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This thesis highlights data from MINOS, a long-baseline accelerator neutrino experiment, and details one of the most sensitive searches for the sterile neutrino ever made. Further, it presents a new analysis paradigm to enable this measurement and a comprehensive study of the myriad systematic uncertainties involved in a search for a few-percent effect, while also rigorously investigating the statistical interpretation of the findings in the context of a sterile neutrino model. Among the scientific community, this analysis was quickly recognized as a foundational measurement in light of which all previous evidence for the sterile neutrino must now be (re)interpreted. The existence of sterile neutrinos has long been one of the key questions in the field. Not only are they a central component in many theories of new physics, but a number of past experiments have yielded results consistent with their existence. Nonetheless, they remain controversial: the interpretation of the data showing evidence for these sterile neutrinos is hotly debated.
The MiniBooNE experiment was designed to perform a search for Vu --> Ve oscillations in a region of A[delta]sin 2 20very different from that allowed by standard, three neutrino oscillations, as determined by solar and atmospheric neutrino experiments. This search was motivated by the LSND experimental observation of an excess of F/e events in a 1,1 beam which was found compatible with two-neutrino oscillations at [delta]m 2 ~ 1 eV2 and sin2 20 1%. If confirmed, such oscillation signature could be attributed to the existence of a light, mostly-sterile neutrino, containing small admixtures of weak neutrino eigenstates. In addition to a search for Vu -- Ve oscillations, MiniBooNE has also performed a search for Vu -->Ve oscillations, which provides a test of the LSND two-neutrino oscillation interpretation that is independent of CP or CPT violation assumptions. This dissertation presents the MiniBooNE Vu -->Ve and Vu --> Ve analyses and results, with emphasis on the latter. While the neutrino search excludes the two-neutrino oscillation interpretation of LSND at 98% C.L., the antineutrino search shows an excess of events which is in agreement with the two neutrino Vu -->Ve oscillation interpretation of LSND, and excludes the no oscillations hypothesis at 96% C.L. Even though the neutrino and antineutrino oscillation results from MiniBooNE disagree under the single sterile neutrino oscillation hypothesis, a simple extension to the model to include additional sterile neutrino states and the possibility of CP violation allows for differences between neutrino and antineutrino oscillation signatures. In view of that, the viability of oscillation models with one or two sterile neutrinos is investigated in global fits to MiniBooNE and LSND data, with and without constraints from other oscillation experiments with similar sensitivities to those models. A general search for new physics scenarios which would lead to effective non-unitarity of the standard 3 x 3 neutrino mixing matrix, or mixing freedom, is also performed using neutrino and antineutrino data available from MiniBooNE.
The MiniBooNE experiment was designed to perform a search for [nu]{sub {mu}} 2![nu]{sub e} oscillations in a region of [Delta]m2 and sin2 2[theta] very different from that allowed by standard, three-neutrino oscillations, as determined by solar and atmospheric neutrino experiments. This search was motivated by the LSND experimental observation of an excess of {bar [nu]}{sub e} events in a {bar [nu]}{sub {mu}} beam which was found compatible with two-neutrino oscillations at [Delta]m2 H"1 eV2 and sin2 2[theta]
There is accumulating evidence for a difference between neutrino and antineutrino oscillations at the H" eV2 scale. The MiniBooNE experiment observes an unexplained excess of electron-like events at low energies in neutrino mode, which may be due, for example, to either a neutral current radiative interaction, sterile neutrino decay, or to neutrino oscillations involving sterile neutrinos and which may be related to the LSND signal. No excess of electron-like events ( -0.5 ± 7.8 ± 8.7), however, is observed so far at low energies in antineutrino mode. Furthermore, global 3+1 and 3+2 sterile neutrino fits to the world neutrino and antineutrino data suggest a difference between neutrinos and antineutrinos with significant (sin2 2[theta]{sub [mu]{mu}} H"35%) {bar [nu]}{sub {mu}} disappearance. In order to test whether the low-energy excess is due to neutrino oscillations and whether there is a difference between [nu]{sub {mu}} and {bar [nu]}{sub {mu}} disappearance, we propose building a second MiniBooNE detector at (or moving the existing MiniBooNE detector to) a distance of H"00 m from the Booster Neutrino Beam (BNB) production target. With identical detectors at different distances, most of the systematic errors will cancel when taking a ratio of events in the two detectors, as the neutrino flux varies as 1/r2 to a calculable approximation. This will allow sensitive tests of oscillations for both {nu}{sub e} and {bar {nu}} appearance and {nu}{sub {mu}} and {bar {nu}}{sub {mu}} disappearance. Furthermore, a comparison between oscillations in neutrino mode and antineutrino mode will allow a sensitive search for CP and CPT violation in the lepton sector at short baseline ([Delta]m2> 0.1 eV2). Finally, by comparing the rates for a neutral current (NC) reaction, such as NC [pi]° scattering or NC elastic scattering, a direct search for sterile neutrinos will be made. The initial amount of running time requested for the near detector will be a total of H"E20 POT divided between neutrino mode and antineutrino mode, which will provide statistics comparable to what has already been collected in the far detector. A thorough understanding of this short-baseline physics will be of great importance to future long-baseline oscillation experiments.
MINOS, Main Injector Neutrino Oscillation Search, is a long-baseline neutrino oscillation experiment in the NuMI muon neutrino beam at the Fermi National Accelerator Laboratory in Batavia, IL. It consists of two detectors, a near detector positioned 1km from the source of the beam and a far detector 734km away in Minnesota. MINOS is primarily designed to observe muon neutrino disappearance resulting from three avor oscillations. The Standard Model of Particle Physics predicts that neutrinos oscillate between three active avors as they propagate through space. This means that a muon type neutrino has a certain probability to later interact as a di erent type of neutrino. In the standard picture, the neutrino oscillation probabilities depend only on three neutrino avors and two mass splittings, [Delta]m2. An anomaly was observed by the LSND and MiniBooNE experiments that suggests the existence of a fourth, sterile neutrino avor that does not interact through any of the known Standard Model interactions. Oscillations into a theoretical sterile avor may be observed by a de cit in neutral current interactions in the MINOS detectors. A distortion in the charged current energy spectrum might also be visible if oscillations into the sterile avor are driven by a large mass-squared di erence, m2s 1 eV2. The results of the 2013 sterile neutrino search are presented here.
This thesis will present a search for sterile neutrinos using data taken with the MINOS experiment between 2005 and 2012. MINOS is a two-detector on-axis experiment based at Fermilab. The NuMI neutrino beam encounters the MINOS Near Detector 1km downstream of the neutrino-production target before traveling a further 734km through the Earth's crust, to reach the Far Detector located at the Soudan Underground Laboratory in Northern Minnesota. By searching for oscillations driven by a large mass splitting, MINOS is sensitive to the existence of sterile neutrinos through looking for any energy-dependent perturbations using a charged-current sample, as well as looking at any relative deficit in neutral current events between the Far and Near Detectors. This thesis will discuss the novel analysis that enabled a search for sterile neutrinos covering five orders of magnitude in the mass splitting and setting a limit in previously unexplored regions of the parameter space $\left\{\Delta m^{2}_{41}, \sin^2\theta_{24}\right\}$, where a 3+1-flavour phenomenological model was used to extract parameter limits. The results presented in this thesis are sensitive to the sterile neutrino parameter space suggested by the LSND and MiniBooNE experiments.
Tension among recent short baseline neutrino experiments has pointed toward the possible need for the addition of one or more sterile (non-interacting) neutrino states into the existing neutrino oscillation framework. This thesis first presents the motivation for sterile neutrino models by describing the short-baseline anomalies that can be addressed with them. This is followed by a discussion of the phenomenology of these models. The MiniBooNE experiment and results are then described in detail, particularly the most recent antineutrino analysis. This will be followed by a discussion of global fits to world data, including the anomalous data sets. Lastly, future experiments will be addressed, especially focusing on the MicroBooNE experiment and light collection studies. In particular, understanding the degradation source of TPB, designing the TPB-coated plates for MicroBooNE and developing lightguide collection systems will be discussed. We find an excess of events in the MiniBooNE antineutrino mode results consistent with the LSND anomaly, but one that has a different energy dependence than the low-energy excess reported in neutrino mode. This disagreement creates tension within global fits which include up to three sterile neutrinos. The low-energy excess will be addressed by the MicroBooNE experiment, which is expected to start taking data in early 2015. Tension among existing experiments calls for additional, more decisive future experiments.
This thesis, encompassing both theory to experiment, guides the reader in a pedagogical way through the author’s attempts to resolve the mystery of the so-called MiniBooNE anomaly, where unexpected neutrino oscillations were reported, potentially explainable by the existence of light sterile neutrinos, but in contradiction with several null results. Within this context, this thesis reports one of the first analyses searching for an excess of electrons in the MicroBooNE experiment finding no excess of events and narrowing down the possible explanations for the anomaly. Additionally, this thesis explores non-minimal heavy neutral leptons as potential explanations for the MiniBooNE excess. To search for evidence for this particle, the author performs an analysis using data from the T2K experiment, which searched for pairs of electrons using a gas argon time projection. This thesis provides a comprehensive explanation of the MiniBooNE anomaly and test of its possibile explanation with liquid and gas time projection chambers.
The MiniBooNE Experiment has contributed substantially to beyond standard model searches in the neutrino sector. The experiment was originally designed to test the $\Delta m^2$~1 eV$^2$ region of the sterile neutrino hypothesis by observing $\nu_e$ ($\bar\nu_e$) charged current quasi-elastic signals from a $\nu_\mu$ ($\bar\nu_\mu$) beam. MiniBooNE observed excesses of $\nu_e$ and $\bar\nu_e$-candidate events in neutrino and anti-neutrino mode, respectively. To date, these excesses have not been explained within the neutrino Standard Model ($\nu$SM), the Standard Model extended for three massive neutrinos. Confirmation is required by future experiments such as MicroBooNE. MiniBooNE also provided an opportunity for precision studies of Lorentz violation. The results set strict limits for the first time on several parameters of the Standard Model-Extension, the generic formalism for considering Lorentz violation. Most recently, an extension to MiniBooNE running, with a beam tuned in beam-dump mode, is being performed to search for dark sector particles. This review describes these studies, demonstrating that short baseline neutrino experiments are rich environments in new physics searches.