Search for a Doubly Charged Scalar at the LHC and FCC-hh

TL;DR

The paper assesses the collider reach for doubly charged scalars $H^{\pm\pm}$ in type‑II seesaw scenarios, focusing on leptonic decays under a narrow‑width assumption. Using a LO Lagrangian implementation and Drell–Yan production, it validates against ATLAS data at $\sqrt{s}=13$ TeV and projects Run II limits to $\mathcal{L}=139$ fb$^{-1}$, including a photon–photon fusion channel. It then forecasts FCC‑hh prospects at $\sqrt{s}=100$ TeV, finding sensitivities up to $M_{H^{\pm\pm}}\approx 4.8$–$5.3$ TeV for 1 ab$^{-1}$ and up to $\sim 7$ TeV for 3 ab$^{-1}$, with larger reach for flavor configurations involving muons. The results highlight FCC‑hh as the most powerful future collider to probe the neutrino‑mass generation mechanism via doubly charged scalars.

Abstract

Doubly charged scalars frequently emerge in many well-motivated extensions of the Standard Model, particularly in frameworks that aim to explain the origin of neutrino masses. Their distinct electric charge and clean leptonic signatures make them especially compelling from the standpoint of experimental searches. In this work, we explore the sensitivity of the LHC full run II, including photon-photon fusion, and Future Circular Collider in its hadron-hadron configuration (FCC-hh) to such states, assuming they decay promptly and exclusively into charged leptons either conserving or violating lepton flavor. We find that the FCC-hh, operating at 100 TeV, is uniquely positioned to probe doubly charged scalars with masses up to 7 TeV and possibly establish the mechanism behind neutrino masses.

Figures (9)

• Figure 1: Feynman diagram of the resonant pair production process of the doubly charged scalar decaying into charged lepton pairs.
• Figure 2: Production cross section as a function of the doubly charged scalar mass assuming $\text{BR}(H^{--}\rightarrow ee)=100\%$, $\text{BR}(H^{--}\rightarrow \mu\mu)=0\%$, and $\text{BR}(H^{--}\rightarrow \mu e)=0\%$. We overlay the current and projected ATLAS limits at 13 TeV center-of-mass energy with $36.1\,\text{fb}^{-1}$ and $139\,\text{fb}^{-1}$ of integrated luminosity.
• Figure 3: Production cross section as a function of the doubly charged scalar mass assuming $\text{BR}(H^{--}\rightarrow ee)=0\%$, $\text{BR}(H^{--}\rightarrow \mu\mu)=100\%$, and $\text{BR}(H^{--}\rightarrow \mu e)=0\%$.
• Figure 4: Production cross section as a function of the doubly charged scalar mass assuming $\text{BR}(H^{--}\rightarrow ee)=0\%$, $\text{BR}(H^{--}\rightarrow \mu\mu)=0\%$, and $\text{BR}(H^{--}\rightarrow \mu e)=100\%$.
• Figure 5: Production cross section as a function of the doubly charged scalar mass assuming $\text{BR}(H^{--}\rightarrow ee)=30\%$, $\text{BR}(H^{--}\rightarrow \mu\mu)=40\%$, and $\text{BR}(H^{--}\rightarrow \mu e)=30\%$.