Susanna CostanzaUniversità degli Studi di Pavia - INFN Pavia
on behalf of the A2 Collaboration @ MAMI
International School of Nuclear Physics – 37th Course
«Probing Hadron Structure with Lepton and Hadron Beams»
Erice, 16 – 24 Settembre 2015
2Erice, 20/09/2015
Introduction
Susanna Costanza
- Perturbative QCD → MEANINGLESS!!- Low energy regime ↔ non perturbative approach
- Phenomenological Quark Models → LIMITED SUCCESS
- Based on internal degrees of freedom
- Three equivalent constituent quarks, quark-diquark structures, quark andflux tubes
- Based on residual interactions of the quarks
- Gluon exchange, Goldstone boson exchange
- Can serve as:
- approximation of the nucleon structure
- guidance about the relevant interaction properties by comparison withthe observed excitation spectrum
How can we study the excited states of the nucleon?
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«Where have all the resonances gone?»
Susanna Costanza
R. Koniuk and N. Isgur, Phys. Rev. Lett. 44 (1980) 845
Agreement between modelpredictions and experimentalfindings in the low massregion
Discrepancies at largermasses: experiments have notfound as many resonances aspredicted
Pictures from wwwnew.hiskp.uni-bonn.de/cb
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Looking for missing resonances
Where is the mismatch between experimentand model predictions rooted?
Data analyses relying entirelyon meson induced reactionswill miss states that couple
only weakly to Nπ
Inappropriate internaldegrees of freedom in the
model
Experimental alternative:Use of reactions induced by the electromagnetic interaction
=Photoproduction of mesons
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Meson photoproduction
PROs:
Prime tool for the experimental investigation of the excitation spectrum ofthe nucleon
Accuracy in measuring photon induced reactions is comparable to that ofhadron induced reactions
Possible due to the large progress in accelerator and detector technology
Possibility to explore multiple meson production reactions (ππ, πη, …)
Access to resonances that decay via intermediate excited states
Electromagnetic couplings are related to spin-flavour correlations of thestates → information about configuration mixing
Susanna Costanza
CONs:
Electromagnetic cross sections are much smaller than the hadronic ones
Photon induced reactions can have significant non-resonant «background»
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A2 Collaboration
- The collaboration has been established in 2004-2005- ~ 70 collaborators- Germany, Italy, Great Britain, Russia, Switzerland, USA- LoI signed and approved in 2004- MoU in 2005
- Location:MAMI Accelerator(Mainz)
- Beam:• photon beam produced bybremsstrahlung process andtagged by the magneticspectrometer• Eγ < 1.5 GeV• ΔEγ = 2 - 4 MeV• Linear and circularpolarisations available
Susanna Costanza
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A2@MAMI: detector overviewCrystal Ball
Photon spectrometer672 NaI-detectorsLarge acceptance (93%)20° < θ < 160°
TAPSForward wall510 BaF2 detectors2° < θ < 21°
MWPC2 cylindrical wire
chambers480 wires, 320 strips
PIDParticle separation24 thin plastic counters
Susanna Costanza
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A2@MAMI: main physics goals
- Precision spectroscopy of low lying baryonic states- Δ(1232) from γp → π0γ’p and π+γ’n
- S11(1535) from γp → η γ’p reaction
- Threshold meson production (test of LET/ChPT)
- Strangeness (γN → ΛK)
- π0 photoproduction at threshold
- Ambiguity free amplitude analysis of meson photoproduction
- Requires double polarisation measurements: γN → Nπ(π), Nη(ρ, …)
- Tests of fundamental symmetries (C, CP, CPT, …)
- Rare η, η’ decays
- Studies of nuclear structure (γA → pX)
- In medium properties of hadrons
- Meson photoproduction on nuclei
Susanna Costanza
The GDH sum rule
Physics motivation
Results
γ p, γ d, γ 3He → X, nπ+, pπ0
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The GDH sum rule- Proposed by Gerasimov – Drell – Hearn in 1966
- Fundamental connection between the ground state properties of aparticle and a moment of the entire excitation spectrum
- Gives a prediction on the absorption of circularly polarised photons bylongitudinally polarised nucleons/nuclei:
Photon energyνth = π production threshold (nucleons)
photodisintegration threshold (nuclei)
Spin
Mass
Anomalous magnetic moment
Photon spin Baryon spin
= a= p
A measurement of the GDH integral constitutes a fundamentalcheck of our knowledge of both the photon and the nucleon
(nucleus)
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The GDH sum rule on the proton
MAMI data: J. Ahrens et al., PRL 87 (2001) 022003ELSA data: H. Dutz et al., PRL 91 (2003) 192001, H.Dutz et al., PRL 93 (2004) 032003
First experimental evaluation on the proton (0.2 < Eg < 2.9 GeV)
IGDH (p) = 211 ± 5 ± 12 µb
Δ(1232)
D13(1520)
F15(1680)
F35(1905)
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Susanna Costanza Erice, 20/09/2015 13
Eγ (GeV) Who IGDH (b)
< 0.20 MAID/SAID -28.5 ± 2
0.20 – 2.90 MAMI+ELSA (measured) 254 ± 5 ± 12
> 2.90 (Regge approach)
Simula et al.Bianchi–Thomas
-13-14
Total 211 ± 5 ± 12
GDH sum rule 205
IGDH (p) = 211 ± 5 ± 12 b
Agreement (within errors) between the GDH sum rule value and the experimental one:
GDH sum rule experimentally (almost) demonstrated!!!
The GDH sum rule on the proton
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Partial channels Models IGDH (p) [b] IGDH (n) [b]
γp → Nπ SAID-FA07K [MAID07] 172 [164] 147 [131]
γp → NππFix, Arenhoevel
EPJA 25, 114 (2005)94 82
γp → Nη MAID -8 -6
γp → KЛ(Σ) Sumowidagdo et al.PRC 65, 0321002 (02)
-4 2
γp → Nρ(ω) Zhao et al.PRC 65, 032201 (03)
0 2
Regge contribution
Bianchi-ThomasPLB 450, 439(99)
-14 20
Total 239 [231] 244 [231]
GDH sum rule 205 233
Comparing the GDH sum rule value and the theoretical predictions:NO AGREEMENT for the proton … but …
AGREEMENT for the neutron
The GDH sum rule: theoretical estimates
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The GDH sum rule on the neutron
3He: System of two protons with spins paired off and an“active” unpaired neutron, in relative s states ( 90% probability)
Allows direct access to the free neutron cross sectionprevented by nuclear structure effects and FSI
The proton contribution is small: µ ≈ µn
2H: System of one proton and one neutron with pairedspins, in relative s states (96% probability)
µ ≈ µp + µn
Susanna Costanza
GDH Mainz – PLB 672, 328 (09)
GDH Bonn – PRL 94, 162001 (05)
AFS model – PRL 93, 202301 (04)
(AFS: Arenhoevel, Fix and Schwamb)
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The GDH sum rule on the deuteron
Xdγ
Helicity dependent total inclusive cross section: Δσ = σp-σa (μb)
)(93.0expp
GDHnGDH
deuteron III bI de 249452xp
bI pGDH
125255
with:
From GDH sum rule: bI nGDH
233 200 MeV < Eγ < 1800 GeV
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The GDH sum rule on the neutron
3He: System of two protons with spins paired off and an“active” unpaired neutron, in relative s states ( 90% probability)
Allows direct access to the free neutron cross sectionprevented by nuclear structure effects and FSI
The proton contribution is small: µ ≈ µn
2H: System of one proton and one neutron with pairedspins, in relative s states (96% probability)
µ ≈ µp + µn
Susanna Costanza
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The GDH sum rule on the neutron
For νth > mπ (π photoproduction threshold on free nucleon), IGDH(3He) IGDH(n);
For 8 MeV < νth < mπ (photodisintegration region), contribution ofnuclear structure effects to IGDH(3He):
Precise experimental data on 3He are required from break-up thresholdupwards:
@ HIgS, from photodisintegration threshold to 60 MeV
@ MAMI, from π photoproduction threshold
to test the GDH sum rule on the neutron and 3He modelsboth through the inclusive and the partial channel measurements.
nGDH
pGDHm
HeGDH
III
87.0026.023
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A2’s contribution to GDH Precise measurements of all partial γN→Nπ(π) channels both on the
proton and on the neutron
Total inclusive cross section: more statistics, especially on the neutron
Reliable extraction of the free–neutron information from deuteriumand 3He targets
A part of the experimental program was recently carried out by the A2@MAMI Collaboration
• 3He data takings: July 2009• proton data takings: November 2013 - May 2014 – May 2015
• deuteron data takings: January 2014 – March 2015
Example:
γ p, γ d, γ 3He → X, pπ0, nπ+
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• A2 data• GDH data
PRL 84 (2000) 5950-5954
MAID model
Susanna Costanza
and : σp–σa (μb)γ p→π0Xγ p→X
γ p→π0X
γ p→X
• A2 data• GDH data
PLB 672, 328 (09)PRL 94, 162001 (05)
Whole statistics fromNovember 2013 beamtime
About 25% of the statistics from November 2013 beamtime(Moeller radiator)
Moeller
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γ p→π0X: (dσp/dΩ)-(dσa/dΩ) (μb/sr)
• A2 data • MAID π0p model
• A2 data• GDH data
PRL 94, 162001 (05)PLB 672, 328 (09)
MAID model(sum of free nucleons)
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and : σp–σa (μb)
γ d→π0X
About 30% of the statistics(Moeller radiator)
About 30% of the statistics(Moeller radiator) • A2 data
• GDH dataPRL 94, 162001 (05)PLB 672, 328 (09)
γ d→X
γ d→π0Xγ d→X
Susanna Costanza
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• A2 data • MAID model
γ d→π0X: (dσp/dΩ)-(dσa/dΩ) (μb/sr)
Moeller
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─── FA model─── PWIA model
P. Aguar Bartolomé et al, PLB 723 (2013) 71-77
FA (Fix-Arenhövel) model: Fermi motion + nuclear structure effectsPWIA (Plane-Wave Impulse Approximation) model:
incoherent sum of quasi-free single nucleon contributions (from MAID multipole analysis smeared with Fermi motion)
─── FA model─── PWIA model
UNPOLARISED POLARISED
γ 3He→πX: σunpol and σp−σa (μb)
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γ 3He→π0X: (dσp/dΩ)–(dσa/dΩ) (μb/sr)
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S. Costanza et al,EPJA (2014) 50: 173
Susanna Costanza
• CB dataPWIA modelFA model π03HeFA model π0ppnFA model π0pd
“Inclusive” analysismethod
PWIA predictions: Δσp2ΔσpΔσ ppnnPWIA
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Total inclusive cross section blablabla
Reasonable agreement between data and PWIA predictions
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P. Aguar Bartolomé et al, PLB 723 (2013) 71-77
─── PWIA prediction
• Running IGDH (3He)─── PWIA prediction
Running IGDH for 3He for Eγ = (200, 500) MeV:
IGDH = 135 ± 20 ± 12 µb≈ Ip
γ 3He→X
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Summary
An overview was given on recent results from thephotoproduction of mesons from nucleons and nuclei at A2@MAMI
The technical/instrumental challenges of the measurements ofsuch reactions are well under control
With respect to similar facilities (CLAS), MAMI can be:
competitive, thanks to the high energy resolution ΔEγ andhigh intensity of the photon beam
complementary, thanks to the almost 4π-covering elmcalorimeters, allowing to reconstruct mixed charged and all-neutral final states
A2 can contribute to many topics of nuclear physics
A lot of collected data to be analysed… and a lot to come!
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Rough derivation of IGDH(n)
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Δσneutron =Δσdeuteron
0.93 −Δσproton Δσneutron =Δσ3He + 0.052·Δσproton
0.87
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Cylindrical cell:length: 20 cmdiameter: 6 cm
Made of quartz glass (thickness: 2 mm)
Titanium enter and exit windows (50 m)provide the necessary gas tightness (4 bar)give long relaxation time (20 hrs) of the gas
polarisation3He polarisation measurements carried out
via NMR technique; field provided by Helmholtzcoils
Developed by P.I. Mainz
3He target
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Susanna Costanza 33
Cylindrical cell:length: 20 cmdiameter: 6 cm
Made of quartz glass (thickness: 2 mm)
Titanium enter and exit windows (50 m)provide the necessary gas tightness (4 bar)give long relaxation time (20 hrs) of the gas
polarisation3He polarisation measurements carried out
via NMR technique; field provided by Helmholtzcoils
Developed by P.I. Mainz
g-beam
Vacuumchamber
Helmholtzcoils
3He target
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Susanna Costanza 34
MEOP: Metastability Exchange Optical Pumping
Ground state
Metastable state
11S0
23S1
23P0
B = 0 B 0
mF = +½
mF = -½
mF = +½
mF = -½
Excited state
(F = ½)
(F = ½)
3He polarisation
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Susanna Costanza 35
MEOP: Metastability Exchange Optical Pumping
Ground state
Metastable state
11S0
23S1
23P0
1083 nm+ transition
B = 0 B 0
mF = +½
mF = -½
mF = +½
mF = -½
Excited state
(F = ½)
(F = ½)
3He polarisation
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Susanna Costanza 36
MEOP: Metastability Exchange Optical Pumping
Ground state
Metastable state
11S0
23S1
23P0
B = 0 B 0
mF = +½
mF = -½
mF = +½
mF = -½
Excited state
(F = ½)
(F = ½)
Polarisation transfer to the 3He groundstate by atomic collisions
0
131
330
131
33 1212 SeHSHeSHeSeH
3He polarisation
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