Rice physicists go deep for ‘dark matter’

Rice physicists go deep for ‘dark matter’
Search for elusive particles takes scientists to Geneva, Chicago and Rome

BY JADE BOYD
Rice News Staff

From subterranean caverns in Europe to the plains of Illinois, Rice physicists traveled the Earth this summer in very different quests to answer the same fundamental question: What’s the unseen cosmic glue that keeps galaxies from flying apart?

COURTESY PHOTO

Paul Padley is pictured at the Large Hadron Collider at the European Organization for Nuclear Research, also known as CERN.

Their search is taking place in far-flung labs across the world, including the world’s most powerful atom smasher in Batavia, Ill.; its heir-apparent in the Swiss Alps; and an Italian national laboratory that’s buried under 9,000 feet of granite.

Though the tools and settings for the experiments vary, the goal is shared: to go beyond the “standard model,” the theory that describes the forces — except for gravity — that account for the interactions between all known elementary particles of matter. “Known” is the key word, because while the standard model is the crowning achievement of modern physics, it accounts for only 4 percent of the known mass in the universe.

Dark matter

Dark matter and dark energy account for the unseen 96 percent of the universe. This 96 percent is said to be “dark” because scientists have yet to observe it. But a number of Rice physicists — including Paul Padley, Jabus Roberts, Marjorie Corcoran and Uwe Oberlack — are working on experiments that could reveal the first direct evidence for dark matter and the forces that created it.

So if dark matter has never been seen, how do physicists know it’s there? Because a lot of gas clouds orbiting distant galaxies, including the Milky Way galaxy, are moving way too fast for dark matter not to exist. In fact, these gas clouds are moving so fast they should be flying away from their host galaxies. Something is holding them in orbit, something “dark” that cannot be seen with telescopes.

Big science’s biggest

PAUL PADLEY

One place physicists hope to find clues about dark matter is the Large Hadron Collider, or LHC, an awe-inspiring $2.5 billion European particle accelerator that is in its final phase of construction. Built by the European Organization for Nuclear Research (CERN), the LHC is poised to become the world’s most powerful particle accelerator when it begins collecting data next year.

For the past decade, Rice’s Paul Padley, associate professor of physics and astronomy, and Jabus Roberts, professor of physics and astronomy, have been extensively involved in the design and construction of the muon detector in the giant Compact Muon Solenoid at the LHC. Padley’s been chosen to lead the scientific operations of a piece of the detector built by U.S. institutions, the $40 million Endcap Muon System. Roberts has spent four months each summer for several years at CERN working with students and engineers installing the experiment. The muon system is just about installed and ready to see the first beam from the LHC next year.

“This is the largest science experiment in history,” Padley said. “More than 2,000 scientists from 34 countries are at the LHC. The fact that Rice has been tapped to lead portions of the project, and the fact that Rice students — both undergraduate and graduate — have been invited to help with construction, are an indication of the superb reputation that Rice has in the physics community.”

Housed in a sprawling 27-kilometer ring of subterranean tunnels on the border between France and Switzerland, the LHC, once finished, will smash together beams of protons traveling near the speed of light to recreate the high-energy conditions that existed during the universe’s infancy.

Smashing atoms

JABUS ROBRTS

High-energy particle colliders like the LHC demonstrate in spectacular fashion the relationship between mass and energy that was summed up in the famous Einstein equation E=mc2. When fast-moving particles smash headlong into one another, a significant portion of their combined energy is converted instantly into mass. The faster and more massive the colliding particles, the heftier the particles created in the smashup.

Supermassive particles made by the collisions are unstable. In nature, they previously existed in the dense, hot soup of energy and matter right after the Big Bang. Because they are unstable, the particles decay quickly, settling into the protons, neutrons and electrons that make up everything seen in the world. But as they cool they decay, spewing out a stream of high-energy byproducts that give telltale signs of their existence. These byproducts are the scientific data that’s collected in particle accelerators, and it’s where scientists at LHC and other particle colliders hope to find telltale signs of massive new particles or other new phenomena that can explain the nature of dark matter.

The reigning king

One place physicists are looking for telltale signs of dark matter or other new phenomena is the Tevatron particle accelerator at Fermi National Accelerator Laboratory (Fermilab) near Chicago.

The Tevatron smashes opposing beams of protons and antiprotons in collisions that weigh in with energies around 1 trillion electron volts. Scaled up, the energy density of these collisions would be roughly akin to packing the energy of 10,000 Hiroshima-sized A-bombs into a space the size of a fingertip. The LHC is expected to generate about 14 times that much power, but until it goes online, the Tevatron is the reigning king of particle colliders.

MARJORIE CORCORAN

Rice’s Marjorie Corcoran, professor of physics and astronomy, began a yearlong sabbatical at Fermilab in July. She’ll spend much of her year helping to run the D0 experiment.. Even for Corcoran, who’s been making pilgrimages to Fermilab for more than 15 years, sitting at the controls of such an experiment is never old hat.

“It looks a lot like Mission Control at NASA,” Corcoran said. “There are four experimenters on duty around the clock, seven days a week in the D0 control room.”

D0, pronounced “D-zero,” is one of the major experiments currently under way at the Tevatron. D0 and the Tevatron’s other major experiment, CDF, are best known for the 1995 codiscovery of the “top” quark, the most massive of the six quarks predicted by the standard model.

Quark combos

Quarks are fermions, and the lightest of the six varieties — the “up” and ‘”own'”quarks — account for all of the matter humans can see. The second generation of quarks — dubbed “strange” and “charm” — are heavy and unstable, existing only briefly inside particle colliders. So too for the third generation, the “bottom” and “top” quarks, which are even heavier and more unstable.

Though physicists have been smashing things together in particle colliders for more than 60 years, Corcoran said there is still far more that physicists can discover with tools like the Tevatron. For example, D0’s latest find, the “triple-scoop baryon,” is the first observed particle that contains a quark from each of the three generations, she said.

“There are 600 physicists looking at D0 data,” Corcoran said. “We have groups looking for evidence of supersymmetry, a leading candidate for the dark matter observed by astronomers. We are studying the bottom quark and the strong and electroweak forces. We also have a strong effort searching for evidence of the Higgs boson, the last remaining unobserved particle of the standard model. We have no idea which of these efforts will pan out, but we actually have more physics topics to explore than we have physicists.”

Trapping WIMPs

Beneath a mountain of solid granite two hours west of Rome, Rice’s Uwe Oberlack, the William V. Vietti Assistant Professor of Physics and Astronomy, is working with an international team that’s looking for “weakly interacting massive particles,” or WIMPs, dark matter particles predicted by supersymmetry.

UWE OBERLACK

“Some explanations for the missing mass in the universe — like lots of massive planet-sized objects in the galaxy, have been ruled out,” Oberlack said. “The most promising theory holds that the mass is made up of WIMPs.”

Theoretically, WIMPs interact so weakly with ordinary matter that they can be seen only with carefully designed experiments that filter out all of the background interactions between known particles.

Oberlack and his XENON Experiment collaborators are racing to become the first to measure WIMPs. XENON took the lead in the race in April when it reported its first results; the team’s initial search found no confirmed WIMPs, despite searching with five times the sensitivity of previously used instruments.

Housed at Gran Sasso National Laboratory beneath central Italy’s tallest mountain, XENON uses a time-projection chamber — a copper vessel containing about 15 kilograms of super-cooled liquid and gaseous xenon. The chamber is designed to act like a 3-D camera, capturing images of subatomic particles in flight. The goal is to measure the collision of a single WIMP with a nucleus of xenon inside the chamber. The granite of the mountain filters out false signals that are created by some cosmic rays. A shield of lead and high-density polyethylene further reduces background signals. To insure low internal radioactivity, the innermost layer of lead was taken from ancient Roman ships that rested on the seafloor for centuries.

“Ultimately, we hope to find direct evidence for WIMPs, but our work is already proving useful,” Oberlack said. “The measurements we’ve already completed have lowered the upper limit of the possible mass of WIMPs, a finding that has important theoretical implications, not only for our work but also for some of the things they’ll be looking for at the LHC. In that way, our systems are very complementary.”

Oberlack said the NSF has approved a plan to replace the current XENON detector with an improved and larger upgrade, called XENON100. This second phase of the project will push the sensitivity limit by another order of magnitude by the end of 2008, just in time for the first scientific results from LHC.