Cosmic clues from astrophysical particles

Doctoral Thesis in Physics

Cosmic clues from astrophysical particles

FRANCESCA CAPEL

Stockholm, Sweden 2020 www.kth.se

ISBN 978-9-7873-548-8 TRITA-SCI-FOU 2020:16

KTH ROYAL INSTITUTE OF TECHNOLOGY

FRANCESCA CAPEL Cosmic clues from astrophysical particlesKTH 2020

Doctoral Thesis in Physics

Cosmic clues from astrophysical particles

Francesca Capel

Particle and Astroparticle Physics, Department of Physics Royal Institute of Technology, SE-106 91 Stockholm, Sweden

Stockholm, Sweden 2020

Cover illustration: Generative astroparticles, Francesca Capel.

https://observablehq.com/@cescalara/generative-astroparticles.

Akademisk avhandling som med tillst˚and av Kungliga Tekniska h¨ogskolan i Stock- holm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie doktorsexa- men den 12 juni 2020 kl. 13.00 i sal FB42, AlbaNova Universitetscentrum, Roslagstulls- backen 21, Stockholm.

Avhandlingen f¨orsvaras p˚a engelska.

ISBN 978-91-7873-548-8 TRITA-SCI-FOU 2020:16

© Francesca Capel, May 2020 Printed by Universitetsservice US-AB

Abstract

Ultra-high-energy cosmic rays (UHECRs) are charged particles that have been ac- celerated to extreme energies, such that they are effectively travelling at the speed of light. Interactions of these particles with the Earth’s atmosphere lead to the development of extensive showers of particles and radiation that can be measured with existing technology. Despite decades of research, the origins of UHECRs re- main mysterious. However, they are thought to be accelerated within powerful astrophysical sources that lie beyond the borders of our Galaxy. This thesis ex- plores different ideas towards the common goal of reaching a deeper understanding of UHECR phenomenology. Part I concerns the development of a novel space-based observatory that has the potential to detect unprecedented numbers of these enig- matic particles. The feasibility of such a project is demonstrated by the results from the Mini-EUSO instrument, a small ultraviolet telescope that is currently on-board the International Space Station. In Part II, the focus is on fully exploiting the available information with advanced analysis techniques to close the gap between theory and data. UHECRs are closely connected to the production of neutrinos and gamma rays, so frameworks for the joint analysis of these complementary cosmic messengers are also developed. The results presented herein demonstrate that to progress, it is crucial to invest in the development of both detection and analysis techniques. By taking a closer look at the existing data, new clues can be revealed to reach a more comprehensive understanding and better inform the design of future experiments.

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Sammanfattning

Ultrah¨og energetisk kosmisk str˚alning (UHECR) ¨ar laddade partiklar som har ac- celererats till extrema energier, s˚a att de i praktiken f¨ardas med ljusets hastighet.

Det ¨ar m¨ojligt att uppt¨acka dessa partiklar n¨ar de v¨axelverkar med jordens atmosf¨ar d˚a omfattande skurar med partiklar och str˚alning utvecklas, vilka kan m¨atas med befintlig teknik. Trots decennier av forskning f¨orblir UHECR:s ursprung dold. Men de tros vara accelererade inom kraftfulla astrofysiska k¨allor som ligger utanf¨or v˚ar galax. Denna avhandling utforskar olika id´eer med det gemensamma m˚alet att n˚a en djupare f¨orst˚aelse av UHECR-fenomenologin. Del I handlar om utvecklingen av ett nytt rymdbaserat observatorium som har potential att uppt¨acka ett stort antal av dessa g˚atfulla partiklar. Genomf¨orandet av ett s˚adant projekt demonstr- eras av resultaten fr˚an Mini-EUSO-instrumentet som f¨or n¨arvarande ¨ar ombord p˚a den Internationella rymdstationen. I Del II ligger fokus p˚a att utnyttja tillg¨ang- lig information med avancerade analystekniker f¨or att minska klyftan mellan teori och data, f¨or att n˚a en djupare f¨orst˚aelse av aktuella observationer. UHECR:er

¨ar n¨ara kopplade till produktionen av neutriner och gammastr˚alning. Ramar f¨or gemensam analys av dessa komplement¨ara kosmiska budb¨arare utvecklas. Resul- taten som presenteras h¨ar visar att det ¨ar avg¨orande att investera i utvecklingen av b˚ade detekterings- och analystekniker f¨or att g˚a vidare. Genom att titta n¨armare p˚a befintliga data kan nya ledtr˚adar avsl¨ojas i sammanhanget med s˚a kallade multi- budb¨arare och ger information f¨or att b¨attre utforma framtida experiment.

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Contents

Abstract iii

Sammanfattning v

Contents vii

Introduction xi

Publication list xv

Author’s contribution xvii

Acknowledgements xxi

I Detection 1

1 The detection and measurement of energetic astroparticles 3

1.1 Ultra-high-energy cosmic rays . . . . 3

1.1.1 The detection of extensive air showers . . . . 4

1.1.2 Current experiments . . . . 10

1.1.3 Observations . . . . 11

1.2 Astrophysical neutrinos . . . . 15

1.2.1 Optical Cherenkov detection . . . . 16

1.2.2 The IceCube neutrino observatory . . . . 18

1.2.3 Observations . . . . 20

1.3 Future upgrades and experiments . . . . 22

2 The space-based detection of UHECRs 25 2.1 Motivation . . . . 25

2.2 The EUSO program . . . . 27

2.3 Status and prospects . . . . 30 vii

viii Contents

3 The Mini-EUSO instrument 33

3.1 Introduction (Paper I) . . . . 33

3.2 The photodetector module . . . . 37

3.2.1 Photomultiplier tubes . . . . 38

3.2.2 Characterisation and sorting . . . . 40

3.2.3 Elementary cell units . . . . 43

3.3 Data acquisition (Paper II) . . . . 47

3.3.1 Firmware implementation . . . . 49

3.3.2 Validation and optimisation . . . . 53

3.3.3 Offline triggers for slow events . . . . 56

3.3.4 Machine learning triggers for sophisticated classification . . 57

3.4 Control software (Paper III) . . . . 59

3.5 Calibration . . . . 65

4 Towards an orbital observatory 67 4.1 Analysis considerations . . . . 68

4.2 Preliminary results . . . . 68

4.3 Future plans . . . . 73

II Analysis 77 5 Multi-messenger astrophysics 79 5.1 Particle acceleration . . . . 80

5.1.1 Acceleration mechanisms . . . . 82

5.1.2 Cosmic accelerators . . . . 85

5.2 UHECR Propagation . . . . 88

5.2.1 Interactions and energy losses . . . . 88

5.2.2 Journey through a magnetised universe . . . . 94

5.3 The multi-messenger context . . . . 98

5.3.1 Neutrinos . . . . 98

5.3.2 Gamma rays . . . . 101

5.4 Current physical picture . . . . 105

5.5 Open questions . . . . 107

6 Statistical methods 109 6.1 Bayesian inference . . . . 109

6.2 Hierarchical modelling . . . . 112

6.3 Statistical computation . . . . 114

6.4 The modelling workflow . . . . 118

Contents ix 7 Association of UHECRs with candidate sources (Paper IV) 121

7.1 Paper IV summary . . . . 121

7.2 Exposure of a ground-based UHECR observatory . . . . 124

7.3 A model for the UHECR arrival directions . . . . 127

7.4 Propagation of ultra-high-energy protons . . . . 131

7.5 Detection effects and the GZK horizon . . . . 136

7.6 Extensions to a more physical model and multi-messenger data . . 138

8 Constraints on the neutrino source population (Paper V) 141 8.1 Paper V summary . . . . 141

8.2 Normalisation of a bounded power law . . . . 145

8.3 Description of cosmological populations . . . . 146

8.4 Simulation of point source detection in IceCube . . . . 148

8.5 A hierarchical model for the neutrino event data . . . . 153

8.6 Extensions to specific source models and multi-messenger data . . 155

III Conclusions 157

Acronyms 163

Code repositories 165

Figures 167

Bibliography 179

IV Papers 205

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Introduction

For thousands of years, humans have studied the night sky using photons of optical light. Advances in high-energy astronomy during the 20^th century have expanded this view to include the whole electromagnetic spectrum, from radio to gamma rays. These multi-wavelength observations have unveiled a universe of energetic and diverse sources that can be analysed from vast distances and ancient epochs.

Over the last few decades, energetic particles have been added to the set of mes- sengers that can be used to explore the cosmos. Ultra-high-energy cosmic rays and high-energy neutrinos present new challenges for astronomers to detect and interpret, but promise to enable the investigation of some of the most extreme as- trophysical environments. With the recent advent of gravitational wave astronomy, it is now possible to harness all four fundamental forces in the pursuit of deeper understanding. This multi-messenger approach lies at the intersection of particle physics, astronomy, astrophysics and gravitation and has wide-ranging implica- tions for our basic theories of nature, from the standard model to general relativity.

This thesis primarily concerns the astronomy and astrophysics of the energetic “as- troparticles,” cosmic rays and neutrinos, and their relevance and relation to the other complementary messengers.

Cosmic rays were first discovered by Victor Hess in 1912, who used an electro- scope to measure an increase in the ionisation of air with altitude on a series of hot air balloon flights reaching up to 5 km. These results were controversial at the time, as it was thought that the ionisation of air was due to some form of “penetrating radiation” coming from deep within the Earth. Hess’ observations were later cor- roborated and the ionisation was attributed to charged particles of cosmic origin.

In 1938, Pierre Auger made the important finding that most of the observed cosmic rays were in fact secondary particles produced through the interaction and decays of higher energy primary particles in the upper atmosphere, now known as particle showers. Over 20 years later, the Volcano Ranch experiment led by John Linsley and Livio Scarsi, was used to reconstruct these particle showers, thus revealing the existence of primary cosmic rays with energies greater than 10²⁰ eV.

To this day, the origin of the highest energy particles ever observed remains one of the most important unanswered questions in astrophysics. Ultra-high-energy cosmic rays (UHECRs) are defined as charged particles that have been accelerated to energies of over 10¹⁸ eV, such that they are almost travelling at the speed of

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xii Introduction light. These particles lie at the very end of the cosmic ray energy spectrum and are extremely rare, with expected fluxes at Earth down to below one particle/km²/year.

The detection of these elusive particles is made possible by harnessing the volume of the Earth’s atmosphere as a giant calorimeter and studying the products of the particle showers initiated within it. The fact that we can detect particles with such extreme energies here on Earth implies that their cosmic accelerators must be out there somewhere, and are thought to be connected to powerful and violent astrophysical processes outside of our own Galaxy. UHECRs interact with the background matter and radiation fields as they propagate towards the Earth, resulting in stochastic energy losses and deflections. At the highest energies and lightest compositions, the UHECRs are minimally deflected and must come from nearby due to the increased energy losses, but it is still challenging to trace the observations back to their sources. The confirmed identification of UHECR sources is a tantalising prospect as it will enable the study of particle acceleration and interactions at otherwise inaccessible energies, as well as deepen our understanding of the high-energy universe.

UHECRs undergo interactions both within their source enviroments and as they propagate through the universe. These interactions lead to the production of sec- ondary gamma rays and neutrinos. While the details of the connection between cosmic rays, gamma rays, and neutrinos are model-dependent, the three messengers are inextricably linked at a fundamental level. The neutral secondaries are unaf- fected by magnetic fields and can provide a direct probe of the source locations.

However, their detection is faced with challenges of a different nature. Neutrinos are weakly interacting and can reach us from distant sources, but have extremely low cross-sections, impeding their detection on Earth. Interactions in the background radiation fields attenuate high-energy gamma rays, so it is not possible to detect them from beyond distances of a few Mpc. Lower energy gamma-ray and X-ray observations can also be used to study the spectra of known sources, constraining possible UHECR acceleration and neutrino production mechanisms. However, to build up a complete picture of the source emission and its local environment, it is necessary to use multi-wavelength photon data from across the electromagnetic spectrum. The connection between UHECRs and gravitational wave emission is less direct, but future observations will provide relevant results on the populations and dynamics of compact object binary mergers and supermassive black holes, which will play a role in constraining related source scenarios. Indeed, as each type of messenger bears useful and complementary information on their origin, the search for the UHECR sources greatly benefits from a multi-messenger approach.

The era of multi-messenger astronomy is well underway, with observations of astrophysical neutrinos, UHECRs and gravitational waves now complementing the wealth of available data covering the electromagnetic spectrum. From covering thousands of square kilometres with particle detectors in regions of high-altitude desert, to instrumenting a cubic kilometre with a grid of photomultiplier modules deep within the South Pole ice sheet, the international community has invested a tremendous amount of time and resources into the development of large-scale exper-

xiii iments and observatories. Despite these substantial efforts now coming to fruition, many open questions remain. A plethora of models exist for the common origins of the cosmic messengers, including production in active galaxies, gamma-ray bursts and galaxy clusters. The general picture is that cosmic rays are accelerated to ultra- high energies by astrophysical shocks in energetic sources that have strong enough magnetic fields to confine these particles within the acceleration region. There are many publicly available astronomical catalogues over a range of wavelengths that can be used to characterise the proposed sources in terms of their photon emis- sion. However, the complex underlying physics of the production, propagation and detection of the cosmic messengers makes it extremely difficult to compare these candidate sources to the UHECR and neutrino data.

The search for the sources of UHECRs is the underlying motivation that has driven the doctoral research presented in this thesis. My work addresses this open question in two complementary ways. Firstly, the development of a novel space- based detector to enable the observation of a higher number of UHECR events, and secondly, the analysis of the existing rich datasets with advanced statistical methods to make use of all the available information. I explore these directions in detail in the two separate parts of this thesis, but as we will see, they are really intricately connected. The extensive experimental efforts over the past few decades have revolutionised the field. New and improved detectors will surely pave the way for discovery, but also involve considerable planning and financial investments from the scientific community, with expected timescales of 5 to 10 years or more. It is therefore timely to exploit the existing data fully, update our understanding of the standard physical picture and optimise the design of future experiments accordingly.

Part I of the thesis concerns the development of a space-based observatory for UHECRs. We begin with a review of the detection techniques for high-energy astrophysical particles, then motivate the need for an orbital observatory. We then discuss the Mini-EUSO instrument, a pathfinder for larger space-based detectors that was recently launched to the International Space Station in 2019. The work on Mini-EUSO forms a major part of this thesis and Papers I–III are associated with this chapter. Part I of the thesis is then concluded with a presentation of Mini- EUSO’s first results and a reflection on the future of this technology in light of recent scientific developments. Part II continues with a review of the multi-messenger context for UHECR observations and the physical interpretation of recent results in the field. We emphasise the potential benefit of bringing together information from the different sub-fields and then introduce statistical analysis methods that can achieve this goal in a principled and coherent way. Following this, we present the work that has been driven by this motivation in Papers IV and V, on the analysis of UHECR and high-energy neutrino data respectively. Finally, In Part III, we bring together the previous two parts of the thesis in conclusion and consider which directions could be fruitful to follow in future work.

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Publication list

Publications included in the thesis

Paper I

Capel, F. et al., 2018, Mini-EUSO: A high resolution detector for the study of terrestrial and cosmic UV emission from the International Space Station. Advances in Space Research, 62, 2954.

10.1016/j.asr.2017.08.030 arXiv:1709.00405

Paper II

Belov, A., Bertaina, M. Capel, F.* et al., 2018, The integration and testing of the Mini-EUSO multi-level trigger system. Advances in Space research, 62, 2966.

* Capel, F. is corresponding author, but author list is alphabetically ordered.

10.1016/j.asr.2017.10.044 arXiv:1711.02376

Paper III

Capel, F. et al., 2019, Mini-EUSO data acquisition and control software. Journal of Astronomical Telescopes, Instruments and Systems, 5(4), 044009.

10.1117/1.JATIS.5.4.044009 arXiv:1907.04938

Paper IV

Capel, F. & Mortlock, D. J., 2019, Impact of using the ultra-high-energy cosmic ray arrival energies to constrain source associations. Monthly Notices of the Royal Astronomical Society, 484, 2324.

10.1093/mnras/stz081 arXiv:1811.06464

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xvi Publication list

Paper V

Capel, F., Mortlock, D. J. & Finley, C., 2020, Bayesian constraints on the as- trophysical neutrino source population from IceCube data. Submitted to Physical Review D.

arXiv:2005.02395

Additional publications not included in the thesis

Paper 6

Abdellaoui, G. et al. (The JEM-EUSO Collaboration), 2017. Meteor studies in the framework of the JEM-EUSO program. Planetary and Space Science, 143, 245.

10.1016/j.pss.2016.12.001

Paper 7

Abdellaoui, G. et al. (The JEM-EUSO Collaboration), 2017. Cosmic ray oriented performance studies for the JEM-EUSO first level trigger. Nuclear Instruments and Methods in Physics Research Section A, 866, 150.

10.1016/j.nima.2017.05.043

Paper 8

Abdellaoui, G. et al. (The JEM-EUSO Collaboration), 2018. First observations of speed of light tracks by a fluorescence detector looking down on the atmosphere.

Journal of Instrumentation, 13(05), 05023.

10.1088/1748-0221/13/05/P05023

Paper 9

Abdellaoui, G. et al. (The JEM-EUSO Collaboration), 2018. EUSO-TA – First results from a ground-based EUSO telescope. Astroparticle Physics, 102, 98.

10.1016/j.astropartphys.2018.05.007

Paper 10

Abdellaoui, G. et al. (The JEM-EUSO Collaboration), 2019. Ultra-violet imaging of the night-time earth by EUSO-Balloon towards space-based ultra-high energy cosmic ray observations. Astroparticle Physics, 111, 54.

10.1016/j.astropartphys.2018.10.008

Author’s contribution

The work completed for this thesis falls into two main parts: the development of novel UHECR detector technology, and the analysis of existing multi-messenger data relevant to the study of cosmic ray accelerators.

I joined the Joint Experiment Missions - Extreme Universe Space Observatory (JEM-EUSO) Collaboration in 2015 upon starting my PhD and I have actively participated throughout my doctoral work. My main contributions have been to the development of the Mini-EUSO instrument that was launched to the ISS in August 2019. For this project, I have assisted in the final instrument design and the writing of associated technical documents, characterised and sorted the photo- multiplier tubes, integrated the different instrument subsystems and made a strong contribution to the various laboratory testing campaigns. I have also designed and implemented part of trigger firmware in close collaboration with the electrical en- gineers, supervised a total of six bachelor- and master-level students on related simulation and analysis projects, and held responsibility for the design and im- plementation of the instrument control software. Aside from my contributions to Mini-EUSO, I have also participated in two weeks of shift operations in November 2015 for the EUSO-TA experiment, a ground-based fluorescence telescope located on the site of the Telescope Array experiment in the Utah desert. Additionally, I have assisted in the characterisation of photomultiplier tubes and integration of the photodetector module for EUSO-SPB1, a fluorescence telescope that flew on a NASA super-pressure balloon from Wanaka, New Zealand in the spring of 2017.

My further contributions include an algorithm for searching for low energy cosmic ray hits in the EUSO-SPB1 data, the development of various open-source tools that have been useful to the collaboration, and participation in the internal review of joint collaboration publications. My work for the JEM-EUSO Collaboration is ongoing. I actively maintain and document the Mini-EUSO instrument control software to support Mini-EUSO’s operations on-board the International Space Sta- tion, and additionally support the adaptation of the software to other experiments.

I highlight my contributions to the papers included in this thesis that have resulted from my work for the JEM-EUSO Collaboration in detail below. For the remain- ing JEM-EUSO full author list publications, my contributions have generally been in the form of the development of the instrumentation used, the operation of the detector in acquiring data, or participation in the internal review process.

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xviii Author’s contribution In the second half of my doctoral work, I independently proposed two data analysis projects that make use of the available UHECR and high-energy neutrino data that have resulted in two corresponding publications. Through discussions around the realisation of these projects with colleagues at the Oskar Klein Centre in Stockholm, I initiated collaboration with Daniel Mortlock and Chad Finley. Daniel and I have worked closely together on both of these projects, and he eventually became co-supervisor of my PhD in 2019. The three of us have collaborated on the second project. I describe my individual contributions to the two papers in more detail below.

Paper I

I gathered all the material for the paper from technical documentation that I had contributed to. I wrote the paper, with contributions in the form of comments from other members of the Mini-EUSO team. My co-authors are the Italian and Russian PIs of Mini-EUSO and other members of the collaboration also contributed with simulations of the optical system, Monte Carlo simulations of the level one trigger efficiency, nuclearites and space debris. I am responsible for the remaining figures.

Paper II

I contributed to the level two trigger design and implementation, and participated in the trigger testing in the laboratory at Lomonosov Moscow State University in September 2016. I also implemented code for integrating raw data packets that is used in various places in the firmware design of the front-end data acquisition. I wrote the paper based on an early draft by my co-author, Federico Fausti, and all co-authors contributed to improving the paper with comments. Given that there were also strong contributions from Alexander Belov and Federico Fausti in the firmware design, we decided on an alphabetical author list with the three of us as corresponding authors.

Paper III

I was the main person responsible for the design and implementation of the in- strument control software, and wrote the majority of the code myself (over 10,000 lines), with small contributions from certain co-authors. I also produced and main- tain the code’s repository on GitHub and online documentation. I wrote all the text in the paper and produced all the figures. My co-authors all contributed to either the code, discussion on the software design or the functional testing of the software in the laboratory.

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Paper IV

I proposed the main idea for the paper to Daniel and we started meeting regularly to discuss the work. I wrote the code and performed the work carried out in the paper. I was also responsible for writing the manuscript and producing the figures.

Daniel gave regular feedback and motivated the development of the first part of the paper, in which we perform calculations and simulations to assess the influence of the energy uncertainty on the effective GZK horizon.

Paper V

Following the completion of Paper IV, I became interested in modelling the IceCube neutrino data and started to discuss some ideas with Chad. Daniel, Chad and I then started having regular meetings, in one of which I proposed the idea for Paper V.

Following this, we all collaborated on the paper, with Daniel contributing to the development of the statistical model, and Chad giving insight into the IceCube results and implementation of the point source search algorithm. I wrote all the code behind the presented work. I also wrote the paper and produced the figures, with feedback from Daniel and Chad.

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Acknowledgements

First and foremost, I would like to thank my main supervisor Christer Fuglesang.

I really appreciate all the opportunities that you’ve given me over the years and, in particular, the freedom to branch out into my own projects. Thank you for all the support, guidance, and inspiration despite your busy schedule. I have really enjoyed our easygoing but straight-talking meetings and learned from your different perspective on my work.

I’m incredibly grateful to my de facto mentor and eventual co-supervisor Daniel Mortlock. Thank you for taking an interest in my work at a critical time in my PhD and helping me turn my ideas into real projects. Working together has been great fun, and I can’t thank you enough for all the advice and time spent talking about science and statistics over cups of tea.

I would also like to extend my sincere thanks to my EUSO colleagues. Many of you have made the chaotic journey of launching a detector into orbit a rich and vibrant learning experience. In particular to Marco Casolino and Lech Piotrowski who have been my effective supervisors and shared this adventure over long working hours in both Rome and Tokyo. Huge thanks to Guillaume Pr´evˆot and Etienne Parizot for hosting me in Paris and tolerating my strong language in the lab for several months. Also to Pavel Klimov and Alexander Belov for your amazing hos- pitality in Moscow as well as for teaching me about FPGAs and how to drink vodka, amongst other things. Many thanks to Mario Bertaina for hosting me in Turin and for sending motivated students to Stockholm to keep me busy during the summertime.

During my doctoral studies, I am extremely fortunate to have met and discussed with many exceptional scientists who have generously contributed to my work in some form. I want to thank my co-author, Chad Finley, for being so open to collaboration and for our many enlightening discussions on neutrino detection. I am very grateful to J. Michael Burgess and Jochen Greiner for valuable advice in difficult times and teaching me what it means to be a scientist. I would also like to acknowledge useful advice and support from Elisa Resconi, Paolo Padovani, Jens Jasche, Mark Pearce, Felix Ryde and Angela Olinto. You have all encouraged me to continue my career in research.

To all friends and colleagues in KTH, the Oskar Klein Centre, and beyond, thank you for the support and good times over the years. Living abroad is always

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xxii Acknowledgements a challenging but rewarding experience, and I will leave Stockholm with so many great memories.

Last but far from least, I want to thank my parents, Sue and Marcus, and my brother, Max. Your support is truly immeasurable and I look up to you all in different ways. Thank you for reminding me to take it easy sometimes. I can’t tell you how much it means to me that no matter where I end up in the world, I know that you guys have my back.

Cheers!

Francesca Capel

Stockholm, 14^th May 2020.

The true delight is in the finding out rather than in the knowing.

—Isaac Asimov

Part I

Detection

1

2

Chapter 1

The detection and

measurement of energetic astroparticles

The techniques used to detect UHECRs and high-energy neutrinos follow similar principles. In both cases, large volume detectors are required, due to the extremely low fluxes of UHECRs, and the minute cross-sections of weakly interacting neu- trinos. In these large volumes, the properties of the primary particle are inferred through the detection of secondary products resulting from its interaction. In this chapter, we review the important principles concerning the detection and measure- ment of these energetic astroparticles, as well as the current status of experimental observations and planned upgrades.

1.1 Ultra-high-energy cosmic rays

Cosmic rays are ionized nuclei that have been accelerated to relativistic energies.

Around 90% or cosmic rays are protons, with heavier nuclei making up the re- maining fraction. The cosmic ray energy spectrum extends over many orders of magnitude, and is relatively well-described by a single power law of the form

dN/dE ∝ E^−α, (1.1)

with the spectral index α ∼ 2.7 over this large range, as shown in Fig 1.1. The spectrum is often plotted multiplied by E⁻²to represent the power density, and this also has the effect of highlighting deviances from a continuous power law. These spectral features are typically used to divide the spectrum into broad energy regions of different physical importance. In fact, the cosmic ray energy spectrum is often referred to as a leg, with a “knee” at∼ PeV energies, where the spectrum steepens

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4 Chapter 1. The detection and measurement of energetic astroparticles to α∼ 3.1, and an “ankle” at ∼ EeV energies, after which the spectrum flattens to α∼ 2.6. However, the suitability of this nice analogy breaks down when considering the hotly debated existence of a less significant “second knee” at around 100 PeV (Apel et al., 2011). At the lowest energies, the cosmic ray flux is thought to be dominated by the Sun, with modulation seen in accordance with the solar cycle.

Below the knee, most cosmic rays are thought to be Galactic, with a transition to an extra-Galactic dominated flux thought to occur somewhere between the knee and the ankle. UHECRs are defined as having energies of above 1 EeV, and so the region of greatest relevance to this thesis lies at the very end of the cosmic ray spectrum.

Beyond the ankle, the flux decreases dramatically and finally a significant cutoff is seen at around 5× 10¹⁹eV¹.

The detail and structure of the cosmic ray spectrum are key to our astrophysical understanding in terms of acceleration and propagation of these particles. At high energies, the theoretical interpretation of these results is still an active field of research and will be discussed later in Chapter 5. Here, we focus on the fact that at energies above 10¹⁹eV, the UHECR flux drops to below 10⁻⁸GeV cm⁻²sr⁻¹s⁻¹. This is equivalent to an all-sky flux of 10⁻¹⁷ cm⁻² s⁻¹, and can be approximately expressed in more familiar units as∼ 1 km⁻² year⁻¹. These low fluxes present a huge challenge to the detection and understanding of UHECRs. Already at energies of around 10¹⁴ eV, it becomes impractical to directly detect cosmic rays with an instrument that can be mounted on a balloon or satellite system, as the incident number of events is simply too low. Instead, the Earth’s atmosphere must be used as an active detector volume. Incident high-energy cosmic rays initiate extensive air showers upon interacting in the upper atmosphere, and the secondary radiation produced in these air showers can be used to infer the properties of the primary particle.

1.1.1 The detection of extensive air showers

An extensive air shower is a giant particle and radiation cascade initiated by an energetic primary cosmic ray nucleus interacting in the Earth’s atmosphere. The cascade forms along the trajectory of the primary, with some lateral dispersion due to the non-zero transverse momentum of the secondaries. Initially, the huge energy of the primary is dissipated and the number of particles in the shower rapidly increases. As the shower develops, the subsequent number of secondary particles with high enough energies to continue producing new particles decreases and the shower subsides.

The cascade can be described by three main components based on the kind of interactions occurring: hadronic, electromagnetic and muonic, as shown in Fig. 1.2.

The shower has a hadronic core dominated by pions and kaons, which decay into muons and neutrinos before interacting, making up the highly penetrating muonic

1Mercifully, given that a continuation in the spectrum would require the contrivance of a label even more extreme than “ultra-high-energy” to describe the observations.

1.1. Ultra-high-energy cosmic rays 5

Challenges for Cosmic-ray Experiments

Thomas Gaisser^1,?

1Bartol Research Institute and Department of Physics and Astronomy University of Delaware

Newark, DE 19716 USA

Abstract. This paper is a commentary on presentations at ISVHECRI 2016 related to cosmic-rays, gamma-rays and neutrinos. Its goal is to highlight the unanswered questions raised during the conference about the sources of these cosmic particles and the relations among them.

1 Introduction

A current theme of astro-particle physics is "multi- messenger astronomy," which emphasizes use of gamma- rays and neutrinos to address still unanswered questions on the origin of high-energy cosmic rays. With this theme in mind, I review the presentations on cosmic-ray spectra and composition, gamma-ray astronomy and neutrino astron- omy presented at ISVHECRI 2016. The role of hadronic interactions is addressed in the review paper of Tanguy Pierog [1].

2 Some cosmic-ray questions

Figure 1 gives a global view of the cosmic-ray spec- trum. The abundant elements of the primary cosmic- ray spectrum are measured accurately to energies higher than a TeV per nucleon with spectrometers in space [2–

4]. Calorimetric measurements with balloon-borne detec- tors [5, 6] extend direct measurements to higher energy but with somewhat less precision. This means that we have good coverage of the composition with direct mea- surements up to about 100 TeV energy per nucleus. Indi- rect measurements with large detectors on the surface are needed for the higher energy cosmic rays. There are sev- eral questions of current interest associated with the vari- ous features in the energy spectrum:

• What is the composition in the knee region and how does it connect with direct measurements at lower en- ergy?

• What is the cause of the hardening of the spectrum around 20 PeV?

• Where is the transition from Galactic to extra-galactic cosmic rays and how is it related to composition around the ankle?

• What is responsible for the apparent end of the spectrum around 100 EeV?

?e-mail: gaisser@bartol.udel.edu

10^-10 10^-8 10^-6 10^-4 10^-2 10⁰

10⁰ 10² 10⁴ 10⁶ 10⁸ 10¹⁰ 10¹² E2 dN/dE (GeV cm-2 sr-1 s-1 )

E_tot (GeV / particle) Energies and rates of the cosmic-ray particles

HERA

RHIC TEVATRON LHC Fixed target

protons only

all-particle electrons

positrons

antiprotons

Grigorov Akeno MSU KASCADE Tibet KASCADE-Grande IceTop73 HiRes1&2 TA2013 Auger2013 Model H4a CREAM all particle

Figure 1. Overview of the energy spectra of various components of the cosmic radiation (Fig. 2.1 of [7]).

• Does the di↵erence between the Auger and the TA spec- trum in the cuto↵ region show that the cosmic-ray spec- trum is di↵erent in di↵erent regions of the sky?

2.1 The knee region

Most air shower measurements have a threshold around a PeV, while direct measurements extend only to ⇠ 100 TeV.

An exception is the ARGO-YBJ RPC carpet detector at high altitude in Tibet, which has the potential to cover the gap between direct and indirect measurements [8].

TAIGA-HISCORE is also starting to measure the spec- trum down to ⇠ 300 TeV [9]. ARGO-YBJ have reported measurements of both the all-particle spectrum and the spectrum of the light (p + He) component [10]. The mea-

arXiv:1704.00788v1 [astro-ph.HE] 3 Apr 2017

Knee

Ankle

Figure 1.1. The cosmic ray energy spectrum shown using data from a range of experiments. The spectrum is plotted as E²dN/dE to reflect the power density.

Measurements of electrons, positrons and antiprotons are also indicated. At lower energies, these could be of secondary origin, but above ∼ 10 GeV it seems that another explanation is required; either the annihilation of dark matter particles or the contribution of astrophysical accelerators. In addition, the maximum equivalent laboratory energies that can be attained by major particle physics experiments are shown with red arrows. Figure adapted from Chapter 2 of Gaisser, Engel, and Resconi (2016).

6 Chapter 1. The detection and measurement of energetic astroparticles

Figure 1.2. Simple schematic of the components of an extensive air shower. The development of air showers is highly complex and this diagram is only intended to summarise the key relevant interactions. Figure adapted from Haungs, Rebel, and Roth (2003).

component of the shower. The electromagnetic component is primarily fed by high- energy photons from decaying neutral pions, which in turn produce electromag- netic cascades governed by pair production and bremsstrahlung processes (Haungs, Rebel, and Roth, 2003). This component dominates the shower in terms of particle number, as shown in the right panel of Fig. 1.3. The energetic electron and positron pairs go on to produce Cherenkov radiation and eventual fluorescence from the de- excitation of atoms in the air. The electromagnetic component of the shower also generates coherent radio emission.

The evolution of air showers is inherently stochastic and complicated, depending on the competition between the decay and interaction of the secondaries in the var- ious components. Because of this complexity, detailed Monte Carlo simulations or numerical integration of the transport equations are typically performed to model shower development. However, using a simple toy model developed by Heitler (1954) for electromagnetic cascades and generalised by Matthews (2005) to the hadronic case, some insight into the general features of air showers can be de- rived (see also Chapter 16 of Gaisser, Engel, and Resconi 2016). For a hadronic cascade, we assume that the interaction of a hadron with energy E produces ntot

new particles, among which the energy is equally divided. The produced particles are either charged or neutral, in the ratio 2:1 respectively. Neutral particles are assumed to immediately decay and contribute to the electromagnetic cascade. The secondary particles then interact similarly every mean hadronic interaction length, λ, as long as their energy is above some threshold energy, Eth. Below Eth, the re-

1.1. Ultra-high-energy cosmic rays 7 sulting particles instead decay, producing one muon per hadron. After n interaction steps, the resulting energy in the shower is

Eshower= 2 3

!n

E0+

"

1− 2 3

!n#

E0, (1.2)

where the first and second terms are the hadronic and electromagnetic contribu- tions, respectively. The shower quickly becomes dominated by the electromagnetic contribution. In this case, the cascade is mediated by the critical energy, Ec, which is the energy at which the energy loss of electrons due to ionisation equals that of bremsstrahlung. The shower maximum is reached when all secondaries have en- ergy Ec, thus the number of particles is NEM,max= E0/Ec. The slant depth of the shower maximum is approximately given by

Xmax≈ X⁰+ λ ln E0

2ntotEc

!

, (1.3)

where X0is the slant depth in units of g cm⁻²of the first interaction. The number of muons produced is given by

Nµ= E0

Eth

!η

, (1.4)

where η = ln nch/ ln ntotand nchis the number of charged particles. The number of muons is sensitive to the hadronic multiplicity through its dependence on η (Engel, Heck, and Pierog, 2011). Generally speaking, higher-energy primary cosmic rays result in showers that reach deeper into the atmosphere and spread over a wider region. As the total number of particles created is proportional to the primary energy, it makes sense to think of the Earth’s atmosphere as a vast calorimeter.

The aim of air shower detectors is to sample this calorimeter and use knowledge of the air shower development in order to reconstruct properties of the primary cosmic rays.

This result can be extended to the case of a primary nuclei using the superpo- sition model. As the typical binding energy per nucleon is ∼ 5 MeV, much less than the interaction energies considered here, a nucleus of mass A and energy E0

can be considered as A independent hadrons with energy Eh= E0/A. This simple approximation leads to the following expressions

N_EM,max^A = AN_EM,max^h (Eh)≈ N^EM,max(E0) X_max^A ≈ X^max(E0/A)∝ λ ln E0

AEc

!

N_µ^A= A^1−η E0

Eth

!η

.

(1.5)

8 Chapter 1. The detection and measurement of energetic astroparticles

NS61CH19-Engel ARI 15 September 2011 8:38

10^–3 10^–2 10^–1 1 10 10² 10³

1 Core distance (km) Particle density (m–2)

γ

Hadrons

× 10¹⁰ Particle number

20

10

7 5

3

1 0

200

400

600

800

0 1 2 3 4

1,000

Atmospheric depth (g cm–2) Altitude (km)

μ^± γ (× 100) μ^±

e^± (× 5) e^±

Hadrons (× 100)

a b

Figure 2

Average (a) lateral and (b) longitudinal shower profiles for vertical, proton-induced showers at 10¹⁹eV. The lateral distribution of the particles at ground is calculated for 870 g cm⁻², the depth of the Pierre Auger Observatory. The energy thresholds of the simulation were 0.25 MeV forγ and e^±and 0.1 GeV for muons and hadrons.

shower of secondary particles. The most frequently produced secondary hadrons are charged and neutral pions. Whereas neutral pions (cτ = 25 nm) immediately decay into two photons, charged pions (cτ = 7.8 m) interact again before decaying (π^± → μ^±+ νμ/¯νμ) once Eπ  30 GeV.

Charged kaons with a slightly shorter lifetime (cτ = 3.7 m) decay at higher energies. The long- lived secondary hadrons (baryons, charged pions, and kaons) form the hadronic shower core.

Photons fromπ⁰decay are the dominant source of the electromagnetic (EM) shower component, which by itself produces only a very small number of hadrons or muons through photoproduction or muon pair production. The muons in an air shower, of which 90% are produced in the hadronic cascade due to the decay of pions and kaons, propagate through the atmosphere with small energy losses and reach the surface of the Earth almost unattenuated. In showers with very large zenith angles (θ > 65^◦), this muonic shower component and the EM particles produced in the decay of muons are the only particles that can be detected at ground.

Figure 2 shows the lateral (i.e., transverse to the shower axis) and longitudinal particle profiles of the different shower components, simulated with CORSIKA (23) for proton-induced showers of 10¹⁹eV. The longitudinal profile is typically studied as a function of the traversed column density (i.e., slant depth) X =

ρ(l)dl, where ρ is the density of air and the integral must be taken along the shower trajectory.

2.1. Electromagnetic Showers

There is extensive literature on the theory of EM showers [see, for example, the seminal articles by Rossi & Greisen (24) and Nishimura (25)], and reliable simulation tools are also available [see, e.g., EGS (26), FLUKA (27), and GEANT4 (28)]. Here, we describe only those features of EM showers that are needed for the discussion of hadron-induced showers, below.

470 Engel

·

·

Annu. Rev. Nucl. Part. Sci. 2011.61:467-489. Downloaded from www.annualreviews.org Access provided by KTH Royal Institute of Technology (Sweden) on 02/29/20. For personal use only.

Figure 1.3. The average lateral (left) and longitudinal (right) profiles for the dif- ferent components of a vertical shower induced by a 10¹⁹ eV proton. Figure from Engel, Heck, and Pierog (2011).

The first result shows that the total number of particles in the electromagnetic component at the shower maximum is independent of the primary mass. However, the depth of the shower maximum and number of muons are both dependent on the mass of the primary nucleus. Heavier nuclei result in showers that develop slightly higher in the atmosphere and produce more muons. Despite the simplified assumptions of the superposition model, these features are a good description of the average behaviour of air showers that are used to reconstruct the properties of the primary particles, as discussed further in Section 1.1.3.

The phenomenon of extensive air showers was first noticed by Bruno Rossi in 1934, who was using Geiger-M¨uller (GM) counters to observe cosmic radiation in Asmara, Eritrea. During the calibration of his detectors, he reported the obser- vation of “very extensive groups of particles, which produce coincidences between counters rather distant from each other” (Rossi 1934, translation taken from Linsley 1998). Rossi’s results were later confirmed by Pierre Auger and his student, Roland Maze. Together, they developed an array of GM counters with a time resolution of

∼ 5 µs to demonstrate that these observations were not chance coincidences or de- tector effects (Auger, Maze, and Grivet-Meyer, 1938). Auger and his collaborators then used these detectors at mountain altitudes and applied early electromagnetic cascade theory to estimate the energy of the shower-inducing primary radiation to be∼ 10¹⁵ eV. This energy was around 5 orders of magnitude greater than that of the cosmic radiation previously studied, so this result was a breakthrough for the indirect detection of high-energy cosmic rays.

Following Auger’s discovery, numerous particle detector arrays were developed all around the world with increasing sizes and improving technology. Most of these arrays covered a surface area of less than one square kilometre and were used to

1.1. Ultra-high-energy cosmic rays 9 study cosmic rays with energies between 10¹⁴ and 10¹⁷eV (Kampert and Watson, 2012). The Volcano Ranch experiment in New Mexico was the first giant air shower array to be constructed and led to the observation of UHECRs, including a par- ticle with an energy of greater than 10²⁰ eV (Linsley, 1963). The Volcano Ranch experiment employed 20 plastic scintillators in a hexagonal array with a separation of 884 m. These scintillators were used to measure the “footprint” at ground level of the secondary muons and electrons produced by extensive air showers. The pri- mary cosmic ray energy can then be inferred from the number of detected particles, by assuming a stage of shower development. Several other giant air shower arrays were developed: Haverah Park in the UK, Narribri in Australia, Yakutsk in Russia and the 100 km² Akeno Giant Air Shower Array (AGASA) in Japan (Nagano and Watson, 2000).

The suspicion that cosmic rays could possibly attain energies of beyond 10²¹eV in the early 1960s motivated the development of alternative detection methods (Greisen, 1965). Relativistic charged particles from extensive air showers excite nitrogen molecules as they pass through the atmosphere. These nitrogen molecules then de- excite, producing fluorescence photons with wavelengths in the 300-450 nm range, corresponding to an ultraviolet (UV) flash on the time scale of µs. The number of emitted fluorescence photons is proportional to the energy of the primary particle, and the constant of proportionality is known as the fluorescence yield. The yield depends on the atmospheric conditions, meaning that calorimetric energy measure- ments are still challenged by these sources of uncertainty (Rosado, Blanco, and Arqueros, 2014). The measurement of the fluorescence emission was developed in the 1960s and 70s, culminating in successful measurements in coincidence with the ground array at Volcano Ranch (Bergeson et al., 1977). This technique was attrac- tive as it can be used to observed a much larger volume of atmosphere, and was put to good use by the Fly’s Eye detector, and later on, the high resolution Fly’s Eye (HiRes) experiment.

Around one third of the relativistic particles in extensive air showers will also produce beamed Cherenkov radiation in the forward direction, with the strongest contribution from the electromagnetic component of the shower (Kieda, Swordy, and Wakely, 2001). This Cherenkov light can be used to reconstruct the shower profile and to estimate the properties of its electron distribution, and thus the elec- tromagnetic shower component. Cherenkov radiation from the primary particle itself may also be observed prior to its first interaction, and used to estimate the composition, since the rate of Cherenkov emission is proportional to Z², where Z is the charge of the primary. Energetic gamma rays that initiate electromagnetic cascades upon interaction with the Earth’s atmosphere are also detected in a sim- ilar way. Imaging Atmospheric Cherenkov Telescopes (IACTs) are used for the observation of cosmic rays in TeV energy range and gamma rays in the GeV–TeV range, including experiments such as the High Energy Stereoscopic System (HESS, Aharonian et al. 2007), the Major Atmospheric Gamma Imaging Cherenkov Tele- scope (MAGIC, Ferenc et al. 2005), and the Very Energetic Radiation Imaging Telescope Array System (VERITAS, Krennrich et al. 2004). At ultra-high ener-

10 Chapter 1. The detection and measurement of energetic astroparticles

Figure 1.4. The hybrid detection technique. The development of the extensive air shower is observed in stereo with multiple fluorescence telescopes. Secondary particles produced in the air shower are sampled by the surface array. This array is made up of individual particle detector modules, such as scintillator detectors shown here. Figure from The Telescope Array Project (2020).

gies, the showers are usually viewed at too wide an angle to capture the direct Cherenkov emission from UHECR showers, but Cherenkov light scattered off air molecules can contribute to the signal seen in fluorescence telescopes and must be accounted for in the event reconstruction.

The combination of a fluorescence telescope with a ground array of particle de- tectors gives an extremely powerful hybrid method for the study of extensive air showers, as illustrated in Fig. 1.4. The fluorescence detectors are able to detect the total calorimetric energy of a shower, whereas the surface array is only able to sample part of the shower front. When used in stereo, fluorescence detectors give a three-dimensional picture of the shower profile, useful for reconstructing the incoming direction and composition of the primary particle. The surface array is only able to achieve this through the relative arrival times of particles in different detectors. However, the fluorescence technique requires clear, dark night-time ob- servation conditions, and thus only has a duty cycle of∼ 15% in comparison with the near 100% duty cycle of the surface array that is unaffected by atmospheric conditions. These complementary measurements can also be used to cross-check and reconcile the two datasets for an improved understanding of air shower mea- surements.

1.1.2 Current experiments

Modern UHECR observations are dominated by two large-scale hybrid experiments:

The Pierre Auger Observatory and the Telescope Array project. Together, these