Precise predictions for $t\bar{t}\gamma/t\bar{t}$ cross section ratios at the LHC

TL;DR

This paper provides highly precise NLO QCD theoretical predictions for the ratio of $t\bar{t}\gamma$ to $t\bar{t}$ production cross sections at the LHC, enabling detailed studies of top quark properties and potential new physics.

Contribution

It introduces the first high-precision NLO QCD predictions for the $t\bar{t}\gamma/t\bar{t}$ cross section ratio, including all relevant effects and uncertainties, at 13 TeV LHC energies.

Findings

Achieved 1-3% precision on the cross section ratio ${\cal R}$.

Estimated theoretical uncertainties for differential ratios range from 1% to 6%.

Validated the use of dynamical scales for improved accuracy.

Abstract

With the goal of increasing the precision of NLO QCD predictions for the $pp\to t\bar{t} \gamma$ process in the di-lepton top quark decay channel we present theoretical predictions for the ${\cal R}= \sigma_{t\bar{t}\gamma}/\sigma_{t\bar{t}}$ cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of $\sqrt{s} = 13$ TeV. Fully realistic NLO computations for $t\bar{t}$ and $t\bar{t}\gamma$ production are employed. They are based on matrix elements for $e^+\nu_e \mu^- \bar{\nu}_\mu b\bar{b}$ and $e^+\nu_e \mu^- \bar{\nu}_\mu b\bar{b}\gamma$ processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the $W$ gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain $1\%-3\%$ percent precision on ${\cal R}$. Moreover, for differential cross section ratios theoretical uncertainties in the range of $1\%-6\%$ have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, ${\cal R}$ at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the $t$-$\bar{t}$-$\gamma$ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.