Abstract
Top-quark pair production is central to many facets of LHC physics. At leading order, the top and anti-top are produced in a back-to-back topology, however this topology accounts only for a minority of the events with TeV-scale momentum transfer that contain a toverline{t} pair. The remaining events instead involve the splitting of an initial or final-state gluon to toverline{t} . We provide simple quantitative arguments that explain why this is the case, and examine the interplay between different topologies and a range of variables that characterise the event hardness. We then develop a method to classify the topologies of individual events and use it to illustrate our findings in the context of simulated events, using both top partons and suitably defined fiducial tops. For events with large toverline{t} invariant mass, we comment on additional features that have important experimental and theoretical implications.
Highlights
Studies of top-quark production at Fermilab’s Tevatron and CERN’s Large Hadron Collider (LHC) were restricted to configurations where the top quarks had a transverse momentum that was comparable to the top-quark mass
It shows that, despite their being suppressed by a power of αs, the (NLO) flavour excitation (FEX) and Gluon splitting (GSP) topologies are numerically comparable to the leading order (LO) flavour creation (FCR) topology at the TeV scale
Since the hvq process is next-to-leading order (NLO) for ttproduction, we expect to have FCR topologies accurate to NLO, and FEX and GSP accurate to LO, while other topologies are at best generated by the shower, so do not have any formal perturbative accuracy
Summary
We start by examining measures to characterise large momentum transfer in ttevents, i.e. the event hardness, including a discussion of their leading-order distributions. The set of observables provides measures of the hard scale of the event that at LO include the top-quark mass and transverse momentum. Note that at the large ∆yttvalues that dominate the gluon-fusion contribution to eq (2.3), the top-quark transverse momentum pT,t is much smaller than its ∆ytt = 0 value of approximately mtt/2. This is the reason why one should be wary of using mttas a renormalisation and factorisation scale for calculating the mttdistribution at large mttvalues, and why one would expect the use of μ = mttto lead to poor stability, as observed in ref.
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