We propose and demonstrate that peripheral neutron-$\alpha$ scattering at low energies can serve as a sensitive and clean probe of the long-range three-nucleon forces. To this aim, we perform ab initio quantum Monte Carlo calculations using two- and three-nucleon interactions derived in chiral effective field theory up to third expansion order. We show that the longest-range three-nucleon force stemming from the two-pion exchange plays a crucial role in the proper description of the neutron-$\alpha$ $D$-wave phase shifts. Our Letter reveals the predictive power of chiral symmetry in the few-body sector and opens a new direction for probing and constraining three-nucleon forces.

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• Elastic scattering reactions
• Few-body systems
• Nuclear forces
• A ≤ 5
• Ab initio calculations