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Progress in Particle and
Nuclear Physics
PERGAMON Progress in Particle and Nuclear Physics 44 (2000) 453454 -
The GDH-Experiment at MAMI/Mainz
B. LANNOY
http:l/www.elsevier.nl/locate/ppartnuclphys
On Behalf of the AZ/GDH Collaboration Labo voor Subatomaire en Stralingsfysica, Universiteit Gent, Proeftuinstraat 86, 9000 Gent, Belgium
The GDH experiment at Mainz has taken its first data in May 98. The first prelim-
inary results are presented using only a small subset of the data in the acceptance
of the central DAPHNE detector.
1 Introduction
The Gerasimov-Drell-Hearn (GDH) sum rule connects static properties of the nucleon like the anoma-
lous magnetic moment IC, the nucleon mass M and charge e, with the helicity dependent total absorption
cross sections ui/z and ~312 for the absorption of circularly polarized photons on longitudinally polar-
ized nucleons. The integral over the photon energy w of the cross section difference, weighted by the
inverse of w, is related to K, in the following way:
s O3 ~3/2bJ) -w2(4dw _ 27r2an2
0 W m2
The forward spin polarizability 70 can be written in a similar way:
(1)
70 = / O” a3/2(W) - ‘?/dw)dW
0 W3
The derivation of the sum rule is based on fundamental physical principles (Lorentz and gauge invari-
ance, causality, unitarity and the non-subtraction hypothesis). The verification of this sum rule has
become possible with the recent developments in doubly-polarized experiments.
The aim of the GDH collaboration is to test the validity of the sum rule on both the proton and neutron
at MAMI (Mainz) (m, SE, 5800 MeV) and ELSA (Bonn) (500 MeV 5 E, 5 3 GeV) accelerators. In
the following an overview of the Mainz experiment will be presented together with preliminary results
for the cross section difference crai2 - (~~1~.
2 Experimental Setup
The circularly polarized photons were produced by bremsstrahlung of longitudinally polarized electrons
(x 70 - 80% polarization, strained GaAs source) coming from the MAMI accelerator (Mainz). The
0146-6410/00/$ - see front matter 0 2000 Published by Elsevier Science BY All rights reserved. PII: SO146-6410(00)00095-8
454 B. Lannoy / Prog. Part. Nucl. Phys. 44 (2000) 453-454
polarized nucleons were contained in a frozen spin butanol (C4HaOH) target (Bonn, Bochum, Nagoya)
(50 mK, max. 87 %, T z 200 hours). The detector setup consisted of the central detector DAPHNE
800
600
0
200 400 600 800 1000 1200 1400 1600
E, IMeV
Figure 1: Preliminary data on the helicity difference 1~312 - c71/2 compared to MAID
(Saclay, Pavia), complemented by several forward detector components to increase the accepted angular
range. The DAPHNE-detector is made up of 3 cylindrical multi-wire proportional chambers surrounded
by a segmented AE- E- AE plastic scintillator telescope and a double scintillator-absorber sandwich.
This detector is capable of detecting charged particles with high efficiency and neutral particles with
a moderate efficiency. The additional forward detectors are the silicon microstrip device MIDAS v
(Pavia), a Cerenkov counter (Gent) for the suppression of electromagnetic background, the annular
ring detector STAR (Tiibingen) and a forward sandwich counter (Mainz).
3 Preliminary results and discussion
The following preliminary results were produced using only = 10 % of the data obtained with the
central DAPHNE detector. Fig. 1 shows the helicity difference of the total cross section 0312 - ~112
from 200 to 800 MeV. The data are compared with the MAID model. The integration of the data
according to eq.1 gives a value of 230 f 20 pb in the 200 - 800 MeV range. Using estimates from
the HDT model for low and high energy contributions, one obtains a value of 225 f 20 pb, which is
compatible with the theoretical value of 204 pb.
The value for the forward spin polarizability yo in the measured energy range yields (170 f 30).10M6 fm4
Using again HDT calculations to evaluate missing contributions (-1OO.1O-6 for low energies and 2.10-”
for higher energies), a value of (72 f 30).10e6 fm4 is obtained.
The analysis of the Mainz data results in a value for the GDH sum rule close to the theoretical
value. Contributions from higher energies need to be measured in Bonn (end of 1999) to obtain more
information on the high energy behaviour of the integral.