Discovery prospects for photophobic axion-like particles in the $WWjj$ final state at the High-Luminosity LHC

Abstract

We evaluate discovery prospects for photophobic axion-like particles (ALPs) in the $WWjj$ final state at the High-Luminosity LHC (HL-LHC; $\sqrt{s}=14$ TeV, $L=3$ ab$^{-1}$). In the photophobic limit ($g_{a\gamma\gamma}=0$), ALPs couple to electroweak gauge bosons and are produced in association with two jets ($pp\to jj a$) via both $s$-channel electroweak exchange and vector-boson-fusion (VBF)-like topologies, followed by $a\to W^+W^-$ decay. We target the different-flavour dilepton mode $W^+W^-\to e^\pm\mu^\mp\nu \bar{\nu}$ with two jets and moderate missing transverse momentum. The analysis employs a two-step strategy: an initial preselection that defines the signal-like final state, followed by a multivariate analysis (MVA) trained on dijet and dileptonic-$WW$ kinematics from both the $s$-channel and VBF-like production mechanisms to separate signal from background; the MVA threshold is optimized independently at each $m_a$. We present $2\sigma$ and $5\sigma$ discovery sensitivities for the ALP-$W$ coupling $g_{aWW}$ across the $170$--$4000$ GeV mass range. For $260$ GeV $\le m_a \le 1500$ GeV, the $2\sigma$ ($5\sigma$) sensitivity is approximately flat around 0.61 (0.76) TeV$^{-1}$. We also report model-independent discovery thresholds for the fiducial quantity $\sigma(pp\to jj a)\,\mathrm{Br}(a\to W^+W^-)$ over the same mass range to enable reinterpretation for other models. These results indicate that the $WWjj$ topology offers competitive and complementary sensitivity to heavy photophobic ALPs at the HL-LHC.