142 research outputs found
Dynamical NNLO parton distributions
Utilizing recent DIS measurements (\sigma_r, F_{2,3,L}) and data on hadronic
dilepton production we determine at NNLO (3-loop) of QCD the dynamical parton
distributions of the nucleon generated radiatively from valencelike positive
input distributions at an optimally chosen low resolution scale (Q_0^2 < 1
GeV^2). These are compared with `standard' NNLO distributions generated from
positive input distributions at some fixed and higher resolution scale (Q_0^2 >
1 GeV^2). Although the NNLO corrections imply in both approaches an improved
value of \chi^2, typically \chi^2_{NNLO} \simeq 0.9 \chi^2_{NLO}, present DIS
data are still not sufficiently accurate to distinguish between NLO results and
the minute NNLO effects of a few percent, despite of the fact that the
dynamical NNLO uncertainties are somewhat smaller than the NLO ones and both
are, as expected, smaller than those of their `standard' counterparts. The
dynamical predictions for F_L(x,Q^2) become perturbatively stable already at
Q^2 = 2-3 GeV^2 where precision measurements could even delineate NNLO effects
in the very small-x region. This is in contrast to the common `standard'
approach but NNLO/NLO differences are here less distinguishable due to the much
larger 1\sigma uncertainty bands. Within the dynamical approach we obtain
\alpha_s(M_Z^2)=0.1124 \pm 0.0020, whereas the somewhat less constrained
`standard' fit gives \alpha_s(M_Z^2)=0.1158 \pm 0.0035.Comment: 44 pages, 15 figures; minor changes, footnote adde
Variable Flavor Number Parton Distributions and Weak Gauge and Higgs Boson Production at Hadron Colliders at NNLO of QCD
Based on our recent NNLO dynamical parton distributions as obtained in the
`fixed flavor number scheme', we generate radiatively parton distributions in
the `variable flavor number scheme' where also the heavy quark flavors (c,b,t)
become massless partons within the nucleon. Only within this latter
factorization scheme NNLO calculations are feasible at present, since the
required partonic subprocesses are only available in the approximation of
massless initial-state partons. The NNLO predictions for gauge boson production
are typically larger (by more than 1 sigma) than the NLO ones, and rates at LHC
energies can be predicted with an accuracy of about 5%, whereas at Tevatron
they are more than 2 sigma above the NLO ones. The NNLO predictions for SM
Higgs boson production via the dominant gluon fusion process have a total (pdf
and scale) uncertainty of about 10% at LHC which almost doubles at the lower
Tevatron energies; they are typically about 20% larger than the ones at NLO but
the total uncertainty bands overlap.Comment: 28 pages, 3 tables, 6 figure
First heavy flavor contributions to deeply inelastic scattering
In the asymptotic limit , the heavy flavor Wilson coefficients
for deep--inelastic scattering factorize into the massless Wilson coefficients
and the universal heavy flavor operator matrix elements resulting from
light--cone expansion. In this way, one can calculate all but the power
corrections in . The heavy flavor operator matrix elements
are known to . We present the last 2--loop result missing in the
unpolarized case for the renormalization at 3--loops and first 3--loop results
for terms proportional to the color factor in Mellin--space. In this
calculation, the corresponding parts of the anomalous dimensions
\cite{LARIN,MVVandim} are obtained as well.Comment: 6 pages, Contribution to the Proceedings of "Loops and Legs in
Quantum Field Theory", 2008, Sondershausen, Germany, and DIS 2008, London, U
The 1958 Pekeris-Accad-WEIZAC Ground-Breaking Collaboration that Computed Ground States of Two-Electron Atoms (and its 2010 Redux)
In order to appreciate how well off we mathematicians and scientists are
today, with extremely fast hardware and lots and lots of memory, as well as
with powerful software, both for numeric and symbolic computation, it may be a
good idea to go back to the early days of electronic computers and compare how
things went then. We have chosen, as a case study, a problem that was
considered a huge challenge at the time. Namely, we looked at C.L. Pekeris's
seminal 1958 work on the ground state energies of two-electron atoms. We went
through all the computations ab initio with today's software and hardware, with
a special emphasis on the symbolic computations which in 1958 had to be made by
hand, and which nowadays can be automated and generalized.Comment: 8 pages, 2 photos, final version as it appeared in the journa
The uses of Connes and Kreimer's algebraic formulation of renormalization theory
We show how, modulo the distinction between the antipode and the "twisted" or
"renormalized" antipode, Connes and Kreimer's algebraic paradigm trivializes
the proofs of equivalence of the (corrected) Dyson-Salam,
Bogoliubov-Parasiuk-Hepp and Zimmermann procedures for renormalizing Feynman
amplitudes. We discuss the outlook for a parallel simplification of
computations in quantum field theory, stemming from the same algebraic
approach.Comment: 15 pages, Latex. Minor changes, typos fixed, 2 references adde
The massless higher-loop two-point function
We introduce a new method for computing massless Feynman integrals
analytically in parametric form. An analysis of the method yields a criterion
for a primitive Feynman graph to evaluate to multiple zeta values. The
criterion depends only on the topology of , and can be checked
algorithmically. As a corollary, we reprove the result, due to Bierenbaum and
Weinzierl, that the massless 2-loop 2-point function is expressible in terms of
multiple zeta values, and generalize this to the 3, 4, and 5-loop cases. We
find that the coefficients in the Taylor expansion of planar graphs in this
range evaluate to multiple zeta values, but the non-planar graphs with crossing
number 1 may evaluate to multiple sums with roots of unity. Our
method fails for the five loop graphs with crossing number 2 obtained by
breaking open the bipartite graph at one edge
) Polarized Heavy Flavor Corrections}to Deep-Inelastic Scattering at
We calculate the quarkonic massive operator matrix elements
and
for the twist--2 operators and the associated heavy flavor Wilson coefficients
in polarized deeply inelastic scattering in the region to
in the case of the inclusive heavy flavor contributions. The
evaluation is performed in Mellin space, without applying the
integration-by-parts method. The result is given in terms of harmonic sums.
This leads to a significant compactification of the operator matrix elements
and massive Wilson coefficients in the region derived previously
in \cite{BUZA2}, which we partly confirm, and also partly correct. The results
allow to determine the heavy flavor Wilson coefficients for to
for all but the power suppressed terms . The results in momentum fraction -space are also presented. We also
discuss the small effects in the polarized case. Numerical results are
presented. We also compute the gluonic matching coefficients in the two--mass
variable flavor number scheme to .Comment: 58 pages Latex, 12 Figure
O ( α ) polarized heavy flavor corrections to deep-inelastic scattering at Q ⫠m
We calculate the quarkonic O(α) massive operator matrix elements A (N),A(N) and A,(N) for the twistâ2 operators and the associated heavy flavor Wilson coefficients in polarized deeply inelastic scattering in the region Q â« m to O(Δ) in the case of the inclusive heavy flavor contributions. The evaluation is performed in Mellin space, without applying the integration-by-parts method. The result is given in terms of harmonic sums. This leads to a significant compactification of the operator matrix elements and massive Wilson coefficients in the region Q â« m derived previously in [1], which we partly confirm, and also partly correct. The results allow to determine the heavy flavor Wilson coefficients for g(x, Q) to O(α ) for all but the power suppressed terms â (m/Q) , k â„ 1. The results in momentum fraction z-space are also presented. We also discuss the small x effects in the polarized case. Numerical results are presented. We also compute the gluonic matching coefficients in the twoâmass variable flavor number scheme to O(Δ)
The massless two-loop two-point function
We consider the massless two-loop two-point function with arbitrary powers of
the propagators and derive a representation, from which we can obtain the
Laurent expansion to any desired order in the dimensional regularization
parameter eps. As a side product, we show that in the Laurent expansion of the
two-loop integral only rational numbers and multiple zeta values occur. Our
method of calculation obtains the two-loop integral as a convolution product of
two primitive one-loop integrals. We comment on the generalization of this
product structure to higher loop integrals.Comment: 22 pages, revised version, eq. 9, 10 and 53 correcte
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