Advances in High Energy Physics

Volume 2017 (2017), Article ID 1572053, 8 pages

https://doi.org/10.1155/2017/1572053

## Probing the Anomalous FCNC Couplings at Large Hadron Electron Collider

^{1}Department of Energy Systems Engineering, Giresun University, 28200 Giresun, Turkey^{2}European Organization for Nuclear Research (CERN), Geneva, Switzerland^{3}Department of Electric and Electronics Engineering, Giresun University, 28200 Giresun, Turkey^{4}Department of Physics, Abant Izzet Baysal University, 14280 Bolu, Turkey^{5}Department of Physics, Ankara University, Tandogan, 06100 Ankara, Turkey

Correspondence should be addressed to I. Turk Cakir; hc.nrec@rikac.krut.yakli

Received 6 May 2017; Accepted 14 August 2017; Published 14 September 2017

Academic Editor: Jinmian Li

Copyright © 2017 I. Turk Cakir 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 investigate the anomalous flavour changing neutral current (FCNC) interactions of top quark through the process . We calculate the signal and background cross sections in electron-proton collisions at Large Hadron electron Collider (LHeC) with a 7 TeV proton beam from the LHC and a new 60 GeV electron beam from energy recovery linac (ERL). We study the relevant background processes including one electron and three jets in the final state. The distributions of the invariant mass of two jets and an additional jet tagged as -jet are used to account for signal and background events after the analysis cuts. We find upper bounds on anomalous FCNC couplings of the order of at LHeC for a luminosity projection of together with the fast simulation of detector effects. As a matter of interest, we analyze the sensitivity to the couplings , and find an enhanced sensitivity to at the LHeC when compared to the results from the HERA.

#### 1. Introduction

The top quark is the most massive of all observed elementary particles. The top quark interacts with the other quarks of Standard Model (SM) via gauge and Yukawa couplings. It interacts primarily by the strong interaction, but can only decay through the weak interaction. The top quark decays to a boson and either a bottom quark (most frequently), a strange quark, or a down quark (rarely). It is also a unique probe to search for the dynamics of electroweak symmetry breaking. Being the heaviest quark, effects of new physics on its couplings are expected to be larger than that for other fermions; hence it is expected that possible deviations with respect to predictions of the SM might be found with precise measurements of its couplings. It is known that the flavour changing neutral current (FCNC) transitions are absent at tree level and highly suppressed at loop level due to the Glashow-Iliopoulos-Maiani (GIM) mechanism [1]. The branching ratios for the top quark FCNC decays through the process are of the order in the SM [2]. However, various scenarios beyond the standard model (BSM), such as two Higgs doublet model, supersymmetry, and technicolor, predict much larger rates at the order of [3, 4]. Therefore, an observation of the large FCNC induced couplings would indicate the existence of the BSM.

The experimental limits on the branching ratios of the rare top quark decays were established by the experiments at Tevatron and LHC. The CDF experiment set upper bound on branching ratio for top quark FCNC decay as [5]. Presently, the most powerful upper limits on top quark FCNC branching ratios from different channels are and at confidence level (CL) given by the CMS experiment [6].

The direct single top quark production channels through the photon emission at the LHC have been studied to probe the top quark FCNC interactions [7, 8]. Search for flavour changing neutral currents in the single top association with a photon at the LHC has been studied in [9]. In the same sign top quark production at the LHC the top FCNC couplings have been studied in [10]. The top FCNC couplings at future circular hadron electron colliders have been studied in [11].

The main physics goals of future high energy particle colliders are to examine the fundamental interactions within the SM and to search for possible effects of new physics beyond the SM. The electron-proton colliders are based on a ring type proton accelerator with an intersecting electron beam accelerator. Currently, a projected future collider, namely, Large Hadron electron Collider (LHeC), has been discussed [12–14]. It comprises a 60 GeV electron beam that will collide with the 7 TeV proton beam, having an integrated luminosity of per year, and it is planned to collect over the years. Recently, the new physics capability and potential of colliders through di-Higgs boson production have been studied in [15].

When we study anomalous top quark FCNC interactions at the future colliders, we take also into account bounds from measured low energy processes where the loops include top quarks. For example, the FCNC transition processes are considered to be the valuable probe of top quark anomalous couplings [16, 17]. The bounds [18] on the top quark real FCNC couplings are lower than the current direct limit but still accessible at the high luminosity run of LHC.

In this work, we study the physics potential of LHeC collider two parameters for probing top quark FCNC couplings through single top production. We focus on two parameters analysis of the signal and background including Delphes detector simulation. We discuss the sensitivity and bounds for anomalous top quark couplings (, ) for different luminosity projections.

#### 2. Calculation Framework

The top quark FCNC interactions would be a good test of new physics at present and future colliders. The new physics effects can be described by a set of higher order effective operators in a model independent framework. The anomalous FCNC interactions of top quark with up-type quarks (, ) and a photon can be described in a model independent effective Lagrangianwhere is the electromagnetic coupling constant, are the strength of anomalous FCNC couplings and the values of these vanish at the lowest order in the SM, and denotes the left (right) handed projection operators. The photon field strength tensor is and anomalous term is related to . The effective Lagrangian is used to calculate both production cross section and the decay widths for the channels. At the colliders anomalous single top quark production can be achieved through the scattering of incoming electron beam with the quark current of the proton via the production channel (-channel) as shown in diagrams, Figure 1. For the event generation and cross section calculations, we have used event generator MadGraph5_aMC@NLO [19] with built in parton distribution function NNPDF23 [20]. The effective interactions described by (1) have been implemented through FeynRules [21] and the model file is used within MadGraph5_aMC@NLO. This event generator provides the tools to perform the simulation of both signal and backgrounds within the same framework. In this study, an allowed range of values for the FCNC parameters is taken into account.