Results from the GDH experiment at Mainz and Bonn

Alessandro Braghieri, INFN Pavia

A2 and GDH Collaborations

• First measurements in double polarization: physical motivation• Experimental set-up• Results: Gerasimov-Drell-Hearn sum rule and resonance contribution• Comments

October 12-15, 2005

Proton = 205 b

Neutron = 233 b

Derivation of the GDH sum rule

Forward ( =0) Compton scat tering amplitude (Lorentz and gauge invariance)

)()()()( 21 ifif fff

Dispersion relation without subtraction

022

22 )(

)(Im2)(Re

d

ff

Optical theorem

Low energy theorem

)()(8

)(Im 2/32/12 f

222

0 2)(lim

mf

22

2

0

213/2 κm

α2πdν

ν

(νσ(νσ

GDH sum rule:

Fundamental check of our knowledge of the N interaction

Important comparison for nucleon models

Valid for any system with (2H, 3He) . “Bridge” between nuclear and particle physics

The same formalism leads to a relation for the so-called forward spin polarizability

03

2/30

)(

4

1

dEE

Connection with the ‘Spin Crisis’ Measurement of the polarized structure function on the nucleon (D.I.S., EMC 1985 …)

114.0),(1

0

2

1 xdQxgp

How the proton spin is shared between partons?

gqqqspin Fraction carried by valence q

EMC QCD npQCD

0!!! 75% 10-50%

Anselmino , Ioffe, Leader, Sov. J. Nucl. Phys. 49:136, 1989

),(2

)( 21

0

12

22 Qxgdx

Q

mQI GDH)(0 22 QIQ

1991, OctHELICITY STRUCTURE OF PION PHOTOPRODUCTIONSpokeperson: D. Sober (CUA),Proposal E91-015 at CEBAF

MEASUREMENT OF POLARIZED STRUCTURE FUNCTIONS IN INELASTIC ELECTRON PROTON SCATTERING USING THE CEBAF LARGE ACCEPTANCE SPECTROMETERSpokepersons: V. Burkert (Jlab), D. Crabb(Virginia U.), Minehart (Virginia U.)Proposal E91-023 at CEBAF

1992, AprPROPOSAL TO MEASURE THE DRELL-HEARN-GERASIMOV SUM RULEApril 1992, proposal at ELSA

1993, AprEXPERIMENTAL CHECK OF THE DRELL-HEARN-GERASIMOV SUM RULESpokeperson: J. Ahrens (Mainz U.)Proposal A2/2-93

2000HELICITY DEPENDENCE OF p --> N BELOW 450-MeV AND CONTRIBUTION TO THE GERASIMOV-DRELL-HEARN SUM RULEBy GDH and A2 Collaborations (J. Ahrens et al.),Published in Phys. Rev. Lett. 84:5950-5954, 2000

…Milestones

The GDH collaboration

J.Ahrens9,S.Altieri15,16,J.R.M.Annand6,G.Anton3,H.-J.Arends9,K.Aulenbacher9,R.Beck9, C.Bradtke1,A.Braghieri15,N.Degrande4, N.d'Hose5, H.Dutz2, S.Goertz1, P.Grabmayr17, K.Hansen8, J.Harmsen1, S.Hasegawa13,T.Hasegawa11, E.Heid9, K.Helbing3, H.Holvoet4, L.VanHoorebeke4, N.Horikawa14,T.Iwata13, P.Jennewein9, T.Kageya14, B.Kiel2,F.Klein2, R.Kondratiev12,K.Kossert7,J.Krimmer17,M.Lang9,B.Lannoy4,R.Leukel9,V.Lisin12, T.Matsuda11,J.C.McGeorge6,A.Meier1,D.Menze2,W.Meyer1,T.Michel3,J.Naumann3, A.Panzeri15,16, P.Pedroni15, T.Pinelli15,16, I.Preobrajenski9,12, E.Radtke1, E.Reichert10, G.Reicherz1,Ch.Rohlof2,D.Ryckbosch4,F.Sadiq6,M.Sauer17,B.Schoch2, M.Schumacher7, B.Seitz7,T.Speckner3,M.Steigerwald9, N.Takabayashi13, G.Tamas9, A.Thomas9, R.van de Vyver4, A.Wakai14, W.Weihofen7, F.Wissmann7, F.Zapadtka7, G.Zeitler3

1Institute of Experimental Physics, Ruhr-University, Bochum, Germany 2 Physics Institute, University of Bonn, Germany 3 Physics Institute, University of Erlangen-Nuernberg, Erlangen, Germany 4 Nuclear Physics Laboratory, Gent, Belgium 5 CEA Saclay, DSM/DAPNIA/SPhN, Gif-sur-Yvette, France 6 Department of Physics & Astronomy, University of Glasgow, U.K. 7 II.Physics Institute, University of Goettingen, Germany 8 Department of Physics, University of Lund, Sweden 9,10 Institute of Nucl. Physics and Inst. of Physics, University of Mainz, Germany 11 Faculty of Engineering, Miyazaki University, Miyazaki, Japan 12 INR, Academy of Science, Moscow, Russia 13,14 Department of Physics and CIRSE, Nagoya University, Nagoya, Japan 15,16 INFN Sezione di Pavia and Dept. of Nucl. Physics of the University , Pavia, Italy 17 Physics Institute, University of Tuebingen, Germany

Experimental set-up

Tagged photon beam Mainz: m E 800 MeV

Bonn: 0.6 GeV E 2.9 GeV

Circularly polarized photons

Bremsstrahlung of linearly polarized electrons

Longitudinally polarized Protons/Deuterons

Frozen spin butanol target (Bonn, Bochum, Nagoya)

Mainz: DAPHNE detector (Pavia, Saclay) + forward angle detectors (Pavia, Mainz, Tübingen, Gent)

Bonn: GDH Detector (Erlangen, Tübingen, Gent)

Large acceptance hadron detectors

Experimental Programme at MAMI and ELSA

Measurements of 3/2- 1/2 over a large photon energy range for proton and neutron (D/3He)

MAMI: m < E < 0.8 GeV

• Inclusive cross section • Partial channels:

00

0

0

p

n

p

n

p

p

00

0

0

n

p

n

p

n

n

ELSA: 0.7 < E < 3.1 GeV

• Inclusive cross section

1997 H 2001-02

(1998) 2003 D 2002

Status and planning

p0,n 200-450 PRL 84,2000

inclusive 200-800 GDH PRL 87,2001

p0 d/d 450-800 2nd res PRL 88,2002

Inclusive <1800 GDH PRL 91,2003

n

p

p

d/d

2nd res PLB 551,2003 ...

PL B624,2005

p d/d 800 2nd res EPJ A17,2003

np0 d/d 200-450 EPJ A21,2004

Inclusive <2900 GDH PRL 93,2004

Summary of results on the Proton

)(2

12/12/3

2/12/3

UNPOL

D13(1520)F15(1680

) F35(1905)

(1232)

Exclusive reactions

and

An important tool to understand resonances

Multipole Analysis

SAID & MAID

Interference enhances contribution of some

multipoles

SAID SM95UIM

J. Ahrens et al, PRL 88, 232002 (2002)

SAID SM02K

mod. UIM

20%:E

11%:M1/22

1/22

0pπpγ

nπpγ nπpγ

Change of sign / interference terms between multipoles

nπpγ

nπpγ

Differential cross sections:

• h=3/2• h=1/2

— MAID 03— SAID (SM02K)

The GDH sum rule for the neutron

bM

d

233

2)()( 22

2

0

2/12/3

• No free n target ► 2H and 3He• Extract GDH from bound Nucleon• Kinematics and Nuclear corrections

• 3He : n ↑ ↑ ↓ (S-state 90%)

• 2H : p + n ↑ ↑ (Partial channels)

• Data analysis in progress. Preliminary Results

D = p+n ??

Comparison between p and nH. Dutz et al, PRL 94 (2005)

Arenhövel, Fix, Schamb, Phys. Rev. Lett. 93 (2004)

npd

Ref.: Schwamb et al., Nucl. Phys. A690 (2001) 647, 682and Nucl. Phys. A696 (2001) 556

npd

Ref.: Schwamb et al., Nucl. Phys. A690 (2001) 647, 682and Nucl. Phys. A696 (2001) 556

Model features:

•NN-Ncoupled channelapproach,

•Complete retardation inhadronic interaction andpion-MEC

•Offshell contributionsto e.m. current

•No free parametersconcerning NNd

• Deut.data▬ Deuteron modelA.Fix Priv. communication▲ Free proton p -> p I.Preobrajenski, PhD Thesis,Mainz, 2001

spectatornpd 0

M.Martinez-Fabregate, PhD thesis

2/12/3 d

d

d

d

• GDH preliminary▬ Deuteron modelA.Fix, private communication+

spectatornnd

▼ free data EPJA 21,323 (04)

n

2/12/3 d

d

d

d

spectatorppd

MeVE 408 MeVE 478

Arenhoevel model

GDH data Preliminary

M.Martinez-Fabregate, PhD thesis

Conclusions

• Data taking to check the GDH sum rule was completed• Proton and Deuteron targets have been used• Most of the analysis on the proton finished (inclusive and partial channels)• Analysis on Deuteron in progress• Check of the partial channels on the bound neutron• Check of the methods using the bound proton• Comparison with the results with the (free) proton• Theoretical calculations