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F. Jegerlehner

Publications and source records attributed to F. Jegerlehner.

At least 37 records · Page 2Linked to original sources
arXiv2004

Steps towards full two-loop calculations for 2 fermion to 2 fermion processes: running versus pole masses schemes

Recent progress in the calculation of the two-loop on-shell mass counterterms within the electroweak Standard Model (SM) for the massive particles are discussed. We are in progress of developing a package for full two-loop SM calculations of 2 -> 2 fermion processes, with emphasis on the analytical approach where feasible. The complete two-loop on-shell renormalization is implemented. Substantial progress has been made in calculating the master integrals. We are able to compute in an efficient and stable manner up to a few thousands of diagrams of very complex mass structure.

hep-ph↗
arXiv2003

O(αα_s) relation between pole- and MS-mass of the t-quark

The O(αα_s) contribution to the relationship between the MS- and the pole-mass of the t-quark propagator within the Standard Model is reviewed. At the same order also the corrections to the top-Yukawa coupling is discussed. We furthermore present the exact analytic expression for the gaugeless limit.

hep-ph↗
arXiv2003

O(αα_s) correction to the pole mass of the t-quark within the Standard Model

We have calculated the O(αα_s) contributions to the relationship between the MS-mass and the pole of the t-quark propagator in the Standard Model in the limit of a diagonal CKM matrix and for a massless b-quark. Analytical results for the so far unknown master-integrals appearing in the calculation are also given.

hep-ph↗
arXiv2003

Theoretical precision in estimates of the hadronic contributions to (g-2)_mu and alpha_QED(M_Z)

I review recent estimates of the non-perturbative hadronic vacuum polarization contributions. Since these at present can only be evaluated in terms of experimental data of limited precision, the related uncertainties pose a serious limitation in our ability to make precise predictions. Besides e+e- - annihilation data also tau decay spectra can help to get better predictions. Here, it is important to account for all possible iso-spin violations in tau - decay spectra, from which e+e- cross sections may be obtained by an iso-spin rotation. The observed 10% discrepancy in the region above the rho may be understood as a so far unaccounted iso-spin breaking effect.

hep-ph↗
arXiv2003

Hadronic vacuum polarization effects in alpha_{em}(M_Z)

Recent evaluations of the hadronic vacuum polarization contributions to the effective fine-structure constant alpha_{em}(M_Z) are summarized and commented. A new update based on corrected CMD-2 data is presented. My new estimates are Delta alpha_{had}^{(5)}(M_Z) = 0.027773 +/- 0.000354 (e^+e^- - data based) and Delta alpha_{had}^{(5)}(M_Z) = 0.027664 +/- 0.000173 (via the Adler-function and extended use of pQCD). Prospects of further possible progress is discussed.

hep-ph↗
arXiv2003

A New Hypergeometric Representation of One-Loop Scalar Integrals in $d$ Dimensions

A difference equation w.r.t. space-time dimension $d$ for $n$-point one-loop integrals with arbitrary momenta and masses is introduced and a solution presented. The result can in general be written as multiple hypergeometric series with ratios of different Gram determinants as expansion variables. Detailed considerations for $2-,3-$ and $4-$point functions are given. For the $2-$ point function we reproduce a known result in terms of the Gauss hypergeometric function $_2F_1$. For the $3-$point function an expression in terms of $_2F_1$ and the Appell hypergeometric function $F_1$ is given. For the $4-$point function a new representation in terms of $_2F_1$, $F_1$ and the Lauricella-Saran functions $F_S$ is obtained. For arbitrary $d=4-2ε$, momenta and masses the $2-,3-$ and $4-$point functions admit a simple one-fold integral representation. This representation will be useful for the calculation of contributions from the $ε-$ expansion needed in higher orders of perturbation theory. Physically interesting examples of $3-$ and $4-$point functions occurring in Bhabha scattering are investigated.

hep-ph↗
arXiv2003

MS Versus Pole Masses of Gauge Bosons II: Two-Loop Electroweak Fermion Corrections

We have calculated the fermion contributions to the shift of the position of the poles of the massive gauge boson propagators at two-loop order in the Standard Model. Together with the bosonic contributions calculated previously the full two-loop corrections are available. This allows us to investigate the full correction in the relationship between MS and pole masses of the vector bosons Z and W. Two-loop renormalization and the corresponding renormalization group equations are discussed. Analytical results for the master-integrals appearing in the massless fermion contributions are given. A new approach of summing multiple binomial sums has been developed.

hep-ph↗
arXiv2002

Measuring the FSR--inclusive pi+pi- cross section

Final state radiation (FSR) in pion--pair production cannot be calculated reliably because of the composite structure of the pions. However, FSR corrections have to be taken into account for a precise evaluation of the hadronic contribution to g-2 of the muon. The role of FSR in both energy scan and radiative return experiments is discussed. It is shown how FSR influences the pion form factor extraction from experimental data and, as a consequence, the evaluation of a_mu^had. In fact the O(alpha) FSR corrections should be included to reach the precision we are aiming at. We argue that for an extraction of the desired FSR--inclusive cross section sigma^(gamma)_had a photon--inclusive scan measurement of the ``e+e- to pi+pi- + photons'' cross section is needed. For exclusive scan and radiative return measurements in contrast we have to rely on ad hoc FSR models if we want to obtain either sigma^(gamma)_had or the FSR--exclusive cross section sigma^(0)_had. We thus advocate to consider seriously precise photon--inclusive energy scan measurements at present and future low energy e+e- facilities. Then together with radiative return measurements from DAFNE and BABAR and forthcoming scan measurements at VEPP-2000 we have a good chance to substantially improve the evaluation of a_mu^had in the future.

hep-ph↗
arXiv2002

FIRCLA, one-loop correction to e+ e- to nu anti-nu H and basis of Feynman integrals in higher dimensions

An approach for an effective computer evaluation of one-loop multi-leg diagrams is proposed. It's main feature is the combined use of several systems - DIANA, FORM and MAPLE. As an application we consider the one-loop correction to Higgs production in e+ e- to nu anti-nu H, which is important for future e+ e- colliders. To improve the stability of numerical evaluations a non-standard basis of integrals is introduced by transforming integrals to higher dimensions.

hep-ph↗
arXiv2002

Hadronic Contributions to the Photon Vacuum Polarization and their Role in Precision Physics

I review recent evaluations of the hadronic contribution to the shift in the fine structure constant and to the anomalous magnetic moment of the muon. Substantial progress in a precise determination of these important observables is a consequence of substantially improved total cross section measurement by the CMD-2 and BES II collaborations and an improved theoretical understanding. Prospects for further possible progress is discussed.

hep-ph↗
arXiv2002

MS vs. Pole Masses of Gauge Bosons: Electroweak Bosonic Two-Loop Corrections

The relationship between MS and pole masses of the vector bosons Z and W is calculated at the two-loop level in the Standard Model. We only consider the purely bosonic contributions which represents a gauge invariant subclass of diagrams. All calculations were performed in the linear $R_ξ$ gauge with three arbitrary gauge parameters utilizing the method of asymptotic expansions. The results are presented in analytic form as series in the small parameters $\sin^2θ_W$ and mass ratio $m_Z^2/m_H^2$. As a byproduct we obtain the bosonic two-loop contributions to the renormalization of the weak mixing parameter $\sin^2θ_W$ and of the Fermi constant. The running of Fermi constant will become important at high energy colliders.

hep-ph↗
arXiv2002

Contents of Lattice 2001 Proceedings

We give here the contents pages for the Proceedings of the Lattice 2001 conference (19th International Symposium on Lattice Field Theory) that took place in August 2001 in Berlin, Germany. The contents are in HTML form with clickable links to the papers that exist on the arXiv, mostly in hep-lat. Corrections and comments should be sent to bunk@physik.hu-berlin.de.

hep-lat↗
arXiv2002

Results for all reactions e+ e- -> 4f, 4f + gamma with nonzero fermion masses

We accomplish our efforts to obtain predictions for all four-fermion final states of e+ e- annihilation and the corresponding bremsstrahlung reactions which are possible in the framework of the Standard Model. For this purpose we have developed a program EE4FGAMMA. Our predictions are valid for fermions of arbitrary masses and we can obtain results for total cross sections without any collinear cut. Keeping exact fermion masses is of course required for top quark production. We give a detailed phenomenological analysis of fermion mass effects and real photon radiation for all channels of four-fermion production at LEP-II and next linear collider energies.

hep-ph↗
arXiv2002

Pion Pair Production with Higher Order Radiative Corrections in Low Energy e+e- Collisions

The complete one-loop QED initial state, final state and initial--final state interference corrections to the process e+e- -> pi+pi- are presented. Analytic formulae are given for the virtual and for the real photon corrections. The total cross section, the pion angular distribution and the pi+pi- invariant mass distribution are investigated in the regime of experimentally realistic kinematical cuts. It is shown that in addition to the full one-loop corrections also two-loop initial state corrections and even the resummation of higher order soft photon logarithms can be necessary if at least per cent accuracy is required. For the data analysis we focus on an inclusive treatment of all photons. The theoretical error concerning our treatment of radiative corrections is then estimated to be less than 2 per mille for both the measurement of the total cross section and the pi+pi- invariant mass distribution. In addition we discuss the model uncertainty due to the pion substructure. Altogether the precision of the theoretical prediction matches the requirements of low energy e+e- experiments like the ones going on at DAFNE or VEPP-2M.

hep-ph↗
arXiv2001

The Effective Fine Structure Constant at TESLA Energies

We present a new estimate of the hadronic contribution to the shift in the fine structure constant at LEP and TESLA energies and calculate the effective fine structure constant. Substantial progress in a precise determination of this important parameter is a consequence of substantially improved total cross section measurements by the BES II collaboration and an improved theoretical understanding. In the standard approach which relies to a large extend on experimental data we find $Δ\al_{\rm hadrons}^{(5)}(\mz) = 0.027896 \pm 0.000395$ which yields $α^{-1}(\mz) = 128.907 \pm 0.054$. Another approach, using the Adler function as a tool to compare theory and experiment, allows us to to extend the applicability of perturbative QCD in a controlled manner. The result in this case reads $Δα^{(5)}_{\rm had}(M_Z^2) = 0.027730 \pm 0.000209$ and hence $α^{-1}(\mz) = 128.930 \pm 0.029$. At TESLA energies a new problem shows up with the definition of an effective charge. A possible solution of the problem is presented. Prospects for further progress in a precise determination of the effective fine structure constant are discussed.

hep-ph↗