Sympathetic and swap cooling of trapped ions by cold atoms in a MOT

A mixed system of cooled and trapped, ions and atoms, paves the way for ion assisted cold chemistry and novel many body studies. Due to the different individual trapping mechanisms, trapped atoms are significantly colder than trapped ions, therefore in the combined system, the strong binary ion$-$atom interaction results in heat flow from ions to atoms. Conversely, trapped ions can also get collisionally heated by the cold atoms, making the resulting equilibrium between ions and atoms intriguing. Here we experimentally demonstrate, Rubidium ions (Rb$^+$) cool in contact with magneto-optically trapped (MOT) Rb atoms, contrary to the general expectation of ion heating for equal ion and atom masses. The cooling mechanism is explained theoretically and substantiated with numerical simulations. The importance of resonant charge exchange (RCx) collisions, which allows swap cooling of ions with atoms, wherein a single glancing collision event brings a fast ion to rest, is discussed.Comment: 10 pages, 3 figure

Population differentiation in the white-fronted chat (Epthianura albifrons) at a continental scale: implications for dispersal, biogeography and conservation

The capacity for dispersal is an important determinant of a species’ resilience to decline from the serial extinction of local populations. Dispersal of animals can be difficult to measure directly but population genetics provides a powerful tool for identifying dispersal limits. This study used microsatellite markers to investigate population structuring of the white-fronted chat (Epthianura albifrons) across its geographic range. Because the species is threatened in the north-eastern part of its range, the primary aim was to identify barriers limiting dispersal. Isolation by distance was not apparent across the 3000 km mainland range but populations on three oceanic islands, and two mainland sites surrounded by urbanisation, were genetically distinct from each other and the other mainland sites. The small populations that were surrounded by urbanised land had significantly lower genetic diversity (allelic diversity and heterozygosity) than the other mainland populations, with the oceanic island populations showing intermediate levels of diversity. These results suggest that although being a partially nomadic species, which facilitates its dispersal over continuous habitat, the species does not have the flight capacity of some other small passerines and is sensitive to habitat discontinuity. Populations inhabiting small areas of coastal saltmarsh in areas with large human populations are likely to face increasing threat levels.</jats:p

Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum