The Johns Hopkins Gazette: January 26, 1998

Jan. 26, 1998
VOL. 27, NO. 19


Physicists Attack The Ultimate Puzzle

Hopkins scientists at the center of search for entities that might account for the material world

Emil Venere
News and Information

Johns Hopkins Gazette Online Edition

A worldwide scientific effort is under way to build a powerful machine aimed at answering a fundamental question: Why does matter have substance?

In essence, current theories simply cannot explain why things in the material world exist. But a series of recent triumphs in the field of high-energy particle physics have brought science closer to understanding the forces and composition of the universe.

One possible solution to the puzzle may be the presence of Higgs bosons, hypothetical subatomic particles unlike any others found so far.

A team of Johns Hopkins scientists is at the center of an unprecedented international effort to search for Higgs particles, or any other entities that might account for the material world.

The physicists are designing and building one of the most important pieces of a gargantuan particle accelerator called the Large Hadron Collider, at the European Laboratory for Particle Physics, known as CERN, near Geneva, Switzerland. Close to 4,000 scientists and engineers from 45 nations in six continents are working on LHC.

"It's truly remarkable," said Chih-Yung Chien, a professor in the Department of Physics and Astronomy. "We have all kinds of nations that used to be enemies working together."

Professor Chih-Yung Chien is leading the team of Hopkins scientists working on the gargantuan accelerator that will search for Higgs particles.

On Dec. 8, 1997, officials from Europe and the United States signed an agreement for American physicists to participate in the LHC. Under the agreement, the United States will contribute about $531 million toward the project, roughly 10 percent of the LHC's total cost of nearly $6 billion.

Chien is working with Hopkins physicists Bruce Barnett, David Gerdes and Aihud Pevsner to design a device called the forward pixel detector, which will be critical to the success of LHC.

The accelerator is scheduled to become operational in 2005.

"It will probably be the largest and most complex piece of scientific equipment ever built," said Gerdes, an assistant professor in the Physics and Astronomy Department.

"It will probably be the largest and most complex piece of scientific equipment ever built," says Assistant Professor David Gerdes of the accelerator.

The LHC will be an ideal tool with which to prove the existence of Higgs particles.

Beams of protons will speed in opposite directions around a subterranean pipe 27 kilometers, or 16 miles, in circumference. The protons, accelerated and guided by powerful magnets, will smash together in head-on collisions that will split matter into its most basic components. Tracks left by the particles will be recorded on a complex nest of detectors.

The forward pixel detector will be the closest thing to the point of impact.

"We picked the innermost detector" to design, said Chien, who is leading the team of Hopkins scientists.

The detector will be a mere 2 inches away from the proton collisions, which will repeat 100 million times every second, bombarding the equipment with heavy doses of radiation. Not only must the detector be able to withstand the radiation, it will have to transmit data ultrafast in order to keep up with the rapid barrage of collisions. And, on top of all that, its signals must not be affected by the substantial magnetic forces generated inside the accelerator.

Hopkins scientists expect to finish a prototype detector within two years.

"It can be made," said Chien, who has been working at CERN since 1984. "The real challenge is to build it cheaply and on time."

Barnett, Gerdes and Hopkins physicist Barry Blumenfeld presently are working at today's most powerful particle collider, operated at the Fermi National Accelerator Laboratory, near Chicago.

Fermilab is being upgraded to make it possibly powerful enough to detect the Higgs particle. But LHC will be about seven times more powerful--meaning the energy in each collision is seven times higher--and the number of collisions per second will be up to 1,000 times greater.

"So, in one year of running, we will get roughly 1,000 times the radiation damage," compared to the damage inflicted by the Fermilab experiment, Barnett said.

Says Professor Bruce Barnett of the international endeavor, "Anytime you get a nation to look beyond its own borders, I think it's good for humanity."

He and Gerdes are on the Fermilab team that in 1994 discovered a subatomic particle called the top quark. The particle had been predicted by theorists and is believed to be one of the last fundamental building blocks of matter to be confirmed by science.

Proving the top quark's existence has provided crucial support for the Standard Model of physics, a widely accepted theory about the nature of matter and energy. The theory was originated in the 1960s and has steadily gained acceptance as physicists have found one after another of its tenets to be correct.

It states that all matter consists of elementary particles called leptons and quarks, with the six varieties of quarks grouped into three sets of "twins": the up and down, the strange and charm, the top and bottom.

Scientists have no reason to doubt the Standard Model because it has led to predictions that have been proved correct by experiments in recent years, said Jonathan Bagger, a theoretical physicist at Hopkins.

But there is one important problem with the Standard Model. At extremely high energy levels, such as those that would have existed at the birth of the universe, the model breaks down.

"Certainly, during the first moments after the Big Bang, this theory is wrong," Bagger said. "The mathematical predictions are nonsense."

Because particles have mass, the theory no longer makes sense at such high energies. Therefore, other particles or forces must exist. One possibility is that Higgs bosons endow the other particles with mass.

"Everyone has been looking for Higgs," Bagger said. "If Higgs exists, great. If it doesn't exist, it's even more important. That means something truly important is missing. That is, some very fundamental understanding of the universe is lacking."

Although Higgs particles alone could resolve the Standard Model's inconsistency, many theorists prefer a more elaborate solution, called supersymmetry.

The LHC is expected to reach energies at which the model breaks down. So theoretical physicists hope the accelerator will yield solid evidence for the reality of supersymmetric particles, Bagger said.

Physicists had hoped one day to use an advanced accelerator called the Superconducting Supercollider, which was to have been built in Texas, to hunt for the Higgs and supersymmetric particles. But budgetary concerns canceled that project several years ago.

Scientists in the United States realized that they had to do something; by 2005, they would no longer have the most powerful accelerator. Without the LHC agreement, the consequences could have been dire for American science.

"It would have been a blow for international collaboration of scientific experiments at all levels," Barnett said. "It's very significant. Anytime you get a nation to look beyond its own borders and cooperate more, I think that's good for humanity."

The SSC would have been three times as powerful as LHC, comfortably reaching the energies needed to produce reactions supporting the reality of the Higgs and supersymmetric particles, or whatever else might instead exist.

To make up for the LHC's lower energy, the proton-beam intensity--not the strength but the number of particles colliding per second--will have to be increased 10 times.

But cranking up the intensity proportionately increases the radiation levels and the sheer amount of data produced, making it more difficult to design reliable detectors.

One way to ensure the effectiveness of the detector is to convert its electrical signals into optical signals, which are faster and are not distorted by magnetic fields.

The Johns Hopkins scientists are working now to develop a laser device that converts electrical signals to light signals and transmits the information over a fiber-optic line. They are part of a CERN team involved in an experiment called the Compact Muon Solenoid, or CMS, which will compete against another CERN team working on an experiment called ATLAS, an acronym for a toroidal LHC apparatus.

The LHC work is a natural progression from the Hopkins research at Fermilab and CERN.

At Fermilab, Gerdes is heading a Hopkins group that is improving a device called the outer tracker, a key component in a huge tracking system used to identify top quarks; the device measures the momentum of subatomic particles created in the collisions.

Barnett and several students are improving a vital Fermilab device known as the silicon vertex detector. The pixel detector being developed for LHC is an outgrowth of the silicon vertex detector, Barnett said.

Upgrading the Fermilab equipment will enable scientists to explore mysteries surrounding the top quark and the other particles. For example, physicists do not know why the various particles have such a large range of masses; the top quark is 35,000 times as massive as the up quark.

"Basically, the moral of the story is that theorists don't know why yet," Bagger said. "We have no idea."

The Fermilab improvements might even make the accelerator powerful enough for scientists to detect the telltale signature of the Higgs particle.

"Fermilab is poaching on the territory where the theory is inconsistent," Bagger said. "It's not really there yet. Imagine a big space of all the possible things that could happen. The upgraded Fermilab accelerator is carving out a small corner of this space, and if what nature chooses was in this corner, we'll be very lucky. The LHC will probe the whole space."

Hopkins' involvement in the LHC will provide a practical benefit to the university as well.

"It will help us recruit and keep top-notch young scientists," Chien said.

Meanwhile, Bagger is serving on a panel reviewing how the United States should position itself in the future of particle physics. The LHC may have a lifetime of 20 years, after which it will be superseded by the next generation of colliders. American physicists will strive to be major players in future experiments, but it is not easy to predict what types of experiments will be best suited to the science of 2025.

"You can see that there is some danger that the U.S. is going to become a backwater," Bagger said. "The question is, what should the U.S. be doing now to be ready to actually propose something in the early years of the next decade?"