Jet substructure: An analytical approach

Abstract

In the past few years, detailed study of the internal structure of hadronic jets has become an active area of research. In particular, jet substructure information has been used to distinguish signal from QCD initiated jets, which constitute a significant background for many physics searches. Prior to the work undertaken in this thesis, theoretical research on jet substructure was largely Monte Carlo based, with limited analytical input. This work presents an analytical approach to the study of jet substructure techniques in the context of high-p_T heavy resonance searches at the LHC.In this thesis, we compute the mass distribution of QCD initiated jets after application of several jet substructure algorithms using approximate fixed-order perturbative QCD at leading and next-to-leading order. This is sufficient to extract the leading logarithmic structure for each technique, which we compare to exact fixed-order results. Using this analytical insight, we propose modifications to some of these algorithms and use our results to discuss the phenomenological impact of different parameter choices.We also perform analytical calculations and Monte Carlo studies to examine the impact of QCD radiation on jets that arise from boosted Higgs decay after application of several jet substructure algorithms. Understanding the action on signal jets is important when two techniques perform similarly on background jets. An example studied here is the Y-splitter and Y-pruning techniques, which both perform well at rejecting background; however, the former retains signal jets which are subject to significant radiative and non-perturbative corrections. We demonstrate that the combination of Y-splitter with trimming ameliorates the poor signal tagging efficiency of Y-splitter whilst retaining effective background rejection. Consequently, we find that this combination outperforms the other techniques studied here, at high p_T. We use our analytical expressions to perform an approximate optimisation of parameters for each algorithm and compare our results to Monte Carlo simulation.Finally, we undertake an analytical fixed-order and resummed study of the mass distribution for QCD jets for the combination of Y-splitter with trimming. We demonstrate that the trimming has a numerically subleading effect on the Y-splitter distribution for typical parameter choices and discuss why such techniques can prove to be superior when compared to the currently proposed individual methods.

Date of Award	1 Aug 2016
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mrinal Dasgupta (Supervisor)