arXiv subjects
Publications and source records attributed to W. Bernreuther.
We discuss a calculation of the next-to-leading order QCD corrections to the process $e^+e^-\to 3$ jets with massive quarks, and show numerical results for the three jet fraction and the differential two jet rate.
We analyze some signatures of neutral Higgs bosons, produced in high energy proton (anti)proton collisions, which decay primarily to top quarks. This channel is clouded by a large irreducible $t\bar t$ background. We investigate the $t\bar t$ invariant-mass distribution in the lepton+jets decay mode of $t\bar t$ and a $t\bar t$ spin-spin correlation in the dilepton decay mode. At the LHC such Higgs bosons can be detected in the $t\bar t$ invariant-mass spectrum, and the spin-spin correlation studied by us is also sensitive to a Higgs boson if this particle has a sizeable pseudoscalar component.
We compute the order $α_s$ QCD corrections to the density matrix for the decay of a neutral Higgs boson $ϕ$ with (in)definite CP parity into a quark antiquark pair, respectively the QED corrections for the decay into a pair of charged leptons. We classify and calculate single spin asymmetries and spin-spin correlations which are generated by the scalar and pseudoscalar Yukawa couplings. These spin effects can be traced in $ϕ\to τ^-τ^+$ and, for heavy Higgs bosons, in $ϕ\to t\bar{t}$. We also calculate resulting correlations among the final states and estimate, for the respective decay modes, the number of events needed to measure the Yukawa couplings with these correlations at the 3 sigma level.
We show that the CP-violating dipole form factors of the tau lepton can be of the order of $α/π$ in units of the length scale set by the inverse $Z$ boson mass. We propose a few observables which are sensitive to these form factors at LEP2 and higher e^+e^- collision energies.
Tests of CP violation by appropriate momentum correlations in $Z\to 3$ jets and in particular in $Z\to b\bar bX$ probe CP-violating effective couplings -- that manifest themselves as form factors -- which conserve the quark chirality and quark flavour. By giving two examples we show that such couplings can be induced at one-loop order in extensions of the Standard Model with CP violation beyond the Kobayashi-Maskawa phase. In one of the models we compute the chirality-conserving part of the CP-violating $Zb{\bar b}$-gluon amplitude for massless $b$ quarks, determine the resulting effective dimension $d=6$ couplings in the local limit, and discuss the possible size of the effects. Finally we show that in models with excited quarks the chiral-invariant CP-violating effective interactions could be quite large if appropriate couplings are of a size characteristic of a strong interaction
We discuss some issues arising in the search for CP violation in the decays Z -> tau+ tau- and Z -> b \bar b X. The form-factor and the effective Lagrangian approach to parametrize CP-violating effects are compared. We emphasize the interest to study both real and imaginary parts of CP-violating form factors like the weak dipole moment form factor d_tau^Z(m^2_Z) of the tau-lepton. We propose an 'optimal' way to search for CP violation when Z -> b \bar b X events have to be selected on a statistical basis from the total number of Z hadronic decays.
We report on an investigation of CP violation in (un)polarized $e^+e^-\to{\bar t}t$ resulting from an extended neutral Higgs sector or from the minimal SUSY extension of the Standard Model (SM). We consider c.m. energies from the ${\bar t}t$ threshold to the TeV range. In addition sensitivity estimates for CP-violating form factors of the top quark are made. Further we discuss the prospects of probing Higgs sector CP violationin $γγ\to{\bar t}t$.
Review of prospects for discovery of new physics signals at LEP2. The areas covered include SUSY, exotic fermions, BESS models, leptoquarks, virtual effects and CP violating observables.
We investigate for two-Higgs doublet models with intrinsic CP violation in the scalar potential CP-nonconserving effects in unpolarized $γγ\rightarrow t \bar{t} $ for a range of neutral Higgs boson masses which includes resonant $φ$ production and the subsequent decay of $φ\rightarrow t \bar{t}$. The importance of taking into account, even in the resonant case, the interference with the nonresonant background is shown. Further, we propose and calculate three asymmetries which efficiently trace CP-violating effects in $γγ\rightarrow t \bar{t} $ using semileptonic $t \bar{t}$ decays.
We discuss the effect of CP-violating $Zb\bar{b}$, $Zb\bar{b}G$ and $Zb\bar{b}γ$ couplings on the width $Γ(Z\rightarrow b\bar bX)$. The presence of such couplings leads in a natural way to an increase of this width relative to the prediction of the standard model. Various strategies of a direct search for such CP-violating couplings by using CP-odd observables are outlined. The number of $Z$ bosons required to obtain significant information on the couplings in this way is well within the reach of present LEP experiments.
The matching condition which determines the effect of a heavy quark threshold on the running of the QCD coupling $α_{\overline{MS}}$ is reviewed. The matching scale is arbitrary to some extent. However, this affects the value of $α_{\overline{MS}}$ away from the threshold region only marginally.
At future $γγ$ colliders copious production of $t \bar{t}$ pairs is possible. This would allow a detailed investigation of the interactions involving the top quark. We propose some orrelations which are sensitive to $t \bar{t}$ final state interactions and we compute the QCD and standard model Higgs boson contributions to these correlations. QCD induced transverse polarization of top quarks is found to be sizeable and measurable at a high-energy $e^+e^-$ collider with an integrated luminosity of 10 $({\rm fb})^{-1}$ which is converted into a photon collider by backscattering of laser photons.
We investigate the possibilities of searching for non-standard CP violation in $pp\to t\bar{t}X$ at multiple TeV collision energies. A general kinematic analysis of the underlying partonic production processes $gg\to t\bar{t}$ and $q\bar{q}\to t\bar{t}$ in terms of their density matrices is given. We evaluate the CP-violating parts of these matrices in two-Higgs doublet extensions of the standard model (SM) and give results for CP asymmetries at the parton level. We show that these asymmetries can be traced by measuring suitable observables constructed from energies and momenta of the decay products of $t$ and $\bar{t} $. We find CP-violating effects to be of the order of $10^{-3}$ and show that possible contaminations induced by SM interactions are savely below the expected signals.