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The LHC: worth the wait?

Professor Jonathan Butterworth Professor Jonathan Butterworth

I have never seen so many physicists in the media as there were in September 2008. There we were, often nervous, always excited, trying to explain what the Large Hadron Collider would do (teach us more about the universe) and what it wouldn't do (destroy the universe). One particularly bizarre memory is of retiring to a pub in Westminster, finally exhausted by the LHC event I was helping with, and continuing to get updates ON MY OWN EXPERIMENT from the BBC news ticker on the TV in the corner. Beams have gone both ways round the LHC... Beams successfully stored... Ah, those were the days! It doesn't get much better than this.

Sadly of course, it didn't. It got a lot worse.

The truth is, while you may have thought we were nervous and excited about being on Breakfast TV, meeting the Minister, blinking in the glare of unaccustomed publicity, we were really nervous and excited about the LHC. No one had done this before. The machine, as Lynn Evans the project leader constantly and correctly reminded everyone, is its own prototype. It pushes the boundaries of technology quite as aggressively as it pushes the boundaries of fundamental physics. The desperately disappointing failure nine days after the glorious startup could have happened on day one, while I was on stage shaking John Denham's hand. It really was a huge relief and triumph when beams were successfully circulated and stored, and it was by no means a guaranteed one. When you do something really new, the unpredictable is always possible.

So the "Big Bang Machine" made the wrong sort of bang. But now, here we are again, excitement is again mounting, beam is imminent and a big physics experiment is once again firing the popular imagination, even to the point that whacky time-travel theories and birds with baguettes are worldwide news. A good point to look at what went wrong, what did we do during the past year, and what effect has this had on the science programme of the LHC.

First of all, what went wrong?

The LHC has hundreds of superconducting magnets, which keep the beam curving in a nice arc. As Newton first noticed, particles continue in a straight line unless acted on by some force. And the faster they are going, the harder it is to bend them out of a straight line and into a circle, even a circle as gently curved as the 27km circumference of the LHC. Since the protons in the LHC will be the highest energy particles ever contained in a lab, the magnets needed to bend them are stupendous. They produce their huge magnetic force-field by virtue of massive electrical currents, and in order to keep these currents flowing they are "superconducting", meaning the resistance to the current is zero. Unfortunately, to keep them superconducting we have to keep them VERY cold - hence the famous tag that the LHC is the coolest place in the universe. They are kept cold bymasses of liquid helium, in what is the single biggest cryogenic facility ever built.

If some small resistance to the electrical current develops in a superconductor, a bad chain reaction can happen. The current heats the resistor, this warms the material, it stops being a superconductor, the resistance gets worse and suddenly you have a lot of electrical energy looking for somewhere to go very quickly. This is known as a "quench", and like other high powered superconducting systems, the LHC magnets have sophisticated quench-protection mechanisms to get rid of the energy in a controlled fashion without doing any damage. Sadly, some of the joints between the magnets didn't. One of these developed a resistance to the huge current and was instantly vaporised. This in itself would have been an annoyance and caused a relatively short delay. However, an electrical arc formed over the gap left behind, and this punctured the helium containment vessel. Suddenly about a ton of pressurised liquid Helium was neither pressurised nor liquid anymore. The explosive pressure wave caused by the expansion from liquid to gas crushed the huge magnets against each other, ripped them out of the concrete floor, and consigned the LHC to a year of laborious repair work. The protection and monitoring systems which allowed the accident to become so serious also clearly needed to be overhauled and improved.

A very busy year for the accelerator experts at CERN. But many physicists, including me, don't work on the accelerator itself, we work on the detectors, the massive digital cameras which will track the particles produced by the proton collisions in the LHC. These detectors were ready & working for the first beam. What did we do with an extra, unwanted year of waiting?

Well, we did have some data. The beams had not collided, but they had deposited some particles in the detectors, initially from collisions with stoppers in the beam-pipe, and then with the collimators at the edge of the beam or residual atoms in the almost-perfect vacuum. Also, nature's astrophysical accelerators provide us with a source of particles passing through the detectors, coming from the cosmic rays which continually impact the upper atmosphere (and which, not incidentally, would have destroyed the Earth long ago if such a thing were within the capability of the LHC). All these data have been used to understand how our detectors perform, to align them correctly so they make the most accurate measurements they can, to tune up and improve our software and so on.

There were a number of other things which needed doing - things we'd postponed because of the imminent beam but which now we could spend some time on getting right. Examples close to my work included migrating all the reconstruction computers to a new distribution of the linux operating system and moving all the code to a more robust and modern management system. There were dozens of similar improvements in the detectors, electronics and software.

One improvement I am particularly pleased with is that we started using new "jet finders". Jets are the sprays of particles you get when a collision knocks a quark or a gluon out of the inside of the proton. Jet finders are algorithms which allow us to relate the particles we see back to the fundamental objects - quarks, gluons, maybe even the Higgs - which have a fleeting existence just after the collision. These algorithms had been improved a lot since the LHC was designed, due to theoretical work and data from previous experiments at CERN, Fermilab in Chicago (which still has the world's highest energy beams until the LHC gets going) and at DESY in Hamburg, where I did my PhD research. Along with a student of mine and some other colleagues, I also published and followed up a neat new way of finding the Higgs using these jets.

So, not exactly an idle year. But how have the problems affected the ambitions of the LHC programme? What of this Higgs thingy then?

There is an energy scale in nature which we know about. Above this energy scale, the weak nuclear force looks very much like the electromagnetic force. Below this energy scale, where we live now, the two forces are very very different. The weak force drives the sun and is only manifest on Earth in relatively rare radioactivity - rare at least compared to light, radio waves, microwaves and the rest of electromagnetism. The LHC will still be the first machine to do physics above this energy scale. This is a whole new landscape of physics, one which played a crucial role in the Big Bang. The LHC will still give us access to this landscape, although we spent an extra year at the border waiting for a visa.Plus, since the LHC will start more cautiously this time, our trek into the landscape will be somewhat slower and it will take a little longer for the fog to clear. But over the next few years we will nevertheless map out the features and learn what surprises nature has in store, be they mini-black holes, extra dimensions, supersymmetry, the Higgs, or an "unknown unknown".

The Higgs is the thing which breaks the symmetry between the weak and electromagnetic forces according to the "Standard Model" of physics. It does this by giving mass to the force carriers, and indeed to all fundamental particles. The LHC will either find the Higgs particle or prove that the Standard Model is wrong. Somewhere in that landscape is the clue to how these forces are unified, be it the Higgs or something else. And it's a big landscape, there may be much more out there. Bruised, somewhat chastened, a bit slower than we might have hoped, the LHC is once again poised to take us there. It will have been worth the wait.

Prof. Jonathan Butterworth,
Physics and Astronomy Department,
ATLAS, CERN,
University College London.



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