Accelerator Neutrinos

Abstract:

This paper presents a comprehensive optimisation study to maximise the light collection efficiency of scintillating cube elements used in the SoLid detector. Very short baseline reactor experiments, like SoLid, look for active to sterile neutrino oscillation signatures in the anti-neutrino energy spectrum as a function of the distance to the core and energy. Performing a precise search requires high light yield of the scintillating elements and uniformity of the response in the detector volume. The SoLid experiment uses an innovative hybrid technology with two different scintillators: polyvinyltoluene scintillator cubes and $^6$LiF:ZnS(Ag) screens. A precision test bench based on a $^{207}$Bi calibration source has been developed to study improvements on the energy resolution and uniformity of the prompt scintillation signal of antineutrino interactions. A trigger system selecting the 1~MeV conversion electrons provides a Gaussian energy peak and allows for precise comparisons of the different detector configurations that were considered to improve the SoLid detector light collection. The light collection efficiency is influenced by the choice of wrapping material, the position of the $^6$LiF:ZnS(Ag) screen, the type of fibre, the number of optical fibres and the type of mirror at the end of the fibre. This study shows that large gains in light collection efficiency are possible compared to the SoLid SM1 prototype. The light yield for the SoLid detector is expected to be at least 52$\pm$2 photo-avalanches per MeV per cube, with a relative non-uniformity of 6 %, demonstrating that the required energy resolution of at least 14 % at 1 MeV can be achieved.

Abstract:

One of the main goals of the Long Baseline Neutrino Observatory (LBNO) is to study the $L/E$ behaviour (spectral information) of the electron neutrino and antineutrino appearance probabilities, in order to determine the unknown CP-violation phase $\delta_{CP}$ and discover CP-violation in the leptonic sector. The result is based on the measurement of the appearance probabilities in a broad range of energies, covering t he 1st and 2nd oscillation maxima, at a very long baseline of 2300 km. The sensitivity of the experiment can be maximised by optimising the energy spectra of the neutrino and anti-neutrino fluxes. Such an optimisation requires exploring an extended range of parameters describing in details the geometries and properties of the primary protons, hadron target and focusing elements in the neutrino beam line. In this paper we present a numerical solution that leads to an optimised energy spectra and study its impact on the sensitivity of LBNO to discover leptonic CP violation. In the optimised flux both 1st and 2nd oscillation maxima play an important role in the CP sensitivity. The studies also show that this configuration is less sensitive to systematic errors (e.g. on the total event rates) than an experiment which mainly relies on the neutrino-antineutrino asymmetry at the 1st maximum to determine the existence of CP-violation.

Abstract:

T2K is an experiment designed to study the ways in which neutrinos evolve as they propagate through space. The experiment uses an artificial beam of muon neutrinos, produced at the J-PARC facility in Japan. The spontaneous mixing of these beam neutrinos with other flavours is measured after 295 kilometres, at the location of the Super-Kamiokande detector. The initial beam composition is measured after 280 metres, using a combination of two near detectors. T2K is optimised for measurements of electron neutrino appearance, and muon neutrino survival. Essential for these measurements is a detailed knowledge of the initial beam composition, before neutrino oscillations had occurred. The production of intense neutrino beams at accelerator facilities is challenging, and requires exceptional understanding of chains of hadronic interactions initiated within thick targets. Most of T2K neutrinos are produced from in-flight decays of focused pions and kaons, emitted from an extended graphite target (90 cm) bombarded with a 30 GeV proton beam. Besides internal constraints from near detector data, T2K relies on hadron production measurements collected by other experiments, most importantly NA61, for accurate modelling of the initial neutrino flux.

This thesis presents a new T2K neutrino flux prediction, using the NA61 replica-target dataset from 2009. The earlier flux prediction relied on NA61 measurements collected with protons incident on a thin graphite target (2 cm). The following analysis incorporates NA61 data collected on the full length replica of the T2K target in 2009. In light of this new data, the flux calculation technique has been modified. A dramatic reduction in the unoscillated muon neutrino fractional flux uncertainty has been achieved, from ~10% to ~5% around the T2K flux peak. The impact of the improved flux prediction on oscillation measurements has been examined, by performing separate fits to ND280 data, and joint ND280 and Super-K data, in the well-established Bayesian Markov Chain Monte Carlo analysis framework.