5,382 research outputs found
The cosmological constant and the relaxed universe
We study the role of the cosmological constant (CC) as a component of dark
energy (DE). It is argued that the cosmological term is in general unavoidable
and it should not be ignored even when dynamical DE sources are considered.
From the theoretical point of view quantum zero-point energy and phase
transitions suggest a CC of large magnitude in contrast to its tiny observed
value. Simply relieving this disaccord with a counterterm requires extreme
fine-tuning which is referred to as the old CC problem. To avoid it, we discuss
some recent approaches for neutralising a large CC dynamically without adding a
fine-tuned counterterm. This can be realised by an effective DE component which
relaxes the cosmic expansion by counteracting the effect of the large CC.
Alternatively, a CC filter is constructed by modifying gravity to make it
insensitive to vacuum energy.Comment: 6 pages, no figures, based on a talk presented at PASCOS 201
Thermal non-equilibrium effects in quantum reflection
We show that the quantum reflection coefficient of ultracold heavy atoms
scattering off a dielectric surface can be tuned in a wide range by suitable
choice of surface and environment temperatures. This effect results from a
temperature dependent long-range repulsive part of the van der
Waals-Casimir-Polder-Lifshitz atom-surface interaction potential
Numbers of the form
For a function , define
N^{\times}_{f}(x)=\#\{n\leq x: n=kf(k) \mbox{ for some k} \}. Let
be the divisor function, be the
prime divisor function, and be
Euler's totient function. We prove that
\begin{gather*} \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! 1) \quad
N^{\times}_{\tau}(x) \asymp \frac{x}{(\log x)^{1/2}}; \\ 2) \quad
N^{\times}_{\omega}(x) = (1+o(1))\frac{x}{\log\log x}; \\ \!\!\!\!\!\!\!\!\! 3)
\quad N^{\times}_{\varphi}(x) = (c_0+o(1))x^{1/2}, \end{gather*} where
\,.Comment: The error term in Theorem 1.2 is improved in this version of the
pape
The Free Particle in Deformed Special Relativity
The phase space of a classical particle in DSR contains de Sitter space as
the space of momenta. We start from the standard relativistic particle in five
dimensions with an extra constraint and reduce it to four dimensional DSR by
imposing appropriate gauge fixing. We analyze some physical properties of the
resulting theories like the equations of motion, the form of Lorentz
transformations and the issue of velocity. We also address the problem of the
origin and interpretation of different bases in DSR.Comment: 15 page
Differential Cross Sections for Higgs Boson Production at Tevatron Collider Energies
The transverse momentum distribution is computed for inclusive Higgs
boson production at TeV. We include all-orders resummation of
large logarithms associated with emission of soft gluons at small . We
provide results for Higgs boson and masses from to 200 GeV. The
relatively hard transverse momentum distribution for Higgs boson production
suggests possibilities for improvement of the signal to background ratio.Comment: 12 pages, latex, 7 figure
Top-mass effects in differential Higgs production through gluon fusion at order \alpha_s^4
Effects from a finite top quark mass on differential distributions in the
Higgs+jet production cross section through gluon fusion are studied at
next-to-leading order in the strong coupling, i.e. . Terms
formally subleading in are calculated, and their influence on the
transverse momentum and rapidity distribution of the Higgs boson are evaluated.
We find that, for the differential K-factor, the heavy-top limit is valid at
the 2-3% level as long as the transverse momentum of the Higgs remains below
about 150 GeV.Comment: 21 pages, 12 figure
Nonlocal vs local vortex dynamics in the transversal flux transformer effect
In this follow-up to our recent Letter [F. Otto et al., Phys. Rev. Lett. 104,
027005 (2010)], we present a more detailed account of the superconducting
transversal flux transformer effect (TFTE) in amorphous (a-)NbGe nanostructures
in the regime of strong nonequilibrium in local vortex motion. Emphasis is put
on the relation between the TFTE and local vortex dynamics, as the former turns
out to be a reliable tool for determining the microscopic mechanisms behind the
latter. By this method, a progression from electron heating at low temperatures
T to the Larkin-Ovchinnikov effect close to the transition temperature Tc is
traced over a range 0.26 < T/Tc < 0.95. This is represented by a number of
relevant parameters such as the vortex transport entropy related to the
Nernst-like effect at low T, and a nonequilibrium magnetization enhancement
close to Tc. At intermediate T, the Larkin-Ovchinnikov effect is at high
currents modified by electron heating, which is clearly observed only in the
TFTE
Polarized Lambda-Baryon Production in pp Collisions
We study the production of longitudinally polarized Lambda-baryons in
single-spin pp collisions at RHIC and HERA-N as a means of determining the
spin-dependent Lambda fragmentation functions. It is shown that a measurement
of the rapidity distribution of the Lambda's would provide an excellent way of
clearly discriminating between various recently suggested sets of polarized
Lambda fragmentation functions that are all compatible with present e^+e^-
data. We also address the main theoretical uncertainties, which appear to be
well under control.Comment: 11 pages, LaTeX, 2 figures, uses epsfig and amssymb style
Finding and counting vertex-colored subtrees
The problems studied in this article originate from the Graph Motif problem
introduced by Lacroix et al. in the context of biological networks. The problem
is to decide if a vertex-colored graph has a connected subgraph whose colors
equal a given multiset of colors . It is a graph pattern-matching problem
variant, where the structure of the occurrence of the pattern is not of
interest but the only requirement is the connectedness. Using an algebraic
framework recently introduced by Koutis et al., we obtain new FPT algorithms
for Graph Motif and variants, with improved running times. We also obtain
results on the counting versions of this problem, proving that the counting
problem is FPT if M is a set, but becomes W[1]-hard if M is a multiset with two
colors. Finally, we present an experimental evaluation of this approach on real
datasets, showing that its performance compares favorably with existing
software.Comment: Conference version in International Symposium on Mathematical
Foundations of Computer Science (MFCS), Brno : Czech Republic (2010) Journal
Version in Algorithmic
Flux Periodicities in Loops of Nodal Superconductors
Supercurrents in superconducting flux threaded loops are expected to
oscillate with the magnetic flux with a period of hc/2e. This is indeed true
for s-wave superconductors larger than the coherence length xi_0. Here we show
that for superconductors with gap nodes, there is no such strict condition for
the supercurrent to be hc/2e rather than hc/e periodic. For nodal
superconductors, the flux induced Doppler shift of the near nodal states leads
to a flux dependent occupation probability of quasi-particles circulating
clockwise and counter clockwise around the loop, which leads to an hc/e
periodic component of the supercurrent, even at zero temperature. We analyze
this phenomenon on a cylinder in an approximative analytic approach and also
numerically within the framework of the BCS theory. Specifically for d-wave
pairing, we show that the hc/e periodic current component decreases with the
inverse radius of the loop and investigate its temperature dependence
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