neutrons

• composite particle, a baryon: one up quark and two down quarks (udd)
• no electric charge, spin 1/2
• mass $1.675\times 10^{-27}$ kg (939.6 MeV/c²), about 0.14% heavier than the proton
• stable inside stable nuclei, free neutrons β-decay: n → p + e⁻ + ν̄ₑ
• free lifetime ≈ 880 s (~15 min), with bottle (≈ 878 s) and beam (≈ 888 s) measurements still disagreeing (CERN Courier, 2025)
• heavy nuclei have more neutrons than protons (²⁰⁸Pb: 82 p, 126 n), since neutrons add strong attraction without electric repulsion