Discovery prospects for photophobic axion-like particles at a 100 TeV proton--proton collider

Abstract

We study heavy photophobic axion-like particles (ALPs) in the limit of an effectively vanishing diphoton coupling, $g_{a\gamma\gamma}\simeq 0$, for which diphoton production and decay are suppressed and collider phenomenology is driven by electroweak interactions ($aWW$, $aZ\gamma$, $aZZ$). We perform detector-level searches at a future $\sqrt{s}=$ 100 TeV $pp$ collider (SppC/FCC-hh), with an integrated luminosity of $\mathcal{L} =$ 20 ab$^{-1}$. We consider $a\to Z\gamma$ and $a\to W^+W^-$ decays. For $pp\to jj\,a$ we include both $s$-channel electroweak exchange and vector boson fusion (VBF)-like topologies, while the tri-$W$ signature arises from associated production $pp\to W^\pm a$ (via $s$-channel exchange) followed by $a\to W^+W^-$. We analyze three final states--$Z\gamma jj$ with $Z\to\ell^+\ell^-$, tri-$W$ ($W^\pm W^\pm W^\mp$) with same-sign dimuons plus jets, and $W^+W^-jj$ with opposite-sign, different-flavor dilepton ($e^\pm\mu^\mp$) plus jets. Among the two $WW$ final states, the VBF-assisted $jj\,a(\to W^+W^-)$ channel overtakes the purely $s$-channel tri-$W$ mode for $m_a \stackrel{>}{\sim}$ 1 TeV, reflecting 100~TeV signal/background kinematic shifts beyond naive energy/luminosity rescaling. A boosted-decision-tree (BDT) classifier built from kinematic observables provides the final signal--background separation, using detector-level simulations of signal and high-statistics SM backgrounds. At $\sqrt{s}=100$ TeV and $\mathcal{L} =$ 20 ab$^{-1}$, we present discovery sensitivities to the ALP--$W$ coupling $g_{aWW}$ over $m_a\in[100,\,7000]$ GeV. In parallel, we report model-independent discovery thresholds on $\sigma\times\mathrm{Br}$ for $pp\to jj\,a$ with $a\to Z\gamma$ and $a\to W^+W^-$, as well as for associated production $pp\to W^\pm a$ with $a\to W^+W^-$....