Probing anomalous top-Higgs couplings at the HL-LHC via H→W W ∗ decay channels
aa r X i v : . [ h e p - ph ] N ov Probing anomalous top-Higgs couplings at the HL-LHCvia H → W W ∗ decay channels Yao-Bei Liu ∗ Henan Institute of Science and Technology, Xinxiang 453003, P.R.China
Stefano Moretti † School of Physics & Astronomy, University of Southampton,Highfield, Southampton SO17 1BJ, UK
Abstract
We study the prospects of probing the anomalous tHq ( q = u, c ) couplings via SS2L or 3L signaturesat the High Luminosity (HL-LHC) run of the 14 TeV CERN collider. We focus on signals of the tH associated production followed by the decay modes t → bℓ + ν ℓ and H → W W ∗ , and t ¯ t productionfollowed by the decay modes t → bℓ + ν ℓ and ¯ t → H ( → W W ∗ )¯ q , where ℓ = e, µ . Based on two typesof H → W W ∗ decay topologies, one assuming the semileptonic decay mode H → W W ∗ → ℓ + νjj andthe other the fully leptonic decay mode H → W W ∗ → ℓ + νℓ − ¯ ν , we perform a full simulation for signalsand backgrounds. It is shown that, at the future HL-LHC, the branching ratio Br ( t → uh ) ( Br ( t → ch )) can be probed to .
17 (1 . × − for the same-sign di-lepton channel, and to . × − (1 . × − ) for the 3L channel at σ sensitivity. ∗ E-mail: [email protected] † E-mail: [email protected] . INTRODUCTION Processes mediated by Flavor Changing Neutral Currents (FCNCs) are very rare in theStandard Model (SM) due to the Glashow-Iliopoulos-Maiani (GIM) mechanism [1]. How-ever, because of the extended flavor structures existing in many New Physics (NP) models, thetwo-body FCNC decays t → qX ( q = u/c and X = g/γZ/H ) can be greatly enhanced:for example, in the Minimal Supersymmetric Standard Model (MSSM) with branching ra-tio Br ( t → cH ) ∼ − [2], in R-parity violating Supersymmetry (SUSY) with branchingratio Br ( t → cH ) ∼ − [3], in 2-Higgs-Doublet Models (2HDMs) with branching ratio Br ( t → cH ) ∼ − − − [4], in the little Higgs model with T-parity and the warped extradimensions both with branching ratio Br ( t → cH ) ∼ − [5, 6] and so on. Thus any exper-imental signatures of such FCNC processes will serve as a clear signal for NP Beyond the SM(BSM) [7]. Up to now, top-Higgs FCNC interactions have been studied widely via anomaloustop decays or anomalous production processes of single top quark [8–16].Currently, the ATLAS and CMS collaborations have carried out searches [17–21] for tqH interactions with 7, 8 and 13 TeV data from the LHC. For example, using 13 TeV data, theATLAS and the CMS experiments have studied the tqH FCNC processes in top quark pairevents with H → γγ for ATLAS and H → b ¯ b for CMS. The resulting observed (expected)limits for Br ( t → qH ) at Confidence Level (CL) have been found to be [19, 20]: Br ( t → Hu ) ≤ . . × − ATLAS4 . . × − CMS Br ( t → Hc ) ≤ . . × − ATLAS4 . . × − CMS (1)Very recently, a search for production of top pairs in which one top quark decays via t → qH is reported by the ATLAS Collaboration [21], with the subsequent Higgs boson decay tofinal states with at least one electron or muon. The upper limits on the branching fractions Br ( t → Hc ) < . and Br ( t → Hu ) < . at CL are obtained (with expectedlimits of . in both cases). Apart from direct collider measurements, the upper limits of Br ( t → qH ) < × − and Br ( t → qH ) < . can be obtained by bounding the tqH vertex from the observed D − ¯ D mixing [22] and Z → c ¯ c [23], respectively.The upcoming project of the HL-LHC is expected to reach 3 ab − . Preliminary sensitivity2tudies for the HL-LHC suggest the upper bound on Br ( t → qH ) to become about . × − at CL by the ATLAS Collaboration [24]. Further, many phenomenological studieswithin model-independent methods have been performed from different channels [25–33]. Inthis work, we study the prospects of probing the anomalous tHq couplings by considering theprocesses of tH associated production and t ¯ t production at the HL-LHC. We analyze two kindsof final states through leptonic top quark decays and H → W W ∗ , one with Same Sign 2-Lepton (SS2L) and the other with 3-Lepton (3L) topology, where the Higgs boson decays intoa semi-leptonic ( H → W W ∗ → ℓ + νjj ) or fully leptonic ( H → W W ∗ → ℓ + νℓ − ¯ ν ) mode. Theadvantage of these channels is that their final states including the SS2L or 3L topologies canbe used to significantly suppress QCD backgrounds [34], which have not been fully studied inprevious literature.The organization of this paper is as follows. In Sec. II, we discuss two kinds of final statesfor the processes of tH associated production with the decay chain t → W + b → ℓ + νb and H → W W ∗ as well as t ¯ t production with the decay chain t → ℓ + ν ℓ b and ¯ t → H ( → W W ∗ )¯ q .Then we discuss the HL-LHC sensitivity to the anomalous tHq couplings. We summarize inSec. III. II. NUMERICAL CALCULATIONS AND DISCUSSIONS
The general Lagrangian for FCNC top interactions with the Higgs boson can be written as L = κ tuH ¯ tHu + κ tcH ¯ tHc + h.c., (2)where the FCNC coupling parameters, κ tuH and κ tcH , are real and symmetric since we do notconsider here the CP violating effects.We perform systematic Monte Carlo (MC) simulations and study the sensitivity to theanomalous tHq couplings through the associated tH and t ¯ t → tH ¯ q processes at HL-LHC.We first extract the relevant Feynman rules via the FeynRules package [35] and generate theevents with MadGraph5-aMC @ NLO [36]. The signal and backgrounds samples are simulatedat parton level with the NN23LO1 Parton Distribution Function (PDF) set [37] and then passedthrough PYTHIA6.4 [38] and DELPHES 3 [39] for parton shower and detector simulations,with the MLM matching scheme [40] adopted. Finally, event analysis is performed by usingMadAnalysis5 [41]. 3 . Analysis of the SS2L channel
For the final states including the SS2L topology, the signals are generated through the fol-lowing processes, pp → t ( → W + b → ℓ + νb ) H ( → W W ∗ → ℓ + νjj ) , (3) pp → t ( → W + b → ℓ + νb )¯ t ( → Hq → W W ∗ q → ℓ + νjjq ) , (4)where ℓ = e, µ . The representative Feynman diagrams are shown in Fig. 1. qg q H W + ℓ + ν ℓW − jj ′ t b W + ℓ + ν ℓ (a) gg g ¯ t ¯ q H W + ν ℓ ℓ + W − j ′ jt b W + ν ℓ ℓ + (b) FIG. 1: Representative Feynman diagrams for the associated tH process (left) and the FCNC decay ofthe top pair production process (right). Here q = u, c . For this channel, the typical signal is exactly two same-sign leptons plus at least three jets,with at least one jet identified as b -jet, and missing transverse energy. The main backgroundsare t ¯ tV ( V = W, Z ), W + W + jj and W + Zjj . The t ¯ t process, which has large cross section,may also contribute to background if the a same-sign lepton pair comes from a B -hadron semi-leptonic decay in the b -jet. We do not consider other backgrounds from t ¯ tH , t ¯ tt ¯ t , tri-bosonevents and tHj . They are neglected because the cross sections are all negligible after applyingthe selection cuts.The cross sections of dominant backgrounds at Leading Order (LO) are adjusted to Next-to-LO (NLO) by means of K -factors, which are 1.04 for W + W + jj jets [42], 1.24 for t ¯ tW [43]and 1.39 for t ¯ tZ [44]. The dominant t ¯ t background is normalized to the NNLO QCD cross4ection of 953.6 pb [45]. For the tH production cross section, the K-factor is taken as 1.5 at the14 TeV LHC [12].The decay chain H → W W ∗ → ℓνjj may result in soft leptons and light jets, especiallywhen they are coming from an off-shell W boson. To analyze the signal sensitivity, we thusemploy the following basic cuts to select the events: • Basic cuts: p T ( ℓ ) >
10 GeV , p T ( j, b ) >
15 GeV , | η ℓ,j,b | < . , where ℓ = e, µ . (GeV) jj M E v e n t s ( s c a l e d t o o n e ) ugthcgththjVtt jj + W + W Zjj + Wtt (GeV) jj l M E v e n t s ( s c a l e d t o o n e ) ugthcgththjVtt jj + W + W Zjj + Wtt (GeV) b l M E v e n t s ( s c a l e d t o o n e ) ugthcgththjVtt jj + W + W Zjj + Wtt (GeV) T H E v e n t s ( s c a l e d t o o n e ) ugthcgththjVtt jj + W + W Zjj + Wtt
FIG. 2: Normalized distributions for the signals and the backgrounds.
In order to choose appropriate kinematic cuts, we plot in Fig. 2 examples of kinematic dis-tributions for the signal and backgrounds. Based on these distributions, we impose a further setof cuts. • Cut-1: Exactly two same-sign leptons ( N ( ℓ + ) = 2 ) with p T ( ℓ ) > GeV and p T ( ℓ ) > GeV ( ℓ and ℓ denote the higher and lower p T lepton, respectively) plus exactly one b -tagged jet ( N ( b ) = 1 ). To remove contamination from hadron decay chains including5 + ℓ − and Z boson, we choose the invariant mass larger than 12 GeV and | M ℓℓ − m Z | > GeV. • Cut-2: At least two jets in the events are required to be successfully reconstructed, i.e., N ( j ) ≥ . Among those reconstructed jets, there are at least one pair of jets which couldcome from a W boson either on-shell or off-shell. Thus the invariant mass of the W boson is required to be M jj < GeV. • Cut-3: The invariant mass of M ℓ jj is required to be smaller than 120 GeV. • Cut-4: Since the first lepton, ℓ , is assumed to originate from the leptonically decayingtop quark, the invariant mass of the b -jet and the leading lepton should be M bℓ < GeV. • Cut-5: The scalar sum of transverse momenta, H T , is to be smaller than 250 GeV. TABLE I: The cut flow of the cross sections (in fb) for the signal and SM backgrounds for the SS2Lchannel. The coupling parameters are taken as κ tuH = 0 . or κ tcH = 0 . while fixing the other to zero.Cuts Signal Backgrounds ug cg t ¯ t → tHq t ¯ tV W W jj W Zjj t ¯ t Basic cuts 3.12 0.34 3.77 6.73 6.42 20.9 61004Cut 1 0.48 0.056 0.69 0.85 0.21 0.25 6.52Cut 2 0.225 0.027 0.34 0.27 0.04 0.046 2.54Cut 3 0.18 0.022 0.28 0.092 0.016 0.011 1.7Cut 4 0.15 0.019 0.24 0.058 0.009 0.0063 1.36Cut 5 0.14 0.017 0.21 0.048 0.007 0.005 1.16
The effects of the cuts on the signal and background processes are illustrated in Tab. I for theSS2L channel, where the anomalous coupling parameters are taken as κ tuH = 0 . or κ tcH = 0 . while fixing the other to zero. From Tab. I we can see that, after all these cuts, the t ¯ t backgroundsfor the SS2L channel, with fake leptons from heavy-flavor jets or charge mis-identifications canbe significant. 6bviously, the non-prompt backgrounds may also be significant, where non-prompt lep-tons are from heavy-flavor decays, mis-identified hadrons, muons from light-meson decays orelectrons from un-identified conversions of photons into jets. Recently, the CMS collaborationsearched for SS2L signatures [46] and found that the overall non-prompt backgrounds are about1.5 times the t ¯ tW background after all cuts. These non-prompt backgrounds are not properlymodeled in our MC simulations. Therefore, for simplicity, we add a non-prompt backgroundthat is 1.5 times t ¯ tW [46] after selection cuts to the overall background. Accounting for thetheoretical and experimental systematic uncertainties on the background predictions would cer-tainly improve the reliability of the results, yet they can only be neglected in our simulation. B. Analysis of the 3L channel
Next, we consider the final states including 3L via the following processes: pp → t ( → W + b → ℓ + νb ) h ( → W W ∗ → ℓ + νℓ − ¯ ν ) , (5) pp → t ( → W + b → ℓ + νb )¯ t ( → Hq → W W ∗ q → ℓ + νℓ − ¯ νq ) , (6)where ℓ = e, µ .The dominant SM backgrounds are t ¯ tV ( V = W, Z ), t ¯ tH , W Z + jets and t ¯ t . The multi-jet backgrounds (where jets can fake electrons) are not included since they are negligible inmulti-lepton analyses [47].The pre-selection cuts are taken as follows: there must exist exactly three isolated leptons( ℓ = e, µ ) and exactly one b -tagged jet with p T ( ℓ ) >
20 GeV , p T ( ℓ , ) >
10 GeV , p T ( j, b ) >
20 GeV , /E T >
100 GeV and | η ℓ,j,b | < . . These cuts can strongly reduce the t ¯ t backgroundand di-boson components.In Fig. 3, we show the invariant mass distribution of M ℓ ℓ and M bℓ from the signal andbackgrounds at the 14 TeV LHC. To remove contamination from hadron decay chains including ℓ + ℓ − pairs and resonant Z bosons, we choose the invariant mass M ℓ ℓ cuts • < M ( ℓ ℓ ) <
55 GeV .Similarly, the invariant mass of the b -jet and the leading lepton, M bℓ , should be smaller than140 GeV. The effects of the cuts on the signal and backgrounds processes are illustrated in7 (GeV) l l M E v e n t s ( s c a l e d t o o n e ) ugthcgthppthjWZjjVtt htttt (GeV) bl M E v e n t s ( s c a l e d t o o n e ) ugthcgththjWZjjVtt htttt FIG. 3: Normalized invariant mass distributions of M ℓ ℓ (left) and M bℓ (right). Table 2 for the 3L channel. One can see that significant backgrounds also come from the toppair production process with fake leptons or charge mis-identifications.
TABLE II: The cut flow of the cross sections (in fb) for the signal and background processes for the 3Lchannel. Cuts Signals Backgrounds ug cg t ¯ t → tHq t ¯ t t ¯ tV W Zjj t ¯ th Basic cuts 1.39 0.17 2.05 21843 1.85 46.2 0.025After cuts 0.14 0.018 0.106 0.23 0.024 0.021 . × − Using the Poisson formula SS = p L int [( S + B ) ln(1 + S/B ) − S ] [48] we estimate theSignal Significance ( SS ) with fixed coupling parameters κ tqH and a given integrated luminosity L int . In Figs. 4 and 5, we plot the contours of SS = 3 and SS = 5 , respectively, for twochannels in the plane of L int − κ tqH . It is clear that, for an integrated luminosity of 3000fb − , the FCNC couplings κ tuH ( κ tcH ) can be probed to 0.045 (0.052) and 0.035 (0.049) at σ statistical sensitivity for the SS2L and 3L channels, respectively. After neglecting the massesof light quarks, the branching ratio of t → qH is approximately given by [13, 49] Br ( t → qH ) = κ tqH √ m t G F (1 − x h ) (1 − x W ) (1 + 2 x W ) λ QCD ≃ . κ tqH , (7)8 .04 0.05 0.06 0.07 0.08 0.09 0.10050010001500200025003000 L ( f b - ) tqH tuH tcH L ( f b - ) tqH tuH tcH FIG. 4: The σ contour plots for the signal in the L int − κ tqH plane for the SS2L (left) and 3L (right)channels at the 14 TeV LHC. L ( f b - ) tqH tuH tcH L ( f b - ) tqH tuH tcH FIG. 5: The σ contour plots for the signal in the L int − κ tqH plane for the SS2L (left) and 3L (right)channels at the 14 TeV LHC. in terms of the Fermi constant G F and with x i = m i /m t ( i = W, h ) . In our numericalcalculation, the relevant SM input parameters are taken as [50]: m H = 125 GeV , m t = 173 . , m W = 80 .
379 GeV , (8) m Z = 91 . , α s ( m Z ) = 0 . , G F = 1 . × − GeV − . σ CL upper limits on Br ( t → qH ) are about Br ( t → uH ) = 1 . × − and Br ( t → cH ) = 1 . × − for the SS2L channel, and Br ( t → uH ) = 7 . × − and Br ( t → cH ) = 1 . × − for the 3L channel. The projected limits from different channelsare summarized in Tab. III. We can see from the table that our results are comparable with thesensitivity limits at the HL-LHC as Br ( t → uH ) < . via the H → γγ channel [29], Br ( t → uH ) < . via the multi-lepton channel and Br ( t → uH ) < . via thedi-photon channel [51]. TABLE III: The projected limits on Br ( t → qH ) from different channels. The last two lines of the tableare the results of this work. Channels Data Set Limits tH → ℓνbτ + τ − LHC, 100 fb − @ 13 TeV, CL Br ( t → uH ) < .
15 % [13] tH → ℓνbℓ + ℓ − X LHC, 100 fb − @ 13 TeV, CL Br ( t → uH ) < .
22 % [13] t ¯ t → W b + Hc → jjb + τ τ c LHC, 100 fb − @ 13 TeV, CL Br ( t → cH ) < .
25 % [14] tH → jjbb ¯ b LHC, 100 fb − @ 13 TeV, CL Br ( t → uH ) < .
36 % [13]
W t → W Hq → ℓνbγγq LHC, 3000 f b − @ 14 TeV, σ Br ( t → qH ) < .
24 % [28] tH → ℓνbγγq LHC, 3000 f b − @ 14 TeV, σ Br ( t → uH ) < .
036 % [29] t ¯ t → W bqH → ℓνbγγq LHC, 3000 f b − @ 14 TeV, σ Br ( t → uH ) < .
23 % [30] e − p → ν e ¯ t → ν e H ( → b ¯ b )¯ q LHeC, 200 f b − @ 150 GeV ⊕ CL Br ( t → qH ) < .
013 % [31] t ¯ t → tqH → ℓνbb ¯ bq ILC, 3000 fb − @ 500 GeV, CL Br ( t → qH ) < .
112 % [32] t ¯ t → tqH → ℓνbb ¯ bq ILC (unpolarized), 500 fb − @ 500 GeV, σ Br ( t → qH ) < .
119 % [33] t ¯ t → tqH → ℓνbb ¯ bq ILC (polarized), 500 fb − @ 500 GeV, σ Br ( t → qH ) < .
088 % [33] t ¯ t → W b + Hq → ℓνb + γγq LHC, 3000 fb − @ 14 TeV, CL Br ( t → qH ) < .
02 % [51] t ¯ t → W b + hq → ℓνb + ℓℓqX LHC, 3000 fb − @ 14 TeV, CL Br ( t → qH ) < .
05 % [51]This work for the SS2L channel LHC, 3000 fb − @ 14 TeV, σ Br ( t → uH ) < .
117 % , Br ( t → cH ) < .
156 %
This work for the 3L channel LHC, 3000 fb − @ 14 TeV, σ Br ( t → uH ) < .
071 % , Br ( t → cH ) < .
139 % II. CONCLUSIONS
The discovery of the 125 GeV Higgs boson opens the door to probe NP processes that involveHiggs boson associated production or decay. In this paper, we have investigated the signal of tH associated production via FCNC tqH couplings and t ¯ t production with ¯ t → H ¯ q at the14 TeV LHC. We focused on the final states including SS2L and 3L signals from the decaymodes t → bℓ + ν ℓ , H → W W ∗ → ℓ + νjj or H → W W ∗ → ℓ + νℓ − ¯ ν . We have then shownthat, at σ level, the branching ratios Br ( t → uH ) and Br ( t → cH ) are, respectively, about Br ( t → uH ) ≤ . × − and Br ( t → cH ) ≤ . × − for the SS2L channel, and Br ( t → uH ) ≤ . × − and Br ( t → cH ) ≤ . × − for the 3L channel at the futureHL-LHC. Acknowledgments
The work of Y-B Liu is supported by the Foundation of Henan Institute of Science andTechnology (Grant no. 2016ZD01) and the China Scholarship Council (201708410324). Thework of SM is supported in part by the NExT Institute and the STFC CG ST/L000296/1. [1] S. L. Glashow, J. Iliopoulos, and L. Maiani,
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