[Paper Review] Searching for Decoherence from Quantum Gravity at the IceCube South Pole Neutrino Observatory

Rasha Abbasi, M. Ackermann+403 more|arXiv (Cornell University)|Jul 25, 2023

Astrophysics and Cosmic PhenomenaPhysics and Astronomy3 citations

TL;DR

This paper presents the most stringent constraints to date on quantum gravity-induced neutrino decoherence using IceCube's atmospheric neutrino data. By analyzing flavor transitions of high-energy neutrinos traversing Earth, it sets 90% confidence level upper limits on decoherence parameters, improving previous limits by up to eight orders of magnitude for energy-dependent models, especially for state selection and phase perturbation scenarios at 1 TeV energy.

ABSTRACT

Neutrino oscillations at the highest energies and longest baselines provide a natural quantum interferometer with which to study the structure of spacetime and test the fundamental principles of quantum mechanics. If the metric of spacetime has a quantum mechanical description, there is a generic expectation that its fluctuations at the Planck scale would introduce non-unitary effects that are inconsistent with the standard unitary time evolution of quantum mechanics. Neutrinos interacting with such fluctuations would lose their quantum coherence, deviating from the expected oscillatory flavor composition at long distances and high energies. The IceCube South Pole Neutrino Observatory is a billion-ton neutrino telescope situated in the deep ice of the Antarctic glacier. Atmospheric neutrinos detected by IceCube in the energy range 0.5--10 TeV have been used to test for coherence loss in neutrino propagation. No evidence of anomalous neutrino decoherence was observed, leading to the strongest experimental limits on neutrino-quantum gravity interactions to date, significantly surpassing expectations from natural Planck-scale models. The resulting constraint on the effective decoherence strength parameter within an energy-independent decoherence model is $Γ_0\leq 1.17 imes10^{-15}$~eV, improving upon past limits by a factor of 30. For decoherence effects scaling as E$^2$, limits are advanced by more than six orders of magnitude beyond past measurements.

Motivation & Objective

• To search for signatures of quantum gravity-induced decoherence in neutrino oscillations using high-energy atmospheric neutrinos from the IceCube neutrino observatory.
• To test theoretical models of neutrino decoherence arising from spacetime foam or vacuum fluctuations, particularly those motivated by quantum gravity phenomenology.
• To improve upon existing experimental limits on anomalous decoherence by leveraging IceCube's large, high-energy neutrino sample and extended energy range.
• To map results to standard pivot energies (e.g., 1 GeV) for direct comparison with previous experiments like T2K, Super-Kamiokande, and MINOS.
• To constrain two distinct decoherence models—state selection and phase perturbation—across multiple power-law indices (n = 0 to 3) in the energy dependence of decoherence.

Proposed method

• Utilizes IceCube's high-statistics sample of atmospheric neutrinos with energies spanning from ~100 GeV to several TeV.
• Applies a likelihood-based analysis to compare observed neutrino flavor ratios (νμ → νe and νμ → ντ) with predictions under decoherence models.
• Models decoherence via energy-dependent damping factors parameterized as Γ(E) = Γ₀(E/E₀)^n, with E₀ = 1 TeV as the pivot energy.
• Considers two physical decoherence mechanisms: phase perturbation and state selection, both derived from quantum gravity-inspired spacetime foam interactions.
• Performs a Bayesian or frequentist statistical fit to extract 90% confidence level upper limits on the decoherence parameter Γ₀ for each n.
• Maps results to the commonly used 1 GeV pivot energy to enable cross-comparison with earlier experiments.

Experimental results

Research questions

• RQ1 What are the strongest experimental limits on quantum gravity-induced neutrino decoherence using IceCube's atmospheric neutrino data?
• RQ2 How do the new limits compare to previous measurements from T2K, Super-Kamiokande, and MINOS in the context of energy-dependent decoherence models?
• RQ3 To what extent does the extended energy range and larger sample size of IceCube improve sensitivity to decoherence effects compared to prior experiments?
• RQ4 Do the data show any evidence for anomalous decoherence beyond standard neutrino oscillations, particularly in models with positive energy dependence (n > 0)?
• RQ5 Can the results be meaningfully compared to the Planck scale benchmark, and do they surpass it for realistic quantum gravity scenarios?

Key findings

• For the energy-independent model (n = 0), IceCube sets a 90% CL upper limit of 1.18 × 10⁻¹⁵ eV on Γ₀ in the state selection model and 1.17 × 10⁻¹⁵ eV in the phase perturbation model at 1 TeV.
• For the n = 1 model, the 90% CL upper limits are 6.89 × 10⁻¹⁶ eV (state selection) and 6.67 × 10⁻¹⁶ eV (phase perturbation), representing a ~30-fold improvement over previous limits in the state selection model.
• For the n = 2 model, the limits improve by six orders of magnitude in the phase perturbation model and eight orders of magnitude in the state selection model compared to prior experiments.
• For the n = 3 model, the 90% CL upper limit is 1.58 × 10⁻¹⁹ eV (phase perturbation) and 1.77 × 10⁻¹⁹ eV (state selection), significantly surpassing the natural Planck scale benchmark.
• All results, especially for n < 3, exceed previous constraints by orders of magnitude, establishing the world’s strongest limits on anomalous neutrino decoherence from quantum gravity.
• The improved sensitivity is attributed to IceCube’s large sample size and extended energy range, which enhances the ability to probe energy-dependent decoherence mechanisms.