CDF HWW Dilepton Analysis

We study the sources of dileptons in p pbar collisions in 1 fb $^{-1}$ of CDF Run II data, looking for evidence of a contribution of Higgs production from gg fusion and semileptonic decay to WW pairs where final state leptons are electrons and muons. We first measure the cross section times branching ratio for ppbar -> Z0 -> e+ e- and ppbar -> Z0 -> mu+ mu-

Searches for evidence of the contribution of the Higgs to events observed at the Tevatron are intensifying as the data sample is increasing. A previous analysis of dileptons [6] put limits on the cross section at about a factor of 14 higher than the standard model prediction at

GeV/c

. With improvements in NNLO computations [2] this became a factor of 10. Results for the direct search of a Standard Model Higgs presented in summer of 2006 for CDF alone as well as with CDF and D0 combined are shown in Figure 1. The limit on the cross section limit for production of a standard model higgs having

GeV/c

comes to within a factor of 4 of the Standard Model Prediction. CDF improvements and increased data volume for the low mass values of the Higgs have resulted in signficant improvements such that all values are now a within a factor of 20 of the Standard Model cross section in the range $114 < M_{h} < 200$ GeV/c

**Figure 2:** Summer 2006 Electoweak working group fits to the Standard Model Higgs.
$\begin{figure}\vspace*{-0.1cm} \begin{center} \mbox{\epsfig{file=v230/img/s06_blueband.eps,width=0.65\textwidth}} \end{center}\end{figure}$

The latest Electroweak working group fits [4] shown in Figure 2 place the preferred value at $M_h= 85 ^{+39}_{-28}$ GeV/c

, at 68% confidence level. Precision electroweak measurements indicate that that

166 GeV/c

at 95% confidence. Direct searches indicate that

114.4 GeV/c

at 95% confidence [1] [5]. Combining this with the precision electroweak measurements gives a limit of

199 GeV/c

at 95% confidence.

At the Tevatron the dominant SM Higgs boson production mechanism is gluon-gluon fusion, gg $\rightarrow$ H (Figure 3). A second Higgs production mechanism, vector boson fusion, produces the same final state plus two forward, hard jets, and contributes an extra 7% to the WW final state. A third Higgs production mechanism, quark-quark fusion, which produces a Higgs in association with a W boson, is about 2 orders of magnitude smaller. The branching ratios of the Higgs to W pairs dips to 15% at 120GeV/c

and rises to 90% at 160GeV/c

, contributing to the search over the full range from 110 GeV/c

to 200 GeV/

. This is the range considered in this analysis.

**Figure 3:** SM Higgs production versus the mass of the Higgs boson.
$\begin{figure}\vspace*{-0.1cm} \begin{center} \mbox{\epsfig{file=v230/old/HWWxsection.eps,width=0.65\textwidth}} \end{center}\end{figure}$

**Figure 4:** Feynman diagram for the dominant production mechanism and for a secondary contribution to the same final state of both final state W's decaying leptonically.
$\begin{figure}\vspace*{-0.1cm} \begin{center} \begin{tabular}{c} \epsfig{file=... ...ile=v230/img/Vbf, width=0.65\textwidth}\\ \end{tabular}\end{center}\end{figure}$

Table 1: The SM Higgs production cross section and branching ratios of H $\rightarrow$ WW are presented. The NNLO computations [3] are given in the columns $gg\rightarrow H^0$ and Vbf. These are multiplied by the branching ratios /citethwg in the column BR(WW) to obtian the values in the columns $gg\rightarrow H^0 \rightarrow WW$ and Vbf $\rightarrow H^0 \rightarrow WW$ . Since the leptons can be e, $\mu$ or $\tau$ , the values must be multiplied by 9 $\times \rm{BR}(W^-\rightarrow e^- \bar{\nu}_e)= 9\times 0.1054$ . This results in the values in the columns marked gg $\rightarrow H^0 \rightarrow WW \rightarrow llvv$ and Vbf $\rightarrow H^0 \rightarrow WW \rightarrow llvv$ . These columsn are summed to give the final column labelled ``Tot''. This is the number of events produced in 1 $fb^{-1}$ .

	$gg\rightarrow H^0$	Vbf	Br(WW)	$gg\rightarrow H^0$	Vbf $\rightarrow H^0$	gg $\rightarrow H^0$	Vbf $\rightarrow H^0$	Tot
				$\rightarrow WW$	$\rightarrow WW$	$\rightarrow WW$	$\rightarrow WW$
						$\rightarrow llvv$	$\rightarrow llvv$
GeV/c	fb	fb		fb	fb	fb	fb	fb
100	1657	99.5	0.1000	165.7	9.95	17.46	1.05	18.51
110	1281		0.0440	56.36	9.71	5.94	1.02	6.96
120	1006	71.7	0.1320	132.79	9.46	13.99	1	14.99
130	801		0.2869	229.81	17.32	24.21	1.83	26.04
140	646	52.1	0.4833	312.21	25.18	32.9	2.65	35.55
150	525		0.6817	357.89	29.8	37.71	3.14	40.85
160	431	38.2	0.9011	388.37	34.42	40.92	3.63	44.55
170	357		0.9653	344.61	30.43	36.31	3.21	39.52
180	297	28.3	0.9345	277.55	26.45	29.24	2.79	32.03
190	249		0.7761	193.25	20.94	20.36	2.21	22.57
200	211	21	0.7347	155.02	15.43	16.33	1.63	17.96

In this analysis the data sample has been tripled and more sophisticated multidimensional techniques attempt to use the dilepton events more efficiently. The standard model prediction of signal cross-sections and branching ratios, and the expected number of events in 1 $fb^{-1}$ are shown in Table 1, for different Higgs masses for the gluon as well as vector boson fusion processes¹ The detector acceptances needs to be taken into account. The dilepton acceptance is of order 12% at

GeV/c

an
isometric view of the CDF II detector

This analysis is carried out in Fermilab, where the TeVatron collides proton and antiproton beams at center-of-mass energy of 1.96 TeV in Run II.

This analysis is carried out using the data collected from May 02 to September 03 at CDF, one of the two collision points at which a delicate detector is constructed and well-maintained to get data. Shown to the right is an isometric view of the CDF II detector. Counting outwards from the beampipe central line, the detector is comprised of a silicon vertex detector (SVX II), a multiwire drift chamber (COT) for particle tracking, lead-scintillator electromagnetic calorimeters ({C/P}E{M/S}), iron-scintillator hadronic calorimeters ({C/W/P}HA) and drift-tube chambers and scintillators (C{M/S}{U/P/X}) for muon detection. Radiation hazards are dissolved with concrete shielding.

High Pt Leptons Event selection consists first of finding dilepton candidates which consist of

Elimination of b's: A Drell-Yan Sample We then proceed to study the dominant sources of dilepton events; however we do not try to compute and compare the b sources of events so in order to eliminate the b's, we require that the leptons:

The isolation cut for the tracking is the sum of all transvers momenta of all tracks in a cone of 0.4 around the lepton, excluding the lepton track to the p_t of the lepton.

Drell-Yan Removal: A WW sample This yields a sample of dilepton events that are dominated by Drell-Yan production. Drell-Yan production is characterized by the lack of missing energy because they tend to an angle between the leptons of 180 degrees and a balance of energy between the leptons. Hence we require that there be missing energy in the event. Since we are ultimately trying to isolate the contribution of Higgs decays, we are mindful of the Higgs masses and place a series of missing energy cuts on the dilepton samples as we work towards enhancing the contribution of the Higgs to the dileptons. Therefore the missing energy cut is placed at

The remaining events are dominated by WW production and some top. The top contribution is reduced by applying jet cuts:

Higgs Discrimination To enhance the contribution of the Higgs decays to WW, the following cuts are applied to the events.

The angular correlations of the dileptons in the WW final states are different when mediated by the spin-0 Higgs than when produced by other electroweak interactions. We therefore fit the predicted angular distribution for a contribution from the spin-0 and continuum production in order to determine the sensitivity of the analysis to a contribution from Higgs production.