Exploring the Invisible Universe

by Kurt Riesselmann

The sun. The moon. The stars. For thousands of years people have tried to understand the things they see in the sky. Little did they know that there is much more than meets the eye.

During the last decade, astronomers and physicists have discovered that about ninety-five percent of our universe is invisible even to the best telescopes. Developing indirect observation techniques, scientists have “seen” black holes, dark matter, three types of ghost-like particles called neutrinos and a mysterious, dark force that speeds up the expansion of the universe. We still know very little about all this “stuff,” except that it is invisible to the naked eye.

Scientists at Fermilab are deeply involved in unraveling the secrets of this invisible universe. An experiment called the Cryogenic Dark Matter Search looks for new types of particles that could be building blocks of dark matter. The Sloan Digital Sky Survey has begun to record a detailed three-dimensional map of the sky, providing vast amounts of data on clusters of galaxies that seem to harbor black holes and dark matter. And the MiniBooNE and MINOS collaborations will take a close look at the properties of hard-to-detect neutrinos, which may account for as much mass as all the stars combined: a few percent of all mass and energy in the entire universe.

In the past few years the Super-Kamiokande experiment has overturned the long-standing dogma that neutrinos are massless. Studying neutrinos created in the earth’s atmosphere, the experiment has found evidence for neutrino oscillations, a phenomenon that can only occur if neutrinos have mass. Although the mass of a single neutrino is believed to be tiny, much less than the mass of an electron, the abundance of neutrinos in our universe could make them an important factor in the formation of galaxies and the evolution of the universe.

The Main Injector Neutrino Oscillation Search will build on the success of Super-Kamiokande. Instead of working with atmospheric neutrinos, however, scientists will create an intense neutrino beam using protons from the powerful Main Injector accelerator at Fermilab. To study the neutrinos, MINOS scientists are building two large detectors, which they will place at two different points along the neutrino beam.

Creating ghost particles

Scientists classify neutrinos as weakly interacting, an optimistic view given the fact that a neutrino can easily traverse the breadth of our planet without leaving a trace. To increase the chance of witnessing a collision of a neutrino with an atomic nucleus, scientists prefer to use intense, man-made neutrino beams. Fermilab initiated the Neutrinos at the Main Injector (NuMI) project, beginning the construction of underground beam lines and halls in May 2000. When NuMI is complete, scientists will send a package of 20,000 billion protons every two seconds to a hall 150 feet underground, where the protons will smash into a target and create neutrinos.

“The proton beam is only several millimeters wide at the target location,” explained Bruce Baller, who manages the beam line design and installation. “The proton beam will travel through a series of little graphite plates, a total length of about one meter. Scientists at the Institute for High Energy Physics in Russia are building this target.”

The target converts the protons into bursts of particles with exotic names such as kaons and pions. Like a beam of light emerging from a flashlight, the particles form a wide cone when leaving the target. A set of two special lenses, called horns, is the key instrument to focus the beam and send the particles in the right direction.

Drawing an electric current of 200,000 amps, about twenty thousand times more than a hairdryer, the horns produce powerful electromagnetic fields that keep the pions and kaons from spreading too far.

To save energy, scientists operate the horns in pulsed mode, creating a focusing field two milliseconds long every time a proton package hits the target. The interiors of the horns have the shape of parabolic cones made of 2.5-millimeter-thick aluminum.

Switching the electromagnetic fields on and off creates stresses and vibrations inside each horn, a bit like the forces occurring inside a musician’s horn. To ensure top performance, the different horn sections are welded together with tolerances only slightly larger than the thickness of a hair. In the last year, engineers have successfully tested a prototype horn for about five million pulses.

“Kris Anderson has been working on the design of the horns for several years,” said project engineer Dave Pushka, who coordinates the work of more than a dozen mechanical engineers on the NuMI and MINOS projects. “It’s a wonderful, very thoroughly analyzed device. We are pushing the envelope in terms of technology. Kris has developed the welding procedure for the horns. It is extremely high quality, like that used for the space shuttle and fighter jets.”

Studying a magic trick

After leaving the horns, the pions and kaons decay into muon neutrinos, one of three types of neutrinos known to physicists. Muon neutrinos, however, seem to be capable of a special trick: traveling at almost the speed of light they can slowly disappear. Scientists have reason to believe that no magical forces are at play however. Instead, they suspect that the muon neutrinos change their identity, transforming into either electron neutrinos or tau neutrinos, and reappearing at a later time. According to the laws of quantum physics, this behavior, called neutrino oscillation, is quite acceptable—assuming that neutrinos have mass.

“I think that essentially everybody is convinced that there is a disappearance effect in the Super-Kamiokande data,” said physicist Stan Wojcicki, professor at Stanford University and spokesperson for the MINOS experiment. “The neutrino oscillation hypothesis gives a perfect fit to the data. But there are also other models that explain the data.”

Some theorists have suggested that muon neutrinos may instead decay into new particles not identified so far. Other physicists wonder whether muon neutrinos could oscillate into sterile neutrinos, a fourth type of neutrino that interacts with matter even less than the three types already known.

The MINOS collaboration is determined to uncover the truth using the Fermilab muon neutrino beam. To check the oscillation hypothesis, physicists will measure the number of muon neutrinos at two points along a 450-mile long path that takes the neutrinos from Batavia, Illinois to an iron mine in Soudan, Minnesota. When construction is complete in 2004, the collaboration can verify the Super-Kamiokande disappearance result in a few months. It will take about two years to carry out the more important part of the experiment, investigating whether the disappearance is due to oscillations.

Sandwiches of steel

The unique setup of the MINOS experiment allows scientists to determine the shape of the oscillation wave. Tuning the energy of the man-made neutrino beam, a feat not possible with atmospheric neutrinos, scientists will measure the strength of the oscillation curve at various points, locating the maxima and minima of the curve by comparing the neutrino events observed in the two detectors.

To succeed with their experiment, MINOS scientists need to catch enough neutrinos at both the Fermilab site and at Soudan. The MINOS collaboration is building a 1,000-ton neutrino detector at Fermilab, which will sample the muon neutrino beam about 3,400 feet from the target, where the oscillation curve is still very close to its maximum. This near detector consists of 282 planes, each containing a one-inch steel plate and a layer of scintillating material, which collects the light caused by a neutrino collision inside the steel.

“The scintillator modules are being made right now at Argonne National Lab,” said Fermilab physicist Catherine James, who leads the team constructing the near detector. “At Fermilab, we have begun to put the first modules on one-inch-thick steel plates, twenty-one feet wide, twelve feet tall.”

James expects that the near detector will observe tens of neutrino events every two seconds, corresponding to the proposed design rate of protons hitting the target. Up in Soudan, where the larger far detector is under construction, it will be more difficult to test the beam. Despite the use of top-quality focusing horns, the cone of the neutrino beam will increase significantly in diameter along the 735-kilometer trip through the earth.

“By the time it gets up there, the neutrino beam will be four kilometers wide,” James said. “The detector in Soudan is only eight meters wide. We will be lucky to get one thousand neutrino events per year.”

Fortunately, simulations have shown that such a sample will suffice to map out the whole oscillation curve as a function of energy.

“The Super-K results determined what beam energy we should start with,” James said. “Based on the most recent results and the distance between our two detectors, we’ve optimized the initial neutrino beam energy.”

The construction of the far detector is progressing faster than expected, and technicians will have installed the first of two 2,700-ton supermodules by the end of June. The components for the near detector will be ready in December. Scientists, however, won’t have access to the new MINOS detector hall until next year due to delays in the excavation schedule (Ferminews, vol.24, no.16, Sept. 28, 2001). Nevertheless, James is excited about the prospects for the MINOS experiment.

“Even with the unfortunate delays that occurred,” she said, “the results will be a great and timely contribution to the field.”

Cover photo: Chris White of Illinois Institute of Technology, leads a team that assembles planes, like the one shown here, for the MINOS near detector. Each of the detector’s 282 planes consist of scintillator modules inside aluminum boxes, and 12-foot-tall steel plates.