Rolf-Dieter Heuer

Rolf-Dieter Heuer

Former Director-General of CERN, the European Organization for Nuclear Research, Geneva, Switzerland

Particle physics doesn’t just expand our horizons – it helps to explain the universe by examining the nature of matter, both seen and unseen. Rolf-Dieter Heuer (1948) works with the Large Hadron Colider (LHC) at CERN, which is poised to provide experimental evidence either confirming or refuting the existence of the Higgs boson, an elementary particle predicted to exist by the Standard Model of particle physics. The Higgs boson is a turning point - its discovery would complete the Standard Model, one of the crowning achievements of 20th century science. But the Standard Model deals only with the ordinary matter that makes up the visible universe. The LHC is also poised to break the wall of the hidden universe - the 96% of it that takes the form of dark matter and energy.   In Heuer’s career as a physicist, he has worked on the construction and operation of several large particle detector systems, among them JADE at the German Electron Synchrotron in Hamburg, Germany. In early 2009, he took up the position of Director-General of CERN.

Breaking the Wall of the Hidden Universe. How Particle Physics Can Explain the Nature of Matter.


On November 9th I was overwhelmed.

Twenty years ago: I am not sure where I really was on the 9th of November, but I know I was somewhere in France; I would guess hundred metres below ground, enabling an experiment, which started at that time. So, I could only watch the fall of the Wall from the far, but it was nonetheless exciting and very nice- especially nice for a German.

Today we are breaking the wall of the hidden Universe- at CERN- not in Berlin. Before I tell you about that, let me spend one minute on CERN to rectify a little bit the view of CERN: it is not ancient sentiments; it is not like the Big Bang here; it is about the Big Bang. But it is also about people and about breaking walls. Let me remind you that CERN was founded 55 years ago out of a movement, which was called “Atoms for Peace”. I think CERN is meanwhile a model organization, which really brings together people. We are really breaking the walls between cultures and nations since 1954.

This slide shows you, and only look at the colours, the distribution of all CERN users by nationality: Member States, Observer States, and other States. You see we have around 10,000 scientific users representing 97 nationalities. So, we have more than half of the United Nations on our campus.

That is one thing. We were breaking another wall- twenty years ago. Who of you is not using the Internet? One. Well, ok, such people exist. Anyhow, it is 20 years ago that the World Wide Web was born at CERN, because we needed to have a reliable platform for communication and information exchange. We were also breaking the wall of communication.

Let us now break the wall to the hidden Universe. This slide shows the evolution of the Universe from the Big Bang to today. The Universe expanded over roughly 14 billion years from essentially point-like to now 1028 centimetres. Don’t ask me what 1028 centimetres mean; I cannot imagine. It is just a number, but that is today the size of the Universe. This slide indicates again the development from the Big Bang to today: 10-32, point-like, to today 1028. The development of the Universe: 60 orders of magnitude. It is very difficult to imagine. During its time of existence, it cooled down to 2.7 Kelvin today in the microwave background.

How to learn about the Universe? We have astronomy and astrophysics with space-based telescopes or ground-based telescopes, which look into the Universe. By looking into the Universe, you can see the history of the Universe and the development of the Universe. Particle Physics is using powerful accelerators and studies the physics laws of the microcosm and at the same time of the early Universe. You see we are coming pretty, pretty close- four orders of magnitude below the proton diameter. We are coming rather close to the Big Bang.

In the past decades we have got a detailed understanding of the microcosm and of the visible Universe. We have developed the Standard Model of Particle Physics, butwith a missing cornerstone, the Higgs-Boson which is supposed to answer a key question: what gives mass to particles? Because this Standard Model, this mathematical formalism works only for massless particles; therefore, Mr Higgs has introduced a very nice additional formalism. We have to find this Boson, this Higgs-Boson, to get him his Nobel Prize.
But the Standard Model has a big, big other problem. It only explains roughly 5% of the matter and energy density of the Universe. 95% of the Universe are “dark”. One quarter is dark matter, which clumps like normal matter, but interacts very little with normal matter, and three quarters are dark energy, which drives the Universe apart. We don’t know what dark matter is, and we even know much, much less about dark energy.

But we know that there is dark matter, because of this picture from the Hubble Space Telescope: The Galaxy Cluster Abell 370. You see sometimes very strange patterns here, and if I zoom in you see there are multiple images of the same distant galaxies. Here are two, here are even three, that look a snake. This is gravitational lensing through normal and through dark matter.

The question is: what is this dark matter? Well, astronomy and astrophysics over the next two decades will use more powerful new telescopes, and they will tell us how dark matter has shaped, together with normal matter, the stars and the galaxies that we see in the night sky. But only particle accelerators can produce this dark matter in the laboratory and can understand exactly what it is. Is it composed only of one kind of particle, or is it more rich and more varied as is the visible world? We don’t know.

The favourite candidate for dark matter is supersymmetry. For each particle that exists, a supersymmetric partner of opposite spin statistics is introduced. So we double the number of particles. Now you will say, “If the physicists don’t know what to do, then they double the number of particles, they double the number of parameters and that explains everything.” But don’t forget: around 80 years ago, Paul Dirac was brave enough to introduce the concept of antimatter. With the concept of antimatter, he doubled the number of particles. We gained a tremendous amount of knowledge, and today we even use it in hospitals with the Positron Emission Tomography. Ok, so far, so much to doubling the particles.

The lightest supersymmetric particle is stable in most models, and therefore a fantastic candidate for dark matter. The Large Hadron Collider (LHC) covers a large range of masses of such hypothetical particles. It is maybe, therefore, the perfect machine to study dark matter, and it is the ideal machine for finding the Higgs-Boson, because if it exists we will find it there.

But let’s concentrate on the dark matter. The Large Hadron Collider is a unique machine in the fantastic surroundings of Geneva, 100 metres underground. It gets us closest to the Big Bang, roughly 10-13 seconds. It breaks the wall of light, because with light telescopes you only get to 300,000 years close to the Big Bang; only then atoms could be formed, and light could get out of the Universe. It breaks the wall of neutrinos, because with neutrino telescopes you only come up to one second close to the Big Bang. We come to 10-13 seconds- very, very close.

How do we do that? Well, we take protons, hydrogen nuclei, run them one direction, and protons running in the other direction with up to 7 TeV on 7 TeV = 14 TeV. That will bring us so close to the Big Bang. The start of the Large Hadron Collider is, to my mind, one of the most exciting turning points in Particle Physics. As I just said, we explore a new energy frontier. Proton-proton collisions at a centre of mass energy of up to 14 TeV and up to 40 million times per second.

Who knows what 14 TeV means? Ok, 7 TeV on 7 TeV: this is the stored energy in the beam. It is like 120 elephants running onto 120 elephants at full escape speed. That is a huge energy. But it is not enough for the Big Bang. If one proton hits another proton, that is like a mosquito hitting a mosquito. How do I get to the Big Bang? A mosquito is hitting the mosquito on a tiny, tiny area: 10-16 centimetres, nearly point-like. It is the energy density that does it in that collision. That brings us close to the Big Bang, close to the early Universe in one of the collisions.

We have four big experiments at the four collision points. Two multi purpose experiments. One experiment, which is special, because it studies the question why are we here? After all, at the beginning of the Universe matter and antimatter were created in equal quantities. That means that we all should be energy. Well, that would be good seeing the first talks and next talks. But shortly after the Big Bang, there started a small asymmetry between matter and antimatter, one part in ten billion - and that is us. But how did that happen and why? It is this experiment in which we study that question.

The LHC is the most empty place in the solar system, because in order to run the particles through the LHC, a vacuum is needed, which is below the pressure on the surface of the moon. I think the pressure is roughly a factor of ten lower than on the moon. It is one of the coldest places in the Universe, because the magnets run at a temperature of 1.9 Kelvin above absolute zero. It is colder than outer space. It is -271 C cold.It is also one of the hottest places in the galaxy, because a collision of the two proton beams generates temperature densities of 1000 million times larger than those at the centre of the sun, but in a much more confined space.