Introduction Results Conclusions equivalent constituent quarks quark ... • initial state: incident...
29
Photoproduction of Mesons of Quasifree Nucleons ————————————————————————– ————————————————————————– - selected results - ————————— ————————— B. Krusche, U. Basel for the CBELSA/TAPS and A2 collaborations Introduction Experimental setups Results Conclusions B. Krusche, Hirschegg, January 2014
Transcript of Introduction Results Conclusions equivalent constituent quarks quark ... • initial state: incident...
Photoproduction of Mesons of Quasifree Nucleons————————————————————————–————————————————————————–
- selected results -——————————————————
B. Krusche, U. Basel for the CBELSA/TAPS and A2 collaboratio ns
Introduction
Experimental setups
Results
Conclusions
B. Krusche, Hirschegg, January 2014
Structure of the Nucleon————————————————————————complex 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?
Progress in Baryon Spectroscopy————————————————————————————————
M [GeV]
3.0
2.5
2.0
1.5
1.0
4 5 3 1
N (938)
2 2 1 0
2 3 2 1
∆(1232)
1 1 0 0
N=0 N=1,2g.s. bands
Nucleon resonances from Lattice QCD:(R.G. Edwards et al., PRD84 (2011) 074508)
Basic features agree with expectations from SU(3) ⊗ O(3) symmetry:• counting of levels consistent with non-relativistic quark model
• Lattice results of course in very early state, mπ =400 MeV...
B. Krusche, Hirschegg, January 2014
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
P11 S11
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
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
.
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, P , T
−→ E, G, H , F
−→ ...
elm. excitationsisospin dependent
quasifree (coherent)off the deuteron
electromagnetic excitations of 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):
• coincident detection of recoil nucleons
• Fermi motion, nuclear effects like FSI, coherent contribut ions
B. Krusche, Hirschegg, January 2014
mesurements off quasifree nucleons bound in the deuteron————————————————————————————–————————————————————————————–Complications:
(1) detection of recoil nucleons mandatory
(2) reaction kinematics modified by Fermi motion - smears out all structures
(3) possible influence of meson - nucleon and nucleon-nucleo n FSI on cross sectionsSolutions:
(1,2) Typical neutron detection efficiencies for CB and TAPS in the range 10% - 30%,CB cannot measure energies (TAPS via ToF); but kinematics co mpletely defined:
• initial state: incident photon and deuteron at restknown/measured: Eγ, md, ~pd = 0
• final state: meson, participant, and spectator nucleonknown/measured: ms, mp, Θp, Φp, mm, ~pmnot measured: Tp, ~ps (four variables)
• four constraints from energy/momentum conservation
(3) comparison of quasifree production off protons and prod uction off free protonsto study FSI effects
0
0.2
0.4
0.6
0.8
1
0 100 200 300
neutron spectator
coun
ts[a
.u.]
ps[MeV]
datasimulationdeuteron wf.
0 100 200 300 400
proton spectator
B. Krusche, Hirschegg, January 2014
MAMI accelerator in Mainz————————————–————————————–
4. Stage: Harmonic Double Sided Microtronmaximum energy: 1.5 GeV
B. Krusche, Hirschegg, January 2014
TAPS Crystal Ball - at MAMI——————————————————————————
B. Krusche, Hirschegg, January 2014
Calorimeters: Crystal Barrel & Crystal Ball with TAPS——————————————————————————————————————————————————
Inner detector513 fibers
Barrel1230 CsIPD readout →
Forward Plug90 CsI PM readout
Mini-TAPS216 BaF2 PM readout →
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, Hirschegg, January 2014
Results - Example I: Photoproduction of π0-mesons——————————————————————————————————————————————————
photoproduction of single pions one ofbest studied meson production reactions
backbone of partial wave analyseslike SAID, MAID, BnGn,...for extraction of resonance properties
reaction with neutral pions of great interest
impact of π0-production off the neutron?
Existing data base/ new resultscross section data for different isospin channels isospin decompostion of pion photoproduction
γp→pπo γn→nπo γp→nπ+ γn→pπ-
1
1.5
2
2.5
3
-1 0 1
W[G
eV]
-1 0 1 -1 0 1cos(Θπ)
-1 0 1
MAMI 2013A(γp → π+n) = +
√
23AV 3 +
√
13(AIV − AIS)
A(γp → π0p) = −√
13AV 3 +
√
23(AIV − AIS)
A(γn → π−p) = +√
13AV 3 −
√
23(AIV + AIS)
A(γn → π0n) = +√
23AV 3 +
√
13(AIV + AIS)
B. Krusche, Hirschegg, January 2014
γN → Nπ0 - reaction-model fits, predictions——————————————————————————————————————————Results from partial wave, reaction models: — SAID — MAID — BnGa
W [MeV]1400 1600 1800
b]µ [σ
0
10
20
30
40
50
60
70
80
γp → pπ0
—- MAID
—- SAID
—- BnGn
W [MeV]1400 1600 1800
b]µ [σ
0
10
20
30
40
50
60
70
80
γn → nπ0
—- MAID
—- SAID
—- BnGn
results agree for proton target (because fitted to proton dat a)
predictions for neutron target disagree completely
data from γn → pπ− do not sufficiently constrain the fits for neutron target(completely different non-resonant backgrounds)
B. Krusche, Hirschegg, January 2014
γn → nπ0 - quasifree π0-production off neutrons————————————————————————————————————————————————(M. Dieterle et al., submitted to PRL)
Total cross sections compared to PWA results: — SAID — MAID — BnGa
W [MeV]1400 1600 1800
b]µ [σ
0
10
20
30
40
50
60
70
80
W [MeV]1400 1600 1800
pσ/ nσ
0.4
0.6
0.8
1
1.2
1.4
1.6
W [MeV]1400 1600 1800
b]µ [σ
0
10
20
30
40
50
60
70
80
W [MeV]1400 1600 1800
pσ/ nσ
0.4
0.6
0.8
1
1.2
1.4
1.6
significant effects from final state interactions in proton d ata
neutron data corrected under assumption of identical FSI fo r both reactions
poor agreement between neutron data and PWA predictions
B. Krusche, Hirschegg, January 2014 MAMI
γN → Nπ0 - angular distributions——————————————————————————————————(M. Dieterle et al.)
γ′p′ → pπ0 γ′n′ → nπ0 coefficients
1300 MeV 1312 MeV 1324 MeV 1336 MeV 1348 MeV 1360 MeV
1372 MeV 1384 MeV 1396 MeV 1408 MeV 1420 MeV 1432 MeV
-1 -0.5 0 0.5 1
1444 MeV
-1 -0.5 0 0.5 1
1456 MeV
-1 -0.5 0 0.5 1
1
1468 MeV
-1 -0.5 0 0.5 1
1
1480 MeV
-1 -0.5 0 0.5 1
1492 MeV
-1 -0.5 0 0.5 1
1504 MeV
-1 -0.5 0 0.5 1
1516 MeV
-1 -0.5 0 0.5 1
1528 MeV
-1 -0.5 0 0.5 1
1540 MeV
-1 -0.5 0 0.5 1
1552 MeV
-1 -0.5 0 0.5 1
1564 MeV
-1 -0.5 0 0.5 1
1576 MeV
-1 -0.5 0 0.5 1
1588 MeV
-1 -0.5 0 0.5 1
1600 MeV
-1 -0.5 0 0.5 1
1
1612 MeV
-1 -0.5 0 0.5 1
1
1624 MeV
-1 -0.5 0 0.5 1
1636 MeV
-1 -0.5 0 0.5 1
1648 MeV
1660 MeV 1672 MeV
1
1684 MeV
1
1696 MeV 1708 MeV 1720 MeV
-1 -0.5 0 0.5 1
1732 MeV
-1 -0.5 0 0.5 1
1744 MeV
-1 -0.5 0 0.5 1
1
1756 MeV
-1 -0.5 0 0.5 1
0.5
1
1.5
2.5
1768 MeV
-1 -0.5 0 0.5 1
1780 MeV
-1 -0.5 0 0.5 1
1792 MeV
-1 -0.5 0 0.5 1
1804 MeV
-1 -0.5 0 0.5 1
1816 MeV
-1 -0.5 0 0.5 1
1
1828 MeV
-1 -0.5 0 0.5 1
0.5
1
1.5
2.5
1840 MeV
-1 -0.5 0 0.5 1
1852 MeV
-1 -0.5 0 0.5 1
1864 MeV
1876 MeV 1888 MeV
1
1900 MeV
-1-0.5 0 0.5 -1-0.5 0 0.5 -1-0.5 0 0.5
0
5
10
15
024
012345
0
2
4
012
012
00.5
11.5
2
00.5
11.5
2
00.5
11.5
22.5
* )0πθcos(
b]µ [Ω
/dσd
1300 MeV 1312 MeV 1324 MeV 1336 MeV 1348 MeV 1360 MeV
1372 MeV 1384 MeV 1396 MeV 1408 MeV 1420 MeV 1432 MeV
-1 -0.5 0 0.5 1
1
1444 MeV
-1 -0.5 0 0.5 1
1456 MeV
-1 -0.5 0 0.5 1
1
1468 MeV
-1 -0.5 0 0.5 1
1
1480 MeV
-1 -0.5 0 0.5 1
1492 MeV
-1 -0.5 0 0.5 1
1504 MeV
-1 -0.5 0 0.5 1
1
1516 MeV
-1 -0.5 0 0.5 1
1528 MeV
-1 -0.5 0 0.5 1
1
1540 MeV
-1 -0.5 0 0.5 1
1
1552 MeV
-1 -0.5 0 0.5 1
1564 MeV
-1 -0.5 0 0.5 1
1576 MeV
-1 -0.5 0 0.5 1
1
1588 MeV
-1 -0.5 0 0.5 1
1600 MeV
-1 -0.5 0 0.5 1
1
1612 MeV
-1 -0.5 0 0.5 1
1
1624 MeV
-1 -0.5 0 0.5 1
1636 MeV
-1 -0.5 0 0.5 1
1648 MeV
1
1660 MeV 1672 MeV
1
1684 MeV
1
1696 MeV 1708 MeV 1720 MeV
-1 -0.5 0 0.5 1
1
1732 MeV
-1 -0.5 0 0.5 1
1744 MeV
-1 -0.5 0 0.5 1
0.5
1
1.5
1756 MeV
-1 -0.5 0 0.5 1
1
1768 MeV
-1 -0.5 0 0.5 1
1780 MeV
-1 -0.5 0 0.5 1
1792 MeV
-1 -0.5 0 0.5 1
1
1804 MeV
-1 -0.5 0 0.5 1
1816 MeV
-1 -0.5 0 0.5 1
0.5
1
1.5
2.5
1828 MeV
-1 -0.5 0 0.5 1
1
1840 MeV
-1 -0.5 0 0.5 1
1852 MeV
-1 -0.5 0 0.5 1
1864 MeV
1
1876 MeV 1888 MeV
1
1900 MeV
-1-0.5 0 0.5 -1-0.5 0 0.5 -1-0.5 0 0.5
05
1015
0246
01234
0123
012
012
00.5
11.5
00.5
11.5
2
012
* )0πθcos(
b]µ [Ω
/dσd
-1
0
1 0/A1A
-1
0
1 0/A2A
-1
0
1
0/A3A
-1
0
1
0/A4A
-1
0
0/A5A
-1
0
0/A6A
1400 1600 1800 1400 1600 1800
-1
0
1
-1
0
1
-1
0
W [MeV]
Coefficients of Legendre polynomials from dσdΩ = Σ AiPi(cos (Θ⋆))
B. Krusche, Hirschegg, January 2014 MAMI
polarization observables - beam - target——————————————————————————————————————completely model independent multipole analysis requires measurement of:
• 4 single polarization observables ( σ,Σ, T, P ) Chiang & Tabakin PRC 55 (1997)
• 4 carefully chosen double polarization observables
photon target polarizationpolarization - x y z
unpolarized
linearly
circularly
σ - T -
Σ H -P -G- F - -E
dσ
dΩ= dσ0
dΩ 1 − PlΣcos(2φ)
+Px [−PlHsin(2φ) + PcF ]
−Py [−T + PlP cos(2φ)]
−Pz [−PlGsin(2φ) + PcE]
B. Krusche, Hirschegg, January 2014
polarization observables for γn → nπ0——————————————————–——————————————————–(Th. Strub et al., very preliminary)
first, preliminary results for T (target asym.) and F (trans. pol. target, circ. pol. beam)
T : γp → pπ0 (blue: qf. p , black: free p) F : γp → pπ0 (blue: qf. p , black: free p )
Invariant mass W [MeV]1200 1340 1480 1620 1760 1900
Asy
mm
etry
T
-1
-0.5
0
0.5
1 0.067±) = 0.000 θcos( free proton (V. Kashevarov)
q.f. proton
SAID
MAID
Invariant mass W [MeV]1200 1340 1480 1620 1760 1900
Asy
mm
etry
F-1
-0.5
0
0.5
1 0.067±) = 0.000 θcos(
free proton (V. Kashevarov)
q.f. proton
SAID
MAID
T : γn → nπ0 (blue: qf. p , red: qf. n ) F : γn → nπ0 (blue: qf. p , red: qf. n )
Invariant mass W [MeV]1200 1340 1480 1620 1760 1900
Asy
mm
etry
T
-1
-0.5
0
0.5
1 0.067±) = 0.000 θcos( q.f. neutron
q.f. protonSAID neutronMAID neutronSAID protonMAID proton
Invariant mass W [MeV]1200 1340 1480 1620 1760 1900
Asy
mm
etry
F
-1
-0.5
0
0.5
1 0.067±) = 0.000 θcos(
q.f. neutronq.f. protonSAID neutronMAID neutronSAID protonMAID proton
asymmetries for freeand quasi-freeprotons agree!
significant deviationsbetween PWApredictions andneutron data
B. Krusche, Hirschegg, January 2014 MAMI
η-photoproduction off the proton: resonance contributions ?————————————————————————————–————————————————————————————–branching ratios and elm. couplings (PDG):
state bη [%] Ap1/2 Ap
3/2 An1/2 An
3/2
• D13(1520): 0.23±0.04 -24 150 -59 -139• S11(1535): 42±10 90 -46• S11(1650): 5 − 15 53 -15• D15(1675): 0±1 19 15 -43 -58• F15(1680): 0±1 -15 133 29 -33• D13(1700): 0±1 -18 -2 0±5 -3• P11(1710): 10 − 30 24 -2• P13(1720): 4±1 -10 -19 4 -10
dominent contribution from S 11 states,interference structure?
D15(1675) has stronger electromagneticcoupling to neutron than to proton
complicated pattern around 1.7 GeV
PWA’s agree excellently with data in S 11 range, less so at higher energies
γp → ηp
1
10
1.4 1.6 1.8 2 2.2 2.4 2.6
1.0 1.5 2.0 2.5 3.0
σ[µb
]W[GeV]
Eγ[GeV]
TAPS 95TAPS 95
GRAAL 02 GRAAL 07
CLAS 02 CLAS 09
Crystal Barrel 05 Crystal Barrel 09
LNS 06 Crystal Ball 10
D13(1520) S11(1535)
S11(1650)
D15(1675)
P11, P13
1
10
1.5 1.75 2
σ[µb
]
W[GeV]
MAID 1MAID 2SAIDBnGn
B. Krusche, Hirschegg, January 2014
angular distributions for γp → pη———————————————–———————————————–typical angular distributions fitted coefficients
750 MeV 790 MeV 925 MeV 1025 MeV 1125 MeV
1225 MeV 1425 MeV
1625 MeV 2025 MeV 2525 MeV
0
0.5
1
1.5
-1 0 1
dσ/d
Ω[µ
b/sr
]
0
0.5
1
1.5
-1 0 1 0
0.2
0.4
0.6
0.8
-1 0 1 0
0.2
0.4
-1 0 1 0
0.2
0.4
-1 0 1
0
0.1
0.2
0.3
0.4
-1 0 1 0
0.1
0.2
0.3
0.4
-1 0 1 0
0.1
0.2
0.3
0.4
-1 0 1cos(Θη)
0
0.1
0.2
0.3
0.4
-1 0 1 0
0.1
0.2
0.3
-1 0 1
A1/A0
A2/A0
A3/A0
A4/A0
0
1
-0.5
0
0.5
0.5
1
1.5
0
2
Ai/A
0
-0.5
-0.25
0
0
1
2
0
1
-0.2
0
0
0.5
0
0.5
1
1.5 2 2.5
-0.1
0
0.1
1.5 1.6 1.7 1.8
W[GeV]
0
0.5
1.8 2 2.2 2.4
fitted with:
dσdΩ = Σ AiPi(cos(Θ⋆))
typical s-wave behavior at threshold
fast variation - interesting structuresaround W ≈ 1.7 GeV
diffractive ( t-channel)at highest energies
B. Krusche, Hirschegg, January 2014
quasifree γn → nη: more surprises—————————————————–—————————————————–(I. Jaegle et al., D. Werthmuller et al., L. Witthauer et al.)
ELSA: MAMI: neutron/proton ratioγd → (n)(p)η γd → (n)(p)η γ3He → X(n)(p)η
0
5
10
1 1.5 2 2.5
σ[µb
]
Eγ[GeV]
pη
nη/(2/3)
pη, nη, MAID
nη, Shklyar et al.
1.5 1.7 1.9 2.1 W[GeV]
0
2.5
5
7.5
10
0.6 0.8 1 1.2 1.4
σn
σnp-σp
σ[µb
]
0
5
10
15
1.5 1.6 1.7 1.8W[GeV]
σ[µb
]
σp
(3/2)*σnσp, free
0.5
1
1.5
2
1.6 1.8
σ n/σ p
W[GeV]
2H, this work
2H, Jaegle et al.
0
5
10
1.5 1.6 1.7 1.8W[GeV]
σ[µb
]
σp(3/2)*σn
1
2
1.6 1.8
σ n/σ p
W[GeV]
3He, this work
2H, Jaegle et al.
0.5
1
1.5
2
1.5 1.6 1.7 1.8 1.9 2
σ n/σ p
W[GeV]
3He, Witthauer 13
2H, Jaegle 082H, Werthmueller 13
2/3
pronounced, narrow structure in neutron excitation functi on close to W=1.68 GeVwidth of structure ≈ 30 MeVneutron/proton ratios in agreement for all measurements:- in S11(1535) region 2/3 ratio- peak close to 1.7 GeV- very close to threshold almost unity, no distinction betwe en participant and spectatorfree and deuteron quasifree proton data agree;quasifree 3He data suppressed by ≈ 25%
B. Krusche, Hirschegg, January 2014
γn → nη - excitations functions for different angular bins———————————————————————————–———————————————————————————–(D. Werthmuller and L. Witthauer et al., Phys. Rev. Lett. 111 (2013) 232001; EPJA 49 (2013) 154)
deuteron target 3He target Legendre coefficients
1500 1600 1700 18000
0.5
1 [-1.00,-0.90]
1500 1600 1700 18000
0.5
1 [-0.90,-0.80]
1500 1600 1700 18000
0.5
1 [-0.80,-0.70]
1500 1600 1700 18000
0.5
1 [-0.70,-0.60]
1500 1600 1700 18000
0.5
1[-0.60,-0.50]
1500 1600 1700 18000
0.5
1[-0.50,-0.40]
1500 1600 1700 18000
0.5
1[-0.40,-0.30]
1500 1600 1700 18000
0.5
1[-0.30,-0.20]
0
0.5
1[-0.20,-0.10]
0
0.5
1[-0.10,0.00]
0
0.5
1[0.00,0.10]
0
0.5
1[0.10,0.20]
1500 1600 1700 18000
0.5
1[0.20,0.30]
1500 1600 1700 18000
0.5
1[0.30,0.40]
1500 1600 1700 18000
0.5
1[0.40,0.50]
1500 1600 1700 18000
0.5
1[0.50,0.60]
1500 1600 1700 18000
0.5
1 [0.60,0.70]
1500 1600 1700 18000
0.5
1 [0.70,0.80]
1500 1600 1700 18000
0.5
1 [0.80,0.90]
1500 1600 1700 18000
0.5
1 [0.90,1.00]
1500 1600 1700 1800 1500 1600 1700 1800 1500 1600 1700 1800 1500 1600 1700 1800
0
0.5
1
0
0.5
1
0
0.5
1
0
0.5
1
0
0.5
1
W[MeV]
b/sr
]µ [
Ω/dσd
1500 1600 1700 1800 19000
0.5
[-1.00,-0.80]
1500 1600 1700 1800 19000
0.5
[-0.80,-0.60]
1500 1600 1700 1800 19000
0.2
0.4
0.6
[-0.60,-0.40]
1500 1600 1700 1800 19000
0.2
0.4
0.6
[-0.40,-0.20]
0
0.2
0.4
0.6 [-0.20,0.00]
0
0.2
0.4
0.6 [0.00,0.20]
1500 1600 1700 1800 1900
0
0.2
0.4
0.6
[0.20,0.40]
1500 1600 1700 1800 1900
0
0.2
0.4
0.6
[0.40,0.60]
1500 1600 1700 1800 1900
0
0.2
0.4
[0.60,0.80]
1500 1600 1700 1800 1900
0
0.2
0.4
[0.80,1.00]datatotal func.signal BWall BGBG contrib.
1500 1600 1700 1800 1500 1600 1700 1800 1900
0
0.5
0
0.2
0.4
0.6
0
0.2
0.4
0.6
0
0.2
0.4
0.6
0
0.2
0.4
W [MeV]
b/sr
]µ [
Ω/dσd
1500 1600 1700 1800 1900
1
pη→pγMcNicoll et al.MAID
1500 1600 1700 1800 1900
1
pη→pγthis work
Jaegle et al.MAID
1500 1600 1700 1800 1900
1
0Anη→nγthis work
Jaegle et al.MAID
1500 1600 1700 1800 1900
-0.5
0
1500 1600 1700 1800 1900
-0.5
0
1500 1600 1700 1800 1900
1A
-0.5
0
0.5
-0.5
0
0.52A
-0.4
-0.2
0
0.2
0.4
-0.4
-0.2
0
0.2
0.4
3A
1500 1600 1700 1800 1500 1600 1700 1800 1500 1600 1700 1800 1900
1
-0.5
0
-0.5
0
0.5
-0.4
-0.2
0
0.2
0.4
W [MeV]
b/sr
]µ[
non-trivial angular dependence observed
B. Krusche, Hirschegg, January 2014 MAMI
photoproduction of meson pairs———————————————–———————————————–ππ- & πη-pairs: clean reaction identificationaccess to cascade decays in invariant mass spectra
N*, ∆
N(938)
N*, ∆
N* ∆
∆
N*
N*
N*,∆
N*
ππ πη
ρ σπ
π
π
η
π
η
η
π
1000 1500 2000 2500 3000
310×1000
1500
2000
2500
3000
310×
0
200
400
600
)0π(p2m
)0 π(p2
m
= 1500 - 1600 MeVγE
(1232)∆
13D (1520)
13D (1520)
1000 2000 3000 4000
310×1000
2000
3000
4000
310×
0
20
40
60
80
100
)0π(p2m
)0 π(p2
m
= 2000 - 2100 MeVγE
(1232)∆
13D (1520)
13D (1520)
15F (1680)
15F (1680)
1000 2000 3000 4000 5000
310×1000
2000
3000
4000
5000
310×
0
50
100
150
200
250
)0π(p2m
)0 π(p2
m
= 2400 - 2500 MeVγE
13D
13D (1520)
(1232)∆
15F (1680)
15F (1680)
Dalitz-plotanalysis ofπ0π0-pairs
B. Krusche, Hirschegg, January 2014 ELSA
γN → Nπoπo - total cross sections————————————————————————————————(M. Oberle et al., preliminary)
free & quasi-free proton quasi-free proton & neutron
W [MeV]1300 1400 1500 1600 1700 1800 1900
b]µ [σ
0
2
4
6
8
10
MAMI-TAPS-2008MAMI-TAPS-2007p MAIDp BoGa
)n0π0π (pexcσ)0π0π (pexcσ
)0π0π (incσ
W [MeV]1300 1400 1500 1600 1700 1800 1900
b]µ [σ
0
2
4
6
8
10
)n0π0π(pexcσ)p0π0π(nexcσ
moderate FSI effects found for quasi-free proton cross sect ion
proton & neutron excitation functions similar,largest differences around W=1600 MeV between resonance peaks
B. Krusche, Hirschegg, January 2014 MAMI
γN → Nπoπo - invariant mass & angular distributions—————————————————————————–—————————————————————————–(M. Oberle et al., preliminary)
pion -nucleon invariant mass angular distr. - Θ⋆ polar angle of ππ-system
900 1400 19000
20
40
60
80
quasi free pquasi free nfree pp MAIDn MAIDp Bonn
900 1400 19000
20
40
60
80
W = 1330 MeV
900 1400 19000
20
40
60
80
1360 MeV
900 1400 19000
20
40
60
80
1390 MeV
900 1400 19000
20
40
60
80
1420 MeV
900 1400 19000
25
50
75
100
1450 MeV
900 1400 19000
25
50
75
100
1480 MeV
900 1400 19000
25
50
75
100
1510 MeV
900 1400 19000
25
50
75
100
1540 MeV
900 1400 19000
25
50
75
100
1570 MeV
900 1400 19000
12
24
36
48
1600 MeV
900 1400 19000
12
24
36
48
1630 MeV
900 1400 19000
12
24
36
48
1660 MeV
900 1400 19000
12
24
36
48
1690 MeV
900 1400 19000
12
24
36
48
1720 MeV
900 1400 19000
7
14
21
28
1750 MeV
900 1400 19000
7
14
21
28
1780 MeV
900 1400 19000
7
14
21
28
1810 MeV
900 1400 19000
7
14
21
28
1840 MeV
900 1400 19000
7
14
21
28
1870 MeV
900 1400 900 1400 900 1400 900 1400 900 1400 1900
0
20
40
60
80
0
25
50
75
0
12
24
36
0
7
14
21
)0πm(N
/dm
[nb/
MeV
]σd
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
quasi free pquasi free nfree excfree incp MAIDn MAIDp BoGa
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
W = 1330 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1360 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1390 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1420 MeV
-1 -0.5 0 0.5 10
0.4
0.8
1.2
1.6
2
1450 MeV
-1 -0.5 0 0.5 10
0.4
0.8
1.2
1.6
2
1480 MeV
-1 -0.5 0 0.5 10
0.4
0.8
1.2
1.6
2
1510 MeV
-1 -0.5 0 0.5 10
0.4
0.8
1.2
1.6
2
1540 MeV
-1 -0.5 0 0.5 10
0.4
0.8
1.2
1.6
2
1570 MeV
-1 -0.5 0 0.5 10
0.3
0.6
0.9
1.2
1.5
1600 MeV
-1 -0.5 0 0.5 10
0.3
0.6
0.9
1.2
1.5
1630 MeV
-1 -0.5 0 0.5 10
0.3
0.6
0.9
1.2
1.5
1660 MeV
-1 -0.5 0 0.5 10
0.3
0.6
0.9
1.2
1.5
1690 MeV
-1 -0.5 0 0.5 10
0.3
0.6
0.9
1.2
1.5
1720 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1750 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1780 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1810 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1840 MeV
-1 -0.5 0 0.5 10
0.2
0.4
0.6
0.8
1
1870 MeV
-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
00.20.40.60.8
1
00.40.81.21.6
00.30.60.91.2
00.20.40.60.8
)* θcos(
b]µ [θ/dσd
invariant mass distributions show contributions from∆⋆, N⋆ → π∆(1232) & ∆⋆, N⋆ → πD13(1520); very similar for p & n
proton & neutron angular distributions different for large W
→ different resonance contributions for ∆⋆, N⋆ → πD13(1520) ?
B. Krusche, Hirschegg, January 2014 MAMI
example for polarization observables for pion-pairs————————————————————————–————————————————————————–beam-helicity asymmetries - circularly polarized beam, un polarized target
———————————————————————————————————–———————————————————————————————————–
N
p3
p1
p2
Φγ z
z’
y
p1, p2, p3 = π1, π2, N′
e.g. (p1, p2, p3) = (π+, πo, n)
I⊙(Φ) ≡ dσ+−dσ−
dσ++dσ−
I⊙(Φ) = −I⊙(2π − Φ)
I⊙(Φ) = Σ∞n=1Ansin(nΦ)
B. Krusche, Hirschegg, January 2014
beam-helicity asym. for γN → Nπoπo & γN → Nπoπ±——————————————————————————————————————————————————————M. Oberle et al., PLB 721(2013) 237, M. Oberle et al., submitte d to EPJA
γp → pπ0π0 (blue: qf. p , black: free p) γp → nπ0π+ (blue: qf. p , black: free p)
0 90 180 270 360-45
-27
-9
9
27
45W = 1416 - 1512 MeV
free p
quasifree p0 90 180 270 360-45
-27
-9
9
27
451512 - 1573 MeV
p (Fix)p (Fix 2)p (BnGa)p (BnGa 2)
0 90 180 270 360-45
-27
-9
9
27
451573 - 1617 MeV
0 90 180 270 360-25
-15
-5
5
15
251617 - 1646 MeV
0 90 180 270 360-25
-15
-5
5
15
251646 - 1674 MeV
0 90 180 270 360-25
-15
-5
5
15
251674 - 1702 MeV
0 90 180 270 360-25
-15
-5
5
15
251702 - 1729 MeV
0 90 180 270 360-25
-15
-5
5
15
251729 - 1769 MeV
0 90 180 270 360-25
-15
-5
5
15
251769 - 1872 MeV
0 90 180 270 0 90 180 270 0 90 180 270 360
-45
-27
-9
9
27
45
-25
-15
-5
5
15
-25
-15
-5
5
15
]o [Φ
A [%
]
0 90 180 270 360-40
-24
-8
8
24
40 = 1348 - 1416 MeVrW
0 90 180 270 360-40
-24
-8
8
24
401416 - 1465 MeV
0 90 180 270 360-40
-24
-8
8
24
401465 - 1512 MeV
p (A.Fix) free p quasifree p
0 90 180 270 360-40
-24
-8
8
24
401512 - 1543 MeV
0 90 180 270 360-25
-15
-5
5
15
251543 - 1573 MeV
0 90 180 270 360-25
-15
-5
5
15
251573 - 1602 MeV
0 90 180 270 360-25
-15
-5
5
15
251602 - 1631 MeV
0 90 180 270 360-25
-15
-5
5
15
251631 - 1660 MeV
0 90 180 270 360-25
-15
-5
5
15
251660 - 1688 MeV
0 90 180 270 360-25
-15
-5
5
15
251688 - 1715 MeV
0 90 180 270 360-25
-15
-5
5
15
251715 - 1769 MeV
0 90 180 270 360-25
-15
-5
5
15
251769 - 1872 MeV
0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 360
-40
-24
-8
8
24
40
-25
-15
-5
5
15
-25
-15
-5
5
15
]o [Φ
A [%
]
γn → nπ0π0 (red: qf. n , black: free p) γn → pπ0π− (red: qf. n , black: free p)
0 90 180 270 360-40
-24
-8
8
24
40W = 1417 - 1513 MeV
n (Fix) free p quasifree n
0 90 180 270 360-40
-24
-8
8
24
401513 - 1574 MeV
0 90 180 270 360-40
-24
-8
8
24
401574 - 1618 MeV
0 90 180 270 360-25
-15
-5
5
15
251618 - 1647 MeV
0 90 180 270 360-25
-15
-5
5
15
251647 - 1675 MeV
0 90 180 270 360-25
-15
-5
5
15
251675 - 1703 MeV
0 90 180 270 360-25
-15
-5
5
15
251703 - 1731 MeV
0 90 180 270 360-25
-15
-5
5
15
251731 - 1771 MeV
0 90 180 270 360-25
-15
-5
5
15
251771 - 1874 MeV
0 90 180 270 0 90 180 270 0 90 180 270 360
-40
-24
-8
8
24
40
-25
-15
-5
5
15
-25
-15
-5
5
15
]o [Φ
A [%
]
0 90 180 270 360-32.5
-19.5
-6.5
6.5
19.5
32.5 = 1348 - 1416 MeVrW
0 90 180 270 360-32.5
-19.5
-6.5
6.5
19.5
32.51416 - 1465 MeV
0 90 180 270 360-32.5
-19.5
-6.5
6.5
19.5
32.51465 - 1512 MeV
n (A.Fix) free p quasifree n
0 90 180 270 360-32.5
-19.5
-6.5
6.5
19.5
32.51512 - 1543 MeV
0 90 180 270 360-30
-18
-6
6
18
301543 - 1573 MeV
0 90 180 270 360-30
-18
-6
6
18
301573 - 1602 MeV
0 90 180 270 360-30
-18
-6
6
18
301602 - 1631 MeV
0 90 180 270 360-30
-18
-6
6
18
301631 - 1660 MeV
0 90 180 270 360-30
-18
-6
6
18
301660 - 1688 MeV
0 90 180 270 360-30
-18
-6
6
18
301688 - 1715 MeV
0 90 180 270 360-30
-18
-6
6
18
301715 - 1769 MeV
0 90 180 270 360-30
-18
-6
6
18
301769 - 1872 MeV
0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 360
-32.5
-19.5
-6.5
6.5
19.5
32.5
-30
-18
-6
6
18
-30
-18
-6
6
18
]o [Φ
A [%
]
N
N’
π1
π2
Φγ z
z’
y
pions ordered byinvariant mass:
m(π1, N) ≥ m(π2, N)
B. Krusche, Hirschegg, January 2014 MAMI
‘charge ordered’ asym.: γN → Nπoπ±————————————————————————————————————(M. Oberle et al., preliminary)
N
p3
p1
p2
Φγ z
z’
y
(p1, p2, p3) = (π±, πo, N) (p1, p2, p3) = (πo, N, π±) (p1, p2, p3) = (π±, N, πo)proton neutron proton neutron proton neutron
1349.1 1475.5 1601.9 1728.3 1854.7-40
-20
0
20
40
p (Fix) free p p (Fix full) quasifree p
1A
1349.1 1475.5 1601.9 1728.3 1854.7-40
-20
0
20
40
n (Fix) free p n (Fix full) quasifree n
1A
1349.1 1475.5 1601.9 1728.3 1854.7-16
-8
0
8
16
2A
1349.1 1475.5 1601.9 1728.3 1854.7-16
-8
0
8
16
2A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
3A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
3A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
4A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
4A
1350 1475 1600 1725 1350 1475 1600 1725 1850
-40
-20
0
20
40
-16
-8
0
8
-5
-2.5
0
2.5
-5
-2.5
0
2.5
W [MeV]
[%]
nA
1349.1 1475.5 1601.9 1728.3 1854.7-40
-20
0
20
40
free p p (A. Fix) quasifree p
1A
1349.1 1475.5 1601.9 1728.3 1854.7-40
-20
0
20
40
free p n (A. Fix) quasifree n
1A
1349.1 1475.5 1601.9 1728.3 1854.7-16
-8
0
8
16
2A
1349.1 1475.5 1601.9 1728.3 1854.7-16
-8
0
8
16
2A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
3A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
3A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
4A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
4A
1350 1475 1600 1725 1350 1475 1600 1725 1850
-40
-20
0
20
40
-16
-8
0
8
-5
-2.5
0
2.5
-5
-2.5
0
2.5
W [MeV]
[%]
nA
1349.1 1475.5 1601.9 1728.3 1854.7-40
-20
0
20
40
free p p (A. Fix) quasifree p
1A
1349.1 1475.5 1601.9 1728.3 1854.7-40
-20
0
20
40
free p n (A. Fix) quasifree n
1A
1349.1 1475.5 1601.9 1728.3 1854.7-16
-8
0
8
16
2A
1349.1 1475.5 1601.9 1728.3 1854.7-16
-8
0
8
16
2A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
3A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
3A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
4A
1349.1 1475.5 1601.9 1728.3 1854.7-5
-2.5
0
2.5
5
4A
1350 1475 1600 1725 1350 1475 1600 1725 1850
-40
-20
0
20
40
-16
-8
0
8
-5
-2.5
0
2.5
-5
-2.5
0
2.5
W [MeV]
[%]
nA
coefficients of sine-series compared to Fix modellarge discrepancies for second resonance regionexcellent agreement between free and quasi-freeproton data, no FSI effects
B. Krusche, Hirschegg, January 2014 MAMI
resonance contributions to photoproduction of πη-pairs—————————————————————————————————————————————————————–I. Horn et al., PRL 101 (2008) 202002; EPJA 38 (2008) 173, V. Ka shevarov et al., EPJA 42 (2009) 141; PLB 693 (2010) 551
total cross section Invariant mass distributions
dominant final states: −− ∆(1232)η, −.− N(1535)π, ... pao(980)
dominant process close to threshold: γp →D33(1700)→ ηP33(1232) → ηπop
B. Krusche, Hirschegg, January 2014 ELSA
isospin decomposition of πη-photoproduction————————————————————————————————————————————(A. Kaeser et al., preliminary)
total cross sections cross section ratios
0
1
2
3
1600 1700 1800 1900
W[MeV]
σ[µb
]
γd→(n)pπ0η
γd→(p)nπ0η
γd→(n)nπ+η
γd→(p)pπ-η
threshold
1
1700 1800 1900W[MeV]
ratio
s
0.5
2σ(nηπ0)/σ(pηπ0)=1σ(pηπ-)/σ(nηπ+)=1
σ(nηπ+)/σ(pηπ0)=1/2σ(pηπ-)/σ(nηπ0)=1/2 N(938)
∆
∆
∆
N*
N*
N*
N*
N*
πηpγ=nγ pγ,nγ
π
η
η
π
π
η
η
π
σ(n) = σ(p)
σ(π+/-) = 1/2σ(π0)
σ(n)/nγ=σ(p)/pγ
σ(π+/-) = 2σ(π0)
cross section ratios agree with γN → ∆⋆ → η∆ → ηπN reaction chain
analysis of invariant mass distributions and polarization observables for allisospin channels under way
B. Krusche, Hirschegg, January 2014 MAMI
Summary——————————measurement of final states with coincident neutrons, in par ticular‘all neutral’ final states like nπ0, nη, nη′, nπoπo... mandatory for analysisof N⋆ properties
effects from Fermi motion under control via kinematic recon struction
effects from FSI:
experimental access via comparison of free and quasi-free p roton results
develpment of models for FSI in progress
FSI effects strongly channel dependent, e.g. small/neglig ible for η, η′,moderate for πoπo, substantial for πo, ηπ
for channels so far investigated FSI effects seem to be much l ess import-ant for polarization observables than for cross sections
experiments at MAMI taking data or are under analysis,expriments at ELSA will start after detector upgrade
B. Krusche, Hirschegg, January 2014
Conclusions————————————excitation spectrum of nucleons is one of the most important testing groundsof non-perturbative QCD, but not yet understood
progress on theory side from lattice expected, but still in v ery early state
progress in experiment currently mainly from photoproduct ion of mesonsresting on three pillars:
exploraration of polarization observables (beam, target, recoil) toestablish a data base allowing almost model independent ana lyses
investigation of different final states including multi-me son productionso that coupled channel analyses can identify excited state s decoupledfrom dominant decays like π0 emission to the nucleon ground-state
investigation of reactions off quasi-free neutrons to esta blish also thephotocouplings for neutron resonances
first impact is demonstrated, resonance listings in PDG (so f ar stronglybiased to elastic pion reactions) become more and more influe nced byresults from the photon induced reactions
B. Krusche, Hirschegg, January 2014
