Multi-Loop Results, Charm- and Bottom-Quark Masses and the Strong Coupling Constant
The impact of recent multi-loop calculations on precise determinations of charm- and bottom-quark masses and the strong coupling constant is discussed.
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arXiv subjects
J. H. Kuehn
Publications and source records attributed to J. H. Kuehn.
QCD Corrections to Hadronic Z and tau Decays
We present a brief (mainly bibliographical) report on recently performed calculations of terms of order O(α_s^4 n_f^2) and O(α_s^4 n_f^2 m_q^2) for hadronic Z and τdecay rates. A few details about the analytical evaluation of the masters integrals appearing in the course of calculations are presented.
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Radiative return at e+e- factories
The energy dependence of the electron - positron hadronic cross section can be measured not only by a straightforward energy scan, but also by means of the radiative return method. To provide extensive comparisons between theory and experiment a Monte Carlo event generator is an indispensable tool. We have developed such a generator called PHOKHARA, which simulates e+e- -> mesons + photon(s) processes. In this paper we present its latest tests and upgrades.
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Quartic mass corrections to R_had at order α_s^3
The total cross section for the production of massive quarks in electron positron annihilation can be predicted in perturbative QCD. After expansion in m^2/s the quartic terms, i.e. those proportional to m^4/s^2, are calculated up to order α_s^3 for vector and axial current induced rates. Predictions relevant for charm, bottom and top quarks production are presented. The α_s^3 corrections are shown to be comparable to terms of order α_s and α_s^2. As a consequence, the predictions exhibit a sizeable dependence on the renormalization scale. The stability of the prediction is improved and, at the same time, the relative size of the large order terms decreases by replacing the running mass \bar{m}(μ) with the scheme independent invariant one \hat{m}. By combining these results with the prediction for massless case and the quadratic mass terms the cross section for massive quark production at electron positron colliders is put under control in order α_s^3 from the high energy region down to fairly low energies.
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Four Pion Final States with Tagged Photons at Electron Positron Colliders
A Monte Carlo generator has been constructed to simulate the reaction e^+e^- \to γ+ 4 π, where the photon is assumed to be observed in the detector. Only initial state radiation is considered. Additional collinear photon radiation has been incorporated with the technique of structure functions. Predictions are presented for cms energies of 1GeV, 3GeV and 10GeV, corresponding to the energies of DAPHNE, BEBC and of B-meson factories. The event rates are sufficiently high to allow for a precise measurement of R(Q^2) in the region of Q between approximately 1GeV and 2.5GeV. For the construction of the program we employ isospin relations between the amplitudes governing tau decays into four pions and electron positron annihilation into four pions. Estimates of the kinematic breaking of these isospin relations as a consequence of the π^- -- π^0 mass difference are given.
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RunDec: a Mathematica package for running and decoupling of the strong coupling and quark masses
In this paper the formulae are collected which are needed for the computation of the strong coupling constant and quark masses at different energy scales and for different number of active flavours. All equations contain the state-of-the-art QCD corrections up to three- and sometimes even four-loop order. For the practical implementation {\tt Mathematica} is used and a package containing useful procedures is provided.
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Quark mass effects to σ(e^+e^- -> hadrons) at O(α_s^3)
The calculation of the quartic mass terms at order α_s^3 to the hadronic R ratio is described. It is based on the operator product expansion for the quark current correlator, combined with an application of the renormalization group equation.
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Measuring sigma(e^+e^- \to hadrons) using tagged photon
We propose to use events with radiated photons in e^+e^- collisions to measure the total cross section of e^+ e^- \to hadrons as a function of the center of mass energy. The Monte Carlo simulation for the collider DAPHNE shows that a competitive accuracy can be achieved with this method.
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Expansion Techniques in Massive Quark Production: Results and Applications
Recent progress in the calculation of multi-loop, multi-scale diagrams is reviewed. Expansion techniques combined with new developments in Computer algebra allow to evaluate the R ratio for massive quarks up to order $α_s^2$ and, partly, even $α_s^3$. Similar techniques can be applied to Higgs- or Z-boson decays and mixed QCD and electroweak interactions.
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The Tau Lepton: Particle Physics in a Nutshell
Recent theoretical results in tau lepton physics and their implications for the experimental analysis are presented. The paper is split into the following chapters: 1. Introduction, 2. Weak Couplings, 3. Inclusive Decays, Perturbative QCD and Sum Rules, 4. Exclusive Decays, 5. Beyond the Standard Model.
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Theoretical Aspects of $τ\to Kππν_τ$ Decays
Predictions based on the chirally normalized vector meson dominance model for decay rates and distributions of $τ$ decays into $Kππν_τ$ final states are discussed. Disagreements with experimental results can be traced back to the $K_1$ widths.
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CP Violation from Charged Higgs Exchange in Hadronic Tau Decays with Unpolarized Beams
CP violating signals in semileptonic $τ$ decays induced by an exotic scalar exchange are studied in a completely model-independent way. These can be observed in decays of unpolarized single $τ$'s even if their rest frame cannot be reconstructed. No beam polarization is required. The importance of the two-meson channel, in particular the $Kπ$ final state is emphasized.
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Theory of Top Quark Production and Decay
Direct and indirect information on the top quark mass and its decay modes is reviewed. The theory of top production in hadron- and electron-positron-colliders is presented.
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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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Heavy Flavor Production and Decay
Recent theoretical results on heavy flavor production and decay in the framework of perturbative QCD are reviewed. This includes calculations for top production at hadron colliders, inclusive charmonium production and the comparison between the singlet and octet mechanisms. Predictions for heavy flavor production in $e^+e^-$ annihilation will be discussed in some detail, covering both the threshold and the high energy region. The first results in NLO for heavy flavor decays will also be reviewed.
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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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CP Violation in Semileptonic $τ$ Decays with Unpolarized Beams
CP violating signals in semileptonic $τ$ decays are studied. These can be observed in decays of unpolarized single $τ$'s even if their rest frame cannot be reconstructed. No beam polarization is required. The importance of the two meson channel, in particular the $Kπ$ final state is emphasized.
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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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