## Thursday, 24 May 2012

### Science and art

Today's post is slightly off-topic for this blog, but I couldn't help myself. I promise it won't happen too often. I just received, via the departmental mailing list, an advertisment for what sounds like quite a nice event at the Tate Modern, in London. They are showing a film about mathematics and mathematicians, which will be followed by a discussion with Michael Atiyah and Cédric Villani, two very successful mathematicians, from different generations, countries, and sub-fields. This is great, but it is part of a broader series of events, going by the name Topology.

If you follow the link above, you will find some of the most extraordinary pseudo-intellectual bullshit I have ever had the displeasure of reading.

## Tuesday, 22 May 2012

### Collective Marvelling

I am pleased to announce that Cross Sections is now part of a small blog network, consisting of blogs written by young researchers (meaning PhD students and post-docs). For now, this is taking the rudimentary form of a separate blog called Collective Marvelling, where you can find information about each of the blogs, and snippets of the latest posts from each, linking to the main articles themselves. There is a permanent link at the top of this page.

The name was not my idea, but I rather like it. Becoming a research scientist is, in a sense, a way to get paid to just marvel at the natural world!

## Friday, 18 May 2012

### How fast are neutrinos?

I realise that I have not been making any effort to write at a level understandable to non-physicists. I don't apologise for this, but I thought it was time for another post aimed at a broader audience. There will be some equations, because I want to be quantitative, but nothing too complicated!

Neutrinos are the most elusive of sub-atomic particles, interacting only very weakly with other matter. In fact, according to Wikipedia, the total flux of solar neutrinos (neutrinos produced by the Sun) at the distance of the Earth is about 65 billion neutrinos per square centimetre per second(!), and these pass straight through without us noticing them. Nevertheless, they do very occasionally 'bounce off' an atom, and this allows us to detect them, and do experiments with them. Indeed, neutrinos made headlines last year, when the Opera experiment claimed to measure them moving faster than light. This turned out (as most of us expected all along) to be due to an error in the equipment, rather than a genuine physical effect, which would have implied a breakdown of special relativity.

Judging by various comments on blogs and the like, the whole 'superluminal neutrino' affair did raise one puzzling point for some people. Since the 1990s, we have known that neutrinos have mass, albeit very small, and special relativity tells us (as we will see below), that no particle with mass can travel at the speed of light. Yet all the news stories reported that the neutrinos were expected to travel at the speed of light! The point is that they were expected to travel so close to the speed of light that no difference could be measured, and in this post, I want to explain why.

In relativity, we define the 'gamma factor' for a massive particle to be $\gamma = \frac{E}{m\,c^2}$, where $E$ is the energy of a particle, and $m$ is its mass.

## Thursday, 10 May 2012

### Preprint roundup

The sheer number of new preprints which are listed every day on the arXiv makes it very difficult to read everything which might be of interest, and harder still to write blog posts about every paper which one might wish to discuss! So today I offer a short list of recent papers which have grabbed my attention, with little in the way of commentary (and in no particular order):

• First a bit of self-promotion. I have a new paper out this morning, about Dirac gauginos in F-theory. It really only takes the first basic steps in the study of such models, but I'm hoping it rouses some interest in them. Dirac gauginos are one way in which reasonably natural low-energy supersymmetry might be 'saved' from the null results reported so far from the LHC, but as far as I can tell, they have received zero attention from the string model building community until now. Believe it or not, Luboš beat me to reporting this.

• Also released this morning was a study by a number of authors, of putative string vacua containing anti-branes in a warped throat. This has been a widely-accepted way to break supersymmetry at a reasonably low scale, and in a reliable way, since the so-called KKLT paper. The new results seem to throw doubt on the whole idea (although I'm no expert in this subject, and I only skimmed the paper). Usually, the anti-brane is treated in the probe approximation, where its backreaction is ignored, and in this formalism, it is found that the apparent singularity of the geometry is resolved by 'polarisation' of the anti-brane. The authors claim that this cannot occur if the backreaction is properly taken into account, making the existence of these type of vacua somewhat more doubtful.

## Wednesday, 9 May 2012

### A dark matter signal from Fermi-LAT?

The Fermi gamma-ray telescope is a space-based gamma ray observatory. Somewhat unusually, for those of us more used to discussing collider experiments, the data it collects is freely available for anybody to analyse. Last month, Christoph Weniger, a dark matter theorist from the Max Planck Institute in Munich, released a paper in which he claimed to have discovered a peak in the gamma-ray flux, possibly corresponding to the annihilation of pairs of dark matter particles with a mass around 130 GeV. (I'm not sure, now, why it couldn't be from decay of a 260 GeV particle, and a quick skim over the paper hasn't enlightened me.) One interesting aspect of this research was that the data in which Weniger found a signal had already been analysed by the experimentalists themselves, and they concluded that there was no signal. Weniger's trick was to focus on those regions of the sky where models of the galaxy and its dark matter halo predict the signal-to-noise ratio to be high. Using the full dataset washes out the signal, hiding it in a (relatively) larger background.

This was actually discussed at length at our journal club at Oxford last week, and the general conclusion seemed to be that the work was very interesting, but far from convincing.