Matrix element analysis update
In recent updates presented at the Oxford Higgs meeting, significant advancements in matrix element analysis have been made by Gemma Wooden and colleagues, focusing on Higgs PT distributions and event selection. A comparison between two implementations revealed improvements in likelihood calculations across all Higgs masses. Preselection criteria to reduce background, such as requiring opposite sign leptons and specific MET thresholds, have been established. Current efforts include systematics analysis and limit setting, with plans for detailed write-ups and integration of final results.
Matrix element analysis update
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Presentation Transcript
Matrix element analysis update Gemma Wooden, Chris Hays, Shih-Chieh Hsu (LBNL), Daniel Whiteson(University of California, Irvine) Oxford Higgs meeting 01/04/11
Improvements • Currently have two implementations (mine & Shih-Chieh’s) • Have carried out dedicated comparisons in past three weeks in terms of: • MCFM vsMadGraph • LHAPDF tools • Following Higgs Yellow Report recommendation for MH and GH • See complete agreement • Have added in some improvements: • Constructed KT distribution from MC (to account for ISR) for 0 and 1 jet channels • Constructed and verified transfer functions to account for detector effects for: • Leptons • MET • Compared two sets of integration variables: • MH, nux, nuy, nuz vs. MH, MW1, nux, nuy • The first set appears to perform better (large peak at zero for second set which isn’t understood yet) Gemma Wooden, Oxford Higgs meeting
Higgs PT distributions • Plotted the Higgs PT distribution for 0, 1 and 2 jet channels after preselection • Use this in the matrix element calculation: • Draw at random from these distributions • Draw from 0 jet distribution for 0 jet events etc. • Apply a weight to each event based on the Higgs PT which is drawn MH = 170 GeV, 1 jet channel MH = 170 GeV, 0 jet channel MH = 170 GeV, 2 jet channel
New integration ranges • Had been using a CDF parameterisation for integration to check with Shih-Chieh • Now switch to ATLAS parameterisation – assume flat distribution for nuX and nuY between -50 and 50 • Use fit to truth distribution for nuZ MH = 170 GeV, nuX MH = 170 GeV, nuZ Gemma Wooden, Oxford Higgs meeting
Event selection • In order to get the best reduction in background, apply some preselection: • Two opposite sign, well-identified leptons • Require Mll > 15 GeV and |Mll – Mz| > 15 GeV • METproj > 35 GeV to reduce the Z background which is significant • Dphill < 2.25 • Finally calculate the ME likelihood: • PH(MH)/ (PH(MH) + PWW) • Use this to get the best possible separation between signal and background
Matrix element likelihoods MH = 120 GeV MH = 160 GeV
Matrix element likelihoods • Matrix element method shows nice improvement in significance wrt cuts-based method • Improvement is seen at all Higgs masses MH = 200 GeV
Next steps • Running systematics samples as we speak: • JES • JER • Lepton resolution • Should add in lepton efficiency – this was found to be a large uncertainty in BDT analysis • Limit setting: • Can calculate the limits using a binned maximum likelihood fit without systematics • Adding in systematics could turn out to be some work • Planning on looking at RooStats in more detail • Plan: • Write up all of the description in the MVA note while the systematics samples are running • Also look into limit setting in the meantime • Update MVA note with final results as soon as possible Gemma Wooden, Oxford Higgs meeting
