Nuclear reaction Q-value ($Q=(m_{\mathrm{initial}}-m_{\mathrm{final}})c^2$)

The initial minus final rest energy in a nuclear reaction; positive values represent energy available for outgoing kinetic energy in addition to the incident kinetic energy.

The same compound-state energy is measured from different rest-energy thresholds in different entrance channels. Subtracting those thresholds converts the required centre-of-momentum kinetic energies. For a stationary target and nonrelativistic projectile, the laboratory kinetic energy is $T_{\mathrm{lab}}=T^{\ast }(m_{\mathrm{projectile}}+m_{\mathrm{target}})/m_{\mathrm{target}}$. A negative converted $T^{\ast }$ means that entrance channel is closed at that resonance energy.