Sensitivities to New Resonance Couplings to $W$-Bosons at the LHC

TL;DR

This work addresses the challenge of discovering a heavy neutral state that couples to $W$-bosons by leveraging the tri-$W$ final state in $pp$ collisions at the HL-LHC, focusing on events with two same-sign muons and a hadronically decaying $W$. It combines detector-level simulations with a boosted decision tree-based multivariate analysis to optimize signal-background separation over a broad mass range $m_X\in [170,3000]$ GeV, using the heavy photophobic ALP as a benchmark. A key contribution is the provision of model-independent sensitivities to $\sigma(pp\to W^{\pm} X)\times\mathrm{Br}(X\to W^{+}W^{-})$ and to the coupling $g_{aWW}$, alongside detailed ML discriminants and kinematic observables that strengthen discovery prospects, particularly for $m_X>300$ GeV. The results show substantial improvements over CMS reinterpretations at 13 TeV, highlighting the HL-LHC's potential to probe beyond-SM electroweak dynamics and to constrain heavy ALP scenarios.

Abstract

We propose a search strategy at the HL-LHC for a new neutral particle $X$ that couples to $W$-bosons, using the process $p p \rightarrow W^{\pm} X (\rightarrow W^{+} W^{-})$ with a tri-$W$-boson final state. Focusing on events with two same-sign leptonic $W$-boson decays into muons and a hadronically decaying $W$-boson, our method leverages the enhanced signal-to-background discrimination achieved through a machine-learning-based multivariate analysis. Using the heavy photophobic axion-like particle (ALP) as a benchmark, we evaluate the discovery sensitivities on both production cross section times branching ratio $σ(p p \rightarrow W^{\pm} X) \times \textrm{Br}(X \rightarrow W^{+} W^{-})$ and the coupling $g_{aWW}$ for the particle mass over a wide range of 170-3000 GeV at the HL-LHC with center-of-mass energy $\sqrt{s} = 14$ TeV and integrated luminosity $\mathcal{L} = 3$ $\textrm{ab}^{-1}$. Our results show significant improvements in discovery sensitivity, particularly for masses above 300 GeV, compared to existing limits derived from CMS analyses of Standard Model (SM) tri-$W$-boson production at $\sqrt{s} = 13$ TeV. This study demonstrates the potential of advanced selection techniques in probing the coupling of new particles to $W$-bosons and highlights the HL-LHC's capability to explore the physics beyond the SM.

Figures (20)

• Figure 1: Production and decay of new particle $X$ coupling to di-$W$ boson at $pp$ colliders, leading to the tri-$W$ final state. The same-charged $W^{\pm}$ bosons are considered to decay into $\mu^{\pm}$ and $\nu_{\mu}$ (or $\overline{\nu}_{\mu}$), while the remaining $W^\mp$ boson decays into di-jet.
• Figure 2: Signal production cross section $\sigma(p \, p \,\rightarrow {W^{\pm}} \, a)$ multiplied by the branching ratio ${\rm Br}(a \to W^+W^-)$ as a function of ALP mass $m_{_{a}}$ from 170 GeV to 3000 GeV at the HL-LHC with $\sqrt{s} = 14$ TeV, assuming the coupling $g_{_{aWW}} =$ 1 TeV$^{-1}$.
• Figure 3: BDT response distributions for total SM background and the signal when ALP mass $m_a$ = 400 GeV (left) and 900 GeV (right) at the HL-LHC with $\sqrt{s} =$ 14 TeV.
• Figure 4: Discovery sensitivities on the production cross section $\sigma(p p \to W^{\pm} \, X)$ times branching ratio Br$(X \to W^{+} W^{-})$ in the mass range of 170 - 3000 GeV at the HL-LHC with $\sqrt{s} =$ 14 TeV and $\mathcal{L} =$ 3 $\rm ab^{-1}$. Red and green curves correspond to 2-$\sigma$ and 5-$\sigma$ significances, respectively.
• Figure 5: Discovery sensitivities with 2-$\sigma$ and 5-$\sigma$ significances on the coupling $g_{_{aWW}}$ in the mass range of 170 - 3000 GeV at the HL-LHC with $\sqrt{s} =$ 14 TeV and $\mathcal{L} =$ 3 $\rm ab^{-1}$ and 35.9 fb$^{-1}$. The 95% C.L. limit (blue curve) Aiko:2024xiv derived from reinterpreting CMS analyses of SM production $pp \to W^\pm W^\pm W^\mp$ at $\sqrt{s} =$ 13 TeV and $\mathcal{L} =$ 35.9 fb$^{-1}$ is displayed for comparison.