2 Collaboration a Quasi-Free Photoproduction of Mesons off 3 He - - PowerPoint PPT Presentation

Photoproduction of η Mesons off Light Nuclei

Lilian Witthauer

Hadron 2011

Munich, June 13th 2011

Collaboration

2

a

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Outline

1 Light η-Mesic Nuclei

Motivation Experiment Identification of the Coherent Reaction Results

2 Quasi-Free Photoproduction of η Mesons off 3He

Motivation Identification of the Quasi-Free Reaction Results

3 Conclusion & Outlook

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Light η-Mesic Nuclei (F. Pheron et al.)

Do the properties of the strong interaction allow the formation of meson-nucleus bound states? p p n η

◮ Several model predictions:

◮ Bhalerao and Liu (1985): attractive s-wave ηN-interaction for

A > 12.

◮ Liu and Haider (1986): interaction migth lead to the formation

• f quasi-bound η-nucleus states for A > 10 (η-mesic nuclei)

◮ new analyses prefer larger values for the scattering length

(above 0.5fm) ➢ existence of very light η-mesic nuclei (2H, 3H, 3He, 4He)?

◮ Quasi-bound states should give rise to an enhancement at

threshold of the cross section relative to the expectation for phase space behavior.

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Light η-Mesic Nuclei: Experimental Evidence

➢ hadronic induced reactions: ➢ photon induced reactions:

• T. Mersmann et al. dp → 3Heη

(COSY-ANKE) ▲ H.-H. Adam et al. dp → 3Heη ▼

• B. Mayer et al. pd → 3Heη

■

• J. Berger et al. dp → 3Heη
• M. Pfeiffer et al.
• Phys. Rev. Lett. 92 (2004) 252001

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

MAinzer MIcrotron: Electron Accelerator

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

MAinzer MIcrotron: Electron Accelerator

Stage: Turns: Energy: LINAC − 3.97 MeV RTM1 18 14.9 MeV RTM2 51 180 MeV RTM3 90 855.1 MeV

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

MAinzer MIcrotron: Electron Accelerator

Stage: Turns: Energy: HDSM 43 1508 MeV

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

MAinzer MIcrotron: Electron Accelerator

A2 Experimental Hall Bremsstrahlung Tagging Eγ = E beam

e−

− E tagged

e−

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Experiment: Crystall Ball and TAPS

◮ Circularly polarised tagged photon

beam (0.45 GeV to 1.4 GeV)

◮ Crystal Ball: Highly segmented sphere

made of NaI

◮ PID: Cylinder of scintillation counters

surrounds target, charged particle detector

◮ TAPS: Forward wall, BaF2 & PbWO4

crystals

◮ 3He Target: 5.3 cm long, density:

0.069 g/cm3

geometrical exeptance close to 4π

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Identification of γ3He→3Heη (F. Pheron et al.)

◮ Decay channels: η → 2γ, η → 6γ ◮ Invariant mass analysis for each energy

and meson cm-polar angle

◮ 6γ: photons combined via χ2-test to 3

pairs (best solution for 3π0 IM) → 110 MeV - 150 MeV cut

◮ use overdetermined kinematics to

separate coherent from breakup

• reactions. → missing energy

➢ strong coherent component visible ➢ recoil taken by quasi-free nucleon ➢ recoil taken by di-nulceon

604 MeV 612 MeV 621 MeV 638 MeV 655 MeV 672 MeV η→2γ η→6γ

∆E[MeV]

20 40 10 20 10 20 30 5 10 15 20 40 10 20 20 40

counts/4 MeV

10 20 20 40 60 10 20 30 20 40 60

• 40 -20

20 10 20 30

• 40 -20

20 40

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

γ3He→3Heη Cross Section (F. Pheron et al.)

0.1 0.2 0.3 600 620 640 660 680 700

Eγ[MeV] σ[µb] η→2γ η→6γ

Tiator et al. Shevchenko et al. Fix et al.

◮ agreement between 2γ and 6γ ◮ disagreement between existing models and data ◮ Tiator et al.: PWIA ◮ Fix and Arenh¨

• vel: 1. PWIA 2.

DWIA → strong FSI effects, full four-body model ◮ Shevchenko et al.: 2 examples for different FSI modelling → strong threshold effects

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

γ3He→ 3Heη Cross Section (F. Pheron et al.)

604.2 MeV 612.7 MeV 621.1 MeV 629.6 MeV 638.1 MeV 646.6 MeV 655.0 MeV 663.5 MeV 671.9 MeV 680.3 MeV

cos(Θη) cos(Θ*

20 40 20 40 20 40 60

dσ/dΩ[nb/sr]

25 50 75 25 50 75

• 1
• 0.5

0.5

• 1
• 0.5

0.5 1

0.1 0.2 600 620 640 660 680 700 720

Eγ[MeV] σ[µb]

1 10 600 650 700

σ/σPWIA

• average of 2γ and 6γ F. Pheron et al.

▼

• M. Pfeiffer et al., Phys. Rev. Lett. 92 (2004) 252001.

 PWIA:

dσPWIA dΩ

(Eγ, q) = q(A)

η

k(A)

γ

·

• k(N)

γ

q(N)

η

dσelem dΩ

• · F 2

A(q2)

F 2

p (q2)

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

π0-p back-to-back (F. Pheron et al.)

• γ

p p n p p n η S11 p n π p n p

0.2 0.4 0.6 0.8 1 400 600 800 1000

Eγ[MeV] counts*Eγ6[a.u.]

• M. Pfeiffer et al., Phys. Rev. Lett. 92 (2004) 252001.

Peak is an artefact arising from the complicated background structure of quasi-free π0 production!

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Quasi-Free η Photoproduction

GRAAL collaboration: narrow structure in the cross section:

γ + d → η + n(p)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.5 1.6 1.7 1.8 1.9 dσ/dΩ, µb/str 0.2 0.4 0.6 0.8 1 1.5 1.6 1.7 1.8 1.9 W, GeV 0.2 0.4 0.6 0.8 1 1.5 1.6 1.7 1.8 1.9 W,GeV dσ/dΩ, µb/str

GRAAL, V.Kuznetsov et al., hep-ex 0606065 Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Quasi-Free η Photoproduction

GRAAL collaboration, CBELSA/TAPS collaboration, LNS-Sendai: narrow structure in the cross section:

γ + d → η + n(p)

W = 1.66 GeV Γ ≈ 25 ± 12 MeV

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.5 1.6 1.7 1.8 1.9 dσ/dΩ, µb/str 0.2 0.4 0.6 0.8 1 1.5 1.6 1.7 1.8 1.9 W, GeV 0.2 0.4 0.6 0.8 1 1.5 1.6 1.7 1.8 1.9 W,GeV dσ/dΩ, µb/str

GRAAL, V.Kuznetsov et al., hep-ex 0606065 ELSA, I.Jaegl´ e et al. EPJA accepted Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Nature of this Structre is unknown

• 1. etaMAID:

Large contribution of the D15(1675) ➢ high value for the branching ratio of ΓηN/Γtot = 17% (PDG: ΓηN/Γ ≃ 0 − 1%)

(L.Tiator, NSTAR2005)

• 2. Chiral Soliton model:

Predicts the existence of a nonstrange member of the baryon antidecuplet (P11-like state). Photoexcitation of this state is thought to have a bigger coupling to the neutron than to the proton

(D.Diakonov et al., arXiv:hep-ph/9703373v2) Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Why on 3He ?

Exclude possibility that the structure could arise from nuclear effects: rescattering of mesons, final state interaction

◮ study it for a nucleon system with different momentum

distribution and different neutron/proton ratio: 3He deuterium p n helium-3 p p n

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Reaction Identification of the Quasi-Free Reaction

p(np) γ +3 He → n(pp) +η

➢ Reconstruct η out of 2 photons with invariant mass technique: Mγγ =

• 2Eγ1Eγ2(1 − cos(φγ1γ2))

➢ Clean invariant mass because of

• ther cuts on decay photons:

Coincidence cuts, random subtraction...

Invariant Mass [MeV] 100 200 300 400 500 600 700 800 900 1000 Counts [a.u.] 200 400 600 800 1000 1200

3

10 × =134.8 MeV

π

M =547.3 MeV

η

M

30 ×

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Background Suppression

Main background from γ +3 He → ηπX Missing Mass cut: M2 = (PBeam + PN − Pη)2

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Background Suppression

Main background from γ +3 He → ηπX Missing Mass cut: M2 = (PBeam + PN − Pη)2 Coplanarity cut: Cut on ∆φ between 170◦ and 190◦

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Total Cross Sections on p/n

[MeV]

B

W 1400 1500 1600 1700 1800 1900 b] µ [ σ 5 10 15 20 25 30

N η → +N γ N Pfeiffer et al. η → +N γ p η → +p γ n η → +n γ N (from sum) η → +N γ

PRELIMINARY

[MeV]

B

W

1400 1500 1600 1700 1800 1900

b] µ [ σ

1 2 3 4 5 6 7 8 p η → +p γ n η → +n γ

PRELIMINARY no coplanarity cut

[MeV]

B

W

1400 1500 1600 1700 1800 1900

b] µ [ σ

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 p η → +p γ n η → +n γ

PRELIMINARY with coplanarity cut

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

WB versus WR

Cross Sections as function of...

◮ WB (Eγ) : √s calculated with 4-momenta of initial state

particles: W 2

B = (Pγ + PN,i)2 = 2EγmN + m2 N

➢ Structures are smeared out because of Fermi motion

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

WB versus WR

Cross Sections as function of...

◮ WB (Eγ) : √s calculated with 4-momenta of initial state

particles: W 2

B = (Pγ + PN,i)2 = 2EγmN + m2 N

➢ Structures are smeared out because of Fermi motion

◮ WR : √s calculated with measured 4-momenta of final state

particles (η, participant nucleon): W 2

R = (Pη + PN,f )2

➢ No effects from Fermi motion, but experimental resolution for recoil nucleon

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Defolding Fermi Motion

WB WR

[MeV]

B

W

1400 1500 1600 1700 1800 1900

b] µ [ σ

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 p η → +p γ n η → +n γ

PRELIMINARY

[MeV]

R

W

1500 1600 1700 1800 1900

b] µ [ σ

1 2 3 4 5 6 p η → +p γ n η → +n γ

PRELIMINARY En out of kinematics 3He: Calculation of recoil nucleon

momentum is more approximate than in the case of LD2 ➢ calculate En with TOF

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Defolding Fermi Motion

WB WR

[MeV]

B

W

1400 1500 1600 1700 1800 1900

b] µ [ σ

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 p η → +p γ n η → +n γ

PRELIMINARY

[MeV]

R

W

1500 1600 1700 1800 1900

b] µ [ σ

1 2 3 4 5 6 p η → +p γ n η → +n γ

PRELIMINARY En out of kinematics 3He: Calculation of recoil nucleon

momentum is more approximate than in the case of LD2 ➢ calculate En with TOF

[MeV]

R

W 1500 1600 1700 1800 1900 b] µ [ σ 2 4 6 8 Resolution: 45 MeV 11 MeV ± = 45 Γ

PRELIMINARY En out of TOF

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Outline Light η-Mesic Nuclei Quasi-Free Photoproduction of η Mesons off 3He Conclusion & Outlook

Conclusion & Outlook

◮ Coherent photoproduction of η-mesons off 3He:

◮ improved statistical quality ◮ total cross section rises extremely between the coherent and

breakup thresholds

◮ angular distributions at threshold are almost isotropic or have

even an angular dependence opposite to the expectation for the form factor behavior ➢ strong evidence for dominant FSI effects, related to a resonant state at η production threshold.

◮ Quasi-Free photoproduction of η-mesons off 3He:

◮ Narrow structure:

Position and width consistent with deuteron data!

◮ Next step:

Try to identify responsible partial waves ➢ Single/double polarisation observables

Photoproduction of η Mesons off Light Nuclei Lilian Witthauer

Thanks for your attention!

This work is supported by: Swiss National Fund and Deutsche Forschungsgemeinschaft