LHC: In the particles cathedral

Subatomic particles such as electrons and protons are very small, but the devices used to study them are rather large. Consider ATLAS: it is about 45 meters long, over 25 meters high, weights about 7000 tons, was imagined in 1994, has been under construction since January 2003 and has the full attention of about 2000 scientists and engineers from three dozen countries.

ATLAS is one of the two main particle detectors that, starting later this year, will observe and measure the results of the collisions of beams of protons traveling at nearly the speed of light inside the Large Hadron Collider (LHC), the new particle accelerator at CERN, the physics lab situated near Geneva (known at large for being the place where the World Wide Web was invented).

This week I have spent a few hours with two colleagues visiting the LHC construction site before it is closed to the public in the coming months (pictures below). We were accompanied by three CERN scientists -- Brian Cox (see this previous post), Torsten Wengler and Albert de Roeck -- and I can't shake the impression of having glimpsed the preparation of one of the most complex and ambitious scientific experiments ever. At a cost of nearly 6 billion euros, once fully operational (next year) the LHC will be the world's most powerful particle accelerator. It is hosted in an underground circular tunnel of 27 km of diameter circonference that crosses the Swiss-French border at an average depth of about 100 meters.

In the LHC, beams of protons (and, later, ions) will travel in opposite directions inside two pipes surrounded by magnets (cooled to near absolute zero - minus 273 Celsius - by liquid helium) and other machinery and wiring. The protons will be accelerated until they nearly reach the speed of light (almost 300'000 km per second -  over 11'000 LHR laps). "Basically, we will start with a bottle of hydrogen gas, open it up and start accelerating", says Brian Cox. The hydrogen nucleus consists of only a single proton, and "accelerating" in a physicist's terminology means increasing the particle's energy levels. At full speed, the energy stored in a beam of protons "will be close to that of a TGV train traveling at 500 km/h", explains Torsten Wengler, trying to dress the scientific project in understandable metaphors.

Along the 27-km ring the scientists are building five detectors. One is ATLAS, the other is called CMS, and there are three smaller, more specialized devices. When the proton beams transit through these machines, powerful magnets will slightly deflect their trajectory, making them cross each other every 25 nanoseconds (one nanosecond = one-billionth of a second; hence, there will be 40 million collisions a second). Imagine two clouds "entering" each other: during each crossing, a couple dozen protons will frontally collide with protons coming in the opposite direction, spraying thousands of particles into the surrounding detector. ATLAS, CMS and the other detectors are designed to "see" and measure them - and look for one of them in particular: a minuscule, elusive, hypothetical particle - the Higgs boson - whose existence is supposed to explain why there is mass in the Universe.

Modern particle physics is based on a "standard model" that explains the interaction between the building blocks of matter. All the particles in this model have been discovered, except for the Higgs. "We don't know why there is mass", says Cox. Finding the Higgs is target number 1 of the LHC. Which is designed to recreate the conditions of a fraction of a second after the Big Bang, when it is supposed everything in the Universe was weightless - until mass (the Higgs, if the hypothesis is correct) appeared. Hence, if successful the LHC experiment may reveal the origins of mass; and may reveal more. For example: why is the force of gravity so weak that we can lift an apple off a plate even if the whole Earth is pulling in the opposite direction?

I asked Wengler if not finding the Higgs would qualify as a failure. "Not really: we would still learn alot", he said, and the first learning would be that the current physics theories would need serious reconsideration - for the Higgs is the glue that keeps the whole theoretical construct together. The only scenario that Wengler would call "catastrophic" is that of a beam of protons going out of control, and spinning out of the 27-km ring. "There would be no risk for people: the protons would travel a distance of probably 100-200 meters underground, horizontally, punching a hole in the rock, and the energy will dissipate into the Earth. It would wreck our machine - that would be catastrophic - but it will not be noticed outside". That's because the LHC is an accelerator, not a reactor: the protons need constant addition of energy in order to keep circling the LHC. The only distance they would travel if they get out is that allowed by inertia. And the scientists built a few "beam dumps" where they can direct the protons in case of problems in the system, plus other safety systems.

OK, time for the visit. The CERN campus is situated in the countryside north of Geneva, and stretches across the Swiss-French border. Above ground, the LHC structures look mostly like hangars and warehouses. And that's what they are. A small construction hosts the ATLAS control room (click on any picture below to enlarge):

Just besides it, the ATLAS main above-ground building is a big warehouse full of components waiting to be installed:

There aren't many places in the world where machinery carries labels such as this:

They are "cosmic ray certified", explains Wengler, because scientists rely on cosmic radiations, which produce particles called muons, to test the detectors before they are brought underground to be installed. The components are lowered through two 100-meters-deep access shafts like this one:

Mandatory: you put on a hard helmet:

and then an elevator takes you down to the LHC. A door at the end of a narrow tunnel:

leads to the gigantic chamber - bigger than a cathedral - that hosts the ATLAS. A particles cathedral, with concrete walls three meters thick. The ATLAS detector is currently being assembled, and here is one of its "wheels":

(Notice the two men on the scaffolding on the left side to get a sense of the size of the machine). The different components seen here are muon detectors. The logistics, organization and project management involved in building this device are on an industrial scale. Nearly 6000 scientists from CERN and from hundreds of institutions around the world work on the LHC. The challenges are enormous, as well as extremely subtle. For example, the underground chamber that hosts the ATLAS is affected by ground temperature and by the pressure of the rocks around it (before the 7000-tons machine was installed, the ground was slightly higher, for example); by the moon phases; by how much water there is in nearby Lake Geneva. These may be only millimetric movements, but the beams are sensitive to sub-millimetric shifts. "We can't prevent the machine from moving", explains Torsten Wengler, "so we try to know the movements in advance and adjust", through a series of alignment systems built into the detector (laser beams and others). Seen from the other end, the partially-built ATLAS looks like this:

The big pipe-shaped red-striped structures installed in a star arrangement are the magnets that will deflect the proton beams (on top of the scaffolding, with a white helmet, a scientist is at work). Why does it need to be so big, I ask. "It's driven by the energy of the particles and the size of the magnets needed to bend the beams of protons", answers Brian Cox. Despite the extreme scale of the device however, actual collisions will happen within a 20-centimeter portion at the core of ATLAS.

When you are in a place like this, safety of the personnel is priority number one, two and three. So everywhere you go you find a series of interfaces to safety devices: general emergency, gas, fire, etc:

This is an icon that I had never seen before: it signals the danger of lack of oxygen (we are 100 meters below ground):

After exiting the ATLAS complex we have to drive for about 20 minutes across the countryside to get to the location of the CMS detector, which gives an idea of the size of the LHC. The CMS and the ATLAS are, in a way, competing experiments. Their aim is the same - find the Higgs and more - but they are built following two very different strategies for detecting particles. The CMS is more compact (that's the "C" in its name - it is about one-quarter of the size of the ATLAS) but much heavier (12'500 tons). It includes an electromagnetic calorimeter made of crystals that, says Albert De Roeck, "are still growing", and contrary to the ATLAS it is being assembled on surface and then lowered into the underground chamber (mostly for practical and budgetary reasons, but also because the underground geology here is more difficult - streams of water were flowing through the chamber while it was being excavated):



The assembly hall is so big that neighbors have complained because it obscures their view of Mont Blanc: "once the machine has been lowered into the LHC, we will take down the hangar", says De Roeck. Here is how a portion of the CMS looks like:



The complexity of the machine (like that of the ATLAS) is not to be underestimated. Its tech description fills 5000 pages; to the layman, building it looks like an act of art - and it translates into surprising beauty:



ATLAS and CMS, as said, are somehow competing experiments. Two separate teams, composed each of more than 2000 scientists from over 100 different research institutions around the world, are engaged in what De Roeck calls "friendly competition" (while Torsten Wengler defines it "not vicious") to be the first to "see" the Higgs. In fact, scientists happen to move from one team to the other, and there is quite a bit of communication, although "each group has its own worries". CERN, actually, is one of those places that show the world as it should be: in one of the experiments, scientists from the US, Israel, Iran and Pakistan work together, "and even if sometimes things get rough, at the end everybody understands that we're all here pursuing the same goal", says Wengler.

The pipes, the ATLAS, the CMS and the other machines are only a portion of the LHC project however: the hardware. A sizable part of the effort has to do with software. Traditionally, high-energy physics has treated software as an add-on. The construction of the hardware is still the main focus of the LHC (mainly because if a mistake is made, it will be more difficult to fix it), but the software development process is here at least as complex. Because the Higgs cannot really be "seen" - it will have to be modelized by computers tracking the electrical impulses generated by the collisions. And because the amount of data that the collisions will produce is staggering. Less than 0.1% of the data generated by the 40 million collisions per second will be saved and analyzed, the rest will be automatically discarded. But that already amounts to about 300 MB of data per second, or 15 million gigabytes per year, that need to be stored (the ATLAS project alone will have something like 2500 computers) and analyzed to search for the rare signal in the loud noise: tiny anomalies that would indicate the Higgs.

For this, the CERN has been working on developing a global network of computers, linked by high-speed connections, called "grid" - the LHC Grid. Grid computing in its simplest form amounts to splitting the data into chunks and send them out to different computers to be analyzed - a sort of distributed data-mining system - while a meta-software keeps track of it all (see for example the SETI@Home project). With the LHC however, the issue is really the unprecedented quantity of data. Here is a screenshot from the European Grid monitoring tool showing where those linked computers currently are:



This also shows to what extent the LHC project is managed in the open. Detailed technical information is available online, including the system's design specs, a dashboard keeping track of the construction progress, presentations, hundreds of pictures, a news bulletin and much more. You can also keep an eye on the construction through the underground webcams.