Welcome to this tour of the permanent exhibit of CERN, called Microcosm. Its tells us the story of the past, the present and the immediate future of elementary particle physics. CERN is the most important research center in this field in Europe.
Some twenty countries participate in this project, and more than 7000 people work here at a very high level.
Right at the beginning, in the entrance hall, you already can see that the infinitely large and the infinitely small meet in this area: you can see an image of the Big Bang, which took place some 15 billion years ago. The unbelievably high concentration of energy from the beginning spread, as time passed, into an increasing volume and as a result, the temperature decreased. What happened in these first fractions of a second determined the eventual evolution of our entire universe.
If you look more carefully at this picture, you will see that pure radiation turned into matter;
Einstein indicates in this formula that this is really possible. But why this transformation of pure energy into matter happened, nobody knows.
Many kinds of particles appeared, of which we only heard of electrons, protons and neutrons in daily life. Why did the others disappear? Nobody knows!!
Nevertheless, as soon as these protons and electrons moved around slowly enough, they could form atoms, hydrogen atoms.Gravitation, which depends on mass, and therefore did not exist before matter appeared, caused attraction, so that these atoms began to collide, causing nuclear reactions, the origin of all the other elements we know now. But also larger structures appeared: stars, planets, milky ways and even clusters of milky ways.
When the temperature dropped to very low values, these atoms could even combine to form molecules, but we do not want to enter chemistry, nor biology here.
At the end of this introduction, let’s talk about energies and mention some numbers.
The unit that is used in this field is the eV, with a value of 1,6 10-19 J. You need 26 billion billion eV to warm up 1g of water by 1 degree centigrade. So this is a unit that is so small that it is only used in the world of atoms and particles, this is clear.
Watch this energy axis.
A molecule at room temperature has only an energy of 25 meV. To ionize an atom (separating an electron from the rest of the atom), you need a few eV. To split a nucleus, you need a few MeV, and to produce new particles with mass, you need close to a GeV.
As you can see, these energy densities also occurred in the early universe and the higher the energy value, the closer in time you approach the Big Bang. This is the energy range where CERN is active. They investigate the laws of physics as they seemed to have been, 15 billion years ago, and these laws were quiet different!!
It was only in 1932 that the neutron was experimentally discovered. Later in this decade, they discovered other particles, wandering around in our atmosphere. The higher you are in the atmosphere, the more of these particles you find.
It didn’t take too long before they came up with a theory that very fast protons, coming in from the deep outer space collided with molecules in the upper layers of the atmosphere, causing an avalanche of totally unknown, rapidly decaying particles.
Scientists were really fascinated by these charged particles, which even could cause sparks in these chambers.
They suggested building an experiment to investigate what was going on. This was the birth of CERN.
The first thing they came up with was a linear accelerator, like this one, rather primitive.
Protons, which already had an energy of 17 keV, given by this alien instrument, were accelerated to an energy of 2 MeV, during one passage.
They used several electric fields, to give them many pushes, as you can see in the copper inner side of the tube. The beam was then sent to a collision area by magnets, I’ll tell you later. From the first experiments they found more and more strange particles, some of which only lived for a very short time and disappeared again. Instead of getting a better view, these experiments complicated things, and the mystery became larger and larger.
The hunt for more, stranger, and yet shorter living particles was open. Higher energies were necessary. They built circular accelerators, at first small ones, like this one
later on bigger, and larger.
The proton energy increased gradually to 1 GeV.
The largest accelerator now is situated in a circular tunnel, 100 m below ground level, and with a circumference of 27 km. Electrons were originally accelerated up to energy levels of some 50 GeV. The problem is no longer the speed itself, this works out quite well, but these electrons must be kept in a circular path. A force, directed towards the middle of the circle is needed. This is why they need very powerful magnets. As you can see, magnets are capable of steering an electron beam within a television tube.
This centripetal force must increase as the speed and as the mass of the particles increases. To do this, the electromagnets need more electric current, which can heat the coils of the magnets and consume far too much energy. So, they changed the LEP set up and used superconducting magnets. This is LEP2. Now energy levels have raised up to more than 100 GeV.
LEP2 experiments will end by the end of October 2000. The set up will be changed again, now with protons to be accelerated. Since these protons have a mass, 2000 times bigger than electrons, the energy involved will be, in 2005, more than 20000 GeV. This again, will be a giant step forward. 2800 packets of protons will do some 11000 circuits per second in the tunnel, with a speed only 0.003 km/s below the velocity of light. This project is called LHC.
If the energy density is that high, masses as we know in daily life don’t exist.
So gravity didn’t exist either in the early universe. Nor did the other interactions ( electromagnetic and weak interactions, responsible for the decay of nuclei). Theoretical physicists think they were all unified in only one form, which split up into the 4 interactions that we now see, when the temperature in the universe got low enough.
The LHC will make it possible to determine certain aspects of this unified force.
The main goal of all the experiments is to bring about collisions. They try to find out how exactly how all those particles behave, how they interact, what are their properties and so on.
The way they used to measure these properties was with a bubble chamber. This is a huge cylinder, filled with liquid hydrogen. Charged particles leave a track of very small bubbles behind as they pass through the chamber. The experimentalists applied a magnetic field, so the tracks became curved.
The curvature gives information on speed and mass of the particles, the exact form of track patterns shows which particles are involved, at least if you are an experienced investigator
They used to investigate thousands of track pictures looking for all kinds of special events: maybe a new pattern of tracks, maybe small differences of properties, and so on
Nowadays, electronics have taken over. Take for example the wire chambers. Thousands of wires are carefully set up, and a high voltage applied. When a particle passes by, it causes small signals, picked up by sensors at the end of a few wires. From this information computers calculate the tracks. Charpak won the Nobel prize for the invention of this technique. Of course, the less these wires disturb the track of the particle the better.
Thanks to the very fine technique CERN came up with, this apparatus is now used all over the world to check bags in airports, cars, and so on. Even people can be scanned without any harm.
Electronic detectors nowadays detect tracks,
others measure energy
They also need different versions of these detectors to respond to all possible particles,
and furthermore there have to be detectors in every direction around the collision point.
This leads to constructions, higher than a 3-store building.
They need thousands of sensors, all connected to computers, which gather data. The LHC will produce some 40 million collisions per second, causing more than a billion events per second. This means 1 petabyte of data. Software programs select possibly interesting events, which will be only 1 out of a million or so; and investigators will throw away another 99% of these. What is left after that will be stored in memories: some 3000 terabyte of storage capacity is needed per year. Can you imagine what this challenge means for industry of chips, computers, software developing firms etc ? Within 5 years all this problems have to be solved, though these specifications overrule present knowledge by a factor five.
Once these data are gathered, they must be examined.
Scientific programming is required here. They have to order data, visualize them, make calculations.
Very powerful computers, yet to be built, must do the job. All these computers must be interconnected. They must be able to exchange data across the world. You know where this leads to… .
Yes indeed, you are right, the WWW was developed at CERN. Although they could have made a lot of money of this technique, the principle of CERN is to deliver all developed technology freely to the world. The WWW provides a good example of this principle.
But here at CERN, they are one step ahead, once more, with the GRID.
Wait another two years, you will get to know more of it then.
The last part of the exhibition is more theoretical. Interactions between particles take place by interchanging messengers. Photons play that role whenever electromagnetic interactions are involved, gravitons, not yet discovered, for gravitational interactions, and for the weak interactions they predicted that W and Z so called bosons would do the job.
Well, Carlo Rubbia and his team, discovered these particles with LEP, after experimenting for only 16 minutes. Can you imagine how precise the theoreticians had predicted the properties of W and Z?
The beams of electrons had to be so narrow that they can be compared with two needles, shot to one another from a distance of 10 km, hitting each other head to head.
To align these beams so accurately, S. Van der Meer came up with an idea of kicking all particles drifting out of the line, back to their places, the so-called stochastic cooling process. He also got the Nobel Prize for this idea.
The fourth interaction we are dealing with is the strong interaction. To explore this, we have to dig even further, into the protons and neutrons. They discovered even smaller, almost point shaped masses, quarks, 3 to be exact, within these nucleons. The messengers between interactions between quarks are called gluons. To detect those, energies, more than a million times more than the LHC can produce will be necessary.
But, LHC might be able to produce a new state of matter: the gluon-quark plasma.
Do you remember that a few instants after the Big Bang occurred, the first particles with mass appeared? And that we can ask why radiation turned into mass?
Some theories predict that another weird particle could be responsible for this change: the Higgs boson. But, unfortunately, LEP experiments might not be able to detect them: they are short by only a few GeV, if theoretical calculations are right. But if they would be found, we finally would know why we are made of mass, and not of pure radiation energy.
Does the story stop here?
No! There is much more to explore there, many more pictures and interactive programs on computers. So if you would pass by in the neighborhood of Geneva, don’t hesitate to visit Microcosm, it’s free anyway!