sealPurdue News

December 1999

Detector will play crucial role in physics experiment

WEST LAFAYETTE, Ind. – Scientists at Purdue University are winding up a project to design and build a key component of an experiment that aims to help answer a troubling mystery: If the Big Bang that created the cosmos spawned equal amounts of matter and antimatter, as theory predicts, where did all the antimatter go?

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"The big question is, how did the universe survive?" says Ian Shipsey, a professor of physics at Purdue. "If matter and antimatter were created in equal quantities in the early universe, each antiparticle would have annihilated a particle, and the universe, as we know it, would have ceased to exist."

But the universe does not contain equal amounts of matter and antimatter; it is predominantly made of matter. A possible solution to the problem is that there are small differences in the properties of antimatter and matter, setting in motion an evolutionary process that, over billions of years, has resulted in today's matter-dominated universe.

The best way to test this theory is to analyze subatomic particles referred to as bottom quarks, also known as beauty quarks, which a silicon detector being built at Purdue is designed to do.

The detector, called Si3, is part of an overall experiment called CLEO III. It contains 450,000 silicon strips, or "cameras," each about the width of a human hair. The cameras detect the life and death of quarks, which are created when matter and antimatter collide. The tiny silicon strips are attached to 125,000 channels of sensitive electronics, arranged in four concentric cylinders and held in place by an elaborate tripod-like framework made of synthetic diamond and copper.

Shipsey heads the detector work, which involves four universities. The Si3, which is about 21 inches long, was shipped early this month to Cornell University, where it will be connected to other detectors in the CLEO III experiment, an international collaboration of about 20 universities.

Once considered strictly theoretical, antimatter has been steadily emerging into the world of reality. Particles of antimatter look and behave the same as ordinary subatomic particles. But although they have the same mass as their matter counterparts, they have the opposite electrical charge. If particles and antiparticles meet, they instantly annihilate each other, releasing a large amount of energy in the process.

Physicists use particle accelerators to produce antiparticles. At Cornell's Wilson Synchrotron Laboratory, electrons and their antimatter counterparts, positrons, are smashed together in an underground particle accelerator. As they collide, the particles annihilate, producing constituent particles whose properties must be precisely measured to further understand the nature of matter.

Various detectors are located at specific distances and positions from the point of collision. As the newly created particles speed away, they penetrate the silicon in the Si3 detector, which sends signals to a computer detailing the precise positions of the particles. The combined signals from all the detectors in CLEO III will be used to measure data such as momentum, velocity, energy and penetrating power of particles produced in collisions.

Those data will then be used to test the widely accepted "standard model" of physics, which states that matter is made of 12 fundamental building blocks. The theory says that all matter consists of six varieties of leptons, a family that includes electrons, and six varieties of quarks. The quarks are grouped into three sets of "twins": the up and down, the strange and charm, and the top and bottom. The quarks are held together by particles called gluons. For example, a proton is made of two up quarks and one down quark, bound together by gluons.

However, more precise information is needed to explain why this structure exists and to solve the matter-antimatter mystery. High-energy collisions at particle accelerators mimic conditions that existed in the early cosmos, yielding data critical to the research.

Source: Ian Shipsey, (765) 494-5390,

Writer: Emil Venere, (765) 494-4709,

Purdue News Service: (765) 494-2096;


Technician Tom Smith, Purdue physicist Ian Shipsey and mechanical engineer Kirk Arndt, (left to right) work on a silicon detector, which will become part of the CLEO III physics experiment operated at Cornell University, where electrons and their antimatter counterparts, positrons, are smashed together in an underground particle accelerator.
A publication-quality photograph is available at the News Service Web site and at the ftp site. Photo ID: Shipsey.quarks

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