What are specific topics in particle physics

For centuries, philosophers and natural scientists have been looking for the basic building blocks that make up all the diversity and beauty of our everyday world - and at present the surprisingly simple answer is: basically six types of particles are sufficient. They are called electron, up and down quark, gluon, photon and Higgs boson. Eleven additional particles are enough to describe the most exotic phenomena that particle physicists are investigating. This is not mere speculation like the four elements of the ancient Greeks - earth, water, air and fire - but follows from the most refined mathematical theory ever devised to describe nature. Despite the name "Standard Model of Particle Physics", it is not just a model, but a comprehensive theory that characterizes the elementary particles and describes their interactions. Everything that happens in our world - with the exception of the effect of gravity - ultimately obeys the rules and equations for particles of the Standard Model.

The standard model was formulated in the 1970s and was first supported by experiments in the early 1980s. For almost three decades, the theory has stood up to every test, no matter how tough. On the one hand, this success is gratifying because it confirms that we actually understand more deeply than ever before how nature works. On the other hand, there is also something paralyzing about success. Before the Standard Model existed, physicists were used to the fact that the experiments, as soon as the chalk dust settled on the theory that had just been established, provided novel particles or other clues to a new theory. They have now been waiting thirty years for this to happen with the standard model.

The wait should be over soon. Experiments with previously unattainable collision energies or with unexpectedly high measurement accuracy are about to go beyond the standard model. These results will not overturn the model, but rather expand it through the discovery of new particles and forces. A relevant experiment has been taking place at the improved Tevatron collider at the Fermi National Accelerator Laboratory in Batavia (Illinois) since 2001. This could immediately create hypothetical particles called Higgs bosons, which complete the Standard Model, or particles that are predicted by the most plausible extension of the theory, the so-called super partners of the known particles.

Important data also come from the "B factories". These particle hurlers in California and Japan are designed to generate billions of times the bottom quark - one of the eleven additional particles - and its antiparticle to which the so-called CP- Study injury (see Spektrum der Wissenschaft 12/1998, p.90). CPstands for charge parity (charge parity), the symmetry relationship between matter and antimatter. CP-Injury means that antimatter is not the exact mirror image of matter in its behavior. The extent of the previously observed in particle decays CP-Injury is compatible with the Standard Model, but there are good reasons to expect a much more severe injury than the model allows. Only physics that go beyond the standard model are capable of additional ones CP- Injury to generate.

The physicists also study the precise electrical and magnetic properties of the particles. According to the Standard Model, electrons and quarks behave like microscopic magnets of a specific strength, and in an electric field their behavior can only depend on their electric charge. Most of the extensions to the Standard Model predict certain deviations in magnetic strength and electrical behavior. New experiments enable measurements that are so sensitive that even these tiny effects will be detectable.

Only recently, while studying the neutrinos that come to earth from the sun and cosmic rays, astrophysicists have proven beyond doubt that these ghostly particles, which traverse the globe with practically no interaction, have a certain mass (see Spectrum of Science 10/2002 , P. 21). This had long been expected by theorists working on extensions to the Standard Model. The next series of experiments will clarify what kind of theory can explain the observed neutrino mass.

Attempts are also underway to discover the hypothetical particles that make up the cold dark matter in the universe, and to investigate even more precisely than before whether protons decay. A success of either project would be a milestone in physics beyond the Standard Model.

Around 2007 the great hadron collider (Large Hadron Collider, LHC), a machine with a circumference of 27 kilometers that is under construction at Cern, the European laboratory for particle physics near Geneva (see Spektrum der Wissenschaft 9/2000, p. 68). A thirty-kilometer-long linear electron-positron collider, which will complement the results of the LHC, is in the design phase.

While physics beyond the Standard Model is only beginning to emerge in outline, newspaper reports often give the impression that the Standard Model has proven to be wrong, has collapsed and will soon be abandoned. But that's not how it works in physics.

Let us take Maxwell’s equations as an example; they were set up at the end of the 19th century to describe the electromagnetic force. At the beginning of the 20th century it turned out that a quantum version of Maxwell’s equations is necessary for atomic orders of magnitude. Today the Standard Model includes these quantized Maxwell equations as a subset of its equations. But that doesn't mean that Maxwell's equations are wrong. They have been expanded - and are still used to construct countless electronic devices.

The standard model will also be retained. It is a complete mathematical theory - a multiple coherent and highly stable building. It will turn out to be part of an even bigger building, but it cannot be "wrong". No fundamental element of the theory can fail without the entire structure collapsing. If the theory were wrong, many successful tests would have been pure coincidence. The model will continue to describe strong, weak, and electromagnetic interactions at low energies.

A permanent building

The standard model has been tried and tested very well. It said the existence of the W-and Z-Bosons ahead as well as the gluon and two heavier quarks called the charm and top quarks. All of them were then proven experimentally and exactly matched the predictions.

(Editor's note: Normally only three quarks combine to form baryons or two quarks combine to form mesons in nature. The standard model also predicts combinations of five. In fact, such a "pentaquark" was discovered a few months ago by Takashi Nakano at Osaka University in Japan. The exotic particle, which almost instantly decays into a neutron and a K meson, has exactly the theoretically predicted mass of 1.54 GeV.)

A second important test is related to the electroweak mixing angle; this parameter plays a role in describing the weak and electromagnetic interactions. According to the standard model, the mixing angle must have the same value for every electroweak process. In fact, according to all observations, this is the case with an accuracy of one percent.

Third, the Large Electron Positron Collider LEP at Cern measured around 20 million from 1989 to 2000 Z-Bosons. Practically every one of them disintegrated in the manner prescribed by the Standard Model - both in terms of the frequency of each type of decay and in terms of the energies and directions of the end products. These tests are just a few examples of the many that have confirmed the Standard Model in every respect.

In all its glory, the Standard Model contains 17 particles and roughly the same number of free parameters, for example for the particle masses and for the strength of the interactions, the so-called coupling constants. In principle, these quantities can assume any value, and we can only find out the correct value through measurements.

Occasionally, hasty critics compare the many parameters of the Standard Model with the systems of epicycles used by medieval theorists to describe planetary orbits. They believe that the standard model has little informative value or that it can explain everything by adjusting one or the other parameter.

In fact, the opposite is true: once the masses and coupling constants have been measured in any process, they are fixed for the whole theory and for any other experiment without leaving the slightest margin. In addition, the form of each equation in the Standard Model is determined in detail by the theory. Every parameter except the mass of the Higgs boson has been measured. As long as we do not go beyond the standard model, new results can only further refine our knowledge of the parameters - and this makes it not easier, but more difficult to bring all the experimental data under one roof, because the measured quantities have to agree more precisely.

Looking for super partners

It seems that extending the Standard Model by adding more particles and interactions brings a lot more freedom into play, but that's not necessarily the case. The currently favored extension is the Minimal Supersymmetric Standard Model (MSSM). Supersymmetry assigns a super partner to each particle type. We know little about their masses, but their interactions are limited by supersymmetry. Once the masses are measured, the MSSM's predictions will be even more limited than the Standard Model because of the mathematical relationships of supersymmetry.

If the standard model works so well, why expand it? A clear hint arises from the long-sought goal of uniting the forces of nature. In the standard model we can extrapolate the forces and ask how they would behave at much higher energies. For example: What did the forces look like when the temperatures were extremely high shortly after the Big Bang? In the case of deep energies, the strong force is around thirty times stronger than the weak force and more than a hundred times stronger than electromagnetism. If we extrapolate, it turns out that the strengths of these three forces become very similar, but never exactly the same. Only when we extend the standard model to the MSSM will the forces become practically identical at a certain high energy. It gets even better: At a slightly higher energy, gravity approaches the same strength - an indication of a connection between the forces of the standard model and gravity. These results seem to speak clearly in favor of the MSSM.

When, when listing the "ten riddles", I keep saying that the Standard Model cannot explain a phenomenon, I don't mean that the theory just hasn't explained it yet, but will one day manage it. The Standard Model is a highly restricted theory and it will never be able to explain the phenomena enumerated.

However, explanations are possible. One reason many physicists like the supersymmetric extension is that it can handle all but the second and the last three puzzles. String theory, in which the particles are not represented by punctiform objects, but by tiny one-dimensional structures, deals with the last three puzzles (see Spektrum der Wissenschaft 2/2003, p. 24).

Big Bang as a particle spinner

That the Standard Model cannot answer all questions is not surprising - every successful scientific theory increases the number of problems solved, but leaves some unsolved. And although with better understanding new questions arise that could not be formulated beforehand, the number of unresolved fundamental questions continues to decrease.

Some of the ten puzzles provide another argument that a new era in particle physics is about to begin. As it turns out, the solution to many fundamental cosmological problems lies in particle physics; one speaks of "particle cosmology". Only from cosmological studies have we been able to find out that space is made up of matter and not antimatter, or that around a quarter of it consists of cold, dark matter. Any theoretical explanation must trace these phenomena back to the development of the universe after the Big Bang. But cosmology alone does not reveal which particles make up cold dark matter, how the matter asymmetry is actually created or how inflation arises. For this, the knowledge of the greatest and the smallest phenomena must come together.

While the physicists are already addressing all of these puzzles that go beyond the standard model, one essential aspect of the model itself remains to be added. About the leptons, quarks and the W-and Z-Giving bosons a mass, the theory postulates the Higgs field, which has not yet been directly proven.

This field is fundamentally different from all others. To see the difference, let's take the electromagnetic field. Electric charges are sources of electromagnetic fields that surround us everywhere and can be heard on the radio, for example. Electromagnetic fields transport energy. An area of ​​space has its lowest possible energy when the electromagnetic field disappears everywhere there. The zero field is the natural state in the absence of charged particles. Surprisingly, the Standard Model requires that the lowest energy exist when the Higgs field has a certain non-zero value. As a result, a non-zero Higgs field fills the universe and the particles interact with it like people wading through water. This interaction gives them their inertial mass.

The quantum particle belonging to the Higgs field is the Higgs boson. In the Standard Model we cannot derive any particle mass from basic principles - not even the mass of the Higgs boson itself. However, some masses, such as the W.- and Z-Calculate bosons and the top quark from other measured quantities. These predictions have been confirmed and support the underlying Higgs physics.

Physicists already know a lot about the Higgs mass. In experiments with the LEP collider, around twenty variables were measured, which the standard model relates to one another. All parameters required to calculate these quantities are already known - with the exception of the mass of the Higgs boson. One can therefore draw conclusions from the data and ask which Higgs mass best fits the twenty quantities. The answer is that the Higgs mass must be less than 200 giga electron volts (GeV, billions of electron volts). For comparison: the mass of the proton is around 0.9 GeV, that of the top quark 174 GeV. That there is an answer at all is a strong indication of the existence of the Higgs. Otherwise, due to a remarkable coincidence, the twenty sizes would have to be related in such a way that they fit a certain Higgs mass. Our confidence in this approach is justified, because the mass of the top quark was precisely predicted in a very similar way before a single one of these particles could be directly detected.

The LEP researchers also looked for the Higgs particles themselves, but were only able to find a mass up to a maximum of 115 GeV. At this upper limit of the energies attainable with LEP, particles appeared in a few events that behaved like Higgs bosons. But the data was too sparse to be conclusively proven. All in all, the results speak for a Higgs mass between 115 and 200 GeV.

The Large Hadron Collider - a Higgs factory

LEP has meanwhile been dismantled to make way for the construction of the LHC, which is slated to begin data collection in four years. Until then, the search for Higgs particles will continue at the Fermilab's Tevatron. If the Tevatron achieves the planned intensity and energy and does not lose operating time due to technical or financial difficulties, it could confirm a 115 GeV Higgs in two to three years. If the Higgs boson is heavier, it will take longer to filter a clear signal from the background. If everything goes as planned, the Tevatron will produce a total of more than 10,000 Higgs bosons, and it could investigate whether they behave as predicted by the model. The LHC will even be a real Higgs factory that will allow extensive studies on millions of these particles.

There is some evidence that some of the super partner particles provided by the MSSM also have sufficiently small masses to be produced by the Tevatron. Supersymmetry could be directly confirmed in this way in the next few years. The lightest super partner is considered the main candidate for cold dark matter in the universe; he could be seen for the first time in the Tevatron.But only the LHC will produce super partners - if they exist - in large quantities and unambiguously clarify whether nature is actually supersymmetrical.

In order to fully grasp the relationship of the Standard Model to the rest of physics, as well as its strengths and weaknesses, it is useful to introduce the concept of effective theories. An effective theory is a description of an aspect of nature whose input data - at least in principle - can be calculated using a deeper theory. In nuclear physics, for example, the mass, charge and spin of the proton are used as input data. In the standard model, these quantities can be calculated by using the properties of the quarks and gluons as input data. Nuclear physics is an effective theory of atomic nuclei, while the Standard Model is the effective theory of quarks and gluons.

From this point of view, any effective theory is incomplete and equally fundamental - that is, actually not fundamental at all. Will the scale of effective theories continue? The MSSM solves some problems that the Standard Model does not, but again it is an effective theory because it in turn has input data. These in turn can perhaps be calculated in string theory.

Even from the standpoint of effective theories, particle physics may have a special status. It could advance our understanding of nature so far that the theory can be formulated without input data. With string theory or one of its relatives, perhaps all inputs can be calculated - not only the electron mass and similar quantities, but also the structure of spacetime and the rules of quantum theory. But we are still an effective theory or two away from that goal.


Supersymmetry: Unveiling the Ultimate Laws of Nature. By Gordon Kane. Perseus Publishing, 2001.

The Little Book of the Big Bang: A Cosmic Primer. By Craig J. Hogan. Copernicus Books, 1998.

The Rise of the Standard Model: A History of Particle Physics from 1964 to 1979. By Lillian Hoddeson et al. (Ed.). Cambridge University Press, 1997.


- The Standard Model of Particle Physics is the most successful theory in the history of science. But now the signs are increasing that it has to be expanded to include new particles that occur in high-energy reactions.

- Large-scale experiments will soon provide direct clues to these new particles. Particle physics is entering a new phase after thirty years of consolidation. Numerous puzzles could be solved with an extension of the standard model.

- A component of the Standard Model - the Higgs boson - has not yet been observed. The Tevatron Collider in the USA or the Large Hadron Collider at Cern could generate and measure Higgs particles in a few years.

Ten puzzles

Good reasons for expanding the Standard Model arise from phenomena which it cannot explain or which are even incompatible with it:

1. All of our theories today seem to say that the universe contains a tremendous concentration of energy - even in the emptyest regions of space. The gravitational effects of this so-called vacuum energy should have long either rolled up the universe tightly or expanded it much more. The Standard Model cannot solve this riddle - the problem of cosmological constants.

2. For a long time cosmologists believed that the expansion of the universe must slow down because it is slowed down by the mutual gravitational attraction of matter. Only recently have we known that the expansion is accelerating and that the cause - called "dark energy" - is inconsistent with the physics of the Standard Model.

3. There are strong indications for the so-called cosmic inflation: In the first fractions of a second after the Big Bang, the universe underwent an extremely rapid expansion. The fields responsible for inflation cannot come from the standard model.

4. If space began with the big bang, that is, with a gigantic burst of pure energy, exactly the same amount of matter and antimatter should have arisen due to the charge parity - the symmetry between particles and antiparticles. Instead, the stars and nebulae consist of protons, neutrons and electrons - and not their antiparticles. This asymmetry cannot be explained with the standard model.

5. Around a quarter of the universe consists of cold dark matter; this invisible, exotic substance cannot be composed of particles of the Standard Model.

6. In the standard model, the particles acquire their mass through interaction with the Higgs field, the quanta of which are the Higgs bosons, which have not yet been proven experimentally. However, the Standard Model cannot explain the peculiar form of this Higgs interaction.

7. The mass of the Higgs boson, calculated by quantum theory, is enormous. But this would also make the masses of all particles much too large. This result cannot be avoided in the standard model and thus causes a fundamental problem.

8. The Standard Model cannot include gravity because it differs fundamentally from the other three forces.

9. The Standard Model is unable to explain the values ​​for the masses of quarks and leptons - for example of electrons and neutrinos.

10. The Standard Model groups the particles into three families. The everyday world consists only of particles of the first family, and they seem to have a closed theory of their own. The standard model describes all three families, but cannot explain why there is more than one family.

From: Spektrum der Wissenschaft 9/2003, page 26
© Spektrum der Wissenschaft Verlagsgesellschaft mbH

This article is included in Spectrum of Science 9/2003