972 research outputs found
Revisiting the Naturalness Problem -- Who is afraid of quadratic divergences? --
It is widely believed that quadratic divergences severely restrict natural
constructions of particle physics models beyond the standard model (SM).
Supersymmetry provides a beautiful solution, but the recent LHC experiments
have excluded large parameter regions of supersymmetric extensions of the SM.
It will now be important to reconsider whether we have been misinterpreting the
quadratic divergences in field theories. In this paper, we revisit the problem
from the viewpoint of the Wilsonian renormalization group and argue that
quadratic divergences, which can always be absorbed into a position of the
critical surface, should be simply subtracted in model constructions. Such a
picture gives another justification to the argument that the scale invariance
of the SM, except for the soft-breaking terms, is an alternative solution to
the naturalness problem. It also largely broadens possibilities of model
constructions beyond the SM since we just need to take care of logarithmic
divergences, which cause mixings of various physical scales and runnings of
couplings.Comment: 22 pages, 6 figures, version published in Phys. Rev.
QED vertex form factors at two loops
We present the closed analytic expression of the form factors of the two-loop
QED vertex amplitude for on-shell electrons of finite mass and arbitrary
momentum transfer . The calculation is carried out within the
continuous -dimensional regularization scheme, with a single continuous
parameter , the dimension of the space-time, which regularizes at the same
time UltraViolet (UV) and InfraRed (IR) divergences. The results are expressed
in terms of 1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 53 pages, 3 figure
Master Integrals for the 2-loop QCD virtual corrections to the Forward-Backward Asymmetry
We present the Master Integrals needed for the calculation of the two-loop
QCD corrections to the forward-backward asymmetry of a quark-antiquark pair
produced in electron-positron annihilation events. The abelian diagrams
entering in the evaluation of the vector form factors were calculated in a
previous paper. We consider here the non-abelian diagrams and the diagrams
entering in the computation of the axial form factors, for arbitrary space-like
momentum transfer Q^2 and finite heavy quark mass m. Both the UV and IR
divergences are regularized in the continuous D-dimensional scheme. The Master
Integrals are Laurent-expanded around D=4 and evaluated by the differential
equation method; the coefficients of the expansions are expressed as
1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 38 pages, 6 figures, typos corrected, version accepted by Nucl. Phys.
Second-order equation of state with the full Skyrme interaction: toward new effective interactions for beyond mean-field models
In a quantum Fermi system the energy per particle calculated at the second
order beyond the mean-field approximation diverges if a zero-range interaction
is employed. We have previously analyzed this problem in symmetric nuclear
matter by using a simplified nuclear Skyrme interaction, and proposed a
strategy to treat such a divergence. In the present work, we extend the same
strategy to the case of the full nuclear Skyrme interaction. Moreover we show
that, in spite of the strong divergence ( , where is
the momentum cutoff) related to the velocity-dependent terms of the
interaction, the adopted cutoff regularization can be always simultaneously
performed for both symmetric and nuclear matter with different
neutron-to-proton ratio. This paves the way to applications to finite nuclei.Comment: 15 figure
Vertex diagrams for the QED form factors at the 2-loop level
We carry out a systematic investigation of all the 2-loop integrals occurring
in the electron vertex in QED in the continuous -dimensional regularization
scheme, for on-shell electrons, momentum transfer and finite squared
electron mass . We identify all the Master Integrals (MI's) of the
problem and write the differential equations in which they satisfy. The
equations are expanded in powers of and solved by the
Euler's method of the variation of the constants. As a result, we obtain the
coefficients of the Laurent expansion in of the MI's up to zeroth
order expressed in close analytic form in terms of Harmonic Polylogarithms.Comment: A few misprints have been corrected. The results are now available at
http://pheno.physik.uni-freiburg.de/~bhabha, as FORM input file
Amine-oxide adsorbents for post-combustion COâ capture
Amine functionalized silicas are promising chemisorbent materials for post-combustion COâ capture due to the high density of active sites per unit mass of adsorbent that can be obtained by tuning the synthesis protocol, thus resulting in high equilibrium COâ adsorption capacities. However, when compared to physisorbents, they have a few disadvantages. Firstly, oxidative degradation of the amine groups reduces the lifetime of these adsorbent materials. Furthermore, rapid heat release following the reaction between amines and COâ results in large local temperature spikes which may adversely affect adsorption equilibria and kinetics. Thirdly, there is a lack of fundamental understanding of COâ-amine adsorption thermodynamics, which is key to scaling up these materials to an industrial-scale adsorption process. In this dissertation the qualitative and quantitative understanding of these three critical aspects of aminosilica adsorbents have been furthered so these materials can be better evaluated and further tuned as adsorbents for post-combustion COâ capture applications.Ph.D
Dimensional Reduction applied to QCD at three loops
Dimensional Reduction is applied to \qcd{} in order to compute various
renormalization constants in the \drbar{} scheme at higher orders in
perturbation theory. In particular, the function and the anomalous
dimension of the quark masses are derived to three-loop order. Special emphasis
is put on the proper treatment of the so-called -scalars and the
additional couplings which have to be considered.Comment: 13 pages, minor changes, references adde
Higher loop corrections to a Schwinger--Dyson equation
We consider the effects of higherloop corrections to a Schwinger--Dyson
equations for propagators. This is made possible by the efficiency of the
methods we developed in preceding works, still using the supersymmetric
Wess--Zumino model as a laboratory. We obtain the dominant contributions of the
three and four loop primitive divergences at high order in perturbation theory,
without the need for their full evaluations. Our main conclusion is that the
asymptotic behavior of the perturbative series of the renormalization function
remains unchanged, and we conjecture that this will remain the case for all
finite order corrections.Comment: 12 pages, 2 imbedded TiKZ pictures. A few clarifications matching the
published versio
The Casimir energy of skyrmions in the 2+1-dimensional O(3)-model
One-loop quantum corrections to the classical vortices in 2+1 dimensional
O(3)-models are evaluated. Skyrme and Zeeman potential terms are used to
stabilize the size of topological solitons. Contributions from zero modes,
bound-states and scattering phase-shifts are calculated for vortices with
winding index n=1 and n=2. For both cases the S-matrix shows a pronounced
series of resonances for magnon-vortex scattering in analogy to the
well-established baryon resonances in hadron physics, while vortices with n>2
are already classically unstable against decay. The quantum corrections
destabilize the classically bound n=2 configuration. Approximate independence
of the results with respect to changes in the renormalization scale is
demonstrated.Comment: 24 pages LaTeX, 14 figure
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