Measuring spin and CP from semi-hadronic ZZ decays using jet substructure

TL;DR

The paper investigates measuring the spin and CP quantum numbers of a heavy resonance X produced at the LHC and decaying to ZZ, focusing on the semihadronic final state ZZ→ℓ+ℓ−jj. By employing jet-substructure techniques and a shape-analysis of spin- and CP-sensitive angular observables, it assesses how well these observables can discriminate among J^CP hypotheses in a realistic background environment. The study finds strong CP sensitivity for a scalar X (0^±) even in the semihadronic channel, while discriminating tensorial and vector resonances is more challenging in the boosted regime; the method shows that the semihadronic channel can supplement the purely leptonic X→ZZ→4ℓ analyses. Overall, jet-substructure provides a viable path to access resonance spin/CP information beyond traditional fully leptonic channels, albeit with notable systematic considerations and channel-dependent limitations.

Abstract

We apply novel jet techniques to investigate the spin and CP quantum numbers of a heavy resonance X, singly produced in pp -> X -> ZZ -> l(+)l(-)jj at the LHC. We take into account all dominant background processes to show that this channel, which has been considered unobservable until now, can qualify under realistic conditions to supplement measurements of the purely leptonic decay channels X -> ZZ -> 4l. We perform a detailed investigation of spin- and CP-sensitive angular observables on the fully-simulated final state for various spin and CP quantum numbers of the state X, tracing how potential sensitivity communicates through all the steps of a subjet analysis. This allows us to elaborate on the prospects and limitations of performing such measurements with the semihadronic final state. We find our analysis particularly sensitive to a CP-even or CP-odd scalar resonance, while, for tensorial and vectorial resonances, discriminative features are diminished in the boosted kinematical regime.

Figures (7)

• Figure 1: Spin- and ${\cal{CP}}$-sensitive angles of Ref. Dell'Aquila:1985ve in $pp\rightarrow X \rightarrow ZZ \rightarrow \mu^+\mu^- jj$. Details on the angles' definition and on the assignment of $j_\alpha$ and $j_\beta$ are given in the text. An angle analogous to $\Phi_1$ can be defined with respect to the leptonic decay plane. We refer to this angle as $\tilde{\Phi}$.
• Figure 2: Cosine of the helicity angle $\theta_h$, Eq. (\ref{['eq:costhetahel']}), calculated from the hadronically decaying $Z$ at different steps of the analysis: inclusive Monte Carlo generation level (top, left), Monte Carlo generation level including selection cuts Eqs. (\ref{['eq:mupt']})-(\ref{['eq:fatjetcrit']}) (top, right), after the full subjet analysis including Monte Carlo-truth information (bottom, left), and after the full analysis (bottom, right).
• Figure 3: Cosine of the helicity angle $\theta_\ell$, Eq. (\ref{['eq:costhetahel']}), calculated from the hadronically decaying $Z$ at different steps of the analysis: inclusive Monte Carlo generation level (top, left), Monte Carlo generation level including selection cuts Eqs. (\ref{['eq:mupt']})-(\ref{['eq:fatjetcrit']}) (top, right), after the full subjet analysis including Monte Carlo-truth information (bottom, left), and after the full analysis (bottom, right).
• Figure 4: Cosine of the angle $\theta^\star$, Eq. (\ref{['eq:costhetastar']}), calculated from the hadronically decaying $Z$ at different steps of the analysis: inclusive Monte Carlo generation level (top, left), Monte Carlo generation level including selection cuts Eqs. (\ref{['eq:mupt']})-(\ref{['eq:fatjetcrit']}) (top, right), after the full subjet analysis including Monte Carlo-truth information (bottom, left), and after the full analysis (bottom, right).
• Figure 5: Angle $\Phi$, Eq. (\ref{['eq:costhetastar']}), calculated from the hadronically decaying $Z$ at different steps of the analysis: inclusive Monte Carlo generation level (top, left), Monte Carlo generation level including selection cuts Eqs. (\ref{['eq:mupt']})-(\ref{['eq:fatjetcrit']}) (top, right), after the full subjet analysis including Monte Carlo-truth information (bottom, left), and after the full analysis (bottom, right).