General search for new phenomena in ep scattering at HERA [Elektronische Ressource] / Deutsches Elektronen-Synchrotron in der Helmholtz-Gemeinschaft. By M. Wessels

Martin Wessels

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2004
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 5 Mo

Extrait

General Search for New Phenomena
in ep Scattering at HERA
Von der Fakultat fur Mathematik, Informatik und Naturwissenschaften
der Rheinisch-Westfalischen Technischen Hochschule Aachen
zur Erlangung des akademischen Grades eines Doktors
der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker
Martin Wessels
aus Hannover
Berichter: Universitatsprofessor Dr. Ch. Berger
apl. Professor Dr. W. Braunschweig
Tag der mund lichen Prufung: 29. Juli 2004
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfugba r.Abstract
A model-independent search for deviations from the Standard Model predic-
+tion is performed in e p and e p collisions at HERA using H1 data corre-
1sponding to an integrated luminosity of 117pb . For the rst time all event
topologies involving isolated electrons, photons, muons, neutrinos and jets
with high transverse momenta are investigated in a single analysis. Events
are divided into exclusive event classes according to their nal state. A novel
statistical algorithm is used to search for deviations from the Standard Model
in the distributions of the scalar sum of transverse momenta and invariant
mass of nal state particles and to quantify their signi cance. A good agree-
ment with the Standard Model prediction is observed in most of the event
classes and one interesting event is measured with four jets and an electron.
The most signi cant deviation is found in a topology containing an isolated
muon, missing transverse momentum and a jet, where a deviation has been
previously reported.
Kurzfassung
In dieser Arbeit wird eine modellunabh angige Suche nach Abweichungen von
+der Vorhersage des Standardmodells in e p und e p Kollisionen bei HERA
durchgefuhr t. Die analysierten H1-Daten entsprechen einer integrierten Lu-
1
minosit at von 117pb . Erstmals werden alle Ereignistopologien, die isolierte
Elektronen, Photonen, Myonen, Neutrinos und Jets mit hohen Transversal-
impulsen enthalten, in einer einzigen Analyse untersucht. Entsprechend ihres
Endzustandes werden alle Ereignisse in exklusive Ereignisklassen unterteilt.
Ein neuartiger statistischer Algorithmus wird benutzt, der in den Verteilun-
gen der skalaren Summe der Transversalimpulse und der invarianten Masse
nachAbweichungenvomStandardmodellsuchtundderenSigni kanzenquan-
ti ziert. In den meisten Ereignisklassen wird eine gute Ubereinstimmung mit
der Standardmodellvorhersage beobachtet, und ein interessantes Ereignis mit
vier Jets und einem Elektron wird gemessen. Die signi kanteste Abweichung
wirdineinerEreignistopologiemiteinemisoliertenMyon,fehlendemTransver-
salimpuls und einem Jet festgestellt, einer Topologie, in der schon in fruheren
Analysen Abweichungen beobachtet wurden.Contents
Introduction 1
1 Standard Model Physics at HERA 5
1.1 Inclusive Electron-Proton Scattering . . . . . . . . . . . . . . . . . . . . . 5
1.1.1 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.2 Cross Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1.3 Virtual Photon-Proton Scattering . . . . . . . . . . . . . . . . . . . 9
1.1.4 Structure Functions in QPM and QCD . . . . . . . . . . . . . . . . 11
1.2 Exclusive Final States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.2.1 Jet Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.2.2 Photon Production . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.3 Lepton Pair Production . . . . . . . . . . . . . . . . . . . . . . . . 18
1.2.4 W Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 The H1-Experiment at HERA 21
2.1 The Electron-Proton Accelerator HERA . . . . . . . . . . . . . . . . . . . 21
2.2 The H1 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.1 General Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 Inner Tracking System . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.2.3 Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2.4 Central Muon Detector . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2.5 Luminosity System . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3 Trigger and Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4 Reconstruction of the Kinematics . . . . . . . . . . . . . . . . . . . . . . . 34
3 Monte Carlo Simulation 37
3.1 Monte Carlo Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4 Data Selection and Classi cation 41
4.1 Analysis Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.3 Particle Identi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
iii Contents
4.3.1 Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3.2 Photons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.3.3 Muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3.4 Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.3.5 Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4 Event Classi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5 Experimental Performance 67
5.1 Energy Measurement and Calibration . . . . . . . . . . . . . . . . . . . . . 67
5.1.1 Electromagnetic Energy Scale . . . . . . . . . . . . . . . . . . . . . 67
5.1.2 Hadronic Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.2 Trigger E ciencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3 Limitations of the Measurement . . . . . . . . . . . . . . . . . . . . . . . . 75
5.3.1 The e- Event Class . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.3.2 The - Event Class . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.4 Resolutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.5 Purities and E ciencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.6 Systematic Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.7 Multi-Jet Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6 Experimental Results 91
7 Search for Deviations 101
7.1 Search Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
7.2 Monte Carlo Experiments and Global Signi cance . . . . . . . . . . . . . . 105
7.3 Sensitivity to New Physics Signals . . . . . . . . . . . . . . . . . . . . . . . 107
7.4 Search Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
Summary 121
A Resolutions 123
B Purities and Eciencies 127
C Systematic Uncertainties 131
D Event Display and Tables of the Results 133
List of Figures 137
List of Tables 141
Bibliography 143Introduction
Today, the knowledge gained by scientists about the structure of matter is summarised
in the Standard Model of particle physics, which has been developed in the second half of
the last century and has proved very successfully in describing all experimentally results
in the eld of high energy physics. The basic assumptions of the Standard Model are
simple and can be written down in a few lines.
1Matteriscomposedofelementaryfermionswithspin ,quarksandleptons,eachoccurring
2
in three families consisting of two particles. Each of the 12 particles has a corresponding
anti-particle with opposite charge but otherwise identical properties.

u c te
leptons: quarks: .
e d s b
These elementary particles are subject to three fundamental forces, which are the strong,
the electromagnetic and the weak force. Gravity, the forth known fundamental interac-
tion, has not yet been included in the Standard Model. The interactions between the
particles composing matter are mediated by bosons carrying spin 1. These exchange
0 particles are the massless photon for the electromagnetic force, the massive Z and W
bosonsfortheweakforceandeightmasslessgluonsmediatingthestronginteraction. The
third kind of particle in the Standard Model is the Higgs boson, which is responsible for
the creation of particle masses.
Towardsagranduni cationofthefundamentalforces,theelectromagneticandweakforce
are combined in the Standard Model into the electroweak interaction. The gauge theory
describing the strong interaction is the Quantum Chromodynamics.
The complex experiments nowadays arranged within the eld of high energy physics to
verify the predictions of the Standard Model are based on the same principle, which has
already been utilised by Rutherford and his assistants Geiger and Marsden in their ex-
periments revolutionising physics at the start of the 20th century. Aiming alpha particles
on a thin gold foil, Rutherford deduced from the rate at which the alpha particles have
beenscatteredatspeci cangles, thatgoldatomsmustbelargelymadeupofemptyspace
containing a small heavy core, which carries nearly all of the mass and the total positive
charge of the gold atom. The atomic nucleus had nally been discovered.
Since then our understanding of nature has improved, nally resulting in the concept