A search of Charged Higgs

A search of charged Higgs in the decay products of pair-produced top quarks.

Introduction:

Two Higgs doublet extensions of the Standard Model predict the existence of charged Higgs bosons ( H^{+/-}). In such models the branching fraction for top quarks B(t->H^{+}b) can be large thus competing with the SM decay t->W^{+}b.

This study presents the results of a search of charged Higgs in the decay products of the pair-produced top quarks. This search is based on the cross section measurements of ttbar production in four exclusive decay channels; the "dilepton" , "lepton + jets" with exaclty one tag, "lepton + jets" with two or more tags and "lepton + hadronic tau" channel.

Assuming the charged Higgs decays to csbar, taunu, t*b and Wh0, limits in the t->Hb branching ratio are obtained. The results are shown in the context of the MSSM.

Authors : Ricardo Eusebi, Andy Hocker , Paul Tipton.
University of Rochester
FOR THE CDF COLLABORATION

Contact person : Ricardo Eusebi

Link to public note

Go to General Methodology (in this page)

MSSM Methodology
Taounic Higgs Methodology
Worst BR combination Methodology

Go to Results

MSSM results in the (mH,tan(b)) plane

Results assuming H-->tn only results

Worst BR combination result

General Methodology

This analysis takes advantage of the Dilepton , Lepton+Jets with exactly one tag, Lepton+Jets with two or more tags and the Lepton+Hadronically decaying tau.
If the top is allowed to decay via t->H+b, in addition to the SM decay t->Wb, the number of candidates in each of the aforementioned cross section measurements changes depending to what the charged Higgs decays to.
The charged Higgs may decay on a variety of channels, including H+->{t*b,csbar, taunu, Wh0} with very different final states. This results in 15 different decay channels of the ttbar pair.

Based on MC simulation we compute, for a certain mass of the charged higgs, the efficiency for each of the decay channels in each of the cross section measurements.
If the mass and the BR's of the charged Higgs were known we can predict the expected number of events for each of the 4 cross section, just by computing the total MC efficiency based on the BR's and the effiencies for that charged Higgs mass of each decay channel.

We show three types of results, each one obtained assuming different things for the charged Higgs BR's.

MSSM Methodology:

The Minimal Supersymmetric Standard Model (MSSM) predicts, for a specifc set of more than 15 input parameters, what the BR's of the Higgs to each of the decays is. In particular, the ratio of Vacuum expectation values of the two doublets, defined as tan(b), is one important parameter. For a certain mass of the charged Higgs and tan(b) (and a specific set of other MSSM parameters) we can predict these branching ratios. The following plot shows, for a specific set of MSSM parameters named Bench#1, the behavior of the BR's as a function of tan(b) for different values of the charged Higgs mass. This was obtained using the program CPsuperH.

( eps version here )	(eps version here)
( eps version here )	(eps version here)

(Another plot you may take a look at is here, where the behavior of the masses is also shown.)

With the BR at hand we can calculate the total number of expected events in each cross section.
The following plot shows (eps version here)what the number of expected events is (for Bench#1) for a mass of the charged Higgs of 120 GeV and as a function of tan(b).

Expected Events as a funciton of tan(b)

The right plot shows what the posterior probability density looks like if we assume that the number of obtained candidates in each cross section is 11 (Dilep) ,54 (LJets 1 tag) ,2 (Lepton+TauHad) and 10 (Lepton+Jets 2 or more tags), which are the numbers predicted using a ttbar production cross section of 6.7+/-0.7 and assuming the top decays only to Wb. The posterior was obtained using a flat prior in log10(tan(b)).
The plot also show the limits obtained in tan(b) for the charged Higgs mass considered, in this case 120 GeV.

By introducing the real number of candidates and repeting for different masses of the charged Higgs we obtain the result plots shown in "MSSS results in the (mH,tan(b)) plane" below.

See the MSSM results in the (mH,tan(b)) plane here

Tauonic Higgs Model :

As shown in the MSSM section above, the charged Higgs decays dominantly to tau nu in the high tan(b) region . This region is also favored by current theoretical arguments.
In this Tauonic Higgs section we assume that the Higgs Br are such that the charged Higgs decay to tau nu 100% of the time, and all its other decays are thus set to zero. In this scenario we evaluate, for a certain charged Higgs mass, the posterior probability density as a function of BR(t->Hb), and obtain the 95%CL. We use a flat prior in BR(t->Hb) and between 0 and 1. See the results in "Results assuming H-->tn only" section.

See the results assuming H-->tn only here

Worst BR combination :

The tauonic Higgs Model is just a specific combination of BR's. In this section we scan over all the possible BR's ( we need to bin the BR phase space) and for each repeat the procedure done in the Tauonic Higgs Model (i.e. get the posterior in BR(t-->Hb) and obtain a 95 % CL). The distribution of limits for a mass of 120 GeV is shown below. ( eps file here)
Limits mH120

The red line indicates the worst limit obtained from all the combination s considered. The green line indicates that we can never do better than that value, no matter what combination of BR is chosen.
We repeat this for all charged Higgs masses quoting the worst limit in each , and obtain the results showed in "Worst BR combination result". This limits are therefore model independent.

See the worst BR combination results here

Last updated : June 6, 2005

Ricardo Eusebi (eusebi@fnal.gov)