB 74 (2) 334) 8

19 η -mesic state spectroscopy with FAIR detect decay of η mesic states η C η p + X in semi-exclusive reaction 9

20 η N M B search for η -mesic states - exclusive measurement BGO-OD@ELSA H. Schmieden, P. Levi Sandri 2 C(γ,p) η 2.8 GeV BGO-OD ideally suited for exclusive measurement 4π acceptance charged and neutral particle ID BGO ball: a highly segmented calorimeterideal for neutral meson detection Forward spectrometer: tracking detectors, dipole magnet, drift chambers and TOF walls - charged particle ID and momentum reconstruction Δp/p -2 % η N η p - exclusive measurement η in coincidence with a forward going p proposal: ELSA/3-22-BGO 2

21 summary. Imaginary part of the η - nucleus optical potential determined from transparency ratio measurements: W(ρ=ρ)= Γ/2 - MeV 2. Real part of the η - nucleus optical potential determined from measurement of: a. excitation function b. momentum distribution data favour V(ρ=ρ) -5±2 MeV - preliminary result! outlook promising parameters for search of η -bound states width ( MeV) binding energy (5 MeV) 2