Detection of Trapped Antihydrogen

181 Pages·2013·5.977 MB·English

by  Richard Hydomako (auth.)

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Richard Hydomako
Detection of Trapped
Antihydrogen
Doctoral Thesis accepted by
University of Calgary, Canada
123
Author Supervisor
Dr. Richard Hydomako Prof.RobertI.Thompson
Department of Physicsand Astronomy Department of Physicsand Astronomy
Universityof Calgary Universityof Calgary
Calgary Calgary
Canada Canada
ISSN 2190-5053 ISSN 2190-5061 (electronic)
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Supervisors’ Foreword
For more than eight decades, antimatter has been a material of fascination and
conjecture for both physicist and the general public. From its postulation in the
1930s, through its initial generation in the pre- and post-war years, through more
than half a century of study, antimatter’s exotic nature, fleeting existence in our
laboratories, and highly reactive character have drawn the attention of everyone
from science fiction authors to Hollywood producers to experimental physicists.
Thetwomajorchallengesofstudyingantimatterare(1)itscreationand(2)storage
in isolation from the matter world surrounding us. The simplest forms of anti-
matter, the positron and the antiproton, are relatively straightforward to produce,
through radioactive decay in the former case and through high energy particle
collisions in the latter. Further, their charged nature makes them relatively
straightforward to store, through the use of strong ion traps that employ electro-
magnetic containment fields to hold the particles within vacuum chambers.
However, the simple structure and nonzero charge of these species make them
unsuitableforalmostanyprecisematter–antimattercomparisonsotherthancharge
or magnetic moment. Therefore, it is neutral atomic antimatter that provides the
bestopportunityforsuchprecisionstudies;thesimplestofwhichisantihydrogen,
made up of a positron orbiting an antiproton.
Found in 2005, the ALPHA Collaboration has been a CERN-based effort of
roughly 40 scientists from 15 international institutions created to pursue the
answers to some of the above listed questions through production, trapping, and
spectroscopic investigation of cold antihydrogen. Working within the Antiproton
DeceleratorfacilityatCERN,theALPHAcollects,cools,manipulates,andmixes
antiprotons and positrons in an effort to generate ultra-low temperature antihy-
drogen,forstorageinamagneticcontainmentfield.Oncestored,opportunitiesfor
microwaveandoptical spectroscopy and/or gravitational studies offer tremendous
possibilities for investigation of matter–antimatter symmetries or asymmetries.
In2010,TheALPHACollaborationachievedafirstforhumankind,thestable,
long-term storage of atomic antimatter, a project carried out at the Antiproton
Decelerator facility at CERN. This result caught worldwide attention, including
being ranked as the 2010s most important physics breakthrough by the Physics
v
vi Supervisors’Foreword
World Magazine. R. Hydomako’s Ph.D. thesis outlines the vital roles that he
playedintheachievementofthismilestone.Inadditiontobeingamemberofthe
core team that ran the experiment, his contributions to this result include: (a)
development of the software for tracking and reconstruction of antihydrogen
annihilations with the Si vertex detector, (b) the analysis of the detector data,
including development of a technique to distinguish the signal of trapped anti-
hydrogen from the main background process due to cosmic rays, (c) operation,
maintenance, and calibration of Si vertex detector hardware and the data acqui-
sition system. The reconstruction of antihydrogen annihilation events, and back-
ground rejections, were critical tasks, which had to withstand the scrutiny by the
international community, when we announced the result. As described in his
thesis,hehasdoneatrulyfinejobintheseimportanttasks,significantlyexceeding
the standard expectations for a graduate student. Furthermore, the technique of
identifying antihydrogen annihilations with the single atom sensitivity, which
Hydomako developed, forms an essential base for future precision studies of
matter–antimatter symmetries with trapped antihydrogen atoms.
Calgary, August 2012 Robert I. Thompson
Supervisor
University of Calgary
Makoto C. Fujiwara
Co-supervisor
TRIUMF/University of Calgary
Acknowledgments
Firstofall,IwouldliketoacknowledgemycolleaguesontheALPHAexperiment.
Working on this experiment has shaped me as a scientist, and I have benefited
enormously from the knowledge and support of everyone involved. To the senior
scientists, including: Dr. Paul Bowe, Prof. Joel Fajans, Dr. Dave Gill, Prof. Jeff
Hangst,Prof.NielsMadsen,Prof.ScottMenary,andProf.ArtOlin,Ihavelearned
atremendousamountfromallofyou.Toallthestudentsandpost-docs:Dr.Gorm
Andresen, Mohammad Ashkezari, Marcelo Baquero-Ruiz, Dr. Eoin Butler, Dr.
Will Bertsche, Steve Chapman, Tim Friesen, Andrea Gutierrez, Andrew
Humphries, Alex Povilus, Dr. Petteri Pusa, Dr. Matt Jenkins, Prof. Daniel de
MirandaSilveira,Dr.JamesStorey,SarahSeifElNasr-Storey,andChukmanSo—
I could not have asked for a better group. I have fond memories with all of you.
Particular thanks goes to my supervisors: Prof. Robert Thompson and Prof.
Makoto Fujiwara. Both of you have inspired and pushed me with your drive,
enthusiasm,andknowledge.Ihavereceivedphenomenalopportunitiesunderyour
tutelage, all of which have helped me grow as an academic and as a person.
I have enjoyed my time in the Department of Physics and Astronomy at the
University of Calgary. The students, staff, and faculty have all been wonderful. I
havemademanyfriendsandwehavesharedsomegreattimesinCalgary.Ithank
you all.
This work was supported by the Alberta Ingenuity Fund, the Natural Sciences
and Engineering Research Council of Canada (NSERC), and the Killam Trust. I
am grateful to all of these funding bodies and scholarship organizations, whose
support allowed me to focus fully on this research.
I must thank my family. Everyone has provided support and encouragement,
which has meant a great deal. It saddens me that my father, William Hydomako,
and grandfather, Lloyd McDonald, could not be here to share this with me. Their
memory and influence remains, and has helped me through this whole endeavor.
Last, but not least, to my wife Kelly, without whom this could not have hap-
pened.Nobodybelievedinmemore,orhelpedmeasmuchasyouhave.Idedicate
this to you.
vii
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Motivation for the Trapping and Study of Antihydrogen . . . . . . 1
1.1.1 Precision Comparison of the Properties
of Hydrogen and Antihydrogen. . . . . . . . . . . . . . . . . . . 2
1.1.2 Gravitational Interaction Between
Matter and Antimatter. . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Short Review of Antimatter Studies
and Antihydrogen Experiments. . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Dissertation Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Contributions of the Author . . . . . . . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 Formation of Antihydrogen with Trapped Plasmas . . . . . . . . . . 9
2.1.1 Three-Body Recombination . . . . . . . . . . . . . . . . . . . . . 12
2.2 Charged (Anti)Particle and Neutral-Atom Trapping. . . . . . . . . . 13
2.2.1 Penning-Malmberg Trap for Charged Particles. . . . . . . . 13
2.2.2 Neutral-Atom Trapping . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Particle-Antiparticle Annihilation. . . . . . . . . . . . . . . . . . . . . . . 21
2.3.1 Electron-Positron Annihilation . . . . . . . . . . . . . . . . . . . 22
2.3.2 Antiproton Annihilation. . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Semiconductor Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.1 Double-Sided Silicon Microstrip Detectors. . . . . . . . . . . 25
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3 The ALPHA Apparatus and Procedures. . . . . . . . . . . . . . . . . . . . 29
3.1 Infrastructure Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 Diagnostic and Particle Detection Devices . . . . . . . . . . . . . . . . 31
3.2.1 Charged Particle Temperature Measurement. . . . . . . . . . 33
ix
x Contents
3.3 Production and Accumulation of Antiparticles . . . . . . . . . . . . . 34
3.3.1 Antiproton Decelerator (AD) . . . . . . . . . . . . . . . . . . . . 34
3.3.2 Positron Accumulator . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.4 Charged Particle Manipulation and Trapping . . . . . . . . . . . . . . 39
3.4.1 Penning-Malmberg Trap . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.2 Catching and Cooling of Antiprotons . . . . . . . . . . . . . . 39
3.4.3 Rotating Wall Compression and Electron Kick-Out. . . . . 45
3.4.4 Evaporative Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.5 Autoresonant Mixing. . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.5 Magnetic Neutral-Atom Trap . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.5.1 Axial Magnetic Field. . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.5.2 Radial Magnetic Field. . . . . . . . . . . . . . . . . . . . . . . . . 54
3.5.3 Fast Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.6 Trapping Experiment Overview. . . . . . . . . . . . . . . . . . . . . . . . 57
3.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4 The ALPHA Silicon Detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.1 Detector Hardware and Software for Control
and Data Collection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.1.1 Silicon Detector Modules. . . . . . . . . . . . . . . . . . . . . . . 62
4.1.2 VA1TA ASIC Readout Chips. . . . . . . . . . . . . . . . . . . . 64
4.1.3 Detector Configuration . . . . . . . . . . . . . . . . . . . . . . . . 65
4.1.4 Readout Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.1.5 TA and Readout Triggers. . . . . . . . . . . . . . . . . . . . . . . 67
4.1.6 Analog Readout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.1.7 Summary of Detector Hardware . . . . . . . . . . . . . . . . . . 78
4.2 Monte Carlo Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.2.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.2.2 Materials and Geometry. . . . . . . . . . . . . . . . . . . . . . . . 80
4.2.3 Event Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.2.4 Digitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5 Event Reconstruction in the ALPHA Detector . . . . . . . . . . . . . . . 89
5.1 Challenges for the ALPHA Event Reconstruction . . . . . . . . . . . 89
5.2 Overview of the Event Reconstruction. . . . . . . . . . . . . . . . . . . 92
5.3 Enumeration and Filtering of Track Candidates. . . . . . . . . . . . . 92
5.4 Determining the Helix Parameters. . . . . . . . . . . . . . . . . . . . . . 98
5.4.1 Helix Parametrization . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.4.2 Radial Helix Parameters . . . . . . . . . . . . . . . . . . . . . . . 98
5.4.3 Axial Helix Parameters . . . . . . . . . . . . . . . . . . . . . . . . 100
5.5 Track Pruning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Contents xi
5.6 Vertexing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
5.6.1 Closest Approach Between Two Helices . . . . . . . . . . . . 105
5.6.2 Closest Approach Between N [2 Helices. . . . . . . . 105
tracks
5.6.3 Mean Distance of Closest Approach to the Vertex . . . . . 106
5.6.4 Track Exclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
5.7 Vertex Distributions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.7.1 Annihilation on Background Gas . . . . . . . . . . . . . . . . . 108
5.7.2 Octupole-Induced Antiproton Annihilation. . . . . . . . . . . 109
5.7.3 Antihydrogen Formation in the Neutral-Atom
Trap Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.7.4 Reconstructed Vertex Position Resolution . . . . . . . . . . . 113
5.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6 Rejection of Background Events. . . . . . . . . . . . . . . . . . . . . . . . . . 121
6.1 Backgrounds to the Antihydrogen Annihilation Signal. . . . . . . . 121
6.1.1 Environment and Hardware Background . . . . . . . . . . . . 122
6.1.2 Mirror-Trapped Antiprotons. . . . . . . . . . . . . . . . . . . . . 124
6.1.3 Cosmic Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.2 Importance of Background Rejection. . . . . . . . . . . . . . . . . . . . 126
6.2.1 Blind Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.3 Cosmic Background Rejection . . . . . . . . . . . . . . . . . . . . . . . . 127
6.3.1 Discriminating Variables . . . . . . . . . . . . . . . . . . . . . . . 128
6.3.2 ‘Training’ Datasets . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.3.3 Cut Placement and Optimization. . . . . . . . . . . . . . . . . . 130
6.3.4 Results of the Background Rejection. . . . . . . . . . . . . . . 132
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
7 The Trapping of Antihydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.1 Mirror-Trapped Antiprotons . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.2 Simulation of Particle Trajectories During Magnetic
Trap Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
7.3 Control Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
7.3.1 Neutral Trap Magnets Disengaged . . . . . . . . . . . . . . . . 139
7.3.2 Without Antiprotons and/or Positrons . . . . . . . . . . . . . . 140
7.3.3 Electric Potential Biasing During Fast
Magnetic Shutdown. . . . . . . . . . . . . . . . . . . . . . . . . . . 140
7.3.4 Heated Positrons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
7.4 Initial Search for Trapped Antihydrogen (2009) . . . . . . . . . . . . 142
7.5 Observation of Trapped Antihydrogen (2010). . . . . . . . . . . . . . 146
7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149