Search for the Higgs Boson in the All-hadronic Channel


Current searches for a low mass Higgs boson at CDF have focused on the Higgs decaying to a pair of b-quarks accompanied by a combination of leptons and missing energy.  This analyses exploits the fully hadronic mode which has 2 major advantages:

  1. Large signal yield as it profits from the largest cross-section x branching ratio

  2. Complete event information.  No missing energy to infer

But the major challenge of this analysis is to model the overwhelmingly large QCD background which is notoriously hard to model.

The analysis presented here is an update of the 2fb-1 result  where the following improvements were added:

  1. 4fb-1 of data used (x2 compared to the previous 2fb-1 result)

  2. Additional b-tagging categories to improve acceptance.

  3. Additional channel added to analysis

When all these improvements are brought together, this yielded a x2 improvement on the Higgs sensitivity for the all-hadronic Higgs channel

Key plots for talk

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Multi-jet Trigger Study

  1. CDF Note 9954 (Internal) (PDF)

All Hadronic Higgs Search

  1. CDF Note 9955 (Internal) (PDF)

  2. CDF Note 10010 (Public)

This analysis is the search for the Higgs boson in the all hadronic bbjj final state. The two channels which are studied in this analysis are associated Higgs production and vector boson fusion (VBF).

The advantage of this channel is the large signal yield as the hadronic final state has the largest cross-section X branching ratio.  The major challenge to this analysis is to accurately model the large QCD background.

VH Channel

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VBF Channel

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The data for this analysis was collected by the TOP_MULTIJET and VH_MULTIJET triggers. These triggers are designed to select events with multiple jets and large Sum-Et; characteristic of the all hadronic Higgs event. The triggers are complex and the trigger efficiency is sample dependent.  A detailed trigger study was performed and further information can be found Trigger Studies Details page.

The event selection consists of two parts:

  • Initial event selection:
    • Pass CDF multi-jet trigger and good run selection
    • Position of reconstructed primary vertex in the luminous region
    • Veto events with reconstructed leptons to reduce ttbar background contribution and to ensure orthogonality with other Higgs searches.
    • Selected events should have 4 or 5 jets with ET > 15 GeV
    • Events should have small missing transverse energy significance to reduce contribution from ttbar background.
  • Events that pass the initial selection will be further filtered by the final selections:
    • Amoung the first 4 leading jets, exactly two jets must be tagged as b-jets. CDF uses two b-jet tagging algorithms: SecVtx and JetProbability. In this analysis, we consider two b-taggging combinations.
      • (SS) Two jets identified (tagged) by the SecVtx algorithm.
      • (SJ) One jet is tagged by SecVtx and the other jet by JetProbability.
    • The event SumEt (based on the sum of the transverse energy of the selected jets) must be greater than 220 GeV. This cut is to improve the signal to QCD multi-jet background significance.
  • The events which pass final selection are selected for the VH or VBF signal regions. The two signal regions are defined by the invariant mass of the 2 b-tagged jets (Mbb) and the invariant mass of the 2 non b-tagged jet (Mqq) from the 4 leading jets.
    • VH Signal Region: 75 < Mbb 175 GeV/c2 & 50 < Mqq < 120 GeV/c2
    • VBF Signal Region: 75 < Mbb 175 GeV/c2 & Mqq > 120 GeV/c2

The number of signal and background events which pass the events selection are given in the tables below. The number of events for each signal region and b-tagging category are given.

Download as PDF or PNG

Download as PDF or PNG

Two new variables added to the analysis are Jet-Shapes. Gluon jets tend to be broader than light-flavor quark jets. The QCD multi-jet background tends to have jets from both gluons and quarks. Whereas the Higgs signal consists of mainly quark jets, with large fraction coming from light-flavor quarks. The jet shape variables are among the input variables to the Neural Net algorithm which is trained to discriminate between the Higgs signal from the QCD multi-jet background. Further details on the studies can be found in the Jet Shapes Details Page.

The sources of background for the all-hadronic Higgs search are:

  • QCD (98%)
  • ttbar
  • Z + jets (where Z decays to b/c quarks)
  • Single-Top
  • W + bb/cc jets
  • WW/WZ/ZZ

All the non-QCD backgrounds are estimated using Monte-Carlo simulation. The QCD background is modelled using a data-based method. Further information on the background modelling can be found in the Background Prediction Details page.

A neural net is trained to separate the background from the Higgs signal. As 98% of the background is from QCD, the neural net is trained against QCD and Higgs signal. The QCD events are from the TRF prediciton. Further information on the neural net training can be found in the Neural Net Details Page.

Neural net distributions for VH and VBF channels are shown below for Higgs Mass of 120 GeV/c2.

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Download as PDF or PNG

Download as PDF or PNG

Download as PDF or PNG

Neural net distributions for all masses & Higgs masses can be found in the Neural Net Details page.

We consider systematics which affect the shapes & normalisation of the Neural Net distributions for the signal & background.

The systematics which are considered are:

  • QCD Modelleing
  • Luminosity
  • b-tag scale factor
  • Jet Energy Scale
  • Initial & fnal state radiation.
  • Parton distribution uncertainties.

Further details of the systematic errors can be found in the public note CDF-Note 10010.

After examining the data, there was no evidence of a Higgs signal. So 95% confidence-limits on the upper cross-section were calculated for Higgs masses from 100 GeV/c2 to 150 GeV/c2. The results are summarised in the table & plot below.

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Download as PDF or PNG

The results for the individual channels can be found in the All Results Page.