[Paper Review] Counting Electrons to Probe the Neutrino Mass Hierarchy

J. Brünner|arXiv (Cornell University)|Apr 23, 2013

Neutrino Physics Research9 references12 citations

TL;DR

This paper proposes using a 2600 km muon neutrino beam from IHEP Protvino to a megaton-scale underwater detector in the Mediterranean Sea to determine the neutrino mass hierarchy by counting electron neutrino-induced cascade events. The method exploits a 'magic' baseline where oscillation probabilities for νμ→νe differ significantly between normal and inverted hierarchy, enabling a 3σ mass hierarchy sensitivity with 1.5×10²¹ protons on target and 3–4% systematic uncertainty.

ABSTRACT

After the successful measurement of the mixing angle $θ_{13}$, the determination of the neutrino mass hierarchy has become a priority for future neutrino experiments. We propose a conventional $ν_μ$ beam with neutrino energies in the range 2-8 GeV aimed at a Mton underwater detector at the "magic" baseline of 2600 km. In this constellation it is sufficient to distinguish ($ν_μ$ induced) track-like interactions from cascade-like interactions with moderate purity to determine the mass hierarchy.

Motivation & Objective

• To determine the neutrino mass hierarchy (MH) using a conventional νμ beam and a large-volume underwater detector.
• To overcome challenges in atmospheric neutrino-based MH measurements due to neutrino-antineutrino cancellation and detector resolution.
• To demonstrate that counting νe-induced cascade events at a 2600 km baseline provides a robust, high-significance method for MH determination.
• To show that moderate purity in distinguishing track-like from cascade-like events suffices for MH sensitivity, reducing technical complexity.
• To provide a complementary approach to atmospheric neutrino measurements in the same detector, potentially enabling unambiguous MH determination.

Proposed method

• A conventional νμ beam with energies 2–8 GeV is directed at a 1 Mton underwater detector at a baseline of 2600 km, near the 'magic' baseline where MH-dependent oscillation effects are maximized.
• Oscillation probabilities P(νμ→νe) are calculated using the Globes package in a three-flavor scheme with global-fit parameters, showing a 13% peak in NH and strong MH dependence in the 3–8 GeV range.
• Neutrino cross sections are modeled using parton-scaled approximations: σCC_νμ ∝ Eν, with σCC_ντ derived from Ref. [12] and NC cross sections scaled as one-third of CC values.
• Detector response is simulated by separating events into track-like (from νμ CC) and cascade-like (from νe CC and ντ CC) channels, with background contributions from misreconstructed events, ντ, NC, and misclassified signal.
• Event rates are computed per energy bin, with signal reduced by 15–20% due to detection inefficiencies, and background contributions estimated using migration and cross-section models.
• Statistical significance of MH discrimination is evaluated using the total event count difference between NH and IH, with systematic uncertainties of 3–4% for cascade rates and flux normalization via νμ CC counting.

Experimental results

Research questions

• RQ1 Can a conventional νμ beam with a 2600 km baseline enable a high-significance determination of the neutrino mass hierarchy through νe cascade counting?
• RQ2 Does the 'magic' baseline of 2500–2600 km provide sufficient MH-dependent oscillation contrast in P(νμ→νe) to allow discrimination with moderate detector purity?
• RQ3 To what extent do systematic uncertainties—particularly in background estimation and flux normalization—affect the statistical significance of MH determination?
• RQ4 How does the performance of cascade-like event counting compare to track-like event counting in terms of MH sensitivity and background robustness?
• RQ5 Can this method be used in conjunction with atmospheric neutrino measurements in the same detector to achieve unambiguous MH determination?

Key findings

• The event rate difference between normal and inverted hierarchy for cascade-like events is 9–18% at 1.5×10²¹ protons on target, with a statistical uncertainty of 1.2%.
• The statistical significance of the MH hypothesis test remains above 7σ, and stays at the 3σ level even when a 3–4% systematic uncertainty is added for cascade event rate determination.
• Background contributions from NC, misreconstructed CC, ντ, and misclassified signal events sum to 15–18% of the total rate, with minimal MH dependence due to cancellation of effects.
• The signal contribution to cascade events is 1134–1547 for NH and 350–519 for IH, showing a strong MH-dependent signal that dominates over background.
• The method remains robust across varying CP-violating phases φCP, with no significant overlap between MH bands in the 3–8 GeV range, confirming the 'magic' baseline's effectiveness.
• A complementary measurement of νμ CC events can control flux normalization, reducing systematic uncertainty and improving MH sensitivity.