Large Hadron Collider? But I just met her!
Ashton Mills
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Nov 19, 2008 2:55 PM
The Large Hadron Collider is the most expensive experiment in history of our planet. What does it aim to find, and what could this mean for our future?
Particle accelerators are nothing new: we’ve been playing with accelerators since 1929, and even your old CRT monitor is a form of particle accelerator. But atom smashers, as they are sometimes called, are designed to accelerate particles to incredible speeds for the sole purpose of colliding them together – just to see what happens.
That’s what the Large Hadron Collider (LHC) is all about, except it happens to be the biggest and most powerful particle accelerator ever built.
Inside the LHC
Started in 2001, the Large Hadron Collider is a joint project by CERN, Europe’s leading nuclear research organisation. The acronym comes from the French translation, Organisation Européenne pour la Recherche Nucléaire. To give you an idea just how important CERN is: the web (as distinct from the internet), including the world’s first web server, began at CERN.
The LHC itself occupies a 27km tunnel, based 100M underground and crossing both French and Swiss borders, that used to house the LEP (large electron positron collider) built in the 1980s, and in operation from 1989 to 2000. The LHC was built to replace it, and it comes at a cost of some six billion euros – at least so far, as running costs add to the construction costs – making it the most expensive experiment in history.
And with it the promise to reveal answers to some of physics’ most mysterious questions.
One of the experiments that will be carried out includes looking for elusive dark matter and dark energy. Physicists can see the effects of dark matter and dark energy through the gravitational forces they exert, but haven’t been able to actually detect them. It’s an important investigation – all matter, from that bit of crud under your fingernail to the distant suns millions of light years away, comprises only about four per cent of the universe. The rest, it is theorised, is dark matter and dark energy – but where is it? This is one question the LHC should shed light on.
Then there’s the question of anti-matter. Physicists believe that at the time of the Big Bang equal portions of matter and anti-matter existed and, even if large portions of matter and anti-matter cancelled each other out, there was clearly enough matter left in the universe to create the universe we have today – but we can’t find the remaining anti-matter that should also exist. The LHC may be able to answer the question of where it went, or where it is now.
But perhaps the most important discovery waiting to happen however is the more publicised one – the Higgs-boson particle. While the other mysteries are (clearly!) well worth exploring, the Higgs-boson is a problem right in front of our eyes, quite literally: what gives matter its mass? We know that matter has mass, but we don’t know why, and whatever it is, it’s holding the entire universe together. The Higgs-boson is theorised to be the key, and the LHC can find it.
First hypothesised in 1964, the Higgs-boson particle is a neat theory for a piece of the puzzle we don’t yet have.
The accepted ‘Standard Model’ of physics (for which we are well short of space to go into here) dictates that universe is made up of twelve fundamental particles governed by four fundamental forces. As a model, it’s become a standard because it’s successfully explained not only the results of many experiments over the years, but also successfully predicted a wide variety of phenomena that were later discovered.
So it works pretty well for us.
However it’s not complete: of the four forces it only successfully describes the strong force, the weak force, and the electromagnetic force through their corresponding force carrier particles that have been discovered (particles that transfer force energy between matter, and belong to a group of particles called ‘bosons’).
Gravity, though we know it exists, hasn’t yet been explained by a corresponding force particle. The Standard Model theorises it should exist, but it hasn’t yet been found.
So there’s still more work to be done.
Another missing component, which much of the model depends on, is the Higgs-boson. It’s just a theory, but one that satisfies an inexplicable loophole. It runs something like this:
The Standard Model states that electricity, magnetism, light and some types of radioactivity are all manifestations of a single underlying force called the ‘electroweak force’ – one of the four fundamental forces. Mathematically for this to be true the theory for force particles requires they should have no mass, but this has already been disproven. Which means although the Standard Model seems to accurately describe what we see in the universe, it’s also missing something.
One of numerous physicists working on it, a physicist called Peter Higgs, proposed a theory by which matter gets its mass and which later became known as the Higgs mechanism. The Higgs mechanism defines a gigantic field created in the Big Bang that spreads across all time and space called (not surprisingly) the Higgs field, indistinguishable from empty space, and through which all matter interacts. And it’s the Higgs field that gives matter its mass, with particles that interact with it as they move through it gaining more mass than those that don’t.
A good analogy here is to imagine an object moving through a viscous substance like honey – the honey slows it down, and the bigger the object the more honey it will interact with and the more it will be slowed. The theory goes that some particles, like photons in light, aren’t being affected much at all by the field while others, like the particles that form the building blocks of the atoms in your body, are. And the magic ingredient that endows mass from the Higgs field to the interacting particle is the Higgs-boson.
It may sound like a convinient theory, but it’s the best we have – physicists can’t currently explain, for example, why one particle has a different mass to another, or even why particles have mass at all. There are many characteristics we know about particles, like charge or spin, but none of these dictate or create mass. Said another way: as far as our understanding of particle physics goes, mass doesn’t exist. But you only have to hold your hand in front of your face to see it does – so how does mass get there?
If the Higgs-boson particle exists, the LHC should be able to find it. If it does, it will complete a great void in our understanding of the universe and open up a whole new school of physics. If it doesn’t that’s good too – it means the Higgs field theory is wrong and we can go back to the drawing board and see what else we can come up with. Either way is progress, and why the LHC is so fundamental to the understanding of physics today.
To run the experiments on the LHC two separate but adjacent pipes run the 27km course, which combine at only four intersection points. Each pipe contains a proton beam, running in opposite directions to each other around the ring. It’s at the intersection points that the beams are guided to collide, and where the various detectors are placed to measure the results.
In order to contain and guide the beams tremendous magnetic forces are used – in all there are 1,232 superconducting dipole magnets, each 15 meters long, to bend the beams around the ring, while another 392 quadruple superconducting magnets, each 5-7 meters long, focus the beams. Many of these magnets weigh over 27 tonnes, and in order to operate at maximum efficiency are cooled using liquid helium to -271 degrees ceclius (absolute zero).
Before collisions take place the beams are accelerated to just shy of the speed of light (some 99.999 per cent the speed of light), at which time the proton beams lap the 27km circuit more than 11,000 times per second (!). In order to facilitate timed collisions, the beams aren’t continuous but instead are produced in ‘bunches’, which are estimated to contain some 280 trillion protons each, all running at near the speed of light, squeezed into a width thinner than a human hair. The energy contained within the beams is so potent that stray particles could destroy the superconducting magnets, and stopping the beams is a science unto itself.
When beams are crossed and collide at the intersection points, a number of detectors measure the results. The four key detectors include ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid), which are designed to look for evidence of the Higgs-boson, dark matter, and even extra dimensions; ALICE (A Large Ion Collider Experiment) which will look for evidence of quark-gluon plasma, which is supposed to exist in the first moments of the Big Bang and could help reveal how matter was made; and the LHCb (Large Hadron Collider beauty – yes the acronyms don’t mean much!) which will analyse the interaction between matter and anti-matter in an attempt to determine what happened to the universe’s missing anti-matter.
Then there’s TOTEM (TOTal Elastic and diffractive cross section Measurement) and LHCf (Large Hadron Collider forward) – the results from which are designed to complement the other detectors, and shed light on the nature of cosmic rays (which, even before the first beams were sent around the LHC, could be seen being picked up by the detectors – rays passing through planet Earth, as it were). Of all the detectors the CMS is the biggest, weighing 12,500 tonnes.
The tremendous energy contained in the beams is so strong that stray particles could melt the superconducting magnets and damage or destroy the LHC. If enough stray particles hit the same magnet in succession, its operating temperature could raise from -271 degrees Celsius to 700 degrees in less than a second, causing a chain reaction as beams become unconstrained. As a result the ring is constantly monitored and as soon as temperature fluctuations occur a ‘quench’ is ordered which, in effect, causes the beams to be stopped and power to the affected magnets immediately cut. Then, to protect them, powerful heaters kick in and heat the 15 meter long magnets to 300 degrees Celsius in the space of two minutes.
Beams have a maximum cycle life of about ten hours in the ring, and stopping them at the next cycle or if a quench occurs is another matter entirely – the energy contained in a single beam could melt through 40 meters of copper in less than a second. If this dissipation of energy occurred anywhere inside the ring, it would destroy whatever it penetrated.
So, in a process known as ‘dumping’, beams are directed through exit segments in the ring by ‘kicker magnets’ that propel them into special dump blocks designed to absorb the energy. After first kicking them, the beams are then ‘diluted’ by a series of ten special magnets that scatter the beam and reduce its intensity by some 100,000 times. At this stage they’ll still bore a hole in most any substance, and so another set of magnets directs the diluted beam in a scanned pattern (similar to the way a CRT monitor is scanned) to dissipate heat over the surface area of what’s known as a dump block – a rather large eight meter long and one meter in diameter block of graphite composite, all secured within 1000 tonnes of concrete on all sides. Dumping takes just 80 millionths of a second, and heats the graphite to around 750 degrees Celsius in the process but does not melt it.
What have we found?
At time of writing the LHC had just gone online with prelimary tests and, unfortunately, an electrical fault between two magnets caused a shutdown that’s going to take a few months to repair.
But even then, answers may not come quickly. If the Standard Model is correct, it’s estimated a Higgs-boson may be produced every few hours, but even at this rate may take up to three years to collect enough statistics to confirm one way or the other its existence.
To learn more about the LHC, the various detectors (each of which has its own website), and the physics being explored a good place to start is CERN's Large Hadron Collider homepage.