916 research outputs found
Dimensional renormalization: ladders to rainbows
Renormalization factors are most easily extracted by going to the massless
limit of the quantum field theory and retaining only a single momentum scale.
We derive factors and renormalized Green functions to all orders in
perturbation theory for rainbow graphs and vertex (or scattering diagrams) at
zero momentum transfer, in the context of dimensional renormalization, and we
prove that the correct anomalous dimensions for those processes emerge in the
limit D -> 4.Comment: RevTeX, no figure
Singlet Polarization Functions at O(\alpha_s^2)
We consider the three-loop singlet diagrams induced by axial-vector, scalar
and pseudo-scalar currents. Expansions for small and large external momentum
are presented. They are used in combination with conformal mapping and
Pad\'e approximations in order to arrive at results for the polarization
functions valid for all . Results are presented for the imaginary parts
which are directly related to physical quantities like the production of top
quarks or the decay of scalar or pseudo-scalar Higgs bosons.Comment: LaTeX, 15 pages, 10 figures included as ps-files. The complete paper
is also available via anonymous ftp at ftp://ttpux2.physik.uni-karlsruhe.de/
, or via www at http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints
Virtual Top-Quark Effects on the H->bb-bar Decay at Next-to-Leading Order in QCD
By means of a heavy-top-quark effective Lagrangian, we calculate the
three-loop corrections of O(alpha_s^2 G_F M_t^2) to the H->bb-bar partial decay
width of the standard-model Higgs boson with intermediate mass M_H<<2M_t. We
take advantage of a soft-Higgs theorem to construct the relevant coefficient
functions. We present our result both in the MS-bar and on-shell schemes of
mass renormalization. The MS-bar formulation turns out to be favourable with
regard to the convergence behaviour. We also test a recent idea concerning the
naive non-abelianization of QCD.Comment: 8 pages (Latex), 5 figures (Postscript
Cloning Hubble Deep Fields I: A Model-Independent Measurement of Galaxy Evolution
We present a model-independent method of quantifying galaxy evolution in
high-resolution images, which we apply to the Hubble Deep Field (HDF). Our
procedure is to k-correct all pixels belonging to the images of a complete set
of bright galaxies and then to replicate each galaxy image to higher redshift
by the product of its space density, 1/V_{max}, and the cosmological volume.
The set of bright galaxies is itself selected from the HDF, because presently
the HDF provides the highest quality UV images of a redshift-complete sample of
galaxies (31 galaxies with I<21.9, \bar{z}=0.5, and for which V/V_{max} is
spread fairly). These galaxies are bright enough to permit accurate
pixel-by-pixel k-corrections into the restframe UV (\sim 2000 A). We match the
shot noise, spatial sampling and PSF smoothing of the HDF data, resulting in
entirely empirical and parameter-free ``no-evolution'' deep fields of galaxies
for direct comparison with the HDF. In addition, the overcounting rate and the
level of incompleteness can be accurately quantified by this procedure. We
obtain the following results. Faint HDF galaxies (I>24) are much smaller, more
numerous, and less regular than our ``no-evolution'' extrapolation, for any
interesting geometry. A higher proportion of HDF galaxies ``dropout'' in both U
and B, indicating that some galaxies were brighter at higher redshifts than our
``cloned'' z\sim0.5 population.Comment: 51 pages, 23 figures, replacement includes figures not previously
include
Detection of Evolved High-Redshift Galaxies in Deep NICMOS/VLT Images
A substantial population of high redshift early-type galaxies is detected in
very deep UBVRIJHK images towards the HDF-South. Four elliptical profile
galaxies are identified in the redshift range z=1-2, all with very red SEDs,
implying ages of >2 Gyrs for standard passive evolution. We also find later
type IR-luminous galaxies at similarly high redshift, (10 objects with z>1,
H1 Gyr. The number
and luminosity-densities of these galaxies are comparable with the local
E/SO-Sbc populations for \Omega_m>0.2, and in the absence of a significant
cosmological constant, we infer that the major fraction of luminous
Hubble-sequence galaxies have evolved little since z~2. A highly complete
photometric redshift distribution is constructed to H=25 (69 galaxies) showing
a broad spread of redshift, peaking at z~1.5, in reasonable agreement with some
analyses of the HDF. Five `dropout' galaxies are detected at z~3.8, which are
compact in the IR, ~0.5 kpc/h at rest 3500\AA. No example of a blue IR luminous
elliptical is found, restricting the star-formation epoch of ellipticals to
z>10 for a standard IMF and modest extinction.Comment: 7 pages, 3 figures, accepted for publication in Astrophysical Journal
Letters, discussion of clustering added, color image available at
http://astro.berkeley.edu/~tjb/nic3.htm
The redshift of the gravitationally lensed radio source PKS1830-211
We report on the spectroscopic identification and the long awaited redshift
measurement of the heavily obscured, gravitationally lensed radio source PKS
1830-211, which was first observed as a radio Einstein ring. The NE component
of the doubly imaged core is identified, in our infrared spectrum covering the
wavelength range 1.5-2.5 microns, as an impressively reddened quasar at
z=2.507. Our redshift measurement, together with the recently measured time
delay (Lovell et al.), means that we are a step closer to determining the
Hubble constant from this lens. Converting the time delay into the Hubble
constant by using existing models leads to high values for the Hubble constant.
Since the lensing galaxy lies very close to the center of the lensed ring,
improving the error bars on the Hubble constant will require not only a more
precise time delay measurement, but also very precise astrometry of the whole
system.Comment: 11 pages, 2 figures, Accepted ApJ
Lattice Green functions in all dimensions
We give a systematic treatment of lattice Green functions (LGF) on the
-dimensional diamond, simple cubic, body-centred cubic and face-centred
cubic lattices for arbitrary dimensionality for the first three
lattices, and for for the hyper-fcc lattice. We show that there
is a close connection between the LGF of the -dimensional hypercubic lattice
and that of the -dimensional diamond lattice. We give constant-term
formulations of LGFs for all lattices and dimensions. Through a still
under-developed connection with Mahler measures, we point out an unexpected
connection between the coefficients of the s.c., b.c.c. and diamond LGFs and
some Ramanujan-type formulae for Comment: 30 page
An Old Cluster in NGC 6822
We present spectroscopy of two clusters in the dwarf irregular galaxy NGC
6822. From these we deduce an age for Cluster VII of 11 Gyr and [Fe/H] = -1.95
+/- 0.15 dex. Cluster VII appears to be an analog of the metal-poor galactic
globular clusters. Cluster VI is found to be much younger and more metal rich,
with an age of approximately 2 Gyr. Its derived metallicity, [Fe/H], of
approximately -1.0 dex is comparable to that of the gas seen today in NGC 6822.
The existence of a metal-poor old cluster in NGC 6822 rules out models for the
chemical evolution of this galaxy with significant prompt initial enhancement.
We find that a star formation rate which is constant with time and is within a
factor of two of the present star formation rate can reproduce the two points
on the age-metallicity relationship for NGC 6822 over the past 10 Gyr defined
by these two clusters.Comment: 8 pages; accepted for publication in A
Matching QCD and HQET heavy--light currents at two loops and beyond
Heavy--light QCD currents are matched with HQET currents at two loops and
leading order in . A single formula applies to all current matchings. As a
by--product, a master formula for the two--loop anomalous dimension of the QCD
current is obtained, yielding a
new result for the tensor current. The dependence of matching coefficients on
prescriptions is elucidated. Ratios of QCD matrix elements are
obtained, independently of the three--loop anomalous dimension of HQET
currents. The two--loop coefficient in is
with light flavours, and a
correction, , that takes account of the non--zero ratio
. Fastest apparent convergence would entail at ~MeV. ``Naive non--abelianization'' of large--
results, via , gives reasonable approximations to
exact two--loop results. All--order results for anomalous dimensions and
matching coefficients are obtained at large . Consistent
cancellation between infrared-- and ultraviolet--renormalon ambiguities is
demonstrated.Comment: 26 pages, preprint OUT-4102-52, LaTeX with epsf style option,
uuencoded PostScript figure (compressed by gzip) appended at the en
Infrared renormalons and the relations between the Gross-Llewellyn Smith and the Bjorken polarized and unpolarized sum rules
It is demonstrated that the infrared renormalon calculus indicates that the
QCD theoretical expressions for the Gross-Llewelln Smith sum rules and for the
Bjorken polarized and unpolarized ones contain an identical negative twist-4
1/Q^2 correction. This observation is supported by the consideration of the
results of calculations of the corresponding twist-4 matrix elements. Together
with the indication of the similarity of perturbative QCD corrections to these
three sum rules, this observation leads to simple new theoretical relations
between the Gross-Llewellyn Smith and Bjorken polarized and unpolarized sum
rules in the energy region . The validity of this relation is
checked using concrete experimental data for the Gross-Llewellyn Smith and
Bjorken polarized sum rulesComment: 7 pages, LaTeX, minor misprints corrected, results unchanged, to be
published in JETP Letters, vol 81, issue 1
- …