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Advances in High Energy Physics
Volume 2017 (2017), Article ID 4693213, 9 pages
Searches for the Anomalous FCNC Top-Higgs Couplings with Polarized Electron Beam at the LHeC
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
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.
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.
The Large Hadron Electron Collider (LHeC) is the second electron-hadron collider following HERA . 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  and a new 60 GeV electron beam  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 %  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 . Using data observed on , the upper limit of can be achieved. Furthermore, through decay and electroweak observables, the upper limit of %  can be obtained.
On the other hand, based on the experimental data, many phenomenological studies are performed from different channels. For instance,  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  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  obtained to be 0.112% based on the process of . The process of has been studied in  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.
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 single top background, misidentification of one or more of the final state light jets to -jet makes this process a reducible background.
2. Tools and Method
During the simulation, we first extract the Feynman Rules by using the FeynRules package  and generate the event with MadGraph@NLO . PYTHIA6.4  was set to solve the initial and final state parton shower, hadronization, heavy hadron decays, and so forth. We use CTEQ6L [17, 18] as the parton distribution function and set the renormalization and factorization scale to be . We take the input heavy particle masses as , , , and , respectively. We employ the following basic preselections cuts to select the events: where is the separation with and in the rapidity-azimuth plane and and are the transverse momentum and the pseudorapidity of jets, -jets, and leptons, while is the missing transverse momentum. Then we adopt a cut-based method and a multivariate analysis- (MVA-) based method for signal and background analysis, respectively.
2.1. Cut-Based Method
In order to distinguish between signal-related events and background-related events as much as possible, we set a series of cuts. We list all the cut-based selections here:(i)Cut 1: the basic preselection cuts(ii)Cut 2: the selection (iii)Cut 3: missing transverse energy (iv)Cut 4: the reconstructed top quark mass window (v)Cut 5: the reconstructed boson mass window (vi)Cut 6: the reconstructed boson mass window (vii)Cut 7: the reconstructed Higgs mass window
2.2. MVA-Based Method
We implemented the MVA method using the Root Toolkit for Multivariate Analysis (TMVA) . After cut 1, cut 2, and cut 3, we especially select several input variables to discriminate the signal and background events, thus resulting in better signal significance. Specifically, we define a set of totally 44 kinematic variables and choose the most effective ones for Boosted Decision Trees (BDT) training, which are the -jet number (), the separation in the plane between jets (, ), the difference in azimuthal angle between jets (, ), the transverse momentum of the jet (), and the difference in within Higgs jet system (). It is worth noting that -beam polarization is considered in both cut-based method and MVA-based method.
In Figure 2 ((60) GeV) and Figure 3 ((120) GeV), we show the dependence of the cross section on the top-Higgs FCNC couplings at = 60 (120) GeV with = 0.6 electron beam polarization and combination of a 7 (50) GeV proton beam for three different cases: (I) , (II) , and (III) . Obviously, the cross section of can be 100 times larger than that of , and the cross section of 50 TeV can be 9.1 (6.6) times larger than that of 7 TeV with a 60 (120) GeV electron beam. We also find that the cross sections between polarized and unpolarized electron beam cases are related as and , independent of being case I, case II, or caseIII. Here , , and represent the right, left, and without electron beam polarization, respectively.
The cross section of the signal and backgrounds (in units of fb) are summarized in Table 2 (cut-based method) and Table 3 (MVA-based method). From these tables, we calculate the signal significance as 4.191 (15.341) and 6.652 (19.236) for 7 and 50 TeV by cut-based method and 4.921 (16.934) and 7.874 (20.785) by MVA-based method after imposing all the relevant event selections (only the first three selections in MVA-based method), respectively. Obviously, compared to the cut-based method, the MVA-based method can get better signal significance. As expected, with the -beam polarization, the results are improved as 5.302 (19.404) and 8.414 (24.335) for cut-based method and 6.224 (21.420) and 9.960 (26.291) for MVA-based method. In addition to effective cuts, enhancing the -tagging efficiency together with reducing the jet misidentification rates is one of the other ways to improve the signal significance. It is confirmed that the signal significance can be increased from 4.191, 6.652, 15.341, and 19.238 to 8.366, 13.840, 33.750, and 44.154 with %, %, and % with the same value of the input parameters and kinematic cuts.
In order to estimate the sensitivity to the anomalous tqH couplings, we used chi-square () function [20, 21]: where is the total cross section and is the statistical error. In Figure 4 (cut-based analysis) and Figure 5 (MVA-based analysis) , we plot the contours of 1 limits to at 7 (50) GeV LHeC and 60 (120) GeV electron beam with different polarization. The red, blue, and black curves represent the 0.6, , and without electron beam polarization. From these figures, we can see that the branching ratio can be probed to , , , 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. As expected, the MVA-based method has a great advantage and also the 50 TeV high energy can get better results compared to the 7 TeV ones. Furthermore, it is clear that the limits can be probed down to 0.090 (0.073%), 0.056 (0.045%), 0.024 (0.018%), and 0.019 (0.015%) with the -beam polarization of .
Finally, we give precise integrated luminosity () corresponding to the critical limits obtained by the experimental results (Table 4) and other phenomenological studies (Table 5). With the -beam polarization , needed to get the upper bounds on is reduced significantly. A detailed comparison between the LHeC collider(s) and the LHC or linear colliders is given.
In this paper, we investigated the anomalous FCNC Yukawa interactions between the top quark, the Higgs boson, and either an up or charm quark with a channel at the LHeC. The signal significance can be obtained as 4.191 (4.921), 6.652 (7.874), 15.341 (16.934), and 19.238 (20.785) with the cut-based (MVA-based) method at (, ) = (7 TeV, 60 GeV), (, ) = (7 TeV, 120 GeV), (, ) = (50 TeV, 60 GeV), and (, ) = (50 TeV, 120 GeV). Similarly, our results show 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%), and with the -beam polarization , the expected limits can be greatly reduced. Finally, a detailed comparison between our study and the critical limits obtained by the experiments and other phenomenological studies is shown. We thus give an overview of the search potential on the anomalous top-Higgs couplings with polarized electron beam at the LHeC.
Conflicts of Interest
The authors declare that they have no Conflicts of interest.
This project is supported by the National Natural Science Foundation of China (Grant no. 11675033) and by the Fundamental Research Funds for the Central Universities (Grant no. DUT15LK22).
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