Search for the Standard Model Higgs boson in the ttH channel

This is a condensed description with plots for the analysis CMS-PAS-HIG-12-025

Abstract

We present a search for the Standard Model Higgs boson produced in association with a top quark pair in 5 fb^-1 of 7 TeV collision data. We look for events where the Higgs boson decays to and the top quark pair decays to either lepton plus jets () or dileptons (), where is an electron or muon. The major background to our signal is top pair production. We use artificial neural networks to discriminate between background and signal events. We perform a simultaneous fit for signal and background fractions using the neural network output distributions. Our expected limit on Higgs boson production cross section times branching ratio for a Higgs boson mass of 125 GeV/c^2 is 4.6 times the Standard Model expectation, while the observed limit is 3.8 times the Standard Model expectation.

CMS Combination of ttH results

Figure	Label	Description
	png pdf	The best-fit values of the signal strength parameter μ = σ/σSM for each ttH channel at MH = 125.7 GeV . The signal strength in the four-lepton final state is not allowed to be below approximately 6 by the requirement that the expected signal-plus-background event yield must not be negative in any of the two bins of jet multiplicity.
	png pdf	The observed and expected 95% CL upper limits on the signal strength parameter μ = σ/σSM for all ttH channels combined, as a function of the mass of the Higgs boson.

Limits

Lepton + jets channel

The observed and expected 95% CL upper limits on the signal strength parameter μ = σ/σ(SM).

Higgs mass(GeV/c^2)	observed	median expected	68% CL band	95% CL band
110	2.5	3.1	[2.2,4.3]	[1.7,5.7]
115	2.8	3.6	[2.6,4.9]	[1.9,6.6]
120	3.1	4.1	[2.9,5.7]	[2.2,7.5]
125	3.8	4.9	[3.5,6.8]	[2.7,9.0]
130	4.4	6.3	[4.5,8.7]	[3.4,11.5]
135	5.6	7.8	[5.6,10.8]	[4.2,14.4]
140	6.7	10.5	[7.6,14.6]	[5.7,19.4]

Dilepton channel

The observed and expected 95% CL upper limits on the signal strength parameter μ = σ/σ(SM).

Higgs mass(GeV/c^2)	observed	median expected	68% CL band	95% CL band
110	7.5	7.2	[5.2,10.0]	[3.9,13.3]
115	11.4	8.9	[6.4,12.4]	[4.8,16.4]
120	11.4	9.6	[6.9,13.3]	[5.2,17.7]
125	14.7	11.7	[8.4,16.2]	[6.3,21.5]
130	15.6	12.8	[9.3,17.8]	[7.0,23.6]
135	20.4	15.8	[11.4,22.0]	[8.58,29.2]
140	23.8	20.6	[14.8,28.6]	[11.2,37.9]

Lepton + jets channel and dilepton channel combined

The observed and expected 95% CL upper limits on the signal strength parameter μ = σ/σ(SM).

Higgs mass(GeV/c^2)	observed	median expected	68% CL band	95% CL band
110	2.3	2.9	[2.1,4.0]	[1.6,5.4]
115	2.8	3.5	[2.4,4.7]	[1.8,6.2]
120	3.1	3.8	[2.8,5.3]	[2.1,7.1]
125	3.8	4.6	[3.3,6.4]	[2.5,8.5]
130	4.4	5.7	[4.1,7.9]	[3.1,10.5]
135	5.7	7.0	[5.1,9.8]	[3.8,13.0]
140	6.6	9.5	[6.8,13.2]	[5.1,17.5]

Figures from CMS-HIG-12-025

figure	label	description
	Figure 1 (a) png,pdf	ANN output for lepton + jet events for 6 or more jets and 2 b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 1 (b) png,pdf	ANN output for lepton + jet events for 4 jets and 3 b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 1 (c) png,pdf	ANN output for lepton + jet events for 5 jets and 3 b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 1 (d) png,pdf	ANN output for lepton + jet events for 6 or more jets and 3 b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 1 (e) png,pdf	ANN output for lepton + jet events for 4 jets and 4 b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 1 (f) png,pdf	ANN output for lepton + jet events for 5 jets and 4 or more b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 1 (g) png,pdf	ANN output for lepton + jet events for 6 or more jets and 4 or more b-tagged jets. Background-like events have a low neural network output value. Signal-like events have a high neural network output value. The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 2 (a) png,pdf	Data/MC comparison of the ANN output shape for events with two electrons or two muons, 2 jets and 2 b-tags . The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 2 (b) png,pdf	Data/MC comparison of the ANN output shape for events with two electrons or two muons and 3 or more b-tags . The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 2 (c) png,pdf	Data/MC comparison of the ANN output shape for events with one electron, one muon, 2 jets and 2 b-tags . The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 2 (d) png,pdf	Data/MC comparison of the ANN output shape for events with one electron, one muon, and 3 or more b-tags . The uncertainty band includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ttH signal (mH = 120 GeV/c^2) is normalized to the total background yield.
	Figure 3 png,pdf	The expected and observed 95% CL upper limits on the signal strength parameter μ = σ/σ(SM) for lepton + jets channel.
	Figure 4 png,pdf	The expected and observed 95% CL upper limits on the signal strength parameter μ = σ/σ(SM) for dilepton channel.
	Figure 5 png,pdf	The expected and observed 95% CL upper limits on the signal strength parameter μ = σ/σ(SM) for lepton + jets and dilepton channels combined.

Tables from CMS-HIG-12-025

Table 3: List of systemtatic effects considered in this analysis. This table lists both the size of the systematic effect on the yields of signal and background processes (or a range when the size of the effect varies with category) and indicates whether the source of systematic uncertainty also affects the ANN output shape.

Source of uncertainty	Rate	Shape?	Notes
Luminosity	2.2%	No	All signal and backgrounds
Lepton ID/Trig	1.8%	No	All signal and backgrounds
Pileup	1%	No	All signal and backgrounds
Jet Energy Resolution	1.5%	No	All signal and backgrounds
Jet Energy Scale	0-66%	Yes	All signal and backgrounds
QCD Scale ($t\bar{t}H$)	12.5%	No	Scale uncertainty for NLO $t\bar{t}H$ prediction
QCD Scale (t&bar{t})	2-12%	No	Scale uncertainty for NLO $t\bar{t}$, $t\bar{t}V$, and single top predictions
QCD Scale ($V$)	1.2-1.3%	No	Scale uncertainty for NNLO $W$ and $Z$ prediction
QCD Scale ($VV$)	3.5%	No	Scale uncertainty for NLO diboson prediction
pdf ($gg$)	9%	No	Pdf uncertainty for $gg$ initiated processes ($t\bar{t}$, $t\bar{t}Z$, $t\bar{t}H$)
pdf ($q\bar{q}$)	4.2-7%	No	Pdf uncertainty for $q\bar{q}$ initiated processes ($t\bar{t}V$, $W$, $Z$)
pdf ($qg$)	4.6%	No	Pdf uncertainty for $qg$ initiated processes (single top)
Factorization scale ($t\bar{t}$)	0-20%	Yes	Uncorrelated between $t\bar{t}+$jets$/b\bar{b}/c\bar{c}$; varies by jet bin
Factorization scale ($V$)	20-60%	No	Varies by jet bin
$b$-Tag SF ($b/c$)	0-15.2%	Yes	All signal and backgrounds
$b$-Tag SF (mistag)	0-10.6%	Yes	All signal and backgrounds

Additional plots for public talks

Lepton + jets channel

You can find all lepton + jets images in tarballs in png or pdf format.

Dilepton channel

You can find all dilepton images in tarballs in png or pdf format.