A couple of particle related bits

Neutrinos cause a lot of problems. These ‘ghostly’ particles have a habit of only interacting very weakly with ordinary matter, making their detection rather difficult. They are produced in massive numbers by fusion reactions inside the Sun, potentially giving us detailed information on what’s going on, but when we measure the type (or ‘flavour’) of neutrinos coming from the Sun, the ratios of each flavour don’t add up to what we’d expect (although the overall numbers are fine now we can add up the numbers of each flavour). A lot of head scratching later and the idea of neutrino oscillations came about. This focuses on wave-particle duality which essentially means every particle in the universe has some level of wave-like actions and some particle like characteristics. So photons of light hitting metals don’t impart their energy continuously like waves breaking on the shore, they give one pulse of energy, then another (meaning electrons requiring a certain amount of energy to leave the metal won’t do so until photons over a certain energy come at them). It also means particles like electrons can undergo wave like interactions like diffraction and interference. It was also suggested that neutrinos – particles – could interfere with one another in a wavelike way such that a group of different flavours would appear to contain a different ratio of flavours to the actual amount that were present when the beam came together. This is neutrino oscillation.

The oscillations required something that hadn’t at the time been measured – the neutrinos needed to have mass. An experiment was soon performed that didn’t show the masses of the neutrinos, but instead showed that the flavours – electron, tau and muon (plus their anti particles, named after the particles whose creation spawned them) – all had differences in their mass. If there were differences, then at least some of them had to have mass, which was enough to allow oscillations to occur. But some doubts still persisted as the oscillations themselves had never been seen – until now.

At the OPERA facility in Italy, they measure the numbers of muon neutrinos spawned by decays in the LHC in CERN, 730km away. They have an idea how many are being produced and where they’re headed over there and how many should be arriving where they are. They also know how many in theory are going to change from being muons to tau neutrinos. Now they have announced that a tau has been produced in a pure muon stream, the first tentative step to showing that oscillation happens – though a few more detections would be nice just to show there’s a trend here.

Meanwhile, in other astroparticle news, the IceCube detector has been looking not at muon neutrinos, but at muons themselves. Muons are a class of particle called Leptons that includes electrons and tauon particles, their anti-particles and their neutrinos. They are the heaviest of the leptons and so the least likely to pop into existence. They are created in high energy interactions and can be accelerated to high energies. The creation and acceleration of such exotic particles by magnetic fields and shockwaves associated with supernovae and stellar remnants such as neutron stars. But there’s been little evidence that this is so. There are limits to the energies of cosmic rays (hugh energy particles born outside of Earth) coming in from outside the solar system. On the low energy scale, the magnetic fields of the Earth and the solar wind deflect those charged particles that aren’t travelling our way with enough momentum. At the highest energies, charged particles are slowed by brehmstrallung radiation that opposes acceleration. Travelling through the cosmos, particles from various sources encounter the magnetic fields of interstellar space and are slowly massaged into a fairly constant background. But some relatively close structures can still be seen lurking in images of the sky taken using particles rather than light.

The map of incoming muons produced by IceCube, which is a nascent neutrino observatory, has shown there to be a slight excess of cosmic rays from the direction of the Vela supernova remnant. This remnant is 800 light years from us and 10,000 years old. Between it and us there is a second supernova remnant, often missed as it shines less brightly across most wavelength ranges (but still more brightly in gamma rays). Vela was the place where an observed neutron star was linked to a supernova remnant directly for the first time, could it now be the place that the first tentative observational proof of cosmic ray acceleration in such a system has been achieved?

…and sticking with muons for a moment. Fermilab recently released data on muon production at its particle accelerator facility. The data showed that production of muons outstripped production of anti-muons in proton-anti-proton collisions by about 1%. This is an important discovery, if confirmed, which could show the reason why the local universe is dominated by matter, rather than having equal amounts of matter and anti-matter. It has been hypothesised that this imbalance exists between the amount of matter and anti-matter generated in particle interactions, with subsequent annihilation between the particles leaving the excess of matter behind to form the things we see.


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