Resonance-aware parton-shower matching for off-shell top-antitop production with semi-leptonic decays at electron-positron colliders

Abstract

We present full off-shell NLO corrections in QCD obtained with the MoCaNLO code matched to parton shower. A resonance-aware matching procedure has been devised for the MC@NLO method tuned to the Catani-Seymour dipole subtraction. Specifically, we consider the off-shell production of a top-antitop pair in the semi-leptonic decay channel in electron-positron collisions and match it to the final-state QCD parton shower of PYTHIA8. Distortions of resonances' line shapes are avoided by providing the details of the resonance-cascade chain on an event-by-event basis to the parton shower and by adapting the matching accordingly through the introduction of dedicated counterterms.

Figures (7)

• Figure 1: Selection of dipoles done by Pythia8 to describe the soft--gluon-radiation pattern from an off-shell $\bar{\text{t}\xspace}\text{t}\xspace$ pair. The two blobs separate the squared current $J^2_{\widehat{\bar{\text{t}\xspace}\text{t}\xspace}}$, describing gluon radiation from the top/antitop-quark production, and the two squared currents $J^2_{\widehat{\text{t}\xspace\text{b}\xspace}}$ and $J^2_{\widehat{\bar{\text{t}\xspace}\bar{\text{b}\xspace}}}$, for radiation from the top/antitop-quark decays.
• Figure 2: Representative Feynman diagrams for each of the four resonance histories considered for the process $\text{e}\xspace^+\text{e}\xspace^-\to \mu^+ \nu_\mu\, \text{j}\xspace\, \text{j}\xspace \, \text{j}\xspace_{\text{b}\xspace}\xspace\, \text{j}\xspace_{\text{b}\xspace}\xspace$. Specifically, we show a doubly-resonant topology in the top and antitop quark [\ref{['fig:ps1']}], a singly-resonant topology in the top quark [\ref{['fig:ps2']}], a singly-resonant topology in the antitop quark [\ref{['fig:ps3']}], and a non-resonant topology in the top and antitop quark [\ref{['fig:ps4']}]. Red and blue fermion lines mark shower production and decay dipoles, respectively, that are active in the presence of either a bottom- or an antibottom-quark emitter.
• Figure 3: Distributions in the transverse momentum of the leading b jet [\ref{['fig:val01']}] and in the invariant mass of the reconstructed leptonically decaying top quark [\ref{['fig:val02']}]. Results for different choices of $\alpha$ in Eq. \ref{['eq:damping']} are presented: $\alpha=2$ (in red), $\alpha=1$ (in green), and $\alpha=4$ (in limegreen).
• Figure 4: Distributions in the invariant mass of the two light jets [\ref{['fig:val001']}] and of the reconstructed hadronically decaying top quark [\ref{['fig:val002']}]. Results for different choices of $\beta_h$ in Eqs. \ref{['eq:pt1']}--\ref{['eq:pt2']} are presented: $\beta_h=0.5$ (in red), $\beta_h=0.9$ (in brown), and $\beta_h=0.1$ (in goldenrod).
• Figure 5: Distributions in the cosine of the polar angle between the leading jet and the antimuon [\ref{['fig:val11']}] and in the invariant mass of the b-jet--antimuon system [\ref{['fig:val12']}]. Results for different choices of $\sqrt{t_0}$ are presented: $\sqrt{t_0}=0.5\,$GeV (in red), $\sqrt{t_0}=0.25\,$GeV (in blue), and $\sqrt{t_0}=1\,$GeV (in magenta).