Design and Development of an Acoustic Calibrator for Deep-Sea Neutrino Telescopes and First Search for Secluded Dark Matter with ANTARES

Resum

Neutrino astronomy is a booming field in astroparticle physics. Due to the particular characteristics of neutrinos, these particles offer great advantages as probes for the study of the far and high-energy Universe. Moreover, it is extensively accepted by the scientific community that a multi-messenger approach with the combination of information provided by neutrinos, photons and charged particles (cosmic rays) is possible to obtain a more complete image of the fundamental astrophysics processes taking place in our Universe. Furthermore, neutrinos also provide a unique way to understand some particle physics principles. As an example, the evidences that neutrinos have mass and flavour mixing did come from observations in the first neutrino telescopes. Since neutrinos are neutral and very weak interacting particles they can reach the Earth from astrophysical sources without deflection by magnetic fields and almost without energy losses and absorption, contrarily to the rest of messengers. The other side of the coin of neutrino properties is that detection of neutrinos is very challenging and big highly instrumented detection volumes are needed. Natural media (deep sea, lakes or ice in the Antarctica) host this kind of experiments using the water (or ice) as target material where the neutrino interaction is produced. ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) is the first undersea neutrino telescope, located at 2475 m depth in the Mediterranean Sea. It has been built by an international collaboration with funds from European Union and participating countries. ANTARES is optimized for optical detection of the Cerenkov light induced by relativistic muons produced by high energy neutrino interactions near the detector. The charge, position and arrival time of the photons to the optical modules which compose the detector allows the muon track reconstruction, and thus, knowing the neutrino coming direction with high angular resolution. Some information of the event energy is also derived. In addition, ANTARES is also hosting the AMADEUS (Antares Modules for Acoustic DEtection Under Sea) experiment which is investigating the feasibility of the acoustic detection of Ultra-High Energy (UHE) neutrinos. The framework of this thesis is the ANTARES experiment. In this sense, Chapter 1 describes the telescope and is dedicated to contextualize the work. As commonly done in the thesis developed in this experiment (and in this field), the work has been divided in two different areas. On the one hand, a part more devoted to technological aspects related to the detector and, on the other hand, a part dedicated to ANTARES data analysis. For the context and the characteristics of the activities performed, training in different fields has been necessary: neutrino telescopes, astroparticle physics, etc. Moreover different skills have been developed as well, such as instrumentation, computer applications, simulation techniques, massive data analysis, etc. The first part of the thesis is focused in the development of a calibrator able to reproduce the acoustic signal generated in the UHE neutrino interaction with a water nucleus which, roughly speaking, generates a highly directive bipolar acoustic pulse. Having a good calibrator is crucial to test and tune the telescope response for this kind of signals. Chapter 2 describes the processes that intervene in the acoustic emission due the UHE neutrino interaction, the propagation conditions and background noise existing in the ANTARES site. The concept of parametric acoustic sources is also introduced as starting point for the calibrator design. The initial tests performed to evaluate the parametric acoustic technique for the generation of the characteristic acoustic neutrino pulse are presented in Chapter 3. These works were pioneering in the parametric generation of transient signals using cylindrical symmetry transducers and demonstrated the applicability of this technique for the development of a compact acoustic neutrino detection calibrator. Chapter 4 is devoted to describe the design and tests of the prototype developed, a three element array assembled in a compact structure. It is able to operate in low and high-frequency modes to increase the functionality for both possible uses: in deep-sea or being operated from a vessel. The second part of the thesis, the data analysis part, is centred in the analysis of the ANTARES data in order to constrain possible Dark Matter models. This work is focused on the detection of products resulting of the Dark Matter annihilation trapped in the centre of the Sun. Specifically, the Secluded Dark Matter (SDM) model has been tested by the detection of di-muons (co-linear muon pair) and/or neutrinos coming from Sun direction. Broadly speaking, this model is based on the idea of the existence of a mediator resulting of the Dark Matter annihilation which, subsequently, would decay into standard model particles as muons or neutrinos. These models have been proposed in order to explain some experimental ¿anomalies¿ observed, such as the electron-positron ratio spectrum detected in satellites, measured recently with high accuracy by AMS-II. The study of this thesis constitutes the first search of experimental evidences of this kind of models in neutrino telescopes. The evidences of the Dark Matter in the Universe and the methods to detect it are reviewed in Chapter 5. Dark Matter searches in ANTARES have been summarized, among others, the ones using the Sun as source. The SDM model characteristics are also introduced. Chapter 6 describes the methodology and tools used in this analysis. A dedicated code has been developed for the simulation of di-muon generation from mediator decay, and its detection in ANTARES, in order to know the detector response to this kind of signal. To finalize, Chapter 7 deals with the data analysis process and the interpretation in terms of indirect SDM search. Since the conclusion of the analysis is that there is not a significant signal excess, limits to SDM models in neutrino telescopes have been established being the first time that these models are constrained in neutrino telescopes. The imposed limits to these models are the more restrictive ones for a wide range of values of the parameters to consider: Dark Matter mass and mass, and lifetime, of the mediator. Therefore, these results improve and/or complement the limits obtained by different methods, such as Dark Matter direct detection or positron and gamma ray indirect detection searches.