First measurement of neutrino oscillation parameters using neutrinos and antineutrinos by NOvA (1906.04907v2)

Abstract: The NOvA experiment has made a $4.4\sigma$-significant observation of $\bar\nu_{e}$ appearance in a 2 GeV $\bar\nu_{\mu}$ beam at a distance of 810 km. Using $12.33\times10^{20}$ protons on target delivered to the Fermilab NuMI neutrino beamline, the experiment recorded 27 $\bar\nu_{\mu} \rightarrow \bar\nu_{e}$ candidates with a background of 10.3 and 102 $\bar\nu_{\mu} \rightarrow \bar\nu_{\mu}$ candidates. This new antineutrino data is combined with neutrino data to measure the oscillation parameters $|\Delta m^2_{32}| = 2.48^{+0.11}_{-0.06}\times10^{-3}$ eV$^2/c^4$, $\sin^2 \theta_{23} = 0.56^{+0.04}_{-0.03}$ in the normal neutrino mass hierarchy and upper octant and excludes most values near $\delta_{\rm CP}=\pi/2$ for the inverted mass hierarchy by more than 3$\sigma$. The data favor the normal neutrino mass hierarchy by 1.9$\sigma$ and $\theta_{23}$ values in the upper octant by 1.6$\sigma$.

• The paper presents the inaugural measurement of neutrino oscillation parameters using a combined neutrino and antineutrino dataset.
• It employs a long-baseline setup with near and far detectors to measure 2 GeV neutrino interactions over an 810 km distance.
• Key findings include tight constraints on Δm²₃₂ and θ₂₃, a significant electron-neutrino appearance signal, and implications for the mass hierarchy.

Measurement of Neutrino Oscillation Parameters by NOvA

The NOvA experiment, a prominent long-baseline neutrino oscillation experiment, presents its first measurements of oscillation parameters by examining interactions of neutrinos and antineutrinos. This paper combines new antineutrino data with previous neutrino data to yield important insights into neutrino oscillation physics, a field that probes the fundamental nature of neutrinos and their interactions.

Overview of the Experimental Setup

NOvA utilizes a sophisticated apparatus to measure oscillations in neutrino beams originating from Fermilab's NuMI beamline. The experiment involves two primary detectors: a Near Detector (ND) sited 1 km from the source and a Far Detector (FD) located 810 km downstream. Both detectors capture interactions of an off-axis neutrino beam predominantly composed of 2 GeV neutrinos. Through intricate simulations and detailed modeling, the NOvA experiment aims to compare the observed neutrino interactions against those predicted by the no-oscillation hypothesis, thereby providing insights into oscillation parameters.

Key Oscillation Results and Measurements

The analysis, grounded on a dataset equivalent to $12.33\times10^{20}$ protons on target, and supplemented by previously reported proton deliveries, accounts for neutrino oscillations involving the transition of muon neutrinos ($\nu_\mu$) to electron neutrinos ($\nu_e$) and vice versa. Key findings include:

• The oscillation parameters are measured as $$|\Delta m^2_{32}| = (2.48^{+0.11}_{-0.06})\times10^{-3}$$ eV$^2/c^4$ and $\sin^2 \theta_{23} = 0.56^{+0.04}_{-0.03}$ under the assumption of the normal mass hierarchy.
• A significant $4.4\sigma$ excess over the expected background is recorded for the $\nu_\mu \to \nu_e$ appearance, marking the first such observation using an antineutrino beam over a long baseline.
• The analysis excludes most values near the CP-violating phase $\delta_{\rm CP}=\pi/2$ for the inverted mass hierarchy at a confidence level exceeding $3\sigma$.
• Data favor the normal hierarchy with a $1.9\sigma$ significance and indicate a preference for $\theta_{23}$ in the upper octant by $1.6\sigma$.

Systematic Considerations and Methodology

The sophisticated methodology employed accounts for potential systematic uncertainties related to neutrino flux, cross-sections, detector responses, and energy calibration. These are constrained in a combined fit of data by penalty terms to a comprehensive likelihood function. The results indicate NOvA's careful systematic assessment in its measurements and the robustness of the experimental techniques deployed.

Implications and Theoretical Considerations

This research holds substantial implications for both theoretical and experimental neutrino physics. The insights into the $\Delta m^2_{32}$ and $\theta_{23}$ parameters contribute to the broader understanding of the neutrino mass hierarchy, a pivotal aspect influencing theoretical models of particle physics that explore beyond the Standard Model. Furthermore, the results concerning the CP-violating phase $\delta_{\rm CP}$ could illuminate phenomena related to matter-antimatter asymmetry in the universe.

Prospects for Future Developments

The NOvA experiment's outcomes underscore the importance of continual refinement in experimental approaches. Future analyses could refine these measurements further by integrating additional data and enhancing detector technologies. Additionally, continued comparison with results from other experiments such as T2K and DUNE could further elucidate the intricate nature of neutrino oscillations.

In summary, the NOvA experiment significantly advances the understanding of neutrino oscillation parameters. Its findings lay critical groundwork that informs future explorations in particle physics, offering a deeper glimpse into the properties of neutrinos and their role in the cosmos.