7,501 research outputs found
Relaxation Tribometry: A Generic Method to Identify the Nature of Contact Forces
Recent years have witnessed the development of so-called relaxation
tribometers, the free oscillation of which is altered by the presence of
frictional stresses within the contact. So far, analysis of such oscillations
has been restricted to the shape of their decaying envelope, to identify in
particular solid or viscous friction components. Here, we present a more
general expression of the forces possibly acting within the contact , and
retain six possible, physically relevant terms. Two of them, which had never
been proposed in the context of relaxation tribometry, only affect the
oscillation frequency, not the amplitude of the signal. We demonstrate that
each of those six terms has a unique signature in the time-evolution of the
oscillation, which allows efficient identification of their respective weights
in any experimental signal. We illustrate our methodology on a PDMS
sphere/glass plate torsional contact
QSO Absorption Line Constraints on Intragroup High-Velocity Clouds
We show that the number statistics of moderate redshift MgII and Lyman limit
absorbers may rule out the hypothesis that high velocity clouds are infalling
intragroup material.Comment: 4 pages, no figures; submitted to Astrophysical Journal Letters;
revised version, more general and includes more about Braun and Burton CHVC
Weaving Lighthouses and Stitching Stories: Blind and Visually Impaired People Designing E-textiles
We describe our experience of working with blind and visually impaired people to create interactive art objects that are personal to them, through a participatory making process using electronic textiles (e-textiles) and hands-on crafting techniques. The research addresses both the practical considerations about how to structure hands-on making workshops in a way which is accessible to participants of varying experience and abilities, and how effective the approach was in enabling participants to tell their own stories and feel in control of the design and making process. The results of our analysis is the offering of insights in how to run e-textile making sessions in such a way for them to be more accessible and inclusive to a wider community of participants
Yukawa Textures From Heterotic Stability Walls
A holomorphic vector bundle on a Calabi-Yau threefold, X, with h^{1,1}(X)>1
can have regions of its Kahler cone where it is slope-stable, that is, where
the four-dimensional theory is N=1 supersymmetric, bounded by "walls of
stability". On these walls the bundle becomes poly-stable, decomposing into a
direct sum, and the low energy gauge group is enhanced by at least one
anomalous U(1) gauge factor. In this paper, we show that these additional
symmetries can strongly constrain the superpotential in the stable region,
leading to non-trivial textures of Yukawa interactions and restrictions on
allowed masses for vector-like pairs of matter multiplets. The Yukawa textures
exhibit a hierarchy; large couplings arise on the stability wall and some
suppressed interactions "grow back" off the wall, where the extended U(1)
symmetries are spontaneously broken. A number of explicit examples are
presented involving both one and two stability walls, with different
decompositions of the bundle structure group. A three family standard-like
model with no vector-like pairs is given as an example of a class of SU(4)
bundles that has a naturally heavy third quark/lepton family. Finally, we
present the complete set of Yukawa textures that can arise for any holomorphic
bundle with one stability wall where the structure group breaks into two
factors.Comment: 53 pages, 4 figures and 13 table
A discontinuity in the -radius relation of M-dwarfs
We report on 13 new high-precision measurements of stellar diameters for
low-mass dwarfs obtained by means of near-infrared long-baseline interferometry
with PIONIER at the Very Large Telescope Interferometer. Together with accurate
parallaxes from Gaia DR2, these measurements provide precise estimates for
their linear radii, effective temperatures, masses, and luminosities. This
allows us to refine the effective temperature scale, in particular towards the
coolest M-dwarfs. We measure for late-type stars with enhanced metallicity
slightly inflated radii, whereas for stars with decreased metallicity we
measure smaller radii. We further show that Gaia DR2 effective temperatures for
M-dwarfs are underestimated by 8.2 % and give an empirical
- relation which is better suited for M-dwarfs with between 2600 and 4000 K. Most importantly, we are able to observationally
identify a discontinuity in the -radius plane, which is likely due
to the transition from partially convective M-dwarfs to the fully convective
regime. We found this transition to happen between 3200 K and 3340 K, or
equivalently for stars with masses . We find that in
this transition region the stellar radii are in the range from 0.18 to
0.42 for similar stellar effective temperatures.Comment: 11 pages, 9 figures, accepted in MNRA
Charmonium Suppression and Regeneration from SPS to RHIC
The production of charmonia is investigated for heavy-ion collisions from SPS
to RHIC energies. Our approach incorporates two sources of yield: (i)
a direct contribution arising from early (hard) parton-parton collisions,
subject to subsequent nuclear absorption, quark-gluon plasma and hadronic
dissociation, and (ii) statistical production at the hadronization transition
by coalescence of and quarks. Within an expanding thermal
fireball framework, the model reproduces centrality dependencies
observed at the SPS in Pb-Pb and S-U collisions reasonably well. The study of
the ratio at SPS points at the importance of the hadronic phase
for interactions, possibly related to effects of chiral symmetry
restoration. Predictions are given for the centrality dependence of the
ratio at full RHIC energy. We also calculate the
excitation function of this ratio. The latter exhibits a characteristic minimum
structure signalling the transition from the standard suppression
scenario prevailing at SPS to dominantly thermal regeneration at collider
energies.Comment: 17 pages, 15 figure
Hadron form factors from sum rules for vacuum-to-hadron correlators
We analyse the extraction of the bound-state form factor from
vacuum-to-hadron correlator, which is the basic object for the calculation of
hadron form factors in the method of light-cone sum rules in QCD. We study this
correlator in quantum mechanics, calculate it exactly, and derive the
corresponding OPE. We then apply the standard procedures of QCD sum rules to
isolate the ground-state form factor from this correlator. We demonstrate that
fixing the effective continuum threshold, one of the key ingredients of the
sum-rule calculation of bound-state parameters, poses a serious problem for sum
rules based on vacuum-to-hadron correlators.Comment: 8 page
Combining Lattice QCD Results with Regge Phenomenology in a Description of Quark Distribution Functions
The most striking feature of quark distribution functions transformed to the
longitudinal distance representation is the recognizable separation of small
and large longitudinal distances. While the former are responsible for the
average properties of parton distributions, the latter can be shown to
determine specifically their small- behavior. In this paper we demonstrate
how the distribution at intermediate longitudinal distances can be approximated
by taking into account constraints which follow from the general properties of
parton densities, such as their support and behavior at . We show that
the combined description of small, intermediate, and large longitudinal
distances allows a good approximation of both shape and magnitude of parton
distribution functions. As an application we have calculated low-virtuality C
even and odd (valence) u and d quark parton densities of the nucleon and the
C-even transversity distribution , combining recent QCD sum rules and
lattice QCD results with phenomenological information about their small-
behavior.Comment: LaTeX, 17 pages including 7 figures, shorter version will appear in
Phys. Lett.
RS-invariant all-orders renormalon resummations for some QCD observables
We propose a renormalon-inspired resummation of QCD perturbation theory based
on approximating the renormalization scheme (RS) invariant effective charge
beta-function coefficients by the portion containing the highest power of
=--, for SU() QCD with quark flavours.
This can be accomplished using exact large- all-orders results. The
resulting resummation is RS-invariant and the exact next-to-leading order (NLO)
and next-to-NLO (NNLO) coefficients in any RS are included. This improves on a
previously employed naive resummation of the leading- piece of the
perturbative coefficients which is RS-dependent, making its comparison with
fixed-order perturbative results ambiguous. The RS-invariant resummation is
used to assess the reliability of fixed-order perturbation theory for the
-ratio, the analogous -lepton decay ratio , and
Deep Inelastic Scattering (DIS) sum rules, by comparing it with the exact NNLO
results in the effective charge RS. For the -ratio and , where
large-order perturbative behaviour is dominated by a leading ultra-violet
renormalon singularity, the comparison indicates fixed-order perturbation
theory to be very reliable. For DIS sum rules, which have a leading infra-red
renormalon singularity, the performance is rather poor. In this way we estimate
that at LEP/SLD energies ideal data on the -ratio could determine
to three-significant figures, and for the we
estimate a theoretical uncertainty
corresponding to . This encouragingly small
uncertainty is much less than has recently been deduced from comparison with
the ambiguous naive resummation.Comment: 25 pages, uses LaTeX, 12 Postscript figures, epsfig.sty 'elsart.sty'
and 'elsart12.sty' are available via anonymous-ftp at
ftp://ftp.tex.ac.uk/tex-archive/macros/latex/contrib/supported/elsevie
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