[Paper Review] Optimised sensitivity to leptonic CP violation from spectral information: the LBNO case at 2300 km baseline

LBNO Collaboration, Sanjib Kumar Agarwalla+158 more|arXiv (Cornell University)|Dec 1, 2014

Neutrino Physics Research12 references20 citations

TL;DR

This paper optimizes the neutrino beam spectrum for the Long Baseline Neutrino Observatory (LBNO) at a 2300 km baseline to maximize sensitivity to leptonic CP violation via spectral information. Using a genetic algorithm, it shows that exploiting both the first and second oscillation maxima significantly enhances CP-violation discovery potential, reducing reliance on neutrino-antineutrino asymmetry and improving robustness to systematic errors.

ABSTRACT

One of the main goals of the Long Baseline Neutrino Observatory (LBNO) is to study the $L/E$ behaviour (spectral information) of the electron neutrino and antineutrino appearance probabilities, in order to determine the unknown CP-violation phase $δ_{CP}$ and discover CP-violation in the leptonic sector. The result is based on the measurement of the appearance probabilities in a broad range of energies, covering t he 1st and 2nd oscillation maxima, at a very long baseline of 2300 km. The sensitivity of the experiment can be maximised by optimising the energy spectra of the neutrino and anti-neutrino fluxes. Such an optimisation requires exploring an extended range of parameters describing in details the geometries and properties of the primary protons, hadron target and focusing elements in the neutrino beam line. In this paper we present a numerical solution that leads to an optimised energy spectra and study its impact on the sensitivity of LBNO to discover leptonic CP violation. In the optimised flux both 1st and 2nd oscillation maxima play an important role in the CP sensitivity. The studies also show that this configuration is less sensitive to systematic errors (e.g. on the total event rates) than an experiment which mainly relies on the neutrino-antineutrino asymmetry at the 1st maximum to determine the existence of CP-violation.

Motivation & Objective

• To enhance sensitivity to leptonic CP violation in the neutrino sector using detailed spectral information from long-baseline neutrino oscillations.
• To address the challenge of limited sensitivity in experiments relying solely on the first oscillation maximum and neutrino-antineutrino asymmetry.
• To reduce dependence on total event rate systematic errors by leveraging multi-maxima spectral data.
• To optimize the neutrino beam energy spectrum using a genetic algorithm across complex beamline parameters.
• To demonstrate that both the first and second oscillation maxima are essential for high-precision determination of the CP-violating phase δCP.

Proposed method

• Employing a genetic algorithm to optimize the energy spectra of neutrino and antineutrino beams based on beamline parameters including proton beam, target, and focusing elements.
• Simulating neutrino and antineutrino appearance probabilities across a broad energy range covering the first and second oscillation maxima at 2300 km baseline.
• Using a double-phase liquid argon time projection chamber (TPC) detector with 20–70 kton mass to measure electron-like events and reconstruct energy spectra.
• Comparing sensitivity performance between standard proton beam (SPS) and high-power proton source (HPPS) beams under identical detector configurations.
• Applying energy cuts (e.g., 2.5 GeV) to isolate the impact of the second oscillation maximum on CP-violation sensitivity.
• Quantifying discovery potential via fractional coverage of the δCP parameter space at 3σ and 5σ confidence levels.

Experimental results

Research questions

• RQ1 How does the inclusion of the second oscillation maximum improve sensitivity to leptonic CP violation in long-baseline experiments?
• RQ2 To what extent can beam spectrum optimization via genetic algorithms enhance CP-violation discovery potential in LBNO?
• RQ3 How does the sensitivity of LBNO compare between SPS and HPPS beam configurations in terms of δCP coverage at 3σ and 5σ levels?
• RQ4 What is the impact of removing events below 2.5 GeV (i.e., excluding the second maximum) on the overall CP-violation sensitivity?
• RQ5 How robust is the optimized beam configuration to systematic uncertainties in total event rates compared to conventional approaches?

Key findings

• With a 20 kton LBNO detector and SPS beam, the experiment achieves 3σ sensitivity to CP violation for 45% of δCP values, increasing to 63% with a 70 kton detector.
• At 5σ, the SPS-based setup covers 35% of the δCP parameter space with a 70 kton detector, demonstrating significant discovery potential.
• The HPPS beam configuration enables 5σ sensitivity for 65% of δCP values and 3σ sensitivity for 80% of the parameter space, significantly outperforming SPS.
• Removing events below 2.5 GeV—thereby eliminating information from the second oscillation maximum—reduces 5σ discovery coverage by over half in the HPPS case (from 65% to 28% for 70 kton).
• The second oscillation maximum contributes critically to CP-violation sensitivity, with a 17% loss in signal events causing a disproportionate drop in discovery potential.
• The optimized beam configuration is less sensitive to systematic errors on total event rates than methods relying solely on neutrino-antineutrino asymmetry at the first maximum.