4,513 research outputs found
WIMP searches with gamma rays in the Fermi era: challenges, methods and results
The launch of the gamma-ray telescope Fermi Large Area Telescope (Fermi-LAT)
started a pivotal period in indirect detection of dark matter. By outperforming
expectations, for the first time a robust and stringent test of the paradigm of
weakly interacting massive particles (WIMPs) is within reach. In this paper, we
discuss astrophysical targets for WIMP detection and the challenges they
present, review the analysis tools which have been employed to tackle these
challenges, and summarize the status of constraints on and the claimed
detections in the WIMP parameter space. Methods and results will be discussed
in comparison to Imaging Air Cherenkov Telescopes. We also provide an outlook
on short term and longer term developments.Comment: 72 pages, 7 figures, Invited review for Journal of Experimental and
Theoretical Physics,v3: added a few references, addressed referee comment
Bikesharing and Bicycle Safety
The growth of bikesharing in the United States has had a transformative impact on urban transportation. Major cities have established large bikesharing systems, including Boston, Chicago, Denver, Minneapolis-Saint Paul, New York City, Salt Lake City, the San Francisco Bay Area, Seattle, Washington DC, and others. These systems began operating as early as 2010, and no fatalities have occurred within the US as of this writing. However, three have happened in North America—two in Canada and one in Mexico. Bikesharing has some qualities that appear inherently unsafe for bicyclists. Most prominently, helmet usage is documented to be quite low in most regions. Bikesharing is also used by irregular bicyclists who are less familiar with the local terrain. In this study, researchers take a closer look at bikesharing safety from qualitative and quantitative perspectives. Through a series of four focus groups, they discussed bikesharing usage and safety with bikesharing members and nonmembers in the Bay Area. They further engaged experts nationwide from a variety of fields to evaluate their opinions and perspectives on bikesharing and safety. Finally, researchers conducted an analysis of bicycle and bikesharing activity data, as well as bicycle and bikesharing collisions to evaluate injury rates associated with bikesharing when compared with benchmarks of personal bicycling. The data analysis found that collision and injury rates for bikesharing are lower than previously computed rates for personal bicycling. Experts and focus group participants independently pointed to bikesharing rider behavior and bikesharing bicycle design as possible factors. In particular, bikesharing bicycles are generally designed in ways that promote stability and limited speeds, which mitigate the conditions that contribute to collisions. Data analysis also explored whether there was evidence of a “safety in numbers benefit” that resulted from bikesharing activity. However, no significant impact from bikesharing activity on broader bicycle collisions could be found within the regions in which they operate. Discussion and recommendations are presented in the conclusion
Attosecond time-scale multi-electron collisions in the Coulomb four-body problem: traces in classical probability densities
In the triple ionization of the Li ground state by single photon absorption
the three electrons escape to the continuum mainly through two collision
sequences with individual collisions separated by time intervals on the
attosecond scale. We investigate the traces of these two collision sequences in
the classical probability densities. We show that each collision sequence has
characteristic phase space properties which distinguish it from the other.
Classical probability densities are the closest analog to quantum mechanical
densities allowing our results to be directly compared to quantum mechanical
results.Comment: 9 pages, 10 figure
Femtosecond Photoionization of Atoms under Noise
We investigate the effect of incoherent perturbations on atomic
photoionization due to a femtosecond mid-infrared laser pulse by solving the
time-dependent stochastic Schr\"odinger equation. For a weak laser pulse which
causes almost no ionization, an addition of a Gaussian white noise to the pulse
leads to a significantly enhanced ionization probability. Tuning the noise
level, a stochastic resonance-like curve is observed showing the existence of
an optimum noise for a given laser pulse. Besides studying the sensitivity of
the obtained enhancement curve on the pulse parameters, such as the pulse
duration and peak amplitude, we suggest that experimentally realizable
broadband chaotic light can also be used instead of the white noise to observe
similar features. The underlying enhancement mechanism is analyzed in the
frequency-domain by computing a frequency-resolved atomic gain profile, as well
as in the time-domain by controlling the relative delay between the action of
the laser pulse and noise.Comment: 10 pages, 10 figure
Analytic solutions for a three-level system in a time-dependent field
This paper generalizes some known solitary solutions of a time-dependent
Hamiltonian in two ways: The time-dependent field can be an elliptic function,
and the time evolution is obtained for a complete set of basis vectors. The
latter makes it feasible to consider arbitrary initial conditions. The former
makes it possible to observe a beating caused by the non-linearity of the
driving field.Comment: 8 pages, 4 figure
Influence of prenatal maternal stress, maternal plasma cortisol and cortisol in the amniotic fluid on birth outcomes and child temperament at 3 months
This prospective, longitudinal study aimed to investigate relationships between indicators of maternal prenatal stress, infant birth outcomes and early temperament. We examined the pattern of associations and postulated pathways between physiological (cortisol plasma concentrations) and self-report indices (stress, anxiety) of maternal prenatal stress, cortisol in the amniotic fluid, birth outcomes and infant temperament at 3 months. The sample consisted of 158 women undergoing amniocentesis in the 2nd trimester of pregnancy. Questionnaire measures of maternal stress and anxiety were found to be unrelated to cortisol in plasma or amniotic fluid. Maternal cortisol was related to amniotic cortisol, which in turn was associated with lower birth weight. Birth weight predicted infant fear and distress to limitation at 3 months old. We found trend-like indirect effects of amniotic fluid on infant distress to limitation and fear via birth weight. This is one of the few studies to simultaneously assess the role of maternal and amniotic fluid cortisol on birth outcomes and infant emotional development. The results suggest that foetal cortisol may be an important predictor of infant outcomes and shed light on the mechanisms through which prenatal maternal stress affects infant psychological health
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