Constraining couplings of the top quarks to the Z boson in ttbar+Z production at the LHC

We study top quark pair production in association with a Z boson at the Large Hadron Collider (LHC) and investigate the prospects of measuring the couplings of top quarks to the Z boson. To date these couplings have not been constrained in direct measurements. Such a determination will be possible for the first time at the LHC. Our calculation improves previous coupling studies through the inclusion of next-to-leading order (NLO) QCD corrections in production and decays of all unstable particles. We treat top quarks in the narrow-width approximation and retain all NLO spin correlations. To determine the sensitivity of a coupling measurement we perform a binned log-likelihood ratio test based on normalization and shape information of the angle between the leptons from the Z boson decay. The obtained limits account for statistical uncertainties as well as leading theoretical systematics from residual scale dependence and parton distribution functions. We use current CMS data to place the first direct constraints on the ttbZ couplings. We also consider the upcoming high-energy LHC run and find that with 300 inverse fb of data at an energy of 13 TeV the vector and axial ttbZ couplings can be constrained at the 95% confidence level to C_V=0.24^{+0.39}_{-0.85} and C_A=-0.60^{+0.14}_{-0.18}, where the central values are the Standard Model predictions. This is a reduction of uncertainties by 25% and 42%, respectively, compared to an analysis based on leading-order predictions. We also translate these results into limits on dimension-six operators contributing to the ttbZ interactions beyond the Standard Model.Comment: JHEP version + error in Eq. 2.6 corrected and corresponding higher order operator limits modifie

Probing top-Z dipole moments at the LHC and ILC

We investigate the weak electric and magnetic dipole moments of top quark-Z boson interactions at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). Their vanishingly small magnitude in the Standard Model makes these couplings ideal for probing New Physics interactions and for exploring the role of top quarks in electroweak symmetry breaking. In our analysis, we consider the production of two top quarks in association with a Z boson at the LHC, and top quark pairs mediated by neutral gauge bosons at the ILC. These processes yield direct sensitivity to top quark-Z boson interactions and complement indirect constraints from electroweak precision data. Our computation is accurate to next-to-leading order in QCD, we include the full decay chain of top quarks and the Z boson, and account for theoretical uncertainties in our constraints. We find that LHC experiments will soon be able to probe weak dipole moments for the first time.Comment: 17 pages, 6 figure

Mixed scalar-pseudoscalar Higgs boson production through next-to-next-to-leading order at the LHC

We study the production of a mixed scalar-pseudoscalar Higgs boson in gluon fusion at the LHC, through next-to-next-to-leading order (NNLO) in QCD. We obtain fully differential results, including the decay of the Higgs boson to two charged lepton pairs. We discuss the impact of the interference between the scalar and pseudoscalar states. We also show differential distributions for several kinematic variables whose shape is sensitive to the parity of the Higgs boson, and assess the impact of the NNLO QCD corrections on these shapes

Analytic results for color-singlet production at NNLO QCD with the nested soft-collinear subtraction scheme

We present analytic formulas that describe the fully-differential production of color-singlet final states in qq̄ and gg annihilation, including all the relevant partonic channels, through NNLO QCD. We work within the nested soft-collinear scheme, which allows the fully local subtraction of infrared divergences.We demonstrate analytic cancellation of soft and collinear poles and present formulas for the finite parts of all integrated subtraction terms. These results provide an important building block for calculating NNLO QCD corrections to arbitrary processes at hadron colliders within the nested soft-collinear subtraction scheme

NNLO QCD corrections to weak boson fusion Higgs boson production in the H → bb‾\overline{b} and H → WW* → 4l decay channels

We compute the next-to-next-to-leading order QCD corrections to Higgs boson production in weak boson fusion followed by its decay to a b$\overline{b}$ pair or to a pair of leptonically-decaying W bosons. Our calculation allows us to compute realistic fiducial cross sections and assess the impact of fiducial cuts applied to the Higgs boson decay products on the magnitude of QCD radiative corrections in weak boson fusion

Nested soft-collinear subtractions in NNLO QCD computations

We discuss a modification of the next-to-next-to-leading order (NNLO) subtraction scheme based on the residue-improved sector decomposition that reduces the number of double-real emission sectors from five to four. In particular, a sector where energies and angles of unresolved particles vanish in a correlated fashion is redundant and can be discarded. This simple observation allows us to formulate a transparent iterative subtraction procedure for double-real emission contributions, to demonstrate the cancellation of soft and collinear singularities in an explicit and (almost) process-independent way and to write the result of a NNLO calculation in terms of quantities that can be computed in four space-time dimensions. We illustrate this procedure explicitly in the simple case of O(α2s)O(αs2) gluonic corrections to the Drell–Yan process of qq¯qq¯ annihilation into a lepton pair. We show that this framework leads to fast and numerically stable computation of QCD corrections

Analytic results for decays of color singlets to gg and qqˉq \bar{q} final states at NNLO QCD with the nested soft-collinear subtraction scheme

We present compact analytic formulas that describe the decay of colorless particles to both q (q) over bar and gg final states through next-to-next-to-leading order in perturbative QCD in the context of the nested soft-collinear subtraction scheme. In addition to their relevance for the description of decays like V -> q (q) over bar\u27, V = Z, W, H -> b (b) over bar and H -> gg, these results provide an important building block for calculating NNLO QCD corrections to arbitrary processes at colliders within the nested soft-collinear subtraction scheme

Anomalous couplings in associated VHVH production with Higgs decay to massive bb quarks at NNLO in QCD

We combine the NNLO QCD description of Higgs boson production in association with an electroweak vector boson $V = W~{\rm or}~Z$ with a similarly-precise description of Higgs boson decays into a pair of massive $b$ quarks and with the anomalous couplings that modify interactions of the Higgs and electroweak vector bosons. The resulting numerical code provides the most advanced theoretical tool to investigate such anomalous couplings in the associated Higgs boson production process. We study the impact of anomalous couplings on fiducial cross sections and differential distributions and argue that, with increased QCD precision, smaller anomalous couplings become accessible in kinematic regions where the effects of higher-dimensional operators in the Standard Model Effective Field Theory remain small and the EFT expansion is under control.Comment: 28 pages, 8 figure