Search for the Standard Model Higgs boson in the MET and b-jet Signature



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Analsis Outline


We are analyzing Z-Higgs and W-Higgs associated production when the Z decays into two neutrinos, or the W decays leptonically but the charged lepton escapes the detection. We do not expect isolated leptons in these events. We are performing a unbiased analysis in which all the considerable Standard Model background processes, such as QCD heavy flavor multi-jet production, top production, and electro-weak processes, are simulated.

We use data collected through March 2007, which corresponds to 1.7 fb-1 integrated luminosity. The events are collected by CDF II detector with a trigger that selects events with missing ET>25 GeV at Level 1 at least two Level 2 clusters with ET>10 GeV and missing ET>35 GeV Level 3.

In this analysis, we simulate processes which yield real taggable objects, that is, when a b- or a c-quark pair is created. Events with light flavor jets, after requiring a positive tag in the event during the basic selections, are considered to be mistags and are estimated from the data. All Monte-Carlo samples were generated with Pythia.

The summary of basic cuts used in the analysis is below:

    - Exactly two jets with ET> 20 GeV, |eta|<2.4, and 0.1<EM Fract.< 0.9
    - 1st jet ET> 35 GeV, and |eta|<2.0
    - 2ndjet ET> 25 GeV, and |eta|<2.0
    - MET>50 GeV
   -  deltaR(jet1, jet2) > 1.0
    - 2 loose b-tags or at least 1 tight b-tag
    - no isolated leptons (electron or muon)

We define two control regions to check our modeling of the data:

Control Region 1 (QCD):
    - All events with an identified lepton (by a set of loose electron or isolated track id-cuts) are rejected.
    - Azimuthal angular separation between 2nd leading jet and missing Et 0.4 rad
    - This control region is dominated by heavy flavor QCD dijet events.

Control Region 2 (EWK):
    - At least 1 isolated high-pt lepton is reqired.
    - Azimuthal angular separation between 2nd leading jet and missing Et is greater than 0.4 rad
    - The single tag part of this control region contains all the physical backgrounds (EWK, Top, QCD) in approximately equal amount. The double tag set contains less QCD h.f. It is dominated by top, W+h.f., and mistagged light flavor; therefore, it provides a better control on the top and electroweak background components.

After achieving a good agreement between the simulation and the data in the control regions (see control region plots), a set of cuts are selected that are optimized for the signal Monte Carlo against the background prediction. The optimization yields four consecutive cuts.

    - 1st jet ET> 60 GeV
    - MHT/HT>0.45 (where HT is the scalar sum and MHT is the magnitude of the vectorial sum of the pT's of the two leading jets)
    - dPhi(1st jet, MET) > 0.8
    - MET> 70 GeV

Exclusion limits:
The table below shows the expected and observed limits in the ZH and WH channels relative to SM:



The table below shows the absolute expected and observed limits in the ZH and WH channels:


The limit calculation was done on the invariant dijet mass in the single and double tag samples.


Created by Artur Apresyan.
For problems or questions regarding this website contact apresyan@fnal.gov
Last updated: 16/08/07.