Shower & Hadronisation Uncertainties for Precision Top Physics - PowerPoint PPT Presentation

Shower & Hadronisation Uncertainties for Precision Top Physics Peter Skands (Monash U) Scale Variations : How big and how correlated? → 7-point variations, with (conservative) soft compensation terms Provided automatically as vector of event weights? ME Corrections Estimating sensitivity to process-specific non-singular terms Alternative Shower Models? Relevant variations in baseline PYTHIA + Status of DIRE and VINCIA Colour Reconnections Interesting physics & annoying complication: proposals for top (+ Ambiguity of MC mass definition?) CMS Top Meeting CERN November 2018

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Proportionality to α s ( μ ) ⟹ can get a (misleadingly?) small band if you α s ( k 2 1 µ 2 ) 2 µ 2 ) ∼ 1 − b 0 ln( k 2 1 /k 2 2 ) α s ( µ 2 ) choose central μ scale very large. α s ( k 2 E.g., some calculations use μ ~ H T ~ largest scale in event ?! Worth keeping in mind when considering (uncertainty on) central μ choice Flavour-dependent slope of order 1 b 0 ∼ 0 . 65 ± 0 . 07 Expansion around μ only sensible if this stays ≲ 1 ๏ Mainstream view: • Regard scale dependence as unphysical / leftover artefact of our mathematical procedure to perform the calculations. • Dependence on it has to vanish in the ‘ultimate solution’ to QFT • → Terms beyond calculated orders must sum up to at least kill μ dependence • Such variations are thus regarded as a useful indication of the size of uncalculated terms. (Strictly speaking, only a lower bound!) Note: In PYTHIA you specify k 2 Typical choice (in fixed-order calculations) : k ~ [0.5,1,2] TimeShower:renormMultFac SpaceShower:renormMultFac 3 � P ETER S K A NDS M O N ASH U.