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Thomas Teubner

Publications and source records attributed to Thomas Teubner.

29 records · Page 2Linked to original sources
arXiv2018

Fully-differential Top-Pair Production at a Lepton Collider: From Threshold to Continuum

We present an approach to predict exclusive $W^+bW^-\bar{b}$ production at lepton colliders that correctly describes the top-anti-top threshold as well as the continuum region. We incorporate $t\bar{t}$ form factors for the NLL threshold resummation derived in NRQCD into a factorized relativistic cross section using an extended double-pole approximation, which accounts for fixed-order QCD corrections to the top decays at NLO. This is combined with the full fixed-order QCD result at NLO for $W^+bW^-\bar{b}$ production to obtain predictions that are not only valid at threshold but smoothly transition to the continuum region. Our implementation is based on the Monte Carlo event generator WHIZARD and the code TOPPIK and allows to compute fully-differential threshold-resummed cross sections including the interference with non-resonant background processes. For the first time it is now possible to systematically study general differential observables at future lepton colliders involving the decay products of the top quarks at energies close to the pair production threshold and beyond.

hep-ph↗
arXiv2018

The hadronic vacuum polarisation contributions to the muon $g-2$

The hadronic vacuum polarisation contributions to the anomalous magnetic moment of the muon, $a_μ^{\rm had, VP}$ have been re-evaluated from the combination of $e^+e^-\rightarrow {\rm hadrons}$ cross section data. Focus has been placed on the development of a new data combination method, which fully incorporates all correlated statistical and systematic uncertainties in a bias free approach. Using these combined data have resulted in estimates of the hadronic vacuum polarisation contributions to $g-2$ of the muon of $a_μ^{\rm had, \, LO \, VP} = (693.27 \pm 2.46)\times 10^{-10}$ and $a_μ^{\rm had, \, NLO \, VP} = (-9.82 \pm 0.04)\times 10^{-10}$. The new estimate for the Standard Model prediction is found to be $a_μ^{\rm SM} = (11\ 659 \ 182.05 \pm 3.56) \times 10^{-10}$, which is $3.7σ$ below the current experimental measurement. In addition, the prediction for the hadronic contribution to the QED coupling at the $Z$ boson mass has been calculated to be $Δα_{\rm had}^{(5)}(M_Z^2)= (276.11 \pm 1.11)\times 10^{-4}$, resulting in $α^{-1}(M_Z^2) = 128.946 \pm 0.015$.

hep-ph↗
arXiv2018

Exclusive top production at a Linear Collider at and off the threshold

We review exclusive top pair production including decays at a future high-energy lepton collider, both in the threshold region and for higher energies. For the continuum process, we take complete QCD next-to-leading order matrix elements for the $2\to 6$ process with leptonic W decays into account. At threshold, we match the fixed-order relativistic QCD-NLO cross section to a nonrelativistic cross section with next-to-leading logarithmic (NLL) threshold resummation implemented via a form factor.

hep-ph↗
arXiv2014

The strange and charm quark contributions to the anomalous magnetic moment of the muon from lattice QCD

We describe a new technique (published in Phys. Rev. D89 114501) to determine the contribution to the anomalous magnetic moment of the muon coming from the hadronic vacuum polarisation using lattice QCD. Our method uses Padé approximants to reconstruct the Adler function from its derivatives at $q^2=0$. These are obtained simply and accurately from time-moments of the vector current-current correlator at zero spatial momentum. We test the method using strange quark correlators calculated on MILC Collaboration's $n_f = 2+1+1$ HISQ ensembles at multiple values of the lattice spacing, multiple volumes and multiple light sea quark masses (including physical pion mass configurations). We find the (connected) contribution to the anomalous moment from the strange quark vacuum polarisation to be $a^s_μ=53.41(59)\times 10^{-10}$, and the contribution from charm quarks to be $a^c_μ=14.42(39)\times 10^{-10}$ - 1% accuracy is achieved for the strange quark contribution. The extension of our method to the light quark contribution and to that from the quark-line disconnected diagram is straightforward.

hep-lat↗
arXiv2013

The Muon (g-2) Theory Value: Present and Future

This White Paper briefly reviews the present status of the muon (g-2) Standard-Model prediction. This value results in a 3 - 4 standard-deviation difference with the experimental result from Brookhaven E821. The present experimental uncertainty is $\pm 63 \times 10^{-11}$ (0.54~ppm), and the Standard-Model uncertainty is $\simeq \pm 49 \times 10^{-11}$. Fermilab experiment E989 has the goal to reduce the experimental error to $\pm 16 \times 10^{-11}$. Improvements in the Standard-Model value, which should be achieved between now and when the first results from Fermilab E989 could be available, should lead to a Standard-Model uncertainty of $\sim \,\pm 35 \times 10^{-11}$. These improvements would halve the uncertainty on the difference between experiment and theory, and should clarify whether the current difference points toward New Physics, or to a statistical fluctuation. At present, the (g-2) result is arguably the most compelling indicator of physics beyond the Standard Model and, at the very least, it represents a major constraint for speculative new theories such as supersymmetry, dark gauge bosons or extra dimensions.

hep-ph↗
arXiv2012

Hadronic contributions to the anomalous magnetic moment of the electron and the hyperfine splitting of muonium

Motivated by recent progress of theory and experiment on the anomalous magnetic moment of the electron, a_e, we update the hadronic contributions to a_e. Using our up-to-date compilation of e^+ e^- \to hadrons data, we find the leading order hadronic contribution a_e^{had,LO,VP} = (1.866 \pm 0.010_{exp} \pm 0.005_{rad}) \cdot 10^{-12} and the next-to-leading order hadronic contribution a_e^{had,NLO,VP} = (- 0.2234 \pm 0.0012_{exp} \pm 0.0007_{rad}) \cdot 10^{-12}, where the first and second errors are from the error of the experimental data and the uncertainty in the treatment of radiative corrections, respectively. These values are compatible with earlier evaluations by other groups, but have significantly improved uncertainties due to the more precise input data used. We also update the leading order hadronic contribution to the ground state hyperfine splitting of muonium, obtaining Δν_{Mu}^{had,VP} = (232.68 \pm 1.25_{exp} \pm 0.72_{rad}) Hz. This value is consistent with the most precise evaluation in the literature and reduces its error by a factor of two.

hep-ph↗
arXiv2007

Determination of the strong coupling constant from the CLEO measurement of the total hadronic cross section in $e^+e^-$ annihilation below 10.56 GeV

Using recent CLEO III results for the cross section for $e^+e^- \to {\rm hadrons}$ at seven centre-of-mass energies between 6.964 and 10.538 GeV, we derive a value for the strong coupling constant $α_s(M_Z^2)=0.110_{-0.012}^{-0.010}{}_{-0.011}^{+0.010}$ where the uncertainties are uncorrelated and correlated, respectively. Our result differs significantly from the one derived by CLEO III, as a consequence of inclusion of quark mass effects and the proper matching between the effective theories with four and five flavours. Combining this new result with an analysis based on earlier cross section measurements in the energy region between 2 and 10.6 GeV, we obtain $α_s(M_Z^2)=0.119^{+0.009}_{-0.011}$, well consistent with the current world average.

hep-ph↗
arXiv1999

Top Quark Physics

In this contribution I review the physics of top quarks at a future Linear Collider. Main emphasis is put on the process e+ e- to ttbar close to threshold. Different physical observables, their sensitivity to the basic parameters and their theoretical prediction are discussed. Recent higher order calculations are shown to have a considerable impact on a precise determination of the top quark mass. It is pointed out how the use of mass definitions different from the pole mass scheme become important in this respect. Continuum top quark production above threshold is discussed briefly.

hep-ph↗
arXiv1993

Radiative Corrections to the top quark width

Calculations of radiative corrections to the top quark width are reviewed. QCD effects are discussed for $t-\bar t$ systems produced in $e^+e^-$ annihilation near the energy threshold.}

hep-ph↗