Muons mapped

Phys. Lett. B 692, 83–104 (2010)

One of four detectors built at CERN's Large Hadron Collider, the Compact Muon Solenoid (CMS) is set to make important discoveries at the frontiers of particle physics. But before then, the 14,000-tonne underground detector has collected significant data on more naturally accelerated particles: muons produced by the interaction of cosmic rays in the Earth's atmosphere.

Normally these particles are an inconvenient background to the physics of interest — hence the detector has been assembled in a cavern many tens of metres underground, to reduce their impact. But, ahead of high-energy proton collisions, from cosmic-data-taking runs in 2006 and 2008 CMS has achieved the most precise measurement so far, for muons of momenta lower than 100 GeV/c, of the muon charge ratio, which is the ratio of the number of positively charged atmospheric muons to the number of negatively charged ones arriving at the Earth's surface.

The entire data span the momentum range 5–1,000 GeV/c, and at higher values the ratio — already tipped in favour of positively charged muons as most cosmic rays and the nuclei with which they interact are positively charged — increases as expected, owing to a higher fraction of these muons coming from kaon decays in the cosmic showers.

Absolute fidelity

Phys. Rev. Lett. 105, 080503 (2010)

Much of quantum mechanics defies intuition. One prediction at odds with everyday experience is that a single quantum cannot be cloned — it is impossible to create identical copies of the arbitrary quantum state of, say, a single photon. On the other hand, in the classical limit of a great many photons, copies can in principle be made with perfect fidelity.

Bruno Sanguinetti and colleagues now propose that the no-cloning theorem — and, more generally, the property that the 'fidelity' of the cloning process increases monotonically from a finite value in the quantum regime to unity in the classical limit — can be used to do something distinctly useful: it can provide a means to determine the absolute power of a radiation source.

They propose, and demonstrate, a procedure that directly relates a relative measurement of two orthogonal polarizations to the luminous power of the source. The method works for inputs ranging from single photons to the level of 1011 photons and might lead to an improved standard for spectral radiance.

Inexplicable nuclei

Phys. Rev. Lett (in the press); preprint at http://arxiv.org/abs/1004.2535v3 (2010)

Earlier this year, scientists at the Pierre Auger cosmic-ray observatory in Argentina made an unexpected discovery. On analysing the make-up of the particles in ultrahigh-energy cosmic rays above 2 × 1018 eV, they found that a considerable fraction consisted of nuclei rather than protons (Phys. Rev. Lett. 104, 091101; 2010). Moreover, they found that the higher the energy, the greater the fraction of nuclei. This was surprising: such high-energy particles were always believed to originate from gigantic black holes in galaxies far from our own, and it is unlikely that nuclei could have travelled such a distance without breaking up into protons and neutrons.

But Antoine Calvez and colleagues have an explanation. They find that a simple model to describe the diffusion of high-energy protons and iron nuclei through the galaxy reproduces the observed effect. The model suggests that it is the more effective trapping of heavy nuclei by the galaxy's magnetic fields that concentrates their number relative to protons, which escape more easily on essentially straight-line trajectories. These rays might originate from dramatic events, such as gamma-ray bursts, within our own galaxy; by analysing the directions of the proton component, the authors say it should be possible to determine the location of these events.

Seeing in the dark

Opt. Lett. 35, 2639–2641 (2010)

Credit: © ISTOCKPHOTO/DIRK FREDER

It was recently discovered that the nucleus of photoreceptor cells in the eyes of nocturnal mammals differs greatly from those of animals active during the day. Moritz Kreysing and colleagues now demonstrate how this unusual nuclear structure focuses near-field light to aid low-light vision.

Chromatin is protein-packaged DNA found in the nucleus of eukaryotic cells. In almost all mammalian cells, a dense form of chromatin, heterochromatin, is found near the edges of the nucleus whereas a less dense form, euchromatin, is found at the centre. Nocturnal animals, however, have evolved photoreceptor-cell nuclei that have the reverse arrangement.

Kreysing et al. used Mei-scattering theory and finite-difference time-domain calculations to analyse both nuclear arrangements in three dimensions. The models describe the nuclei as 5-μm-diameter dielectric spheres with a core 4 μm across; the refractive index of heterochromatin, 1.04, is slightly higher than euchromatin, 1.02. Whereas far-field scattering patterns are very similar for the two architectures, there are pronounced differences in the near field. The inverted nucleus strongly focuses incident light — an effect that is largely impervious to any perturbation of the spherical shape of the modelled nucleus.

Stars with influence

Nature 466, 947–949 (2010)

Interstellar molecular clouds of gas and dust come in many shapes and sizes. When clouds are sufficiently dense and large, star formation may take place. Newly born massive stars, in turn, influence their parental clouds, causing them to boil away over millions of years. The stars may also shape the clouds into elongated structures or cause fragments to separate. But until Olivier Berné and colleagues detected waves on the surface of the Orion nebula, the mechanism leading to the cloud sculptures was not clear.

In general, when two fluids are moving at different velocities, waves are generated at the interface. This is known as the Kelvin–Helmholtz instability and occurs, for example, on water surfaces in windy weather. Regular wavelets observed in the nebula by Berné et al. using the Spitzer Space Telescope arise from the mechanical interaction of high-velocity gas and plasma from a massive star blowing past the original molecular gas.