Science Spin November 2008

The Large Hadron Collider, and the God (damn) particle

Imagine trying to figure out how to bake a cake by slamming two together at high speed and then analysing the crumbs? Yet, physicists are doing something just like this at CERN's LHC(Large Hadron Collider) in a bid to understand the structure of matter. Smashing protons together at almost the speed of light, they hope to confirm their current theories about matter and mass, and perhaps make new, even unexpected discoveries.

The LHC is the world's newest and most powerful particle accelerator. It sits in a circular tunnel under the French-Swiss border, is 27 km in circumference, took eight years to build -- fortunately, they could re-use a tunnel from a previous accelerator -- and, all told, cost nearly €7.5 billion. Switched on in a blaze of publicity in September, it was switched off a few days later when an electrical fault caused a helium leak. Glitches like this are inevitable, given the experiment's complexity - it is, arguably, the biggest and most expensive scientific experiment ever - but it could be Spring 2009 before the machine is back in action. (It takes weeks to warm the super-cooled magnets from their chilly operating temperature of -271.3°C ( 1.9° above absolute zero), before repairs can begin, and several weeks more to freeze the magnets again.)

When fully operational, the LHC will take science into new and unexplored realms. It is nearly 10 times more powerful than the current record holder, Fermilab's Tevatron accelerator in Chicago, where the top quark was discovered in 1995. Colliding particles at higher energies brings physics into terra incognita, akin to exploring the far side of the Moon . . . and we usually find interesting and unexpected things in new and unexplored territories.

So why did 20 European countries spend so much money building this high-tech facility? Why are thousands of scientists around the world waiting for the data to start pouring in? And just what are they looking for amid the 'crumbs' of subatomic collisions? After all, they must have some idea of what to look for, not least so that they could design and build this experiment. Theoreticians play a vital role here, but in truth the two go hand in hand: without a theory there is no context, without experiments there are no data. Science is not random, and fortune favours the prepared mind.

Missing piece of jigsaw

Over the last 50 years, physicists carefully pieced together a detailed picture of matter at the subatomic level, called the "Standard Model" of particle physics. It's been very successful - predicting the existence of the top quark, for instance -- and agrees with all the experimental data to date. The model is like a jigsaw, and crucially it is built on the principle of symmetry. Physicists have exploited this symmetry, together with experimental data, to figure out how the jigsaw fits together and even to predict the existence of new jigsaw 'pieces' or new particles.

But, a piece of the jigsaw is missing. The theory, and the symmetry, both predict the existence of something called the Higgs particle. Except, despite repeated experiments, no one has yet seen the 'Higgs' in any particle collision. So scientists designed and built the LHC to look for it, with more energetic collisions than ever before.

The Higgs particle is so elusive that Nobel physicist Leon Lederman called it "the goddamn particle", but it is now popularly (and perhaps unfortunately) called "the God particle". It was first suggested in 1964 by Peter Higgs, a professor of physics at the University of Edinburgh. Higgs was building on work by Yoichiro Nambu, who shared this year's Nobel physics prize for his work on 'broken symmetry'.

But, what is the Higgs particle? To answer that, we need to think about fields and particles. And in quantum theory, the two go hand in hand: a particle accompanies every field, and a field accompanies every particle. Think of the photon, or particle of light, associated with the electric and magnetic fields. The Standard Model actually has a veritable zoo of 'fundamental' particles and associated fields: the electron, various quarks, neutrino and photon, to name a few (protons are not fundamental, being composed of three quarks).

No particles (a vacuum) usually means there is no field, but the Higgs field is different: it is everywhere, we believe, even in a vacuum, even when there are no particles; it permeates inter-galactic space, surrounds you as you read this article, is in the centre of the Sun, and yet has never been detected directly. Although we cannot see it or feel it or smell it, we believe it is there and, if it is, the LHC should detect its associated Higgs particle.

One reason for believing the field exists, is that we think we see its indirect effects on electrons and quarks: the Higgs field hinders these fundamental particles as they move through the vacuum . . . it slows them, and they cannot move as fast as they would if the Higgs field were not there. Or, as the Standard Model would put it: the Higgs field gives these particles their mass, and they must travel at less than the speed of light.

In contrast, because the Higgs field is electrically neutral, it is transparent to photons: these pass effortlessly through at the speed of light, and so have no mass. The Higgs field is not the source of all mass though, only the mass of fundamental particles such as electrons or quarks. (Protons and neutrons are composite particles and their mass arises from their composite nature.)

So why have we never seen a Higgs particle? Especially as the Higgs field is all around us, and physicists have been smashing particles together for nigh on 50 years? The answer lies in the particle's weight and life expectancy. The Higgs is, we predict, very unstable, living for a tiny fraction of a second before decaying into other, more familiar particles. You'd have to be very lucky to see one naturally, and ready with an extremely sensitive 'camera' or detector to capture the event.

Our best hope is to 'make' and detect them in a particle accelerator, by smashing protons together at almost the speed of light. Because Higgs particles are heavy, we haven't been able to produce them in a particle accelerator until now: it takes an awful lot of energy to produce a heavy particle in the fireball explosion that happens when particles collide (the energy, E, being determined by Einstein's famous equation E=mc2). The LHC collisions will hopefully be powerful enough to produce Higgs particles, albeit fleetingly, though it could take several years of sifting through data to find and confirm this.

Black holes and big bangs

There is more to the LHC than making and photographing Higgs particles, however, and physicists have a host of other predictions -- or 'suggestions' -- of things to look for with this powerful new accelerator. So, even if the Higgs particle is found quickly there will still be plenty to do. Some suggestions come from string theory and super-symmetry (SUSY), and include supersymmetric particles (heavier partners of every particle already seen in the standard model plus some more), and even mini-black holes. The latter led to concerns that the LHC could create a black-hole, that might swallow the planet. But rest easy -- cosmic rays from outside the Solar System have been raining down on us for billions of years, often with energies much greater than the LHC will ever achieve, and we are all still here.

And, what about recreating the Big Bang? When high-speed protons collide in the LHC, the resulting fireball could reach temperatures 100,000 times hotter than the centre of the Sun, about a trillion degrees centigrade. The last time temperatures like this were seen in our Universe was a few microseconds after the Big Bang.

This picturesque language is rather misleading, however. The LHC 'fireballs' last for such a short time and disperses so quickly that we couldn't say that it has a definite temperature. The LHC will however also collide lead nuclei, producing larger fireballs that are a soup of quarks and gluons. This also lasts for such a short time that it may never reach thermal equilibrium, and again we probably cannot associate any definite temperature with it. Conditions in the quark-gluon soup or 'plasma', while involving energies similar to those present 100 microseconds after the Big Bang, are unlikely to be identical. Nevertheless, the LHC is designed to probe this unusual state of matter and hopefully shed light on the physics of the very early Universe.

Irish physicists are involved in another LHC experiment, the LHCb detector, which is exploring why there is more matter than anti-matter in our Universe. This experiment will probe the phenomenon of 'time reversal asymmetry', for which this year's Nobel physics prize was awarded. Experimentalists at UCD, led by Dr Ronan McNulty, are involved in developing this detector.

Dark matter mystery

The elusive Higgs particle is not the only piece missing in physics. There are also the twin mysteries of 'dark matter' and 'dark energy'. Evidence for these comes from astrophysics: by watching how peripheral stars orbit around galactic centres, and how galaxies waltz around one another, we can infer that there is more stuff out there than just the protons, neutrons and electrons described by the Standard Model. Because we cannot see this unknown stuff, it is sometimes called "dark matter" (though "mystery matter" would be a better phrase, since a lot of ordinary matter, protons and neutrons, is also dark). We know only that mystery matter is electrically neutral and seems to be almost inert to all the other known forces, except gravity.

Only about 4 per cent of the energy in the observable Universe is due to ordinary matter (protons and neutrons), about 23 per cent is 'mystery matter', and the remaining 73 per cent is something else again, that is often called 'dark energy', to distinguish it from mystery matter. Dark energy exerts a force that counteracts gravity, and is accelerating the rate at which the Universe expands.

So, despite the phenomenal success of the standard model, we still do not know what 96 per cent of the Universe is made of! It's not unlike the state of the physical sciences at the end of the 19th century: gravity, electricity and magnetism were the only known forces then, and Newton's laws, together with Maxwell's equations (combining the dynamics of the electric and magnetic forces into the unified framework of electromagnetism), successfully explained a huge range of phenomena. Many people felt that Nature held no more secrets, it was only a case of calculating the next decimal place to get more accurate answers.

Then Wilhelm Röntgen discovered X-rays in 1895, and a year later Henri Becquerel discovered radioactivity. There followed a virtual revolution in physics: Einstein's theories of relativity, quantum theory in the 1920s, quantum electrodynamics in the 1940s and the standard model of particle physics in the 1960s. The standard model has proved extremely successful to date. The Higgs is the only experimentally missing piece, and the only theoretical problem is to include gravity.

Except, now there is the problem of mystery matter and dark energy. Perhaps these will lead to another major revolution, as their nature, origin and properties are unravelled? And the LHC may prove to be be the first step in this revolution.

CERN

The European Organization for Nuclear Research (CERN is a French acronym) is the world's leading laboratory for particle physics. Founded in the 1950s, there are 20 member states, and India, Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have observer status. Ireland is one of a handful of smaller European countries that is not a member state.

The World Wide Web, was invented at CERN to enable scientists around the world to share documents and files. Now, CERN staff and researchers worldwide are developing the next generation web, called The Grid, to handle the vast volumes of data that the LHC experiments will generate. Use the Web to learn more about CERN and the LHC, by visiting clicking here