1,477 research outputs found
Helicity Probabilities For Heavy Quark Fragmentation Into Excited Mesons
In the fragmentation of a heavy quark into a heavy meson whose light degrees
of freedom have angular momentum , all the helicity probabilities are
completely determined in the heavy quark limit up to a single probability
. We point out that this probability depends on the longitudinal
momentum fraction of the meson and on its transverse momentum
relative to the jet axis. We calculate as a function of scaling
variables corresponding to and for the heavy quark limit of the
perturbative QCD fragmentation functions for quark to fragment into mesons. In this model, the light degrees of freedom prefer to have
their angular momentum aligned transverse to, rather than along, the jet axis.
Implications for the production of excited heavy mesons, like and
, are discussed.Comment: 10 pages, Latex file plus 3 figures with postscript files appended at
the en
Energetics and stability of dangling-bond silicon wires on H passivated Si(100)
We evaluate the electronic, geometric and energetic properties of quasi 1-D
wires formed by dangling bonds on Si(100)-H (2 x 1). The calculations are
performed with density functional theory (DFT). Infinite wires are found to be
insulating and Peierls distorted, however finite wires develop localized
electronic states that can be of great use for molecular-based devices. The
ground state solution of finite wires does not correspond to a geometrical
distortion but rather to an antiferromagnetic ordering. For the stability of
wires, the presence of abundant H atoms in nearby Si atoms can be a problem. We
have evaluated the energy barriers for intradimer and intrarow diffusion
finding all of them about 1 eV or larger, even in the case where a H impurity
is already sitting on the wire. These results are encouraging for using
dangling-bond wires in future devices.Comment: 8 pages, 6 figure
Language and theory of mind in autism spectrum disorder : the relationship between complement syntax and false belief task performance.
This study aimed to test the hypothesis that children with autism spectrum disorder (ASD) use their knowledge of complement syntax as a means of âhacking outâ solutions to false belief tasks, despite lacking a representational theory of mind (ToM). Participants completed a âmemory for complementsâ task, a measure of receptive vocabulary, and traditional location change and unexpected contents false belief tasks. Consistent with predictions, the correlation between complement syntax score and location change task performance was significantly stronger within the ASD group than within the comparison group. However, contrary to predictions, complement syntax score was not significantly correlated with unexpected contents task performance within either group. Possible explanations for this pattern of results are considered
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Free Recall Learning of Hierarchically Organised Lists by Adults with Asperger's Syndrome: Additional Evidence for Diminished Relational Processing
The Task Support Hypothesis (TSH, Bowler et al. Neuropsychologia 35:65â70 1997) states that individuals with autism spectrum disorder (ASD) show better memory when test procedures provide support for retrieval. The present study aimed to see whether this principle also applied at encoding. Twenty participants with high-functioning ASD and 20 matched comparison participants studied arrays of 112 words over four trials. Words were arranged either under hierarchically embedded category headings (e.g. InstrumentsâStringâPluckedâViolin) or randomly. Both groups showed similar overall recall and better recall for the hierarchically organised words. However, the ASD participants made less use of information about relations between words and more use of item-specific information in their recall, confirming earlier reports of relational difficulties in this population
Effects of mesonic correlations in the QCD phase transition
The finite temperature phase transition of strongly interacting matter is
studied within a nonlocal chiral quark model of the NJL type coupled to a
Polyakov loop. In contrast to previous investigations which were restricted to
the mean-field approximation, mesonic correlations are included by evaluating
the quark-antiquark ring sum. For physical pion masses, we find that the pions
dominate the pressure below the phase transition, whereas above T_c the
pressure is well described by the mean-field approximation result. For large
pion masses, as realized in lattice simulations, the meson effects are
suppressed.Comment: 11 pages, 4 figures; version accepted for publication in Yad. Fiz.,
text extended, 1 figure adde
O(N) methods in electronic structure calculations
Linear scaling methods, or O(N) methods, have computational and memory
requirements which scale linearly with the number of atoms in the system, N, in
contrast to standard approaches which scale with the cube of the number of
atoms. These methods, which rely on the short-ranged nature of electronic
structure, will allow accurate, ab initio simulations of systems of
unprecedented size. The theory behind the locality of electronic structure is
described and related to physical properties of systems to be modelled, along
with a survey of recent developments in real-space methods which are important
for efficient use of high performance computers. The linear scaling methods
proposed to date can be divided into seven different areas, and the
applicability, efficiency and advantages of the methods proposed in these areas
is then discussed. The applications of linear scaling methods, as well as the
implementations available as computer programs, are considered. Finally, the
prospects for and the challenges facing linear scaling methods are discussed.Comment: 85 pages, 15 figures, 488 references. Resubmitted to Rep. Prog. Phys
(small changes
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Ensemble prediction for nowcasting with a convection-permitting model - II: forecast error statistics
A 24-member ensemble of 1-h high-resolution forecasts over the Southern United Kingdom is used to study short-range forecast error statistics. The initial conditions are found from perturbations from an ensemble transform Kalman filter. Forecasts from this system are assumed to lie within the bounds of forecast error of an operational forecast system. Although noisy, this system is capable of producing physically reasonable statistics which are analysed and compared to statistics implied from a variational assimilation system. The variances for temperature errors for instance show structures that reflect convective activity. Some variables, notably potential temperature and specific humidity perturbations, have autocorrelation functions that deviate from 3-D isotropy at the convective-scale (horizontal scales less than 10 km). Other variables, notably the velocity potential for horizontal divergence perturbations, maintain 3-D isotropy at all scales. Geostrophic and hydrostatic balances are studied by examining correlations between terms in the divergence and vertical momentum equations respectively. Both balances are found to decay as the horizontal scale decreases. It is estimated that geostrophic balance becomes less important at scales smaller than 75 km, and hydrostatic balance becomes less important at scales smaller than 35 km, although more work is required to validate these findings. The implications of these results for high-resolution data assimilation are discussed
Four new T dwarfs identified in PanSTARRS 1 commissioning data
A complete well-defined sample of ultracool dwarfs is one of the key science
programs of the Pan-STARRS 1 optical survey telescope (PS1). Here we combine
PS1 commissioning data with 2MASS to conduct a proper motion search
(0.1--2.0\arcsec/yr) for nearby T dwarfs, using optical+near-IR colors to
select objects for spectroscopic followup. The addition of sensitive far-red
optical imaging from PS1 enables discovery of nearby ultracool dwarfs that
cannot be identified from 2MASS data alone. We have searched 3700 sq. deg. of
PS1 y-band (0.95--1.03 um) data to y19.5 mag (AB) and J16.5
mag (Vega) and discovered four previously unknown bright T dwarfs. Three of the
objects (with spectral types T1.5, T2 and T3.5) have photometric distances
within 25 pc and were missed by previous 2MASS searches due to more restrictive
color selection criteria. The fourth object (spectral type T4.5) is more
distant than 25 pc and is only a single-band detection in 2MASS. We also
examine the potential for completing the census of nearby ultracool objects
with the PS1 3 survey.Comment: 25 pages, 8 figures, 5 table, AJ accepted, updated to comply with
Pan-STARRS1 naming conventio
The Semileptonic Decays and from Lattice QCD
We present a lattice QCD calculation of the form factors and differential
decay rates for semileptonic decays of the heavy-light mesons and to
the final state . The results are obtained with three methodological
improvements over previous lattice calculations: a matching procedure that
reduces heavy-quark lattice artifacts, the first study of lattice-spacing
dependence, and the introduction of kinematic cuts to reduce model dependence.
We show that the main systematics are controllable (within the quenched
approximation) and outline how the calculations could be improved to aid
current experiments in the determination of~ and~.Comment: 35 pp, 12 fig
Heavy Flavour Baryons in Hyper Central Model
Heavy flavor baryons containing single and double charm (beauty) quarks with
light flavor combinations are studied using the hyper central description of
the three-body problem. The confinement potential is assumed as hyper central
coulomb plus power potential with power index . The ground state
masses of the heavy flavor, and baryons are computed
for different power index, starting from 0.5 to 2.0. The predicted
masses are found to attain a saturated value in each case of quark combinations
beyond the power index .Comment: 10 pages, 4 figure
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