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In anticipation of the unexpected

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The "Large Hadron Collider" opens the door to a new world

by Hans-Christian Schultz-Coulon


For more than half a century, particle physicists have been researching "what holds the world together at its core" - with ever larger facilities, for ever smaller structures. Now particle physics is about to enter a new era: The "Large Hadron Collider" will go into operation at the European research center CERN and open the door to an unexplored area of ​​energy. Physicists all over the world are looking at the new accelerator and the results of the experiments - in anticipation of the unexpected.

The fact that atoms are not - as long assumed - indivisible, but have a nucleus that consists of neutrons and protons and is orbited by electrons, is part of everyday knowledge today. Some non-physicists will also have heard of the fact that protons and neutrons are not elementary, indivisible particles, but are made up of quarks and gluons. However, in addition to the electron and the "up" and "down" quarks bound in protons and neutrons, there are other quarks and leptons that do not contribute to the structure of the matter surrounding us, may seem new and strange to many. And rightly so, because although the existence of these particles has been proven beyond doubt by many experiments, we still have no answer to the question of why they exist and what role they play in our existence.

To answer these questions, an experimental apparatus of unprecedented size is currently being built at the European research center CERN in Geneva: the so-called "Large Hadron Collider" (LHC) with its four detectors ALICE, ATLAS, CMS and LHCb. Starting in 2008, protons will collide with protons at extremely high energies of 14 TeV in order to better understand the world of elementary particles and perhaps to discover new, as yet unknown particles. 14 TeV - according to Einstein's famous formula E = mc2, this is an energy that corresponds to 14,000 times the mass of a single proton. At the LHC - by converting energy into mass - in a single proton-proton collision (theoretically) up to 14,000 protons can be generated.

What holds the world together inside

Our current knowledge about the building blocks of matter and the forces manifests itself in the so-called standard model of elementary particles. According to this model, there are twelve different elementary particles of matter - six quarks and six leptons, of which only the two lightest quarks (u, d) and the electron (e) are responsible for the structure of the atoms.

The repulsive and attractive forces that prevail between the matter particles are mediated in the standard model by four force particles: the photon (?) Responsible for the electrical and magnetic forces, the extremely heavy W and Z bosons as mediators of the weak forces describing the radioactive decay and the Gluons, which ensure the strong cohesion of the quarks in the proton.

The picture that particle physicists have of the mediation of forces through the exchange of force particles is quite easy to understand: Similar to the situation of two ice skaters who, due to the conservation of momentum when throwing a ball, move more and more away from each other with each throw even when a force particle is exchanged, momentum (and energy) are transferred from one particle to another. The fact that behind this simple picture there is also an extremely powerful mathematical formalism justifies the importance of the Standard Model as a theory with exact predictive power, which has withstood all experimental results to this day.

The reader may have already noticed at this point that gravity, which plays a very decisive role in our everyday life, is not included in the standard model. This is because the gravitation is about 40 orders of magnitude weaker compared to the other forces. Only because strong and weak forces have a very short range of less than the diameter of an atomic nucleus and because the atoms are electrically neutral do we feel gravity as the only remaining force. However, due to its small size, it does not play a role in the interaction of elementary particles - we will come back to this aspect later.
 

 

Smaller is almost always possible - at least in particle physics. The atomic nucleus is made up of protons and neutrons. These in turn consist of quarks and leptons. For each of these twelve particles there is an additional antiparticle. But that's not all: The particle zoo comprises a total of 48 elementary building blocks of matter, 13 force particles and the Higgs.

The Higgs - Holy Grail of Particle Physics?

In addition to the matter and force particles, there is another essential element in the Standard Model: the Higgs particle. Its existence has not yet been proven experimentally. Only the introduction of this additional particle allows a consistent formulation of a theory with massive building blocks of matter. Without the Higgs, the Standard Model would only predict massless particles, contrary to all experimental results.

But how does the Higgs ensure that the various elementary particles receive mass? To this end, the physicists postulate an omnipresent Higgs background field: Due to the interaction of the quarks and leptons with this background field, they become inert and thus apparently acquire mass. Here, too, there is a simple analogy: An object that moves through a pot of (invisible) honey instead of through air experiences significant resistance due to the frictional forces that occur: It becomes sluggish and thus apparently heavier.

The two LHC experiments ATLAS and CMS are dedicated to the search for the Higgs. The planning of the detectors was therefore largely based on this goal. But where does the particle physicists' confidence in discovering the Higgs at the LHC come from? Or is the existence of the Higgs particle - like that of the Holy Grail - just a pipe dream?

In fact, based on existing data, the standard model restricts the permitted mass range of the Higgs to values ​​below 200 GeV. If the standard model, which has been confirmed by many experimental measurements, is valid, the discovery of the Higgs with ATLAS and CMS is beyond question. If the model is wrong, however, signs of other mechanisms for generating the particle masses must appear in the energy range of the LHC. It may not be the Standard Model Higgs that particle physicists at the LHC will find - but they will find something new.

The long way to new knowledge

In order to research the particle world, physicists have been building ever larger accelerator facilities for more than 50 years, which are used to study how the world is structured at ever higher energies. The three largest of the last 20 years are HERA, LEP and the Tevatron: HERA, where the internal structure of the proton is examined with electrons, the LEP accelerator, on which the W and Z bosons were measured with high precision, and the Tevatron, the Discovered the last and heaviest quark, the Top, in 1995. In particular, however, the precise data from these accelerator experiments have enormously refined our knowledge of the standard model and established the Higgs as a solid component in our worldview.

The experimental procedure of the particle physicists is briefly explained at this point: The reaction products of the particle collisions generated in the accelerators are electronically registered with huge detector systems. The illustration on page 25 shows an example of an electron-proton scattering event recorded with the H1 detector at HERA, as well as the associated scattering reaction. An electron entering from the left is scattered by a quark in the proton and kicks it out of the proton; Quark and electron are detected in the detector, the electron as a single track, the quark as a particle jet. The analysis of the scattering data allows conclusions to be drawn about the properties of the particles and their interactions. The HERA data of the last 15 years have re-shaped our image of the proton as a dynamically changing union of quarks and gluons and thus created an important prerequisite for the proton-proton scattering experiments at the LHC.

Already in the 1980s, before the experiments mentioned above were started and well before the realization that the Higgs particle would not be discovered with today's accelerators, the planning for the next big step in high-energy physics began: the LHC with its four detectors. The aim was to develop a new machine that would allow penetration into areas of energy, the exploration of which would provide new insights into the structure of our world. The result is a machine that consists of around 8000, mostly superconducting, high-performance magnets, connected in a circle, and installed in the old LEP tunnel with a circumference of 27 kilometers, which is about 100 meters deep in the Swiss Jura. At four points on the ring, magnets, held on their path, collide light-fast protons and the detectors record their thousands of individual components. The giants among these detectors are ATLAS and CMS. They are the size of multi-storey houses, as the illustration on page 28 shows using the example of the ATLAS detector in an early stage of expansion.

The major challenges in building the accelerator were the development of the superconducting magnets with fields of over eight Tesla as well as the error-free handling of the proton beams, in each of which an energy of around 350 MJ is stored - this corresponds to a truck moving at 480 km / h 40 t weight. A beam loss in one of the four detectors would destroy it immediately. Coping with these tasks has employed numerous scientists and has taken more than twenty years.

The situation is similar with the detectors. Thousands of scientists from all over the world have also worked on them for more than ten years. This was necessary in order to meet the high requirements that are necessary, among other things, for the discovery and measurement of the Higgs boson. Heidelberg University has also made several key contributions here. For example, highly integrated special electronics for ATLAS was developed at the Kirchhoff Institute for Physics to preselect the scattering events of interest to physicists.

For a better understanding: At the LHC, the protons collide forty million times per second. A picture is "taken" of each of these collisions, which means that the signals left behind in the detector are temporarily stored. However, the ATLAS detector has more than 40 million channels, corresponding to a camera with 40 megapixels. If you had to permanently save all of these images in order to analyze them later, that would correspond to a data volume of around one and a half petabytes, i.e. well over a million CDs, every second. This is not possible even with the most modern computer systems. However, since only a very small part of the scattered events is relevant for the search for new knowledge, they have to be recognized at an early stage with so-called trigger systems and filtered out at lightning speed.

Together with physicists from Germany, England and Sweden, the Kirchhoff Institute for Physics has built such a trigger system over the past decade. It will be completed later this year along with the rest of the ATLAS detector. The search for new knowledge about the structure of our world should finally start next year.

Even if the Standard Model withstands all experimental tests to this day, the particle physicists believe that it is only the approximation of a more general theory that could reveal itself at energies in the TeV range (i.e. at the LHC) and many of the previously open questions in a natural way Way explained. So it is still not known why there are twelve different types of matter particles, whether these are actually elementary and why they have such different masses. It is also unclear why gravity is so much weaker than the other forces.

 

This is what the decay of a small black hole could look like (simulation). With its spherical signature, it would be clearly detectable at the LHC.

Of new symmetries and extra dimensions

The search for answers to these questions gave physicists a number of strange ideas, the realism of which can be investigated at the LHC. An additional symmetry between matter particles and force particles - the "supersymmetry" - could explain the mass hierarchy that can be observed in the three particle families and perhaps shed light on the mystery of the dark matter suspected by astronomers.

Superordinate symmetries are of central importance in modern physics because - if they are broken spontaneously - they can provide a "natural" explanation for observed asymmetries, similar to a magnet, in which in the ground state all atomic spins point in a certain direction, although none is marked in the underlying theory. The system falls into one (of many) energetically preferred configuration. Similarly, supersymmetry explains the occurrence of a certain (a priori not defined) mass spectrum of the elementary particles.

For particle physics, the experimental proof of supersymmetry would be extremely exciting, as it assigns a supersymmetric partner to each standard model particle and thus predicts a large number of new elementary particles whose masses are very likely to be in the energy range of the LHC. The easiest of them would be a top candidate for the dark matter of the universe.

And how does gravity fit into the picture? Here, too, theoreticians have found conceivable ways out, such as the "supergravity" models or string theory. A particularly fascinating idea is the possible existence of additional, compacted spatial dimensions, which, due to their tiny dimensions, remain hidden from us in everyday life, but which would explain the low strength of gravity at large, experimentally accessible distances. Only below a distance that corresponds to the expansion of the extra dimensions would gravity be as strong as the other interactions.

This could have spectacular consequences for the LHC. The existence of extra dimensions could manifest itself in the observation of microscopic black holes. They would arise when two high-energy building blocks of the colliding protons, gluons or quarks, come so close to each other that the gravitational force between them is strong enough to bind them together. Such tiny black holes would not be dangerous because they disintegrate again within a very short time due to Hawking radiation. Through this decay they would be clearly detectable by the LHC detectors due to their spherical event signature.

The detection of such small black holes would be a scientific sensation and would revolutionize our worldview once more. The tasks of particle physics would change fundamentally, and a new area of ​​research would arise: the measurement of the geometry of additional spatial dimensions - an extremely fascinating idea.

In anticipation of the unexpected

When the LHC starts up next year after twenty years of development work, particle physics will be at the beginning of a new era in which the door to a previously unexplored area of ​​energy will be opened. What exactly the physicists will find is uncertain. With great certainty, however, the LHC will reveal the secret of the electroweak symmetry breaking, the mechanism for generating the particle masses, most likely through the discovery of the Higgs boson.

However, expectations of the LHC go far beyond a Higgs discovery. Because all previous data point to a slight Higgs and, as a result, to the occurrence of new phenomena beyond the standard model in the TeV range that the LHC is exploring. The latter follows, among other things, from theoretical considerations on vacuum stability and the so-called "fine-tuning" problem. So the world of physicists is looking forward to the CERN, the LHC and its experiments - in anticipation of the unexpected.

 

Prof. Dr. Hans-Christian Schultz-Coulon has been working at the Kirchhoff Institute for Physics at Heidelberg University since 2004. His field of work is experimental particle physics at accelerators. His research includes the development of trigger systems, investigations into the structure of the proton and the search for new phenomena beyond the standard model of particle physics. He has recently been involved in the calorimeter development for a detector at the International Linear Collider (ILC). Prof. Schultz-Coulon is a member of the ATLAS, CALICE and H1 collaborations.
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