Fixed-target reaction threshold from invariant energy
For projectile rest mass m_a and target m_A initially at rest, squared center-of-momentum energy is E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a.
Key knowledge
For projectile rest mass m_a and target m_A initially at rest, squared center-of-momentum energy is E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a. If the minimum allowed final invariant mass is M_f, an endothermic channel requires K_thr=[M_f²−(m_a+m_A)²]c²/(2m_A). With Q=(m_a+m_A−M_f)c² this becomes −Q(1+m_a/m_A)+Q²/(2m_Ac²), showing why laboratory threshold generally exceeds −Q.
Values and equations
E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a; K_thr=[M_f²−(m_a+m_A)²]c²/(2m_A)=−Q(1+m_a/m_A)+Q²/(2m_Ac²)
- E_cm,K_a,K_thr,Q
- Center-of-momentum energy, projectile kinetic/threshold energy and rest-energy release J
- m_a,m_A,M_f
- Projectile, target and minimum final invariant masses kg
- c
- Vacuum light speed m s⁻¹
Conditions and scope
- m_A>0; isolated relativistic energy/momentum conservation, fixed target and specified final rest masses/states. For a freely separating final state with at least two massive particles, M_f is their mass sum; otherwise use the actual minimum permitted invariant energy.
- The positive endothermic threshold is a kinematic minimum, not proof of a nonzero rate, sufficient angular-momentum/charge balance, or a universal threshold for all target geometries. The small-|Q| formula neglects the explicit Q² correction; exothermic two-or-more-body channels have no positive energy threshold from this mass condition alone.
Sources · 1
References
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Evidence and review
PDG: Kinematics (2025 edition): Sections 49.1–49.2, printed page 1, equations 49.2–49.3: invariant center-of-momentum energy and fixed-target expression in ħ=c=1 units. Threshold and Q forms are explicit algebraic derivations with c restored; no cross section is inferred.
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