Space, time, matter and forces

Particle-physics experiments will help explain the origin of dark matter and the absence of anti-matter. New aspects of reality, such as extra dimensions and microscopic black holes, could be discovered at the Large Hadron Collider. Next-generation colliders will use new materials and techniques to reach higher energies, and could also be smaller and cheaper than current accelerators.

Particle physics is concerned with both the basic constituents of matter and their interactions, and with the fundamental properties of space and time. The greatest tool for this exploration is the particle accelerator, first developed in the 1960s, which collides particles together at high energies comparable to those found in the early universe, to reveal their constituents and create new particles in the process.

Over the years, ever more powerful accelerators have led to the discovery of quarks — the elementary particles contained in protons and neutrons — and other particles responsible for binding them together as well as particles responsible for radioactive decay. These findings have laid the foundation for the Standard Model of particle physics.

The Standard Model has proved remarkably successful: all experimental data from accelerators have so far validated its predictions. However, one of its most important predictions — that particles derive their masses from the hypothesized Higgs particle — has yet to be verified. One of the primary aims of the most powerful accelerator, the Large Hadron Collider (LHC) near Geneva, Switzerland, is to detect the Higgs particle. Work is underway to develop the mathematical, experimental and computational techniques needed to recognize it1.

Beyond the Standard Model

There are still many issues that the Standard Model cannot explain. The main theoretical mystery is the vast difference in the strengths of the fundamental forces (Fig. 1); for example, the pull of a small magnet on a needle is much stronger than the gravitational pull of the whole Earth. This is extremely unnatural for forces described by quantum theory and this ‘hierarchy problem’ is at the heart of many novel theories that go beyond the Standard Model.

Another major mystery is the source of dark matter — the invisible substance that comprises almost one-quarter of the universe. It might be that dark matter is made from a new type of particle that interacts only weakly with conventional particles. Experiments to search for such particles with crystals and liquid noble gases are already underway2. Astronomers are also hunting for signs of high-energy ?-rays that are released when dark matter is annihilated.

There is a theory that extends the Standard Model and attempts to explain the origin of dark matter. Known as super- symmetry3, it predicts a whole host of new particles, some of which could be constituents of dark matter and could show up in dark-matter searches or at the LHC.

In the Standard Model, the fact that particles are organized into ‘generations’ — there are two heavier versions of the electron, for example — is another aspect in need of verification. Studying rare decays of heavy quarks and the properties of neutrinos, both experimentally and theoretically, could help resolve this mystery.

One of the most celebrated theories for new physics beyond the Standard Model is string theory4, which posits that all fundamental particles are tiny vibrating strings (see image above), helping to explain their generations and symmetry.

At the LHC, physicists hope to find exotic by-products of string theory, including curled-up extra dimensions that are too tiny to be noticed in everyday life, and microscopic black holes. Should these features be found, it will mark a revolution in our understanding of physics.

Investigating the Anti-Matter Mystery

Yet another question that the Standard Model cannot answer is why our universe is composed predominately of matter (Fig. 2). Some 80 years ago, it was proposed that for every known type of particle, there should exist a corresponding anti-particle with the same mass but opposing properties, such as charge. Current theories suggest that equal amounts of matter and anti- matter were produced during the Big Bang, yet matter dominates the universe around us. Where is this missing anti-matter?

There are several possible theories to explain the absence of anti-matter, some of which indicate small deviations from the Standard Model. To test them will require precise monitoring of the production of anti-matter in experiments at the LHC and at the KEK electron-positron accelerator facility in Japan.

The answer might be tied to a fundamental particle called the neutrino5, which could well be its own anti-particle. Different types of neutrino have slightly different masses, and neutrinos can spontaneously oscillate among these types. High-precision measurements of the oscillation parameters are planned in the next decade, which might elucidate the role of neutrinos in the evolution of the universe. It has been proposed that the asymmetry between matter and anti-matter could be connected to the question of whether the neutrino is its own anti-particle; this idea will be tested by measuring rare radioactive decays in a dedicated experiment at the Gran Sasso National Laboratory in Italy6.

Next-Generation Colliders

The LHC, and the searches for dark matter and neutrinos, are expected to provide many answers about the constituents of the universe; however, they will also raise new questions that particle physicists are already preparing to address.

The next large-scale accelerator project on the horizon is the International Linear Collider (ILC). In contrast to the LHC, the ILC will collide electrons rather than protons. Unlike protons, electrons do not have any substructure, making it easier to analyse their collision products.

Looking beyond the ILC, there are proposals to accelerate muons, which are another type of fundamental particle. Muons, like electrons, have no substructure, but they are 200-times heavier than electrons, so they can be accelerated in a circular collider. Muons decay quickly, however, and it is a big challenge to produce a beam from such short-lived particles.

It is clear that attempts to build ever larger accelerators will eventually hit a financial and practical wall. One strategy to drastically reduce the size and cost of accelerators is to exploit the properties of plasmas, as they can support much stronger electric fields than conventional accelerators7. Tests of the proposed plasma-wakefield accelerators are planned for the coming years.

Particle physics has been — and will remain — an international enterprise. By investigating some of the smallest elements of the universe, particle physics promises to uncover some of its biggest secrets.

Emission of particles into extra dimensions, and the production of microscopic black holes and vibrating strings, are exciting ideas investigated theoretically at the Max Planck Institute for Physics. A new wave of particle-physics experiments in neutrino physics, dark-matter searches and heavy-quark production, and at the highest energies, is set to test such new theories of matter, space and time (Dvali, G. & Redi, M. Phys. Rev. D 80, 055001, 2009).

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