Rare k-decays

This article reviews the current situation in the field of rare K decays: the relevant phenomenology, the present experimental situation, and prospects for the near future. Study of rare K decays can make a significant contribution in a number of different frontier areas of research in high-energy physics. In the area of CP violation, study of such rare decays as K(L)0 --> pi0e+e-, K(L)0 --> pi0mu+mu-, K(L)0 --> pi0nunuBAR, and muon polarization in K(L)0 --> mu+mu- can provide important complementary information to what has been learned from the decay K(L)0 --> pipi. Even though experiments with sufficient accuracy to make a meaningful study of CP violation are still a few years away, significant progress has been made in this general area during the last decade. A second major area of interest in the field of rare K decays is the search for processes forbidden in the Standard Model, e.g., K(L)0 --> mue and K+ --> pi+mu+e-. Various extensions of the Standard Model predict that these processes will occur with branching fractions in the range of 10(-10) to 10(-15). Experiments of the last decade have pushed the limits into the 10(-10) to 10(-11) range, and further improvements in sensitivity of one to two orders of magnitude can be expected in the next few years. K decays allow one also to study higher-order weak-interaction processes such as K(L)0 --> mu+mu-, K(L)0 --> e+e-, K+ --> pi+nunuBAR, which are forbidden to first order in the Standard Model. Because of strong suppression, these decay modes offer potential windows on new physics; in addition, they may offer the most reliable measurement of V(td), one of the elements of the weak mixing matrix in the quark sector. The studies of the mu+mu- channel have achieved data samples of close to 1000 events; the other two modes should be observed for the first time in the next few years. Finally, as a byproduct of these studies, one has been able to look simultaneously for new light particles into which the K meson could decay. Limits obtained for various hypothetical particles are summarized.Physic

Subdiffusive motion in kinetically constrained models

We discuss a kinetically constrained model in which real-valued local densities fluctuate in time, as introduced recently by Bertin, Bouchaud and Lequeux. We show how the phenomenology of this model can be reproduced by an effective theory of mobility excitations propagating in a disordered environment. Both excitations and probe particles have subdiffusive motion, characterised by different exponents and operating on different time scales. We derive these exponents, showing that they depend continuously on one of the parameters of the model.Comment: 12 pages, 5 figure

Entrepreneurship as nexus of change: the syncretistic production of the future

This paper deals with the issue of how the future is created and the mechanisms through which it is produced and conceived. Key to this process appears to be social interaction and how it is used to bring about change. Examining the entrepreneurial context by qualitative longitudinal research techniques, the study considers the situations of three entrepreneurs. It demonstrates that the web of relationships in which individuals are engaged provide the opportunity to enact the environment in new ways, thus producing organizations for the future. It further provides empirical evidence for a Heideggerian reading of strategy-as-practice, extending this conceptualization to account for the temporal dimension

Use of high-resolution measurements for the retrieval of temperature and gas-concentration profiles from outgoing infrared spectra in the presence of cirrus clouds

We explore ways in which high-spectral-resolution measurements can aid in the retrieval of atmospheric temperature and gas-concentration profiles from outgoing infrared spectra when optically thin cirrus clouds are present. Simulated outgoing spectra that contain cirrus are fitted with spectra that do not contain cirrus, and the residuals are examined. For those lines with weighting functions that peak near the same altitude as the thin cirrus, unique features are observed in the residuals. These unique features are highly sensitive to the resolution of the instrumental line shape. For thin cirrus these residual features are narrow (≤0.1 cm-1), so high spectral resolution is required for unambiguous observation. The magnitudes of these unique features are larger than the noise of modern instruments. The sensitivities of these features to cloud height and cloud optical depth are also discussed. Our sensitivity studies show that, when the errors in the estimation of temperature profiles are not large, the dominant contribution to the residuals is the misinterpretation of cirrus. An analysis that focuses on information content is also presented. An understanding of the magnitude of the effect and of its dependence on spectral resolution as well as on spectral region is important for retrieving spacecraft data and for the design of future infrared instruments for forecasting weather and monitoring greenhouse gases

On the detection of characteristic optical emission from electronically coupled nanoemitters

Optical emission from an electronically coupled pair of nanoemitters is investigated, in a new theoretical development prompted by experimental work on oriented semiconductor polymer nanostructures. Three physically distinct mechanisms for photon emission by such a pair, positioned in the near-field, are identified: emission from a pairdelocalized exciton state, emission that engages electrodynamic coupling through quantum interference, and correlated photon emission from the two components of the pair. Each possibility is investigated, in detail, by examination of the emission signal via explicit coupling of the nanoemitter pair with a photodetector, enabling calculations to give predictive results in a form directly tailored for experiment. The analysis incorporates both near- and far-field properties (determined from the detector-pair displacement), so that the framework is applicable not only to a conventional remote detector, but also a near-field microscope setup. The results prove strongly dependent on geometry and selection rules. This work paves the way for a broader investigation of pairwise coupling effects in the optical emission from structured nanoemitter arrays

A record current account deficit: causes and implications

The U.S. current account deficit was at a record level in 1999 and is expected to increase further in 2000. How large can this deficit get? Will an eventual adjustment in the deficit place the U.S. economy at risk? This article examines three arguments often put forth to explain the increase in the deficit--a consumption boom, the U.S. as a safe haven for short-term foreign capital and technological change affecting the U.S. economy. The authors find the strongest evidence in support of technological change and suggest why, under these conditions, an economic adjustment to the deficit need not to have as adverse an impact as some observes fear.Deficit financing ; Economic conditions - United States

The remotely piloted vehicle as an earth science research aircraft

A brief study was conducted at the Goddard Space Flight Center to identify existing remotely piloted vehicle (RPV) capabilities and to determine if the use of an RPV was advantageous and practical for Earth science investigations. A total of 17 instrument systems were identified. It was found that RPV's were considered especially valuable for dangerous missions, e.g., flights through volcano plumes and hurricanes, long duration profiles over inaccessible regions such as the Antarctic, and very low altitude ocean profiling missions

Structure and dynamics in glass-formers: predictability at large length scales

Dynamic heterogeneity in glass-formers has been related to their static structure using the concept of dynamic propensity. We re-examine this relationship by analyzing dynamical fluctuations in two atomistic glass-formers and two theoretical models. We introduce quantitative statistical indicators which show that the dynamics of individual particles cannot be predicted on the basis of the propensity, nor by any structural indicator. However, the spatial structure of the propensity field does have predictive power for the spatial correlations associated with dynamic heterogeneity. Our results suggest that the quest for a connection between static and dynamic properties of glass-formers at the particle level is vain, but they demonstrate that such connection does exist on larger length scales.Comment: 7 pages; 4 figs - Extended, clarified versio

Fluctuation-dissipation ratios in the dynamics of self-assembly

We consider two seemingly very different self-assembly processes: formation of viral capsids, and crystallization of sticky discs. At low temperatures, assembly is ineffective, since there are many metastable disordered states, which are a source of kinetic frustration. We use fluctuation-dissipation ratios to extract information about the degree of this frustration. We show that our analysis is a useful indicator of the long term fate of the system, based on the early stages of assembly.Comment: 8 pages, 6 figure