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, $g_{a{\hat{I}}^3{\hat{I}}^3}\simeq 0$, for which diphoton production and decay are suppressed and collider phenomenology is driven by electroweak interactions ($aWW$, $aZ{\hat{I}}^3$, $aZZ$). 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 $a\to Z{\hat{I}}^3$ and $a\to W^{+}{W}^{-}$ decays. For $pp\to jja$ 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--$Z{\hat{I}}^{3}jj$ 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 ($e^{\pm }\hat{I}{¼}^{\mp }$) plus jets. Among the two $WW$ final states, the VBF-assisted $jja(\to W^{+}{W}^{-})$ 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 $\ddot{I}ƒ\times \mathrm{Br}$ for $pp\to jja$ with $a\to Z{\hat{I}}^3$ and $a\to W^{+}{W}^{-}$, as well as for associated production $pp\to W^{\pm }a$ with $a\to W^{+}{W}^{-}$....

41 pages, 18 figures, 8 tables; v2: clarifies 100 TeV vs 14 TeV differences beyond naive scaling and strengthens the detector-level signal+background analysis