[Paper Review] Future Neutrino Oscillation Sensitivities for LBNE

Matthew Bass, D. Cherdack+1 more|arXiv (Cornell University)|Oct 25, 2013

Neutrino Physics Research4 references3 citations

TL;DR

This paper evaluates the future neutrino oscillation sensitivity of the Long-Baseline Neutrino Experiment (LBNE) using simulated event spectra and the GLoBES software framework. It demonstrates that LBNE will achieve $1 ext{--}20^{ extdegree}$ resolution on the CP-violating phase $\delta_{\text{CP}}$, detect CP violation at $3\sigma$ for 67% of $\delta_{\text{CP}}$ values, and determine the neutrino mass hierarchy with $\Delta\chi^2 \geq 25$, significantly advancing the precision of neutrino mixing parameter measurements beyond current experiments.

ABSTRACT

The primary goal of the Long-Baseline Neutrino Experiment (LBNE) is to measure the neutrino mixing matrix parameters. The design, optimized to search for CP violation and to determine the neutrino mass hierarchy, includes a large $\mathcal{O}(10$ kt) Liquid Argon Time Projection Chamber (LAr TPC) at 1300 km downstream of a wide-band neutrino beam. A brief introduction to the neutrino mixing parameters will be followed by a discussion of sensitivity study analysis methods and a summary of the results for LBNE. The studies include comparisons with the Tokai-to-Kamioka (T2K) and NuMI Off-axis electron-neutrino Appearance (NO$ν$A) experiments as well as combined sensitivities. Finally, the impact of including a realistic set of systematic uncertainties will be presented.

Motivation & Objective

• To assess the sensitivity of the Long-Baseline Neutrino Experiment (LBNE) to key neutrino oscillation parameters, including $\delta_{\text{CP}}$, the neutrino mass hierarchy, and the $\theta_{23}$ octant.
• To evaluate the impact of systematic uncertainties—particularly signal and background normalization errors—on parameter resolution and discovery potential.
• To compare LBNE's projected performance with existing experiments (T2K, NOvA) and quantify the added sensitivity from combined global fits.
• To determine the required exposure and detector configuration (LBNE10 vs. full LBNE) for achieving high-precision measurements of CP violation and mass hierarchy.
• To validate statistical methods for mass hierarchy determination, addressing limitations of standard frequentist approaches.

Proposed method

• Sensitivity studies use the GLoBES software library to compute $\Delta\chi^2$ between true and test oscillation hypotheses across parameter space.
• Event spectra for $\nu_e$ appearance and $\nu_\mu$ disappearance are simulated using parameterized neutrino fluxes, cross sections, energy resolutions, and detection efficiencies.
• The analysis accounts for $1\%$ normalization uncertainty on signal and $5\%$ on background events, with oscillation parameters updated from the Fogli et al. 2012 global fit.
• The probability of $\nu_\mu \to \nue$ transitions is modeled using a matter-corrected approximation that includes $\delta_{\text{CP}}$, $\theta_{23}$, and $\Delta m^2_{31}$.
• Systematic uncertainties are varied to assess their impact on $\delta_{\text{CP}}$ resolution and mass hierarchy determination, with results visualized via $\Delta\chi^2$ contours.
• Combined sensitivity is evaluated by fitting LBNE with T2K and NOvA data, showing improved performance in low-exposure regimes.

Experimental results

Research questions

• RQ1 What is the expected resolution on the CP-violating phase $\delta_{\text{CP}}$ for LBNE and its initial stage (LBNE10), and how does it depend on true $\delta_{\text{CP}}$ and $\theta_{23}$?
• RQ2 Can LBNE detect CP violation at the $3\sigma$ confidence level across a broad range of $\delta_{\text{CP}}$ values, and how does this sensitivity vary with $\theta_{23}$?
• RQ3 What is the sensitivity of LBNE to determine the neutrino mass hierarchy, and how does $\Delta\chi^2$ for hierarchy discrimination scale with $\delta_{\text{CP}}$?
• RQ4 How do systematic uncertainties—especially normalization errors—affect the precision of $\delta_{\text{CP}}$ and mass hierarchy measurements?
• RQ5 To what extent does combining LBNE data with T2K and NOvA data enhance sensitivity to $\delta_{\text{CP}}$ and mass hierarchy?

Key findings

• LBNE is projected to resolve $\delta_{\text{CP}}$ to within $10^\circ$ to $20^\circ$ at $1\sigma$ for a wide range of true $\delta_{\text{CP}}$ values, assuming normal mass hierarchy and current parameter uncertainties.
• LBNE will detect CP violation at the $3\sigma$ level for $67\%$ of $\delta_{\text{CP}}$ values when $\sin^2\theta_{23} = 0.39$, significantly outperforming LBNE10, which achieves $3\sigma$ sensitivity for $40\%$ of $\delta_{\text{CP}}$ values.
• The experiment will determine the neutrino mass hierarchy with $\Delta\chi^2 \geq 25$ for nearly all $\delta_{\text{CP}}$ values, indicating high confidence in hierarchy determination.
• For the worst-case scenario in mass hierarchy determination (around $\delta_{\text{CP}} = 90^\circ$), the inclusion of T2K and NOvA data improves $\Delta\chi^2$ from $\sim 9$ to $\sim 16$, demonstrating synergy in early phases.
• With high exposure, the combined T2K+NOvA+LBNE fit provides little additional sensitivity gain over LBNE alone, indicating LBNE's dominance in high-precision regime.
• The study confirms that standard frequentist methods may misrepresent mass hierarchy determination probabilities, and highlights the need for more robust statistical treatments as proposed by Qian et al.