[Paper Review] Sensitivity to eV-scale Neutrinos of Experiments at a Very Low Energy Neutrino Factory

C. Tunnell, J.H. Cobb+1 more|arXiv (Cornell University)|Nov 28, 2011

Neutrino Physics Research1 references3 citations

TL;DR

This paper proposes a Very Low Energy Neutrino Factory (VLENF) using a 3 GeV muon storage ring to test eV-scale sterile neutrino hypotheses, particularly the unconfirmed LSND anomaly. By measuring wrong-sign muon appearance via $ ue \to \nu_\mu$ oscillations at 800 m baseline, the VLENF achieves $7\sigma$ sensitivity to the LSND and MiniBooNe sterile neutrino fits, exceeding the $5\sigma$ threshold for confirmation or exclusion.

ABSTRACT

The results of LSND have yet to be confirmed at the $5 σ$-level. An experiment is proposed utilizing a 3 GeV muon storage ring that would allow for both disappearance and appearance channels to be explored at short-baselines. The appearance channel could provide well over $5 σ$ confirmation or rejection of the LSND result. Other physics could also be performed at such a facility such as the measurement of electron-neutrino cross sections. The sensitivity of experiments at a Very Low Energy Neutrino Factory (VLENF) to neutrinos at the eV-scale is presented.

Motivation & Objective

• To provide a $5\sigma$ confirmation or refutation of the unconfirmed LSND anomaly involving eV-scale sterile neutrinos.
• To measure electron-neutrino cross sections with high precision using a near detector at 20–50 m baseline.
• To explore short-baseline oscillations in the (3+1) neutrino model with a dedicated experiment at a Very Low Energy Neutrino Factory (VLENF).
• To assess the sensitivity of a VLENF to sterile neutrino parameters, particularly $|U_{e4}|^2$ and $|U_{\mu4}|^2$, using realistic detector and beam systematics.
• To demonstrate that a VLENF with 3 GeV muons and 800 m baseline can achieve $7\sigma$ sensitivity to the LSND and MiniBooNe $\bar{\nu}$ data, surpassing the $5\sigma$ requirement.

Proposed method

• Utilizes a fixed-field alternating gradient (FFAG) muon storage ring optimized for 2 GeV muons to produce a directed neutrino beam via $\mu^+ \to e^+ \bar{\nu}_\mu \nu_e$ decay.
• Employs a near detector (20–50 m) for measuring neutrino-nucleon cross sections, including the first precision measurement of $\nu_e$ cross sections.
• Deploys a far detector at 800 m baseline to observe $\nu_e \to \nu_\mu$ appearance via wrong-sign muon events, the 'golden channel' for sterile neutrino detection.
• Applies the $\chi^2$-based pull method with marginalized systematics (2% signal, 20% background normalization errors) and spectral information to compute sensitivity.
• Uses Monte Carlo integration to compute the phase-space-averaged neutrino flux, accounting for detector size and baseline comparable to accelerator straight.
• Extends GLoBES software with the SNU add-on to model $4 \times 4$ mixing matrices in the (3+1) sterile neutrino framework for oscillation probability calculations.

Experimental results

Research questions

• RQ1 Can a VLENF at 3 GeV achieve $5\sigma$ sensitivity to the LSND anomaly, thereby confirming or refuting eV-scale sterile neutrinos?
• RQ2 What is the optimal baseline and muon energy for maximizing sensitivity to $\nu_e \to \nu_\mu$ appearance in the (3+1) model?
• RQ3 How does the inclusion of spectral information and systematics affect the sensitivity to sterile neutrino parameters in the VLENF setup?
• RQ4 To what extent can the VLENF distinguish between the LSND and MiniBooNe $\bar{\nu}$ data within a (3+1) sterile neutrino fit?
• RQ5 What are the dominant backgrounds to the $\nu_e \to \nu_\mu$ appearance channel, and how do they impact the statistical significance of the signal?

Key findings

• The VLENF achieves $7\sigma$ sensitivity to the 99% confidence interval of the LSND and MiniBooNe $\bar{\nu}$ sterile neutrino fits, exceeding the $5\sigma$ requirement for confirmation or exclusion.
• The optimal baseline for short-baseline oscillation sensitivity is approximately 800 meters, with sensitivity remaining robust above 2 GeV muon energy.
• After applying cuts, the signal yields 27 events with only 2 background events, indicating a high signal-to-background ratio for the golden channel.
• The far detector's sensitivity is maximized when the muon energy is fixed at 3 GeV, aligning with cross-section physics needs and optimizing oscillation sensitivity.
• Systematic uncertainties (2% signal, 20% background normalization) are marginalized over, and the sensitivity remains strong due to spectral information and the pull method.
• The study identifies potential challenges, including cosmic muon backgrounds and charge misidentification at low energies, and suggests RF bunching as a mitigation strategy.