The properties of quark and gluon jets, and the differences between them, are increasingly important at the LHC. However, Monte Carlo event generators are normally tuned to data from e + e − collisions which are primarily sensitive to quark-initiated jets. In order to improve the description of gluon jets we make improvements to the perturbative and the non-perturbative modelling of gluon jets and include data with gluon-initiated jets in the tuning for the first time. The resultant tunes significantly improve the description of gluon jets and are now the default in Herwig 7.1.

We present a phenomenological study of angularities measured on the highest transverse-momentum jet in LHC events that feature the associate production of a Z boson and one or more jets. In particular, we study angularity distributions that are measured on jets with and without the SoftDrop grooming procedure. We begin our analysis exploiting state-of-the-art Monte Carlo parton shower simulations and we quantitatively assess the impact of next-to-leading order (NLO) matching and merging procedures. We then move to analytic resummation and arrive at an all-order expression that features the resummation of large logarithms at next-to-leading logarithmic accuracy (NLL) and is matched to the exact NLO result. Our predictions include the effect of soft emissions at large angles, treated as a power expansion in the jet radius, and non-global logarithms. Furthermore, matching to fixed-order is performed in such a way to ensure what is usually referred to as NLL′ accuracy. Our results account for realistic experimental cuts and can be easily compared to upcoming measurements of jet angularities from the LHC collaborations.

Soft drop has been shown to reduce hadronisation effects at e + e − colliders for the thrust event shape. In this context, we perform fits of the strong coupling constant for the soft-drop thrust distribution at NLO+NLL accuracy to pseudo data generated by the Sherpa event generator. In particular, we focus on the impact of hadronisation corrections, which we estimate both with an analytical model and a Monte-Carlo based one, on the fitted value of α s (m Z ). We find that grooming can reduce the size of the shift in the fitted value of α s due to hadronisation. In addition, we also explore the possibility of extending the fitting range down to significantly lower values of (one minus) thrust. Here, soft drop is shown to play a crucial role, allowing us to maintain good fit qualities and stable values of the fitted strong coupling. The results of these studies show that soft-drop thrust is a promising candidate for fitting α s at e + e − colliders with reduced impact of hadronisation effects.3. if the splitting fails the soft-drop condition, the softer subjet is discarded (groomed away) and the steps are repeated for the resulting jet (the harder subjet); 4. if instead the subjets pass the condition, the procedure is terminated and the resulting jet is the combination of subjets i and j.The soft-drop algorithm features two parameters: z cut and β. The first determines how stringent the cut on the subjet energies is, whereas the latter provides an angular suppression to grooming. While β → ∞ corresponds to no grooming, for β = 0 no angular dependence is taken into account and the soft-drop algorithm reduces to the modified Mass-Drop Tagger (mMDT) [29,30]. For practical purpose, we have implemented the above procedure using FastJet [31] for the jet clustering and additional manipulations.The event shape thrust [17] is defined aswhere p i labels the three-momentum of particle i and the sum extends over all particles in the event E. The resulting vector n defines the thrust axis. Often the related variable τ ≡ 1 − T = min n

We present for the first time resummed predictions at NLO + NLL accuracy for the Durham jet-resolution scales y n,n+1 in multijet production in e + e − collisions. Results are obtained using an implementation of the well known CAESAR formalism within the SHERPA framework. For the 4-, 5-and 6-jet resolutions we discuss in particular the impact of subleading colour contributions and compare to matrix-element plus parton-shower predictions from SHERPA and VINCIA.