21,762 research outputs found
Conformal and non Conformal Dilaton Gravity
The quantum dynamics of the gravitational field non-minimally coupled to an
(also dynamical) scalar field is studied in the {\em broken phase}. For a
particular value of the coupling the system is classically conformal, and can
actually be understood as the group averaging of Einstein-Hilbert's action
under conformal transformations. Conformal invariance implies a simple Ward
identity asserting that the trace of the equation of motion for the graviton is
the equation of motion of the scalar field. We perform an explicit one-loop
computation to show that the DeWitt effective action is not UV divergent {\em
on shell} and to find that the Weyl symmetry Ward identity is preserved {\em on
shell} at that level. We also discuss the fate of this Ward identity at the
two-loop level --under the assumption that the two-loop UV divergent part of
the effective action can be retrieved from the Goroff-Sagnotti counterterm--
and show that its preservation in the renormalized theory requires the
introduction of counterterms which exhibit a logarithmic dependence on the
dilaton field.Comment: LateX, 50 pages. Several points clarified; references added. New
section on Weyl invariant renormalisation adde
Hokupa'a-Gemini Discovery of Two Ultracool Companions to the Young Star HD 130948
We report the discovery of two faint ultracool companions to the nearby
(d~17.9 pc) young G2V star HD 130948 (HR 5534, HIP 72567) using the Hokupa'a
adaptive optics instrument mounted on the Gemini North 8-meter telescope. Both
objects have the same common proper motion as the primary star as seen over a 7
month baseline and have near-IR photometric colors that are consistent with an
early-L classification. Near-IR spectra taken with the NIRSPEC AO instrument on
the Keck II telescope reveal K I lines, FeH, and water bandheads. Based on
these spectra, we determine that both objects have spectral type dL2 with an
uncertainty of 2 spectral subclasses. The position of the new companions on the
H-R diagram in comparison with theoretical models is consistent with the young
age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity,
lithium abundance and fast rotation. HD 130948 B and C likely constitute a pair
of young contracting brown dwarfs with an orbital period of about 10 years, and
will yield dynamical masses for L dwarfs in the near future.Comment: 10 pages, 3 figures, (13 total pages
A Renormalization Group Analysis of the NCG constraints m_{top} = 2\,m_W},
We study the evolution under the renormalization group of the restrictions on
the parameters of the standard model coming from Non-Commutative Geometry,
namely and . We adopt the point of
view that these relations are to be interpreted as {\it tree level} constraints
and, as such, can be implemented in a mass independent renormalization scheme
only at a given energy scale . We show that the physical predictions on
the top and Higgs masses depend weakly on .Comment: 7 pages, FTUAM-94/2, uses harvma
- …