Analysis of neutrino event generators for KM3NeT/ORCA Bachelorarbeit vorgelegt von Patrick Heuer Erlangen Centre for Astroparticle Physics Physikalisches Institut 4 Friedrich-Alexander-Universität Erlangen-Nürnberg 1. Gutachter: Dr. Thomas Eberl 2. Gutachter: Prof. Dr. Gisela Anton Tag der Abgabe: 09. 07. 2018 Contents 1. Introduction 3 2. Neutrino detection5 2.1. Neutrino physics . .5 2.1.1. Elementary particles and fundamental forces . .5 2.1.2. Neutrino oscillation . .6 2.1.3. Neutrino-nucleus interactions . .7 2.1.4. Cross-sections . .9 2.2. Detection principles . 10 2.2.1. Cherenkov light . 11 2.2.2. Neutrino telescopes . 11 2.2.3. KM3NeT/ORCA . 12 3. Neutrino simulation 14 3.1. gSeaGen . 14 3.2. GiBUU . 15 3.2.1. Job card . 15 3.2.2. Output file . 16 3.2.3. Cross-sections . 17 3.3. Comparison . 17 3.3.1. yb-distributions . 19 3.3.2. yb-φν,µ distributions . 20 3.3.3. Mean yb ....................................... 22 3.3.4. Scatter type ratio . 23 3.3.5. Particle multiplicities . 26 3.3.6. Mean particle production . 26 3.3.7. Mean pion production . 26 3.4. Differences in GiBUU by changing the use of PYTHIA . 28 3.4.1. Cross-sections . 29 3.4.2. yb-distributions . 30 3.4.3. Mean yb ....................................... 30 1 2 Contents 3.4.4. Scatter type ratio . 32 3.4.5. Particle-production . 32 4. Summary and conclusion 34 List of Figures 36 List of Tables 39 Bibliography 40 A. example job card 43 B. yb distributions 45 C. yb-φν,µ distributions 53 D. Particle production 55 E. New PYTHIA settings 59 1 Introduction The detection of neutrinos is a challenge in experimental physics dating back to 1930, when Wolfgang Pauli postulated them. From there, it took over 20 years to finally prove the existence of the elusive particle in the Cowan?Reines neutrino experiment.[1] To learn about the properties of the neutrino, one has to detect the particles produced in interactions with matter. Once believed to be massless in the Standard Model of particle physics, we know today that neutrinos have a rest mass. This results in an effect called neutrino oscillation, where a neutrino changes its flavour during propagation. This is firmly established by a number of experiments measuring the corresponding squared mass differences and mixing angles occurring in the theory of neutrino oscillation.[2–7] One of the remaining questions is the hierarchy of the mass eigenstates or more 2 precisely the sign of the larger of the two mass-squared differences ∆m23. One approach to learn about the phenomenon of neutrino oscillations, is the detection of neutrinos created in air-showers, induced by cosmic rays in the atmosphere, after propagating through the Earth. This approach is pursued by KM3NeT/ORCA, a future multi-megaton underwater Cherenkov detector.[8, 9] To understand the measurements of ORCA and generally all neutrino telescopes, we rely on the use of simulations of the interaction of neutrinos, in the detector material. In the first step in such simulations, a so-called event generator is employed, simulating the first interaction and the final state particles of the reaction. Over the last decade it has become evident that the standard approach used in neutrino simulations, namely the Relativistic Fermi Gas Model (RFGM), conspicuously fails to account for the complexity of nuclear dynamics and the variety of reaction mechanisms contributing to the detected signals.[10] The currently used event generator in the KM3NeT collaboration is gSeaGen, a home-brew generator based on GENIE, which relies on the RFGM. An improvement of these simulations could be achieved using the new generator GiBUU. GiBUU was written over the last decades and is aiming to provide a unified theoretical transport framework in the MeV and GeV energy regimes for a wide array of reaction types like hadron-, photon-, electron-, neutrino- and heavy-ion-induced reactions on nuclei.[11] The goal of this thesis is to understand the usage of GiBUU for KM3NeT/ORCA and compare its output to the one of gSeaGen. To do this it will first introduce the physical principles involved in neutrino detection, following with an introduction of the two generators and a comparison of the 3 4 1. Introduction respective output. It will end with an evaluation, summarizing the found contradictions between the generators and search for explanations of these. 2 Neutrino detection Neutrinos are electrically neutral, extremely light particles with spin 1/2 that only have weak and gravitational interaction. They were postulated in 1930 by Wolfgang Pauli as a remedy to conserve energy in the beta decay. First assumed to be massless, neutrinos have the ability to change their flavour, in a process called neutrino oscillation. This behaviour can only be explained if neutrinos have mass. To better understand this phenomenon and to determine the mass hierarchy of neutrino masses, neutrino telescopes, like the future KM3NeT/ORCA, measure atmospheric neutrinos in the few-GeV range. 2.1. Neutrino physics Covering all physical principles involved in neutrino physics is beyond the scope of this thesis. A short summary of relevant principles will be given with a focus on those most relevant for the thesis. 2.1.1. Elementary particles and fundamental forces Elementary particles are particles that have no known constituents. These particles and their fundamental interactions are explained in the Standard Model of particle physics. The current elementary particles are grouped into three classes: quarks, leptons and gauge bosons. There are six known quarks: up, down, charm, strange, top and bottom. They are the only particles to carry a fractional electric charge and are the constituents of hadrons, held together by the strong force. These are grouped into baryons (three quarks) and mesons (two quarks). The most commonly known baryons are protons, consisting of two up and one down-quark and neutrons, consisting of two down and one up-quark. Mesons are unstable products of particle interactions involving quarks. The lightest meson is the pion (π) consisting of a quark-anti-quark pair. Leptons can be categorized into two groups, electrically charged (with a charge of -1e, where e is the elementary charge) and electrical-neutral leptons. The best-known charged lepton is the electron (e−), whereas muons (µ−) and taus (τ −) can be described as heavier versions of the e−. Every charged lepton has a corresponding neutral lepton, the neutrino (νe, νµ, ντ ). The correspondence of the 5 6 2. Neutrino detection Family Name Charge (e) Interaction Quark up 3/2 em, strong, weak, gravitation down -1/2 em, strong, weak, gravitation charm 3/2 em, strong, weak, gravitation strange -1/2 em, strong, weak, gravitation top 3/2 em, strong, weak, gravitation bottom -1/2 em, strong, weak, gravitation Lepton e− 1 em, weak, gravitation νe 0 weak, gravitation µ− 1 em, weak, gravitation νµ 0 weak, gravitation τ − 1 em, weak, gravitation ντ 0 weak, gravitation Family Name Charge (e) Force carrier of Gauge boson γ 0 em gluon 0 strong W ± ±1 weak Z0 0 weak Table 2.1.: Elementary particles and the force carriers of the fundamental forces. neutrino to one of the charged lepton is called flavour. Each of these particles interact only via certain fundamental forces. There are four known fundamental forces: the electromagnetic (em) force, the strong force, the weak force and the gravitational force. While the electromagnetic and the gravitational force act on infinite scales the strong and the weak force only act on subatomic levels. All these forces interact via the exchange of gauge bosons. Every force is mediated via another one of these bosons. The boson mediating the electromagnetic force is the photon (γ), the ones mediating the weak force are the Z0, W + and W −, the ones mediating the strong force are gluons and the one mediating the gravitational force, called graviton, is, to this day, still hypothetical. An overview of the different particles, their charge and their interactions can be found in Table 2.1. Every particle has one corresponding anti-particle with same mass but opposite charge. For example, the anti-particle of the e− is the positron e+, with same mass but a positive charge. For further reading about the Standard Model of particle physics see [12] and [13]. 2.1.2. Neutrino oscillation In the last decade a number of experiments were conducted that showed neutrinos change their flavour over time. This can be explained with neutrino oscillations, resulting from the flavour eigenstates (νe, νµ and ντ ) being linear combinations of the mass eigenstates (ν1, ν2 and ν3). If we only consider the 2.1. Neutrino physics 7 eigenstates νµ, ντ , ν2 and ν3 the linear combinations would be νµ = ν2 cos θ23 + ν3 sin θ23 ντ = −ν2 sin θ23 + ν3 cos θ23 with θ23 the so-called mixing angle. If the eigenstates are evolved in time, one can show the probability of a νµ staying in its flavour is 2 ! 2 2 1.27∆m23L P (νµ → νµ) = 1 − sin (2θ23) sin , Eν 2 2 2 with m23 = m3 − m2 the difference in the squared masses, L the distance the neutrino travelled and Eν the energy of the neutrino.[14] In the 3-ν framework the mixing between the states is given in the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix U: −iδ 1 0 0 c13 0 e s13 c12 s12 0 U = 0 c23 s23 × 0 1 0 × −s12 c12 0 iδ 0 −s23 c23 −e s13 0 c13 0 0 1 with δ, the complex CP phase, cij = cos θij and sij = sin θij. Contradictory to the Standard Model of particle physics, the existence of neutrino oscillations imply a non-zeo neutrino rest mass.