1. Helicity dependence of g n ® Nπ(π) and the GDH integral on the neutrom
Update of the
Proposal A2-9/2005
Spokepersons:
P. Pedroni INFN-Sezione di Pavia, Italy
J. Annand University of Glasgow, UK
W.J. Briscoe GWU, Washington, USA
J.Krimmer IP, University of Mainz, Germany
O.Jahn IKP, University of Mainz, Germany

2. Goals of the experiment
First measurement of the total helicity dependent cross section on 3He [ MeV]
(Substantial) statistical improvement of the existing total helicity dependent deuteron data in the photon energy range [ MeV]
First measurement of the helicity dependent g n ® Np(p) channels (mainly using deuterons) [ MeV]
NEW

3. Experimental verification of the GDH sum rule
Proposed in 1966
Prediction on the absorption of circularly polarized photons by longitudinally polarized hadrons
photon-spin
hadron-spin
photon-spin
hadron-spin sa sp
Anomalous magnetic moment

4. Why could the GDH sum rule be violated ?
The only “weak” hypothesis is the assumption that Compton scattering gN ® g’ N’ becomes spin independent when n ® ¥
A violation of this assumption can not be easily explained
Possible hypotheses for violation:
Exchange of a1-like (J=1+) mesons between g and N
Non pointlike (constituent) quarks ?
Photoproduction of gravitons ?

5. GDH sum rule predictions
3He m
2.79
-1.91
0.86
-2.13 k
1.79
-1.92
-0.14
-8.37
IGDH
204
233
0.65
498
(n.m.)
(mb)
“naive” expectations

6. AFS model Arenhoevel, Fix, Schwamb, PRL 93, 202301 (04) pNN
pN from MAID +nuclear effects
ppNN
EPJA 25,114 (05)
p0d
PLB 407,1 (97) pn
NPA 690,682 (01)

7. Model from Golak-Gloeckle

8. GDH sum rule on the proton
Eg (GeV)
IGDH (mb) *
MAID03
SAID04
-29
-28
Measured
(Mainz+Bonn)
254±5±12
> 2.90
(Regge approach)
Simula et al.
Bianchi-Thomas
-13
-14
Total
211±5±12
GDH sum rule
205
* Low energy theorems in the Np threshold region (multipole analyses not very wrong ...)

9. GDH sum rule: predictions (2009)
Proton IGDH (mb)
Neutron IGDH (mb)
g p ® Np [164]
g p ® Npp
g p ® Nh
g p ® KL (S)
g p ® Nr(w)
Regge contrib
(Eg > 2 Gev)
g n ® Np [131]
g n ® Npp
g n ® Nh
g n ® K L(S)
g n ® Nr(w)
Regge contrib
TOTAL
239 [231]
244 [231]
GDH
205
233
Np : SAID-FA07K [MAID07]
KL(S) : Sumowidagdo et al., PRC 65, (02)
Npp : Fix, Arenhoevel EPJA 25, 114 (2005)
Nh : MAID
Nr : Zhao et al., PRC 65, (03)
Regge : Bianchi-Thomas , PLB 450,439(99)

10. GDH data Sensitivity to E2-*M2- PRL 88, 232002, 2002 SAID (SM01)
to reproduce the data:
MAID2000
Sensitivity to E2-*M2-

11. GDH sum rule on the neutron
No Free neutron target available
Model dependent results from nuclear targets
Experimental goal: to have a “small” and “realiable” model dependence
Two different (and complementary) targets
=) deuterated butanol
=) high pressure 3He target
Measurement of partial channels like

12. GDH sum rule on the neutron
p n
2H: m ~ mp + mn Þ ­ ­
IGDHneutr. ~ IGDHdeut.-IGDHprot. -INuclear
3He: m ~ mn Þ ­ ­ ¯ (S-state with ~ 90% prob.)
IGDHneutr. ~ IGDHHe3-a·IGDHprot. -INuclear
(a ~ 10%)
3He better suited to get IGDHneutr (inclusive method)
2H better suited to measure partial channels

13. Status on the deuteron results

14. Results on the deuteron
Statistical precision to be improved
AFS
MAID

15. Status on the 3He results

16. Measurements of different observables on many partial channels
Partial channel measurement
e.m. interaction does not conserve isospin
3 different reaction amplitudes A0 A1/2 A3/2
measurements on both the proton and the neutron are needed
Different N* decay modes:
Determination of the baryon resonance properties
Measurements of different observables on many partial channels

17. Complete characterization of the different isospin components
Different resonance excitations

18. Helicity dependent cross sections

19. Unpolarized cross sections

20. ° GDH g p -> n p+ Preliminary · GDH coll Preliminary
Fix -Arenhoevel
Levchuk et al.
° GDH g p -> n p+
EPJA 26, 135 (05)
Preliminary

21. Experimental set-up CB+TAPS+PID+MWPC
Longitudinally polarized Deuterated Butanol
Longitudinally polarized 3He gas target (6 bars)
(presentation by J.Krimmer)
Moeller Polarimeter
Cerenkov detector (online suppression of e.m. background / very open CB-TAPS trigger)

22. Total inclusive cross section
(not feasible)
(inclusive method)
For each partial reaction channel, at least one reaction product has to be detected with (almost) complete acceptance (solid angle & efficiency)
detector with a very high acceptance/particle detection efficiency (CB+TAPS: 97% of 4p)
Suppression of e.m. events (pair prod./Compton) at forward angles

23. Beam time estimate Deuteron: 950 hours
related to unpolarized target nucleons
3 different electron beam energies: 525, 850, 1500 MeV
Deuteron: hours
3He (6 bars) : hours (preliminary evaluation)
now
Total inclusive:
Single pion: