Particle Detectives


MEN IN WHITE: Surrounded by testing and fabrication equipment in the SCIPP laboratory are (clockwise from lower left) Abraham Seiden, professor of physics and director of SCIPP, Bruce Schumm, assistant professor of physics, Alex Grillo, research physicist, Hartmut Sadrozinski, adjunct professor of physics, Robert Johnson, associate professor of physics, and David Dorfan, professor of physics.

Photo: R. R. Jones

UCSC physicists are building supersensitive high-tech tools to track quarks, mesons, and other denizens of the subatomic world

   When Queen Victoria asked the great English experimentalist Michael Faraday what good was electricity, he replied (or so the story goes), "Madam, what good is a baby?"

Faraday had been able to induce a trickle of electric current, but at that time there were no practical uses for it and its eventual applications could not even be imagined.

The assumption behind all basic scientific research is that there is inherent value in understanding how nature works, even though the practical applications cannot be known in advance. Furthermore, increasing our knowledge of the world around us has aesthetic and intellectual value, regardless of any practical spinoffs.

At the Santa Cruz Institute for Particle Physics (SCIPP), UCSC researchers are trying to answer questions that most people wouldn't even think to ask: Why are we made out of matter and not antimatter? What gives matter mass? How does gravity work?

Particle physicists grapple with these cosmic mysteries by peering into the subatomic world of quarks, mesons, and other esoteric particles. Ultimately, they hope to grasp the fundamental forces that govern the physical world and to explain how the universe evolved from the Big Bang, about 15 billion years ago, into what we observe today.

"We have a 'standard model' explanation for things such as how mass is generated, and our experiments are designed to test that model," says physicist Abraham Seiden, director of SCIPP. "But the results could be totally different from what we expect to see, so the experiments are really exploratory in terms of figuring out how things work."

While they are not trying to cure cancer or build a faster computer, physicists can certainly take credit for many advances in those areas. Medical imaging technologies (X-rays, CT, MRI), lasers, transistors (the basis for the computer industry), and even the World Wide Web are among the by-products of the pioneering work of physicists.

"The technology we are developing in the lab makes use of the latest developments in industry," Seiden says. "In turn, our work generates spinoffs that have practical value to industry as well as to other fields of research."

The driving force behind research in particle physics, however, is primarily an insatiable curiosity about the nature of the universe.

   In a typical high-energy physics experiment, a particle accelerator boosts beams of subatomic particles to near the speed of light and brings them together in a head-on collision, creating a burst of energy and a shower of new particles.

The most interesting particles produced from these collisions tend to be very short-lived, rapidly decaying into other particles. The challenge for researchers is to work backwards from the evidence recorded by sensitive detectors to reconstruct what kinds of particles were produced in the collision and how they subsequently decayed.

These experiments involve international collaborations among hundreds of investigators and are performed inside only a handful of major accelerator facilities. Within these huge projects, SCIPP's team of about 30 physicists has carved a niche as a leading developer of special detectors and electronics used to track subatomic particles.

Made from silicon, the ubiquitous semiconductor of the computer age, SCIPP's detectors are fabricated using techniques perfected in the integrated-circuit industry.

   Scipp scientists developed the first silicon-based detector used in a colliding-beam experiment. That experiment was conducted at the Stanford Linear Accelerator Center (SLAC) in Palo Alto in the mid-1980s, and SCIPP teams continue to be an integral part of research at SLAC.

NASA, meanwhile, has SCIPP researchers working on a space-based telescope to be launched within the next decade (see next page). SCIPP detectors mounted on the Gamma-ray Large Area Space Telescope will record the direction of gamma rays emitted by neutron stars, black holes, pulsars, supernova remnants, and other intriguing astro-physical sources.

In addition, SCIPP physicists, engineers, and technicians have been tapped to develop detectors for a particle accelerator at the European Particle Physics Laboratory (known by its French initials, CERN) in Geneva. When CERN's Large Hadron Collider (LHC) begins operation in 2005, it will rank as the most powerful particle accelerator in the world.

Inside the LHC, two beams of protons speeding in opposite directions through a tunnel 16 miles in circumference will cross paths, producing as many as a billion collisions each second between protons moving at close to the speed of light. One of the LHC's main detectors--ATLAS (A Toroidal LHC Apparatus)--will sort through the debris from those collisions and record the data of interest to researchers.

As its name implies, ATLAS will be huge: the size of a five-story building. And the effort to build the detector is of the same scale: ATLAS is being assembled by 1,700 collaborators from 144 institutions around the world.

"There is plenty of room for different people to make contributions," Seiden says. "We're involved in the inner tracking of particles as they emerge from the site of the collisions."

At the heart of the ATLAS detector, built around the point where the proton beams collide, will be a cylindrical array of SCIPP's silicon strip detectors, sporting a total of about 10 million readout channels. These detectors will track the paths and measure the momentum of charged particles. Additional layers of detectors will record other properties of particles or identify particular types of particles.

Silicon strip detectors can measure the position of a particle to within 10 microns (one hundredth of a millimeter) or less. This represents a major advance over detectors used in earlier experiments, which were 10 to 20 times less sensitive.

Made out of thin wafers of silicon, the detectors have electrodes finely etched into the surface in parallel strips. When a charged particle passes through the silicon, it generates an electrical signal in the nearest electrode. Mounted on each silicon wafer are electronic chips that read the signals.

"It's like having a small computer on each detector module to process information," Seiden says.

Some of the collisions inside the LHC will create conditions comparable to those that prevailed in the universe just one trillionth of a second after the Big Bang, when the temperature was 10 quadrillion degrees Celsius. Although these conditions will only exist within a space about the size of a proton, physicists will be able to study how matter behaves at such high energies. Particles that researchers could never observe under ordinary conditions may be produced in these high-energy collisions.

A primary goal of the ATLAS project, for example, is to detect the Higgs boson, a particle physicists believe endows matter with mass. The LHC may also enable physicists to detect the particles responsible for so-called "dark matter," the existence of which can only be inferred from its gravitational effects on the structure of the universe.

Part of the excitement in these experiments lies in their unpredictability. The results may confirm the standard model, or they could turn the whole field of particle physics on its head and send theorists scrambling for an explanation. Either possibility holds the promise of a new and deeper understanding of the universe in which we live.

--Tim Stephens

Gamma-ray Large Area Space Telescope

<-- The telescope, topped by 25 tower modules


<-- Gamma ray


<-- Detector panel, comprised of SCIPP's silicon strip detectors


<-- Tower module, with multiple layers of panels


Illustration elements courtesy Terry Anderson / Stanford linear accelerator center.


SCIPP DETECTORS play a major role in a number of high-energy physics projects, including the planned Gamma-ray Large Area Space Telescope (diagram, above) and the ATLAS detector at the European Particle Physics Laboratory in Geneva, Switzerland (diagram, below).

In the photo, SCIPP's Robert Johnson holds a silicon strip detector (right) and a detector panel that will be housed in the space-based telescope.

Photo: R. R. Jones

ATLAS detector, European Particle Physics Laboratory

SCIPP detectors will be located in the heart of the five-story-high ATLAS project


Graphic courtesy Lawrence Berkeley National Laboratory

Babar Meets The Mesons

The fact that the universe is full of matter and not antimatter is a puzzling observation still unexplained by modern physics. How this relates to BaBar the little elephant is a story in itself.

Researchers at the Santa Cruz Institute for Particle Physics (SCIPP) will participate in an experiment this spring that may help resolve the antimatter puzzle by creating and tracking the decay of ephemeral particles called B mesons. Produced along with each B meson is an anti-B meson, the shorthand notation for which is the letter "B" with a bar over it, pronounced "bee-bar."

That's close enough for the researchers to name the detector that will track these particles "BaBar." They even obtained permission from children's book author Laurent de Brunhoff to use the image of the little elephant as a sort of logo for the detector project.

SCIPP researchers are designing a critical component of the BaBar detector.

"Antimatter is just like ordinary matter except that the charges are reversed, so an antielectron, which we call a positron, has the same mass and energy as an electron but has a positive charge," explains Abraham Seiden, director of SCIPP.

The existence of antimatter was predicted by quantum theory and later confirmed in experiments. Scientists now know that each of the subatomic particles that make up matter has a corresponding antiparticle. In addition, one of the key properties of particles and anti-particles is that when they collide they annihilate each other, converting their entire mass into energy.

So why did the Big Bang, which should have produced equal amounts of particles and antiparticles, result in a universe full of matter?

In the BaBar experiment, which will be conducted at the Stanford Linear Accelerator Center, researchers hope to observe in B and anti-B mesons a violation of symmetry that may explain how matter came to dominate the universe.

--Tim Stephens

Babar™ © L. De Brunhoff

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