Advances in High Energy Physics

Volume 2016, Article ID 2613187, 9 pages

http://dx.doi.org/10.1155/2016/2613187

## Single Top and Higgs Associated Production in the Minimal Model at the LHC

^{1}College of Physics & Electronic Engineering, Henan Normal University, Xinxiang 453007, China^{2}School of Physics and Electromechanical Engineering, Zhoukou Normal University, Henan 466001, China^{3}Basic Teaching Department, Jiaozuo University, Jiaozuo 454000, China

Received 17 April 2016; Accepted 31 July 2016

Academic Editor: Juan José Sanz-Cillero

Copyright © 2016 Bingfang Yang 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. The publication of this article was funded by SCOAP^{3}.

#### Abstract

We study the single top production in association with a Higgs boson in the extension of the Standard Model at the LHC. We calculate the production cross sections of the processes in this model. We further study the observability of the process through and find that it is still challenging for the 14 TeV LHC with high luminosity to detect this signal.

#### 1. Introduction

In July 2012, a Higgs-like resonance with mass has been discovered by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) [1, 2]. So far, all the measurements of the discovered new particle [3–10] are well compatible with the scalar boson predicted by the Standard Model (SM) [11–15].

It is well known that the SM cannot be the final theory of nature. Theoretically, successful explanation of some problems, such as the hierarchy problem, requires new physics beyond the SM near the TeV scale. Experimentally, the solid evidence for neutrino oscillation is one of the firm hints for new physics. The minimal extension of the SM that we consider in this paper is that the SM gauge groups are augmented by a symmetry, where and represent the baryon number and lepton number, respectively. The gauge symmetry can explain the presence of three right-handed neutrinos and provide a natural framework for the seesaw mechanism [16, 17]. In addition, it is worth noting that symmetry breaking can take place at the TeV scale, hence giving rise to new and interesting TeV scale phenomenology.

The Yukawa couplings play an important role in probing the new physics since they are sensitive to new flavor dynamics. The top quark is the heaviest particle discovered and owns the strongest Yukawa coupling. The top quark Yukawa coupling is speculated to be sensitive to the electroweak symmetry breaking (EWSB) mechanism and new physics. The production process is a golden channel for directly probing the top Yukawa coupling; however, this process cannot provide the information on the relative sign between the coupling of the Higgs to fermions and to vector bosons. As a beneficial supplement, the production process can bring a unique possibility [18–21] and many relevant works have been carried out [22–34].

The model predicts heavy neutrinos, a TeV scale extra neutral gauge boson, and an additional heavy neutral Higgs, which makes the model phenomenologically rich. The heavy Higgs state mixes with the SM Higgs boson so that some Higgs couplings are modified and this effect can also influence the process of single top and Higgs associated production. Besides, the process of single top and heavy Higgs associated production deserves attention, which is equally important for understanding the EWSB and probing new physics. Performing the detailed analysis on this process may provide a good opportunity to probe the model signal.

The paper is structured as follows. In Section 2 we review the model related to our work. In Section 3 we first calculate the production cross sections of the single top and associated production at the LHC and then explore the observability of -channel process through by performing a parton-level simulation. Finally, we make a summary in Section 4.

#### 2. A Brief Review of the Model

The minimal extension of the SM [35–42] is based on the gauge group with the classical conformal symmetry. Under this gauge symmetry, the invariance of the Lagrangian implies the existence of a new gauge boson. In order to make the model free from all the gauge and gravitational anomalies, three generations of right-handed neutrinos are necessarily introduced.

The Lagrangian for Yang-Mills and fermionic sectors is given by where , , and are, respectively, the and gauge fields, and the fields’ charges are the usual SM and ones. The non-Abelian field strengths not included here are the same as in the SM. In this field basis, the covariant derivative is

In this model, the most general gauge-invariant and renormalizable scalar Lagrangian can be expressed as with the scalar potential given by To determine the condition for the potential to be bounded from below, the couplings , , and should be related as We denote the vacuum expectation values (VEVs) of and by and , respectively, and the nonzero minimums are given by where and are the EWSB scale and the symmetry breaking scale, respectively.

From the mass terms in the scalar potential, the mass matrix between the two Higgs bosons in the basis can be given by The mass eigenstates are related via the mixing matrix where the mixing angle () satisfies The masses of the physical Higgs bosons and are given by where and are light SM-like and heavy Higgs bosons, respectively.

To complete the discussion on the Lagrangian, we write down the Yukawa term, which in addition to the SM terms has interactions involving the right-handed neutrinos : where and run within 1~3. The VEV of the field breaks the symmetry and generates the Majorana masses for the right-handed neutrinos and the Dirac masses for the light neutrinos.

In terms of the mixing angle , the couplings of and with the fermions and gauge bosons can be expressed as follows: where denotes the SM fermions and with is the usual Weinberg angle.

#### 3. Numerical Results and Discussions

For the single top and Higgs associated production, the three processes of interest are characterized by the virtuality of the boson in the process [43]: (i) -channel, where the is spacelike; (ii) -channel, where the is timelike; (iii) -associated production channel, where there is emission of a real boson. In the model, the lowest-order Feynman diagrams of the -channel process are shown in Figure 1, the -channel process is shown in Figure 2 and the -associated production channel process is shown in Figure 3. We can see that the Feynman diagrams for these processes are the same as the corresponding SM processes. Moreover, the conjugate processes where is replaced by have been included in our calculations.