Abstract
The CP nature of the Higgs coupling to top quarks is addressed in this paper, in single charged lepton final states of $t\bar{t}h$ events produced in proton-proton collisions at the LHC. Pure scalar ($h=H$) and pseudo-scalar ($h=A$) Higgs boson signal events, generated with MadGraph5_aMC@NLO, are fully reconstructed using a kinematic fit. Angular distributions of the decay products, as well as CP-sensitive asymmetries, are exploited to separate and gain sensitivity to possible pseudo-scalar components of the Higgs boson and reduce the contribution from the dominant irreducible background $t\bar{t}b\bar{b}$. Significant differences are found between the pure CP-even and -odd signal hypotheses as well as with respect to the Standard Model background, in particular the $t\bar{t} b\bar{b}$ contribution. Such differences survive the event reconstruction, allowing to define optimal observables to extract the Higgs couplings parameters from a global fit. A dedicated analysis is applied to efficiently identify signal events and reject as much as possible the expected Standard Model background. The results obtained are compared with a similar analysis in the dilepton channel. We show that the single lepton channel is more promising overall and can be used in combination to study the CP nature of the Higgs coupling to top quarks.
Highlights
A new particle with mass around 125 GeV, consistent with the Higgs boson predicted by the Standard Model (SM), was discovered by ATLAS [1] and CMS [2] at the LHC
We study the experimental sensitivity to the CP nature of the Higgs boson Yukawa couplings to top quarks, which can be obtained through the use of several angular observables using tth events produced at the LHC
Semileptonic final states from tth decays were fully reconstructed by a kinematic fit performed with KLFITTER
Summary
A new particle with mass around 125 GeV, consistent with the Higgs boson predicted by the Standard Model (SM), was discovered by ATLAS [1] and CMS [2] at the LHC. Since the Higgs Yukawa coupling to the top quark is expected to be close to unity [9], much larger than the other Yukawa couplings, its impact on the vacuum stability is expected to be more important The study of this process allows a direct measurement of the vertex and, in particular, provides sensitivity to the CP nature of the Higgs couplings to top quarks [10]. The observation of the associated production of top quarks with a Higgs boson was recently announced independently by the CMS and the ATLAS collaborations, using a combination of analyses, resulting in significances in excess of five standard deviations [13,14] These analyses employed LHC data collected at 7, 8, and 13 TeV and are sensitive to tth final states, with the Higgs boson decaying to bb , WWÃ, τþτ−, γγ, and ZZÃ.
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