Photoproduction of mesons off light nuclei

Transcription

1 Photoproduction of mesons off light nuclei B. Krusche, U. Basel Introduction Experimental setups Crystal Ball & MAMI Crystal Barrel & ELSA Experimental results photoproduction off quasi-free nucleons:η,η,π o η,π ± η,π o,π o π o,π o π ±,... coherent photoproduction of mesons off light nuclei: π o η, π o π o, π o π o π o Conclusions Collaboration B. Krusche, Symposium, Universität Mainz, Feb.

2 complex many body system valence quarks Structure of the Nucleon comparison: known excited states - constituent quark model (Capstick & Roberts) sea quarks N(I=/) Mass/(MeV/c ) (I=3/) gluons models - effective dof s: 3 equivalent constituent quarks quark - diquark models (fewer states) quarks - flux tubes etc. (more states) G 9 () H 9 () D () G 7 (9) P () S (9) D 3 (8) F () F 7 (99) P 3 (9) P 3 (7) P (7) D 3 (7) F (68) D (67) S (6) S (3) D 3 () P () exp QM 8 6 QM exp H 3, () F 37 (39) D 3 (3) H 39 (3) S 3 () F 3 () F 37 (9) D 33 (9) D 3 (93) P 33 (9) P 3 (9) F 3 (9) S 3 (9) P 3 (7) D 33 (7) S 3 (6) P 33 (6) chiral soliton models (anti-decuplet states) P (939) P 33 (3) coupled channel dynamics (molecule-like states)... ordering of (low-lying) states? missing resonance problem?

3 Experimental Options: Final. states: Observables: Isospin: neutron targets single meson production: γp pπ,η,η,ω...;σk ( )... σ[µb] ρ 3 γp pπ o D 3 P S F γp pη photon energy S (3) S (6) multiple meson production: N *, ππ π N *, π N * σ η π N(938) πη N * η N * π N *, π N * η ang. distributions dσ/dω Dalitz plots M(N,m i ), M(m,m ) polarization dof: - linearely pol. beams - circularly pol. beams - longitudinally pol. targets - transversely pol. targets - recoil polarization I Σ, R, T E, G, H, F... elm. excitations isospin dependent quasifree off the deuteron coherent off the deuteron

4 electromagnetic excitations of the neutron importance of measurements off the neutron: different resonance contributions needed for extraction of iso-spin composition of elm. couplings complications due to use of nuclear targets (deuteron): coincident detection of recoil nucleons Fermi motion, nuclear effects like FSI, coherent contributions B. Krusche, Symposium, Universität Mainz, Feb.

5 σ tot [µb] most basic example: γd π o np in the -resonance 8 6 Darwish et al. 3; Levchuk et al. π np 3 ω γ lab dσ/dω π [µb/sr] dσ/dω π [µb/sr] ω γ lab =3 MeV θ π [deg] π - pp 6 8 θ π * [deg] π np ω lab γ =8 MeV dσ/dω π [µb/sr] dσ/dω π [µb/sr] 3 π - pp ω γ lab =33 MeV θ π [deg] π np ω γ lab =3 MeV 6 8 θπ * [deg] completely dominated by excitation of (3) resonance naive expectation: proton/neutron cross sections equal but: plain-wave modelling does not at all fit data large effects from FSI (in particular NN-FSI) σ tot, µb 6 3 γd π np 3, MeV B. Krusche, Symposium, Universität Mainz, Feb.

6 σ [µb] π o -mesons in the -resonance: coherent - breakup - FSI 6 large coherent cross section, strong FSI γd ppπ d(γ,π o )np d(γ,π o )d 3 3 photon energy dσ/dω π [µb/sr] dσ/dω π [µb/sr] π - pp lab 3 ω γ =3 MeV 6 8 θ π [deg] 6 π np lab ω γ =8 MeV 6 8 dσ/dω π [µb/sr] π - pp lab 3 ω γ =33 MeV 6 8 θ π [deg] γd npπ o dσ/dω π [µb/sr] 6 π np lab ω γ =3 MeV 6 8 θ π * [deg] θπ * [deg] γd Xπ o vs. γp pπ o (/A)dσ/dΩ[µb/sr] = MeV =39 MeV =6 MeV =6 MeV =3 MeV =3 MeV cos(θ π ) strong FSI: breakup cross sections no good estimate for free nucleon xs breakup + coherent quasi-free without FSI γ π N N N N a γ π N N N N b γ N N c π N N B. Krusche, Symposium, Universität Mainz, Feb.

7 π o in the second resonance: not yet understood models (MAID, SAID): cross sections similar for proton and neutron (D 3 ()) again large disagrement between inclusive data and plain-wave predictions again only due to FSI? σ[µb] p(γ,π o )p d(γ,π o )np (/A)dσ/dΩ [µb/sr] cos(θ* N π )=-.9 cos(θ* N π )=-.7 cos(θ* N π )= cos(θ* N π )=-.3 cos(θ* N π )=-. cos(θ* N π )=. cos(θ* N π )= * N cos(θ* π )= cos(θ π )=.3 photon energy B. Krusche, Symposium, Universität Mainz, Feb.

8 exclusive single π o quasi-free and coherent production - b] σ [µ 8 6 total cross sections PRELIMINARY σ [µ b] 3 3 D 3 M. Dieterle et al. F σ(π n) σ(π p) σ(π d) x D quasi-free off proton and neutron: contributions from different N -resonancs coherent off deuteron: only -resonances, N suppressed due to isospin conservation B. Krusche, Symposium, Universität Mainz, Feb. MAMI

9 Experiments: Crystal Ball & Crystal Barrel with TAPS Bonn ELSA accelerator: Crystal Barrel (CsI), TAPS (BaF ) forward wall, inner detectors 3. GeV, lin. pol.: available, circ. pol.: available Mainz MAMI accelerator: Crystal Ball (NaJ), TAPS (BaF ) forward wall, inner detectors. GeV, lin. pol.: available, circ. pol.: available cm B. Krusche, Symposium, Universität Mainz, Feb.

10 B. Krusche, Symposium, Universität Mainz, Feb. TAPS Crystal Ball - at MAMI

11 η-photoproduction off the proton: resonance contributions? branching ratios and elm. couplings (PDG): state b η [%] A p / A p 3/ A n / A n 3/ D 3 ():.3± S (3): S (6): D (67): ± F (68): ± D 3 (7): ± P (7): 6.±. P 3 (7): ± σ tot / [µb] S S γp ηp.. P, P 3 D? / [GeV] D (67) has stronger electromagnetic coupling to neutron than to proton but parameters quite uncertain: A n / =-(-7), An 3/ =-(3-77) b η =-% (PDG), b η =7% (ETA-MAID, Chiang et al.) interference structure in S -sector? Data: W / [GeV] TAPS: B. Krusche et al., PRL7 (99) 3736 GRAAL: F. Renard et al., PLB8 () CLAS: M. Dugger et al., PRL89 () Crystal Barrel: V. Crede et al., PRL9 () B. Krusche, Symposium, Universität Mainz, Feb.

12 what is expected for n(γ,η)n - why is it interesting? total cross sections for proton and neutron from MAID model with and without D (67)) previous data from MAMI only at lower incident photon energies GRAAL experiment reported narrow structure in neutron excitation function σ[µub] full model MAID, proton no D (67), proton. d(γ,η)x, σ n /σ p ( ) d(γ,η)x, σ * n /σ p ( ) He(γ,η)X, σ n /σ p ( ) He(γ,η)X, σ * n /σ p ( ) dσ/dω, a.u W,GeV dσ/dω, a.u W,GeV full model MAID, neutron no D (67), neutron σ n /σ p Kaiser et al. MAID full model MAID only S(3) dσ/dω, a.u W,GeV dσ/dω, a.u W,GeV W dσ/dω, a.u W,GeV dσ/dω, a.u W,GeV predictions from chiral soliton models: P -like state of the anti-decuplet has strong photon-coupling to the neutron and large ηn decay branching ratio B. Krusche, Symposium, Universität Mainz, Feb.

13 control over Fermi motion and FSI effects? test with help of proton data: example: excitation functions pη σ[µb] σ[µb] σ(ηp), quasi-free 7 σ[µb] σ(ηp), free proton σ(ηp), quasi-free proton σ(ηp), free proton.9.8 σ(ηp), quasi-free, unfolded W excellent agreement after correction of Fermi motion with corrrection factors calculated from ratio of free and Fermi folded excitation function no FSI etc., only Fermi motion good agreement for excitation functions depending on final state invariant mass of meson-nucleon pair model independent correction of Fermi motion B. Krusche, Symposium, Universität Mainz, Feb. ELSA

14 control over detection efficiency of recoil nucleons? internal consistency: σ incl = σ p + σ n + (σ d ) coherent part σ d often negligible: σ n σ incl σ p example: η photoproduction excitation functionsη distribution of differences of dσ dσ (n), dω dω (incl p) σ[µb] σ p σ n σ np σ p +σ n σ n /(σ np -σ p ).. counts 6 3 fitted Gauss standard Gauss σ =. σ =. µ =.3 µ = δσ i / σ i no indication of any systematic effects from neutron detection efficiency! B. Krusche, Symposium, Universität Mainz, Feb. ELSA

15 quasifree η-photoproduction off the deuteron cross section for γn ηn from analyses with very different systematics: () η in coincidence with recoil neutrons () difference of inclusive data and η in coincidence with recoil protons σ[µb] σ(npη) σ(pη) σ(nη) σ(npη)-σ(pη) σ[µb] σ n /σ p. σ n /σ p. pη, nη, MAID pη nη*3/ I. Jaegle et al. B. Krusche, Symposium, Universität Mainz, Feb. ELSA

16 Bonn-Gatchina-Model analysis σ tot [µb] σ tot [µb] σ tot [µb] CB-ELSA CB-ELSA CB-ELSA M(γn) M(γn) M(γn) different scenarios to reproduce bump structure: left: interference in S -sector: adjusting phases etc. center: introduction of conventional (broad) P resonance right: introduction of very narrow P resonance A. Anisovich et al., EPJA (9) 3 B. Krusche, Symposium, Universität Mainz, Feb.

17 de-folding of Fermi motion for events with neutron in TAPS (cos(θ η ) < -.) neutron energy from time-of-flight comparsion: W from photon energy (Fermi smeared) - W from nucleon - meson -vectors (resolution smeared) de-folded proton cross section similar to free proton, de-folded neutron cross section shows structure around.7 GeV: position: W =683 MeV width: Γ=6± MeV (resolution dominated) dσ/dω acc [µb/sr] I. Jaegle et al., Phys. Rev. Lett. (8) ηp ηn W B W R B. Krusche, Symposium, Universität Mainz, Feb. ELSA

18 de-folding of Fermi motion - kinematical re-construction reaction kinematics completely determined even without neutron ToF: initial state: incident photon and deuteron at rest known/measured:, m d, p d = final state: η-meson, participant, and spectator nucleon known/measured: m s, m p, Θ p, Φ p, m η, p η not measured: T p,p s (four variables) four constraints from energy/momentum conservation four-vectors of participant and spectator determined structure in neutron excitation function width (±) MeV (resolution dominated). spectator momentum in quantitative agreement with deuteron wave function σ n /σ p. counts[a.u.].8.6. neutron spectator data simulation deuteron wf. proton spectator σ[µb]. 6 8 W σ(ηp), free proton σ(ηp), quasi-free proton 3/σ(ηn), quasi-free neutron. 3 3 p s 6 8 W B. Krusche, Symposium, Universität Mainz, Feb. ELSA

19 Fermi de-folded proton and neutron data excitation functions for different bins of polar angle fit with: free proton data ( ): McNicoll et al. PRC 8 (); curves: MAID model cosθ cosθ -.. cosθ. +. cosθ dσ dω = q k ΣA i P i (cos(θ )) σ[µb] σ(ηp) σ(ηn) A i [µb/sr] γp pη A o γn nη A -. σ n /σ p. A W A 3 peak in neutron cross section related to dip in proton cross sections (?) only dip reproduced by MAID! W[GeV] B. Krusche, Symposium, Universität Mainz, Feb. ELSA

20 preliminary excitation functions from MAMI W = f( ), 3 < Φ < W = f(n,η), 3 < Φ < σ [µb] gp ηp 8 6 gn ηn σ [µb] gp ηp 8 6 gn ηn W beam W true W = f( ), 7 < Φ < 9 W = f(n,η), 7 < Φ < 9 σ [µb] gp ηp 8 6 gn ηn σ [µb] gp ηp 8 6 gn ηn W beam B. Krusche, Symposium, Universität Mainz, Feb. MAMI W true

21 similar results for 3 He target L. Witthauer et al. quasi-free proton, W from p η final state σ [µb] Total cross section (W) W nucleon via time-of-flight, (only nucleons in TAPS, backward η) Total cross section (W) 8 W = 6 ± MeV Γ = 36 ± 7 MeV 6 Total cross section (W) quasi-free neutron, W from n η final state σ [µb] σ [µb] W W R B. Krusche, Symposium, Universität Mainz, Feb. MAMI

22 s-wave coupled channel model s-wave coupled channel model with dynamical generation of S (3) pole free cross sections KΛ KΣ quasi-free cross sections σ [µb] σ n / σ p. cross section ratio 7 σ [µb] s / =m N +m η s / =3 MeV structure in σ n /σ p due to KΛ, KΣ threshold effects (?) (intermediate strangeness states in loop-diagrams) M. Döring. K. Nakayama, nucl-th:99.338v B. Krusche, Symposium, Universität Mainz, Feb.

23 other decay channels: Compton scattering (?!) a resonance with strong electromagnetic coupling to the neutron could (should) show up in γn nγ experimentally extremely challenging! positive result reported in: V. Kuznetsov et al. Phys. Rev. C83 () (R) 3 N. γp γp γn γn W σ [a.u.] W true MAMI data under analysis first very preliminary results (less than % statistics, still contaminated with π o decay photons) not yet conclusive B. Krusche, Symposium, Universität Mainz, Feb. MAMI

24 Summary narrow structure in excitation function of γn nη: GRAAL: Tohoku-LNS: ELSA: MAMI-C: W 68 MeV, Γ <3 MeV W 666 MeV, Γ < MeV W 68 MeV, Γ < MeV W 67 MeV, Γ < MeV so far no information about quantum numbers of possible resonance or whatever nature of the structure B. Krusche, Symposium, Universität Mainz, Feb.

25 Results from other reaction channels... photoproduction of η -mesons off the neutron I. Jaegle et al. EPJA 7 () Photoproduction of π o -pairs off the neutron (in preparation) σ [µb] W σ(π pn) σ(π σ(π p) σ(π n) σ(π σ(π p) + σ(π n) p) - GRAAL n) - GRAAL B. Krusche, Symposium, Universität Mainz, Feb. ELSA

26 Conclusions - Systematic investigation of meson production off H and 3 He η-photoproduction off deuteron: large difference for resonance contributions to p(γ,η)p and n(γ,η)n narrow structure in excitation function off neutron other channels: iso-spin dependence of the elm. nucleon excitation outlook: quasi-free (double) polarization observables I. Jaegle I. Keshelashvili T. Rostomyan M. Dieterle A. Käser Y. Maghrbi M. Oberle F. Pheron D. Werthmüller L. Witthauer Th. Challand R. Trojer B. Krusche, Symposium, Universität Mainz, Feb.