Photos from CERN

As I mentioned a couple of posts back, we were lucky enough to go the 100m down into the chamber for the CMS detector. I got a few pictures while we were there.

First of all, on the surface before going into the cavern and indeed all around the CERN sites you see huge gas tanks. These tanks are for storing helium. The detectors and accelerator have to be cooled to just above absolute zero (there are superconducting magnets for the accelerator which only work at very low temperatures). If there is a problem with any of the components they may have to be taken apart, or at least looked at in detail. In order to do this, the helium will be released and the parts will warm back up. CERN has the largest supply of helium in the world and this is both expensive stuff and not easy to produce. It is still being produced now in order that they have a large enough supply. So, when you release the helium you want to store it somewhere, not just let it off into the atmosphere, and that is what these enormous tanks are for. The circumference of the tanks is, I guess, around 2 or 3 meters.

100m down into the cavern and we were lucky enough to see the last two pieces of the CMS detector being readied to slot into place. The following panorama is from 6 shots, which don't go together perfectly, but give a pretty good impression of the complexity of the detector:

The largest component of the CMS detector weighs in at 2000 tonnes. Your average crane cannot carry this sort of weight and so in order to lower it into the cavern they had to construct a specially designed pulley system. The two towers on top of the CMS building housed the wheels for this pulley. In the background of this picture is Mont Blanc:

Mont Blanc can be seen more clearly in this picture which I took as we were taking the bus to the site. The new polarising filter for my camera works very well at removing the reflections from the glass in the bus. the extreme blue in the sky in the picture of the helium tanks also comes from using a polarising filter which helps to remove any haze:

See here for a couple more from the detector.

RTN recap part 1 and gauged supergravities

I'm back in Santiago after a week at the RTN winter school in CERN. I have a full list of topics that I want to go over from the school, and this time I REALLY will do it (I may have said that before, but this time I really mean it). In particular Henning Samtleben gave an excellent course on gauged supergravity and flux compactifications which was extremely clearly presented for what is not an easy topic to get your head around. There's a reading list which was supposed to be read before the lectures but I feel that I will get more out of them now. I'll put up a link as soon as the videos are up online.

I'll see if I can give a basic overview of gauged supergravities, if only to get it clear in my own head.

Gauged supergravities can be understood from two points of view.

The first route is to take an ungauged supergravity which comes from a compactification of 10 dimensional supergravity on a torus of some number of dimensions. This will give you an effective theory in D=10-d dimensions where d is the dimension of the torus. The second route is to compactify on a less trivial space and to turn on n-form fluxes in your theory.

For the first route, the D-dimensional theory which you obtain from compactifying on a torus will have both gauged and global symmetries. The gauged symmetries will be abelian and there will be a gauge field for each of these U(1) symmetries. Then there will be a global symmetry, which typically will take the form of an exceptional group (one subset of the semi-simple Lie groups).

The theory you now have is an ungauged supergravity (although it does have gauge fields, the name comes from the fact that there are no non-abelian gauge symmetries). A gauged supergravity is formed by gauging some subset of the global symmetries.

There may be many ways to do this (to choose the subset of global symmetries to gauge) which will put the abelian gauge fields that you already have into the adjoint representation of the now non-abelian gauge group. The way in which you do the gauging (choose the subgroup of the global symmetries to gauge) is all defined in terms of a single tensor. This tensor has a series of constraints which must be imposed in order to form a closed algebra when you perform the gauging and also to keep supersymmetry (though you may wish to break some number of the supersymmetries in the gauging - giving you a maximally or non-maximally supersymmetric gauged supergravity).

As I said, there are two routes to gauged supergravity. The second route is to go back to the 10 dimensional supergravity that you started with, and rather than simply compactifying on a torus, you compactify on a more complex manifold - a d-sphere, a torus with torsion, or you may turn on some fluxes which wrap closed cycles in the compactified directions. This process results in the same gauged supergravity theories that you would have obtained by gauging a subgroup of the global symmetries in your 'trivially' compactified theory.

The point about these theories is that you have gone from a 10 dimensional theory to a lower dimensional theory with a non-abelian gauge group. This is clearly an interesting area to explore and has many applications, from the AdS/CFT correspondence to model building and more...

Anyway, that's the very basics (the flow chart as I have picked it up over the last week) of gauged supergravities.

As far as I can tell, the main subtleties come from 1) applying the constraints on the tensor which encodes the gauging of your global symmetries and 2) finding a realisation where the gauge transformations act in a manifestly covariant manner on your fields.

I may try and do the same thing for the other lectures if I get some time. For now I'll upload some photos from the trip.

Strings in CERN

I'm currently in CERN, in Geneva at the RTN winter school on Strings, Supergravity and Gauge Theories which is proving to be both extremely interesting and exhausting in equal measure. With five hours of lectures a day and several hours spent every evening working with my collaborator, it feels like we've had weeks here already although the whole thing will be over on Friday. Though I hadn't been particularly looking forward to the topics, the lecturers are all excellent and I'm learning more from these courses than I have from a school in some time. Many of the lecture notes are up online and the videos I presume will follow.

Today we were lucky enough to take a trip to one of the two main detectors for the LHC, the 27km circumference proton accelerator to be turned on later this year. The CMS detector had its final segment lowered 100m into the ground yesterday and so the bulk of the machine is now in place, getting ready to start taking data from the particle collisions some time in the summer (data is already being taken in the detectors from cosmic ray events). At 12,500 tonnes, and around 21 by 16 meters, this is in essence a huge piece of electronics which will be a general purpose detector (one of the two, the other being ATLAS) monitoring some of the 40 million collisions per second when the machine is running at full pelt.

I'll get my own photos up online when I'm back in Santiago. Having spent some time working on Babar it was impressive to see the next generation of machine which we hope to give us some of the missing pieces which we know must be out there to complete our understanding of of elementary particle physics.

In the time outside lectures I'm working as quickly as possible with my collaborator on a project which has taken far too long and we are finally trying to get it finished off. Having spent almost all of the time working on this project in different continents we're taking full advantage of our current state. Will update more when I'm back home.