For every answer two new questions arise. Particle physics experiments at the Fermilab explore the fundamental constituents and architecture of the universe. Here accelerators produce the highest-energy particles ever made by man. Experimentalists use particle collisions to search for undiscovered particles and interactions and to learn more about known particles and interactions. One of the ways that researchers search for signals of new physics is to observe rarely interacting particles, such as neutrinos, and their corresponding antimatter particles. Some of these experiments search for evidence of the process theorists hypothesize allowed our universe full of matter to bloom rather than being annihilated by an equal amount of antimatter created in the big bang. Other experiments seek to observe rare processes that can give researchers a glimpse of unknown particles and unobserved interactions. For every answer physicists find, two new questions arise.

Neutrinos are some of the most fascinating of the known particles. They abound in the universe but interact so little with other particles that trillions of them pass through our bodies each second without leaving a trace. Neutrinos come in three types, called flavors: muon, electron and tau. They have no electric charge. Their mass is so small that the heaviest neutrino is at least a million times lighter than the lightest charged particle.

At Fermilab, physicists use a beam of protons from the Main Injector accelerator to create the most intense high-energy neutrino beam in the world. Magnets direct the protons onto a graphite target. When the protons strike this target, they take the form of new particles called pions. A magnetic lens called a horn focuses and collects the positively charged pions and discards the negatively charged ones. The positively charged pions travel through a long, empty space and ultimately decay into antimuons and muon neutrinos. Experimentalists filter the resulting mix of debris, antimuons, undecayed pions and muon neutrinos through a steel and concrete absorber, which stops all but the weakly interacting neutrinos. To make a beam of antineutrinos, they reverse the magnetic field of the horn to collect negatively charged pions that decay to negatively charged muons and muon antineutrinos.

The facility that creates Fermilab's neutrino beam is called NuMI, for Neutrinos at the Main Injector. The neutrinos travel between two detectors for an experiment called MINOS, or Main Injector Neutrino Oscillation Search. One sits at Fermilab; the other is located 450 miles away in the Soudan Underground Laboratory in Minnesota. The NuMI Beamline is aimed downward at a 3.3 degree angle toward the underground laboratory. Neutrinos interact so rarely with other particles that they can pass untouched through the entire Earth.

Although the beam starts out at 150 feet below ground at Fermilab, it passes as much as 6 miles beneath the surface as it travels through the earth toward Soudan. Neutrinos travel at the speed of light and make the trip from Illinois to Minnesota in just two and a half thousandths of a second. Researchers at Fermilab use the NuMI beamline as a source of neutrinos for other intensity-frontier experiments as well.

Muon. Physicists also search intense beams of particles for signs of virtual particles. According to the Heisenberg Uncertainty Principle, even massive particles can pop briefly in and out of existence as virtual particles. The more massive the particle, the less frequently this happens. When virtual particles drop in, they are much less massive than usual, but physicists can detect them in intensity-frontier experiments by the effect they have on interactions between other particles like kaons and muons. By colliding two concentrated beams of one of these types of particles into one another or by firing them into a target, physicists bring them into proximity in the hopes that they will interact. They then study those interactions with ultra-precise detectors, looking for unusual outcomes. They can identify the presence of virtual particles involved in an interaction by the effects they have.

Even if the particle colliders like the Large Hadron Collider grants physicists the chance to observe the particles physicists seek directly, they will need experiments like those at the intensity frontier to make precise measurements of their parameters.

Cosmos as a laboratory: Dark Matter and Dark Energy. Scientists of Fermilab use the cosmos as a laboratory to investigate the fundamental laws of physics. Researchers use detectors to study particles from space as they approach and enter our atmosphere in forms such as cosmic rays, gamma rays and neutrinos emitted by the sun. These experiments allow researchers to test theories about how the universe was formed, what it is made of and what its future holds.

Experiments at the Cosmic Frontier may have the best chance of discovering the nature of dark matter and dark energy. Theorists have concluded that these two mysterious materials constitute 96 percent of the universe and may be responsible for its formation and accelerating expansion.

No one has ever directly observed dark matter, but two clues led astronomers to suspect its existence. First, when researchers measured the masses of all the stars and planets that make up galaxies, they discovered that the gravity of those objects alone would not be great enough to hold them together. Something they could not see must have been contributing mass and therefore gravitational pull. Second, they observed in space the kind of distortions of light usually caused by large masses in areas that seemed empty.

The composition of dark matter is unknown, and its existence shows that the Standard Model of particle physics is incomplete. Several theories of particle physics, such as supersymmetry, predict that weakly interacting massive particles (WIMPs) exist with properties suitable for explaining dark matter.

In the 20th century, astronomers first discovered that the universe was getting bigger. They found this by observing something similar to the Doppler effect in the light coming from distant galaxies. The Doppler effect is what causes a car horn to change in pitch from high to low as it approaches and passes. This happens because the sound waves are compressed as the car moves toward you, resulting in a higher pitch, and are stretched as it recedes, resulting in a lower pitch. As an object approaches you, the light waves coming from it compress. Astronomers call this blueshift. When light waves stretch as an object moves farther away, astronomers call it redshift.

By measuring the spectrum of an astronomical object, astronomers can tell how much the space between the object and observer has stretched as the light traveled through it. When astronomer Vesto Slipher measured light coming from other galaxies, he found that almost all were redshifted, or moving away. He found that those that seemed dimmer and farther away had even higher redshifts. The universe was expanding. This led astronomers to the idea of the big bang. Astronomers assumed, however, that the force of gravity from all of the matter in the universe would slow the expansion. They were in for a surprise in 1998 when they discovered that the expansion was actually speeding up. Astronomers discovered this when they measured the brightness of the light coming from a certain type of supernova that always explodes with roughly the same energy. The dimmer the light from the supernova, the farther the distance it had traveled to Earth. They discovered that the supernovae were farther away than their redshift measurements predicted. The universe was expanding at an accelerating rate.

Some particle astrophysicists think this is happening because a force with a repulsive gravity is pushing the universe apart. They call this force dark energy. Many experiments at the cosmic frontier seek to study the nature of dark energy.

Fermi National Accelerator Laboratory advances the understanding of the fundamental nature of matter and energy by providing leadership and resources for qualified researchers to conduct basic research at the frontiers of high energy physics and related disciplines.

By Vasil Sidorov on February 2, 2009 from Fermilab  

E-mail: sidorovvasil@gmail.com

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