arXiv subjects

M. Steinhauser

Publications and source records attributed to M. Steinhauser.

107 records · Page 6Linked to original sources

arXiv1997

Higgs Decay to Order α_s^4

We present in analytic form the three-loop O(α_s^2) correction to the H -> gg partial width of the standard-model Higgs boson with intermediate mass M_H << 2 M_t. Its knowledge is required because the O(α_s) correction is so sizeable that the theoretical prediction to this order is unlikely to be reliable. For M_H=100 GeV, the resulting QCD correction factor reads $1+(215/12)α_s^{(5)}(M_H)/π+150.419 (α_s^{(5)}(M_H)/π)^2 \approx 1+0.66+0.21$. The new three-loop correction increases the Higgs-boson hadronic width by an amount of order 1%.

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arXiv1997

Higgs Decay to Top Quarks at O(α_s^2)

Three-loop corrections to the scalar and pseudo-scalar current correlator are calculated. By applying the large momentum expansion mass terms up to order (m^2/q^2)^4 are evaluated analytically. As an application O(α_s^2) corrections to the decay of a scalar and pseudo-scalar Higgs boson into top quarks are considered. It is shown that for a Higgs mass not far above the $t\bar{t}$ threshold these higher order mass corrections are necessary to get reliable results.

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arXiv1997

Mass Corrections to the Vector Current Correlator

Three-loop QCD corrections to the vector current correlator are considered. The large momentum procedure is applied in order to evaluate mass corrections up to order $(m^2/q^2)^6$. The inclusion of the first seven terms to the ratio $R=σ(e^+e^- \to hadrons)/σ(e^+e^- \to μ^+μ^-)$ leads to reliable predictions from the high energy region down to relatively close to threshold.

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arXiv1997

Three-Loop O(alpha_s^2 G_F M_t^2) Corrections to Hadronic Higgs Decays

We calculate the top-quark-induced three-loop corrections of O(alpha_s^2 G_F M_t^2) to the Yukawa couplings of the first five quark flavours in the framework of the minimal standard model with an intermediate-mass Higgs boson, with mass M_H << 2M_t. The calculation is performed using an effective-Lagrangian approach implemented with the hard-mass procedure. As an application, we derive the O(alpha_s^2 G_F M_t^2) corrections to the H -> q q-bar partial decay widths, including the case q=b. The couplings of the Higgs boson to pairs of leptons and intermediate bosons being known to O(alpha_s^2 G_F M_t^2), this completes the knowledge of such corrections in the Higgs sector. We express the results both in the MS-bar and on-shell schemes of mass renormalization. We recover the notion that the QCD perturbation expansions exhibit a worse convergence behaviour in the on-shell scheme than they do in the MS-bar scheme.

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arXiv1996

Automatic Computation Of Three-Loop Two-Point Functions In Large Momentum Expansion

We discuss the calculation of two-point three-loop functions with an arbitrary number of massive propagators and one large external momentum. The relevant subdiagrams are generated automatically. The resulting massless two-point integrals and massive tadpoles are transformed on-line to FORM-expressions ready to be used by existing FORM packages which calculate them analytically. As an example we compute the quartic mass corrections to the photon polarization function.

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arXiv1996

Virtual Top-Quark Effects on the H->bb-bar Decay at Next-to-Leading Order in QCD

By means of a heavy-top-quark effective Lagrangian, we calculate the three-loop corrections of O(alpha_s^2 G_F M_t^2) to the H->bb-bar partial decay width of the standard-model Higgs boson with intermediate mass M_H<<2M_t. We take advantage of a soft-Higgs theorem to construct the relevant coefficient functions. We present our result both in the MS-bar and on-shell schemes of mass renormalization. The MS-bar formulation turns out to be favourable with regard to the convergence behaviour. We also test a recent idea concerning the naive non-abelianization of QCD.

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arXiv1996

Higher Moments of Heavy Quark Vacuum Polarization

We present analytical calculation of the first seven moments of the heavy quark vacuum polarization function in the three-loop order. The obtained results are compared against the asymptotic formulas following from the threshold singularities. We also discuss the $μ$ dependence of the moments within the BLM procedure.

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arXiv1996

Three-Loop Polarization Function and ${\cal O}(α_s^2)$ Corrections to the Production of Heavy Quarks

The three-loop vacuum polarization function $Π(q^2)$ induced by a massive quark is calculated. A comprehensive description of the method is presented. From the imaginary part the ${\cal O}(α_s^2)$ result for the production of heavy quarks $R(s)=σ(e^+e^-\to \mbox{hadrons})/σ(e^+e^-\to μ^+μ^-)$ can be extracted. Explicit formulae separated into the different colour factors are given.

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arXiv1996

Top Quark Production to ${\cal{O}}(α_s^2)$

The cross-section for the production of $t\bar{t}$ pairs in $e^+e^-$-annihilation via a virtual photon is determined up to order $α_s^2$. For the cm-energies of interest the effect of the top mass may not be considered as a small parameter and the full mass dependence must be included.

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arXiv1996

Double Bubble Corrections to Heavy Quark Production

Second order ${\cal O}(α_s^2)$ corrections to the heavy quark production cross-section due to massless quarks and coloured scalars are calculated for all energies above threshold. Based on the method introduced in this letter also the gauge non-invariant second order corrections due to the pure gluonic selfenergy insertion and a certain class of ${\cal{O}}(α_s^3)$ and ${\cal{O}}(α_s^4)$ corrections are determined. For the special choice of the gauge parameter, $ξ=4$, the leading threshold and high energy behaviour of the pure second order gluonic corrections to the cross-section are governed by the gluonic self energy insertion.

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arXiv1995

Heavy Quark Vacuum Polarisation to Three Loops

The real and imaginary part of the vacuum polarisation function $Π(q^2)$ induced by a massive quark is calculated in perturbative QCD up to order $α_s^2$. The method is described and the results are presented. This extends the calculation by Källén and Sabry from two to three loops.

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arXiv1995

Three-Loop O(alpha_s^2 G_F M_t^2) Corrections to Higgs Production and Decay at e^+e^- Colliders

We evaluate the next-to-leading-order QCD corrections of O(alpha_s^2 G_F M_t^2) to the Standard-Model l^+l^-H, ZZH, and W^+W^-H couplings in the heavy-top- quark limit. Exploiting these results together with knowledge of Delta rho to the same order, we analyze a variety of production and decay processes of low- mass Higgs bosons at e^+e^- colliders. Specifically, we consider H->l^+l^-, H->l^+l^-l'^+l'^-, e^+e^-->ZH, Z->ff-barH, and e^+e^-->ff-barH, with f=l,nu. We work in the electroweak on-shell scheme formulated with G_F and employ both the on-shell and MS-bar definitions of the top-quark mass in QCD. As expected, the scheme and scale dependences are greatly reduced when the next-to-leading- order corrections are taken into account. In the on-shell scheme of top-quark mass renormalization, the O(alpha_s^2 G_F M_t^2) corrections act in the same direction as the O(alpha_s G_F M_t^2) ones and further increase the screening of the O(G_F M_t^2) terms. The coefficients of (alpha_s/pi)^2 range from -6.847 for the ZZH coupling to -16.201 for the l^+l^-H coupling. This is in line with the value -14.594 recently found for Delta rho.

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arXiv1995

Virtual Top Effects on Low-Mass Higgs Interactions at Next-to-Leading Order in QCD

We present the next-to-leading-order QCD corrections of O(alpha_s^2 G_F M_t^2) to the low-M_H effective l^+l^-H, ZZH, and W^+W^-H interaction Lagrangians in the high-M_t limit. In the on-shell scheme formulated with G_F, the O(alpha_s^2 G_F M_t^2) corrections support the O(alpha_s G_F M_t^2) ones and further increase the screening of the O(G_F M_t^2) terms. The coefficients of (alpha_s/pi)^2 range from -6.847 to -16.201, being in line with the value -14.594 recently found for Delta rho. All four QCD expansions converge considerably more rapidly, if they are written with mu_t=m_t(mu_t), where m_t(mu) is the MS-bar mass, rather than the pole mass, M_t.

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arXiv1995

QCD Corrections from Top Quark to Relations between Electroweak Parameters to Order $α_s^2$

The vacuum polarization functions $Π(q^2)$ of charged and neutral gauge bosons which arise from top and bottom quark loops lead to important shifts in relations between electroweak parameters which can be measured with ever-increasing precision. The large mass of the top quark allows approximation of these functions through the first two terms of an expansion in $M_Z^2/M_t^2$. The first three terms of the Taylor series of $Π(q^2)$ are evaluated analytically up to order $\as^2$. The first two are required to derive the approximation, the third can be used to demonstrate the smallness of the neglected terms. The paper improves earlier results based on the leading term $\propto G_F M_t^2 \as^2$. Results for the subleading contributions to $\dr$ and the effective mixing angle $\sineff$ are presented.

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arXiv1995

Corrections of order ${\cal O}(G_F M_t^2 \as^2)$ to the $ρ$ parameter

The three-loop QCD corrections to the $ρ$ parameter from top and bottom quark loops are calculated. The result differs from the one recently calculated by Avdeev et al. As function of the pole mass the numerical value is given by $\drho=\frac{3G_F M_t^2}{8\sqrt{2}π^2}(1- 2.8599 \api - 14.594 (\api)^2 )$.

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arXiv1994

Corrections of Order ${\cal O}(G_Fα_s m_t^2)$ to the Higgs Decay Rate $Γ(H\to b\bar{b})$

QCD corrections to the electroweak one-loop result for the partial width $Γ(H\rightarrow b\bar{b})$ are studied. For the decay channel into bottom quarks the rate is affected by a virtual top quark through electroweak interactions. The calculation of QCD corrections to this quantity is performed for an intermediate range Higgs mass in the heavy top mass limit. The leading correction of order ${\cal O}(G_Fα_s m_t^2)$ is estimated. Numerically the contribution is of comparable size as the electroweak correction, but of opposite sign.

hep-ph↗

arXiv1994

${\cal O}(α\as \ln m_t^2)$ Non-Universal Corrections to the Decay Rate $Γ(Z\rightarrow b\bar{b})$

The partial decay rate $Γ(Z\to b\bar{b})$ is significantly influenced by the mass of the top quark due to electroweak radiative corrections. The leading $\sim m_t^2$ and the next-to-leading contribution $\sim \ln m_t^2$ are known to be numerically of similar size. In this work we calculate the QCD corrections to the logarithmic correction using the heavy top mass expansion. The ${\cal O} (α\as \ln m_t^2)$ corrections are of the same order as the QCD corrections to the quadratic top mass term, but of different sign.

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