Large-Width New Physics at Colliders: A Gauge-Invariant Resummation Approach

Abstract

Broad resonances challenge the standard Monte-Carlo treatment of unstable particles, which introduces a Breit-Wigner width into leading-order matrix elements and can generate unphysical gauge artifacts. We develop a gauge-consistent framework that combines a Dyson-resummed propagator with Slavnov-Taylor-identity-implied resummed vertices, enabling a consistent implementation in MadGraph5. In the Type-I seesaw model, heavy Majorana neutrinos naturally satisfy $Γ\sim m$, leading to strong departures from the Breit-Wigner lineshape, distorted angular correlations, and significant modifications to both $s$- and $t$-channel dynamics. Comparing with the normal and complex-mass schemes, we find that standard treatments can substantially misestimate cross sections and kinematic distributions in the large-width regime. Our results show that existing collider limits on heavy neutrinos-and, more generally, on any broad resonance-should be revisited within a fully resummed framework, opening new opportunities for both experimental searches and theoretical model building.

Figures (5)

• Figure 1: Representative diagrams for $pp \to \mu \nu_\mu jj$ arising from the $s$-channel (left) and the $t$-channel (right) processes.
• Figure 2: Cross sections for $pp\to\mu\nu_\mu jj$ and $pp\to\mu\mu jj$ versus $m_N$ in the normal, complex-mass, and resummed schemes.
• Figure 3: Normalized probability densities obtained from the squared propagator denominators for a heavy neutrino with $m_N=750\,\mathrm{GeV}$, comparing the Breit–Wigner and fully resummed lineshapes.
• Figure 4: The $\Delta\eta(j_1 j_2)$ distributions (normalized to 1) for $pp\to\mu\nu_\mu jj$ at $m_N=10\,\mathrm{TeV}$ in the normal, complex-mass, and resummed schemes.
• Figure 5: The $\Delta\phi(E^\mathrm{miss} \ell)$ distributions (normalized to 1) for $pp\to\mu\nu_\mu jj$ at $m_N=10\,\mathrm{TeV}$ in the normal, complex-mass, and resummed schemes.