Towards the mysteries of the Big Bang

ALICE in Plasma land

Dr David Evans, leader of the UK team on the ALICE experiment, bears witness to a modern-day marvel of science and engineering.

On 8 November 2010, after nearly 20 years of planning, design and construction, two beams of lead nuclei were circulating in the LHC tunnel at almost the speed of light, lapping the 27 km LHC accelerator over 11,000 times a second.

The control room of the ALICE (or 'A Large Ion Collider Experiment') detector was crammed with physicists, eagerly scanning the vast array of monitors and awaiting the first collisions. At 11:20am local time, the first lead nuclei smashed together in the heart of the 10,000-tonne ALICE detector, situated about 80m underground, and a great cheer came from the ALICE control room.

For my part, the cheer was not just due to the excitement of achieving success after all these years, but of relief too. We built a crucial part of the electronics and there's always the worry that it won't work. Of course, it did, so hopefully the Science and Technology Facilities Council (STFC), who fund my team, won't be asking for their money back.

Seconds later, my attention was focused on the event display monitor, which showed thousands of particles streaming through the ALICE detector – the debris of the violent sub-atomic explosion caused by the collision of two lead nuclei. The event represented the massive advances in both physics and engineering made over the past few decades and opened a new chapter in physics.

Mini Big Bangs

These collisions created tiny sub-atomic fireballs, reaching the highest temperatures and densities ever achieved in an experiment and recreating the conditions that existed a millionth of a second after the Big Bang.

Temperatures of about ten trillion degrees, a million times hotter than the centre of the Sun, are achieved in these collisions, along with densities similar to a neutron star – imagine one of the great pyramids of Egypt crushed to the size of a pinhead.

Under these conditions protons and neutrons, which make up atomic nuclei, melt and a new state of matter, an exotic primordial soup of quarks and gluons, called a quark-gluon plasma, is formed. In essence, a tiny amount of the very early Universe is recreated.

We cannot study the quark-gluon plasma directly, as it only lasts for a fleeting moment before cooling and turning into thousands of particles. It is these particles that are detected in the ALICE detector and after studying millions of such collisions we can start to probe the secrets of the quark-gluon plasma.

The strongest force

By studying this quark-gluon plasma, we hope to learn a lot more about the 'Strong Force', the most powerful of the four known fundamental forces of nature. It is a thousand times stronger than the electromagnetic force and is responsible for holding protons and neutrons together in atomic nuclei. Protons and neutrons are not fundamental particles, but are themselves made up of particles called quarks.

Again, it is the strong force which binds these quarks inside protons and neutrons. Perhaps the most interesting aspect of the strong force is that it generates about 98 per cent of the mass of protons and neutrons (the other 2 per cent comes from the quarks themselves). That means that about 98 per cent of the mass all atomic matter, including the Earth and everything on it, comes from this force.

Early results

After intensive analysis, the first results have shown that the current theoretical models do not work. The most surprising result so far is that the quark-gluon plasma does not behave like a super-hot gas, as predicted, but more like a liquid.

However, this is not the kind of liquid that most people would be familiar with; this is a super-hot, super-dense mother of all liquids – quite literally, as this is what the early Universe would have resembled ten microseconds after the Big Bang when the first protons and neutrons were being born.

Pushing out the frontiers of physics also requires pushing current technologies to the limit or, in many cases, creating new technologies. This is true not just for the LHC accelerator but for the giant particle detectors too; put together, the LHC project represents one of the Worlds technological masterpieces.

Sometimes, however, problems can be solved with low-technology solutions. Such an example of innovation is the ALICE Time-of-Flight detector, which is designed to measure the time a particle takes to travel from the collision point to the detector.

The known technology, with the required timing precision, would have cost more than the budget for the entire ALICE experiment, so something new had to be developed. The result was a detector made from five layers of thin glass, with a 250 micron gap between the layers – the exact diameter of standard nylon fishing line, which is used as a spacer to keep the distance between these glass plates fixed.

Although this must surely be the only particle detector ever to use fishing line, this relatively low-tech, low-cost solution produced a new type of detector with an intrinsic timing resolution of about 50 picoseconds.

Last year was a great year for ALICE and something I have worked towards for the past ten years. The next ten years are going to be a lot more exciting, with many new discoveries and quite a few surprises along the way – it's a great time to be studying high-energy physics.

Hot stuff

Phil Schewe, chief science writer at the American Institute of Physics, takes a historical view, and argues that the Quark Age begins at four trillion degrees.

High temperature and high civilisation go together. The Bronze Age takes its name from a sequence of technological developments made possible by the alloying of tin and copper. Alloying requires high heat, first to separate (smelt) metal from its rocky ore and second to melt the crystalline solid into a liquid that can be cast into useful shapes. Later still, better furnaces and higher temperatures facilitated the production of molten iron, and along with it all the implements and structures associated with the Iron Age.

We now live in the Silicon Age, a time characterised by microchip farmsteads sown with billions of transistors that are bundled into a wide variety of electronic devices. The melting point of silicon, 1,411°C, is actually a little lower than that of iron, which points to the fact that technological progress, over the past half century or so anyway, has come not directly from the use of ever higher temperatures.

Nowadays we must do more than merely melt atoms. Instead, innovation comes from taking atoms apart or at least using the inner parts of atoms – their electrons or their nuclei – to carry out information processing or to unleash gigawatts of power from teaspoons of uranium.

A new age

We take computers for granted just as we earlier took steel bridges and bronze arrowheads for granted. What will we take for granted a century from now? What will be the next 'Age'? Progress will most likely involve a further journey into the heart of matter, into the heart of atoms.

Historically, the inward journey occurred in stages. First the ancient Greeks and Chinese speculated that the matter all around them consisted of combinations of a small number of elements. For the Greeks it was earth, water, air, and fire; for the Chinese it was earth, water, wood, metal, and fire. Through the efforts of alchemists, the number of elements was enlarged. Then in the 17th, 18th, and 19th centuries, with the coming of scientific chemistry, the idea of matter as made of combinations of elements in fixed amounts took hold.

In the 20th century, melting matter into elements wasn't the primary aim any more. Now the goal was to transmute one element into another or – with the application of more energy – to disintegrate the atom or even its nuclear core. With the use of particle accelerators, which smash high-speed beams of particles against other particles, new kinds of matter could be produced, new forces not seen before could be explored, and new arrangements of known particles could be sought.

Disintegrating the atom

One of the most important of these explorations into the inner realm of atoms is being carried out in two places. At the Brookhaven National Laboratory near New York City and at the CERN laboratory near Geneva, heavy atoms are being accelerated to very high energies and then collided head on, creating a miniature fireball a million times hotter than the sun, at least for a very brief moment, and a 100 times denser than ordinary nuclei.

At the moment of collision, one nucleus strikes another. Each nucleus contains hundreds of protons and neutrons, and each of these, in turn, consists of a tiny swarm of particles called quarks and another swarm of particles called gluons, which yoke the quarks together.

Just as the Bronze or Iron Ages could not begin before the advent of furnaces capable of reliably reaching high temperatures, so the melting of protons into quarks and gluons could not be achieved until the machines at Brookhaven and CERN were designed and built.

At Brookhaven the main machine is called the Relativistic Heavy Ion Collider, or RHIC. At CERN the machine is called the Large Hadron Collider, or LHC. Both machines passed significant milestones in 2010.

In February 2010, RHIC scientists announced the results of an experiment in which gold ions, gold atoms from which all electrons have been stripped (with a greater positive charge the ions are easier to accelerate), are crashed together. Not only did the two nuclei burst apart during the collision, but the protons themselves melted into their constituent quarks and gluons. Most of these particles sort themselves back into familiar particles like protons and electrons, which fly out from the scene of the fireball.

Before the sorting-out process can take place, though, the molten quark-gluon mixture exists for a short time as a unique nuclear fluid. This fluid is sometimes called a quark-gluon plasma in analogy with an electrical plasma – the clouds of particles in which electrons have been stripped from atoms, leaving not one cloud of neutral atoms but two clouds, one positively charged and one negatively charged.

Colour charge

The nuclear analogue of electrical charge is called 'colour charge', and normally protons and other nuclear particles (known collectively as 'hadrons') are 'colour neutral'.

However, in the heat of the fireball, when all hadrons are melted down into an even more basic collection of quarks and gluons, the quarks each carry a net colour charge. Consequently, the quark soup is considered a form of plasma.

The fireball only lasts for a few trillion-trillionths of a second, but this is long enough for surrounding detectors to gather plenty of information. For instance, the piercing beams of light (called gamma rays) emerging from the fireball indicate that the effective temperature of the fluid was four trillion degrees centigrade, the highest temperature ever definitely produced and measured in an experiment.

This was well above the point where, theorists said, protons should melt. For several years the RHIC scientists had suspected that they had reached the proton melting point, but couldn't prove it without the gamma ray evidence.

One of the biggest surprises was how the quarks behaved once they were liberated from inside protons. Most theorists had suspected that the quarks sprung from inside their protonic cocoons would act like a gas. Instead the quark swarm created in RHIC collisions actually looks more like a liquid, a liquid in which the quarks interact strongly with each other.

This is the hottest liquid on Earth. An even greater energy density can be achieved amid proton-antiproton collisions at Fermilab, near Chicago, and amid proton-proton collisions at LHC, performed recently with proton beams at an energy of 3.5 trillion electron volts. But when two protons (each containing three quarks) collide what you have is a fireball with six quarks.

If you include the gluons running among the quarks, you have an ensemble of perhaps a few tens of particles. Although impressive, this tiny cloud doesn't have enough moving parts to be considered a gas or a liquid. You can't study the collective flow of particles needed if you're going to say something thermodynamic about the quark fluid. The very concept of 'temperature' requires more than tens of particles.

Within the fireball

'I think it's safe to say that when you have 1,000 particles you can talk temperature,' says Barbara Jacak, a physicist and spokesperson for the PHENIX detector group at RHIC.

For the collisions at RHIC between two whole gold nuclei, each fireball consists of hundreds of quarks; including gluons, the particle count is around a thousand, sufficient for one to measure true liquid qualities. Scientists at RHIC hope to extend their experiment over the next few years in order to learn more about their quark liquid.

At Brookhaven the experiment continues. The scientists there are actually lowering the temperature of their fireball in an effort to see what their quark liquid looks like just before and just after melting, whereas at CERN scientists are moving in the opposite direction.