Optimal determination of New Physics couplings: A comparative study

Abstract

We study the determination of new physics (NP) parameters using the optimal observable technique (OOT) in situations where the standard model (SM) dominates over the NP effects, and when the NP dominates over the SM contribution, using the 2-Higgs doublet model as an illustrative example; for the case of SM domination we extend our results using an effective theory parameterization of NP effects. For the case of SM dominance we concentrate on $t \bar{t}$ production in an $e^+e^-$ collider, while for the case of NP dominance we consider both $t \bar{t}$ production and pair production of charged scalars, also in an $e^+e^-$ collider. We discuss the effects of the efficiency of background reduction, luminosity and beam polarization, and provide a comparison of the optimal uncertainties with those obtained using a standard $χ^2$ analysis of (Monte Carlo generated) collider data.

Figures (15)

• Figure 1: Top-quark pair production at $e^+\,e^-$ collider. Left: SM contribution; Right: SMEFT contribution.
• Figure 2: Plots of the $e^+ e^-\rightarrow\bar{t} t$ cross-section. Top row: angular dependence for CM energy $\sqrt{s} = 500$ GeV, and different values of $\Lambda$ for unpolarized (left) and polarized with $P_{e^\pm}=_{+80\%}^{-5\%}$ (right) beams. Second row: total cross section as a function of $\sqrt{s}$ for $c_1= c_2=1$ and several values of $\Lambda$, for unpolarized (left) and unpolarized (right) beams. Third row: dependence on the beam polarization for the SM (left) and the SM + EFT with $c_1=c_2=1$ and $\Lambda =4$ TeV (right). Bottom row: comparison of the SM total cross section (black horizontal line) with the SM + EFT with $c_1=c_2=1$ as a function of $\Lambda$ for unpolarized (left) and polarized (right) beams; the region labeled "BSM small" corresponds to $\sigma_{\tt SM + EFT} > 2 \sigma_{\tt SM}$.
• Figure 3: Production and decay of top-quark at $e^+ \, e^-$ colliders for $2l~+2b$ + missing energy signal.
• Figure 4: Invariant di lepton mass ($m_{ll}$) (left), Invariant di b-jet mass ($m_{b b}$) distributions (right) for $2l ~+2b$ + missing energy final state coming from $t\bar{t}$ signal with EFT ($\Lambda=4$ TeV) as well as dominant SM backgrounds at the $e^+\,e^-$ collider with $\sqrt s$ = 500 GeV. Top panel: Unpolarized beams; bottom panel: polarized beams $P_{e^\pm}=^{-5\%}_{+80\%}$.
• Figure 5: Optimal 1-$\sigma$ regions in $\Delta c_1-\Delta c_2$ plane for various choices of EFT parameters and choices of beam polarization. See figure inset and heading for details.