Searches for the Anomalous FCNC Top-Higgs Couplings with Polarized Electron Beam at the LHeC

Searches for the Anomalous FCNC Top-Higgs Couplings with Polarized Electron Beam at the LHeC XiaoJuan Wang, Hao Sun, and Xuan Luo Institute of Theoretical Physics, School of Physics & Optoelectronic Technology, Dalian University of Technology, No. 2 Linggong Road, Dalian, Liaoning 116024, China Correspondence should be addressed to Hao Sun; nc.ude.tuld@nusoah Received 29 September 2016; Revised 2 December 2016; Accepted 4 January 2017; Published 16 March 2017 Academic Editor: Burak Bilki Copyright © 2017 XiaoJuan Wang 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 SCOAP3. Abstract We study the single top and Higgs associated production in the top-Higgs FCNC couplings at the LHeC with the electron beam energy of  GeV and  GeV and combination of a 7 TeV and 50 TeV proton beam. With the possibility of -beam polarization (, ), we distinct the cut-based method and the multivariate analysis- (MVA-) based method and compare with the current experimental and theoretical limits. It is shown that the branching ratio can be probed to 0.113 (0.093%), 0.071 (0.057%), 0.030 (0.022%), and 0.024 (0.019%) with the cut-based (MVA-based) analysis at (, ) = (7 TeV, 60 GeV), (, ) = (7 TeV, 120 GeV), (, ) = (50 TeV, 60 GeV), and (, ) = (50 TeV, 120 GeV) beam energy and 1 level. With the possibility of -beam polarization, the expected limits can be probed down to 0.090 (0.073%), 0.056 (0.045%), 0.024 (0.018%), and 0.019 (0.015%), respectively. 1. Introduction The Large Hadron Electron Collider (LHeC) is the second electron-hadron collider following HERA [1]. With remarkable higher energy and luminosity, the LHeC is a major step towards understanding the Higgs physics and QCD. For the LHeC colliding energy, the 7 TeV proton beam at the LHC as well as the 50 TeV proton beam at the future FCC-he [2] and a new 60 GeV electron beam [1] are envisaged. To probe new physics, the anomalous flavor changing neutral current (FCNC) Yukawa interactions, between the top-Higgs and either an up or charm quark, would provide a clear signal. The SM Lagrangian can be extended by the following terms:where the real parameters and denote the FCNC couplings of the Higgs to up-type quarks. The total decay width of the top quark is where the decay width and can be found in [3, 4], respectively. Thus, the branching ratio for can be approximately given by where is the Fermi constant and . The boson and top quark masses are chosen to be and , respectively. Up to now, the investigation of anomalous couplings has been experimented by many groups, which gives the strong limits on the top-Higgs FCNC couplings. For instance, according to the ATLAS and CMS collaborations, the upper limits of % [5, 6] and % [7] have been set at 95% confidence level (CL). At one-loop level, the mixing observable can receive sizeable contributions with such an unvanishing flavor violating tqH coupling [8]. Using data observed on , the upper limit of can be achieved. Furthermore, through decay and electroweak observables, the upper limit of % [9] can be obtained. On the other hand, based on the experimental data, many phenomenological studies are performed from different channels. For instance, [10] found that the branching ratios can be probed to 0.24% at 3 level at 14 TeV LHC with integrated luminosity of 3000  through the process . Reference [11] explored the top-Higgs FCNC couplings through and found that the branching ratios can be probed to 0.23% at 3 sensitivity at 14 TeV LHC with  . And [12] obtained to be 0.112% based on the process of . The process of has been studied in [13] and the authors therein estimated the upper limits of % at 100  of 13 TeV data for multilepton searches. The results from different experiments and theoretical channels are summarized in Table 1. Table 1: The results from different experimental and phenomenological channels. In this study, we examined at the LHeC, where the Higgs boson decays to at 7 (50) TeV with 60 (120) GeV electron beam and 1000  integrated luminosity. The possibility of -beam polarization is also considered. The Feynman diagram is plotted in Figure 1. The main backgrounds which yield the same or similar final states to the signal are listed as follows: where , and if possible. Notice that is the neutral current multijet QCD background, and all the others belong to charged current (CC) productions. For the single top background , the produced top quark will decay to a boson and a -jet. The boson continues to decay to non--jet final states, which might be mistagged as a -jet. With the same final states, and are the irreducible backgrounds corresponding to associated Higgs jet and jet which contain three QED couplings. is the CC multijet QCD background. Similar to the sin (...truncated)