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

TL;DR

This work investigates heavy photophobic axion-like particles in the limit of negligible diphoton coupling, focusing on electroweak interactions and exploring discovery potential at a 100 TeV pp collider with 20 ab^{-1}. It develops detector-level analyses for three complementary channels—Zγjj, tri-W, and WWjj—each probing distinct production modes (s-channel and VBF topologies) and decays (a→Zγ or a→WW), employing BDT-based multivariate methods to maximize sensitivity. The study delivers both coupling reach in g_{aWW} for m_a between 100 and 7000 GeV and model-independent σ × Br thresholds, highlighting cross-channel consistency and robustness against systematic uncertainties. Overall, the results show a substantial expansion of the ALP discovery reach relative to HL-LHC projections, demonstrating the power of a 100 TeV collider to test photophobic ALP scenarios within a general EFT framework and to constrain the electroweak gauge structure of new pseudoscalars.

Abstract

We study heavy photophobic axion-like particles (ALPs) in the limit of an effectively vanishing diphoton coupling, $g_{aγγ}\simeq 0$, for which diphoton production and decay are suppressed and collider phenomenology is driven by electroweak interactions ($aWW$, $aZγ$, $aZZ$). We perform detector-level searches at a future $\sqrt{s}=$ 100 TeV $pp$ collider (SppC/FCC-hh), with an integrated luminosity of $L =$ 20 ab$^{-1}$. We consider $a\to Zγ$ 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--$Zγjj$ 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 ($e^\pmμ^\mp$) plus jets. A boosted-decision-tree (BDT) classifier built from kinematic observables provides the final signal--background separation. At $\sqrt{s}=100$ TeV and $L =$ 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 $σ\times\mathrm{Br}$ for $pp\to jj\,a$ with $a\to Zγ$ and $a\to W^+W^-$, as well as for associated production $pp\to W^\pm a$ with $a\to W^+W^-$. Taken together, the three channels deliver broad coverage and mutually reinforcing checks across decay modes and production regimes, strengthening robustness against channel-dependent systematics. The 100~TeV program extends the discovery reach well beyond 14~TeV projections within a general effective field theory (EFT) description that does not assume a specific ultraviolet completion.

Figures (44)

• Figure 1: Signal production and decay topologies for the photophobic ALP considered in this work. (I) $pp \to jj\,a(\to Z \gamma)$ with $Z\to\ell^+\ell^-$ ($\ell=e,\mu$); (II) $pp \to W^{\pm} a(\to W^+W^-)$, where the same-sign $W$ bosons decay leptonically into muons and neutrinos, $W^\pm\to\mu^\pm \nu_\mu / \bar{\nu}_\mu$, while the remaining $W$ boson decays hadronically, $W^\mp\to jj$; (III) $pp \to jj\,a(\to W^+W^-)$, where the two $W$ bosons decay leptonically into different-flavor dilepton, $e^\pm\mu^\mp$.
• Figure 2: Assuming benchmark couplings $g_{a\gamma\gamma}=0$ and $g_{aWW}=1~\mathrm{TeV}^{-1}$ at the SppC/FCC-hh with $\sqrt{s}=100$ TeV, the production cross section times branching ratio, $\sigma(pp\to jj\,a)\times\mathrm{Br}(a\to Z \gamma)$, is shown as a function of the ALP mass $m_a$ in the range $100$--$7000$ GeV, while $\sigma(pp\to W^{\pm}a)\times\mathrm{Br}(a\to W^+ W^-)$ and $\sigma(pp\to jj\,a)\times\mathrm{Br}(a\to W^+ W^-)$ are shown as functions of $m_a$ in the range $170$--$7000$ GeV. The red, green, and blue curves correspond to $jj\,a(\to Z \gamma)$, $W^{\pm}a(\to W^+ W^-)$, and $jj\,a(\to W^+ W^-)$, respectively.
• Figure 3: Distributions of the invariant mass of the system formed by the two leading leptons and the leading photon, $m(\ell_1+\ell_2+\gamma_1)$, for the total background (dotted) and for signal benchmarks with various $m_a$, after preselection at $\sqrt{s}=100$ TeV.
• Figure 4: Distributions of BDT responses for the benchmark signal with $m_a=700$ GeV (black, shaded) and the total SM background (red) at the SppC/FCC-hh with $\sqrt{s}=100$ TeV.
• Figure 5: BDT response distributions for the total SM background and the signal $W^\pm a(\to W^\pm W^\mp)$ at the SppC/FCC-hh with $\sqrt{s}=100$ TeV, for $m_a=400$ GeV (left) and $m_a=900$ GeV (right).