3,227 research outputs found
Effective Nonlocal Euclidean Gravity
A nonlocal form of the effective gravitational action could cure the
unboundedness of euclidean gravity with Einstein action. On sub-horizon length
scales the modified gravitational field equations seem compatible with all
present tests of general relativity and post-Newtonian gravity. They induce a
difference in the effective Newton's constant between regions of space with
vanishing or nonvanishing curvature scalar (or Ricci tensor). In cosmology they
may lead to a value for the critical density after inflation. The
simplest model considered here appears to be in conflict with nucleosynthesis,
but generalizations consistent with all cosmological observations seem
conceivable.Comment: 12 pages, LaTe
Competing bounds on the present-day time variation of fundamental constants
We compare the sensitivity of a recent bound on time variation of the fine
structure constant from optical clocks with bounds on time varying fundamental
constants from atomic clocks sensitive to the electron-to-proton mass ratio,
from radioactive decay rates in meteorites, and from the Oklo natural reactor.
Tests of the Weak Equivalence Principle also lead to comparable bounds on
present variations of constants. The "winner in sensitivity" depends on what
relations exist between the variations of different couplings in the standard
model of particle physics, which may arise from the unification of gauge
interactions. WEP tests are currently the most sensitive within unified
scenarios. A detection of time variation in atomic clocks would favour
dynamical dark energy and put strong constraints on the dynamics of a
cosmological scalar field.Comment: ~4 Phys Rev page
Jet measurements by the CMS experiment in pp and PbPb collisions
The energy loss of fast partons traversing the strongly interacting matter
produced in high-energy nuclear collisions is one of the most interesting
observables to probe the nature of the produced medium. The multipurpose
Compact Muon Solenoid (CMS) detector is well designed to measure these hard
scattering processes with its high resolution calorimeters and high precision
silicon tracker. Analyzing data from pp and PbPb collisions at a center-of-mass
energy of 2.76 TeV parton energy loss is observed as a significant imbalance of
dijet transverse momentum. To gain further understanding of the parton energy
loss mechanism the redistribution of the quenched jet energy was studied using
the transverse momentum balance of charged tracks projected onto the direction
of the leading jet. In contrast to pp collisions, a large fraction the momentum
balance for asymmetric jets is found to be carried by low momentum particles at
large angular distance to the jet axis. Further, the fragmentation functions
for leading and subleading jets were reconstructed and were found to be
unmodified compared to measurements in pp collisions. The results yield a
detailed picture of parton propagation in the hot QCD medium.Comment: 7 pages, 5 figures, Quark Matter 2011 conference proceeding
A bottom–up model of spatial attention predicts human error patterns in rapid scene recognition
Humans demonstrate a peculiar ability to detect complex targets in rapidly presented natural scenes. Recent studies suggest that (nearly) no focal attention is required for overall performance in such tasks. Little is known, however, of how detection performance varies from trial to trial and which stages in the processing hierarchy limit performance: bottom–up visual processing (attentional selection and/or recognition) or top–down factors (e.g., decision-making, memory, or alertness fluctuations)? To investigate the relative contribution of these factors, eight human observers performed an animal detection task in natural scenes presented at 20 Hz. Trial-by-trial performance was highly consistent across observers, far exceeding the prediction of independent errors. This consistency demonstrates that performance is not primarily limited by idiosyncratic factors but by visual processing. Two statistical stimulus properties, contrast variation in the target image and the information-theoretical measure of “surprise” in adjacent images, predict performance on a trial-by-trial basis. These measures are tightly related to spatial attention, demonstrating that spatial attention and rapid target detection share common mechanisms. To isolate the causal contribution of the surprise measure, eight additional observers performed the animal detection task in sequences that were reordered versions of those all subjects had correctly recognized in the first experiment. Reordering increased surprise before and/or after the target while keeping the target and distractors themselves unchanged. Surprise enhancement impaired target detection in all observers. Consequently, and contrary to several previously published findings, our results demonstrate that attentional limitations, rather than target recognition alone, affect the detection of targets in rapidly presented visual sequences
Mass freezing in growing neutrino quintessence
Growing neutrino quintessence solves the coincidence problem for dark energy
by a growing cosmological value of the neutrino mass which emerges from a
cosmon-neutrino interaction stronger than gravity. The cosmon-mediated
attraction between neutrinos induces the formation of large scale neutrino
lumps in a recent cosmological epoch. We argue that the non-linearities in the
cosmon field equations stop the further increase of the neutrino mass within
sufficiently dense and large lumps. As a result, we find the neutrino induced
gravitational potential to be substantially reduced when compared to linear
extrapolations. We furthermore demonstrate that inside a lump the possible time
variation of fundamental constants is much smaller than their cosmological
evolution. This feature may reconcile current geophysical bounds with claimed
cosmological variations of the fine structure constant.Comment: 15 pages, 12 figures. Version published in PR
Emergence of slow-switching assemblies in structured neuronal networks
Unraveling the interplay between connectivity and spatio-temporal dynamics in
neuronal networks is a key step to advance our understanding of neuronal
information processing. Here we investigate how particular features of network
connectivity underpin the propensity of neural networks to generate
slow-switching assembly (SSA) dynamics, i.e., sustained epochs of increased
firing within assemblies of neurons which transition slowly between different
assemblies throughout the network. We show that the emergence of SSA activity
is linked to spectral properties of the asymmetric synaptic weight matrix. In
particular, the leading eigenvalues that dictate the slow dynamics exhibit a
gap with respect to the bulk of the spectrum, and the associated Schur vectors
exhibit a measure of block-localization on groups of neurons, thus resulting in
coherent dynamical activity on those groups. Through simple rate models, we
gain analytical understanding of the origin and importance of the spectral gap,
and use these insights to develop new network topologies with alternative
connectivity paradigms which also display SSA activity. Specifically, SSA
dynamics involving excitatory and inhibitory neurons can be achieved by
modifying the connectivity patterns between both types of neurons. We also show
that SSA activity can occur at multiple timescales reflecting a hierarchy in
the connectivity, and demonstrate the emergence of SSA in small-world like
networks. Our work provides a step towards understanding how network structure
(uncovered through advancements in neuroanatomy and connectomics) can impact on
spatio-temporal neural activity and constrain the resulting dynamics.Comment: The first two authors contributed equally -- 18 pages, including
supplementary material, 10 Figures + 2 SI Figure
Asymptotically free four-fermion interactions and electroweak symmetry breaking
We investigate the fermions of the standard model without a Higgs scalar.
Instead, we consider a non-local four-quark interaction in the tensor channel
which is characterized by a single dimensionless coupling . Quantization
leads to a consistent perturbative expansion for small . The running of
is asymptotically free and therefore induces a non-perturbative scale
, in analogy to the strong interactions. We argue that
spontaneous electroweak symmetry breaking is triggered at a scale where
grows large and find the top quark mass of the order of . We also
present a first estimate of the effective Yukawa coupling of a composite Higgs
scalar to the top quark, as well as the associated mass ratio between the top
quark and the W boson.Comment: 24 page
Primordial nucleosynthesis as a probe of fundamental physics parameters
We analyze the effect of variation of fundamental couplings and mass scales
on primordial nucleosynthesis in a systematic way. The first step establishes
the response of primordial element abundances to the variation of a large
number of nuclear physics parameters, including nuclear binding energies. We
find a strong influence of the n-p mass difference (for the 4He abundance), of
the nucleon mass (for deuterium) and of A=3,4,7 binding energies (for 3He, 6Li
and 7Li). A second step relates the nuclear parameters to the parameters of the
Standard Model of particle physics. The deuterium, and, above all, 7Li
abundances depend strongly on the average light quark mass hat{m} \equiv
(m_u+m_d)/2. We calculate the behaviour of abundances when variations of
fundamental parameters obey relations arising from grand unification. We also
discuss the possibility of a substantial shift in the lithium abundance while
the deuterium and 4He abundances are only weakly affected.Comment: v2: 34 pages, 2 figures, typo in last GUT scenario corrected, added
discussion and graph of nonlinear behaviour in GUT scenarios, added short
section discussing binding of dineutron and 8Be, refs added, conclusions
unaltered. Accepted for publication, Phys. Rev.
There are No Nice Interfaces in 2+1 Dimensional SOS-Models in Random Media
We prove that in dimension translation covariant Gibbs states
describing rigid interfaces in a disordered solid-on-solid (SOS) cannot exist
for any value of the temperature, in contrast to the situation in .
The prove relies on an adaptation of a theorem of Aizenman and Wehr.
Keywords: Disordered systems, interfaces, SOS-modelComment: 8 pages, gz-compressed Postscrip
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