# LIB2-160 — The fixed-Z isospin coefficient is selected rather than derived

- class: CLAIM · review_state: PUBLISHED · house_tier: not yet assigned
- labels as printed: math_status NOT_PRINTED · attachment NOT_PRINTED · scorecard tier word none (PDF p.133)
- basis: {"pdf_sha256_prefix": "b64ddcd9e16a6dcd", "release": "Rev33.1_S369"}

## Statement

The isospin Casimir coefficient χ = ϕ⁻⁶T3 is selected on the fixed-Z chains, not derived; the source carries the sector as loaded.

## Caution

The selected coefficient is not a derived isospin magnitude law or a prediction of the neutron–proton mass difference.

## Primary source

Paper 9, §9, Isospin coefficient, PDF p.138: "The isospin Casimir coefficient χ = φ−6 T3 is selected on the fixed-Z chains, not derived."

## Machine

- url: /library-v2/LIB2-160/
- content_sha256: 0c7fdf9ef60393342eb1f1be32b2cfc74ef451732f6ff46b82e32bb835b6bfbf
- candidate_ids: C2-129

## Freshness

- BUILD_STAMP S371a · 2026-09-26T22:53Z · master 01d1a5873ede42f5 · cut 9dcc6ce9a8ee3e02
- Declared volatile fields (R36): BUILD_STAMP