134 research outputs found
Leading Order QCD Shear Viscosity from the 3PI Effective Action
In this article we calculate the leading order shear viscosity in QCD using
the resummed 3PI effective action. We work to 3-loop order in the effective
action. We show that the integral equations that resum the pinching and
collinear contributions are produced naturally by the formalism. All leading
order terms are included, without the need for any kind of power counting
arguments.Comment: 23 pages, 27 figure
Graviton loops and brane observables
We discuss how to consistently perform effective Lagrangian computations in
quantum gravity with branes in compact extra dimensions. A reparametrization
invariant and infrared finite result is obtained in a non trivial way. It is
crucial to properly account for brane fluctuations and to correctly identify
physical observables. Our results correct some confusing claims in the
literature. We discuss the implications of graviton loops on electroweak
precision observables and on the muon g-2 in models with large extra
dimensions. We model the leading effects, not controlled by effective field
theory, by introducing a hard momentum cut-off.Comment: 9 pages + 4 eps figures, JHEP style latex document. The paper is
composed by a theoretical part, followed (after page 21) by a
phenomenological part. v2: version published in JHEP, few typos corrected.
v3: few additional typos corrected in the Appendi
Hadron structure after 25 years of QCD
We briefly review the status of our understanding of hadron structure based
on QCD. This includes the role of symmetries, especially chiral symmetry, and
the insights provided by lattice QCD. The main focus is on baryon structure and
especially the nucleon, but this cannot be treated realistically without
reference to spectroscopy. Our aim is to highlight recent insights and
promising directions for future work.Comment: Invited talk presented at the 16th Int. Conf. on Few Body Problems
(Taipei, March 6-10, 2000); 13 pages, 3 figure
Composite fermions in Electroweak Symmetry Breaking
If the electroweak symmetry is broken by some unspecified strong dynamics,
composite fermions may exist with definite transformation properties under
SU(2)_L x SU(2)_R/SU(2)_{L+R} and may play a role in giving masses by mixing to
all the standard quarks and leptons. Assuming this to be the case, we analyze
the role of Singlets, Doublets and Triplets in the ElectroWeak Precision Tests
and in Flavour Physics. Doublets and Triplets are generically disfavoured. In
the Singlet case, we specify the breaking patterns of the flavour group that
allow to keep the CKM picture of flavour physics and we discuss the effects of
the mixing between composite and elementary fermions. These mixings affect in
particular the rather peculiar LHC phenomenology of the composite fermions.Comment: 18 pages, 2 figures (v2: minor modifications, published version
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