Loic-rene Labit – Phd’s defense – nov. 16, 2021

The neutrino group at LAPP proposes an experimental project on neutrino physics, on the study of very short baseline neutrino oscillations with the STEREO detector. The discovery of neutrino oscillations is one of the major accomplishments in particle physics of the last decades (2015 Nobel prize), and it proves that the neutrinos have a non-zero mass. This fact, whose profound reason remains unexplained today, has nevertheless been embedded in the Standard Model of particle physics. A huge experimental effort is ongoing ever since, aiming at a precise experimental measurement of all the parameters involved. Historically, the first experimental indications of neutrino oscillations came from two different experiments failing to observe the predicted fluxes for the neutrinos coming from the Sun (the Homestake experiment in the 1970s, later beautifully extended and confirmed by SNO in 2002) and from the atmosphere (SuperKamiokande, 1998). Intriguingly, two recent results hint at an unknown neutrino disappearance at short baselines in a very similar way. The calibration runs of the Gallium-based experiments carried out in the late 1990s (SAGE, GALLEX) all systematically point to a shortage of observed neutrinos with respect to the expectations. Neutrino experiments exposed to nuclear reactors observe, too, a lack of neutrinos when compared to the latest computation of neutrino fluxes. The analogy with the historical discovery of neutrino oscillations is readily made: a fraction of the neutrinos from the Gallium experiments calibration sources and from the nuclear reactors would be oscillating into a fourth, unknown type of neutrino. 

This new neutrino would go undetected, which means it would interact with ordinary matter even less than the three known types (electron, muon and tau neutrinos). It is thus said to be a sterile neutrino. Since the neutrino baselines are rather short in the two cases, the neutrino oscillation length at the energies of these experiments would be of only a few meters. For more details, see, for instance, G. Mention et al., arXiv:1101.2755, Phys. Rev. D 83, 073006 (2011). Several experiments are, or will shortly be, underway with the goal of studying these anomalous experimental results. The STEREO experiment, sitting less than 10 m away from the core of the nuclear reactor at the Laue-Langevin Institute in Grenoble, is currently taking data till the end of 2019. Unlike the previous hints, which all rely on flux measurements, the STEREO concept consists in precisely measuring the energy spectrum of the reactor antineutrinos, and looking in it for the tell-tale distortions that a neutrino oscillation would induce. Indeed, the neutrino oscillation formula has a definite energy and baseline dependence that would be imprinted on the spectrum of the unoscillated neutrinos (recall that Posc = sin2(2θnew) sin2(Δm2new L/4 E)). The small reactor core size (diameter < 40 cm), the short neutrino baseline L (8 to 12 meters) and the good energy resolution of the STEREO experiment will allow us to confirm or refute the sterile neutrino hypothesis as an explanation for the anomalous results. The LAPP is in charge of the radioactive source calibration system, which is absolutely essential to the understanding of the absolute energy scale, and, more generally, the whole detector response. The PhD work revolves around two topics. 

The first one is the calibration of the detector, in which the LAPP takes a leading role. This work comprises data-taking responsibilities during the physics and calibration runs, the extraction of the calibration constants and the monitoring of their time evolution, and the implementation of improvements and corrections in the simulations and in the neutrino reconstruction software chain, especially regarding the computation of the neutrino energy. The second topic is the analysis itself of the neutrino data, the goal being to confirm or refute the oscillations to an unknown neutrino state. Apart from the selection criteria, that the student may be led to improve, the PhD candidate will study the signal and the backgrounds, and work on the statistical algorithms and models needed to extract the oscillation frequency and the mixing angle of the hypothetical oscillation, and derive the corresponding exclusion contours.