Improved search for muon-neutrino to electron-neutrino oscillations in MINOS (1108.0015v1)

Abstract: We report the results of a search for $\nu_{e}$ appearance in a $\nu_{\mu}$ beam in the MINOS long-baseline neutrino experiment. With an improved analysis and an increased exposure of $8.2\times10^{20}$ protons on the NuMI target at Fermilab, we find that $2\sin^2(\theta_{23})\sin^2(2\theta_{13})<0.12\ (0.20)$ at 90% confidence level for $\delta\mathord{=}0$ and the normal (inverted) neutrino mass hierarchy, with a best fit of $2\sin^2(\theta_{23})\sin^2(2\theta_{13})\,\mathord{=}\,0.041^{+0.047}_{-0.031}\ (0.079^{+0.071}_{-0.053})$. The $\theta_{13}\mathord{=}0$ hypothesis is disfavored by the MINOS data at the 89% confidence level.

• The paper refines neutrino oscillation measurements using improved LEM algorithms and increased data exposure to constrain the θ13 mixing angle.
• The study enhances background discrimination with a neural network approach applied to MINOS's dual-detector system over a 735 km baseline.
• The results disfavor the null θ13 hypothesis at 89% CL, significantly narrowing the parameter space for neutrino oscillation models.

Analytical Evaluation of Muon-Neutrino to Electron-Neutrino Oscillations in the MINOS Experiment

The paper under investigation details an advanced analysis of muon-neutrino to electron-neutrino oscillations within the framework of the MINOS long-baseline neutrino experiment. The experiment, established to investigate the quantitative characteristics of neutrino oscillation parameters, primarily focuses on refining the constraints on the mixing angle $\theta_{13}$, an essential component of the PMNS matrix, among three-neutrino oscillations.

Within the context of increased exposure, totaling $8.2 \times 10^{20}$ protons on target at the NuMI beamline at Fermilab, the research utilizes substantial improvements in data analysis techniques. The results posit an upper limit for the combined oscillation parameter $2\sin^2(\theta_{23})\sin^2(2\theta_{13})$ to be less than 0.12 (0.20) at a 90% confidence level for a CP-violating phase $\delta = 0$, considering a normal (inverted) neutrino mass hierarchy. The best-fit values for these parameters adjust to $0.041^{+0.047}_{-0.031}$ (normal hierarchy) and $0.079^{+0.071}_{-0.053}$ (inverted hierarchy).

The findings leverage an increased dataset and improvements in analysis methodologies, particularly through the implementation of a nearest-neighbors algorithm termed "library event matching" (LEM). This approach enhances the precision in discriminating between background interactions and signal events, making significant use of an artificial neural network to optimize detection.

From a theoretical standpoint, the paper operates within the well-accepted three-flavor neutrino oscillation model. It addresses the effects of CP violation and matter interactions, incorporating a rigorous statistical handling of systematic uncertainties.

Practically, the analysis stems from the substantial design and execution of the two-detector system of MINOS, comprising a near detector (ND) and a far detector (FD), both constructed to maximize semblance while permitting controlled examination of neutrino interactions over the 735 km beam line. This careful implementation, along with detailed event characterization methods, allows for minimization of systematic backgrounds, enhancing the sensitivity of the detection capabilities.

While previous paradigms had hinted at a potential non-zero value for $\theta_{13}$, this paper corroborates such indications with remarkable confidence, disfavoring the null $\theta_{13}$ hypothesis at the 89% confidence level. It is pertinent to note the precise control over simulated versus observational data alignment, primarily facilitated via the sophisticated LEM classifiers which accurately navigate the convoluted energy patterns characterizing neutrino events in MINOS's scintillator strips.

The implications of these findings are far-reaching. The constraints on $\theta_{13}$ significantly narrow the parameter space available, thereby limiting the oscillation model configurations supported by the data. This information contributes substantially to the global understanding of neutrino physics, having potential ramifications on future neutrino research directives and experiments, particularly those probing CP violation and the neutrino mass hierarchy.

Conclusively, the presented paper in the MINOS framework is a substantial advancement in precise neutrino oscillation measurements. It sets the foundation for prospective experimental inquiries and theoretical interpretations concerning neutrino properties and interactions within established particle physics paradigms. Future work might focus on further augmentations in experimental setup sensitivity and the exploration of complementary experimental avenues to probe CP-violation effects in neutrinos more articulately.