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By Andy Fell

Researchers working at the Large Hadron Collider at CERN, Switzerland, announced earlier this month that they had found a new particle with the characteristics predicted for the long-sought Higgs boson. �������ϲ����Ͽ� Davis physicists have contributed to the Higgs search for more than 20 years and, with the new knowledge, are now exploring the next frontier in physics.

The Higgs boson is associated with the Higgs field, proposed in 1964, which gives mass to other particles in the universe.

"We're extremely excited," Professor Jack Gunion said. In 1990, he co-authored "The Higgs Hunter's Guide," and his theoretical work influenced the design of the detectors that were used to finally capture the fleeting particle.

Following the July 4 announcement, the next step will be to study the properties of the new particle and see what it can tell scientists about current theory — the Standard Model of particle physics — and test new theories, such as supersymmetry, that try to go deeper.

"It closes this chapter, but another one is just beginning," said Mike Mulhearn, assistant professor of physics.

The Higgs boson is the last particle predicted by the Standard Model of particle physics to be detected in a laboratory.

"Until now the Higgs has been a term in an equation that had to be extremely fine tuned for the theory to be stable," said John Conway, professor of physics. Now the properties of the actual particle can be determined.

Added Robin Erbacher, professor of physics: "The question is, is this a Higgs from the Standard Model or a particle that looks like a Higgs and decays in the same way.”

Building the Large Hadron Collider

In 1992, �������ϲ����Ͽ� Davis — led by Professor (now emeritus) Richard Lander and Winston Ko, now dean of Mathematical and Physical Sciences — was among the first four U.S. universities to sign up for the effort to build the Compact Muon Solenoid (CMS), one of the three particle detectors that are part of the Large Hadron Collider in Switzerland. The others were �������ϲ����Ͽ�LA, �������ϲ����Ͽ� Riverside and the University of Texas at Dallas.

Lander's team had developed a concept for a detector based on a high-field magnet for an experiment in Japan, and proposed such a detector for the Superconducting Supercollider, a massive particle accelerator that was to be built in Texas. When their proposal was turned down, Lander and Ko took it to the Large Hadron Collider consortium instead and some of those ideas went into the CMS.

The following year, Congress canceled the Superconducting Supercollider, leading many more U.S. universities to join the CMS project.

The CMS is made up of a layered series of detectors and steel yokes that filter out background noise. A team of about 3,000 scientists from 39 countries works on the CMS alone; the machine is 52 feet in diameter and weighs almost 13,800 tons.

"It had to be a very robust design, capable of handling a very large number of events," said Richard Breedon, a �������ϲ����Ͽ� Davis research physicist who has worked on the CMS since 1994.

Bump hunting

The collider smashes together two streams of protons, producing a spray of other fundamental particles that quickly decay into other particles. Because particles like the Higgs boson are very short-lived, they can only be detected by the characteristic wreckage left behind from their decay.

According to the theory Gunion helped develop, Higgs bosons can decay in several characteristic ways. The detectors at the collider are designed to look for these decay products.

The physicists working on the project are divided into teams that comb through the data from these detectors, before the final results are combined.

To use the parable of the blind men, it's as if one team is looking for a trunk, another for some big ears, a third for a short tail, and so on. Then they discuss whether what they have found could be an elephant.

What they are really looking for is a "bump" in a plot where the data moves a few points out of line, Conway said. But a small bump could also be just a random fluctuation, so the physicists have been trying to collect enough data to say that the chances of a bump being a random event instead of a real particle are very small — one in 3.5 million or less. John Smith, a �������ϲ����Ͽ� Davis research physicist, designed software that showed that background events could be screened out effectively.

Beyond the Standard Model

"What we're seeing might not be a Standard Model Higgs boson, but a Higgs boson from some beyond-the-Standard Model theory of physics," Conway said.

While the Standard Model has worked extremely well, it does require a lot of "fine tuning," comparable to seeing a pencil balanced on its point. Physicists hope they will now be able to see what is holding the pencil in place.

There are also the mysteries of "dark matter," an invisible material that makes up about a quarter of the universe, and "dark energy," which is causing the universe to expand at an accelerating rate.

The new Higgs particle and other experiments at the collider in Switzerland will help physicists test theories such as supersymmetry, which calls for a series of "super" particles that are partners to the Standard Model particles. At about 125 giga electron-volts (GeV), the candidate Higgs boson is at the size predicted by supersymmetry theory, Conway said.

The next piece of evidence would be to find the supersymmetric partner of a particle called the top quark, or "Stop quark," he said.

"It's an elusive target but it should be in reach of the LHC," Conway said.

The collider will continue its run at 8 tera electron-volts (TeV) until December, then shut down for about 18 months while it is upgraded to run at 14 TeV, Breedon said.

If supersymmetry turns out not to be true, physicists will have to turn to other ideas.  It would also rule out some models for dark matter, Mulhearn said.

"What if we don't see supersymmetry and the Higgs is not the Standard Model Higgs?" Gunion asked. "That will be a huge debate."

As Conway put it, “The work is just beginning.”

PARTICLE PARTICIPANTS

�������ϲ����Ͽ� Davis faculty and researchers who have worked at or with the Large Hadron Collider:

• Richard Breedon, research physicist
• Manuel Calderon de la Barca, professor
• Michael Case, software engineer
• Maxwell Chertok, professor
• John Conway, professor
• Timothy Cox, project physicist
• Robin Erbacher, professor
• Britt Holbrook, senior engineer
• Winston Ko, professor and dean
• Richard Lander, professor emeritus
• Michael Mulhearn, assistant professor
• David Pellett, professor emeritus
• John Smith, research physicist
• John Thomson, machine shop manager
• Mani Tripathi, professor

Many graduate students and postdocs have also worked on the project over the years. Among them: Fengcheng Li, now president of Teralane Semiconductor in Shenzhen, China; and Jeff Rowe, a researcher in the �������ϲ����Ͽ� Davis Computer Security Laboratory, whose doctoral theses focused on the design of the Compact Muon Solenoid.