5,764 research outputs found
Mass Effects in Hard Exclusive Photoproduction of Mesons
We report on an attempt to describe hard exclusive photoproduction of
mesons, i.e. the reaction , by means of a
modified version of the hard-scattering approach, in which the proton is
treated as a quark-diquark rather than a three-quark system. In order to
improve the applicability of the model at momentum transfers of only a few GeV
we take into account constituent-mass effects in the calculation of the
perturbative scattering amplitude. With a standard -meson distribution
amplitude and diquark-model parameters adopted from preceding investigations of
other photon-induced reactions our predictions for differential cross sections
overestimate the naive extrapolation of the low-momentum transfer ZEUS data.
Our results, however, reveal the importance of taking into account the
charm-quark mass.Comment: 4 pages, 4 eps-figures, uses espcrc1.st
Double handbag description of proton-antiproton annihilation into a heavy meson pair
We propose to describe the process in a
perturbative QCD motivated framework where a double-handbag hard process factorizes from transition distribution
amplitudes, which are quasi forward hadronic matrix elements of operators, where q denotes light quarks and c denotes the heavy quark.
We advocate that the charm-quark mass acts as the large scale allowing this
factorization. We calculate this process in the simplified framework of the
scalar diquark model and present the expected cross sections for the PANDA
experiment at GSI-FAIR.Comment: 25 page
-production in -collisions within a double handbag approach
We estimate the scattering amplitude of the process within a double-handbag framework where transition distribution
amplitudes, calculated through an overlap representation, factorize from a hard
subprocess. This process will be measured in the
experiment at GSI-FAIR.Comment: Talk given at the 11th International Conference on Low Energy
Antiproton Physics (LEAP2013
Hard exclusive photoproduction of and mesons
We present predictions for differential cross sections for the reaction
and give an outlook to which extent our calculations may
be generalized to the photoproduction of mesons. Our results are
obtained within perturbative QCD treating the proton as a quark-diquark system.Comment: 4 pages, 1 figure, uses Elsevier style espcrc1.st
Witten index, axial anomaly, and Krein's spectral shift function in supersymmetric quantum mechanics
A new method is presented to study supersymmetric quantum mechanics. Using relative scattering techniques, basic relations are derived between Kreinâs spectral shift function, the Witten index, and the anomaly. The topological invariance of the spectral shift function is discussed. The power of this method is illustrated by treating various models and calculating explicitly the spectral shift function, the Witten index, and the anomaly. In particular, a complete treatment of the twoâdimensional magnetic field problem is given, without assuming that the magnetic flux is quantized
Transient Nucleation near the Mean-Field Spinodal
Nucleation is considered near the pseudospinodal in a one-dimensional
model with a non-conserved order parameter and long-range
interactions. For a sufficiently large system or a system with slow relaxation
to metastable equilibrium, there is a non-negligible probability of nucleation
occurring before reaching metastable equilibrium. This process is referred to
as transient nucleation. The critical droplet is defined to be the
configuration of maximum likelihood that is dynamically balanced between the
metastable and stable wells. Time-dependent droplet profiles and nucleation
rates are derived, and theoretical results are compared to computer simulation.
The analysis reveals a distribution of nucleation times with a distinct peak
characteristic of a nonstationary nucleation rate. Under the quench conditions
employed, transient critical droplets are more compact than the droplets found
in metastable equilibrium simulations and theoretical predictions.Comment: 7 Pages, 5 Figure
Electroweak form factors of heavy-light mesons -- a relativistic point-form approach
We present a general relativistic framework for the calculation of the
electroweak structure of heavy-light mesons within constituent-quark models. To
this aim the physical processes in which the structure is measured, i.e.
electron-meson scattering and semileptonic weak decays, are treated in a
Poincar\'e invariant way by making use of the point-form of relativistic
quantum mechanics. The electromagnetic and weak meson currents are extracted
from the 1- and 1--exchange amplitudes that result from a
Bakamjian-Thomas type mass operator for the respective systems. The covariant
decomposition of these currents provides the electromagnetic and weak
(transition) form factors. Problems with cluster separability, which are
inherent in the Bakamjian-Thomas construction, are discussed and it is shown
how to keep them under control. It is proved that the heavy-quark limit of the
electroweak form factors leads to one universal function, the Isgur-Wise
function, confirming that the requirements of heavy-quark symmetry are
satisfied. A simple analytical expression is given for the Isgur-Wise function
and its agreement with a corresponding front-form calculation is verified
numerically. Electromagnetic form factors for and and weak
-decay form factors are calculated with a simple
harmonic-oscilllator wave function and heavy-quark symmetry breaking due to
finite masses of the heavy quarks is discussed.Comment: 20 pages, 14 figure
Genomics and epigenomics: new promises of personalized medicine for cancer patients
Recent years have brought about a marked extension of our understanding of the somatic basis of cancer. Parallel to the large-scale investigation of diverse tumor genomes the knowledge arose that cancer pathologies are most often not restricted to single genomic events. In contrast, a large number of different alterations in the genomes and epigenomes come together and promote the malignant transformation. The combination of mutations, structural variations and epigenetic alterations differs between each tumor, making individual diagnosis and treatment strategies necessary. This view is summarized in the new discipline of personalized medicine. To satisfy the ideas of this approach each tumor needs to be fully characterized and individual diagnostic and therapeutic strategies designed. Here, we will discuss the power of high-throughput sequencing technologies for genomic and epigenomic analyses. We will provide insight into the current status and how these technologies can be transferred to routine clinical usage
Electromagnetic meson form factor from a relativistic coupled-channel approach
Point-form relativistic quantum mechanics is used to derive an expression for
the electromagnetic form factor of a pseudoscalar meson for space-like momentum
transfers. The elastic scattering of an electron by a confined quark-antiquark
pair is treated as a relativistic two-channel problem for the and
states. With the approximation that the total velocity of the
system is conserved at (electromagnetic) interaction vertices this
simplifies to an eigenvalue problem for a Bakamjian-Thomas type mass operator.
After elimination of the channel the electromagnetic meson
current and form factor can be directly read off from the one-photon-exchange
optical potential. By choosing the invariant mass of the electron-meson system
large enough, cluster separability violations become negligible. An equivalence
with the usual front-form expression, resulting from a spectator current in the
reference frame, is established. The generalization of this
multichannel approach to electroweak form factors for an arbitrary bound
few-body system is quite obvious. By an appropriate extension of the Hilbert
space this approach is also able to accommodate exchange-current effects.Comment: 30 pages, 5 figure
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