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W. Bernreuther

Publications and source records attributed to W. Bernreuther.

At least 19 recordsLinked to original sources
arXiv2017

CP-violating top quark couplings at future linear $e^+e^-$ colliders

We study the potential of future lepton colliders to probe violation of the CP symmetry in the top quark sector. In certain extensions of the Standard Model, such as the two-Higgs-doublet model (2HDM), sizeable anomalous top quark dipole moments can arise, that may be revealed by a precise measurement of top quark pair production. We present results from detailed Monte Carlo studies for the ILC at 500~\GeV{} and CLIC at 380~\gev{} and use parton-level simulations to explore the potential of high-energy operation. We find that precise measurements in $e^+e^- \rightarrow t\bar{t}$ production with subsequent decay to lepton plus jets final states can provide sufficient sensitivity to detect Higgs-boson-induced CP violation in a viable two-Higgs-doublet model. The potential of a linear $e^+e^-$ collider to detect CP-violating electric and weak dipole form factors of the top quark exceeds the prospects of the HL-LHC by over an order of magnitude.

hep-ex↗
arXiv2017

New method for a continuous determination of the spin tune in storage rings and implications for precision experiments

A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune - defined as the number of spin precessions per turn - is given by $ν_s = γG$ (gamma is the Lorentz factor, $G$ the magnetic anomaly). For 970 MeV/c deuterons coherently precessing with a frequency of ~120 kHz in the Cooler Synchrotron COSY, the spin tune is deduced from the up-down asymmetry of deuteron carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the order $10^{-8}$, and to $1 \cdot 10^{-10}$ for a continuous 100 s accelerator cycle. This renders the presented method a new precision tool for accelerator physics: controlling the spin motion of particles to high precision is mandatory, in particular, for the measurement of electric dipole moments of charged particles in a storage ring.

physics.acc-ph↗
arXiv2016

Production of heavy Higgs bosons and decay into top quarks at the LHC

We investigate the production of heavy, neutral Higgs boson resonances and their decays to top-quark top-antiquark ($t\bar{t}$) pairs at the Large Hadron Collider (LHC) at next-to-leading order (NLO) in the strong coupling of quantum chromodynamics (QCD). The NLO corrections to heavy Higgs boson production and the Higgs-QCD interference are calculated in the large $m_t$ limit with an effective K-factor rescaling. The nonresonant $t\bar{t}$ background is taken into account at NLO QCD including weak-interaction corrections. In order to consistently determine the total decay widths of the heavy Higgs bosons, we consider for definiteness the type-II two-Higgs-doublet extension of the standard model and choose three parameter scenarios that entail two heavy neutral Higgs bosons with masses above the $t\bar{t}$ threshold and unsuppressed Yukawa couplings to top quarks. For these three scenarios we compute, for the LHC operating at 13 TeV, the $t\bar{t}$ cross section and the distributions of the $t\bar{t}$ invariant mass, of the transverse top-quark momentum and rapidity, and of the cosine of the Collins-Soper angle with and without the two heavy Higgs resonances. For selected $M_{t\bar{t}}$ bins we estimate the significances for detecting a heavy Higgs signal in the $t\bar{t}$ dileptonic and lepton plus jets decay channels.

hep-ph↗
arXiv2014

Unraveling models of CP violation through electric dipole moments of light nuclei

We show that the proposed measurements of the electric dipole moments of light nuclei in storage rings would put strong constraints on models of flavor-diagonal CP violation. Our analysis is exemplified by a comparison of the Standard Model including the QCD theta term, the minimal left-right symmetric model, a specific version of the so-called aligned two-Higgs doublet model, and briefly the minimal supersymmetric extension of the Standard Model. By using effective field theory techniques we demonstrate to what extend measurements of the electric dipole moments of the nucleons, the deuteron, and helion could discriminate between these scenarios. We discuss how measurements of electric dipole moments of other systems relate to the light-nuclear measurements.

hep-ph↗
arXiv2014

Decays of polarized top quarks to lepton, neutrino and jets at NLO QCD

We compute the differential and total rate of the semileptonic decay of polarized top-quarks $t\to \ell ν_\ell + b{\rm jet} + {\rm jet}$ at next-to-leading order (NLO) in the QCD coupling with an off-shell intermediate $W$ boson. We present several normalized distributions, in particular those that reflect the $t$-spin analyzing powers of the lepton, the b-jet and the $W^+$ boson at LO and NLO QCD.

hep-ph↗
arXiv2012

How to pin down the CP quantum numbers of a Higgs boson in its tau decays at the LHC

We investigate how the CP quantum numbers of a neutral Higgs boson or spin-zero resonance, produced at the CERN Large Hadron Collider, can be determined in its tau-pair decay mode. We use a method developed in an earlier paper based on the distributions of two angles and apply it to the major 1-prong tau decays. We show for the resulting dilepton, lepton-pion, and two-pion final states that appropriate selection cuts significantly enhance the discriminating power of these observables. From our analysis we conclude that, provided a Higgs boson will be found at the LHC, it appears feasible to collect the event numbers needed to discriminate between a CP-even and -odd Higgs boson and/or between Higgs boson(s) with CP-conserving and CP-violating couplings after several years of high-luminosity runs.

hep-ph↗
arXiv2006

CP Studies and Non-Standard Higgs Physics

There are many possibilities for new physics beyond the Standard Model that feature non-standard Higgs sectors. These may introduce new sources of CP violation, and there may be mixing between multiple Higgs bosons or other new scalar bosons. Alternatively, the Higgs may be a composite state, or there may even be no Higgs at all. These non-standard Higgs scenarios have important implications for collider physics as well as for cosmology, and understanding their phenomenology is essential for a full comprehension of electroweak symmetry breaking. This report discusses the most relevant theories which go beyond the Standard Model and its minimal, CP-conserving supersymmetric extension: two-Higgs-doublet models and minimal supersymmetric models with CP violation, supersymmetric models with an extra singlet, models with extra gauge groups or Higgs triplets, Little Higgs models, models in extra dimensions, and models with technicolour or other new strong dynamics. For each of these scenarios, this report presents an introduction to the phenomenology, followed by contributions on more detailed theoretical aspects and studies of possible experimental signatures at the LHC and other colliders.

hep-ph↗
arXiv2006

Two-Parton Contribution to the Heavy-Quark Forward-Backward Asymmetry in NNLO QCD

Forward-backward asymmetries, $A_{FB}^Q$, are important observables for the determination of the neutral-current couplings of heavy quarks in inclusive heavy quark production, $e^+ e^- \to γ^*, Z^* \to Q +X$. In view of the measurement perspectives on $A_{FB}^Q$ at a future linear collider, precise predictions of $A_{FB}^Q$ are required for massive quarks. We compute the contribution of the $Q \bar Q$ final state to $A_{FB}^Q$ to order $\as^2$ in the QCD coupling. We provide general formulae, and we show that this contribution to $A_{FB}^Q$ is infrared-finite. We evaluate these two-parton contributions for $b$ and $c$ quarks on and near the $Z$ resonance, and for $t$ quarks above threshold. Moreover, near the $t \bar t$ threshold we obtain, by expanding in the heavy-quark velocity $β$, an expression for $A_{FB}^{t \bar t}$ to order $\as^2$ and NNLL in $β$. This quantity is equal, to this order in $β$, to the complete forward-backward asymmetry $A_{FB}^t$.

hep-ph↗
arXiv2006

Heavy-Quark Form Factors and Threshold Cross Section at O(α_S^2)

During the last year, analytic expressions for the two-loop QCD corrections to the form factors for the vector, axial-vector, scalar and pseudo-scalar vertices involving a pair of heavy quarks, $Q \bar{Q}$, were calculated. The results are valid for arbitrary momentum transfer and mass of the heavy quarks. These form factors have a number of applications, including anomalous couplings, the $e^{+}e^{-} \to Q \bar Q$ cross section, and the forward-backward asymmetry of heavy quarks. Here the $Q {\bar Q}$ threshold cross section is presented with some new second order axial vector contributions.

hep-ph↗
arXiv2005

QCD Corrections to Static Heavy Quark Form Factors

Interactions of heavy quarks, in particular of top quarks, with electroweak gauge bosons are expected to be very sensitive to new physics effects related to electroweak symmetry breaking. These interactions are described by the so-called static form factors, which include anomalous magnetic moments and the effective weak charges. We compute the second-order QCD corrections to these static form factors, which turn out to be sizeable and need to be taken into account in searches for new anomalous coupling effects.

hep-ph↗
arXiv2005

Decays of Scalar and Pseudoscalar Higgs Bosons into Fermions: Two-loop QCD Corrections to the Higgs-Quark-Antiquark Amplitude

As a first step in the aim of arriving at a differential description of neutral Higgs boson decays into heavy quarks, $h \to Q {\bar Q}X$, to second order in the QCD coupling $α_S$, we have computed the $hQ{\bar Q}$ amplitude at the two-loop level in QCD for a general neutral Higgs boson which has both scalar and pseudoscalar couplings to quarks. This amplitude is given in terms of a scalar and a pseudoscalar vertex form factor, for which we present closed analytic expressions in terms of one-dimensional harmonic polylogarithms of maximum weight 4. The results hold for arbitrary four-momentum squared, $q^2$, of the Higgs boson and of the heavy quark mass, $m$. Moreover we derive the approximate expressions of these form factors near threshold and in the asymptotic regime $m^2/q^2 \ll 1$.

hep-ph↗
arXiv2005

Mixed QCD and weak corrections to top quark pair production at hadron colliders

The order $α_s^2 α$ mixed QCD and weak corrections to top quark pair production by quark antiquark annihilation are computed, keeping the full dependence on the $t$ and $\bar t$ spins. We determine the contributions to the cross section and to single and double top spin asymmetries at the parton level. These results are necessary ingredients for precise standard model predictions of top quark observables, in particular of top spin-induced parity-violating angular correlations and asymmetries at hadron colliders.

hep-ph↗
arXiv2005

Two-Loop QCD Corrections to the Heavy Quark Form Factors: Anomaly Contributions

We present closed analytic expressions for the order $α_s^2$ triangle diagram contributions to the matrix elements of the singlet and non-singlet axial vector currents between the vacuum and a quark-antiquark state. We have calculated these vertex functions for arbitrary momentum transfer and for four different sets of internal and external quark masses. We show that both the singlet and non-singlet vertex functions satisfy the correct chiral Ward identities. Using the exact expressions for the finite axial vector form factors, we check the quality and the convergence of expansions at production threshold and for asymptotic energies.

hep-ph↗
arXiv2004

Two-Loop QCD Corrections to the Heavy Quark Form Factors: Axial Vector Contributions

We consider the Z Q Qbar vertex to second order in the QCD coupling for an on-shell massive quark-antiquark pair and for arbitrary momentum transfer of the Z boson. We present closed analytic expressions for the two parity-violating form factors of that vertex at the two-loop level in QCD, excluding the contributions from triangle diagrams. These form factors are expressed in terms of 1-dimensional harmonic polylogarithms of maximum weight 4.

hep-ph↗
arXiv2004

Two-loop QCD Corrections to the Heavy Quark Form Factors: the Vector Contributions

We present closed analytic expressions of the electromagnetic vertex form factors for heavy quarks at the two-loop level in QCD for arbitrary momentum transfer. The calculation is carried out in dimensional regularization. The electric and magnetic form factors are expressed in terms of 1-dimensional harmonic polylogarithms of maximum weight 4.

hep-ph↗