- About this Journal ·
- Abstracting and Indexing ·
- Aims and Scope ·
- Article Processing Charges ·
- Articles in Press ·
- Author Guidelines ·
- Bibliographic Information ·
- Citations to this Journal ·
- Contact Information ·
- Editorial Board ·
- Editorial Workflow ·
- Free eTOC Alerts ·
- Publication Ethics ·
- Reviewers Acknowledgment ·
- Submit a Manuscript ·
- Subscription Information ·
- Table of Contents

ISRN High Energy Physics

VolumeΒ 2012Β (2012), Article IDΒ 341643, 8 pages

http://dx.doi.org/10.5402/2012/341643

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

^{1}Department of Engineering of Physics, Faculty of Engineering, Ankara University, Tandogan, 06100 Ankara, Turkey^{2}Physics Department, Faculty of Sciences, Ankara University, Tandogan, 06100 Ankara, Turkey^{3}Physics Section, Faculty of Sciences and Arts, TOBB Economics and Technology University, 06560 Ankara, Turkey^{4}Institute of Physics, Azerbaijan National Academy of Sciences, H. Javid pr., 33, Baku, AZ 1143, Azerbaijan

Received 26 August 2011; Accepted 9 October 2011

Academic Editors: K.Β Cho and H.Β Hayashii

Copyright Β© 2012 R. ΓiftΓ§i et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 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 14βTeV center of mass energy. The signal is the process , followed by and decays with subsequent and corresponding hermitic conjugates. It is shown that if βGeV, then considered process will provide unique opportunity to discover charged Higgs boson with mass range of 200 to 350βGeV at the LHC.

#### 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 79.2βGeV [2]. The Tevatron CDF excludes masses of a charged Higgs boson below 105 and 130βGeV for and , respectively, by searching 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 [4β9].

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 300β500βGeV. Obviously, the fourth-generation quarks in this mass region will be observed at the first few years of the LHC data taking [11β16]. Meanwhile, data collected at Tevatron experiments set limits on and as 358βGeV and 372βGeV, respectively [17]. Naturally, as the Tevatron searches in -quark decays, the LHC may do the same in -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 14βTeV center of mass energy. In Section 2, 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 , followed by and decays with subsequent , 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 -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]: where denotes the generation index and is defined as ratio of the two Higgs doublets vacuum expectation values. Applying the flavor democracy to three-generation MSSM results in [10], whereas 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 400βGeV (used at the rest of the paper), which is the mid-point of preferable range of mass mentioned at the Section 1, are plotted in Figure 1(a) for βGeV. These plots show that the dominant decay channels of are and at low values; and decays are dominant at region.

Obtained results for branching ratios of decays of the charged Higgs boson into SM fermions are given in Figure 1(b) as a function of . The charged Higgs boson dominantly decays to for almost all 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 , 1 and 40, respectively.

#### 3. Charged Higgs Boson Production at the LHC

We study the (and its hermitic conjugate) production process at the LHC, followed by leptonic decay of one and hadronic decay of the other. The calculated production cross-sections with βGeV are plotted in Figure 3 for charged Higgs boson mass values of 200 and 300βGeV. CTEQ6L1 parton distribution functions [20] are used in numerical calculations. The SM background (6 jet + 1βlepton + 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 -quark jets are tagged.

We choose the following set of selection cuts: βGeV cut for at least one of -jets and βGeV for the rest of the jets and the lepton , where denotes pseudorapidity, a minimum separation of ( 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 invariant mass. It is drawn for sample values of the charged Higgs boson masses of 200, 250, 300, and 350βGeV for . Here, we have included a -tagging efficiency of . 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 are presented in Figures 5(a) and 5(b).

The number of eventsβin a window of 40βGeV around selected valuesβfor signal (), and SM background (), along with the statistical significance () for 100βfb^{β1} and 10 fb^{β1} of integrated luminosity is presented in Tables 1 and 2 for and , respectively. It is seen that the mass regions 200β350βGeV for and 200β300βGeV for are covered with more than even with low integrated luminosity of 10 fb^{β1}. To compare with three SM generation case, for example, we obtain the signal significance with 10βfb^{β1} for the four-family case at and βGeV, whereas is 6.2 with 100 fb^{β1} 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].

#### 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 higher mass values, as well as further optimizations of cuts, detector features, and so forth, is ongoing.

#### Acknowledgments

R. ΓiftΓ§i would like to acknowledge for support from the Scientific and Technical Research Council (TUBITAK) BIDEB-2218 Grant. This work was also supported in part by the State Planning Organization (DPT) under Grant no. DPT-2006Kβ120470 and in part by the Turkish Atomic Energy Authority (TAEA) under Grant no. VII-B.04.DPT.1.05.

#### References

- J. F. Gunion, H. E. Haber, G. L. Kane, and S. Dawson,
*The Higgs Huntersβ Guide*, Addison-Wesley, Reading, Mass, USA, 1999. - W.-M. Yao et al., βReview of particle physics,β
*Journal of Physics G*, vol. 33, article 001, no. 1, pp. 1β1232, 2006. View at Publisher Β· View at Google Scholar - A. Abulencia, D. Acosta, J. Adelman et al., βSearch for charged Higgs bosons from top quark decays in $p\stackrel{{\rm B}\u2015}{p}$ collisions at $\sqrt{s}=1.96$?TeV,β
*Physical Review Letters*, vol. 96, article 042003, no. 4, 2006. View at Publisher Β· View at Google Scholar - V. Barger, R. J. N. Phillips, and D. P. Roy, βHeavy charged Higgs signals at the LHC,β
*Physics Letters, Section B*, vol. 324, no. 2, pp. 236β240, 1994. View at Google Scholar Β· View at Scopus - J. F. Gunion, βDetecting the tb decays of a charged Higgs boson at a hadron supercollider,β
*Physics Letters, Section B*, vol. 322, no. 1-2, pp. 125β130, 1994. View at Google Scholar Β· View at Scopus - D. P. Roy, βThe hadronic tau decay signature of a heavy charged Higgs boson at LHC,β
*Physics Letters, Section B*, vol. 459, no. 4, pp. 607β614, 1999. View at Google Scholar - S. Moretti and D. P. Roy, βDetecting heavy charged Higgs bosons at the LHC with triple b-tagging,β
*Physics Letters, Section B*, vol. 470, no. 1β4, pp. 209β214, 1999. View at Publisher Β· View at Google Scholar Β· View at Scopus - D. J. Miller, S. Moretti, D. P. Roy, and W. J. Stirling, βDetecting heavy charged Higgs bosons at the CERN LHC with four b-quark tags,β
*Physical Review D*, vol. 61, no. 5, Article ID 055011, pp. 1β13, 2000. View at Google Scholar Β· View at Scopus - K. A. Assamagan and N. Gollub, βThe ATLAS discovery potential for a heavy charged Higgs boson in $gg\beta \x86\x92tb{H}^{{\rm B}\pm}$ with ${H}^{{\rm B}\pm}\beta \x86\x92tb$,β
*European Physical Journal C*, vol. 39, supplement 2, pp. s25βs40, 2005. View at Publisher Β· View at Google Scholar - S. Sultansoy, βFlavor democracy in particle physics,β in
*Proceedings of the 6th International Conference of the Balkan Physical Union*, vol. 899, pp. 49β52, August 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus - E. Arik, S. AtaΔ, Z. Z. Aydin et al., βSearch for the fourth family up quarks at CERN LHC,β
*Physical Review D*, vol. 58, no. 11, Article ID 117701, 14 pages, 1998. View at Google Scholar - ATLAS: Detector and Physics Performance Technical Design Report, CERN/LHCC/99-14/15, section 18.2., 1999.
- B. Holdom, βThe discovery of the fourth family at the LHC: what if?β
*Journal of High Energy Physics*, vol. 2006, article 076, no. 8, 2006. View at Publisher Β· View at Google Scholar - B. Holdom, βT' at the LHC: the physics of discovery,β
*Journal of High Energy Physics*, vol. 2007, article 063, no. 3, 2007. View at Publisher Β· View at Google Scholar - B. Holdom, βThe heavy quark search at the LHC,β
*Journal of High Energy Physics*, vol. 2007, article 069, no. 8, 2007. View at Publisher Β· View at Google Scholar - V. E. Ozcan, S. Sultansoy, and G. Unel, βSearch for 4th family quarks with the ATLAS detector,β
*The European Physical Journal C*, vol. 57, pp. 621β626, 2008. View at Google Scholar - A. Ivanov for CDF and D0 Collaborations, βSearches for fourth generation fermions,β FERMILAB-CONF-11-453-E, presented at 8th Conference on Flavor Physics and CP Violation: FPCP 2011, Maale Hachamisha, Israel, 23β27 May 2011, http://arxiv.org/abs/1109.1025.
- A. K. Ciftci, R. Ciftci, and S. Sultansoy, βFourth standard model family neutrino at future linear colliders,β
*Physical Review D*, vol. 72, no. 5, pp. 1β8, 2005. View at Publisher Β· View at Google Scholar - E. Boos, V. Bunichev, M. Dubinin et al., βCompHEP 4.4βautomatic computations from lagrangians to events,β
*Nuclear Instruments and Methods in Physics Research, Section A*, vol. 534, no. 1-2, pp. 250β259, 2004. View at Publisher Β· View at Google Scholar Β· View at Scopus - J. Pumplin, D. R. Stump, J. Huston, H. -L. Lai, P. Nadolsky, and W. -K. Tung, βNew generation of parton distributions with uncertainties from global QCD analysis,β
*Journal of High Energy Physics*, vol. 6, no. 7, pp. 325β371, 2002. View at Google Scholar - T. Stelzer and W. F. Long, βAutomatic generation of tree level helicity amplitudes,β
*Computer Physics Communications*, vol. 81, no. 3, pp. 357β371, 1994. View at Google Scholar Β· View at Scopus