Universe Today | Steve Nerlich | 2012 Feb 05
Today's article is about neutrino astronomy ... on ice.
This paper presents some recent observations from the IceCube neutrino telescope at the South Pole – which acually observes neutrinos from the northern sky – using the Earth to filter out some of the background noise. Cool huh?
Firstly, a quick recap of neutrino physics. Neutrinos are sub-atomic particles of the lepton variety and are essentially neutrally charged versions of the other leptons – electrons, muons and taus – which all have a negative charge. So, we say that neutrinos come in three flavours – electron neutrinos, muon neutrinos and tau neutrinos.
Neutrinos were initially proposed by Pauli (a proposal later refined by Fermi) to explain how energy could be transported away from a system undergoing beta decay. When solar fusion began to be understood in the 1930s – the role of neutrinos was problematic since only a third or more of the neutrinos that were predicted to be produced by fusion were being detected – an issue which became known as the solar neutrino problem in the 1960′s.
The solar neutrino problem was only resolved in the late 1990s when the three neutrino flavours idea gained wide acceptance and each were finally detected in 2001 – confirming that solar neutrinos in transit actually oscillate between the three flavours (electron, muon and tau) – which means that if your detector is set up to detect only one flavour you will detect only about one third of all the neutrinos coming from the Sun.
Ten years later, the IceCube Neutrino Observatory is using our improved understanding of neutrinos to try and detect high energy neutrinos of extragalactic origin. The first challenge is to distinguish atmospheric neutrinos (produced in abundance as cosmic rays strike the atmosphere) from astrophysical neutrinos.
Using what we have learnt from solving the solar neutrino problem, we can be confident that any neutrinos from distant sources have had time to oscillate – and hence should arrive at Earth in approximately equal ratios. Atmospheric neutrinos produced from close sources (also known as ‘prompt’ neutrinos) don’t have time to oscillate before being detected.
When looking for point sources of high energy astrophysical neutrinos, IceCube is most sensitive to muon neutrinos – which are detected when the neutrino weakly interacts with an ice molecule – emitting a muon. A high energy muon will then generate Cherenkov radiation – which is what IceCube actually detects. Unfortunately muon neutrinos are also the most common source of cosmic ray induced atmospheric neutrinos, but we are steadily getting better at determining what energy levels represent astrophysical rather than atmospheric neutrinos.
So, it’s still early days with this technology – with much of the effort going in to learning how to observe, rather than just observing. But maybe one day we will be observing the cosmic neutrino background – and hence the first second of the Big Bang. One day…
So… comments? Are neutrinos the fundamentally weirdest fundamental particle out there? Could IceCube be used to test the faster-than-light neutrino hypothesis?
Astrophysical neutrino results - Thomas K. Gaisser (for the IceCube Collaboration)
- arXiv.org > astro-ph > arXiv:1201.5637 > 26 Jan 2012
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