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

Susanna Costanza

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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)

Susanna Costanza

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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)

Susanna Costanza

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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