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Ev-Scale Sterile Neutrino Investigation with the First Tritium KATRIN Data Ev-Sterile Neutrino Suche Mit Den Ersten Tritium Daten Von KATRIN

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Ev-Scale Sterile Neutrino Investigation with the First Tritium KATRIN Data Ev-Sterile Neutrino Suche Mit Den Ersten Tritium Daten Von KATRIN Technische Universität München Fakultät für Physik Abschlussarbeit im Masterstudiengang Kern-, Teilchen-, und Astrophysik eV-scale Sterile Neutrino Investigation with the First Tritium KATRIN Data eV-sterile Neutrino Suche mit den Ersten Tritium Daten von KATRIN Fotios Megas 29. März 2019 Max-Planck-Institut für Physik Erstgutachter (Themensteller): Prof. Dr. Susanne Mertens Declaration of Authorship English: I hereby declare that I have written the present Master’s thesis in the course Nuclear, Particle and Astrophysics independently, and only with the use of the cited external resources. Deutsch: Ich erkläre hiermit, dass ich die vorliegende Abschlussarbeit im Masterstudiengang Kern-, Teilchen-, und Astrophysik selbstständig und nur mithilfe den zitierten Quellen angefertigt habe. Fotios Megas Munich, 29.3.2019 iii Contents Declaration of Authorship ............................. iii Abstract ....................................... vii 1 Neutrino Physics .................................1 1.1 Neutrino History . .1 1.1.1 Postulation and First Detection . .1 1.1.2 Neutrinos in the Standard Model . .1 1.1.3 Neutrino Oscillations . .2 1.2 Sterile Neutrinos . .3 1.2.1 Theoretical Aspects of Sterile Neutrinos . .4 1.2.2 Experimental Aspects of Sterile Neutrinos . .5 1.2.3 Sterile Neutrinos in Cosmology . .7 1.2.4 Sterile Neutrinos in β-decay . .8 2 The KATRIN Experiment ............................ 11 2.1 Experimental Challenges . 11 2.2 Experimental Setup . 12 2.3 Modelling the Spectrum . 17 3 The First Tritium Measurement Campaign ................. 21 3.1 Analysis Methods . 21 3.1.1 The Covariance Matrix Approach . 23 3.1.2 χ2 Significance Testing . 24 3.2 Data Handling . 24 3.3 Systematics Budget . 26 3.4 Results . 33 3.4.1 Comparison of Sensitivity and Exclusion Limit . 33 3.4.2 Impact of Individual Systematics . 33 3.4.3 Dependence on Statistics and the Fit Range . 36 3.4.4 Comparison to Other Measurements . 37 4 Expected Sensitivity of KATRIN for eV Sterile Neutrinos ......... 39 4.1 Statistical Sensitivity . 39 v Contents 4.2 Impact of Elevated Background . 39 4.2.1 Impact of Elevated Background . 39 4.2.2 Means to Reduce the Background . 41 4.3 Impact of Systematic Uncertainties . 42 4.4 Optimization of Measurement Interval . 45 4.5 Improvements on the Measurement Time Distribution . 46 5 Conclusion .................................... 51 Bibliography ..................................... 53 Appendix A: Comparison of all studies ..................... 59 Acknowledgements ................................ 61 vi Abstract Anomalies observed in reactor and accelerator neutrino oscillation experiments may be resolved by the existence of light sterile neutrinos. These hypothetical particles with a mass scale in the range of eV mix with the active neutrinos, dis- torting their usual oscillation pattern. The KATRIN experiment aims to determine the effective electron antineutrino mass, with an unprecedented sensitivity of 0.2 eV/c2. Being sensitive to these small neutrino masses, KATRIN can possibly also resolve the anomalies in the oscillation sector. The two main topics of this thesis are 1) the search for light sterile neutrinos in the very first tritium data of the KATRIN experiment and 2) the investigation of the impact of an elevated background on the KATRIN sensitivity to light sterile neutrinos. Anomalien in Neutrino Oszillations Experimenten sind mit der Einführung von leichten, hypothetischen Teilchen, die sterilen Neutrinos, zu erklären. Diese Teilchen können das gewöhnliche Oszillationsverhalten von Neutrinos verzerren, indem sie dabei auch teilnehmen. Das KATRIN Experiment ist geplant die Neutrinomasse mit einer Sensitivität von 0.2 eV/c2 zu bestimmen. KATRIN kann also parallel zu der Neutrino- masse Messungen auch möglicherweise diese Anomalien lösen. Die zwei Hauptthemen dieser Arbeit sind 1) die Suche nach sterilen Neutrinos in den ersten Tritium Daten von KATRIN und 2) die Untersuchung der Auswirkung von einer höheren Untergrundsrate auf der Sensitivität von KATRIN bezüglich sterilen Neutrinos. vii Chapter 1 Neutrino Physics 1.1 Neutrino History The neutrino is the lightest fermion and the only particle in the Standard Model of particle physics (SM) that interacts only via the weak interaction. Its elusive nature even nowadays troubles physicists as it still, some 90 years after its postulation by W.Pauli, has some unknown properties. 1.1.1 Postulation and First Detection In 1930, Pauli proposed in a letter the existence of an electromagnetically neutral particle, which he called the "neutron" [1]. This particle had properties of both the neutron and neutrino, but when J.Chadwick discovered the neutron in 1932 and Fermi published his results on beta-decay, the difference between the two neutral particles was made clear. Without the existence of the neutrino, the nuclear beta- decay would just include the nucleus and the emitted electron, making it a two- body problem. This means that the spectrum would effectively be just a sharp line situated above the maximum (endpoint) energy the electron can get from the decay. The observed spectrum was however a continuous one, which indicated a three- body problem, or in other words, the existence of another particle. This particle would have to be neutral and small compared to the nucleus or else it would have been detected. Thus, the term "neutrino" was coined. It would be 24 years later at the Savanah reactor river site, that the neutrino (actually electron-antineutrino) was detected by F.Reines and C.Cowan [2]. 1.1.2 Neutrinos in the Standard Model In the years that followed, the neutrino was found to have different flavours (elec- tron, muon and tau) and was finally added in the SM as a half-integer, neutral, weakly interacting particle. It was established that the helicity of the neutrino was always negative [3], implying not only that the neutrino has left-handed helicity, but also that it is massless. In the case of massless particles, helicity takes the same 1 Chapter 1 Neutrino Physics Figure 1.1: The fermion sector of the SM. Neutrinos are the only fields with only one (left) chirality state. Picture taken from [4]. meaning as chirality, meaning that in the case of neutrinos (antineutrinos) right- chiral (left-chiral) fields are absent. The current knowledge of the fermion sector in the SM is shown in figure 1.1. 1.1.3 Neutrino Oscillations The SM could predict the interactions of neutrinos with great precision until the observation of the so-called neutrino oscillations, where a neutrino produced as a flavor state α has a probability of being detected at a later time at a state β. For this oscillation to take place, the neutrino has to be produced as a definite flavor state and propagate as a mass eigenstate. If neutrinos possess at their birth all three mass eigenstates (which roughly correspond to their three flavors), then these states propagate at different speeds 1, effectively changing the probability of which flavor state the neutrino will later interact as. The mixing of flavor and mass is dictated by the Pontecorvo-Maki-Nakagawa- Sakata (PMNS) matrix, which is a unitary matrix, almost analog to the CKM-matrix of quark mixing [5]. The mixing then takes the form, 1The correct way to phrase this would be that their phases evolve differently with time. 2 1.2 Sterile Neutrinos 0 1 0 10 1 ν U U U ν B e C B e1 e2 e3 CB 1C B C B CB C BνµC = BUµ1 Uµ2 Uµ3CBν2C ; (1.1) @B AC @B AC@B AC ντ Uτ1 Uτ2 Uτ3 ν3 where νe,µ,τ are the flavour eigenstates of the neutrino, ν1;2;3 the mass eigenstates and U the PMNS matrix elements. U can be parametrized to depend on three mixing angles and one phase factor. In case of a Majorana neutrino, two more complex phases are added. Although we will work with an antineutrino, the PMNS parametrization holds, with minor sign changes [6]. For a simplified case of two neutrino families, the oscillation probability is given by, ! L(km) P ν ν = sin2 (2θ)sin2 1:27∆m2 ; (1.2) e ! µ E(GeV ) with θ the mixing angle, ∆m the difference of the mass eigenstates, L the distance to the source, and E the neutrino energy. Neutrino oscillations helped explain the solar neutrino problem, where the neutrino flux detected coming from the sun was not the same as the one predicted by the SM. Nowadays, neutrino oscillations is a well established fact, proven by a number of experiments [7–10]. This means that neutrinos have a mass in contradiction to the Standard Model, because a zero mass entails a zero oscillation probability. There are many theorized mechanisms to generate a mass for the neutrinos, but here we will show only the most prominent, which focuses on the existence of a so-called "sterile" neutrino. 1.2 Sterile Neutrinos The most intuitive way to include a neutrino mass in the SM is to include additional neutrino singlet fields with right-handed chirality, νR. Then, a mass for the neut- rinos can be generated through the Higgs mechanism. This right-handed neutrino would only interact gravitationally, as the weak interaction only couples to left- handed fields. Thus right-handed neutrinos are commonly refered to as "sterile". The only way for this neutrino to take part in any SM process is through mixing with the active neutrinos. In this case, the PMNS matrix can be extended to a 4x4 matrix 2, 2In theory the number of the additional degrees of freedom is not constrained and we could have a (3+N)x(3+N) matrix. Here we just consider the minimal case of one sterile neutrino. 3 Chapter 1 Neutrino Physics 0 1 0 10 1 ν U U U U ν B e C B e1 e2 e3 e4 CB 1C B C B CB C BνµC BUµ1 Uµ2 Uµ3 Uµ4CBν2C B C = B CB C ; (1.3) Bντ C BUτ1 Uτ2 Uτ3 Uτ4CBν3C @B AC @B AC@B AC νs Us1 Us2 Us3 Us4 ν4 where νs is the "flavor" eigenstate corresponding to the right-chiral SM singlet νR, and ν4 the corresponding mass eigenstate.
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