2,665 research outputs found
Non-perturbative QEG Corrections to the Yang-Mills Beta Function
We discuss the non-perturbative renormalization group evolution of the gauge
coupling constant by using a truncated form of the functional flow equation for
the effective average action of the Yang-Mills-gravity system. Our result is
consistent with the conjecture that Quantum Einstein Gravity (QEG) is
asymptotically safe and has a vanishing gauge coupling constant at the
non-trivial fixed point.Comment: To appear in the proceedings of CORFU 200
Renormalization Group Flow of the Holst Action
The renormalization group (RG) properties of quantum gravity are explored,
using the vielbein and the spin connection as the fundamental field variables.
The scale dependent effective action is required to be invariant both under
space time diffeomorphisms and local frame rotations. The nonperturbative RG
equation is solved explicitly on the truncated theory space defined by a three
parameter family of Holst-type actions which involve a running Immirzi
parameter. We find evidence for the existence of an asymptotically safe
fundamental theory, probably inequivalent to metric quantum gravity constructed
in the same way.Comment: 5 pages, 1 figur
The Heavy-Flavour Contribution to Proton Structure
We present theoretical and experimental considerations pertaining to deeply
inelastic heavy-flavour production at HERA. The various theoretical
uncertainties in the cross section calculation are discussed. Cuts are imposed
to determine the fraction of charm production accessible to the detectors. The
production of charm at asymptotic and bottom production are also covered.
Experimental aspects include current charm production data analysis and
prospects for future analyses including anticipated high precision and
distinguishing photon-gluon fusion charm events from excitation from the charm
parton density. The feasibility of measuring is
investigated.Comment: 22 total pages with 16 figures. To appear in abbreviated form in the
proceedings of the workshop ``Future Physics at HERA'', DESY, Hamburg, 199
Extended microsatellite analysis in microsatellite stable, MSH2 and MLH1 mutation-negative HNPCC patients: Genetic reclassification and correlation with clinical features
Background: Hereditary nonpolyposis colorectal cancer (HNPCC) is an autosomal dominant disorder predisposing to predominantly colorectal cancer (CRC) and endometrial cancer frequently due to germline mutations in DNA mismatch repair (MMR) genes, mainly MLH1, MSH2 and also MSH6 in families seen to demonstrate an excess of endometrial cancer. As a consequence, tumors in HNPCC reveal alterations in the length of simple repetitive genomic sequences like poly-A, poly-T, CA or GT repeats (microsatellites) in at least 90% of the cases. Aim of the Study: The study cohort consisted of 25 HNPCC index patients ( 19 Amsterdam positive, 6 Bethesda positive) who revealed a microsatellite stable (MSS) - or low instable (MSI-L) - tumor phenotype with negative mutation analysis for the MMR genes MLH1 and MSH2. An extended marker panel (BAT40, D10S197, D13S153, D18S58, MYCL1) was analyzed for the tumors of these patients with regard to three aspects. First, to reconfirm the MSI-L phenotype found by the standard panel; second, to find minor MSIs which might point towards an MSH6 mutation, and third, to reconfirm the MSS status of hereditary tumors. The reconfirmation of the MSS status of tumors not caused by mutations in the MMR genes should allow one to define another entity of hereditary CRC. Their clinical features were compared with those of 150 patients with sporadic CRCs. Results: In this way, 17 MSS and 8 MSI-L tumors were reclassified as 5 MSS, 18 MSI-L and even 2 MSI-H ( high instability) tumors, the last being seen to demonstrate at least 4 instable markers out of 10. Among all family members, 87 malignancies were documented. The mean age of onset for CRCs was the lowest in the MSI-H-phenotyped patients with 40.5 +/- 4.9 years (vs. 47.0 +/- 14.6 and 49.8 +/- 11.9 years in MSI-L- and MSS-phenotyped patients, respectively). The percentage of CRC was the highest in families with MSS-phenotyped tumors (88%), followed by MSI-L-phenotyped ( 78%) and then by MSI-H-phenotyped (67%) tumors. MSS tumors were preferentially localized in the distal colon supposing a similar biologic behavior like sporadic CRC. MSH6 mutation analysis for the MSI-L and MSI-H patients revealed one truncating mutation for a patient initially with an MSS tumor, which was reclassified as MSI-L by analyzing the extended marker panel. Conclusion: Extended microsatellite analysis serves to evaluate the sensitivity of the reference panel for HNPCC detection and permits phenotype confirmation or upgrading. Additionally, it confirms the MSS status of hereditary CRCs not caused by the common mutations in the MMR genes and provides hints to another entity of hereditary CRC. Copyright (C) 2004 S. Karger AG, Basel
Fractal space-times under the microscope: A Renormalization Group view on Monte Carlo data
The emergence of fractal features in the microscopic structure of space-time
is a common theme in many approaches to quantum gravity. In this work we carry
out a detailed renormalization group study of the spectral dimension and
walk dimension associated with the effective space-times of
asymptotically safe Quantum Einstein Gravity (QEG). We discover three scaling
regimes where these generalized dimensions are approximately constant for an
extended range of length scales: a classical regime where , a
semi-classical regime where , and the UV-fixed point
regime where . On the length scales covered by
three-dimensional Monte Carlo simulations, the resulting spectral dimension is
shown to be in very good agreement with the data. This comparison also provides
a natural explanation for the apparent puzzle between the short distance
behavior of the spectral dimension reported from Causal Dynamical
Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic
Safety.Comment: 26 pages, 6 figure
Quantum Einstein Gravity
We give a pedagogical introduction to the basic ideas and concepts of the
Asymptotic Safety program in Quantum Einstein Gravity. Using the continuum
approach based upon the effective average action, we summarize the state of the
art of the field with a particular focus on the evidence supporting the
existence of the non-trivial renormalization group fixed point at the heart of
the construction. As an application, the multifractal structure of the emerging
space-times is discussed in detail. In particular, we compare the continuum
prediction for their spectral dimension with Monte Carlo data from the Causal
Dynamical Triangulation approach.Comment: 87 pages, 13 figures, review article prepared for the New Journal of
Physics focus issue on Quantum Einstein Gravit
Single \pi^- production in np collisions for excess energies up to 90 MeV
The quasifree reaction np\to pp\pim was studied in a kinematically complete
experiment by bombarding a liquid hydrogen target with a deuteron beam of
momentum 1.85 GeV/c and analyzing the data along the lines of the spectator
model. In addition to the three charged ejectiles the spectator proton was also
detected in the large-acceptance time-of-flight spectrometer COSY-TOF. It was
identified by its momentum and flight direction thus yielding access to the
Fermi motion of the bound neutron and to the effective neutron 4-momentum
vector which differed from event to event. A range of almost 90
MeV excess energy above threshold was covered. Energy dependent angular
distributions, invariant mass spectra as well as fully covered Dalitz plots
were deduced. Sizeable FSI effects were found as were contributions of
and partial waves. The behavior of the elementary cross section
close to threshold is discussed in view of new cross section
data. In comparison with existing literature data the results provide a
sensitive test of the spectator model.Comment: 21 pages, 9 figures, 4 tables, submitted to EPJ
Charm quark and D^* cross sections in deeply inelastic scattering at DESY HERA
A next-to-leading order Monte Carlo program for the calculation of heavy
quark cross sections in deeply inelastic scattering is described. Concentrating
on charm quark and D^*(2010) production at HERA, several distributions are
presented and their variation with respect to charm quark mass, parton
distribution set, and renormalization-factorization scale is studied.Comment: 15 pages including 8 figures. Uses Latex, Revtex, and psfig.
References added - others updated. Several sentences/words added for clarity.
Results/conclusions unchanged. To appear in Phys. Rev.
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