Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop

Measurement of helicity dependent total inclusive γ-3He cross section and the

GDH sum rule on the neutronSusanna Costanza

(INFN Pavia)on behalf of the A2 Collaboration

International Workshop “Meson Production at Intermediate and High

Energies”

Messina, 10-11 November 2011

Susanna Costanza 2Messina, 11 /11/11

The GDH sum rule Proposed by Gerasimov – Drell – Hearn in 1966 Fundamental connection between the ground state properties of a particle and a

moment of the entire excitation spectrum Gives a prediction on the absorption of circularly polarised photons by

longitudinally polarised nucleons/nuclei:

Photon energyth = production threshold (nucleons) photodisintegration threshold (nuclei)

Spin

Mass

Anomalous magnetic moment

It allows to: check our knowledge of the N interaction and of the physics of strongly

interacting systems access new experimental observables and study the baryon resonances through

the helicity dependence of partial channels test photoreaction models

Photon spin Baryon spin

= a= p


The GDH sum rule ∙∙

A measurement of the GDH integrand represents a fundamental test of our knowledge of both the photon and the nucleon

(nucleus)

Could the GDH sum rule be violated?

The only “weak” hypothesis is the assumption that the Compton scattering N ’N’ becomes spin-independent as

… a violation of this assumption can not be explained easily …

Possible explanations could be: exchange of a a1-like (J = 1+) meson between and N non-pointlike (contituent) quarks?

Susanna Costanza Messina, 11 /11/11 4

GDH sum rule on the proton

IGDH (p) = 211 ± 5 ± 12 b

Xp

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


GDH sum rule on the proton ∙∙

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 ± 12GDH sum rule 205

IGDH (p) = 211 ± 5 ± 12 b

Agreement (within errors) between the GDH sum rule value and the experimental one:

GDH sume rule experimentally (almost) proved!!!


GDH sum rule: theoretical estimates

Partial channels Models IGDH (p) [b] IGDH (n) [b]

p N SAID-FA07K [MAID07] 172 [164] 147 [131]

p N Fix, Arenhoevel EPJA 25, 114 (2005) 94 82

p Nh MAID -8 -6

p KL (S) Sumowidagdo et al. PRC 65, 0321002 (02) -4 2

p Nr(w) Zhao et al. PRC 65, 032201 (03) 0 2

Regge contribution Bianchi-Thomas PLB 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

… need of improved N() analyses?


GDH sum rule on the neutronComplication: lack of free neutron targets

Direct access to the free neutron cross section prevented by nuclear structure effects and FSI

Solution: use of 2H (deuterated butanol) or high pressure gas 3He targets

Theoretical models needed to evaluate these effects

Deuteron (2H):

System of one proton and one neutron with paired spins, in relative s states ( 96% probability)

np nGDH

pGDHm

dGDH III

93.093.0

Effective nucleon polarisation (D wave)

GDH Mainz – PLB 672, 328 (09)

GDH Bonn – PRL 94, 162001 (05)

AFS model – PRL 93, 202301 (04)(AFS: Arenhoevel, Fix and Schwamb)


GDH sum rule on the deuteron

Xd

Helicity dependent total inclusive cross section: = p-a (b)

)(93.0 pGDH

nGDH

deuteron III expbI d

e xp 249452 bI p

GDH 125255 with:

From GDH sum rule: bI nGDH 233 200 MeV < Eγ < 1800 GeV


GDH sum rule on the neutron ∙∙Complication: lack of free neutron targets

Direct access to the free neutron cross section prevented by nuclear structure effects and FSI

Solution: use of 2H (deuterated butanol) or high pressure gas 3He targets

Theoretical models needed to evaluate

3He: System of two protons with spins paired off and an “active” unpaired neutron, in relative s states ( 90% probability)

n nGDH

pGDHm

HeGDH III

87.0026.023

Effective nucleon polarisation (S’ and D waves)


GDH sum rule on the neutron ∙∙∙

For th > m ( photoproduction threshold on free nucleon), IGDH(3He) IGDH(n); For 8 MeV < th < m (photodisintegration region), contribution of nuclear

structure effects to IGDH(3He):

From previous considerations, the most accurate evaluation of IGDH for the neutron comes from 3He: the proton contribution is much smaller than in the deuteron case.

3He used as a substitute for a polarised neutron target at MAMI

Precise experimental data on 3He are required from break-up threshold upwards: @ HIS, from photodisintegration threshold to 60 MeV @ MAMI, from photoproduction threshold

to test the GDH sum rule on the neutron and 3He models both through the inclusive and the partial channel measurements.

Facilitytagged photon facility of the MAMI accelerator in Mainz

Beamcircularly polarised photons produced by bremsstrahlung of longitudinally polarised electrons

Ee- = 525 MeV150 < E < 500 MeV

Targetpolarised 3He gas

Detectorthe large acceptance (93%) Crystal Ball (CB) photon spectrometer in combination with the TAPS detector(A. Thomas’s talk)


Experimental setup for 3He exp.

TAPS

CRYSTAL BALL

PID

MWPC


3He polarisationMEOP: 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)

Polarisation transfer to the 3He ground state by atomic collisions

013

133

013

133 1212 SeHSHeSHeSeH

Simplified!


3He target Cylindrical cell:

Length: 20 cm diameter: 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 polarisation

3He polarisation measurements carried out via NMR technique; field provided by Helmholtz coils Developed by P.I. Mainz

-beam

Vacuum chamber

Helmholtz coils


3He target ∙∙

All charged particle events

Event difference (P – A)

3He gas

Ti windows

z–vertex from MWPC analysis

150 MeV < Eγ < 500 MeV

Natoms 1021/cm2 102 times less than in a solid/liquid target

3He target is an “empty” target…

UNPOLARISED CROSS SECTIONS ON 3He

Susanna Costanza 15Messina, 11/11/11

OUTLINE of the Section Total inclusive cross section

No partial channel separation (just hadron counting) Partial/semi-inclusive channels:

3He ± X ±/p separation by dE/dx – E technique (using PID & MWPC)

3He 0 X invariant mass (using only CB)

3He ppn ppn/ppn0 separation by missing mass/energy plots

Comparison: total inclusive vs sum of partial channels

…obtained after subtraction of (the big) empty target contribution...


Total inclusive cross sectionXHe3

“Inclusive” analysis method

(NO partial channel separation)

Extrapolation from quasi-free pion production and MAID cross sections

Extrapolation from Schwamb model for ppn

Good agreement with DAPHNE data


Partial channel I: 3He(γ,)XFirst data

Preliminary

Nuclear structure contribution (FSI, …)more important for the 0X partial channel


Partial channel II: γ 3He ppn

First data

Preliminary

No model estimation available for 3He“Quasi-deuteron” approximation ( 3He pnps)

evaluated from the Schwamb d pn model

dHe 68.13

Discrepancy between data and Schwamb model mostly due to 3-body absorption effects


Total inclusive vs partial channels

First data

Preliminary

Good agreement between inclusive method and the sum of partial channels

POLARISED CROSS SECTIONS ON 3He

Susanna Costanza 20Messina, 11/11/11

OUTLINE of the Section Total inclusive cross section

No partial channel separation Partial/semi-inclusive channels:

3He ± X ±/p separation by dE/dx – E technique (using PID & MWPC)

3He 0 X invariant mass (using only CB)

3He ppn ppn/ppn0 separation by missing mass/energy plots

Comparison: total inclusive vs sum of partial channels

Same analyses as for unpol

ap 2123

…no subtraction of empty target contribution...


Total inclusive cross section

“Total Inclusive” analysis(NO partial channel separation)

XeH 3

Extrapolation from quasi-free pion production and MAID cross sections

Model: A. Fix (X - simplified treatment of FSI) + Schwamb (ppn) Prediction based on MAID

pn 05.087.0

Fix + SchwambMAID

First data

Preliminary

Reasonable agreement between data and empirical prediction


Partial channel I: 3He(γ,)XFirst data

Preliminary

Nuclear structure contribution (FSI, …)less important than for the unpolarised case


Partial channel II: γ 3He ppnFirst data

Preliminary

Discrepancy between data and Schwamb model

mostly due to 3-body absorption effects

dHe 68.13

No model estimation available for 3He“Quasi-deuteron” approximation ( 3He pnps)

evaluated from the Schwamb d pn model


Rough derivation of IGDH(n)


Conclusions

Thank you!

This first test has proved the feasibility of the use of polarised 3He gas target to check the GDH sum rule on the neutron

Unprecedented data and preliminary results for unpolarised and polarised photoabsorption cross section on 3He

Good agreement for the unpolarised inclusive cross section between the CB-MAMI data and the DAPHNE data

Importance of the data in providing additional constraints for nuclear and subnuclear models

Further measurements to improve statistics and to investigate a wider energy range are needed


Backup slides


±/p separation

p

e

dE/dx – E techinque usedEnergy information from PID (corrected with MWPC) and

CB


0 identification invariant mass

Only CB information used


ppn/ppn0 separation


Total inclusive vs partial channels

First data

Preliminary

Good agreement between inclusive method and the sum of partial channels


GDH sum rule on the deuteron ∙∙Running GDH integral for the

deuteron

GDH Mainz GDH BonnAFS model

ν0 = 200 MeV, Eγ = 1.8 GeV

Assuming that, in this measured energy region, the incoherent, quasi-free meson production processes dominate:

93.0)( pGDH

nGDH

deuteron III exp

bI de xp 249452

bI pGDH 125255

bI nGDH 230

with:

From GDH sum rule:

bI nGDH 233

Documents

Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop