Lepton Number Violation in the Nucleus and the Universe

University College London Department of Physics and Astronomy DOCTORAL THESIS LukáˇsGráf Lepton Number Violation in the Nucleus and the Universe Supervisor: Dr. Frank F. Deppisch Programme of Study: Research Degree: Physics and Astronomy Qualification: Doctor of Philosophy London 2018 Acknowledgements First and foremost, I would like to express my gratitude to my supervisor, Frank Deppisch, who has always been very supportive during my Ph.D. studies. Not only I appreciate his professional erudition and pedagogical guidance, but also his patience, kindness and helpfulness in any situation. My sincere thanks also go to Francesco Iachello for his mentorship and hospi- tality he provided me during my two long-term visits at Yale University. Working with him has been a wonderful experience, both professionally and personally. At the same time, I would like to thank my dear friends and collaborators TomásGonzalo, Julia Harz, Wei-Chih Huang and Jenni Kotila, whose contribution to our common research projects has been important for successful comple- tion of this thesis. I also want to thank my parents, brother and grandparents for supporting me in many different ways throughout my studies. Especially, I am grateful to my grandfather, Václav Gráf,for his continuous availability to discuss any aspect of my work. Last but not least, I would like to thank my wife Tereza, without whose unwavering love, support and patience the path through my Ph.D. would have been much harder to follow. 3 4 I, LukáˇsGráfconfirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. September 2018, London LukáˇsGráf 5 6 “Look closely. The beautiful may be small.” IMMANUEL KANT 8 Abstract Conservation of lepton number is an intriguing feature common for all particle interactions so far observed in Nature and it can be understood as a consequence of an anomalous abelian symmetry of the Standard Model. However, there are certain hints, most importantly the experimental evidence of non-zero neutrino masses, pointing at breaking of the lepton number symmetry at certain high energy scale. Although the new lepton number violating physics can generally lie beyond the reach of collider experiments, it is also possible to probe it at low energies. Most importantly, an observation of neutrinoless double beta decay would provide an evidence of non-conservation of lepton number, shedding light on the origin of non-zero neutrino mass at the same time. In this work we concentrate on the effective description of the non-standard mechanisms of this extremely rare nuclear process, drawing its connection to lepton number violation at high energies. Moreover, assuming its hypothetical observation we discuss some of the possible cosmological implications. Specifically, following a brief review of the long-range mechanisms triggered by 6-dimensional operators, we study in detail the short-range mechanisms contributing to neutrinoless double beta decay as 9-dimensional operators at Fermi scale. After deriving the nuclear matrix ele- ments and phase-space factors involved, we determine limits on the respective particle physics parameters. Constraining these lepton number violating couplings allows to estimate the high-energy scale of the corresponding new physics. Unlike in the standard case, non-standard mechanisms yield energies of order of a few TeVs. Presence of lepton number violation at such low scales implies together with sphaleron transitions a potential washout of a primordial lepton and baryon asymmetry. Consequently, restrictions may be imposed on scenarios of the observed matter-antimatter asymmetry generation. We argue that certain models of high-scale baryogenesis can be generally excluded, if a non-standard neutrinoless double beta decay is observed. 9 10 Impact Statement The research carried out in this thesis aims to help determine crucial neutrino properties as precisely and robustly as possible by interpreting experimental searches for so called neutrinoless double beta decay. At the same time, it cor- relates the results from these laboratory experiments with the early universe cosmology and related astrophysical measurements. The hypothetical process of neutrinoless double beta decay involves the simul- taneous transition of two neutrons into protons within an atomic nucleus, with two electrons being emitted. It has not been observed so far and the fact that there are no neutrinos in the final state requires new physics not present in the Standard Model of particle physics. More specifically, if the neutrino is identical to its anti-neutrino, the observation of neutrinoless double beta decay would allow to directly probe the neutrino mass. In this case, neutrinoless double beta decay is the most promising experimental approach to “weigh” neutrinos. On the other hand, there is a large variety of other possible mechanisms originating from yet undiscovered new physics that could lead to neutrinoless double beta decay, and these are the main subject of this work. Their accurate description and a robust determination of the process rates crucially require detailed nuclear physics cal- culations. As we show, this effort consequently allows to draw conclusions not only about new particle physics, but also about cosmological models explaining baryon asymmetry of the Universe. Therefore, the present study features interdis- ciplinary aspects, touching on and interconnecting theoretical and experimental particle physics, nuclear physics and cosmology. Since we directly aim to assess and sharpen the implications that can be deduced from the numerous current and upcoming searches for neutrinoless double beta decay, our analysis will increase their physics impact. Although the academic contribution of the publications based on this work lies primarily in the area of particle physics, the overlaps may have interesting consequences also in the other mentioned fields. More generally, this work is part of basic, fundamental research and as such it does not have immediate technological use. On the other hand, it contributes to a natural endeavour that expands human knowledge, which has to inevitably precede any subsequent applications. As history has taught us, this kind of research also gives rise to a variety of side products that can find use in various areas of industry. Specifically, theoretical research drives experimental efforts which require the development of innovations and new technologies. Moreover, theoretical particle physics and phenomenology in general excites the interest of many, especially young, people, often leading them to pursue a career in the important economic sector of science and technology. 11 12 Contents 1 Introduction 15 2 To the Standard Model and Beyond 19 2.1 Beta Decay & Fermi Theory . 19 2.2 Gauging the World . 21 2.3 The Very Standard Model . 22 2.4 Effective Point of View . 33 3 The ν Physics 37 3.1 Weyl: “Dirac or Majorana?” . 37 3.2 Neutrinos Mix . 39 3.3 Neutrinos Oscillate . 42 3.4 Probing Neutrino Mass and Mixing . 45 3.5 Neutrinoless Double Beta Decay . 47 3.5.1 Experimental View . 48 3.5.2 Black Box Theorem . 54 3.5.3 Discriminating among Different Mechanisms . 55 3.5.4 The Standard (Mass) Mechanism . 56 3.6 Neutrinos May Seesaw . 62 3.7 Left-Right Symmetry . 67 3.8 Going High - Do We Have the GUTs? . 74 3.8.1 SO(10) Unification . 76 3.9 Baryon Asymmetry of the Universe . 80 3.9.1 Conditions for Baryogenesis . 81 3.9.2 Leptogenesis . 82 4 Non-Standard 0νββ Decay Mechanisms 89 4.1 The Effective 0νββ Decay Lagrangian . 89 4.2 Neutrinoless Double Beta Decay Rate . 96 4.3 Leptonic Phase Space Factors . 100 4.4 From Quarks to Nucleons . 105 4.5 Non-Relativistic Expansion . 108 4.6 From Nucleons to the Nucleus . 114 4.7 Know Your NMEs . 115 4.8 Decay Half-life and Angular Correlation . 121 4.9 Bounds on Couplings . 125 4.10 QCD Running of Couplings . 127 13 5 0νββ Decay From SMEFT 131 5.1 ∆L = 2 SM Effective Operators . 131 5.1.1 Dimension 5 . 133 5.1.2 Dimension 7 . 134 5.1.3 Dimension 9 . 135 5.1.4 Dimension 11 . 135 5.2 Relation to the Low-Scale Operators . 138 5.2.1 SU(2)L Decomposition and Effective 0νββ Couplings . 138 5.2.2 Estimation of Wilson Coefficients . 143 5.2.3 Determination of the Operator Scale . 150 6 Falsifying Baryogenesis 153 6.1 Effective Washout . 153 6.1.1 Boltzmann Equations . 153 6.1.2 Approximated Scattering Density . 155 6.1.3 The Minimal Washout Temperature . 157 6.2 Falsification of High-Scale Baryogenesis . 160 6.2.1 Long-Range Contributions . 162 6.2.2 Short-Range Contributions and their Interplay with Long-Range Contributions . 164 6.2.3 Effect of Additional NMEs and QCD Running . 169 6.2.4 Comparison with Standard Mass Mechanism . 170 6.2.5 An s-channel Contribution Example . 175 6.3 Washout in UV-Complete Models . 176 6.4 Caveats and Loopholes . 178 7 Conclusion and Outlook 183 Bibliography 188 14 1 Introduction The endeavour of particle physics to discover, describe and understand the elementary components of matter and their interplay is an integral part of fundamental physics. It sheds light on the deepest theoretical concepts governing the Universe and in the ideal case it is expected to pave the way towards an ultimate all-encompassing theory. Remarkably enough, our present comprehension of matter already contained in the Standard Model (SM) of Fundamental Particles and Interactions [1–7] provides a very detailed and accurate picture of subnuclear physics. It describes a large number of observed phenomena in a simple theory and with it we are able to make very precise predictions on particle properties and process rates, a vast majority of which agree with the experimental data to a great accuracy.

Load more