Top Tagging

TL;DR

Boosted hadronic tops motivate jet-substructure approaches to identify t t decays within fat jets amid dense QCD backgrounds. The review contrasts clustering-history taggers and jet-shape/tagger families, and details grooming techniques (filtering, trimming, pruning) that mitigate soft QCD effects while preserving signal. It catalogs major algorithms (YSplitter, Seattle, Johns Hopkins, HEPTopTagger, Thaler-Wang, N-subjettiness) and discusses performance, data validations, and applications to beyond-Standard-Model searches. The outlook emphasizes data-driven validation and multi-observable strategies to achieve robust boosted-top tagging in the LHC environment.

Abstract

Top tagging is a recent approach to identifying boosted hadronic top quarks. It avoids reconstructing individual top decay products and instead uses a jet algorithm to reconstruct the entire top decay. Quite generally, geometrically large jets including heavy particles (fat jets) can be analyzed on the level of their subjet constituents. LHC data will soon allow us to establish this new analysis method. We discuss different tagging algorithms, their critical QCD aspects, and currently available experimental results. For the development of taggers and their different applications this review should provide a firm theoretical and algorithmic background.

Figures (17)

• Figure 1: Approximate C/A distance of all three top decay products for Standard Model $t\bar{t}$ events as a function of $p_{T,t}$. The left panel shows all events while the right panel shows only tagged events using C/A fat jets of size $R=1.5$. Figure from Ref. heptop1.
• Figure 2: Reconstructed $W$ mass (left) and top mass (right) from cluster (solid and dashed) and cone (dotted and dot-dashed) algorithms. The solid and dotted curves include underlying event. For the top mass we also show the mass reconstructed from partons instead of subjets as the double-dot-dashed line. Figures from Ref. mike.
• Figure 3: Left: Normalized $p_T \sqrt{y}$ distributions in the $W$ candidate jet for a massive $WW$ system decaying to two relativistic semi-leptonic $W$ bosons (dots), $t\bar{t}$ events (triangles), and $W$+jet production (squares). Figure from Ref. butterbrot.
• Figure 4: Three splitting points $y_i$ for fat jets from $Z'$ decays with $m_{Z'} = 2$ TeV (solid) and 3 TeV (dashed). The $x$-axis shows the three leading splitting points of the $k_T$-algorithm as $p_T \sqrt{y}/2$. Figures from Ref. ysplitter.
• Figure 5: Left: Helicity angle distribution for top, gluon, and light-flavor quark jets fulfilling $p_T > 700$ GeV. All other tagging cuts have been already imposed. Right: original tagging and mis-tagging efficiency estimates for top and QCD jets as a function of the fat jet $p_T$. Figures from Ref. hopkins.