arXiv subjects

Werner Bernreuther

Publications and source records attributed to Werner Bernreuther.

At least 19 recordsLinked to original sources

arXiv2024

Binned top quark spin correlation and polarization observables for the LHC at 13.6 TeV

We consider top-antitop quark $(t{\bar t})$ production at the Large Hadron Collider (LHC) with subsequent decays into dileptonic final states. We use and investigate a set of leptonic angular correlations and distributions with which all the independent coefficient functions of the top-spin dependent parts of the $t{\bar t}$ production spin density matrices can be experimentally probed. We compute these observables for the LHC center-of-mass energy 13.6 TeV within the Standard Model at next-to-leading order in the QCD coupling including the mixed QCD-weak corrections. We determine also the $t{\bar t}$ charge asymmetry where we take in addition also the mixed QCD-QED corrections into account. In addition we analyze and compute possible new physics (NP) effects on these observables in terms of a gauge-invariant effective Lagrangian that contains the operators up to mass dimension six that are relevant for hadronic $(t{\bar t})$ production. First we compute our observables inclusive in phase space. In order to investigate which region in phase space has, for a specific observable, a high NP sensitivity, we determine our observables also in two-dimensional $(M_{t{\bar t}},\cosθ_t^*)$ bins, where $M_{t{\bar t}}$ denotes the $t{\bar t}$ invariant mass and $θ_t^*$ is the top-quark scattering angle in the $t{\bar t}$ zero-momentum frame.

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arXiv2023

Top and bottom quark forward-backward asymmetries at next-to-next-to-leading order QCD in (un)polarized electron positron collisions

We consider, at order $α_s^2$ in the QCD coupling, top-quark pair production in the continuum at various center-of-mass energies and $b$-quark pair production at the $Z$ resonance by (un)polarized electron and positron beams. For top quarks we compute the forward-backward asymmetry with respect to the top-quark direction of flight, the associated polar angle distribution, and we analyze the effect of beam polarization on the QCD corrections to the leading-order asymmetry. We calculate also the polarized forward-backward asymmetry. For $b$-quark production at the $Z$ peak we explore different definitions of $A_{\rm FB}$. In particular, we analyze $b$ jets defined by the Durham and the flavor-$k_T$ clustering algorithms. We compute the inclusive $b$-jet and two-jet asymmetry with respect to the $b$-jet direction. For the latter asymmetry the QCD corrections to order $α_s^2$ are small. That predestines it to act as a precision observable.

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arXiv2021

Probing the tau electric dipole moment at BEPC-II collider energies

We investigate the prospects of the search for a nonzero $τ$ EDM form factor $d_τ(s)$ in $τ$ pair production by $e^+ e^-$ collisions at BEPC-II collider energies. We compute the expectation values and covariances of simple and optimal $CP$-odd observables for $τ$-pair production at $\sqrt{s}=4.18$ GeV and $4.95$ GeV with subsequent decays of $τ^\pm$ into major leptonic or semihadronic modes. For the $τ$ decays to two pions and three charged pions we take the full kinematic information of the hadronic system into account. Applying cuts and using realistic assumptions on the eventually attainable integrated luminosities at these energies, ${\cal L}(4.18) = 3\times 10^4~{\rm pb}^{-1}$ and ${\cal L}(4.95) = 10^4~{\rm pb}^{-1}$, respectively, we find the following. By taking into account purely semihadronic and semihadronic-leptonic $τ^+τ^-$ decays one can achieve with optimal $CP$-odd observables the 1 s.d. sensitivities $δ{\rm Re} d_τ= 4.5\times 10^{-18}$ ecm ($5.3\times 10^{-18}$ ecm) and $δ{\rm Im} d_τ= 2.2 \times 10^{-18}$ ecm ($2.7\times 10^{-18}$ ecm) at $\sqrt{s}=4.18$ GeV ($4.95$ GeV).

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arXiv2021

The electric dipole moment of the tau lepton revisited

We reconsider the issue of the search for a nonzero electric dipole form factor (EDM) $d_τ(s)$ using optimal observables in $τ^+τ^-$ production by $e^+ e^-$ collisions in the center-of-mass energy range from the $τ$-pair threshold to about $\sqrt{s} \sim 15$ GeV. We discuss the general formalism of optimal observables and apply it to two $CP$-odd observables that are sensitive to the real and imaginary part of $d_τ(s)$, respectively. We compute the expectation values and covariances of these optimal $CP$ observables for $τ$-pair production at $\sqrt{s}=10.58$ GeV with subsequent decays of $τ^\pm$ into major leptonic or semihadronic modes. For the $τ$ decays to two pions and three charged pions we take the full kinematic information of the hadronic system into account. Assuming that the Belle II experiment at the KEKB accelerator will eventually analyze data corresponding to an integrated luminosity of 50 ab$^{-1}$ and applying acceptance cuts on the final-state pions we find that 1~s.d. sensitivities $δ{\rm Re} d_τ= 6.8 \times 10^{-20}$ e cm and $δ{\rm} Im d_τ= 4.0 \times 10^{-20}$ e cm can be obtained with events where both $τ$'s decay semihadronically. In the ideal case where no cuts on the final-state particles are applied we find with 50 ab$^{-1}$ at $\sqrt{s}=10.58$ GeV corresponding to $4.5 \times 10^{10}$ $τ^+ τ^-$ events the 1 s.d. sensitivities $δ{\rm Re} d_τ= 5.8 \times 10^{-20}$ e cm and $δ{\rm} Im d_τ= 3.2 \times 10^{-20}$ e cm, again for events where both $τ$ leptons decay semihadronically. Furthermore, we analyze the potential magnitude of the $τ$ EDM form factor in the type-II two-Higgs doublet extension and in two scalar leptoquark extensions of the Standard Model.

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arXiv2020

Polarized $q \bar{q} \rightarrow Z +$Higgs amplitudes at two loops in QCD: the interplay between vector and axial vector form factors and a pitfall in applying a non-anticommuting $γ_5$

We consider QCD corrections to two loops for the polarized amplitudes of $q{\bar q}\to Z +$ Higgs boson. First we show how the polarized amplitudes of $b \bar{b} \rightarrow Z h$ associated with a non-vanishing $b$-quark Yukawa coupling and a scalar or pseudoscalar Higgs boson $h$ can be built up solely from vector form factors (FF) of properly grouped classes of diagrams, bypassing completely the need of explicitly manipulating $γ_5$ in dimensional regularization (up to a few "anomalous", i.e., triangle diagrams). We determine the contributions of the triangle diagrams in the heavy top limit. We present the analytic results of the vector FF and the triangle-diagram contributions to the axial vector FF, which are sufficient for deriving the two-loop QCD amplitudes for $b \bar{b} \rightarrow Z h$ with a CP-even and CP-odd Higgs boson $h$. We derive the respective Ward identity for these amplitudes, which are subsequently verified to two-loop order in QCD using these FF. In addition, the FF of a class of corrections to $q \bar{q} \rightarrow ZH$ proportional to the top-Yukawa coupling are obtained analytically to two-loop order in QCD in the heavy-top limit using the Higgs-gluon effective Lagrangian where the top quark is integrated out. We address a pitfall that occurs when applying the non-anticommutating $γ_5$ prescription to this class of contributions that has been overlooked so far in the literature. We attribute this issue to the fact that the absence of certain heavy-mass expanded diagrams in the infinite-mass limit of a scattering amplitude with an axial vector current depends on the particular $γ_5$ prescription in use.

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arXiv2018

Differential decay rates of CP-even and CP-odd Higgs bosons to top and bottom quarks at NNLO QCD

We consider the decay of a neutral Higgs boson of arbitrary CP nature to a massive quark antiquark pair at next-to-next-to-leading order in perturbative QCD. Our analysis is made at the differential level using the antenna subtraction framework. We apply our general set-up to the decays of a CP-even and CP-odd heavy Higgs boson to a top-quark top-antiquark pair and to the decay of the 125 GeV Higgs boson to a massive bottom-quark bottom-antiquark pair. In the latter case we calculate, in particular, the two-jet, three-jet, and four-jet decay rates and, for two-jet events, the energy distribution of the leading jet.

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arXiv2017

Production of heavy Higgs bosons and decay into top quarks at the LHC. II: Top-quark polarization and spin correlation effects

We analyze, within several parameter scenarios of type-II two-Higgs doublet extensions of the standard model, the impact of heavy neutral Higgs-boson resonances on top-quark pair production and their subsequent decay to dileptonic final states at the LHC (13 TeV). In particular, we investigate the effects of heavy Higgs bosons on top-spin observables, that is, the longitudinal top-quark polarization and top-quark spin correlations. We take into account NLO QCD as well as weak interaction corrections and show that top-spin observables, if evaluated in judiciously chosen top-quark pair invariant mass bins, can significantly enhance the sensitivity to heavy Higgs resonances in top-quark pair events.

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arXiv2016

Heavy Higgs boson production and decay into top quarks at the LHC

In this contribution we report on the calculation of the next-to-leading order (NLO) QCD corrections to the hadro-production of heavy neutral Higgs bosons and their decay into top-quark pairs within the type-II two-Higgs-doublet extension of the standard model (SM). We take into account the contributions from resonant Higgs boson production, the non-resonant SM $t\bar{t}$ background as well as the interference of these two contributions. The NLO corrections to the signal and interference contributions are calculated by applying the heavy top-quark mass ($m_t$) limit including an effective rescaling. In our NLO calculation the QCD-Higgs interference is evaluated in the resonance region that provides the dominant part of the heavy Higgs-boson contributions. Evaluating representative $CP$-conserving and $CP$-violating parameter scenarios within the two-Higgs-doublet model (2HDM) we present results for different distributions and, in addition, for observables that depend on the top-quark spin.

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arXiv2016

The forward-backward asymmetry for massive bottom quarks at the $Z$ peak at next-to-next-to-leading order QCD

We compute the order $α_s^2$ QCD corrections to the $b$-quark forward-backward asymmetry in $e^+e^-\to b{\bar b}$ collisions at the $Z$ boson resonance, taking the non-zero mass of the $b$ quark into account. We determine these corrections with respect to both the $b$-quark axis and the thrust axis definition of the asymmetry. We compute also the distributions of these axes with respect to the electron beam. If one neglects the flavor singlet contributions to the $b$-quark asymmetry, as was done in previous computations for massless $b$ quarks, then the second-order QCD corrections for $m_b\neq 0$ are smaller in magnitude than the corresponding corrections for $m_b=0$. Including the singlet contributions slightly increases the magnitude of the corrections. The massive $α_s^2$ corrections to the $b$-quark forward-backward asymmetry slightly diminish the well-known tension between the bare $b$-quark asymmetry and the standard model fit from $2.9σ$ to $2.6σ$.

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arXiv2016

Top-quark pair production at next-to-next-to-leading order QCD in electron positron collisions

We set up a formalism, within the antenna subtraction framework, for computing the production of a massive quark-antiquark pair in electron positron collisions at next-to-next-to-leading order in the coupling $α_s$ of quantum chromodynamics at the differential level. Our formalism applies to the calculation of any infrared-safe observable. We apply this set-up to the production of top-quark top antiquark pairs in the continuum. We compute the production cross section and several distributions. We determine, in particular, the top-quark forward-backward asymmetry at order $α_s^2$. Our result agrees with previous computations of this observable.

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arXiv2016

Heavy Higgs boson resonances and their decay into top quarks at the LHC

We investigate, within the type-II two-Higgs-doublet extension of the standard model (SM), the impact of heavy neutral Higgs boson resonances with unsuppressed Yukawa couplings to top quarks on top-quark pair production at the LHC at next-to-leading order (NLO) in the strong coupling constant. We take into account the resonant Higgs boson contributions, the non-resonant SM $t\bar{t}$ continuum and the interference of these two contributions. The NLO QCD corrections to heavy Higgs production and the interference contributions are calculated in the large top-quark mass ($m_t$) limit, including an effective K-factor rescaling. Our evaluation of the QCD-Higgs interference is focused on the Higgs resonance region. Using representative CP-conserving as well as CP-violating parameter scenarios phenomenological results are presented for different observables.

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arXiv2016

Improved effective vector boson approximation revisited

We reexamine the improved effective vector boson approximation which is based on two-vector-boson luminosities $\mathrm{\mathbf{L}}_{\rm pol}$ for the computation of weak gauge-boson hard scattering subprocesses $V_1 V_2\to {\cal W}$ in high-energy hadron-hadron or $e^-e^+$ collisions. We calculate these luminosities for the nine combinations of the transverse and longitudinal polarizations of $V_1$ and $V_2$ in the unitary and axial gauge. For these two gauge choices the quality of this approach is investigated for the reactions $e^-e^+ \to W^- W^+ ν_e {\barν}_e$ and $e^-e^+ \to t {\bar t} ν_e {\barν}_e$ using appropriate phase-space cuts.

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arXiv2015

A set of top quark spin correlation and polarization observables for the LHC: Standard Model predictions and new physics contributions

We consider top-antitop quark ($t{\bar t}$) production at the Large Hadron Collider (LHC) with subsequent decay into dileptonic and lepton plus jets final states. We present a set of leptonic angular correlations and distributions with which experiments can probe all the independent coefficient functions of the top-spin dependent parts of the $t{\bar t}$ production spin density matrices. We compute these angular correlations and distributions for LHC center-of-mass energies 8, 13, and 14 TeV within the Standard Model at next-to-leading order in the QCD coupling including the mixed QCD-weak corrections and for the transverse top-quark polarization and the $t{\bar t}$ charge asymmetry also the mixed QCD-QED corrections. In addition we analyze and compute the effects of new interactions on these observables in terms of a gauge-invariant effective Lagrangian which contains all operators relevant for hadronic $t{\bar t}$ production up to mass dimension six.

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arXiv2015

Prospects of constraining the Higgs CP nature in the tau decay channel at the LHC

We investigate how precisely the CP nature of the 125 GeV Higgs boson, parametrized by a scalar-pseudoscalar Higgs mixing angle, can be determined in Higgs-to-tau-pair decay with subsequent tau-lepton decays to charged prongs at the Large Hadron Collider (LHC). We combine two methods in order to define an observable which is sensitive to this scalar-pseudoscalar mixing angle: We use the rho-decay plane method for tau-to-rho decays and the impact parameter method for all other major tau decays. For estimating the precision with which the mixing angle can be measured at the LHC (13 TeV) we take into account the background from Drell-Yan production and perform a Monte Carlo simulation of measurement uncertainties on the signal and background distributions. We obtain that the mixing angle can be determined with an uncertainty of 15 degree (9 degree) at the LHC with an integrated luminosity of 150 inverse fb (500 inverse fb), and with ~4 degree with 3 inverse ab. Future measurements of the scalar-pseudoscalar mixing angle yield direct information on whether or not there is an extended Higgs-boson sector with Higgs-sector CP violation. We analyze this in the context of a number of two-Higgs-doublet extensions of the Standard Model, namely the so-called aligned model and conventional two-Higgs-doublet extensions with tree-level neutral flavor conservation.

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arXiv2015

Top quark spin correlations and polarization at the LHC: standard model predictions and effects of anomalous top chromo moments

A number of top-spin observables are computed within the Standard Model (SM), at next-to-leading order in the strong and weak gauge couplings for hadronic top-quark anti-quark (ttbar) production and decay at the LHC for center-of-mass energies 7 and 8 TeV. For dileptonic final states we consider the azimuthal angle correlation, the helicity correlation, and the opening angle distribution; for lepton plus jets final states we determine distributions and asymmetries that trace a longitudinal and transverse polarization, respectively, of the t and t-bar samples. In addition, we investigate the effects of a non-zero chromo-magnetic and chromo-electric dipole moment of the top quark on these and other top-spin observables and associated asymmetries. These observables allow to disentangle the contributions from the real and imaginary parts of these moments.

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arXiv2014

Determination of the Higgs CP mixing angle in the tau decay channels at the LHC including the Drell-Yan background

We investigate how precisely the CP nature of the 125 GeV Higgs boson resonance h can be unraveled at the LHC in its decays to tau pairs. We use a method which allows to determine the scalar-pseudoscalar Higgs mixing angle in this decay mode. This mixing angle can be extracted from the distribution of a signed angle, which we analyze for the major charged-prong tau decays. For definiteness, we consider Higgs-boson production by gluon fusion at NLO QCD. We take into account also the irreducible background from Drell-Yan production at NLO QCD. We compute, for the signal and background reactions, angular and energy correlations of the charged prongs and analyze which type of cuts suppress the Drell-Yan background. An important feature of this background is that its contribution to the distribution of our observable is a flat line, also at NLO QCD. By separating the Drell-Yan events into two different sets, two different non-trivial distributions are obtained. Based on this observation we propose to use these sets for calibation purposes. By Monte Carlo simulation we study also the effect of measurement uncertainties on this distribution. We estimate that the Higgs mixing angle can be determined with our method to a precision of 14 degree (5 degree) at the high luminosity LHC (14 TeV) with an integrated luminosity of 500 inverse fb (3 inverse ab).

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arXiv2014

Determination of the Higgs CP-mixing angle in the tau decay channels

The measurement of possible Higgs sector CP-violation in the tau decay channels at the LHC is investigated. A CP-violating effect would manifest itself in these decay modes in characteristic spin-spin correlations of the tau lepton pairs which can be accessed using the momenta and impact parameters of the charged tau decay particles. We examine a CP-sensitive observable for a 125 GeV Higgs boson resonance in the gluon fusion channel at the LHC. Furthermore, we consider the distribution of this observable for the irreducible Drell-Yan background. By splitting these events into two categories we obtain two different distributions which can be used for calibration purposes. Finally, we estimate the achievable precision of the scalar-pseudo-scalar mixing angle of the tau decay channel for Run II and the high luminosity run of the LHC.

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arXiv2014

The real-virtual antenna functions for $S \to Q\bar{Q} X$ at NNLO QCD

We determine, in the antenna subtraction framework for handling infrared divergences in higher order QCD calculations, the real-virtual antenna functions for processes involving the production of a pair of massive quarks by an uncolored initial state at NNLO QCD. The integrated leading and subleading color real-virtual antenna functions are computed analytically in terms of (cyclotomic) harmonic polylogarithms. As a by-product and check we compute $R_Q=σ(e^+e^-\to γ^*\to Q\bar{Q}X)/σ(e^+e^-\to γ^*\toμ^+μ^-)$ and compare with existing results. Our result for $R_Q$ is exact to order $α_s^2$.

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