QCD at HERA: The Hadronic Final State in Deep Inelastic Scattering

170 Pages·1999·2.771 MB·English

by  Michael Kuhlen (auth.)

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QCD at HERA
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With 99 Figures
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Dr. Michael Kuhlen
Max-Planck-Institut fiir Physik
Werner-Heisenberg-Institut
F6hringer Ring 6
D-8o8o5 Miinchen
Email: [email protected]
Physics and Astronomy Classification Scheme (PACS):
12.38.-t, 13.6o.-r, 13.87.-a
ISSN oo81-3869
ISBN 3-540-65118-7 Springer-Verlag Berlin Heidelberg New York
Library of for. applied Data Cataloging-in-Publication Congress eiD Deutsche kehtoilbiB - PIC Einheitsaufnahme
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Preface
HERA ~ is the world's first electron-proton collider. 2 The large centre-of-mass
(CM) energy of 300 GeV allows the exploration of new regimes: new parti-
cles with masses up to 300 GeV can be produced, the structure of the proton
can be studied with a resolving power varying over 5 orders of magnitude
down to dimensions of 10-is m, and partons with very small fractional pro-
ton momenta (Bjorken x down to 10 -6) become experimentally accessible.
The hadronic final state which emerges when the proton breaks up has an
invariant mass of up to 300 GeV. It provides a laboratory to study quan-
tum chromodynamics (QCD) under varying experimental conditions, which
can be controlled by measuring the scattered electron. The hadronic final
state carries information on the structure of the proton. It is made use of to
study the dynamics of the proton's constituents, complementary to structure
function measurements.
In contrast to "clean" e+e - interactions, the initial state in ep collisions
already contains a strongly interacting particle. That makes the physics more
complicated, but leads also to interesting effects that cannot be studied in
e+e - collisions. At HERA, well-established fields are being studied, exploit-
ing the tunable kinematic conditions. These comprise jet physics and com-
parisons with perturbative QCD to extract the strong coupling a8 and the
density of gluons in the proton, or for example the measurement of fragmen-
tation functions. Other topics have only blossomed with the advent of HERA.
To name a few, a class of events with large rapidity gaps and small x physics
allow longstanding problems in QCD to be addressed, which are connected
with scattering cross-sections at high energies and confinement. There are
also exotic effects like instanton-induced reactions, which, if discovered at
HERA, would alter significantly our view of particle physics.
Purpose
This work provides a review of the hadronic final state measurements at
HERA in deep inelastic scattering (DIS). The emphasis is on experimen-
tal results, because in many cases at HERA, experiment is driving theory.
1 "Hadron-Elekt ron-Ring-Anlage"
2 HERA can operate with either electrons or positrons. In the following, the generic
name electron is used for electrons as well as for positrons.
VI Preface
Many measurements are being performed without any theoretical prediction,
and often they have not yet found an unambiguous theoretical explanation.
Nevertheless, the results are discussed in the context of the theory, where
possible.
The review of the experimental situation is complete up to the fall of
1997, with a few updates concerning the situation in summer 1998. It can
therefore be consulted for quick access to the HERA data. Basic concepts are
explained in a partly pedagogical fashion to serve physicists from outside the
HERA community and newcomers to HERA physics.
Contents
In Chap. 1 the HERA machine and the 1H and ZEUS experiments are intro-
duced, and the kinematic variables are discussed. In Chap. 2 the theoretical
framework of deep inelastic scattering is set up with evolution equations and
a special section on the interest in small x physics. The hadronic final state
should not be discussed without knowledge of the inclusive ep cross-section
and the proton structure functions (Chap. 3). In Chap. 4 we start with simple
models for hadron production. They are subsequently refined in and serve as
the basis for the discussion of the data.
Measurements of basic event properties are presented in Chap. :5 energy
flows, charged particle spectra, charm and strangeness contents, and Bose-
Einstein correlations. The fragmentation of the scattered quark is studied in
Chap. 6 and compared with quark fragmentation in e+e - annihilation and
QCD calculations. Measurements of event shape variables offer a new view
of hadronization properties with "power corrections", offering a potentially
powerful tool for measurements of the strong coupling .s~c Jet production
has been compared to perturbative QCD predictions to measure as and the
gluon density in the proton (Chap. .)7 By measuring jet rates, regions of
phase space have been identified where the measured jet rates are not well
understood yet, and where the underlying physics possibly departs from the
conventional picture of deep inelastic scattering.
Also the energy flow measurements at small x have not yet found an un-
ambiguous theoretical interpretation. In Chap. 8 on low x physics dedicated
searches for "footprints" of new QCD effects (BFKL) are discussed: energy
flows, high PT particles and "forward jets". Chapter 9 deals with the possibil-
ity of discovering QCD instantons at HERA, which would have far reaching
consequences for our understanding of field theories and for cosmology.
Some related topics and neighbouring fields could only be touched upon.
The interested reader is referred to other reviews on structure functions ,1 ,2
rapidity gaps 3, photoproduction 4 and on hadron production at fixed
target experiments .5
Throughout, unless stated otherwise, the data shown have been corrected
for detector effects and QED radiation, and the errors comprise statistical
and systematic errors added in quadrature.
Preface VII
stnemegdelwonkcA
This work has been made possible by a grant from the Deutsche Forschungs-
gemeinschaft, which allowed me to write a "Habilitation". The result is this
book. I would like to thank G. Buschhorn from the Max-Planck-Institut flit
Physik in Munich for the hospitality I have experienced in his group during
the last two years. I have enjoyed very much the close collaboration in the
MPI group, in particular with F. Botterweck, T. Carli and G. Grindhammer.
I wish to thank all my colleagues from the H1 and ZEUS experiments,
on whose research this work is based, for their collaboration and friendly
competition at HERA. I would like to thank J. Dainton, E. Lohrmann, B.
Naroska and A. Wagner for their continuous encouragement and their support
to undertake the Habilitation and to teach at the Universit~it Hamburg.
I am grateful to J. Bartels, E. De Wolf, A. Martin, G. Ingelman, A.
Ringwald and F. Schrempp for their patient explanations of sometimes heavy
theory, and for enlightening discussions, in which some very fruitful ideas were
born. Helpful discussions with B. Andersson, Y. Dokshitzer, D. Graudenz, L.
LSnnblad, M. Seymour, S. Lang, E. Mirkes and B. Webber are gratefully
acknowledged.
Finally, I would like to thank J. Bartels, N. Brook, T. Carli, T. Doyle,
A. De Roeck, E. De Wolf, G. Grindhammer, J. Hartmann, G. Ingelman, H.
Jung, A. Levy, E. Lohrmann, U. Martyn, D. Milstead, B. Naroska, N. Pavel,
J. Repond, A. Ringwald, F. Schrempp, F. Sefkow and V. Shekelyan for their
comments on the manuscript, or parts of it. Their critical reading has been
invaluable.
Hamburg, August 1998 Michael Kuhlen

Contents
1. Introduction ............................................. 1
1.1 The HERA Machine ................................... 1
1.2 The H1 and ZEUS Detectors ............................ 1
1.3 Kinematics ............................................ 3
. Theoretical Framework
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Deep Inelastic Scattering ............................... 9
2.2 Evolution Equations ................................... 11
2.3 The DGLAP Equations ................................. 13
2.4 The BFKL Equation ................................... 16
2.5 The Interest in Small x ................................. 18
2.6 Hadronic Final States .................................. 23
. Inclusive Cross-Sections .................................. 27
3.1 The Structure Function F2 ............................. 27
3.2 QCD Analysis of F2 ................................... 30
3.3 The Longitudinal Structure Function FL .................. 31
3.4 p argot and the Transition
Between DIS and Photoproduction ....................... 34
4. Models for Hadron Production in DIS ................... 35
4.1 Reference Frames ...................................... 35
4.2 Kinematics and Variables ............................... 37
4.3 Simple Mechanisms for Hadron Production ................ 38
4.4 Perturbative QCD Radiation ............................ 41
4.5 Hadronization ......................................... 46
4.6 Rapidity Gaps ......................................... 51
4.7 Monte Carlo Generators ................................ 54
. General Event Properties ................................ 57
5.1 Energy Flow .......................................... 57
5.2 Charged Particle Multiplicities .......................... 60
5.3 Charged Particle Momentum Spectra ..................... 68
5.4 Strangeness Production ................................. 76
X Contents
5.5 Charm Production ..................................... 78
5.6 Bose-Einstein Correlations .............................. 84
. The Quark Fragmentation Region ........................ 87
6.1 Charged Particle Spectra ............................... 87
6.2 Event Shapes .......................................... 97
7. Jets ...................................................... 103
7.1 Introduction .......................................... 103
7.2 Jet Algorithms ........................................ 104
7.3 Jet Shapes ............................................ 104
7.4 The Strong Coupling as ................................ 105
7.5 The Gluon Density ..................................... 111
7.6 Jet Rates at Low Q2 ................................... 113
. Low-x Physics ........................................... 119
8.1 Introduction .......................................... 119
8.2 Energy Flows ......................................... 121
8.3 Transverse Momentum Spectra .......................... 128
8.4 Forward Jets .......................................... 134
8.5 Summary and Outlook ................................. 136
9. Instantons ............................................... 139
9.1 Introduction ........................................... 139
9.2 Instanton Theory ........................................ 139
9.3 Instantons at HERA .................................... 141
9.4 Experimental Signatures ................................ 144
9.5 Searches for Instanton Processes .......................... 147
9.6 Conclusion ............................................ 150
10. Summary ................................................. 151
References .................................................... 155
Index ......................................................... 167