Biomedical subjects

B A Kniehl

Publications and source records attributed to B A Kniehl.

5 recordsLinked to original sources

PubMed2006

Strong-coupling constant with flavor thresholds at five loops in the modified minimal-subtraction scheme.

We present in analytic form the matching conditions for the strong-coupling constant alphas(nf) (mu) at the flavor thresholds to four loops in the modified minimal-subtraction scheme. Taking into account the present knowledge on the coefficient beta4 of the Callan-Symanzik beta function of quantum chromodynamics, we thus derive a five-loop formula for alphas(nf) (mu) together with appropriate relationships between the asymptotic scale parameters Lamda(nf) for different numbers of flavors nf.

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PubMed2006

Reconciling open-charm production at the Fermilab Tevatron with QCD.

We study the inclusive hadroproduction of D0, D+, D*+, and D(s)+ mesons at next-to-leading order in the parton model of quantum chromodynamics endowed with universal nonperturbative fragmentation functions fitted to e+e- annihilation data from CERN LEP1. Working in the general-mass variable-flavor-number scheme, we resum the large logarithms through the evolution of the fragmentation functions and, at the same time, retain the full dependence on the charm-quark mass without additional theoretical assumptions. In this way, the cross section distributions in transverse momentum recently measured by the CDF Collaboration in run II at the Fermilab Tevatron are described within errors.

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PubMed2005

Generalizing the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution of fragmentation functions to the smallest values.

An approach valid to any order which unifies the fixed order Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution of fragmentation functions at large x with soft gluon logarithmic resummation at small x is proposed. At lowest order, this approach, implemented with the double logarithmic approximation, reproduces exactly the modified leading logarithm approximation but is more complete due to the degrees of freedom given to the quark sector and the inclusion of the fixed order terms. We find that data from the largest x values to the peak region can be better fitted than with other approaches.

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PubMed2001

Width and partial widths of unstable particles.

In the gauge theory context, a definition of branching ratios and partial widths of unstable particles is proposed that satisfies the basic principles of additivity and gauge independence. A simpler definition, similar to the conventional one, is examined in the Z0-boson case. In order to establish contact with experiment, we show that it leads to a peak cross section that justifies the expression used by the LEP Electroweak Working Group through next-to-next-to-leading order, provided that the pole rather than the on-shell mass and width of the Z0 boson is employed.

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PubMed2000

Strong coupling constant from scaling violations in fragmentation functions.

We present a new determination of the strong coupling constant alpha(s) through the scaling violations in the fragmentation functions for charged pions, charged kaons, and protons. In our fit we include the latest e+e- annihilation data from CERN LEP1 and SLAC SLC on the Z-boson resonance and older, yet very precise, data from SLAC PEP at center-of-mass energy sqaure root of s = 29 GeV. At next-to-leading order, we find alpha(s)(5)(M(Z)) = 0.1170+/-0.0073. A new world average of alpha(s) is given.

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