The Spin Structure of the Proton

212 Pages·2007·4.173 MB·English

by  Steven Bass

The Spin
Structure
of the Proton
TThhiiss ppaaggee iinntteennttiioonnaallllyy lleefftt bbllaannkk
11 11
== SSDD q q ++ DD gg + + LLqq ++ LLgg
22 22
qq
STEVEN D. BASS
Innsbruck University, Austria
world scientific
NEW JERSEY . LONDON . SINGAPORE . BEIJING . SHANGHAI . HONG KONG . TAIPEI . CHENNAI
Published by
World Scientific Publishing Co. Pte. Ltd.
5 Toh Tuck Link, Singapore 596224
USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601
UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE
British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.
THE SPIN STRUCTURE OF THE PROTON
Copyright © 2008 by World Scientific Publishing Co. Pte. Ltd.
All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or
mechanical, including photocopying, recording or any information storage and retrieval system now known or to
be invented, without written permission from the Publisher.
For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center,
Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from
the publisher.
ISBN-13 978-981-270-946-2
ISBN-10 981-270-946-0
ISBN-13 978-981-270-947-9 (pbk)
ISBN-10 981-270-947-9 (pbk)
Printed in Singapore.
Alvin - The Spin Structure.pmd 1 10/2/2007, 9:22 AM
October2,2007 9:9 WorldScienti(cid:12)cBook-9.75inx6.5in bass
To C(cid:19)eline
v
October2,2007 9:9 WorldScienti(cid:12)cBook-9.75inx6.5in bass
TThhiiss ppaaggee iinntteennttiioonnaallllyy lleefftt bbllaannkk
October2,2007 9:9 WorldScienti(cid:12)cBook-9.75inx6.5in bass
PREFACE
The Spin Structure of the Proton is one of the most challenging open puzzles in
QuantumChromodynamics. Keyparticlephysicsexperimentsindeepinelasticscat-
tering suggest that just 30% of the spin of the proton is carried by the intrinsic
(cid:24)
spin of its quark constituents { considerably less than the prediction of relativistic
constituent quark models ( 60%). This discovery has challenged our understand-
(cid:24)
ing about the internal structure of the proton and inspired vast experimental and
theoreticalactivitytounderstandthe roleofspininthe proton’sinternalstructure:
about 1000 theoretical papers and a new programme of dedicated experiments at
CERN and DESY in Europe, and BNL, JLab and SLAC in the United States.
Until recently, the main experimental activity has focussed on fully inclusive
measurements of the proton’s g spin structure function with longitudinally polar-
1
ized targets. New experiments are underway to measure the separate (cid:13)avour- and
spin-dependent parton distributions for the proton’s valence quark, sea quark and
gluonic constituents, and to investigatethe spin structure of transverselypolarized
protons. The important questions are: How is the spin of the proton built up out
fromtheintrinsicspinandorbitalangularmomentumofitsquarkandgluoniccon-
stituents? What happens to spin and orbital angular momentum in the transition
from current quarks to constituent quarks in low-energy QCD? Does the proton
spinpuzzleinvolvethe suppressionof the valencequarkspincontributionordothe
seaquarksandgluonsconspiretoreducethetotalquarkspincontentintheproton?
Here we givean overviewof the presentstatus of ourunderstanding: How does the
proton spin?
Steven D. Bass
vii
October2,2007 9:9 WorldScienti(cid:12)cBook-9.75inx6.5in bass
TThhiiss ppaaggee iinntteennttiioonnaallllyy lleefftt bbllaannkk
October2,2007 9:9 WorldScienti(cid:12)cBook-9.75inx6.5in bass
CONTENTS
PREFACE vii
1. INTRODUCTION 1
1.1 Spin and the proton spin problem . . . . . . . . . . . . . . . . . . 3
1.2 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2. SPIN EXPERIMENTS AND DATA 11
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Scaling and polarized deep inelastic scattering . . . . . . . . . . . 14
2.3 The QCD parton model . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Spin experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5 Spin data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.6 Regge theory and the small x behaviour of spin structure
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3. DISPERSION RELATIONS AND SPIN SUM RULES 31
3.1 Spin amplitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 Light cone dominance and the operator product expansion . . . . 36
3.3 The QCD parton model . . . . . . . . . . . . . . . . . . . . . . . 39
3.4 Parton distributions and light-cone correlationfunctions . . . . . 43
3.5 Renormalization group and QCD evolution . . . . . . . . . . . . . 45
3.6 Polarized partons and high-energy proton-protoncollisions . . . . 47
4. g SPIN SUM RULES 51
1
4.1 The (cid:12)rst moment of g . . . . . . . . . . . . . . . . . . . . . . . . 51
1
4.2 SU(3) breaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.3 (cid:23)p elastic scattering . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.4 The Burkhardt-Cottingham sum rule . . . . . . . . . . . . . . . . 62
4.5 The Gerasimov-Drell-Hearnsum rule . . . . . . . . . . . . . . . . 64
4.6 The transition region . . . . . . . . . . . . . . . . . . . . . . . . . 67
ix