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J. H. Kühn

Publications and source records attributed to J. H. Kühn.

At least 37 records · Page 2Linked to original sources
arXiv2008

Hadronic Z- and tau-Decays in Order alpha_s^4

Using recently developed methods for the evaluation of five-loop amplitudes in perturbative QCD, corrections of order alpha_s^4 for the cross section of electron-positron annihilation into hadrons and for the decay rates of the Z-boson and the tau-lepton into hadrons are evaluated. The new terms lead to a significant stabilization of the perturbative series, to a reduction of the theory uncertainly in the strong coupling constant alpha_s, as extracted from these measurements, and to a small shift of the central value, moving two central values closer together. The agreement between two values of alpha_s measured at vastly different energies constitutes a striking test of asymptotic freedom. Combining the results from Z and tau decays we find alpha_s(M_Z)=0.1198 \pm 0.0015 as one of the most precise and presently only result for the strong coupling constant in order alpha_s^4.

hep-ph↗
arXiv2006

Four-Loop QCD Corrections to the Rho Parameter

The four-loop QCD corrections to the electroweak $ρ$-parameter arising from top and bottom quark loops are computed. Specifically we evaluate the missing ``non-singlet'' piece. Using algebraic methods the amplitude is reduced to a set of around 50 new master integrals which are calculated with various analytical and numerical methods. Inclusion of the newly completed term halves the final value of the four-loop correction for the minimally renormalized top-quark mass. The predictions for the shift of the weak mixing angle and the W-boson mass is thus stabilized.

hep-ph↗
arXiv2006

Recent progress in computing four-loop massive correlators

We report about recent progress in computing four-loop massive correlators. The expansion of these correlators in the external momentum leads to vacuum integrals. The calculation of these vacuum integrals can be used to determine Taylor expansion coefficients of the vacuum polarization function and decoupling functions in perturbative Quantum chromodynamics. New results at four-loop order for the lowest Taylor expansion coefficient of the vacuum polarization function and for the decoupling relation are presented.

hep-ph↗
arXiv2005

QCD Decoupling at Four Loops

We present the matching condition for the strong coupling contant alpha_s at a heavy quark threshold to four loops in the modified minimal subtraction scheme. Our results lead to further decrease of the theoretical uncertainty of the evolution of the strong coupling constant through heavy quark thresholds. Using a low energy theorem we furthermore derive the effective coupling of the Higgs boson to gluons (induced by a virtual heavy quark) in four- and (partially) through five-loop approximation.

hep-ph↗
arXiv2005

Strange Quark Mass from Tau Lepton Decays with O(alpha_s^3) Accuracy

The first complete calculation of the quadratic quark mass correction to the correlator of the two currents relevant for the strangeness-changing semihadronic tau-decay rate is presented including its real part at the four-loop level. This allows to perform the extraction of the strange quark mass m_s from the decay width of the tau-lepton with full O(alpha_s^3) accuracy. In agreement with previous estimates the newly computed alpha_s^3 term proves to be rather large. This justifies inclusion of the similarly estimated alpha_s^4 term in phenomenological analysis. Combined with an updated value of V_{us} and an ``'improved'' version of the renormalization group improvement of the perturbative series this leads to an increase of the central value of m_s by about 20 % and a partial reduction of the theoretical uncertainty by about 50 %.

hep-ph↗
arXiv2005

Next-to-Next-to-Leading Logarithms in Four-Fermion Electroweak Processes at High Energy

We sum up the next-to-next-to-leading logarithmic virtual electroweak corrections to the high energy asymptotics of the neutral current four-fermion processes for light fermions to all orders in the coupling constants using the evolution equation approach. From this all order result we derive finite order expressions through next-to-next-to leading order for the total cross section and various asymmetries. We observe an amazing cancellation between the sizable leading, next-to-leading and next-to-next-to-leading logarithmic contributions at TeV energies.

hep-ph↗
arXiv2005

Fermionic and Scalar Corrections for the Abelian Form Factor at Two Loops

Two-loop corrections for the form factor in a massive Abelian theory are evaluated, which result from the insertion of massless fermion or scalar loops into the massive gauge boson propagator. The result is valid for arbitrary energies and gauge boson mass. Power-suppressed terms vanish rapidly in the high energy region where the result is well approximated by a polynomial of third order in ln(s/M^2). The relative importance of subleading logarithms is emphasised.

hep-ph↗
arXiv2003

QCD corrections to single top quark production in electron photon interactions

Single top quark production in electron photon interactions provides a clean environment for the measurement of the Cabbibo-Kobayashi-Maskawa matrix element Vtb. Aiming an experimental precision at the percent level the knowledge of radiative corrections is important. In this paper we present results for the radiative corrections in quantum chromodynamics.

hep-ph↗
arXiv2003

Five-loop vacuum polarization in pQCD: O(m_q^2α_s^4 n_f^2) contribution

We present the analytical calculation of the contribution of order m_q^2 α_s^4 n_f^2 to the absorptive part of the vacuum polarization function of vector currents. This term constitutes an important gauge-invariant part of the full O(m_q^2 alpha_s^4) correction to the total cross-section of e^+ e^- annihilation into hadrons. The results are compared to predictions following from various optimization schemes.

hep-ph↗
arXiv2003

Heavy mass expansion, light-by-light scattering and the anomalous magnetic moment of the muon

Contributions from light-by-light scattering to (g_μ-2)/2, the anomalous magnetic moment of the muon, are mediated by the exchange of charged fermions or scalar bosons. Assuming large masses M for the virtual particles and employing the technique of large mass expansion, analytical results are obtained for virtual fermions and scalars in the form of a series in (m_μ/M)^2. This series is well convergent even for the case M=m_μ. For virtual fermions, the expansion confirms published analytical formulae. For virtual scalars, the result can be used to evaluate the contribution from charged pions. In this case our result confirms already available numerical evaluations, however, it is significantly more precise.

hep-ph↗
arXiv2002

Towards Order alpha_s^4 Accuracy in tau-decays

Recently computed terms of orders O(α_s^4 n_f^2) in the perturbative series for the tau decay rate, and similar (new) strange quark mass corrections, are used to discuss the validity of various optimization schemes. The results are then employed to arrive at improved predictions for the complete terms order O(α_s^4) and O(α_s^5) in the massless limit as well as for terms due to the strange quark mass. Phenomenological implications are presented.

hep-ph↗
arXiv2002

The Impact of $σ(e^+e^-\to {hadrons})$ Measurements at Intermediate Energies on the Parameters of the Standard Model

We discuss the impact of precision measurements of $σ(e^+e^-\to {hadrons})$ in the center-of-mass range between 3 and 12 GeV, including improvements in the electronic widths of the narrow charmonium and bottonium resonances, on the determination of parameters of the Standard Model. In particular we discuss the impact of potential improvements on the extraction of the strong coupling constant $α_s$, on the evaluation of the hadronic contributions to the electromagnetic coupling $α(M_Z)$, and the determination of the charm and bottom quark masses.

hep-ph↗
arXiv2002

Determination of $α_s$ and heavy quark masses from recent measurements of $R(s)$

In this paper we compare recent experimental data for the total cross section $σ(e^+e^-\to{hadrons})$ with the up-to-date theoretical prediction of perturbative QCD for those energies where perturbation theory is reliable. The excellent agreement suggests the determination of the strong coupling $α_s$ from the measurements in the continuum. The precise data from the charm threshold region, when combined with the recent evaluation of moments with three loop accurracy, lead to a direct determination of the short distance $\bar{\rm MS}$ charm quark mass. Our result for the strong coupling constant $α_s^{(4)}(5 {GeV})=0.235^{+0.047}_{-0.047}$ corresponds to $α_s^{(5)}(M_Z)=0.124^{+0.011}_{-0.014}$, for the charmed quark mass we find $m_c(m_c)=1.304(27)$. Applying the same approach to the bottom quark we obtain $m_b(m_b)=4.191(51)$ GeV. Whereas our result for $α_s(M_Z)$ serves as a useful cross check for other more precise determinations, our values for the charm and bottom quark masses are more accurate than other recent analyses.

hep-ph↗
arXiv2001

The Cross Section of e^+ e^- Annihilation into Hadrons of Order alpha_s^4 n_f^2 in Perturbative QCD

We present the first genuine QCD five-loop calculation of the vacuum polarization functions: analytical terms of order alpha_s^4 n_f^2 to the absorptive parts of vector and scalar correlators. These corrections form an important gauge-invariant subset of the full O(alpha_s^4) correction to $e^+ e^- annihilation into hadrons and the Higgs decay rate into hadrons respectively. They discriminate between different widely used estimates of the full result.

hep-ph↗
arXiv2000

Subleading Sudakov Logarithms in Electroweak Processes

Recent results for the asymptotic behavior of fermion scattering amplitudes in the Sudakov limit are presented including next-to-leading logarithmic corrections. These are used for the analysis of the dominant electroweak corrections to the fermion-antifermion pair production in $e^+e^-$ annihilation at high energy.

hep-ph↗