Probing the CP nature of the Higgs coupling in tth events at the LHC

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1 January 7, 8 Probing the CP nature of the iggs coupling in tth events at the LC E. Gouveia also on behalf of arxiv:8.4954v [hep-ph] 5 Jan 8 S.P. mor dos Santos, M.C.. Fiolhais,4, R. Frederix 5, R. Gonçalo, R. Martins,. Onofre, C.M.Pease 4,. Peixoto 6,. Reigoto, R. Santos 7,8, J. Silva 6 LIP, Departamento de Física, Universidade do Minho, Braga, Portugal LIP, Departamento de Física, Universidade de Coimbra, 4-56 Coimbra, Portugal LIP, v. Prof. Gama Pinto, n., 649- Lisboa, Portugal; Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Lisboa, Portugal 4 Science Department, Borough of Manhattan Community College, City University of ew York, 99 Chambers St, ew York, Y 7, US 5 Technische Universität München, James-Franck-Str., D Garching, Germany 6 Centro de Física, Universidade do Minho, Campus de Gualtar, Braga, Portugal 7 Instituto Superior de Engenharia de Lisboa - ISEL, Lisboa, Portugal 8 Centro de Física Teórica e Computacional, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Edifício C Lisboa, Portugal The determination of the CP nature of the top quark Yukawa coupling is addressed in this phenomenological work, using tth events at the s = TeV LC, with h bb and through the dilepton decay channel of tt. Signal events were generated for pure CP -even, pure CP -odd and mixed-cp coupling scenarios. Standard Model (SM) backgrounds were also considered. ngular distributions and asymmetries were studied and found to be potentially good probes to the CP nature of the coupling, even after detector simulation and full event reconstruction using a kinematic fit. Expected limits to the tth production cross-section times branching ratio were obtained for a range of top quark Yukawa CP -mixing angles and for different integrated luminosities of the LC. PRESETED T TOP7 - th International Workshop on Top Quark Physics Braga, Portugal, September 7, 7 Funded by Fundação para a Ciência e Tecnologia, through grant PD/BD/8/6

2 Introduction The CP nature of the couplings to gauge bosons of the recently discovered particle compatible with the 5 GeV iggs boson [, ] has been studied by TLS and CMS. So far, no equivalent analyses have been carried out in the fermionic sector alone. The top quark, due to its expected large Yukawa coupling, will be the obvious candidate for such measurements. The top quark Yukawa interaction with the iggs boson can be parametrised as L = κy t t(cos α + iγ 5 sin α)th, () where the SM is recovered for α = and κ =, and α / {, π} implies a non-zero CP -odd component in the coupling. The iggs associated production process tth allows a direct measurement of this coupling. While a non-zero CP -odd component is expected to affect the inclusive tth production cross-section, it has been suggested [ 6] that differential cross-section measurements, exploiting the distinct kinematics and spin correlations among intermediate and final-state particles, can enhance sensitivity to its presence. We show that angular distributions of the kind introduced in [7] are sensitive to the CP nature of the coupling. lso, a survey of previously proposed observables resulted in a reduced set of those which still show significant discrimination power even after detector simulation, event selection and event reconstruction via a kinematic fit [8]. Event generation, simulation and reconstruction Events from pp collisions were generated at s = TeV using MadGraph5 amc@lo [9]. Signal tth events, as well as the irreducible background, were generated at LO in QCD. Signal samples in which the iggs has a non-zero CP -odd component were generated using the C LO X UFO model [6], with cos α varying from - to. Particle decays were handled by MadSpin []. The SM backgrounds tt+jets, tt + V (V = Z, W ), single top, V +jets, V + bb and V V were also generated. Parton shower and hadronisation were performed with Pythia6 [] and followed by a fast detector simulation with the Delphes [] package. Events were selected with at least four jets (of which at least three must be b- tagged) and exactly two opposite-charge leptons, all with p T GeV and η <.5. dditionally, the dilepton mass m ll must meet the requirement m ll 9 GeV > GeV. Kinematic reconstruction of the event starts with the choice of the most likely assignment of jets to b-quarks from t, t and h decay, using a boosted decision tree (BDT). Then, a solution for the neutrino momenta is obtained by combining information from missing E T with a constraint on W and t masses.

3 ngular distributions Let θ tth (t, h) (respectively θ tth (t, h)) be the angle between the direction of h and the direction of t (respectively t), both evaluated in the tth rest-frame. Figure shows two-dimensional distributions of signal events for these angles, calculated at LO after parton-shower effects, prior to detector simulation and event selection. In (t,) tt LC, s = TeV tt events (m =5 GeV).8.6 (t,) tt LC, s = TeV tt events (m =5 GeV).. θ.4 θ tt θ (t,) tt θ (t,) Figure : Parton-level distributions of θ tth (t, h) (x-axis) and θ tth (t, h) (y-axis). The SM signal (h = ) distribution (left) and the pure CP -odd scenario signal (h = ) distribution (right) are shown. the SM scenario (left) the iggs boson tends to travel preferentially along a direction close to that of one of the top quarks and nearly opposite to the other. In the pure CP -odd scenario (right), events are more evenly spread over the allowed range. The angular observables introduced in [7] are constructed from angles θy X, defined as the angle between the direction of Y, in the rest frame of X, and the direction of X in the rest frame of its parent system. In Figure, normalised distributions of sin(θt tth ) sin(θw h ) and sin(θ tth + h ) sin(θ tt ) are shown after cuts and kinematic reconstruction for the SM signal, pure CP -odd scenario signal and irreducible background t events. These are among the most sensitive observables of this kind with respect to the CP nature of the signal. From the survey of observables proposed in previous theoretical or phenomenological studies, the ones found to be more sensitive to the different CP scenarios, after cuts and reconstruction, were β bb θ lh (l +, l ), proposed in [4], and b 4 = p z t p z/p t tp t, proposed in [5]. Distributions of these observables are shown, after cuts and reconstruction, in Figure, for signal events in the SM and in the pure CP -odd scenarios and for the background. Overall, b 4 is found to be the most sensitive observable,

4 d. LC, s = TeV LO+Pythia6 tth (h=) m tth (h=) m =5 GeV =5 GeV d.5 LC, s = TeV LO+Pythia6 tth (h=) m tth (h=) m =5 GeV =5 GeV tth h = sin(θt )*sin(θ ) W+ Exp (seq) x Y tth tt Exp x = sin(θ )*sin(θ ) Y h t d. LC, s = TeV LO+Pythia6 tth (h=) m tth (h=) m =5 GeV =5 GeV d. LC, s = TeV LO+Pythia6 tth (h=) m tth (h=) m =5 GeV =5 GeV Exp x = b Y 4. lh Exp x = β θ (l+,l-) Y bb Figure : ormalised distributions of signal events in the SM (tt) and pure CP -odd (tt) scenarios and of the background, after cuts and reconstruction. with associated forward-backward asymmetries of +.6 for SM signal, -.7 for pure CP -odd signal and. for. 4 Expected limits on tth production cross-section signal-background classification BDT, combining the most sensitive observables, was trained for each value of cos α. Its output was used as a discriminant for extracting the expected 95% CL upper limits on the production cross-section times the h bb branching ratio (σ BR), under the background-only hypothesis. Figure shows, on the left, these limits as a function of cos α, for different integrated luminosities and, on the right, the corresponding limits on signal strength, computed relatively to the prediction for that value of α. From the left plot, we see that the analysis becomes more sensitive as the CP -odd component of the coupling increases. Thus, a search optimized for the SM tth search is not insensitive to production through a mixed-cp coupling. With respect to CP measurements, it was recently suggested that the single-lepton channel is nearly twice as sensitive as the dilepton channel [] and it would seem beneficial to combine the two.

5 95% CL limit on σ BR(h bb)[pb] α 4 LC s = TeV - Lδt = fb - Lδt = fb - Lδt = fb 95% CL limit on µ=σ/σ 4 LC s = TeV - Lδt = fb - Lδt = fb - Lδt = fb cos(α).5.5 cos(α) Figure : Expected 95% CL limits in the background-only scenario as a function of α and for integrated luminosities. Left: σ BR(h bb); right: µ = σ/σ α. CKOWLEDGEMETS I am thankful to the International dvisory Committee for granting me the opportunity to present and discuss this work at the Workshop, and to the Local Organizing Committee for a very nicely organized gathering. References [] G. ad et al. [TLS Collaboration], Phys. Lett. B 76 () [] S. Chatrchyan et al. [CMS Collaboration], Phys. Lett. B 76 () [] P. rtoisenet et al., JEP () 5 [4] F. Boudjema et al., Phys. Rev. D 9 (5) no., 59 [5] J. F. Gunion and X. G. e, Phys. Rev. Lett. 76 (996) 4468 [6] F. Demartin et al., Eur. Phys. J. C 74 (4) no.9, 65 [7] S. P. mor dos Santos et al., Phys. Rev. D 9 (5) no., 4 [8] S. mor Dos Santos et al., Phys. Rev. D 96 (7) no., 4 [9] J. lwall et al., JEP 47 (4) 79 [] T. Sjostrand, S. Mrenna and P. Z. Skands, JEP 65 (6) 6 [] J. de Favereau et al. [DELPES Collaboration], JEP 4 (4) 57 [] D. zevedo,. Onofre, F. Filthaut and R. Gonçalo, arxiv:7.59 [hep-ph]. 4