[Paper Review] Towards Powerful Probes of Neutrino Self-Interactions in Supernovae

Po-Wen Chang, Ivan Esteban+3 more|arXiv (Cornell University)|Jun 24, 2022

Neutrino Physics Research4 citations

TL;DR

This paper proposes that enhanced neutrino self-interactions (νSI) in core-collapse supernovae lead to a tightly coupled, expanding neutrino fluid that can produce either a burst or steady-state wind outflow. The key result is that νSI significantly extend the duration of the neutrino signal, making the observed duration in SN 1987A a powerful, model-independent probe of νSI strength.

ABSTRACT

Neutrinos remain mysterious. As an example, enhanced self-interactions ($ν$SI), which would have broad implications, are allowed. At the high neutrino densities within core-collapse supernovae, $ν$SI should be important, but robust observables have been lacking. We show that $ν$SI make neutrinos form a tightly coupled fluid that expands under relativistic hydrodynamics. The outflow becomes either a burst or a steady-state wind; which occurs here is uncertain. Though the diffusive environment where neutrinos are produced may make a wind more likely, further work is needed to determine when each case is realized. In the burst-outflow case, $ν$SI increase the duration of the neutrino signal, and even a simple analysis of SN 1987A data has powerful sensitivity. For the wind-outflow case, we outline several promising ideas that may lead to new observables. Combined, these results are important steps towards solving the 35-year-old puzzle of how $ν$SI affect supernovae.

Motivation & Objective

• To address the long-standing lack of robust observables for enhanced neutrino self-interactions (νSI) in supernovae despite high neutrino densities.
• To investigate how νSI alter neutrino dynamics, particularly the formation of a tightly coupled fluid that expands relativistically.
• To determine whether νSI lead to a burst-like or steady-state wind outflow, and to assess the observational consequences of each.
• To demonstrate that the duration of the neutrino signal from SN 1987A provides a powerful, model-independent constraint on νSI strength.
• To lay a foundation for future observables in the wind-outflow case, despite the complexity of the physics involved.

Proposed method

• Model the neutrino fluid as a relativistic, strongly coupled system under hydrodynamic expansion, using relativistic hydrodynamics to describe its evolution.
• Analyze two possible outflow morphologies: a burst-outflow (sudden decoupling from an extended fluid) and a steady-state wind (continuous, high-velocity outflow).
• Use the condition that νSI cause neutrinos to decouple collectively over a large spatial extent, increasing the signal duration beyond standard diffusion timescales.
• Apply a simplified model with momentum transfer between neutrinos and baryons via the neutrino-nucleon mean free path λνN to assess steady-state wind solutions.
• Solve the relativistic hydrodynamic equations with modified energy-momentum conservation to derive velocity and density profiles for both outflow types.
• Use the condition v = 1/√3 at the PNS edge as a critical boundary condition for the wind solution, ensuring continuity and physical consistency.

Experimental results

Research questions

• RQ1 How do enhanced neutrino self-interactions alter the dynamics of neutrino outflows in core-collapse supernovae?
• RQ2 Under what conditions does the neutrino fluid form a burst-like outflow versus a steady-state wind?
• RQ3 Can the duration of the neutrino signal from SN 1987A serve as a model-independent probe of νSI strength?
• RQ4 What physical conditions at the PNS edge are required to sustain a steady-state wind outflow with νSI?
• RQ5 What new observables could emerge from the wind-outflow scenario, and how might they be detected?

Key findings

• Neutrino self-interactions lead to a tightly coupled, expanding neutrino fluid that can produce a burst-like outflow with a significantly extended signal duration.
• In the burst-outflow case, the signal duration increases when the fluid decouples over a radial extent ℓ_FS larger than the standard diffusion scale ℓ₀, leading to a longer observed signal.
• A conservative analysis of SN 1987A data already shows strong sensitivity to νSI, with a 30-second signal duration providing a powerful constraint.
• The wind-outflow case requires a sharp transition at the PNS edge where v = 1/√3, and is only physically viable if the neutrino-nucleon mean free path λνN is of order the PNS radius.
• The wind solution requires rapid acceleration from v ≈ 0.14 to v ≈ 0.58 within a single mean free path at the PNS edge, indicating highly non-trivial initial conditions.
• The wind solution is the only continuous, steady-state outflow that satisfies the boundary conditions and energy-momentum conservation, but its stability and formation timescale remain open questions.