3,358 research outputs found
Spin Hall Effect in Atoms
We propose an optical means to realize a spin hall effect (SHE) in neutral
atomic system by coupling the internal spin states of atoms to radiation. The
interaction between the external optical fields and the atoms creates effective
magnetic fields that act in opposite directions on "electrically" neutral atoms
with opposite spin polarizations. This effect leads to a Landau level structure
for each spin orientation in direct analogy with the familiar SHE in
semiconductors. The conservation and topological properties of the spin
current, and the creation of a pure spin current are discussed.Comment: 4 pages, 2 figure; Final versio
Stars quenching stars: how photoionization by local sources regulates gas cooling and galaxy formation
Current models of galaxy formation lack an efficient and physically
constrained mechanism to regulate star formation (SF) in low and intermediate
mass galaxies. We argue that the missing ingredient could be the effect of
photoionization by local sources on the gas cooling. We show that the soft
X-ray and EUV flux generated by SF is able to efficiently remove the main
coolants (e.g., HeII, OV and FeIX) from the halo gas via direct
photoionization. As a consequence, the cooling and accretion time of the gas
surrounding star-forming galaxies may increase by one or two orders of
magnitude. For a given halo mass and redshift, the effect is directly related
to the value of the star formation rate (SFR). Our results suggest the
existence of a critical SFR above which "cold" mode accretion is stopped, even
for haloes with virial masses well below the critical shock-heating mass
suggested by previous studies.The evolution of the critical SFR with redshift,
for a given halo mass, resembles the respective steep evolution of the observed
SFR for z<1. This suggests that photoionization by local sources would be able
to regulate gas accretion and star formation, without the need for additional,
strong feedback processes.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
The 21 Centimeter Forest
We examine the prospects for studying the pre-reionization intergalactic
medium (IGM) through the so-called 21 cm forest in spectra of bright
high-redshift radio sources. We first compute the evolution of the mean optical
depth for models that include X-ray heating of the IGM gas, Wouthuysen-Field
coupling, and reionization. Under most circumstances, the spin temperature T_S
grows large well before reionization begins in earnest. As a result, the
optical depth is less than 0.001 throughout most of reionization, and
background sources must sit well beyond the reionization surface in order to
experience measurable absorption. HII regions produce relatively large
"transmission gaps" and may therefore still be observable during the early
stages of reionization. Absorption from sheets and filaments in the cosmic web
fades once T_S becomes large and should be rare during reionization. Minihalos
can produce strong (albeit narrow) absorption features. Measuring their
abundance would yield useful limits on the strength of feedback processes in
the IGM as well as their effect on reionization.Comment: 9 pages, 5 figures, submitted to MNRA
Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe
Observations of the high-redshift Universe with the 21 cm hyperfine line of
neutral hydrogen promise to open an entirely new window onto the early phases
of cosmic structure formation. Here we review the physics of the 21 cm
transition, focusing on processes relevant at high redshifts, and describe the
insights to be gained from such observations. These include measuring the
matter power spectrum at z~50, observing the formation of the cosmic web and
the first luminous sources, and mapping the reionization of the intergalactic
medium. The epoch of reionization is of particular interest, because large HII
regions will seed substantial fluctuations in the 21 cm background. We also
discuss the experimental challenges involved in detecting this signal, with an
emphasis on the Galactic and extragalactic foregrounds. These increase rapidly
toward low frequencies and are especially severe for the highest redshift
applications. Assuming that these difficulties can be overcome, the redshifted
21 cm line will offer unique insight into the high-redshift Universe,
complementing other probes but providing the only direct, three-dimensional
view of structure formation from z~200 to z~6.Comment: extended review accepted by Physics Reports, 207 pages, 44 figures
(some low resolution); version with high resolution figures available at
http://pantheon.yale.edu/~srf28/21cm/index.htm; minor changes to match
published versio
Progress in D-brane model building
The state of the art in D-brane model building is briefly reviewed, focusing
on recent achievements in the construction of D=4 N = 1 type II string vacua
with semi-realistic gauge sectors. Such progress relies on a better
understanding of the spectrum of BPS D-branes, the effective field theory
obtained from them and the explicit construction of vacua. We first consider
D-branes in standard Calabi-Yau compactifications, and then the more involved
case of compactifications with fluxes. We discuss how the non-trivial interplay
between D-branes and fluxes modifies the previous model-building rules, as well
as provides new possibilities to connect string theory to particle physics.Comment: Using w-art.cls, 27 pages, 6 figures. Based on a talk given at the
RTN `Constituents, Fundamental Forces and Symmetries of the Universe'
Workshop and Midterm meeting in Napoli, October 2006. v2: typos corrected and
references adde
Positive Result in the Early Passive Phase of the Tilt-table Test: A Predictor of Neurocardiogenic Syncope in Young Men
Background/Aims: This study elucidated the prognostic factors for neurocardiogenic syncope in males in their late teens and early twenties. Methods: Tilt-table testing (TTT) was performed on 665 males (age range, 17 to 27 years) following the Italian protocol. The subjects were tilted head-up at a 70 ° angle on a table for 30 minutes during the passive phase. If the passive phase was negative, the subjects were given sublingual nitroglycerin and tilted to the same angle for 20 minutes during the drugprovocation phase. The subjects with positive results were followed without medication. We analyzed factors related to the recurrence rate of syncope. Results: Of 305 subjects (45.8%) with positive results, 223 (age range, 18 to 26 years) were followed for 12 months. The frequency of previous syncopal episodes ℠4 (p = 0.001) and a positive result during the passive phase (p = 0.022) were significantly related to a high recurrence rate. A positive result during the early passive phase ( †12 minutes) was significantly related to a higher recurrence rate than was that during the late passive phase (> 12 minutes; p = 0.011). Conclusions: A positive result during the early passive phase of TTT and frequent previous syncopal episodes were prognostic factors for neurocardiogenic syncope in men in their late teens and early twenties. Keywords: Syncope, vasovagal; Prognosis; Tilt-table tes
Trace and antitrace maps for aperiodic sequences, their extensions and applications
We study aperiodic systems based on substitution rules by means of a
transfer-matrix approach. In addition to the well-known trace map, we
investigate the so-called `antitrace' map, which is the corresponding map for
the difference of the off-diagonal elements of the 2x2 transfer matrix. The
antitrace maps are obtained for various binary, ternary and quaternary
aperiodic sequences, such as the Fibonacci, Thue-Morse, period-doubling,
Rudin-Shapiro sequences, and certain generalizations. For arbitrary
substitution rules, we show that not only trace maps, but also antitrace maps
exist. The dimension of the our antitrace map is r(r+1)/2, where r denotes the
number of basic letters in the aperiodic sequence. Analogous maps for specific
matrix elements of the transfer matrix can also be constructed, but the maps
for the off-diagonal elements and for the difference of the diagonal elements
coincide with the antitrace map. Thus, from the trace and antitrace map, we can
determine any physical quantity related to the global transfer matrix of the
system. As examples, we employ these dynamical maps to compute the transmission
coefficients for optical multilayers, harmonic chains, and electronic systems.Comment: 13 pages, REVTeX, now also includes applications to electronic
systems, some references adde
Constraints on the Redshift Evolution of the L_X-SFR Relation from the Cosmic X-Ray Backgrounds
Observations of local star forming galaxies have revealed a correlation
between the rate at which galaxies form stars and their X-Ray luminosity. We
combine this correlation with the most recent observational constraints on the
integrated star formation rate density, and find that star forming galaxies
account for 5-20% of the total soft and hard X-ray backgrounds, where the
precise number depends on the energy band and the assumed average X-ray
spectral energy distribution of the galaxies below ~20 keV. If we combine the
L_X-SFR relation with recently derived star formation rate function, then we
find that star forming galaxies whose X-ray flux falls well (more than a factor
of 10) below the detection thresholds of the Chandra Deep Fields, can fully
account for the unresolved soft X-ray background, which corresponds to ~6% of
its total. Motivated by this result, we put limits on the allowed redshift
evolution of the parameter c_X \equiv L_X/SFR, and/or its evolution towards
lower and higher star formation rates. If we parametrize the redshift evolution
of c_X ~ (1+z)^b, then we find that b \leq 1.3 (95% CL). On the other hand, the
observed X-ray luminosity functions (XLFs) of star forming galaxies indicate
that c_X may be increasing towards higher redshifts and/or higher star
formation rates at levels that are consistent with the X-ray background, but
possibly at odds with the locally observed L_X-SFR relation.Comment: Accepted to MNRAS after minor revisions. 12 pages, 8 figure
Search for Neutral Higgs Bosons of the Minimal Supersymmetric Standard Model in e+e- Interactions at \sqrt{s} = 189 GeV
A search for the lightest neutral scalar and neutral pseudoscalar Higgs
bosons in the Minimal Supersymmetric Standard Model is performed using 176.4
pb^-1 of integrated luminosity collected by L3 at a center-of-mass energy of
189 GeV. No signal is observed, and the data are consistent with the expected
Standard Model background. Lower limits on the masses of the lightest neutral
scalar and pseudoscalar Higgs bosons are given as a function of tan(beta).
Lower mass limits for tan(beta)>1 are set at the 95% confidence level to be m_h
> 77.1 GeV and m_A > 77.1 GeV
Search for Extra Dimensions in Boson and Fermion Pair Production in e+e- Interactions at LEP
Extra spatial dimensions are proposed by recent theories that postulate the
scale of gravity to be of the same order as the electroweak scale. A sizeable
interaction between gravitons and Standard Model particles is then predicted.
Effects of these new interactions in boson and fermion pair production are
searched for in the data sample collected at centre-of-mass energies above the
Z pole by the L3 detector at LEP. In addition, the direct production of a
graviton associated with a Z boson is investigated. No statistically
significant hints for the existence of these effects are found and lower limits
in excess of 1 TeV are derived on the scale of this new theory of gravity
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