Hints of new physics?

Physicists measuring the parameters involved in neutrino oscillations have seen a suggestion that one of these parameters differs quite markedly from theory.

Neutrinos are ‘ghostly’ particles that travel extremely quickly and interact infrequently with ordinary matter. Despite the Earth being constantly bombarded with the things, they are able to pass through the bulk of the planet without stopping. But as they go their merry way, something does alter the composition of a beam of neutrinos, not an external force, but the wave function of the neutrinos themselves. Like two musical notes played close in pitch, the wave function of two types or flavours of neutrinos will generate the appearance of the third flavour. The same thing happens to the antineutrinos, but with a difference to the extent that it happens. This difference is so small that according to the Standard Model of particle physics, there should be no detectable difference between the parameters involved in neutrino oscillation and those in antineutrino oscillation on the scale of an experiment the size of MINOS. However, a difference might have been seen.

Using the NUMI beam, which can produce neutrinos or antineutrinos, the researchers measured the mass difference parameter delta m squared (delta = the difference in, m = mass, squared = well, squared). The results they got for the neutrino beam were forty percent lower than those they got for the antineutrino beam. However, the experiment hasn’t yet been repeated with sufficient frequency to tie down the error margins, there is still a five percent chance that the results were erroneous and there is no measurable difference at all. The experiment will now be repeated until the results can be announced with sufficient confidence. STFC (funding body for UK involvement in Fermilab) press statement is here.

But neutrinos aren’t the only things acting up at the moment. Probes of the Sun are providing more observables than ever before. We can see our nearest star in the light of several different species, each of which highlights a different aspect – sunspots, filaments, prominences etc. In addition, observations of the vibrations of the Sun allow us to perform seismology on the star, providing some detail about internal movements. As the Sun undergoes quite a deep minimum, this article in New Scientist explains how this sharpens our knowledge of how the internal mechanism of the Sun, and therefore other stars, works.

…and we’re back to neutrinos. The three flavours of neutrinos – electron, muon and tau – have different properties. Although they’re all hard to detect, some are very hard to detect relative to the others. The motivation for studying the possibility of neutrino oscillations came from the fact that there seemed a difference between the numbers of neutrinos of one flavour detected and the number that should be produced by nuclear fusion in the Sun. As it happened, the oscillation of detectable neutrinos into undetectable neutrinos sorted this out – helped along by new detection methods that caught all three flavours. But neutrino oscillation depends on the fact that there are differences between the masses of the three neutrinos, which itself suggests that at least two of the three flavours must have mass. The idea that we have a type of particle that is pretty much undetectable due to its weak interactions, but has mass allowing gravitational interactions is a strong indication of the nature of Dark Matter – the most expected type of which is a Weakly Interacting Massive Particle, or WIMP. WIMPs are said to be likely to be ‘cold’, that is unlike the zippy neutrino, they are relatively sluggish so far as moving particles go. Dark Matter first came about to explain why the rotation rate with distance of galaxies was flatter than expected – ie why it didn’t drop off quicker nearer the edges of the galaxy, where the matter was sparser and so less tightly held by gravity and where greater energy is required to spin up the matter so it keeps pace with stuff closer to the nucleus.

In addition to Dark Matter came Dark Energy, the mysterious force behind the expansion of the Universe. These two factors become very important on larger scales in the Universe and so are more apparent in massive structures. The largest structure is the Cosmic Microwave Background, whose properties – ripple sizes, temperature differences and the like – must be influenced by the ‘Dark Side’ of the Universe. Information taken from the WMAP satellite, which has provided the most sensitive map so far of what’s been called the afterglow of the Big Bang, has been concordant with the predictions of theories involving Dark Matter and Energy, however now a team from Durham University have started to suggest that all might not be well.

Their suggestion is that the resolution of the probe and the software used to smooth the data aren’t actually sufficient to decide on how large the ripples really are. Furthermore, they also suggest that another prediction based on Dark Energy may be wide of the mark. Light travelling through space gets red shifted and blue shifted by a number of factors. One is the relative velocity of the source and the observer, another is gravitational time dilation, which shifts the frequency of light as it enters a gravity well, and puts it back again as it exits. In the case of a massive gravity well, such as a galaxy cluster, the time taken for light to pass through the well is significant – large enough for any acceleration in the expansion of the universe to leave an additional redshift on the light as it leaves the weakening well, all of which travels at a set velocity in this model. The result is a small detectable blue shift in the light passing through massive clusters. But no shift has been detected by this group, leaving two suggestions – either there is no expansion or this thought experiment is flawed. Data for this came from the Sloan Digital Sky Survey, which looked at the northern hemisphere and a southern hemisphere survey is sought to examine the effect there.

Now none of this is conclusive – there is other evidence for the Dark Side of the Universe and the Planck survey will provide a higher resolution map of the CMB – but it does keep the debate on the facts going.

All of this kind of work requires observations of large amounts of data. Traditionally, human eyes have been the best tool we have for identifying patterns of interest and rejecting those that aren’t – allowing citizen science projects such as Galaxy Zoo to perform real science by farming out relatively easy drudgery to the general public. However, now a computer technique called a neural net, which allows a computer to be trained to recognise and reject appropriately has produced results with 90% of the accuracy of users of Galaxy Zoo, allowing a computer to finally start rumbling through these vast amounts of data properly. The work was performed by two people I know, one of whom was (and is) the professor in charge of UCL’s astrophysics group (and a key member of the Dark Energy Survey), the other of which earned her doctorate there. UCL’s press release is here.


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