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?
breakup (quasi-free)
πo
Nγ
γ + A → πo + A′ + N + ...
dσdΩ ∝ ∑ |A|2 × ...
& nuclear effects & FSI & ...
photo-excitation ofquasi-free neutrons
in-medium propertiesof hadrons, meson FSI...
coherent
γ
πo
A, ~q γ + A → πo + A
dσdΩ ∝ | ∑ A|2 × F 2(q2) × ...
& nuclear effects & FSI & ...
spin/iso-spin filters
meson - nucleusbound states...
∆ in-medium properties
nuclear form factors
incoherent
γ
πo
γ′
A, ~q γ + A → πo + A⋆
→ πo + A + γ
transition form factors
∆ in-medium properties
spin/iso-spin selection
Structure of the Nucleoncomplex many body system
models - effective dof’s:
valence quarks
sea quarks
gluons
3 equivalent constituentquarks
quark - diquark models(fewer states)
quarks - flux tubes etc.(more states)
chiral soliton models(anti-decuplet states)
coupled channel dynamics(molecule-like states) ...
comparison: known excited states -constituent quark model (Capstick & Roberts)
1000
1200
1400
1600
1800
2000
2200
2400
P11(939)
P11(1440)
D13(1520)S11(1535)
S11(1650)D15(1675)F15(1680)D13(1700)P11(1710)P13(1720)P13(1900)
F17(1990)F15(2000)D13(2080)S11(2090)P11(2100)G17(2190)D15(2200)H19(2220)G19(2250)
P33(1232)
P33(1600)S31(1620)
D33(1700)P31(1750)S31(1900)F35(1905)P31(1910)P33(1920)D35(1930)D33(1940)F37(1950)F35(2000)S31(2150)
H39(2300)D35(2350)F37(2390)H3,11(2420)
Mass/(MeV/c2)N(I=1/2) ∆(I=3/2)
exp expQM QM
ordering of (low-lying) states?
missing resonance problem?
low lying excited states
P11(939)
P11(1440)
D13(1520)S11(1535)
S11(1650)D15(1675)F15(1680)D13(1700)
P33(1232)
P33(1600)S31(1620)
D33(1700)
N(I=1/2) ∆(I=3/2)
η σ π ρ π
Notation:
L2I2J ; L=0(S),1(P),2(D),...
ππ ηπ
B. Krusche, Giessen, November 2009
Experimental Options:
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:
σ(A) ∝ Aα(T )α ≈ 1: ’volume’, no absorptionα ≈ 2/3: ’surface’, strong absorption
0.5
0.6
0.7
0.8
0.9
1
10 102
α
πo
η
transparent∆-resonance(black)
T[MeV]
πo-mesons: strongly absorbed atenergies sufficient to excite ∆;but only very weak interactionat small momenta−→ no bound-states possible
η-mesons: strong interaction also atvery small momenta due to s-waveS11(1535) resonance at threshold−→ strong enough for
(quasi)-bound states?
B. Krusche, Giessen, November 2009
‘history’ of η-mesic nuclei1985: Bhalerao & Liu:attractive η-nucleus interaction for A ≥12
1986: Liu & Haider:suggestion of η-nucleus bound states
experiments: inconclusive e.g.:Chrien et al. (1988): π+ +16 O → p +15
η O
Johnson et al. (1993): π+ +18 O → π− +18η Ne
1993 - 2002: analysis of newη-production data from the proton:larger ηN-scattering lengths
1991 - 2002: T. Ueda, C. Wilkin,S.A. Rakityanski and others:suggestions of bound2H-, 3H-, 3He-, 4He-η states
experiments:threshold behaviorof η-production
p + d → 3He + η
d + d → 4He + η
pd→3Heηγ3He→3Heη
|f|2 [a
.u.]
1
10
10 2
0 0.5 1 1.5pη[fm-1]
B. Krusche, Giessen, November 2009
GDH
MøllerPolarimeter
Crystal Barrel
M Q BPM
Quadrupole
Combined-Function Magnet
Dipole (horizontal)Dipole (vertical)
Radio FrequencySolenoid
SextupoleSkew Quadrupole
pol. esource
(50 keV)
Mottpolarimeter
LINAC 2
electrongun
(26 MeV)
LINAC 1(20 MeV)
detectortests
skewquadrupoles
tune jumpquadrupole
injection septa
bending magnetbeamlines for
SR experimentsBN3
BN2
BN1
FZK laboratory
DORIS cavity
PETRA cavity
tune jumpquadrupole
extraction septa
superconductingsolenoid
medium energyphysics
experiments
boostersynchrotron
DESY cavity
BN0
0.5 - 1.6 GeV
half cell of ELSA
-
0.5 - 3.5 GeVstretcher ring
EKS
0 m 5 m 10 m 15 m
Electron Stretcher Accelerator (ELSA)
ComptonPolarimeter
MAMI accelerator in Mainz
4. Stage: Harmonic Double Sided Microtronmaximum energy: 1.5 GeV
B. Krusche, Giessen, November 2009
Mainz accelerator lab
experiments: Crystal Ball & Crystal Barrel with TAPS
10 cm
Bonn ELSA accelerator:Crystal Barrel (CsI),TAPS (BaF2) forward wall,inner detectors
Eγ ≤ 3.5 GeV,lin. pol.: available,circ. pol.: available
Mainz MAMI accelerator:Crystal Ball (NaJ),TAPS (BaF2) forward wall,inner detectors
Eγ ≤ 1.5 GeV,lin. pol.: available,circ. pol.: available
B. Krusche, Giessen, November 2009
Experimental setup: Crystal Barrel and TAPS
γ7
30o
magnet
radiator
electronbeam
beamdump
fibers
proportional wire chamber
unscatteredelectrons
beamphoton
scintillators
electronsscattered
B. Krusche, Giessen, November 2009
B. Krusche, Giessen, November 2009
TAPS Crystal Ball - at MAMI
B. Krusche, Giessen, November 2009
resonances coupling to η photoproduction off the protonbranching ratios and elm. couplings (PDG):
state bη [%] Ap1/2 Ap
3/2 An1/2 An
3/2
• D13(1520): 0.23±0.04 -24 166 59 139• S11(1535): 30 - 55 90 -46• S11(1650): 3 - 10 53 -15• D15(1675): 0±1 19 15 -43 -58• F15(1680): 0±1 -15 133 29 -33• D13(1700): 0±1• P11(1710): 6.2±1.0• P13(1720): 4±1
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
0.5
1
1.5
700 800 900 1000
σ n/σ p Kaiser et al.
MAID full model
MAID only S11(1535)
d(γ,η)X, σn/σp(Eγ)d(γ,η)X, σn/σp(Eγ)
d(γ,η)X, σn/σp(E*d(γ,η)X, σn/σp(Eγ)
4He(γ,η)X, σn/σp(Eγ)4He(γ,η)X, σn/σp(Eγ)4He(γ,η)X, σn/σp(E
*4He(γ,η)X, σn/σp(Eγ)
Eγ[MeV]B. Krusche, Giessen, November 2009
quasifree eta photoproduction from p,n: results from GRAAL
recoil neutron recoil proton
0
0.2
0.4
0.6
0.8
1.5 1.6 1.7 1.8 1.9W,GeV
dσ/d
Ω, a
.u.
0
0.5
1
1.5 1.6 1.7 1.8 1.9W,GeV
dσ/d
Ω, a
.u.
0
0.2
0.4
0.6
0.8
1.5 1.6 1.7 1.8 1.9W,GeV
dσ/d
Ω, a
.u.
0
0.5
1
1.5 1.6 1.7 1.8 1.9W,GeV
dσ/d
Ω, a
.u.
0
0.2
0.4
0.6
0.8
1.5 1.6 1.7 1.8 1.9W,GeV
dσ/d
Ω, a
.u.
0
0.5
1
1.5 1.6 1.7 1.8 1.9W,GeV
dσ/d
Ω, a
.u.
V. Kouznetsov for the GRAAL collaboration,proceedings NSTAR 2004, page 197
proton cross section in agree-ment with free proton results
narrow structure in quasifreeneutron cross section atW =1675, width Γ ≈ 40 MeV
predicted properties ofnucleon-like pentaquark
Θ |2 _
[ud] s >+
|2 _
[us] d >3/2+Ξ
N |2 _
[ud] d >+
N |s_
[ud][su] s>+
+
Σ |2 _
[su] s >+
s
Σ |_
[ud][su] d >+
+
|2 _
[ds] u >3/2--Ξ
B. Krusche, Giessen, November 2009
identification of η-meson production (exclusive)
decay channel: η → 3πo → 6γ
select events with 7 hits
invariant mass off all photon pairs
cut on πo invariant mass
select best combination of6γ to 3πo by χ2-test
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
0
5
10
15
1000 1500 2000 2500
σ[µb
]
Eγ[MeV]
ηp, free protonηp
ηn ηnp-ηpσ(ηn)/(2/3)
0.5
1
1.5
2
1000 1500 2000 2500Eγ[MeV]
σ n/σ p
B. Krusche, Giessen, November 2009
quasi-free angular distributions
700 MeV
750 MeV 800 MeV 850 MeV
900 MeV
950 MeV 1000 MeV
1100 MeV 1200 MeV 1300 MeV
1400 MeV 1500 MeV 1600 MeV 1700 MeV 1800 MeV
1900 MeV 2000 MeV 2100 MeV 2200 MeV 2400 MeV
cos(Θη)cos(Θ*
ηp, qf ηn, qf ηp, free folded
0
0.5
1
0
0.2
0.4
0.6
0
0.2
0.4
dσ/d
Ω[µ
b/sr
]
0
0.2
0.4
-1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1
cos(Θ) =- 1.0 - -0.6
cos(Θ) =- 0.6 - -0.2
cos(Θ) =- 0.2 - +0.2
cos(Θ) =+ 0.2 - +0.6
cos(Θ) =+ 0.6 - +1.0
Eγ[MeV]
0
1
0
1
0
1
dσ/d
Ω[µ
b/sr
]
0
1
0
1
1000 1500 2000 2500
I. Jaegle et al., Phys. Rev. Lett. 100 (2008) 252002
B. Krusche, Giessen, November 2009
fit of angular distributionsfit with:
dσdΩ = q⋆
k⋆× Σ AiPi(cos(Θ⋆))
result:
all coefficients similar for protonand neutron above 1.25 GeV
Ao coefficient:dominance of S 11 resonances,for neutron small shoulderaround 1 GeV
A1 coefficient:interference S 11, P11?
A2 coefficient:interference S 11 - D13 resonance
Ao A1
A2 A3
Eγ[GeV]
0
1
2
31.5 1.7 1.9 2.1 W[GeV]
-0.5
-0.25
0
0.25
1.5 1.7 1.9 2.1 W[GeV]
-0.5
0
0.5
1 1.5 2 2.5
Ai [
µb/s
r]
-0.5
-0.25
0
0.25
1 1.5 2 2.5
10-1
1
1 2
• quasifree neutron • quasifree proton — free proton (Ff)
— q.f. neutron (Maid) — q.f. neutron (Shklyar et al.)
B. Krusche, Giessen, November 2009
Bonn-Gatchina model analysis
basis: coupled channel isobar analysis with background ter ms
0
2
4
6
8
10
12
1500 1600 1700 1800 1900 2000 2100
M(γn) [MeV]
σtot [µb]
CB-ELSA
0
2
4
6
8
10
12
1500 1600 1700 1800 1900 2000 2100
M(γn) [MeV]
σtot [µb]
CB-ELSA
0
2
4
6
8
10
12
1500 1600 1700 1800 1900 2000 2100
M(γn) [MeV]
σtot [µb]
CB-ELSA
different scenarios to reproduce ’bump’ structure:
left: interference in S 11-sector: adjusting phases etc.
middle: introduction of conventional (broad) P 11 resonance
right: introduction of very narrow P 11 resonanceA. Anisovich et al., EPJA 41 (2009) 13
B. Krusche, Giessen, November 2009
s-wave coupled channel models-wave coupled channel model with dynamical generation of S 11(1535) pole
free cross sections quasi-free cross sections
700 800 900 1000 1100 1200Eγ [MeV]
0
5
10
15
20
σ [µ
b]
KΣKΛ
700 800 900 1000 1100 1200Eγ [MeV]
0
5
10
15
20
σ [µ
b]
s1/2
=1535 MeVs
1/2=m
N+mη
cross section ratio
700 12000
1
2
700 800 900 1000 1100 1200Eγ [MeV]
0
0.5
1
1.5
2
σ n / σ p
structure in σn/σp due to K Λ, KΣ threshold effects(intermediate strangeness states in loop-diagrams)
M. Doring. K. Nakayama, nucl-th:0909.3538v1
B. Krusche, Giessen, November 2009
‘de-folding’ of Fermi smearing
for events with neutron in TAPS(cos (Θ⋆
η) < -0.1)neutron energy from time-of-flight
comparsion: W from photonenergy (Fermi smeared) -W from nucleon - meson4-vectors (resolution smeared)
de-folded proton cross sectionsimilar to free proton,de-folded neutron cross sectionshows structure around 1.7 GeV:
position: W =1683 MeVwidth: Γ=60±10 MeV(resolution dominated)
WB[MeV] WR[MeV]
0
2
4
1500 1600 1700 1800
dσ/d
Ωac
c[µb/
sr]
ηpηn
0
2
4
1500 1600 1700 1800
I. Jaegle et al.,Phys. Rev. Lett. 100 (2008) 252002
B. Krusche, Giessen, November 2009
new preliminary results from MAMI C: reaction identificatio n
raw invariant mass cut on co-planarity
co-planarity
B. Krusche, Giessen, November 2009
preliminary excitation functions
W = f(Eγ), 130 < ∆Φ < 220 W = f(n, η), 130 < ∆Φ < 220
W = f(Eγ), 170 < ∆Φ < 190 W = f(n, η), 170 < ∆Φ < 190
B. Krusche, Giessen, November 2009
structure also seen for 3He target nuclei
nucleons bound in deuteron nucleons bound in 3He
0
0.5
1
1500 1600 1700 1800 1900
γ3He→(np)pη
γ3He→(pp)nη
W[MeV]σ[
a.u.
]
B. Krusche, Giessen, November 2009
Summary
narrow structure in excitation function of γn → nη:
GRAAL: W ≈ 1680 MeV, Γ <30 MeV
Tohoku-LNS: W ≈ 1666 MeV, Γ <40 MeV
ELSA: W ≈ 1685 MeV, Γ <60 MeV
MAMI-C: W ≈ 1675 MeV, Γ <40 MeV
so far no information about quantum numbersof possible resonanceor whatever nature of the structure
B. Krusche, Giessen, November 2009
coherent η-photoproduction: search for light - η-mesic nuclei
η-photoproduction dominated by excitation of S 11(1535):
γ(E1) + N → S11 → N + η
Jz: -1 +1/2 -1/2 -1/2 0 → spin-flip transition
isospin structure: AIS1/2/A
p1/2 ≈0.09 → dominantly isovector
expectation for light nuclei:
1) 4He: J=0, I=0, isoscalar, non spin-flip → very small signal(not seen, only upper bounds, V. Hejny et al.)
2) 2H: J=1, I=0, isoscalar, spin-flip → small signal(seen, almost in agreement with expectations)
3) 3He: J=1/2, I=1/2, isovector, spin-flip → ’large’ signal
B. Krusche, Giessen, November 2009
η-photoproduction from 3He - threshold behavior
photon energy / MeV
σ tot /
µba
rn
coherent threshold
quasifree threshold
quasifree cross sectioncoherent cross section
10-2
10-1
1
10
600 620 640 660 680 700 720 740
0
20
-1 -0.5 0 0.5 1cos Θη
*
dσ/
dΩ
/ nb
/sr
600 - 625 MeV
expectedangular dependence
cos Θη*
625 - 745 MeV
expectedangular dependence
-25
0
25
50
75
100
-1 -0.5 0 0.5 1
M. Pfeiffer et al., PRL 92 (2005) 252001
evidence for strong finalstate interaction of the η-meson
⇐= threshold enhancement ofcoherent part
istropic angular distributionof coherent part at threshold =⇒
B. Krusche, Giessen, November 2009
search for η-mesic nuclei
G. Sokol et al., search in: γ +12 C → N +η (A − 1) → N + π+ + n + (A − 2)
similar principle for photoproduction from 3He:
0
100
200
300
400
500
600
700
800
250 500 750
50
100
150
200
250
600 800
170o-180o
150o-170o170o-180o
150o-170o
E[MeV]E[MeV]
170o-180o
(150o-170o
subtracted)
-5
0
5
10
15
20
25
30
35
600 800E[MeV]
→ search for back-to-backπo - p pairs
excess of πo-pback-to-backemission at theη-threshold(3.5σ)
B. Krusche, Giessen, November 2009
new 3He experiment - improved statistics
reaction identification:invariant mass analyses for η → 2γ and η → 3πo → 6γ
missing energy analysis for coherent kinematics:
-10 0 100
50
100
< 600 MeVγ592 < E
-10 0 100
50
100 < 608 MeVγ600 < E
-10 0 10 200
50
100
150 < 617 MeVγ609 < E
-60 -40 -20 0 200
200
400
< 697 MeVγ689 < E
Missing Energy[MeV]
Cou
nt(a
.u)
-20 -10 0 10 200
20
40
< 600 MeVγ592 < E
-10 0 10
0
20
40
< 608 MeVγ600 < E
-20 -10 0 10 20
0
20
40 < 617 MeVγ609 < E
-50 00
50
100 < 697 MeVγ689 < E
Missing Energy[MeV]
Cou
nt(a
.u)
η → 2γ η → 6γ
PhD thesis F. Pheron
B. Krusche, Giessen, November 2009
new 3He experiment - coherent η-production
[MeV]γE600 650 700
bµ/ ησ
0
0.1
0.2
0.3
γ2→ηγ6→0π3→η
Coh. Thres.
Qf. Thres.
-1 -0.5 0 0.5 10
0.005
0.01
0.015
0.02 = 602.0 MeVγE
-1 -0.5 0 0.5 10
0.005
0.01
0.015 = 606.3 MeVγE
-1 -0.5 0 0.5 10
0.005
0.01
0.015 = 610.5 MeVγE
-1 -0.5 0 0.5 10
0.02
0.04
= 686.6 MeVγE
)*ηΘ cos(
b/sr
]µ [
Ω/dσd
preliminary cross sections:very steep rise of total cross section at threshold confirmed
rather flat angular distributions at threshold
B. Krusche, Giessen, November 2009
new 3He experiment - coherent η-production
[MeV]γE600 650 700
bµ/ ησ
0
0.1
0.2
0.3
γ2→ηM.Pfeiffer et al[1]
γ6→0π3→η
Coh. Thres.
Qf. Thres.
-1 -0.5 0 0.5 10
0.005
0.01
0.015
0.02 = 602.0 MeVγE
-1 -0.5 0 0.5 10
0.005
0.01
0.015 = 606.3 MeVγE
-1 -0.5 0 0.5 10
0.005
0.01
0.015 = 610.5 MeVγE
-1 -0.5 0 0.5 10
0.02
0.04
= 686.6 MeVγE
)*ηΘ cos(
b/sr
]µ [
Ω/dσd
preliminary cross sections:very steep rise of total cross section at threshold confirmed
rather flat angular distributions at threshold
B. Krusche, Giessen, November 2009
new 3He experiment
πo-p back-to-back pairs:peak structure at coherent threshold also confirmedwith much better statistics
/MeV γ E400 500 600 700 800
yie
ld/ a
.u.
0
100
200
°-180° = 170pπ
ψ
°-170° = 150pπ
ψ
/MeV γ E400 500 600 700 800
yie
ld/ a
.u.
0
5
10
Coh. Thres.η
/MeV γ E400 500 600 700 800
yie
ld/ a
.u.
0
100
200 °-180° = 165pπ
ψ
°-165° = 150pπ
ψ
/MeV γ E400 500 600 700 800
yie
ld/ a
.u.
0
5
10
Coh. Thres.η
Summary: new data confirme previous results with much betterstatistics - further analysis under way
B. Krusche, Giessen, November 2009
the reaction γp → πoηp - πoη-pairs off the protonIdentification
Total cross section
Invariant mass distributions
dominant final states:−− ∆(1232)η, −.− N(1535)π, ... pao(980)
I. Horn et al. (ELSA)
←−
B. Krusche, Giessen, November 2009
threshold region of πoη dominated by D 33(1700)V. Kashevarov et al. (MAMI)
⇐ best model fitsfor P 33 or D35 isobars
⇐ model fitsfor D 33 isobar
analysis of threshold region in terms of isobar model(Fix et al.) indicates strong dominance of D 33(1700)
extracted amplitudes allow specific predictions forcoherent production of πoη-pairs of light nuclei (Fix et al.)
B. Krusche, Giessen, November 2009
coherent photoproduction of πoη-pairs: d(γ, ηπo)d
Proton / Deuteron5 10
Ene
rgy
depo
site
d [M
eV]
200
400
600
Neutron TOF [ns]5 10
200
400
600
[MeV]γγ0 200 400 600
Cou
nts
[a.u
.]
0
50
100
150 2.5× = 12.8 MeVoπσ
= 31.1 MeVησ
m [MeV]∆−50 0 50
Cou
nts
[a.u
.]
0
20
40
= 15 MeVm∆σ
time-of-flight versus energyfor deuteron identification
invariant mass (two-photon)for reaction identificationand missing mass(deuteron treated as missingparticle) for verificationof coherent kinematics
B. Krusche, Giessen, November 2009
d(γ, ηπo)d: total cross section, kinetic energy distributions
total cross section kinetic energy
[MeV]γE1000 1500 2000 2500
b]µ [σ
0
0.05
0.1
0.15 A. Fix
[MeV]kin.cmT
0 50 100 150 200
[nb
/ MeV
]ki
n.cm
/dT
σd
0
1
2 oπη
= 900 − 1100 MeVγEW = 2628 − 2767 MeV
[MeV]kin.cmT
0 200 400
[nb
/ MeV
]ki
n.cm
/dT
σd
0
0.5
1
1.5
)ηps T()oπps T(
)η* T(∆)oπ* T(∆
= 1100 − 1600 MeVγEW = 2767 − 3088 MeV
[MeV]kin.cmT
0 200 400 600 [n
b / M
eV]
kin.
cm/d
Tσd
0
0.2
0.4
0.6
0.8 = 1600 − 2000 MeVγEW = 3088 − 3322 MeV
total cross section in reasonable agreement with predictio ns
T distributions support dominant ∆⋆ → ∆(1232)η → Nηπo contribution:
T (πo) peaks around 100 MeV ( ∆(1232) → Nπ), T (η) rises with E γ
B. Krusche, Giessen, November 2009
very preliminary: 3He(γ, ηπo)3He
identification via missing mass preliminary total cross section
0
0.5
1
0 100 200
data
πoη 3He, simu
πoη NX, simu
sum simulations
Eγ=765 - 1400 MeV
∆m[MeV]
coun
ts[a
.u.]
[MeV]γE800 1000 1200
b]µ [σ
0
0.05
0.1
0.15
0.2He − A23ηoπ→He3γ
d − CBELSA/TAPSηoπ→dγ He − A. Fix3ηoπ→He3γ
d − A. Fixηoπ→dγ
qualitative agreement with isotope dependence from Fix’ mo del
B. Krusche, Giessen, November 2009
quasi-free photoproduction of πoη pairs...preliminary results
[MeV]γE800 1000 1200 1400
b]µ [σ
0
2
4
6 p), I. Horn et al., CBELSAηoπ(pσFree p folded with d Fermi motion
He Fermi motion3Free p folded with , CBELSA−TAPSp)ηoπ(dσ
p),A2ηoπ(dσp)/2ηoπ(He3σ
[MeV]γE1000 1500 2000 2500
b]µ [σ
0
2
4
6 p), I. Horn et al., CBELSAηoπ(pσFree p folded with d Fermi motion
He Fermi motion3Free p folded with , CBELSA−TAPSp)ηoπ(dσ
p),A2ηoπ(dσp)/2ηoπ(He3σ
[MeV]γE800 1000 1200 1400
b]µ [σ
0
2
4
6
8 pn)ηoπ(dσ
p)ηoπ(dσ
n)ηoπ(dσ
p)ηoπ(σpn) − ηoπ(dσp)ηoπ(σpn) − ηoπ(He3σ
pn)/(3/2)ηoπ(He3σ
/2p)ηoπ(He3σ
n)ηoπ(He3σ
[MeV]γE1000 1500 2000 2500
b]µ [σ0
2
4
6
8 pn)ηoπ(dσ
p)ηoπ(dσ
n)ηoπ(dσ
p)ηoπ(σpn) − ηoπ(dσp)ηoπ(σpn) − ηoπ(He3σ
pn)/(3/2)ηoπ(He3σ
/2p)ηoπ(He3σ
n)ηoπ(He3σ
deuteron results fromMAMI and ELSA consistent
quasi-free proton andneutron results fromdeuteron and 3Heconsistent
quasi-free protoncross section roughly50 % smaller than freeproton cross section.Not yet understood. FSI ?
quasi-free proton andneutron cross sectionsvery similar as expected
results for many more channels, e.g. quasi-free η′
0
0.5
1
1.5
1.25 1.5 1.75 2 2.25 2.5
γd→(n)pη,
γd→(p)nη,
γd→(p)(n)η,
γd→dη, x 100
Eγ[GeV]
σ[µb
]
W[GeV] 1.9 2.0 2.1 2.2 2.3
Ao
A1 A2 A3
Eγ[GeV]
0
0.1
0.2
0.3 γp→pη,
γd→(n)pη,
Ai [
µb/s
r]
0
0.1
-0.1
0
0.1
-0.1
0
0.1
0
0.1
0.2
1.5 2 2.5
γd→(p)nη,
0
0.1
1.5 2 2.5
-0.1
0
0.1
1.5 2 2.5
-0.1
0
0.1
1.5 2 2.5
⇐ total cross sections,
model calculations
K. Nakayama et al.
Legendre coefficients
of angular distributions ⇓
very good agreementbetween quasi-freeand free protoncross section
angular distributionsdominated by s-waves
reasonable agreementwith model calculations
broad structure inneutron cross sectionbetween2.0 GeV< W < 2.5 GeVwhich is lesspronounced for proton
...and single πo quasi-free and coherent...
total cross sections
[MeV]γE400 600 800 1000 1200
b]
µ [σ
10
210np)0π(σ
n)0π(σ p)0π(σ
n) Folded0π(σMAID
p) Folded0π(σMAID
[MeV]γE500 600 700 800 900 1000 1100 1200 1300 1400
b]
µ [σ
0
10
20
30
40
50
60
n)0π(σ p)0π(σ d)0π(σ
quasi-free off proton and neutron: contriutions from diffe rent N ⋆-resonancs
coherent off deuteron:only ∆-resonances, N ⋆ forbidden due to isospin conservation
B. Krusche, Giessen, November 2009
... double πo quasi-free and coherent...
total cross sections
[MeV]γE0 200 400 600 800 1000 1200 1400
b)
µ (σ
0
2
4
6
8
10
NeutronProton
cross section ratio
[MeV]γE500 1000
pσ / nσ
0
0.5
1
p)oπoπ(He3σn) / oπoπ(He3σ×2p)oπoπ(He3σp)) / oπoπ(He3σppn) oπoπ(
He3σ(×2
p)oπoπ(dσn) / oπoπ(dσ
coherent contribution
[MeV]γE500 1000
b]µ [σ0
0.02
0.04
0.06
0.08
A2
CBELSA/TAPS
A. Fix
quasi-free off proton and neutron:some difference in resonance bump structure
coherent off deuteron:only N ⋆-resonances, ∆-resonances forbiddendue to isospin conservation
polarization observables - example: beam-helicity for 2 π
beam-helicity asymmetry (circularly polarized beam, unpo larized target)
p
n
π+
πo
Φγ z
z’
y
I⊙(Φ) ≡ dσ+−dσ−
dσ++dσ−
γp → pπ+π− γp → nπoπ+ γp → pπoπo
575 - 605
605 - 635
635 - 665
665 - 695
700 - 730
745 - 815
Φ[o]
-10
0
10
-10
0
10
-10
0
10
0 100 200 300 100 200 300
I⊙[%
]
520 - 570
570 - 620
620 - 670
670 - 720
720 - 770
770 - 820
Φ[o]
-20
0
20
-20
0
20
-20
0
20
0 100 200 300 100 200 300
I⊙[%
]
560 - 620
620 - 680
680 - 745
745 - 810
Φ[o]
-10
0
10
-10
0
10
0 100 200 300 100 200 300
I⊙[%
]proton results so far not understood!
...free and quasi-free (off deuteron) proton results...
preliminary quasi-free data from protons bound in deuteron alreadyin good agreement with free proton data
]o [Φ−150 −100 −50 0 50 100 150
[%]
° I
−15
−10
−5
0
5
10
15700 − 800 MeV
Free pQuasiree pFree p FermiFolded
20
0
20
Free P n+π 0π p > γ
Fermi folded P
[770 820]∈ γE
100 0 100
20
0
20
]o[Φ
[%]
° I
γp → pπoπo γp → nπoπ+
B. Krusche, Giessen, November 2009
...quasi-free proton and quasi-free neutron...
100 0 100
10
0
10
[720 770]∈ γE ProtonNeutron
100 0 100
10
0
10
[770 820]∈ γE ProtonNeutron
100 0 100
10
0
10
[820 900]∈ γE ProtonNeutron
100 0 100
10
0
10
[900 1000]∈ γE ProtonNeutron
100 0 100
10
0
10
[1000 1080]∈ γE ProtonNeutron
100 0 100
10
0
10
[1080 1160]∈ γE ProtonNeutron
100 0 100
10
0
10
[1160 1220]∈ γE ProtonNeutron
100 0 100
10
0
10
[1220 1280]∈ γE ProtonNeutron
100 0 100
10
0
10
[1280 1340]∈ γE ProtonNeutron
100 0 100100 0 100100 0 100
10
0
10
10
0
10
10
0
10
]o[Φ
[%]
° I
20
0
20
n+π 0π p > γ pπ 0π n > γ
[720 770]∈ γE
20
0
20
n+π 0π p > γ pπ 0π n > γ
[770 820]∈ γE
20
0
20
n+π 0π p > γ pπ 0π n > γ
[820 900]∈ γE
100 0 100
20
0
20
n+π 0π p > γ pπ 0π n > γ
[900 1100]∈ γE
100 0 100
20
0
20
n+π 0π p > γ pπ 0π n > γ
[1100 1200]∈ γE
100 0 100
20
0
20
n+π 0π p > γ pπ 0π n > γ
[1200 1400]∈ γE
100 0 100100 0 100100 0 100
20
0
20
20
0
20
]o[Φ
[%]
° I
almost identical results forγp → pπoπo and γn → nπoπo
less similarities forγp → nπoπ+ and γn → pπoπ−
B. Krusche, Giessen, November 2009
ConclusionsSystematic investigation of meson production
off 2H and 3He:
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
η-photoproduction off 3He:evidence for (quasi)-bound η-nucleus state
other channels:iso-spin dependence of the elm. nucleon excitation
outlook:upcoming program to measure quasi-free(double) polarization observables
I. JaegleI. KeshelashviliT. RostomyanF. PheronY. MaghrebiD. WerthmullerTh. ChallandeR. TrojerM. DieterleM. OberleL. Witthauer
B. Krusche, Giessen, November 2009
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