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The future: Particle physics in the 21st century
The last century has seen revolutionary discoveries in particle physics. Quarks emerged as the tiniest building blocks. We learned that antiparticles exist for each type of particle and that forces are transmitted by quantum particles called bosons. Slowly the Standard Model of particles and their interactions came into focus. Yet, although many questions have been answered, fundamental mysteries remain.
The hunt for the Higgs boson
The current theoretical framework of particle physics is deeply connected to a hypothetical particle, the Higgs boson, transmitting a new type of force. Physicists believe it is the last missing piece in the Standard Model, and its discovery would present the key to understanding why some but not all particles have mass. If nature does not provide for a Higgs boson, other particles and forces are needed to save the interactions of the Standard Model components and to explain the origin of mass.
Past experiments have failed to find the Higgs boson, although experimenters at the European Particle Physics Laboratory, CERN, may have seen the first signs of a Higgs signal in their detectors in the year 2000. In the next few years the CDF and DZero experiments at Fermilab are the only experiments in the world capable of discovering the Higgs particle, the key to understanding the property of mass.
The Search for the Higgs (article in Beamline, March 2001, PDF)
What is Electroweak Symmetry Breaking (article in FermiNews, January 1998, PDF)
Solving the neutrino puzzle
Neutrinos are the most elusive of elementary particles. They were postulated in 1931, but it took more than 20 years to observe the first neutrinos in an experiment. Today, we still know very little about these particles; and their biggest secret is their mass. Physicists have identified three different types of neutrinos, all much lighter than an electron. Because they fill the universe, even with their tiny mass neutrinos still could contribute as much as five percent to the total mass of the universe..
The Kamioka Observatory in Japan has found strong evidence for non-zero neutrino mass. New neutrino experiments will search for confirmation and for an understanding of the nature of neutrino mass. The experiments will look for neutrino oscillations, the transformation of one type of neutrino into another, a unique indication for neutrino mass. Fermilab hosts two neutrino experiments, MiniBooNE and MINOS, both currently under construction.
Creating collisions of high-energy particles in particle accelerators is the best way of reproducing – in a tiny region of space – the high-energy conditions of the early universe, when the temperature was billions of degrees and atoms hadn't formed yet. Physicists think that the Big Bang produced a primordial soup of particles containing free-moving quarks and gluons, the building blocks that eventually formed protons and neutrons.
Scientists at the Brookhaven National Laboratory try to recreate these conditions by colliding gold ions at high energies. Though a single proton inside the gold ions has much less energy than the protons circulating the Tevatron, the acceleration of an ion with many protons and neutrons is the ideal method of bringing matter to a "boil."
Where's the antimatter?
Particle physicists can produce antimatter with accelerators, and current experiments even investigate the properties of antihydrogen. Our everyday world, however, is completely dominated by matter. Where did the antimatter go?
Scientists believe that the early universe contained as many particles as antiparticles. As they interacted with each other, a slight asymmetry in the laws of nature favored the survival of matter. Experimenters have observed this misalignment in processes involving kaons, unstable particles that contain strange quarks. The BaBar experiment at the Stanford Linear Accelerator Center and the BELLE experiment at the Japanese KEK laboratory examine whether this effect also occurs in the decay of particles containing bottom quarks. The CDF and DZero experiments at Fermilab, as well as the HERA-B experiment at the German particle physics laboratory DESY, carry out complementary research.
Astrophysics experiments have shown that visible, or luminous, matter accounts for less than 10 percent of the entire mass in the universe. The motion of galaxies and other scientific observations indicate the presence of gravitational forces that seem to come from an unknown type of invisible matter, called dark matter. New astrophysics experiments, including the Sloan Digital Sky Survey and the Pierre Auger Observatory supported by Fermilab, will provide more information on the extent of dark matter and its role in the evolution of the universe. The Cryogenic Dark Matter Search looks for heat created by dark matter particles passing through an ultracold detector.
Physicists hope to identify some of the elementary constituents of dark matter using future high-energy particle accelerators. Research & Development programs study the scientific merit and technological feasibility of various possibilities.
Fulfilling Einstein's dream
The unification of electric and magnetic forces into an electromagnetic theory represented a major achievement at the end of the 19th century. It took about 70 years before theorists achieved a comparable breakthrough: the unification of the electromagnetic force with the weak force, which is responsible for particle decay processes. The resulting electroweak theory fueled many speculations about a Grand Unified Theory that would also incorporate the strong and gravitational forces.
Einstein attempted to construct a theory that would encompass both gravity and electromagnetism – with no success. But mathematical and theoretical advances at the end of the 20th century revived Einstein's dream. Superstring theory may hold the key to constructing a theory of quantum gravity, the essential step in linking gravity to the other fundamental forces. Future experiments will provide guidance as physicists compare theoretical predictions with real-world data.
Nobody knows the actual size and shape of elementary particles such as quarks. Experiments have yielded upper limits on their size, but nobody knows how tiny they really are. Particle physicists usually think of quarks and other fundamental particles as point-like objects that have no volume at all. String theories, however, envision particles as little loops that can vibrate like the strings of a violin.
Though we perceive our world as consisting of one time dimension and three spatial dimensions, each point in the universe could have tiny extra dimensions attached to it. Only certain types of particles and interactions would feel their presence. In the context of string theories, this novel idea could explain why at the atomic level gravity is much weaker than any other force. The latest generation of particle physics experiments is prepared to explore space and time at the smallest scales.
These are just some of the ideas and anticipations that physicists have. Although the Standard Model is one of the most successful and thoroughly tested theories in physics, it cannot be the final answer. Many unsolved mysteries seem to require concepts and mechanisms that go beyond our present knowledge. It will take powerful accelerators, world-class experiments and groundbreaking ideas to unravel the secrets of matter, space and time.
Interactions: Key Questions for the Future
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|last modified 3/25/2004 email Fermilab|