Wednesday, 4 July 2012


CMS and ATLAS both now have conclusive evidence of a Higgs-like particle with a mass of around $125-127$ GeV; this will be covered all over the internet, so I won't go into details here. I will say that it's a thrilling moment, and that we all eagerly await more data, to elucidate the detailed properties of this particle.

But the main reason for this post is to publicly congratulate everybody involved in this work. There are (at least) two distinct groups of people to highlight:

  • The LHC is an extraordinary machine, and has been performing phenomenally well; this discovery is only possible now because the machine has delivered so much integrated luminosity so quickly. It represents an unparallelled engineering achievement, so congratulations to the team who have worked so hard to get it to this stage.

Tuesday, 3 July 2012

Professor Higgs, your boson is ready

As has already been discussed extensively by many bloggers (including Jester, Tommaso, Shaun, and especially Peter Woit), CERN are holding a special seminar and press conference tomorrow morning, to announce the latest results on the hunt for the Higgs boson. It is widely rumoured that the evidence of a $\sim 125$ GeV Higgs from last year has strengthened to the point of being conclusive.

The plausibility of said rumours was only increased by a press release, which came out yesterday, giving the final results from the Higgs search at the Tevatron; the paper itself is available here.

String Phenomenology: Days 4 & 5

Well, my blogging momentum ran out towards the end of last week, but I will wrap up the String Pheno series with some points of note from the last two days.

In a day dominated by F-theory talks (at least in the plenary sessions), I thought the best of the lot was from Eran Palti, who discussed 'ultra-local' model-building in F-theory. The idea is to focus on the neighbourhood of a single point in the compact geometry, where all the important interactions are supported. His main point was that in many string models, we don't actually have enough control to calculate physical coupling constants; often an overall proportionality factor is missing, and simply assumed to be 'of order one' (i.e. of magnitude between about .1 and 10). I think this is a very important point; 'string phenomenology' as it stands is a bit of a misnomer, because as far as I know, nobody has yet been able to do an honest calculation of all quantities like masses and coupling constants in a realistic string model.

Friday, 29 June 2012

String Phenomenology: Day 3

As you will have noticed if you visit this blog, I've not done a very good job of blogging this conference. In order to keep things in order, let me post the only thing I wrote about day 3, so I can move on to the later days!

Mariana Graña discussed the consistency of putative string theory solutions which break supersymmetry via anti-D3-branes in a warped throat (I briefly discussed this in a previous post). She was fairly adamant that this setup is inconsistent, due to induced singularities in the three-form fluxes supporting the throat. The most important approximation she and her collaborators have used seems to be to 'smear' the anti-D3-branes — replace the point-like branes with a continuous charge distribution. This misses one possible resolution of the singularities, which is polarisation of the anti-D3-branes into NS5-branes, but they have arguments to suggest that this won't solve the problem. I doubt that the controversy will be resolved any time soon.

(The latest paper by Graña et al. went on the arXiv the day after this talk.)

Edit: I suppose it's reasonable to also flag my own talk, in which I spoke about my paper from May, which I already mentioned here. I wasn't particularly happy with the talk, although it went okay, and I had a fair-sized audience who seemed to pay attention, so no complaints!

Wednesday, 27 June 2012

String Phenomenology: Day 2

Here are some of the highlights from day 2 of the conference, as I saw it.

Gordy Kane continued to describe his recent collaboration with Bobby Acharya and others (see the post about day 1), focussing on their prediction of the Higgs mass. Specifically, they claim to predict that the Higgs should sit between about 122 and 129 GeV, and most likely at about 125 GeV. This time, the animosity towards these claims was a lot more apparent.

Tuesday, 26 June 2012

String Phenomenology: Day 1

Here is a brief overview of the more interesting points of day 1. I will link to the abstracts of each talk; hopefully, in time, the same pages will also include the slides and video from the talks.

Ben Allanach kicked things off by describing some of the lates experimental results, and what they might mean for supersymmetry (SUSY) in particular (and hence for string model building, basically all of which is supersymmetric). His most important points (I think) were the following:

  • Discovery of a standard-model-like Higgs, with a mass of around 125 GeV, could be just around the corner. In many popular realisations of SUSY breaking, a Higgs mass of 125 GeV is right at, or just beyond, the maximum possible value, assuming that superpartner masses are kept below several TeV.
  • There is an unexplained anomaly in the Tevatron data, in the 'forward-backward asymmetry' in the production of top-anti-top pairs. The Tevatron collided protons and anti-protons, and this variable measures the number of tops which are produced travelling in the same direction as the initial proton, compared to the number travelling in the direction of the anti-proton. The measured value disagrees with the standard model prediction by something like $3\sigma$.

Monday, 25 June 2012

String Phenomenology 2012

This week I am in Cambridge for this year's String Phenomenology conference, and it seems like a good excuse to do some blogging. There are five days of what should be quite interesting talks, and I will try to summarise and pass comment on at least some of them every day. Watch this space!

Friday, 1 June 2012

Citation tracking

I know of no good way to quantify the value of one person's contribution to science, or indeed any other field of intellectual endeavour. Nevertheless, people often try to do so, and the vast majority of these efforts focus on citation counts. There are many suggestions as to what is the most reliable indicator: total citation count, average citations per paper, or something slightly more sophisticated, like the h-index. Needless to say, all of these have major flaws, and tend to favour scientists who have simply been around for a long time, or who work on popular topics, but I get the impression that such quantities are still sometimes used as an aid in decisions on hiring, tenure etc.

Such issues have been discussed exhaustively on the blogosphere and elsewhere (although I'm too lazy to gather any links), and I don't really have anything new to add. But I did want to point out that, as far as I can tell, accurate citation data often simply aren't available.

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.

Friday, 27 April 2012

How to explain the Higgs mechanism

Time and again I read/hear popular-level descriptions of the Higgs mechanism in which it is proclaimed that the Higgs field is "like molasses", offering resistance to particles moving through it. This is an awful analogy, and makes me cringe every time. Even non-physicists should immediately see why: a particle moving through molasses feels a drag force which will ultimately bring it to rest with respect to the molasses (in the absence of some persistent driving force). But the Higgs field fills all of spacetime, and thanks to Galileo and Newton, we know that in empty space, in the absence of forces, particles move with an arbitrary constant velocity (up to the speed limit imposed by Einstein, of course!).

The big difference is that the background value of the Higgs field is Lorentz-invariant — it doesn't define any absolute standard of rest.

Thursday, 19 April 2012


I don't seem to be finding the time to write proper blog posts, so here is a dot-point rundown of a few things I have found interesting lately:

  • Bobby Acharya, Gordon Kane, and Piyush Kumar released a preprint last week in which they discuss 'generic' predictions of string theory for low energy physics. It is well worth a look, and represents a good review of some of the progress of the last ten years, but the word 'generic' concerns me quite a bit in this context; I don't even know a way to define a 'generic' member of a discrete set (of purported string vacua, for example). To be fair, the paper contains a number of caveats pointing out the various assumptions being made.

  • IceCube is a wonderful experiment, which uses the Antarctic ice sheet as a giant neutrino detector. The collaboration has just published a paper in which they report a null result in the search for neutrinos from almost 200 gamma ray bursts, allowing them to set a limit for neutrino production about four times below predictions. In particular, this basically rules out gamma ray bursts as the dominant source of very high energy cosmic rays. For a press-release-level overview, see here; the BBC has also covered the story.

Friday, 6 April 2012

LHC back online for 2012

Yesterday, CERN announced that the proton-proton collisions are again underway at the LHC, for the first time since last year. One quantitative change is that the machine is now accelerating each beam to $4$ TeV per proton, compared to $3.5$ TeV in 2011. (For those who might not know, $1$ TeV is equivalent, by Einstein's famous relation $E = mc^2$, to approximately one thousand times the mass of a proton.)

This year could be a very significant one for particle physics. If the Higgs boson really is sitting at $\sim125$ GeV, then its discovery is likely to be announced, and if low-energy supersymmetry is part of the real world, then we might hope to see at least the first evidence of it.

Saturday, 31 March 2012

Hairy quantum black holes

The arXiv this morning offered another interesting paper, this time by Gia Dvali and Cesar Gomez. These two authors, sometimes with collaborators, have written a number of related papers over the last few years regarding black holes and quantum gravity. Although interested, I'm afraid I have not taken the time to properly understand their papers, but here is a synopsis of a couple of the relevant ones as I understand them:

  1. First, there was the suggestion that quantum Einstein gravity might be self-consistent in the UV, with the naïvely-expected growth of scattering amplitudes being softened by the production of black holes at high energies. Trans-Planckian momentum transfer becomes ill-defined, because horizons form before any such processes can occur. There might therefore be no need for any fancier quantum theory of gravity.

  2. The only other paper I want to mention is this one. Here they put forward an argument that, quantum-mechnically, black holes should be thought of as bound states of gravitons. Let me try to summarise their argument very briefly:
    A gravitating system of mass $M$ sources a gravitational field containing, they say, $N \sim \frac{M^2}{M_P^2}$ gravitons, where $M_P$ is the Planck mass. The typical wavelength of these gravitons is the size of the gravitating source; as the source becomes more compact, this wavelength decreases, corresponding to a greater amount of energy being contained in the gravitational field itself. When the source reaches its Schwarzschild radius, the original source is (classically) hidden behind a horizon, and we can think of the entire rest energy as residing in the gravitons. From the paper:
    "For us the black hole is a bound-state (Bose-condensate) of N weakly-interacting gravitons…"

    They go on to explain Hawking radiation as the quantum depletion of this condensate: interactions between the gravitons will occasionally give one enough of a kick to escape the condensate. Similarly, graviton interactions may pair-produce any particles in the theory, and sometimes one of these will escape.

I am uneasy about the ideas contained in paper 2. One concern is the following: a Bose-Einstein condensate, consisting of some fixed number of particles, is not much like a classical field configuration. Every undergraduate knows about coherent states of the harmonic oscillator, which are given by eigenstates of the annihilation operator — about as far as one can get from a state containing a fixed number of particles. Nevertheless, these are the 'most classical' states. Or take something less trivial: a widely separated kink and anti-kink (so we remain in the topologically-trivial sector) in $\lambda\phi^4$ theory in two spacetime dimensions. Can this sensibly be treated as some bound state of $\phi$ quanta? Even if it can, does the same reasoning apply to gravitons, considering the rather dramatic effects which a strong gravitational field has on spacetime (which is a fixed background for other field theories)? Perhaps these concerns are unimportant, and I am willing to ascribe them to my own ignorance for now, and move on to discussing the new paper.

Wednesday, 28 March 2012

Inflation and quantum gravity

Today's arXiv listing brought an interesting new paper by Joe Conlon of Oxford. In it he discusses constraints on inflation models coming from general principles of quantum gravity.

Inflation in short is the idea that very early in its history, the universe underwent rapid expansion by a factor of something like a billion billion billion (or about 60 'e-folds' in the jargon of the field). This solves certain problems of cosmology, which I don't want to go into here. In the context of general relativity, inflation can be achieved by a scalar field $\phi$ slowly rolling down a potential; inflation stops when it reaches its minimum. This scalar field is called the inflaton. Note that I am deliberately ignoring the fact that multiple fields can play important roles in inflation; this doesn't really matter for what I want to discuss, although see the caveat at the end of section 2 of Joe's paper.

One variable feature of inflation models is the total distance (in field space) which is traversed by the inflaton during inflation. There has been some controversy about whether this can consistently be greater than the Planck scale, because standard effective field theory arguments break down in this case (we can, and probably should, add arbitrary operators $\phi^k/M_P^{k-4}$ to the Lagrangian of the theory, and these all become important if $\phi \sim M_P$, rendering the theory meaningless). This is the sort of problem that one might hope to make some progress on by turning to string theory…

Monday, 26 March 2012

From Perimeter to U.Penn.

All too soon, my visit to Perimeter came to an end yesterday, and I'm now writing from the University of Pennsylvania, in Philadelphia. I'm here all this week on the kind invitation of Ron Donagi, and I'll be giving a seminar tomorrow, which will be very similar to the one I gave at PI. This is my second visit to U.Penn., the first being last year for the fantastic String Math conference.

Let me mention one interesting aspect of the last week. I had the chance to talk at some length with John Dixon (I recommend reading his short profile; he has had a very unconventional career), and in particular he explained a little bit about an idea he has been working on for several years, which he calls 'CyberSUSY'. You can find the papers on the arXiv. It's a rather complicated idea, which I couldn't possibly explain fully here even if I understood it, but I can outline some of the ingredients.

The theory includes a non-standard realisation of supersymmetry, and an infinite tower of arbitrarily high-spin fields, carrying the quantum numbers of the standard model fields (this sounds bizarre, but is reminiscent of the tower of massive modes one finds in string theory). The supersymmetry-invariance of the action depends on the invariance of the superpotential under certain transformations, which are not usually considered because they do not leave the kinetic terms invariant (and therefore are not symmetries of the theory). John's point of view is that this ties supersymmetry together with the gauge symmetry and multiplet structure of the standard model. Furthermore, upon the addition of a dimensionful parameter, electroweak symmetry and supersymmetry are both broken.

Saturday, 17 March 2012

Seminar at Perimeter

On Thursday I arrived at the Perimeter Institute in Waterloo, Canada, for a ten day visit. It is quite a singular institution, its establishment having been funded privately over a decade ago by Mike Lazaridis, the man behind the Blackberry.

Yesterday I gave the string seminar here, and spoke about the recent paper I discussed in this post. Attendance was quite reasonable, and I think it went well enough. One of the many great things about Perimeter is that they record all (I think) of the seminars given here, and make them available for free on their website. You can check out mine here. It starts about two minutes in, but nothing of import is missing. I had a look at a few minutes of it, as I've never actually seen myself give a seminar before, and was mostly struck by just how Australian I sound…

I'm looking forward to the next week here, after which I'll be spending a week at the University of Pennsylvania. There are many excellent people at both institutions, so I hope to have some interesting things to blog about.

Thursday, 8 March 2012

More progress on anti-hydrogen

To put it mildly, this blog has been rather quiet, but I hope that this post signals a return to semi-regular blogging. Let me first quickly sum up the news in high energy physics since December: a standard model-like Higgs is looking more and more likely at about 125 GeV, and low-energy supersymmetry is looking less and less likely, as the models which are still consistent with the data are getting uglier. With that out of the way, let me turn to the subject of today's post…

In the second ever entry in this blog, I discussed the ALPHA experiment at CERN, which is designed to trap and study anti-hydrogen. As a birthday present to me yesterday, a new paper from the collaboration was published online by Nature. Last year they managed to trap anti-hydrogen for several minutes at a time, and they are now beginning to study its energy levels, which is the whole point of the experiment. Theory says that these should be exactly the same as for hydrogen, and any deviation from this would be very big news indeed.

My understanding of the experiment is as follows. They use inhomogeneous magnetic fields to trap the atoms. The magnetic moment of an anti-hydrogen atom in its ground state is dominated by the spin of the positron, and the experiment traps those atoms in which the positron has one polarisation; the others escape. Once the atoms are trapped, the team irradiates them with microwaves, tuned to a frequency which should induce a spin-flip in some of the atoms, resulting in them escaping the trap and annihilating in the surrounding material. They have of course performed control experiments in which the microwaves are off-resonance.

The measurements are somewhat complicated by the external field required to trap the atoms.