244 research outputs found
Rare charm meson decays D->Pl^+l^- and c->ul^+l^- in SM and MSSM
We study the nine possible rare charm meson decays D->Pl^+l^-
(P=pi,K,eta,eta') using the Heavy Meson Chiral Lagrangians and find them to be
dominated by the long distance contributions. The decay D^+ -> pi^+l^+l^- with
the branching ratio 1*10^(-6) is expected to have the best chances for an early
experimental discovery. The short distance contribution in the five Cabibbo
suppressed channels arises via the c->ul^+l^- transition; we find that this
contribution is detectable only in the D->pi l^+l^- decay, where it dominates
the differential spectrum at high-q^2. The general Minimal Supersymmetric
Standard Model can enhance the c->ul^+l^- rate by up to an order of magnitude;
its effect on the D->Pl^+l^- rates is small since the c->ul^+l^- enhancement is
sizable in low-q^2 region, which is inhibited in the hadronic decay.Comment: 17 page
Calorimetric and transport investigations of CePd_{2+x}Ge_{2-x} (x=0 and 0.02) up to 22 GPa
The influence of pressure on the magnetically ordered CePd_{2.02}Ge_{1.98}
has been investigated by a combined measurement of electrical resistivity,
, and ac-calorimetry, C(T), for temperatures in the range 0.3 K<T<10 K
and pressures, p, up to 22 GPa. Simultaneously CePd_2Ge_2 has been examined by
down to 40 mK. In CePd_{2.02}Ge_{1.98} and CePd_2Ge_2 the magnetic
order is suppressed at a critical pressure p_c=11.0 GPa and p_c=13.8 GPa,
respectively. In the case of CePd_{2.02}Ge_{1.98} not only the temperature
coefficient of , A, indicates the loss of magnetic order but also the
ac-signal recorded at low temperature. The residual
resistivity is extremely pressure sensitive and passes through a maximum and
then a minimum in the vicinity of p_c. The (T,p) phase diagram and the
A(p)-dependence of both compounds can be qualitatively understood in terms of a
pressure-tuned competition between magnetic order and the Kondo effect
according to the Doniach picture. The temperature-volume (T,V) phase diagram of
CePd_2Ge_2 combined with that of CePd_2Si_2 shows that in stoichiometric
compounds mainly the change of interatomic distances influences the exchange
interaction. It will be argued that in contrast to this the much lower
p_c-value of CePd_{2.02}Ge_{1.98} is caused by an enhanced hybridization
between 4f and conduction electrons.Comment: 9 pages, 7 figure
Gamma Ray Bursts as Probes of Quantum Gravity
Gamma ray bursts (GRBs) are short and intense pulses of -rays
arriving from random directions in the sky. Several years ago Amelino-Camelia
et al. pointed out that a comparison of time of arrival of photons at different
energies from a GRB could be used to measure (or obtain a limit on) possible
deviations from a constant speed of light at high photons energies. I review
here our current understanding of GRBs and reconsider the possibility of
performing these observations.Comment: Lectures given at the 40th winter school of theretical physics:
Quantum Gravity and Phenomenology, Feb. 2004 Polan
First Observation of Coherent Production in Neutrino Nucleus Interactions with 2 GeV
The MiniBooNE experiment at Fermilab has amassed the largest sample to date
of s produced in neutral current (NC) neutrino-nucleus interactions at
low energy. This paper reports a measurement of the momentum distribution of
s produced in mineral oil (CH) and the first observation of coherent
production below 2 GeV. In the forward direction, the yield of events
observed above the expectation for resonant production is attributed primarily
to coherent production off carbon, but may also include a small contribution
from diffractive production on hydrogen. Integrated over the MiniBooNE neutrino
flux, the sum of the NC coherent and diffractive modes is found to be (19.5
1.1 (stat) 2.5 (sys))% of all exclusive NC production at
MiniBooNE. These measurements are of immediate utility because they quantify an
important background to MiniBooNE's search for
oscillations.Comment: Submitted to Phys. Lett.
Lorentz and CPT Violation in Neutrinos
A general formalism is presented for violations of Lorentz and CPT symmetry
in the neutrino sector. The effective hamiltonian for neutrino propagation in
the presence of Lorentz and CPT violation is derived, and its properties are
studied. Possible definitive signals in existing and future
neutrino-oscillation experiments are discussed. Among the predictions are
direction-dependent effects, including neutrino-antineutrino mixing, sidereal
and annual variations, and compass asymmetries. Other consequences of Lorentz
and CPT violation involve unconventional energy dependences in oscillation
lengths and mixing angles. A variety of simple models both with and without
neutrino masses are developed to illustrate key physical effects. The
attainable sensitivities to coefficients for Lorentz violation in the
Standard-Model Extension are estimated for various types of experiments. Many
experiments have potential sensitivity to Planck-suppressed effects, comparable
to the best tests in other sectors. The lack of existing experimental
constraints, the wide range of available coefficient space, and the variety of
novel effects imply that some or perhaps even all of the existing data on
neutrino oscillations might be due to Lorentz and CPT violation.Comment: 25 pages REVTe
Recurrent mutations in the U2AF1 splicing factor in myelodysplastic syndromes
Myelodysplastic syndromes (MDS) are hematopoietic stem cell disorders that often progress to chemotherapy-resistant secondary acute myeloid leukemia (sAML). We used whole-genome sequencing to perform an unbiased comprehensive screen to discover the somatic mutations in a sample from an individual with sAML and genotyped the loci containing these mutations in the matched MDS sample. Here we show that a missense mutation affecting the serine at codon 34 (Ser34) in U2AF1 was recurrently present in 13 out of 150 (8.7%) subjects with de novo MDS, and we found suggestive evidence of an increased risk of progression to sAML associated with this mutation. U2AF1 is a U2 auxiliary factor protein that recognizes the AG splice acceptor dinucleotide at the 3' end of introns, and the alterations in U2AF1 are located in highly conserved zinc fingers of this protein. Mutant U2AF1 promotes enhanced splicing and exon skipping in reporter assays in vitro. This previously unidentified, recurrent mutation in U2AF1 implicates altered pre-mRNA splicing as a potential mechanism for MDS pathogenesis
Re-structuring of marine communities exposed to environmental change: a global study on the interactive effects of species and functional richness
Species richness is the most commonly used but controversial biodiversity metric in studies on aspects of community stability such as structural composition or productivity. The apparent ambiguity of theoretical and experimental findings may in part be due to experimental shortcomings and/or heterogeneity of scales and methods in earlier studies. This has led to an urgent call for improved and more realistic experiments. In a series of experiments replicated at a global scale we translocated several hundred marine hard bottom communities to new environments simulating a rapid but moderate environmental change. Subsequently, we measured their rate of compositional change (re-structuring) which in the great majority of cases represented a compositional convergence towards local communities. Re-structuring is driven by mortality of community components (original species) and establishment of new species in the changed environmental context. The rate of this re-structuring was then related to various system properties. We show that availability of free substratum relates negatively while taxon richness relates positively to structural persistence (i.e., no or slow re-structuring). Thus, when faced with environmental change, taxon-rich communities retain their original composition longer than taxon-poor communities. The effect of taxon richness, however, interacts with another aspect of diversity, functional richness. Indeed, taxon richness relates positively to persistence in functionally depauperate communities, but not in functionally diverse communities. The interaction between taxonomic and functional diversity with regard to the behaviour of communities exposed to environmental stress may help understand some of the seemingly contrasting findings of past research.Mercator Stiftung via GAMEPostprint4,41
SU(3) Breaking and D0-D0bar Mixing
The main challenge in the Standard Model calculation of the mass and width
difference in the D0-D0bar system is to estimate the size of SU(3) breaking
effects. We prove that D meson mixing occurs in the Standard Model only at
second order in SU(3) violation. We consider the possibility that phase space
effects may be the dominant source of SU(3) breaking. We find that y=(Delta
Gamma)/(2Gamma) of the order of one percent is natural in the Standard Model,
potentially reducing the sensitivity to new physics of measurements of D meson
mixing.Comment: 18 pages; minor corrections, version to appear in Phys. Rev.
Energy Flow in the Hadronic Final State of Diffractive and Non-Diffractive Deep-Inelastic Scattering at HERA
An investigation of the hadronic final state in diffractive and
non--diffractive deep--inelastic electron--proton scattering at HERA is
presented, where diffractive data are selected experimentally by demanding a
large gap in pseudo --rapidity around the proton remnant direction. The
transverse energy flow in the hadronic final state is evaluated using a set of
estimators which quantify topological properties. Using available Monte Carlo
QCD calculations, it is demonstrated that the final state in diffractive DIS
exhibits the features expected if the interaction is interpreted as the
scattering of an electron off a current quark with associated effects of
perturbative QCD. A model in which deep--inelastic diffraction is taken to be
the exchange of a pomeron with partonic structure is found to reproduce the
measurements well. Models for deep--inelastic scattering, in which a
sizeable diffractive contribution is present because of non--perturbative
effects in the production of the hadronic final state, reproduce the general
tendencies of the data but in all give a worse description.Comment: 22 pages, latex, 6 Figures appended as uuencoded fil
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