Atomic Physics: Puzzle Confirmed

research highlights

GAME THEORY Randolph Pohl and colleagues from the only skin within a few hundred nanometres of Tolerate thy neighbour Charge Radius Experiment with Muonic the glass surface. They could therefore reliably New J. Phys. 18, 083021 (2016) Atoms (CREMA) collaboration have now estimate the area of contact and monitor with performed laser spectroscopy measurements high spatiotemporal resolution how that area As things deteriorate in a society, the on muonic deuterium, an exotic atom with changed when the surface was vibrated. temptation to hold fast to one’s beliefs a nucleus consisting of one proton and one How these findings can be of practical intensifies, and opposing those with whom neutron — a deuteron — orbited by a muon. value is not difficult to see: as we increasingly we disagree seems ever more important. But They prepared atoms in the metastable interact with our electronic devices using Attila Szolnoki and Matjaž Perc think there 2S state, and subsequently measured the our fingertips, modulating the ‘touching might be another way. The pair have come up frequencies of 2S–2P transitions. experience’ using ultrasonic vibrations with a spatial public goods game that suggests The resulting values enabled a precise might form the basis for a new form of we all might be better off with healthy doses determination of the root-mean-square haptic feedback. AHT of tolerance. charge radius of the deuteron, which is All the usual players factor into Szolnoki linked to the value of the Rydberg constant. FLUID DYNAMICS and Perc’s game: cooperators, defectors and At 2.12562(78) × 10–15 m, this radius is Call the shots loners. But the pair added another breed significantly smaller than the conventional Phys. Rev. Fluids 1, 043201 (2016) into the mix, incorporating players with a value obtained from electron scattering certain degree of tolerance for the defectors experiments. Together with similar results among them. These players could choose obtained earlier for muonic hydrogen, the to cooperate or abstain, depending on the findings of Pohl et al. imply a smaller proton actions of other players. radius, posing a serious challenge for the A diversity of tolerance proved optimal. standard model of particle physics. BV When tolerant players had different thresholds for withstanding defection, they could actually BIOTRIBOLOGY turn the whole group towards cooperation in Feel the squeeze cases where defection would have otherwise Proc. Natl Acad. Sci. USA 113, 9210–9215 (2016) dominated. Certain stable solutions only emerged when the number of players was large A glass surface feels more slippery to the enough, lying largely undetected for smaller touch when vibrated at ultrasonic frequencies. games. But once they emerged, they remained The phenomenon has long been known, but stable, even in small populations. AK the underlying mechanisms have remained unclear. Michaël Wiertlewski and colleagues PHOTO STOCK ALLRED / ALAMY © NATHAN ATOMIC PHYSICS now suggest that the reduced friction is due The spatter pattern of blood caused by Puzzle confirmed to the skin bouncing on a ‘squeeze film’ that a gunshot wound contains a wealth of Science 353, 669–673 (2016) forms as the air between the fingertip and information that forensic scientists want to the vibrating glass plate is compressed. This tap into in order to reconstruct the scene of a Just over a century ago, by providing an mechanism decreases the contact area and crime. Current methods for analysing blood explanation of atomic emission spectra, thus the friction. spatter patterns are relatively simply however, Bohr’s model of the hydrogen atom helped The picture of load sharing between largely ignoring effects such as gravity and lay the foundations of quantum mechanics. compressed air and the skin emerged from air resistance — assumptions that may not be Today, probing energy levels of atomic a series of tribometric experiments in which valid when blood drops travel large distances. systems remains an important tool in the area of fingertip–surface contact was Patrick Comiskey and colleagues now present further developing our understanding of measured optically. Wiertlewski et al. relied a method for determining the origin of blood fundamental atomic properties. on frustrated total internal reflection to image back-spatter patterns from a gunshot, which takes into account the motion of droplets NEUTRINO PHYSICS through air. Using the Rayleigh–Taylor instability Push the limits Phys. Rev. Lett. 117, 082503 (2016) framework, Comiskey et al. predicted the initial distribution of drop sizes and It’s not yet clear whether the neutrino is a Majorana particle — that is, identical to its velocities, showing that the interaction of antiparticle. The answer could be found in the neutrinoless double-beta decay: a hypothetical the resulting drop cloud with air helps to radioactive process producing two electrons and two antineutrinos that annihilate each other. reduce the drag experienced by each droplet. Azusa Gando and colleagues report the latest results of the KamLAND-Zen experiment, This information can be used to accurately setting 1.07 × 1026 years as the lower limit on the half-life of this decay. This can be translated calculate the trajectory and subsequent impact into a limit on the effective Majorana neutrino mass. And its value almost reaches a very dynamics of the droplets, giving rise to distinct interesting regime — that of the inverted mass hierarchy, one of the two possible orderings of stain distributions. Using their model, they neutrino masses. predicted characteristic stain patterns for For such precision measurements, one needs an extremely clean background. The realistic gunshot scenarios, which agree well KamLAND-Zen experiment uses 13 tons of xenon-loaded liquid scintillator in a nylon balloon with preliminary experiments conducted on inside the KamLAND detector. Gando et al. purified the xenon liquid scintillator in several stages sponges soaked in swine blood. LF to reduce radioactive contamination, but to push sensitivity even further — to the inverted mass hierarchy regime — they will have to upgrade to a larger balloon. IG Written by Luke Fleet, Iulia Georgescu, Abigail Klopper, Andreas H. Trabesinger and Bart Verberck.