Search for High Mass Resonances Using Boosted All Hadronic Top Quarks

Introduction

An effort to identify highly boosted top quark pairs resulting from the production of a massive resonance and decaying into two "fat jets" is summarized here. This is an analysis falling under the Top Cross Section Working Group as well as the 4GT working group.

The analysis strategy is summarized here, along with the people involved and the tasks that they have expressed interest in.

Relevant Published or Released papers

• CDF search for boosted top quarks (5 fb-1): Conference Note
• Top pair production cross section with the l+jets and dilepton channels (2.9 pb-1): Glance entry
• Top pair production cross section with the dilepton channel (35 pb-1): Glance entry
• CMS study of boosted top quark pairs: Search for Anomolous ttbar production in the highly-boosted all-hadronic final state

Relevant Internal Notes

• Note on pileup corrections for 2010: CDS entry
• Paper on jet shapes for 2010: CDS entry for draft paper

Organization

• Contact Person: Pekka Sinervo
• Analysis Group: Trisha Farooque, Alex Martyniuk, Gilad Perez, Michele Pettini, Francesco Spano, Bernd Stelzer, Oliver Stelzer-Holmes
• Mailing list: Pekka can provide you with a list

Overall Strategy

• Select events using the requirement of at least on high pT jet reconstructed with either R=1.0 and with a pTjet1>500 GeV. The jet should be in the central region defined by |eta|<2.0, though one could argue that a narrower "central" interval could be defined.
• Require a recoil jet with pT>pTjet2, where pTjet2 is a fraction of pTjet1. This could notionally be around 50% of pTjet1, though the analysis is not too sensitive to this (either for background or signal). Also require this jet to be reasonable central |eta|<2.0. Need to do some optimization of this cut on the signal MC as a first step.
• Require the events to have no evidence of significant missing Et by making a MetSignificance cut of around 5. The key here is to make this sample efficient for fully hadronic decays and fully complementary to the lepton+jets final state.
• At this point, we'll be dominated by QCD dijet events -- "QCD jets".
• We perform the pileup corrections for mass, which helps improve the S/B between QCD jets and top quark jets.
• There are then at least three different strategies that should be investigated:
  1. B-tagging -- we look at the antikT 0.4 jets that are associated with the two leading jets, and then look at the MV1 results for each of these. We'll be able to use all the standard tools for this, though we'll need to work on understanding the efficiency and mistagging rates once we have a sizeable contribution of jets with pT>200 GeV. I've poked an idea in the ears of the b-tagging conveners as to how to get some constraints on these efficiencies.
  2. A template approach proposed by Almeida et al. (see this article for top jets and this article for an extension to Higgs jets).
  3. A conventional jet substructure requirement like using the jet mass and planar flow (the interesting thing here is that planar flow is substantially affected by pile-up, but Almeida et al. argue that the planar flow from the best template is more effective).
• Do the b-tagging and top templating on both the leading jet and the non-leading jet, separately. In principle, these are independent methods, so using one against the other will allow us to measure fake rates.
• Estimates of t-tbar acceptance would be done via MC for SM ttbar, KKG of various masses and Zprimes of various masses.
• Background estimates would be done by looking at anti-tagged jet samples. We've used an ABCD technique in the past, and that may be the way to go here.
• Optimize the data selection for two purposes: i) set an upper limit on a resonance decaying to t-tbar and ii) measure the top quark cross section above a top quark pT above some threshold. This is work that Sergei and Ben at Argonne are pioneering. The resonance search doesn't require seeing a top signal per se, so long as we understand the backgrounds.

Event Selection

• AKTLC10 jet with leading jet pT>500 GeV in |eta|<2.0
• A good primary vertex with at least 4 tracks (check to make this consistent with other 2011 analyses)
• Second AKTLC10 jet with pT > 300 GeV and |eta|< 2.0
• SMET < 5 (this is the primary anti-lepton+jet selection)
• Jet mass correction for NPV and JES correction to jet
• Identify all AKT4 jets with R=1.0 of each fat jet and with pT>20 GeV
• Define MV1 > 0.5 as "b-tag"
• Run top template and calculate OV2 and OV3 values, identify best template, calculate Pf for best template for each jet
• Optimize selection on OV2 and OV3, using jet mass as the signal. Use sample with mjet2<100 GeV as the "background" control sample.
• Final selection (to be optimized), but likely something like >1 b-tag (or > 2 b-tags); 130 < mjet1 < 210 GeV and 130 < mjet2 < 210 GeV; and OV2>0.5 (perhaps OV3 > 0.5 will be better)
• Background estimated using ABCD technique with correction calculated from QCD MC

Tasks

• Selection of AKT10 jets (Trisha Farooque, Alex Martyniuk)
• Jet mass correction and incorporation of JES corrections (Trisha Farooque)
• b-tagging implementation (Jacob Groth-Jensen of Weizmann is giving some help, but could use more personnel)
• top-templating (Francesco Spano and Michele Petteni)
• MC calculations (Stephen Swedish)
• Systematic uncertainties (Pekka coordinating this effort)
• Limit calculations (Stephen, Michele, Bernd, Oliver?)

Other related links/presentations

• Overview of Analysis at 12 Apr 2012 4GT Meeting: Slides