U.S. Department of Energy

Pacific Northwest National Laboratory

Glossary

A | C | D | E | H | L | N | O | P | S | T | W
Accelerator-based experiments

Physicists probe nuclei with complex devices called accelerators. Cosmic accelerators produce particles with much higher energies than can be produced by particle accelerators on Earth. Studying these particles may give physicists a way of exploring energy scales far beyond what current or future particle accelerators can achieve.

Antimatter

What we know about the history of the cosmos suggests that visible matter could have been destroyed by an equal amount of antimatter long before the present day. Explaining why this did not occur, and therefore, why the cosmos as we know it exists, is a fundamental question scientists are researching.

Antiparticle

A subatomic particle identical to another subatomic particle in mass, but opposite to it in electric and magnetic properties (as sign of charge) that when brought together with its counterpart produces mutual annihilation. A subatomic particle not found in ordinary matter.

Astrophysics

The science of why objects, usually stellar and galactic processes, behave the way they do.

Axions

A subatomic particle of low mass and energy that is postulated to exist by the Peccei-Quinn theory in 1977 because of certain properties of the strong force. The axion originally arose as part of an elegant solution to the “strong CP problem” (charge conjugation and parity) in the Standard Model of particle physics. It was then appreciated that a light axion is an ideal cold dark matter candidate.

Colliders

Colliders are used to discover the identity and properties of the particles that make up the universe, and to understand the forces and interactions between those particles.

Cosmic acceleration

An important part of scientists’ story of the origin and evolution of the Universe is the existence of two periods during which the expansion of the Universe accelerated. A primordial epoch of acceleration, called inflation, occurred during the first fraction of a second of existence. The cause of this inflation is unknown, but may have involved currently unexplored physics at ultra-high energies. A second distinct epoch of accelerated expansion began about nine billion years later and continues today. This expansion is presumed to be driven by some kind of dark energy, which could be related to Einstein’s cosmological constant, or driven by a different type of dark energy that evolves with time.

Cosmology

The science of the origin and development of the universe.

Dark Energy

The surprising discovery in 1998 that the expansion of the universe is accelerating, instead of slowing down due to gravity, has posed another significant question: what is the dark energy that is pushing our universe apart? While it may be an inherent feature of the universe, it could be something dynamic related to new particles or forces, or a failure of Einstein’s theory of General Relativity. Dark energy may explain why the expansion of the universe continues to accelerate rather than slow down, as would be expected if gravity predominated. Both dark matter and dark energy remain among the top mysteries of the universe, key topics of astrophysical research today.

Dark Matter

In 1933, Fritz Zwicky, a faculty member of Caltech, coined the term “dark matter” to describe the unseen matter that must dominate the Coma Galaxy Cluster in order to match his observations of the motion of its galaxies. Scientists have been confident dark matter exists for more than 80 years, although they’re still trying to understand what it’s made of and why there seems to be so much of it. Dark matter is the gravitational glue that holds stars and gas in galaxies and great galaxy clusters together, whose fast-moving stars would otherwise be flung far apart. It is a form of matter that does not emit light and, therefore, is difficult to detect with ordinary observation methods. Scientists know it must exist, because its mass causes gravitational effects that can be observed. Based on the effects of gravity in our galaxy, scientists believe there is a high concentration of dark matter near the galaxy’s center around the supermassive black hole residing there.

Detectors

Developed to identify, track and provide many details about subatomic particles, such as those produced by particle accelerators.

Explore the unknown

There are clear indicators of new phenomena awaiting discovery beyond those motivating the other four drivers. Particle physics searches take two basic forms: directly producing and indirectly detecting evidence for new particles.

Extra Dimensions and/or Supersymmetry (SUSY)

Supersymmetry is an extension of the Standard Model that aims to fill some of the gaps. It predicts a partner particle for each particle in the Standard Model.

Higgs boson

This recently discovered form of matter, never before observed, is mysterious and offers a unique portal into the laws of nature. Scientists want to know what principles determine its effects on other particles; How does it interact with neutrinos or with dark matter; Is there one Higgs particle or many; Is the new particle really fundamental or is it composed of others?

Higgs sector

In particle physics, the Higgs sector is the collection of quantum fields and/or particles that are responsible for the Higgs mechanism, i.e. for the spontaneous symmetry breaking of the Higgs field. The word “sector” refers to a subgroup of the total set of fields and particles.

Leptogenesis

Neutrinos may drive the evolution of matter-antimatter asymmetry through a process known as leptogenesis.

Neutrino mass

Recent discoveries show that neutrinos have mass and they change between types as they travel. Physicists now know that neutrinos exist in three types and they oscillate, i.e., they change type as they move in space and time.

Neutrinos

Mysterious and elusive particles called neutrinos morph from one type into another and back again. These “oscillations” imply that neutrinos have nonzero mass, but are also anomalously light, relative to other particles. Neutrinos may help scientists discover why the cosmos exists as we know it. There are three known neutrino types: electron neutrinos, muon neutrinos and tau neutrinos.

Neutrons

Neutrons have no net charge, thus are much difficult to identify and study. Due to its lack of charge, the neutron wasn’t added to the atlas of subatomic particles until 1932. The neutron’s name originates from the Latin root for “neutral.” But, we know the quarks inside are charged, so nuclear scientists use high-energy electrons to understand the neutron’s interior and answer the question ‘how does that net neutrality come about?’

New physics beyond the Standard Model

Probing more deeply for deviations from the Standard Model predictions, thus providing some other evidence for physics Beyond the Standard Model.

Non-accelerator-based experiments

The use of naturally occurring particles and phenomena to explore particle and astroparticle physics. Cosmic rays in the earth’s atmosphere and neutrinos from the sun, galactic supernovae, and terrestrial nuclear reactors serve as some of the non-accelerator-based particle sources used in this area of research.

Nuclear physics

A journey of discovery into the nucleus of the atom, which is at the heart of our ability to understand the universe. It provides answers and expands our knowledge of both the infinitely small and the extremely large.

Ordinary matter

Stars, planets and all living things are made of matter - everything that makes up the things we ourselves are, as well as what we see and touch. Any object with mass causes gravitational attraction, similar to the way the earth’s mass causes the attraction that holds us to the ground.

Particle

When matter is produced, an equal amount of antimatter should be produced. Particles are produced as matter-antimatter pairs.

Particle candidates

Theoretically favored particles that may exist in dark matter include WIMPS and Axions.

Particle physics

Particle physics advances the understanding of the basic building blocks of the Universe. IT is a discovery science defined by the search for new particles and new interactions, as well as by tests of physical principals. The Office of High Energy Physics promotes a broad, long-term U.S. particle physics program centered on five science drivers that are intertwined.

Protons

The charged proton was discovered in the early days of nuclear physics (1918).

Standard Model

According to the Standard Model, about 13.7 billion years ago, the entire universe was born from an event known as the Big Bang. This event gave birth to all the matter and energy existing in the universe today.

Sterile neutrinos

Scientific experiments seek to confirm or rule out the existence of a fourth type of neutrino that is “sterile,” or doesn’t interact through the weak force like the known neutrinos.

The Big Bang

The Big Bang gave birth to all the matter and energy that exists in the universe today. The Big Bang set the universe in motion and initiated its expansion about 13.7 billion years ago.

The Visible Universe

The Visible Universe includes everything that can be “seen” by the electromagnetic radiation it emits. This includes not just human-visible matter like stars and planets, but also sources such as the cosmic microwave background and astronomical radio, x-ray and gamma ray sources. Dark Matter and Dark Energy are explicitly excluded from the Visible Universe.

Weakly Interacting Massive Particles (WIMPS)

WIMPS, which may exist in dark matter, rarely react with anything, making them difficult to discover. Interactions of WIMPs with one another or particles of ordinary matter may occasionally give off detectable signals, which is what scientists are hoping to see.