[Paper Review] Dark Side of the Neutron?

Bartosz Fornal, Benjaḿın Grinstein|arXiv (Cornell University)|Nov 7, 2018

Atomic and Subatomic Physics ResearchPhysics and Astronomy56 citations

TL;DR

The paper argues the bottle-beam neutron lifetime discrepancy may indicate a neutron dark decay with a ~1% branching fraction into a dark sector, and develops simple models (n→χγ and n→χφ) to illustrate this possibility, plus experimental tests and astrophysical implications.

ABSTRACT

We discuss our recently proposed interpretation of the discrepancy between the bottle and beam neutron lifetime experiments as a sign of a dark sector. The difference between the outcomes of the two types of measurements is explained by the existence of a neutron dark decay channel with a branching fraction 1%. Phenomenologically consistent particle physics models for the neutron dark decay can be constructed and they involve a strongly self-interacting dark sector. We elaborate on the theoretical developments around this idea and describe the efforts undertaken to verify it experimentally.

Motivation & Objective

• Motivate the neutron lifetime discrepancy between bottle and beam experiments as a potential signal of new physics beyond the Standard Model.
• Propose neutron dark decay channels that could account for ~1% of decays without proton final states.
• Construct simple particle-physics models (n→χγ and n→χφ) that realize these decays while respecting baryon number and existing constraints.
• Analyze kinematic and mass-range requirements for dark final states using nuclear stability limits and Be-9 failure thresholds.
• Discuss experimental searches and astrophysical implications, including neutron stars and self-interacting dark sectors.

Proposed method

• Review the neutron decay framework and quantify how a non-proton final state with Br(n→p+anything)<1 can reconcile τn^beam and τn^bottle.
• Introduce effective Lagrangians for neutron-dark decays, matching to hadronic matrix elements (e.g., ΔΓ_n→χγ and ΔΓ_n→χφ).
• Derive mass-cascade constraints using nuclear stability, notably the Be-9 threshold: 937.900 MeV < Mf < 939.565 MeV.
• Present two minimal models: Model 1 (n→χγ) with a SM-singlet χ and a colored scalar Φ, and Model 2 (n→χφ) with χ, φ, and a mediator χ̃; give parameter relations that yield ΔΓ ≈ Γn/100.
• Discuss self-interacting dark sector extensions and compatibility with neutron-star constraints.

Experimental results

Research questions

• RQ1 Can a neutron dark decay with a ~1% branching reconcile the bottle and beam neutron lifetime measurements?
• RQ2 What are the allowed mass ranges for the dark final-state particles given nuclear stability and Be-9 constraints?
• RQ3 What minimal particle-physics models can realize n→χγ or n→χφ decays while preserving baryon number and avoiding existing bounds?
• RQ4 How do dark-sector self-interactions and related astrophysical constraints (e.g., neutron stars, DM) impact the viability of these scenarios?
• RQ5 What experimental signatures and current/future searches can test these neutron-dark-decay hypotheses?

Key findings

• A 1% dark decay branching can reconcile τn^beam with τn^bottle under the assumption of missing protonless decays.
• Mass range for a dark final state in n→χγ or n→χφ is constrained by Be-9 stability to 937.900 MeV < Mf < 939.565 MeV (and for χ alone, 937.9 MeV < mχ < 939.565 MeV).
• Photon energy in n→χγ lies in 0 < Eγ < 1.664 MeV (and 0.782 MeV < Eγ < 1.664 MeV if χ is dark matter and must be stable), with Eγ→0 as mχ→mn.
• Two minimal models realize the idea: Model 1 yields ΔΓ ≈ Γn/100 via n→χγ with a mixing parameter ε and β lattice factor; Model 2 yields ΔΓ ≈ Γn/100 for n→χφ with an intermediate state χ̃ and coupling λφ.
• Collider and low-energy constraints are satisfied for reasonable parameter choices (e.g., MΦ/√|λqλχ| ≈ 400 TeV in Model 1 with MΦ ≳ 1 TeV; similar scales in Model 2).
• Extensions with self-interacting dark sectors can address neutron-star constraints and may contribute to dark matter or baryogenesis, while also motivating searches in nuclear decays and meson decays.