susanna costanza (infn pavia) on behalf of the a2 collaboration international workshop
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Measurement of helicity dependent total inclusive γ - 3 He cross section and the GDH sum r ule on the neutron. Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop “ Meson Production at Intermediate and High Energies ” - PowerPoint PPT PresentationTRANSCRIPT
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
Susanna Costanza 3Messina, 11 /11/11
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
Susanna Costanza Messina, 11 /11/11 5
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!!!
Susanna Costanza Messina, 11 /11/11 6
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?
Susanna Costanza 7Messina, 11 /11/11
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)
Susanna Costanza 8Messina, 11 /11/11
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
Susanna Costanza 9Messina, 11 /11/11
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)
Susanna Costanza 10Messina, 11 /11/11
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)
Susanna Costanza Messina, 11 /11/11 11
Experimental setup for 3He exp.
TAPS
CRYSTAL BALL
PID
MWPC
Susanna Costanza 12Messina, 11 /11/11
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!
Susanna Costanza 13Messina, 11 /11/11
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
Susanna Costanza 14Messina, 11 /11/11
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...
Susanna Costanza 16Messina, 11 /11/11
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
Susanna Costanza 17Messina, 11 /11/11
Partial channel I: 3He(γ,)XFirst data
Preliminary
Nuclear structure contribution (FSI, …)more important for the 0X partial channel
Susanna Costanza 18Messina, 11 /11/11
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
Susanna Costanza 19Messina, 11 /11/11
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...
Susanna Costanza 21Messina, 11 /11/11
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
Susanna Costanza 22Messina, 11 /11/11
Partial channel I: 3He(γ,)XFirst data
Preliminary
Nuclear structure contribution (FSI, …)less important than for the unpolarised case
Susanna Costanza 23Messina, 11 /11/11
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
Susanna Costanza 24Messina, 11 /11/11
Rough derivation of IGDH(n)
Susanna Costanza 25Messina, 11 /11/11
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
Susanna Costanza 26Messina, 11 /11/11
Backup slides
Susanna Costanza 27Messina, 11 /11/11
±/p separation
p
e
dE/dx – E techinque usedEnergy information from PID (corrected with MWPC) and
CB
Susanna Costanza 28Messina, 11 /11/11
0 identification invariant mass
Only CB information used
Susanna Costanza 29Messina, 11 /11/11
ppn/ppn0 separation
Susanna Costanza 30Messina, 11 /11/11
Total inclusive vs partial channels
First data
Preliminary
Good agreement between inclusive method and the sum of partial channels
Susanna Costanza 31Messina, 11 /11/11
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