Higgs_Public

Search for the SM Higgs Boson in the ZH->bb+vv Final State
with 289/pb Run II at CDF

Abstract

We present a search for the Standard Model Higgs boson produced in association with a Z boson in pp collisions at sqrt(s)= 1.96 TeV. We focus our search on the H -> bb and Z->vv decay modes. In the final state, we expect two b-jets and at most a third soft jet radiated off by one of the b-jets, at least one b-tagged jet, and large missing transverse momentum produced by the undetected neutrinos. We processed 289/pb data collected in CDF Run II. The observed number of events is consistent with the Standard Modell prediction for the backgrounds within the errors. A 95% C.L. upper limit is set on the Z-Higgs production cross-section at various Higgs masses. The observed cross-section limits range from 0.9 to 1.09 between 90 GeV and 130 GeV of Higgs masses. The observed limits are also consistent with the Monte Carlo expectations. (The results have been presented at the SUSY 05 conference.)

Authors: V. Veszpremi, O. Gonzalez, D. Bortoletto, A. Garfinkel
Blessed: July 14, 2005
Godparents: none
Data: Run II, 289/pb
Documentation: CDF7719
Internal Web Page
Public: Note CDF7983 PS

Introduction

According to the latest results from LEP2, the lower bound on the mass of the Higgs is 114.4 GeV at a 95 % C.L. [Phys.Lett. B565 (2003) 61-75] The previous results of the CDF collaboration from Run I has been published in [FERMILAB-PUB-05-042-E]. In the Run I analysis, the luminosity of the data was not enough to observe any trace of the Higgs beyond the region excluded by LEP. With the projected total luminosity of Run II, we expect to have a sensitivity to observe or exclude the Higgs near the current lower mass limit [FERMILAB-PUB-03/320-E]. In this analysis we use 289/pb of Run II data, but the CDF detector has already recorded nearly 1 /fb.

In the final state, we expect two b-jets and no more than one additional soft jet, which is radiated off by one of the b-jets in a fairly large fraction of the signal events. Since the two neutrinos escape the detector without being observed, a large amount of transverse momentum appears missing from the events.

Control Region and Extended Signal Region definitions:

For all the regions:

1. Quality cuts:

    - Consider runs from the data in which all the calorimeters, the tracking, and the muon system were functional.

    - A set of quality cuts are applied to partially filter events with large fake missing transverse energy..

2. Basic Selection cuts:

- Select events with two jets:
    - Leading jet Et > 25 GeV
    - Second most energetic jet Et>25 GeV

- No more jets with Et above 15 GeV is allowed

- Missing Et>70 GeV

- At least 1 b-tagged jet is required

3. Definition of Extended Signal Region and the Control Regions:

Control Region 1:
    - All events with an identified lepton (by a set of loose id-cuts) are rejected.
    - Azimuthal angular separation between 2nd leading jet and missing Et must be less than 23 degrees (0.4 rad)
    - This control region is dominated by heavy flavor QCD multi-jet events. These events are simulated by a b-filtered QCD Pythia Monte Carlo sample. The control region also contains mistagged light flavor jets, which is derived from the data using the so-called Mistag Rate Matrix. The sum of these two background components are normalized to the data in this region to get the correct normalization for the QCD background. The plot below shows the distribution of the Missing Et, and demonstrates a correct normalization for the QCD Monte Carlo, as well as the capability of the MC to reproduce the data.

Control Region 2:
    - At least 1 isolated high-pt lepton is reqired.
    - Azimuthal angular separation between 2nd leading jet and missing Et must be more than 23 degrees (0.4 rad)
    - This control region contains all the physical backgrounds (EWK, Top, QCD). Later the optimized cuts are also applied on this region and invariant mass plot is created to gain confidence to open the "blind box" containing the signal region.

Extended Signal Region:
    - Events with isolated leptons are vetoed
    - Azimuthal angular separation between 2nd leading jet and missing Et must be more than 23 degrees (0.4 rad)
    - After a succesful MC simulation in the two other control regions, a cut optimization is performed in this region. The optimization is done on the MC and the Mistag samples and aims to maximize the signal to square-root background ratio. The dijet invariant mass is the last cut among the optimized cuts. For the optimization we use a Higgs sample (ZH->nunu bb) where the mass of the Higgs is 120 GeV. The only cut depending on the mass of the simulated Higgs is the invarian mass. We select events in a window +/-20 GeV around the mean of the reconstructed Higgs mass, and count the expected signal and Standard Model background prediction in the mass window.
The set of events that pass all the optimized cuts but the mass window is called the Signal Region.

Signal and event reduction during cut optimization. We start with events from the Extended Signal Region and apply the optimized cuts sequentially.

The table above lists the various SM background components int the three regions (CR-1, CR-2, and Signal Region) after passing the optimized cuts (except mass window).

Mass(GeV)	Mass window (GeV)	Observed events	SM prediction	Higgs signal acceptance	Expected limit	Observed limit
90	60-100	6	7.18	2.27%	1.26+/-0.24	1.09
100	65-105	7	7.07	2,76%	1.02+/-0.19	1.01
110	75-115	7	5.9	3.21%	0.92+/-0.29	1.04
115	75-115	7	5.9	3.44%	0.86+/-0.27	0.97
120	80-120	6	4.36	3.65%	0.72+/-0.28	0.90
130	90-130	8	4.11	3.87%	0.64+/-0.20	1.05

The predicted and observed limit with 95% C.L. at various Higgs masses. The mass windows for each Higgs mass scenarions are displayed.

Blessed Plots and Tables:

	Leading order diagram of ZH production. BMP
	Production cross section of the Higgs boson BMP
	Branching ratios of the Higgs boson BMP
	SM background components int the three regions (CR-1, CR-2, and Signal Region) after passing the optimized cuts (except mass window) GIF
	Corrected Missing Et in Control Region 1. GIF EPS
	Dijet invariant mass in the Control Region 2 after applying the optimized selection cuts. GIF EPS
	Dijet invariant mass in the Signal Region after applying the optimized selection cuts. GIF EPS
	95% C.L. exclusion plot GIF EPS
	Single Tagged Candidate Event Jet Et1 = 84.7 GeV Jet Et2 = 71.9 GeV - TAGGED Missing Et = 98.0 GeV Dijet mass = 128.6 GeV GIF EPS
	same event... GIF EPS
	Double Tagged Candidate Event Jet Et1 = 100.3 GeV - TAGGED Jet Et2 = 54.7 GeV - TAGGED Missing Et = 144.8 GeV Dijet mass = 82.1 GeV GIF EPS
	same event... GIF EPS

Page was last updated December 13th, 2005 by Viktor Veszpremi
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