# Possible Impact of the Fourth-Generation Quarks on Production of a Charged Higgs Boson at the LHC

Authors: R. Çiftçi; A. K. Çiftçi; S. Sultansoy
Journal: ISRN High Energy Physics
 (2012)

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## Abstract

We investigate the impact of the fourth-generation quarks on production and decays of the charged Higgs boson at CERN Large Hadron Collider (LHC) with 14 TeV center of mass energy. The signal is the processgg→u4¯u4, followed by u4¯→W−b¯ and u4→h+b decays with subsequent h+→tb̅ and corresponding hermitic conjugates. It is shown that if mu4=400 GeV, then considered process will provide unique opportunity to discover charged Higgs boson with mass range of 200 to 350 GeV at the LHC.


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## Body

## 1. Introduction

It is known that two-Higgs doublet model (2HDM), in general, and minimal supersymmetric extension of the standard model (MSSM), in particular, predict the existence of a charged scalar particle as well as two neutral scalar particles in addition to the standard model (SM) Higgs boson [1]. Experimental observations of these particles could be indirect indication of SUSY. Experiments at LEPII limit the mass of a charged Higgs boson from below as 79.2 GeV [2]. The Tevatron CDF excludes masses of a charged Higgs boson below 105 and 130 GeV for tanβ=1 and tanβ=40, respectively, by searching t→h±b decays [3]. Obviously, higher energy reach of the Large Hadron Collider (LHC) will give opportunity to search charged Higgs boson in wider mass region. The production of the charged Higgs boson at the LHC for three SM generation case is considered in a number of papers [4–9].On the other hand, flavor democracy, which is quite natural in the SM framework, predicts the existence of the fourth-generation (see review [10] and references therein). The masses of the fourth-generation quarks and charged leptons are expected to be almost degenerate with preferable range of m4= 300–500 GeV. Obviously, the fourth-generation quarks in this mass region will be observed at the first few years of the LHC data taking [11–16]. Meanwhile, data collected at Tevatron experiments set limits on mu4 and md4 as 358 GeV and 372 GeV, respectively [17]. Naturally, as the Tevatron searches h± in t-quark decays, the LHC may do the same in u4-quark decays.In this paper, we investigate the impact of the fourth-generation quarks on production and decays of the charged Higgs boson of 2HDM at the LHC with 14 TeV center of mass energy. In Section2, the lagrangian describing decays of the charged Higgs is presented and the branching ratios of decays of the fourth SM generation up quark and charged Higgs boson are evaluated. The production of the charged Higgs boson at the LHC via gluon-gluon fusion process gg→u4̅u4, followed by u4̅→W-b̅ and u4→h+b decays with subsequent h+→tb̅, as well as the SM background, is studied in Section 3. The statistical significance of the charged Higgs boson signal at the LHC is estimated assuming three b-quark jets to be tagged. Finally, concluding remarks are made in Section 4.


## 2. Charged Higgs Boson Decays

Interactions involved charged Higgs boson can be described as below [6]:(2.1)L=g2h+(cot⁡βmuiu¯idiL+tan⁡βmdiu¯idiR+tan⁡βmliν¯iliR)+h.c.,
where i=1,2,3,4 denotes the generation index and tanβ is defined as ratio of the two Higgs doublets vacuum expectation values. Applying the flavor democracy to three-generation MSSM results in tanβ=mt/mb≈40 [10], whereas tanβ=mu4/md4≈1 is preferable in four-generation case. The Cabibbo-Kobayashi-Maskawa (CKM) matrix elements are not shown in (2.1). In numerical calculations, we use CKM mixings given in [18].In order to compute decay widths of the charged Higgs boson, above lagrangian has been implemented into the CompHEP [19]. The decay branching ratios of the fourth-generation up quark with mass of 400 GeV (used at the rest of the paper), which is the mid-point of preferable range of u4 mass mentioned at the Section 1, are plotted in Figure 1(a) for mh±=200 GeV. These plots show that the dominant decay channels of u4 are bh+ and sh+ at low tanβ values; bW and sW decays are dominant at tanβ>1 region.Branching ratios for different decay channels of the (a) fourth-generation up quark and (b) charged Higgs boson as a function of thetanβ in the 2HDM. Following mass values are used: mu4=400 GeV, mh±=200 GeV, and mh0=115 GeV.
(a)(b)Obtained results for branching ratios of decays of the charged Higgs boson into SM fermions are given in Figure1(b) as a function of tanβ. The charged Higgs boson dominantly decays to tb for almost all tanβ values. Furthermore, Figures 2(a) and 2(b) present the branching ratios of the charged Higgs boson decays as a function of its mass for two different values of tanβ, 1 and 40, respectively.Branching ratios for decays of the charged Higgs boson as a function of its mass for different values oftanβ: (a) tanβ=1 and (b) tanβ=40.
(a)(b)


## 3. Charged Higgs Boson Production at the LHC

We study thegg→u4̅u4→W-b̅bh+→W-b̅btb̅→W-b̅bb̅bW+ (and its hermitic conjugate) production process at the LHC, followed by leptonic decay of one W and hadronic decay of the other. The calculated production cross-sections with mu4=400 GeV are plotted in Figure 3 for charged Higgs boson mass values of 200 and 300 GeV. CTEQ6L1 parton distribution functions [20] are used in numerical calculations. The SM background (6 jet + 1 lepton + missing energy) cross-sections are computed using MadGraph package [21]. This background is potentially much larger than the signal. However, in order to extract the charged Higgs boson signal and to suppress the SM background, we impose some kinematic cuts. In addition, we assume that three b-quark jets are tagged.Figure 3
σ×BR of the process gg→u4̅u4→W-b̅bh+→W-b̅btb̅→W-b̅bb̅bW+→b̅bb̅blνq̅q′ for a charged Higgs boson with 200 and 300 GeV masses as a function of tanβ.We choose the following set of selection cuts:PT>80 GeV cut for at least one of b-jets and PT>20 GeV for the rest of the jets and the lepton |η|<2.5, where η denotes pseudorapidity, a minimum separation of ΔR=[(Δϕ)2+(Δη)2]1/2>0.4 (ϕ is the azimuthal angle) between the lepton and the jets as well as each pair of jets. The signal and background cross-sections are given in Figure 4 as a function of the reconstructed tb invariant mass. It is drawn for sample values of the charged Higgs boson masses of 200, 250, 300, and 350 GeV for tanβ=1. Here, we have included a b-tagging efficiency of 50%. The signal and SM background cross-sections are shown separately in Figure 4(a), while their sum is presented in Figure 4(b). The signal peaks are clearly visible at all selected mass values. The similar plots for tanβ=40 are presented in Figures 5(a) and 5(b).The signal cross-section distributions of the reconstructed charged Higgs boson mass for four selected mass values attanβ=1 and corresponding SM background: (a) separated and (b) summed.
(a)(b)The same as Figure4 but for tanβ=40.
(a)(b)The number of events—in a window of 40 GeV around selectedmh± values—for signal (S), and SM background (B), along with the statistical significance (S/B) for 100 fb−1 and 10 fb−1 of integrated luminosity is presented in Tables 1 and 2 for tanβ=1 and tanβ=40, respectively. It is seen that the mass regions mh±= 200–350 GeV for tanβ=1 and mh±= 200–300 GeV for tanβ=40 are covered with more than 5σ even with low integrated luminosity of 10 fb−1. To compare with three SM generation case, for example, we obtain the signal significance 9.6σ with 10 fb−1 for the four-family case at tanβ=40 and mh±=300 GeV, whereas S/B is 6.2 with 100 fb−1 in three SM generation case as given in [7]. The signal significance discussed here assumes perfect detector. More realistic detector future such as the effect of the realistic jet-mass resolutions as well as the method of how to choose the best combination is discussed in [9].Table 1
The number of charged Higgs boson signal and SM background events and the statistical significance of the signal fortanβ=1.
mh±±20 GeVSBS/B100 fb−110 fb−1100 fb−110 fb−1100 fb−110 fb−1200171017110510.5166.952.8250154015413413.4133.042.130083383.3757.596.330.435021521.5474.731.39.9Table 2
The same as Table1 but for tanβ=40.
mh±±20 GeVSBS/B100 fb−110 fb−1100 fb−110 fb−1100 fb−110 fb−120063063.010510.561.519.425059959.913413.451.816.430026326.3757.530.49.6350575.7474.78.22.6


## 4. Conclusion

Our study shows that the existence of the fourth SM generation provides new channel for charged Higgs boson search at the LHC. If the fourth-generation quarks and charged Higgs boson have appropriate masses, then this channel will be a discovery mode. More detailed study including highermu4 mass values, as well as further optimizations of cuts, detector features, and so forth, is ongoing.



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*Source: 341643-2012-01-26.xml*