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arXiv · 2609.11364

Massless contraction of massive particles in the infinite boost limit

Kevin Nguyen
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Abstract

Massless particles, defined through Wigner's induced representations of the Poincar\'e group, are obtained as a limit of infinitely boosted massive particles wherein the massive little group undergoes an In\"on\"u--Wigner contraction to the massless one. It is demonstrated that the spin-$s$ multiplet of a massive representation re-organizes into the direct sum of massless particles with helicities in the range $[-s,s]$. This group theoretic result is the Hilbert space counterpart of the St\"uckelberg formulation of massive fields of spin $s$ in the high-energy limit, where the St\"uckelberg fields become independent massless fields of spins $s'\leq s$ in integer steps. It therefore also provides the kinematic setting of the Goldstone equivalence theorem.

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Kevin Nguyen. 2026-09-10. Massless contraction of massive particles in the infinite boost limit. https://arxiv.org/abs/2609.11364

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