Page last updated at 02:22 GMT, Saturday, 21 November 2009

'A long wait', but worth it'

Atlas (Cern/C. Marcelloni)
The Atlas detector will join the hunt for the Higgs boson particle

Particle physicist Professor Jonathan Butterworth describes what it is like to work on the Large Hadron Collider, one of the best known scientific projects in the world.

He explains the trials of waiting for the start-up and what happened to the machine following its switch-on in 2008.

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 -otherwise known as the LHC - 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.

It doesn't get much better than this - sadly, it got a lot worse.

The truth is, while you may have thought we were nervous and excited about being on Breakfast TV, meeting MP John Denham, blinking in the glare of unaccustomed publicity, we were really nervous and excited about the LHC.

The machine, as then project leader Lyn Evans 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.

What went wrong?

The desperately disappointing failure nine days after the glorious start-up could have happened on day one, while I was on stage shaking John Denham's hand.

So the "Big Bang machine" made the wrong sort of bang. But now, here we are again, a big physics experiment is once again firing the popular imagination, even to the point that wacky 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 that 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 extremely powerful.

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.

Under pressure

The LHC magnets have sophisticated quench-protection mechanisms to get rid of energy in a controlled fashion without doing any damage.

Sadly, some of the joints between the magnets didn't. In September 2008, 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" - or plasma discharge - formed over the gap left behind, and this punctured the helium containment vessel. Suddenly, about a tonne 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's engineers to a year of laborious repair work.

Infographic (BBC)
1 - 14 quadrupole magnets replaced
2 - 39 dipole magnets replaced
3 - More than 200 electrical connections repaired
4 - Over 4km of beam pipe cleaned
5 - New restraining system installed for some magnets
6 - Hundreds of new helium ports being installed around machine
7 - Thousands of detectors added to early warning system

The protection and monitoring systems which allowed the accident to become so serious 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 and working for the first beam. What did we do with an extra 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 (which absorb "stray" particles that have spread out) 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.

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.

'Ambitious programme'

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 particles out of the inside of the proton.

Jet finders are mathematical "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.

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, it wasn't exactly an idle year.

But over the next few years we will learn what surprises nature has in store, be they mini-black holes, extra dimensions, a theory called "supersymmetry", the Higgs, or an "unknown unknown".

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.

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