Search for Standard Model Higgs Boson Production in Association with a W Boson using Matrix Element Technique with 5.6 fb-1 of CDF Data

Barbara Alvarez5, Florencia Canelli1, 4, Bruno Casal Laraña2, Javier Cuevas Maestro2,
Peter Dong1, Ricardo Eusebi6, Craig Group1, Martina Hurwitz4,
Enrique Palencia2, Alberto Ruiz2, Bernd Stelzer7, Rocio Vilar2, Jesus Vizan2, Rainer Wallny3
  
1Fermilab, 2IFCA (CSIC-UC), 3UCLA, 4University of Chicago, 5MSU, 6Texas A&M, 7Simon Fraser University
 


- Abstract -
- Event Selection -
- Matrix Element Input Variables -
- Matrix Element Probabilities -
- Dijet Mass -
- Final discriminat -
- Systematics -
- Results -
- Public Note -
Abstract

We present a search for Standard Model Higgs boson production in association with a W boson using 5.6 fb-1 of CDF II data collected between 2002 and 2010. This search is performed using a matrix element technique to calculate event probability densities for the signal and background hypothesis. Based on these probabilities, we build a final discriminant distribution which is fitted to the data using a binned likelihood approach. We observe no evidence for a Higgs boson signal and set 95 % confidence level upper limits on the WH production cross section times the branching ratio of the Higgs boson to decay to bb-bar pairs of &sigma(pp-bar -> WH)xBR(H -> bb-bar)/SM < 2.1 to 35.3 for Higgs boson masses between mH=100 GeV/c2 and mH=150 GeV/c2 . The expected (median) limit estimated in pseudo-experiments is: &sigma(pp-bar -> WH)xBR(H -> bb-bar)/SM < 2.5 to 27.5 at 95 % C.L. With respect to the previous iteration of this analysis, we have now incorporated 0.8 fb-1 of data. This improves the limit by a 5% to 14%, depending on the Higgs mass






Event Selection

This analysis uses events from leptonic decay of the W boson. We require a single, well isolated high-transverse-energy lepton, large missing transverse energy (from the neutrino), and two or three high-transverse-energy jets. Of these jets, we require at least one to be identified as originating from a b-quark by secondary vertex (SecVtx) tagging. The secondary vertex tag identifies tracks associated with the jet originating from a vertex displaced from the primary vertex.
In order to gain sensitivity we consider 3 orthogonal tagging categories:
  • SVSV: Events where two or more jets are tagged by the SecVtx algorithm.
  • SVJP: Events with only one jet tagged by SecVtx and the another one tagged by the JetProbability algortihm.
  • SVnoJP: Events with only one jet tagged by SecVtx (in this case, none of the other jets is tagged by any of the algorithms).
Since each tagging category has different signal to background ratio,the final sensitivity is improved. We further require the missing transverse energy and the jets not to be collinear for low values of missing transverse energy. This requirement removes a large fraction of the non-W background while retaining most of the signal.
Our major backgrounds come from W + heavy flavor jets, Wbb-bar, Wcc-bar, and Wc + jet; mistags which are W + light quark/gluon events that are mistakenly tagged as b-jets due to detector resolution effects; Non-W, which are mostly multijet events in which a jet is mistakenly identified as a lepton and jets are mismeasured, providing a false missing transverse energy signature; and top pair production events in which one lepton or two jets are lost due to detector acceptance.

The number of expected signal and background events, in the 2 and 3 jets bin, in 5.6 fb -1 of CDF data, passing all event selection requirements is shown below.


Although the predictions seem to match too closely the observed number of events, this is just an effect of the fact that all tag categories (SVSV, SVJP, and SVnoPJ) are strongly correlated. In fact, a chi2 between the predicted and observed events taking into account the correlations between tag categories yield a chi2 probability of 2.65% for the 2-jet bin, and of 36.85% for the 3-jet bin.







Matrix Element Input Variables

Input variables used for ME calculation in the 2 jet bin channel are listed and ploted below, from left to right: Total energy, Px, Py, Pz.


-- Lepton (Untagged sample) --




-- Lepton (Signal region) --




-- Leading Jet (Untagged sample) --




-- Leading Jet (Signal region) --




-- Second Leading Jet (Untagged sample) --




-- Second Leading Jet (Signal region) --





Input variables used for ME calculation in the 3 jet bin channel are listed and ploted below, from left to right: Total energy, Px, Py, Pz.

-- Lepton (Untagged sample) --




-- Lepton (Signal region) --




-- Leading Jet (Untagged sample) --




-- Leading Jet (Signal region) --




-- Second Leading Jet (Untagged sample) --




-- Second Leading Jet (Signal region) --




-- Third Leading Jet (Untagged sample) --




-- Third Leading Jet (Signal region) --






Matrix Element Probabilities


Matrix element probabilities for the WH, Wbb, single top (s channel) and ttbar production processes in the 2 jet bin channel.

-- Untagged sample --




-- Signal region --






Dijet mass


Dijet mass in the 2 jet bin channel is ploted below. From left to right: untagged, SVnoJP, SVJP and SVSV samples.







Final discriminant


Event Probability discriminant for events with 2 jets. From left to right: untagged, SVnoJP, SVJP, SVSV samples


-- EPD (WH 115) --




-- EPD (WH 115) in log scale--





Event Probability discriminant for events with 3 jets. From left to right: untagged, SVnoJP, SVJP, SVSV samples

-- EPD (WH 115) --




-- EPD (WH 115) in log scale--






Systematics:

We address systematic uncertainty from several different sources:
  • jet energy scale
  • initial state radiation
  • final state radiation
  • parton distribution functions
  • lepton identification
  • luminosity
  • b-tagging SF
Systematic uncertainties can influence both the expected event yield (normalization) and the shape of the discriminant distribution. Normalization uncertainties are estimated by recalculating the acceptance using Monte Carlo samples altered due to a specific systematic effect. The WH normalization uncertainty is the difference between the systematically shifted acceptance and the default one and are shown in the Table below. Jet energy scale shape systematics in both signal and background (W+jets and ttbar) are included.


Systematic uncertainty SVnoJP SVJP-SVSV
Jet energy scale 2.0 % (15.8 %) 2.0 % (13.5 %)
ISR/FSR + PDF 3.1 % (13.1 %) 5.6 % (21.4 %)
Lepton ID 2.0 % 2.0 %
Luminosity 6.0 % 6.0 %
b-tagging SF 3.5 % 8.4 %

Rate Systematic Uncertainties in the three signal samples. In brackets are the systematic uncertainties for 3-jet events (when different that the numbers for 2-jet events).






Results:

In order to extract the most probable WH signal content in the data we perform the maximum likelihood method. We have analyzed 5.6 fb-1 of CDF Run II data. We observe no evidence for a Higgs boson signal and set 95 % confidence level upper limits on the WH production cross section times the branching ratio, in SM units, of the Higgs boson to decay to bb-bar pairs of &sigma(pp-bar -> WH)xBR(H -> bb-bar)/SM < 2.1 to 35.3 for Higgs boson masses between mH = 100 GeV/c2 and mH = 150 GeV/c2. The expected (median) sensitivity estimated in pseudo experiments is:
&sigma(pp-bar -> WH)xBR(H -> bb-bar)/SM < 2.5 to 27.5 at 95 % C.L.


mH (GeV) Expected (&sigma/SM) Observed (&sigma/SM)
100 2.5 2.1
105 2.7 2.6
110 3.0 3.2
115 3.5 3.6
120 4.4 4.6
125 5.1 5.3
130 6.6 8.3
135 8.7 9.2
140 13.0 14.8
145 17.8 18.9
150 27.5 35.3

Expected and observed upper limit cross sections in SM units for 2 and 3-jet events for different Higgs mass points.



mH (GeV) Expected (&sigma/SM) Observed (&sigma/SM)
100 2.6 2.7
105 2.8 3.3
110 3.2 3.7
115 3.7 4.5
120 4.7 5.9
125 5.5 6.8
130 7.1 9.6
135 9.5 12.0
140 14.2 19.3
145 19.7 24.0
150 30.7 43.2

Expected and observed upper limit cross sections in SM units for 2-jet events for different Higgs mass points.



mH (GeV) Expected (&sigma/SM) Observed (&sigma/SM)
100 12.2 5.1
105 12.9 5.6
110 13.9 8.6
115 15.8 8.5
120 19.5 10.8
125 23.0 12.4
130 28.1 17.3
135 39.5 22.9
140 56.1 33.7
145 77.9 42.5
150 119.8 80.5

Expected and observed upper limit cross sections in SM units for 3-jet events for different Higgs mass points.









Public Note:

For more details about the analysis you can have a look at our public note .ps and .pdf versions.



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