828 research outputs found
Study of Doubly Heavy Baryon Spectrum via QCD Sum Rules
In this work, we calculate the mass spectrum of doubly heavy baryons with the
diquark model in terms of the QCD sum rules. The interpolating currents are
composed of a heavy diquark field and a light quark field. Contributions of the
operators up to dimension six are taken into account in the operator product
expansion. Within a reasonable error tolerance, our numerical results are
compatible with other theoretical predictions. This indicates that the diquark
picture reflects the reality and is applicable to the study of doubly heavy
baryons.Comment: 23 pages, 9 figures, minor corrections in expression
Classes of exact wavefunctions for general time-dependent Dirac Hamiltonians in 1+1 dimensions
In this work we construct two classes of exact solutions for the most general
time-dependent Dirac Hamiltonian in 1+1 dimensions. Some problems regarding to
some formal solutions in the literature are discussed. Finally the existence of
a generalized Lewis-Riesenfeld invariant connected with such solutions is
discussed
A self-calibration rotational stitching method for precision measurement of revolving surfaces
When measuring revolving objects, it is often desired to obtain not only the geometrical form of the workpiece, but also the topography of the surface, as they both affect the performance of the part. However, holistic measurement of the entire three-dimensional surface of a revolving part is challenging since most surface measurement instruments only have limited measurement ability, where the bottom and the side surfaces cannot be measured. One solution to obtain geometrical form and surface topography information simultaneously is to add a precision axis to rotate the object while performing surface topography measurement. However, this solution requires a high-cost precision rotation stage and adjustable mounting and alignment aids. Moreover, errors in the rotation will be added to the measurement result, which can be difficult to compensate. Stitching is a method often used for measuring revolving surfaces without the need for precision motion axes, as the method is applied at the software level, and errors in the rotation can be compensated by the stitching algorithm. Nevertheless, the overall accuracy of stitching is limited when the number of sub-surfaces is large, since the measurement and stitching error accumulate along the stitching chain. In this paper, a self-calibration rotational stitching method is presented which can compensate for the accumulated error. The self-calibration method utilises the inherent nature of a revolving surface and compensates for the registration error by aligning the last dataset with the first dataset. The proposed method is demonstrated by measuring grinding wheels with a coherence scanning interferometer and simultaneously rotating the grinding wheels with a low-cost stepper-motor. It is demonstrated that the proposed stitching measurement method is effective in compensating for accumulated registration error. The proposed self-calibration rotational stitching method can be easily extended to a wide range of applications for measuring revolving surfaces using various measuring instruments
Any-degrees-of-freedom (anyDOF) registration for the characterization of freeform surfaces
This paper presents an any-degrees-of-freedom (anyDOF) registration method for the characterization of freeform surfaces. The method attempts to fill the research gap regarding traditional surface registration methods which are normally dedicated to solving the global optimization problem with all DOF but they lack flexibility. The proposed anyDOF method is capable of registering surfaces with any specified combination of DOF. This is particularly useful when some of the DOF are known to be unchanged according to the a priori knowledge. The anyDOF surface registration method is regarded as a typical optimization problem of finding the minimum distance from target surface to the reference surface, with constraints of the unwanted DOF. The problem is solved by the Levenberg-Marquardt method. Simulated experiments for a two-dimensional (2D) profile and a three-dimensional (3D) surface were undertaken, together with three measurement experiments including a fluid-jet polished surface, a bonnet polished surface and a diamond machined freeform surface. Experimental results show that the anyDOF registration method is highly flexible in the characterization of freeform surfaces
Improved Method of Fatigue Life Assessment for TiAl Alloys
For rapid fatigue life assessment of TiAl alloys,
the new method was proposed based on qualitative
and quantitative analyses. The qualitative
analysis was employed to illustrate the microstructure
effect for TiAl alloys on their fatigue
life. The new formula is derived for estimation
of the interaction forces of dislocations, which
yields quite satisfactory results. The results of
qualitative and quantitative analyses were used
to predict the fatigue life improvement by the
addition of trace elements producing grain refinement.Предложен новый метод оценки усталостной долговечности TiAl сплавов по результатам
количественного и качественного анализа. С помощью количественного анализа определено
влияние микроструктуры TiAl сплавов на их усталостную долговечность. Расчет усилия
между дислокациями проведен по стандартной формуле, с помощью которой можно получить точный результат. Количественный и качественный анализ показал, что усталостную
долговечность можно повысить путем введения микроэлементов, которые способствуют измельчению зерен.Запропоновано новий метод оцінки довговічності від утомленості ТіАl сплавів за
результатами кількісного та якісного аналізу. За допомогою кількісного аналізу
визначено вплив мікроструктури TiAl сплавів на їх довговічність від утомленості.
Розрахунок зусилля між дислокаціями проведено за стандартною формулою, за
допомогою якої можна отримати точний результат. Кількісний та якісний аналіз
показав, що довговічність від утомленості можна підвищити шляхом введення мікроелементів, які сприяють подрібненню зерен
Photoproduction in Scattering
We investigate the + c + photoproduction in
collision at the LEP II energy. The physical motivations for this study are: 1)
such next-to-leading order(NLO) process was not considered in previous
investigations of photoproduction in interaction, and it is
worthwhile to do so in order to make sound predictions for experimental
comparison; 2) from recent Belle experiment results, the process with same
final states at the factory has a theoretically yet unexplainable large
fraction; hence it is interesting to see what may happen at other colliders; 3)
the existing LEP data are marginal in observing such process, and at the planed
Linear Colliders(LCs) this process can be measured with high accuracy; 4) it is
necessary to take this process into consideration in the aim of elucidating the
quarkonium production mechanism, especially in testing the universality of
NRQCD nonperturbative matrix elements via photoproduction in
electron-position collisions.Comment: 15 pages, 3 figure
Heart of Darkness: The Significance of the Zeptobarn Scale for Neutralino Direct Detection
The direct detection of dark matter through its elastic scattering off
nucleons is among the most promising methods for establishing the particle
identity of dark matter. The current bound on the spin-independent scattering
cross section is sigma^SI < 10 zb for dark matter masses m_chi ~ 100 GeV, with
improved sensitivities expected soon. We examine the implications of this
progress for neutralino dark matter. We work in a supersymmetric framework
well-suited to dark matter studies that is simple and transparent, with models
defined in terms of four weak-scale parameters. We first show that robust
constraints on electric dipole moments motivate large sfermion masses mtilde >
1 TeV, effectively decoupling squarks and sleptons from neutralino dark matter
phenomenology. In this case, we find characteristic cross sections in the
narrow range 1 zb 70 GeV. As sfermion masses are
lowered to near their experimental limit mtilde ~ 400 GeV, the upper and lower
limits of this range are extended, but only by factors of around two, and the
lower limit is not significantly altered by relaxing many particle physics
assumptions, varying the strange quark content of the nucleon, including the
effects of galactic small-scale structure, or assuming other components of dark
matter. Experiments are therefore rapidly entering the heart of dark
matter-favored supersymmetry parameter space. If no signal is seen,
supersymmetric models must contain some level of fine-tuning, and we identify
and analyze several possibilities. Barring large cancellations, however, in a
large and generic class of models, if thermal relic neutralinos are a
significant component of dark matter, experiments will discover them as they
probe down to the zeptobarn scale.Comment: 35 pages, 11 figures; v2: references added, figures extended to 2 TeV
neutralino masses, XENON100 results included, published versio
Reheating Temperature and Gauge Mediation Models of Supersymmetry Breaking
For supersymmetric theories with gravitino dark matter, the maximal reheating
temperature consistent with big bang nucleosynthesis bounds arises when the
physical gaugino masses are degenerate. We consider the cases of a stau or
sneutrino next-to-lightest superpartner, which have relatively less constraint
from big bang nucleosynthesis. The resulting parameter space is consistent with
leptogenesis requirements, and can be reached in generalized gauge mediation
models. Such models illustrate a class of theories that overcome the well-known
tension between big bang nucleosynthesis and leptogenesis.Comment: 30 pages, 4 figures; v2: refs adde
Color-singlet and color-octet J/psi production in top quark rare decays
J/psi production in top quark rare decays is investigated under the framework
of NRQCD factorization formalism. Various production channels are studied, and
we find that the contributions from the color-singlet quark fragmentation and
the color-octet gluon fragmentation are both over 3 orders larger than that
from the leading order color-singlet process. The numerical results show that
the branching ratio B(t -> c + J/psi + X) is about 10^{-14} in the SM, and
10^{-10} in the MSSM.Comment: 10 pages, RevTex, 4 postscript figures. To appear in Phys. Rev.
Inelastic production in polarized photon-hadron collisions
Presented here is a calculation of inelastic production in polarized
photon-hadron collisions under the framework of NRQCD factorization formalism.
We consider the photoproduction of \jpsi in the energy range relevant to
HERA. The Weizs\"acker-Williams approximation is adopted in the evaluation of
the cross sections for collisions. We found that this process can give
another independent test for the color-octet mechanism, and the different
features for the two color-octet processes may provide further informations on
the mechanism for inelastic \jpsi photoproduction. And the discrepancy on the
production asymmetry between various sets of polarized gluon distribution
functions is also found to be distinctive.Comment: 14pages, 6 PS figure
- …