https://doi.org/10.1140/epjp/s13360-025-06450-6
Regular Article
A brief history of mass
Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, 61801, Urbana, Illinois, USA
Received:
24
March
2025
Accepted:
17
May
2025
Published online:
8
June
2025
It has been known since the 1950s that an unstable particle is associated with a complex pole in the propagator. This had to be rediscovered twice: in the early 1970s in the context of hadronic resonances, and in the early 1990s in the context of the Z boson. The physical mass of the particle is the real part of the pole in the complex energy plane. In hadronic physics, this replaced the “Breit–Wigner mass,” which was found to depend on the parameterization of the “energy-dependent width.” In Z physics, it replaced the “on-shell” mass, which was found to be gauge dependent. Although the mass defined from the complex pole position has been widely discussed in the literature, it has not yet made its way into quantum field theory textbooks.
© The Author(s) 2025
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