Advances in High Energy Physics

Volume 2016, Article ID 1760219, 12 pages

http://dx.doi.org/10.1155/2016/1760219

## Effects of the FCNC Couplings in Production of New Heavy Quarks within Models at the LHC

^{1}Department of Physics, Dumlupinar University, Merkez, 43100 Kutahya, Turkey^{2}Department of Physics, Ankara University, Tandogan, 06100 Ankara, Turkey

Received 30 December 2015; Revised 20 March 2016; Accepted 3 May 2016

Academic Editor: Michal Kreps

Copyright © 2016 V. Çetinkaya 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 flavor changing neutral current couplings of new heavy quarks through the models at the LHC. We calculate the cross sections for the signal and the corresponding standard model background processes. Considering the present limits on the mass of new heavy quarks and the boson, we performed an analysis to investigate the parameter space (mixing and mass) through different models. For FCNC mixing parameter and the mass GeV and new heavy quark mass GeV at the LHC with TeV, we find the cross section for single production of new heavy quarks associated with top quarks as fb, fb, fb, and fb within the , , , and models, respectively. It is shown that the sensitivity would benefit from the flavor tagging.

#### 1. Introduction

Addition of new heavy quarks would require the extension of the flavor mixing in charged current interactions as well as the extension of Higgs sector in the standard model (SM). A large number of new heavy quark pairs can be produced through their color charges at the Large Hadron Collider (LHC). However, due to the expected smallness of the mixing between the new heavy quarks and known quarks through charged current interactions, the production and decay modes can be affected by the flavor changing neutral current (FCNC) interactions. A new symmetry beyond the SM is expected to explain the smallness of the mixing. We may anticipate the new physics discovery by observing large anomalous couplings in the heavy quark sector. The couplings of the new heavy quarks can be enhanced to observable levels within some new physics models. In numerous phenomenological studies (see [1] and references therein), a lot of extensions of the SM foresee the extra gauge bosons, the boson in particular. Flavor changing neutral currents can be induced by an extra gauge boson . The boson in the models using an extra group can have tree-level or an effective (where and both can be the up-type quarks or down-type quarks) couplings. The , , and models corresponding to the specific values of the mixing angle in the model with different couplings to the fermions and the leptophobic model with the couplings to quarks but no couplings to leptons are among the special names of the models [2].

The ATLAS and CMS collaborations have performed extensive searches of new vector resonances at the LHC. We summarize briefly these searches that exploited data from the run at TeV and TeV, as well as the corresponding constraints on boson masses. The most stringent limits come from searches with leptonic final states (): GeV [3] and GeV [4], GeV [5] (more recently GeV [3]) for the boson predicted by the extensions, also extending to the mass limit of GeV [3, 6] for a gauge boson with sequential couplings. The results from ATLAS experiment exclude a leptophobic decaying to with a mass less than GeV at CL [7], while the CMS experiment excludes a top-color decaying to with a mass less than GeV at CL [8]. These searches assume rather narrow width for the boson (). From the electroweak precision data analysis, the improved lower limits on the mass are given in the range 1100–1500 GeV, which gives a limit on the – mixing about [2]. The limits on the boson mass favors higher center of mass energy collisions for direct observation of the signal. Using dilepton searches with LHC data, the dark matter constraints have been analyzed in [9, 10] in the regime .

A work performed in [11, 12] presents the effects of FCNC interactions induced by an additional boson on the single top quark and top quark pair production at the LHC ( TeV). The relevant signal cross sections have been calculated and particularly the benefit from flavor tagging to identify the signal has been discussed. Considering an existence of sizeable couplings to the new heavy quarks, the boson decay width and branchings, as well as the production rates, can be quite different from the expectations of usual search scenarios.

In the models of interest new heavy quarks can have some mixing with the SM quarks. For example, in composite Higgs model [13] the lightest new heavy quark couples predominantly to the heavier SM quarks (top and bottom quarks). In the models of vector-like quarks (VLQ) [14] they are expected to couple preferentially to third-generation quarks and they can have flavor changing neutral current couplings, in addition to the charged current decays characteristic of chiral quarks. Within the model the isosinglet quarks [15] are predicted and they can decay to the quarks of the SM. The new heavy quarks can be produced dominantly in pairs through strong interactions for masses around 1 TeV in the collisions of the LHC with a center of mass energy of TeV. The single production of new heavy quarks would only be dominant over pair production for the large quark masses [16], it is model dependent, and it could be suppressed if the mixing with SM quarks is small.

There are searches for pair production and single production of new heavy quarks at the LHC. The ATLAS and CMS collaborations focused on decay modes of new heavy quarks into a massive vector boson and a third-generation quark assuming a branching ratio, based on of collision data at TeV, and set lower mass limit for up-type new heavy quark as GeV [17] and GeV [18].

In this work, we investigate the single production of new heavy quarks via FCNC interactions through boson exchange at the LHC. This paper aims at studying the signal and background in detail within the same MC framework, and the relevant interaction vertices are implemented into the MC software. Analyzing the signal observability (via contour plots) for different mass values of the boson and new heavy quarks as well as the mixing parameter through FCNC interactions are another feature of the work. In Section 2, we calculate the decay widths and branching ratios of boson for the mass range 1500–3000 GeV in the framework of different models. An analysis of the parameter space of mass and coupling strength is given for the single production of new heavy quarks at the LHC in Section 3. We analyzed the signal observability for the FCNC interactions. We consider both and single new heavy quark productions for the purpose of enriching the signal statistics even at the small couplings. For the quark decay we consider mode within the interested parameter space. The analysis for the signal significance is given in Section 4 and the work ends up with the conclusions as given in Section 5.

#### 2. FCNC Interactions

In the gauge eigenstate basis, we can write the additional neutral current Lagrangian related to the gauge symmetry by following the formalism given in [11, 19, 20] where are the chiral couplings of boson with fermions and . is the gauge coupling of and Here, it is presumed that there is no mixing between the and bosons as favored by the precision data. If the chiral couplings are nondiagonal matrices flavor changing neutral currents (FCNCs) will arise. FCNC couplings come out by fermion mixing if the couplings are diagonal but nonuniversal. Here, we assume that all FCNCs are in the left-hand sector and only for up-type quarks, and the specific form of the mixing matrix is used. Our assumptions can be considered within a model framework (e.g., a particular class of string models [21, 22]) and an ad hoc illustration of a constrained formalism. In the interaction basis the FCNC for the up-type quarks can be given by where the chiral couplings can be written as

The effects of these FCNCs may always arise in the sectors, both up-type and down-type ones after diagonalizing their mass matrices. For simplicity, we suppose that the neutral current couplings to for the right-handed up-sector and down-sector are family universal and flavor diagonal in the interaction basis. In this case, unitary rotations () can maintain the right-handed couplings flavor diagonal, and left-handed sector becomes nondiagonal. The chiral couplings of in the fermion mass eigenstate basis are given by Here, the matrix can be written as ; due to our supposition that the down-sector has no mixing it becomes . The flavor mixing in the left-handed quark fields is simply relevant to this . One can find the couplings with the parametrization (where the generation index runs from 1 to 3) for the matrix by assuming the up-sector diagonalization and using unitarity of the CKM matrix.

The FCNC effects from the exchange have been studied for the down-type sector and implications in flavor physics through -meson decays [23–28] and -meson mixing [20, 29–33]. These effects have also been studied for up-type quark sector in top quark production [11, 34–38]. The parameters for different models are listed in Table 1. In numerical calculations, the coupling is taken as for the models.