30,634 research outputs found
Non-marginally bound inhomogeneous dust collapse in higher dimensional space-time
We investigate the occurrence and nature of a naked singularity in the
gravitational collapse of an inhomogeneous dust cloud described by a
self-similar higher dimensional Tolman-Bondi space-time. Bound, marginally
bound and unbound space-times are analyzed. The degree of inhomogeneity of the
collapsing matter necessary to form a naked singularity is given.Comment: 5 pages, RevTeX 4, no figures, 2 tables, Accepted in IJMP
Perturbative tests of non-perturbative counting
We observe that a class of quarter-BPS dyons in N=4 theories with charge
vector (Q, P) and with nontrivial values of the arithmetic duality invariant I
:= gcd (Q wedge P) are nonperturbative in one frame but perturbative in another
frame. This observation suggests a test of the recently computed
nonperturbative partition functions for dyons with nontrivial values of the
arithmetic invariant. For all values of I, we show that the nonperturbative
counting yields vanishing indexed degeneracy for this class of states
everywhere in the moduli space in precise agreement with the perturbative
result.Comment: 10 pages, 0 figure
Volatility and growth: credit constraints and productivity-enhancing investment
We examine how credit constraints affect the cyclical behavior of productivity-enhancing investment and thereby volatility and growth. We first develop a simple growth model where firms engage in two types of investment: a short-term one and a long-term productivity-enhancing one. Because it takes longer to complete, long-term investment has a relatively less procyclical return but also a higher liquidity risk. Under complete financial markets, long-term investment is countercyclical, thus mitigating volatility. But when firms face tight credit constraints, long-term investment turns procyclical, thus amplifying volatility. Tighter credit therefore leads to both higher aggregate volatility and lower mean growth for a given total investment rate. We next confront the model with a panel of countries over the period 1960-2000 and find that a lower degree of financial development predicts a higher sensitivity of both the composition of investment and mean growth to exogenous shocks, as well as a stronger negative effect of volatility on growth
Thermal Behaviour of Euclidean Stars
A recent study of dissipative collapse considered a contracting sphere in
which the areal and proper radii are equal throughout its evolution. The
interior spacetime was matched to the exterior Vaidya spacetime which generated
a temporal evolution equation at the boundary of the collapsing sphere. We
present a solution of the boundary condition which allows the study of the
gravitational and thermodynamical behaviour of this particular radiating model.Comment: 10 pages, 3 figure
Characteristics of polar coronal hole jets
High spatial- and temporal-resolution images of coronal hole regions show a
dynamical environment where mass flows and jets are frequently observed. These
jets are believed to be important for the coronal heating and the acceleration
of the fast solar wind. We studied the dynamics of two jets seen in a polar
coronal hole with a combination of imaging from EIS and XRT onboard Hinode. We
observed drift motions related to the evolution and formation of these
small-scale jets, which we tried to model as well. We found observational
evidence that supports the idea that polar jets are very likely produced by
multiple small-scale reconnections occurring at different times in different
locations. These eject plasma blobs that flow up and down with a motion very
similar to a simple ballistic motion. The associated drift speed of the first
jet is estimated to be 27 km s. The average outward speed of
the first jet is km s, well below the escape speed, hence
if simple ballistic motion is considered, the plasma will not escape the Sun.
The second jet was observed in the south polar coronal hole with three XRT
filters, namely, Cpoly, Alpoly, and Almesh filters. We
observed that the second jet drifted at all altitudes along the jet with the
same drift speed of 7 km s. The enhancement in the light curves
of low-temperature EIS lines in the later phase of the jet lifetime and the
shape of the jet's stack plots suggests that the jet material is falls back,
and most likely cools down. To support this conclusion, the observed drifts
were interpreted within a scenario where reconnection progressively shifts
along a magnetic structure, leading to the sequential appearance of jets of
about the same size and physical characteristics. On this basis, we also
propose a simple qualitative model that mimics the observations.Comment: Accepted Astronomy and Astrophysic
Digitized archive of the Kodaikanal images: Representative results of solar cycle variation from sunspot area determination
Photographic images are valuable data resources for studying long term
changes in the solar magnetic field and its influence on the Earth's climate
and weather.
We digitized more than 100 years of white light images stored in photographic
plates and films that are available at Kodaikanal observatory starting from
1904. The digitized images were calibrated for relative plate density and
aligned in such a way that the solar north is in upward direction. A
semi-automated sunspot detection technique was used to identify the sunspots on
the digitized images. In addition to describing the calibration procedure and
availability of the data, we here present preliminary results on the sunspot
area measurements and their variation with time. The results show that the
white-light images have a uniform spatial resolution throughout the 90 years of
observations. However, the contrast of the images decreases from 1968 onwards.
The images are circular and do not show any major geometrical distortions. The
measured monthly averaged sunspot areas closely match the Greenwich sunspot
area over the four solar cycles studied here. The yearly averaged sunspot area
shows a high degree of correlation with the Greenwich sunspot area. Though the
monthly averaged sunspot number shows a good correlation with the monthly
averaged sunspot areas, there is a slight anti-correlation between the two
during solar maximum The Kodaikanal data archive is hosted at
http://kso.iiap.res.in. The long time sequence of the Kodaikanal white light
images provides a consistent data set for sunspot areas and other proxies. Many
studies can be performed using Kodaikanal data alone without requiring
intercalibration between different data sources.Comment: 9 pages, A&A(accepted
Qubit rotation and Berry Phase
A quantized fermion can be represented by a scalar particle encircling a
magnetic flux line. It has the spinor structure which can be constructed from
quantum gates and qubits. We have studied here the role of Berry phase in
removing dynamical phase during one qubit rotation of a quantized fermion. The
entanglement of two qubit inserting spin-echo to one of them results the change
of Berry phase that can be considered as a measure of entanglement. Some effort
is given to study the effect of noise on the Berry phase of spinor and their
entangled states.Comment: 12 page
Cornering pseudoscalar-mediated dark matter with the LHC and cosmology
Models in which dark matter particles communicate with the visible sector through a pseudoscalar mediator are well-motivated both from a theoretical and from a phenomenological standpoint. With direct detection bounds being typically subleading in such scenarios, the main constraints stem either from collider searches for dark matter, or from indirect detection experiments. However., LHC searches for the mediator particles themselves can not only compete with — or even supersede — the reach of direct collider dark matter probes, but they can also test scenarios in which traditional monojet searches become irrelevant, especially when the mediator cannot decay on-shell into dark matter particles or its decay is suppressed. In this work we perform a detailed analysis of a pseudoscalar-mediated dark matter simplified model, taking into account a large set of collider constraints and concentrating on the parameter space regions favoured by cos-mological and astrophysical data. We find that mediator masses above 100-200 GeV are essentially excluded by LHC searches in the case of large couplings to the top quark, while forthcoming collider and astrophysical measurements will further constrain the available parameter space
Controlling the superconducting transition by spin-orbit coupling
Whereas there exists considerable evidence for the conversion of singlet
Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous
magnetic fields, recent theoretical proposals have suggested an alternative way
to exert control over triplet generation: intrinsic spin-orbit coupling in a
homogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple
Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit
coupled ferromagnet owing to structural inversion asymmetry. Unconventional
modulation of the superconducting critical temperature as a function of
in-plane and out-of- plane applied magnetic fields suggests the presence of
triplets that can be controlled by the magnetic orientation of a single
homogeneous ferromagnet. Our studies demonstrate for the first time an active
role of spin-orbit coupling in controlling the triplets -- an important step
towards the realization of novel superconducting spintronic devices.Comment: 11 pages + 4 figures + supplemental informatio
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