Photoproduction of mesons off nuclei B. Krusche, U. Basel, CBELSA/TAPS, CBALL/TAPS collaborations Introduction excited states of the nucleon meson nucleus interactions Experimental setups Crystal Barrel & TAPS @ ELSA Crystal Ball & TAPS @ MAMI Experimental results photoproduction of mesons off quasi-free nucleons: η , η ′ , π o π o , π o η ,... coherent photoproduction of mesons off light nuclei: π o η , π o π o , π o π o π o search for η -mesic nuclei Conclusions Collaboration 2 a B. Krusche, Giessen, November 2009
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Photoproduction of mesons off nuclei - unibas.ch...η-production data from the proton: larger ηN-scattering lengths 1991 - 2002: T. Ueda, C. Wilkin, S.A. Rakityanski and others: suggestions
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Photoproduction of mesons off nucleiB. Krusche, U. Basel, CBELSA/TAPS, CBALL/TAPS collaborations
Introductionexcited states of the nucleon
meson nucleus interactions
Experimental setupsCrystal Barrel & TAPS @ ELSA
Crystal Ball & TAPS @ MAMI
Experimental resultsphotoproduction of mesons off quasi-free nucleons: η, η′, πoπo, πoη,...
coherent photoproduction of mesons off light nuclei: πoη, πoπo, πoπoπo
search for η-mesic nuclei
ConclusionsCollaboration
2a
B. Krusche, Giessen, November 2009
Photoproduction of mesons off nuclei - what can we learn?
Final states: Observables: Isospin: neutron targets
single meson production:γp → pπ, η, η′, ω...; ΣK(⋆)...
0
10
20
30
40
500 1000
σ[µb
]
γp→pπo
D13
P11S11
F15
0
5
10
15
20
500 1000
γp→pηS11(1535)
S11(1650)
photon energy [MeV]
multiple meson production:
N*, ∆
N(938)
N*, ∆
N* ∆
∆
N*
N*
N*,∆
N*
ππ πη
ρ σπ
π
π
η
π
η
η
π
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ang. distributions−→ dσ/dΩ
Dalitz plots−→ M(N, mi), M(m1, m2)
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. excitationsisospin dependent
quasifree off the deuteron
coherent off the deuteron
electromagnetic excitation off the neutron
importance of measurements off the neutron:
• different resonance contributions
• needed for extraction of iso-spin composition of elm. coupl ings
complications due to use of nuclear targets (deuteron):
• Fermi motion
• nuclear effects like FSI, re-scattering, coherent contrib utions
B. Krusche, Giessen, November 2009
πo-mesons in the ∆-resonance: coherent - breakup - FSI
large coherent cross section, strong FSI γd → Xπo vs. γp → pπo
0
1
2
3
4
5
6
200 300 400 500
σ [1
00µb
]
d(γ,πo)d
d(γ,πo)np
photon energy [MeV]
γd→ ppπ−
0
10
20
30
40
0 60 120 180
dσ/d
Ωπ
[µ
b/sr
]
0
10
20
30
40
0 60 120 180
dσ/d
Ωπ
[µ
b/sr
]
θπ [deg] θπ [deg]
ωγlab=310 MeV
π- pp
ωγlab=330 MeV
π- pp
γd→ npπo
0
20
40
60
0 60 120 180
dσ/d
Ωπ
[µ
b/sr
]
θπ* [deg] θπ
* [deg]
ωγlab=285 MeV
π0 np
0
20
40
60
0 60 120 180
dσ/d
Ωπ
[µ
b/sr
]ωγlab=285 MeV
π0 np π0 np
ωγlab=285 MeV
π0 np π0 np
ωγlab=285 MeV
π0 np π0 np
ωγlab=285 MeV
π0 np
ωγlab=324 MeV
π0 np
cos(Θπ)
Eγ=340 MeV
Eγ=260 MeV
Eγ=215 MeV
Eγ=530 MeV
Eγ=460 MeV
Eγ=395 MeV0
2
4
0
10
0
5
10
(1/A
)dσ/
dΩ[µ
b/sr
]
0
5
0
20
-1 -0.5 0 0.5 10
2
4
-1 -0.5 0 0.5 1
strong FSI: breakup cross sections no good estimate for free nucleon xs
N2
N1
N2
N1
γ π
N2
N1
N2
N1
γ π
a bN2
N1
γ π
N1
N2c
breakup + coherent ≈quasi-free without FSI
B. Krusche, Giessen, November 2009
simpler case: quasi-free η-production in the S 11-range
no complication from coherent process, no significant FSI ef fects,controllable Fermi motion effects
γd → ηX γp → pη, γd → pη(n)
10-2
10-1
1
620 640 660 680
coherent threshold
breakup
Eγ [MeV]
σ tot [
µb]
0
5
10
15
20
25
600 700 800
σ tot [
µb]
5
10
15
600 800 1000 1200 1400
σ[µb
]
Eγ[MeV]
σ(ηp), free proton
σ(ηp), quasi-free
σ(ηp), unfolded
significant FSI-effects only very close to threshold!
perfect agreement between free and quasi-free proton cross sections aftercorrection for Fermi motion!
B. Krusche, Giessen, November 2009
resonance excitations in
quasi-free and coherent photoproduction of mesons
motivations:investigate/find resonances that are stronger excited on th e neutronthan on the proton (Moorehouse rules)
investigate spin/iso-spin structure of electromagnetic r esonance excitations
tools & status & problems:quasi-free production ⇒ neutron excitationsso far mainly results for π,η-production (differential cross sections)partly not well understood ‘deuteron’ effects
coherent production ⇒ spin/iso-spin filtersso far almost only for πo and sparse results for η
in most cases very small cross sections, difficult to separat e from breakup
B. Krusche, Giessen, November 2009
interaction of mesons in nuclear matter
results from inclusive (quasi-free) pion photoproductionA-scaling of cross sections as function of kinetic energy T:
D15(1675) has stronger electromagneticcoupling to neutron than to protonbut parameters quite uncertain:
An1/2=-(21-57), An
3/2=-(30-77)
bη=0-1% (PDG), bη=17% (ETA-MAID, Chiang et al.)
interference structure in S 11-sector?
γp → ηp
1.6 1.8 2 2.2 2.4
5
10
1520 1 1.5 2 2.5
W / [GeV]
/ [GeV]γ Eb]µ / [ totσ
Data:
• TAPS: B. Krusche et al., PRL74 (1995) 3736• GRAAL: F. Renard et al., PLB528 (2002) 215• CLAS: M. Dugger et al., PRL89 (2002) 222002• Crystal Barrel: V. Crede et al., PRL94 (2005) 012004
⇑⇓
⇑
⇓
S11
S11
D15?
P11, P13
B. Krusche, Giessen, November 2009
what is expected for n(γ, η)n - why is it interesting?
total cross sections for protonand neutron from MAID modelwith and without D 15(1675)(Eta-MAID, W.T. Chiang et al., NPA 700 (2002) 429)
0
10
1500 1600 1700 1800
σ[µu
b]
W[MeV]
full model MAID, protonno D15(1675), proton
full model MAID, neutronno D15(1675), neutron
previous data from MAMI only atlower incident photon energies
use πo mass as constraint,construct 3 πo invariant mass
cut on 3 πo invariant mass
missing mass analysis toremove ηπ final states etc.treat recoil nucleon as missingparticle:
m2 = (Pγ + PN − Pη)2
[MeV]oπoπoπm400 600 800 1000
Cou
nts
[a.u
.]
800 MeV≤ γ E≤779 MeV
[MeV]neutronm800 1000 1200 1400
Cou
nts
[a.u
.]
data
n(p) +η
sim.πNNη
n(p) sim.η
sim.πNNη
[MeV]oπoπoπm400 600 800 1000
Cou
nts
[a.u
.]
1000 MeV≤ γ E≤950 MeV
[MeV]neutronm800 1000 1200 1400
Cou
nts
[a.u
.]
data with
background
[MeV]oπoπoπm400 600 800 1000
Cou
nts
[a.u
.]
1600 MeV≤ γ E≤1400 MeV
[MeV]neutronm800 1000 1200 1400
Cou
nts
[a.u
.]
B. Krusche, Giessen, November 2009
B. Krusche, Giessen, November 2009
B. Krusche, Giessen, November 2009
quasifree η-photoproduction off the deuteroncross section for γn → ηn from analyses with very different systematics:(1) η in coincidence with recoil neutrons(2) difference of inclusive data and η in coincidence with recoil protons
1
10
500 1000 1500 2000 2500
σ[µb
]
Eγ[MeV]
σ(npη)
σ(pη)
σ(nη)
σ(npη)-σ(pη)
0
5
10
1000 1500 2000 2500
σ[µb
]
Eγ[MeV]
pη
nη*3/2
pη, nη, MAID
0.5
1
1.5
1000 1500 2000 2500
σn/σp
σ n/σ p
B. Krusche, Giessen, November 2009
comparison of free and quasi-free cross sections
quasi-free total cross sections‘corrected’ for Fermi smearing(correction factors from foldingETA-MAID model with momentumdistribution of bound nucleons)
result:in S11(1535) peak below 0.9 GeV perfectagreement between free and quasi-freeproton data and quasi-free neutron datascaled by 2/3
Fit parameters for S 11 Breit-Wigner:
proton:W =1538 MeV, Γ=157 MeV, Ap
1/2=103
neutron:W =1538 MeV, Γ=148 MeV, An
1/2=85
narrow structure around 1 GeV inneutron/proton ratio,width is only upper bound