62,337 research outputs found
Exploring the timelike region for the elastic form factor in the light-front quantization
Even though the Drell-Yan-West formulation is the most rigorous and
well-established framework to compute the exclusive processes, its utility has
been limited only to the spacelike region because of the intrinsic kinematic
constraint . We present an explicit example demonstrating how one may
obtain the necessary information (i.e. nonvalence or so called Z-graph
contribution) in the timelike region of exclusive process without encountering
a formidable task of direct calculation that has hindered so far the progress
in this area. In the analysis of bound state form factors using an
exactly solvable model of dimensional scalar field theory interacting
with gauge fields, the results analytically continued from the spacelike region
coincide exactly with the direct results in the timelike region. This example
verifies that the method of analytic continuation is capable of yielding the
effect of complicate nonvalence contributions. The meson peaks analogous to the
vector meson dominance(VMD) phenomena are also generated at the usual VMD
positions.Comment: 21 pages, 8 figures, we changed the title, added some references and
included some paragraphs in the introduction and conclusions; version to
appear in Nucl. Phys.
Frame-Independence of Exclusive Amplitudes in the Light-Front Quantization
While the particle-number-conserving convolution formalism established in the
Drell-Yan-West reference frame is frequently used to compute exclusive
amplitudes in the light-front quantization, this formalism is limited to only
those frames where the light-front helicities are not changed and the good
(plus) component of the current remains unmixed. For an explicit demonstration
of such criteria, we present the relations between the current matrix elements
in the two typical reference frames used for calculations of the exclusive
amplitudes, i.e. the Drell-Yan-West and Breit frames and investigate both
pseudoscalar and vector electromagnetic currents in detail. We find that the
light-front helicities are unchanged and the good component of the current does
not mix with the other components of the current under the transformation
between these two frames. Thus, the pseudoscalar and vector form factors
obtained by the diagonal convolution formalism in both frames must indeed be
identical. However, such coincidence between the Drell-Yan-West and Breit
frames does not hold in general. We give an explicit example in which the
light-front helicities are changed and the plus component of the current is
mixed with other components under the change of reference frame. In such a
case, the relationship between the frames should be carefully analyzed before
the established convolution formalism in the Drell-Yan-West frame is used.Comment: 14 pages, 4 figure
Scalable Task-Based Algorithm for Multiplication of Block-Rank-Sparse Matrices
A task-based formulation of Scalable Universal Matrix Multiplication
Algorithm (SUMMA), a popular algorithm for matrix multiplication (MM), is
applied to the multiplication of hierarchy-free, rank-structured matrices that
appear in the domain of quantum chemistry (QC). The novel features of our
formulation are: (1) concurrent scheduling of multiple SUMMA iterations, and
(2) fine-grained task-based composition. These features make it tolerant of the
load imbalance due to the irregular matrix structure and eliminate all
artifactual sources of global synchronization.Scalability of iterative
computation of square-root inverse of block-rank-sparse QC matrices is
demonstrated; for full-rank (dense) matrices the performance of our SUMMA
formulation usually exceeds that of the state-of-the-art dense MM
implementations (ScaLAPACK and Cyclops Tensor Framework).Comment: 8 pages, 6 figures, accepted to IA3 2015. arXiv admin note: text
overlap with arXiv:1504.0504
Effective Potentials for Light Moduli
We examine recent work on compactifications of string theory with fluxes,
where effective potentials for light moduli have been derived after integrating
out moduli that are assumed to be heavy at the classical level, and then adding
non-perturbative (NP) corrections to the superpotential. We find that this two
stage procedure is not valid and that the correct potential has additional
terms. Althought this does not affect the conclusion of Kachru et al (KKLT)
that the Kaehler moduli may be stabilized by NP effects, it can affect the
detailed physics. In particular it is possible to get metastable dS minima
without adding uplifting terms.Comment: Minor revisions, References added, Version to be published in PLB, 14
pages 3 figure
Moduli-mixing racetrack model
We study supersymmetric models with double gaugino condensations in the
hidden sector, where the gauge couplings depend on two light moduli of
superstring theory. We perform a detailed analysis of this class of model and
show that there is no stable supersymmetric minimum with finite vacuum values
of moduli fields. Instead, we find that the supersymmetry breaking occurs with
moduli stabilized and negative vacuum energy. That yields moduli-dominated soft
supersymmetry breaking terms. To realize slightly positive (or vanishing)
vacuum energy, we add uplifting potential. We discuss uplifting does not change
qualitatively the vacuum expectation values of moduli and the above feature of
supersymmetry breaking.Comment: 23 pages, 4 eps figure
The degenerate gravitino scenario
In this work, we explore the "degenerate gravitino" scenario where the mass
difference between the gravitino and the lightest MSSM particle is much smaller
than the gravitino mass itself. In this case, the energy released in the decay
of the next to lightest sypersymmetric particle (NLSP) is reduced. Consequently
the cosmological and astrophysical constraints on the gravitino abundance, and
hence on the reheating temperature, become softer than in the usual case. On
the other hand, such small mass splittings generically imply a much longer
lifetime for the NLSP. We find that, in the constrained MSSM (CMSSM), for
neutralino LSP or NLSP, reheating temperatures compatible with thermal
leptogenesis are reached for small splittings of order 10^{-2} GeV. While for
stau NLSP, temperatures of 4x10^9 GeV can be obtained even for splittings of
order of tens of GeVs. This "degenerate gravitino" scenario offers a possible
way out to the gravitino problem for thermal leptogenesis in supersymmetric
theories.Comment: 27 pages, 10 figures and 1 table. Minor typos and references fixed.
Matches published version in JCAP
Interference and binding effects in decays of possible molecular component of X(3872)
It is pointed out that the internal structure of the narrow resonance X(3872)
at the D^0 {\bar D}^{*0} threshold can be studied in some detail by measuring
the rate and the spectra in the decays X(3872) \to D^0 {\bar D}^0 \pi^0 and
X(3872) \to D^0 {\bar D}^0 \gamma. In particular, if this resonance contains a
dominant `molecular' component D {\bar D^*} \pm {\bar D} D^*, this component
can be revealed and studied by a distinct pattern of interference between the
underlying decays of D^{*0} and {\bar D}^{*0} whose coherence is ensured by
fixed (but yet unknown) C parity of the X(3872).Comment: 7 pages, 1 figur
New Developments in Treacherous Points of Light-Front Dynamics
Light-front dynamics(LFD) plays an important role in hadron phenomenology.
Last few years, however, it has been emphasized that treacherous points such as
zero-mode contributions should be taken into account for successful LFD
applications to hadron phenomenology. We discuss examples of treacherous points
and present new progresses made last few years to handle them correctly.Comment: 5 pages, espcrc1.sty. proceedings of FB XVIII (August 2006, Brazil),
to be published in Nucl. Phys.
Numerical shadows: measures and densities on the numerical range
For any operator acting on an -dimensional Hilbert space we
introduce its numerical shadow, which is a probability measure on the complex
plane supported by the numerical range of . The shadow of at point
is defined as the probability that the inner product is equal to ,
where stands for a random complex vector from , satisfying .
In the case of N=2 the numerical shadow of a non-normal operator can be
interpreted as a shadow of a hollow sphere projected on a plane. A similar
interpretation is provided also for higher dimensions. For a hermitian its
numerical shadow forms a probability distribution on the real axis which is
shown to be a one dimensional -spline. In the case of a normal the
numerical shadow corresponds to a shadow of a transparent solid simplex in
onto the complex plane. Numerical shadow is found explicitly for
Jordan matrices , direct sums of matrices and in all cases where the
shadow is rotation invariant. Results concerning the moments of shadow measures
play an important role. A general technique to study numerical shadow via the
Cartesian decomposition is described, and a link of the numerical shadow of an
operator to its higher-rank numerical range is emphasized.Comment: 37 pages, 8 figure
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