119 research outputs found

### Electroweak Sudakov corrections

At energies much larger than the mass of the weak gauge bosons, electroweak
radiative corrections can lead to significant corrections. At 1 TeV the one
loop corrections can be of ${\cal O} (20 %)$ due to large contributions of the
Sudakov type. We summarize recent progress in the evaluation and resummation of
the double and single logarithmic corrections to general scattering amplitudes
for fermions, transversely as well as longitudinally polarized external lines.Comment: contribution to LCWS2000 at Fermila

### Resummation of angular dependent corrections in spontaneously broken gauge theories

Recent investigations of electroweak radiative corrections have revealed the
importance of higher order contributions in high energy processes, where the
size of typical corrections can exceed those associated with QCD considerably.
Beyond one loop, only universal (angular independent) corrections are known to
all orders except for massless $e^+ e^- \longrightarrow f {\overline f}$
processes where also angular dependent corrections exist in the literature. In
this paper we present general arguments for the consistent resummation of
angular dependent subleading (SL) logarithmic corrections to all orders in the
regime where all invariants are still large compared to the gauge boson masses.
We discuss soft isospin correlations, fermion mass and gauge boson mass gap
effects, the longitudinal and Higgs boson sector as well as mixing
contributions including CKM effects for massive quarks. Two loop arguments are
interpreted in the context of the effective high energy effective theory based
on the Standard Model Lagrangian in the symmetric basis with the appropriate
matching conditions to include the soft QED regime. The result is expressed in
exponentiated operator form in a CKM-extended isospin space in the symmetric
basis. Thus, a full electroweak SL treatment based on the infrared evolution
equation method is formulated for arbitrary high energy processes at future
colliders. Comparisons with known results are presented.Comment: 22 pages, 3 eps-figures, uses LaTeX2

### Mass gap effects and higher order electroweak Sudakov logarithms

The infrared structure of spontaneously broken gauge theories is
phenomenologically very important and theoretically a challenging problem.
Various attempts have been made to calculate the higher order behavior of large
double-logarithmic (DL) corrections originating from the exchange of
electroweak gauge bosons resulting in contradictory claims. We present results
from two loop electroweak corrections for the process $g \longrightarrow f_{\rm
R} {\bar f}_{\rm L}$ to DL accuracy. This process is ideally suited as a
theoretical model reaction to study the effect of the mass gap of the neutral
electroweak gauge bosons at the two loop level. Contrary to recent claims in
the literature, we find that the calculation performed with the physical
Standard Model fields is in perfect agreement with the results from the
infrared evolution equation method. In particular, we can confirm the
exponentiation of the electroweak Sudakov logarithms through two loops.Comment: 10 pages, 3 figures, LaTeX2e, uses epsfi

### Two loop mass effects in the static position space QCD-potential

The perturbatively calculable short distance QCD potential is known to two
loops including the effect of massive quarks. Recently, a simple approximate
solution in momentum space was utilized to obtain the potential in coordinate
space. The latter is important in several respects. A comparison with
non-perturbative lattice results is feasible in the overlap regime using light
$\bar{MS}$ masses. This might be even more promising employing the concept of
the force between the heavy color singlet sources, which can be easily derived
from the potential. In addition, the better than two percent accuracy bottom
mass determination from $\Upsilon$-mesons is sensitive to massive charm loops
at the two loop order. We summarize recent results using exact one loop
functions and explicit decoupling parametrizations.Comment: Version to appear in Proceedings of QCD0

### Renormalization Group Improved Heavy Quark Production in Polarized \gamma \gamma Collisions

The experimental determination of the partial width $\Gamma (H
\longrightarrow \gamma \gamma)$ of an intermediate mass Higgs is among the most
important measurements at a future photon photon collider. Recently it was
shown that large non-Sudakov as well as Sudakov double logarithmic (DL)
corrections can be summed to all orders in the background process $\gamma
\gamma (J_z=0) \longrightarrow q \bar{q}$. It was found that positivity and
stability of the cross section was only restored at the four-loop level. One
remaining large source of uncertainty stems from the fact that the scale of the
strong coupling is unspecified within the double logarithmic approximation. In
this paper we include the leading and next-to-leading order running coupling to
all orders. We thus remove the inherent scale uncertainty of both the exact
one-loop and all-orders DL result without encountering any Landau-pole
singularities. The effect is significant and, for the non-Sudakov form factor,
is found to correspond to an effective scale of roughly $\alpha_s(9m_q^2)$.Comment: 23 pages, 10 eps-figure

### The Standard Model Higgs in $\gamma \gamma$ Collisions

For a Higgs boson below the $W^\pm$ threshold, the $\gamma \gamma$ collider
option of a future linear $e^+ e^-$ machine is compelling. In this case one can
measure the ``gold-plated'' loop induced $\Gamma (H \longrightarrow \gamma
\gamma)$ partial width to high precision, testing various extensions of the
Standard Model. With recent progress in the expected $\gamma \gamma$ luminosity
at TESLA, we find that for a Higgs of 115 GeV a statistical accuracy of the two
photon partial width of 1.4 % is possible. The total width depends thus solely
on the accuracy of $BR(H \longrightarrow \gamma \gamma)$ and is of ${\cal O}
(10 %)$.Comment: contribution to LCWS2000 at Fermila

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