Sensitivity to longitudinal vector boson scattering in semileptonic final states at the HL-LHC

Longitudinal vector boson scattering provides an important probe of electroweak symmetry breaking, bringing sensitivity to physics beyond the Standard Model as well as constraining properties of the Higgs boson. It is a difficult process to study due to the small production cross section and challenging separation of the different polarization states. We study the sensitivity to longitudinal $WV$ vector boson scattering at the high-luminosity Large Hadron Collider in semi-leptonic final states. While these are characterized by larger background contributions compared to fully leptonic final states, they benefit from a higher signal cross section due to the enhanced branching fraction. We determine the polarization through full reconstruction of the event kinematics using the $W$ boson mass constraint and through the use of jet substructure. We show that with these techniques sensitivities around three standard deviations at the HL-LHC are achievable, which makes this channel competitive with its fully leptonic counterparts.

Figure 1

Comparison of (left) the $p_T$ balance of the hadronically decaying $V$ decay products, and (right) the $\cos ({\theta }^{*})$ distribution for generator and detector level reconstruction for longitudinally and transversely polarized $V$ bosons, using (top) the leading $p_T$ subjet $q_1$, and (bottom) the quark $q$ from the $V$ boson decay.

Figure 2

Comparison of (left) The $p_T$ balance of the hadronically decaying $V$ decay products, and (right) the $\cos ({\theta }^{*})$ distribution for particle-flow-based and track-based substructure reconstruction for longitudinally and transversely polarized $V$ bosons.

Figure 3

Comparison of (left) the $p_T$ balance of the leptonically decaying $W$ decay products, and (right) the $\cos ({\theta }^{*})$ distribution for generator and detector level reconstruction for longitudinally and transversely polarized $W$ bosons.

Figure 4

Kinematic distributions of the $W_L{V}_L$ signal compared to the three main background processes after the event selection for an integrated luminosity of $3000{\mathrm{fb}}^{-1}$.

Figure 5

Distributions of the different inputs to the multiclass tagger in $W+\text{jets}$, top-quark pair production, background VBS $WV$, and signal VBS $W_L{V}_L$ events.

Figure 6

Signal fit input templates for (left) the multiclass DNN tagger, and (right) $\mathrm{\Delta }\eta (j_1,j_2)$. The error bars represent the statistical uncertainty.

Figure 7

Expected $W_L{V}_L$ significance as a function of the integrated luminosity at the HL-LHC, using the full DNN and the $\mathrm{\Delta }\eta (j_1,j_2)$ observable alone. The “stat only” results are obtained considering statistical uncertainties only, rather than both statistical and systematic uncertainties.