Fermilab TodayWednesday, November 9, 2005  
Crash Course in Luminosity, Key to Discovery at Fermilab
Higgs Predictions
The CDF and DZero experiments might have a chance to find evidence for a Higgs boson, which is quite massive and therefore requires a great deal of energy to produce. As the Tevatron integrated luminosity Lint increases, the experimental energy limits (colored lines) will move closer to the predictions based on current theory (black lines). (Click on image for larger version.)
Why do we care about increasing our chances for collisions?
At the Tevatron, collisions create exotic new particles that may help physicists discover the universe's best-kept secrets. When a proton and antiproton collide almost at the speed of light, they create an enormous burst of energy. Following Einstein's famous equation, E=mc2, some of this energy can convert to brand new particles. "Imagine a car crash," said Steve Holmes, Associate Director for Accelerators. "Two Minis run into each other and, instead of a fender rattling to the pavement, a 1997 Hummer pops out." The freshly-made mass in the Tevatron can take the form of matter that belongs in another era. Some of these particles may give us clues about the evolution of our universe.

"One could almost say that we are bringing extinct states of matter back into existence," said Roger Dixon, who manages the Accelerator Division. Billions of years ago, the universe was denser, hotter and packed with energy, so the particles made in high-energy colliders may have been as common and natural then as carbon is today. Physicists can study the tracks left by these fleeting particles in the Tevatron detectors to learn about the primitive environment they once thrived in. The more energy created by the proton-antiproton collisions, the more ancient the resulting matter. "The energy in the Tevatron is two TeV," said Pushpa Bhat, who manages the run II upgrades of the Tevatron. "That matches the universe's energy one picosecond after the big bang."

So close to the birth of our universe, a million-millionth of a second after the big bang, there was far too much energy for the matter we are familiar with. "We're looking back to a time when only simple things existed, like quarks and gluons," said Dixon. The quarks hadn't yet cooled and condensed to form the protons and neutrons that, together with electrons, are the building blocks of life, stars and planets. Studying quarks, gluons and other elementary particles resurrected from this primordial soup may help to answer huge questions. We might explain mass (rather than just describing its effect), find out what dark matter is made of, find extra dimensions, or pinpoint the single, original source of all known matter and forces.

"It's much more than just looking back in time," said Chris Quigg, a theorist in Fermilab's Particle Physics Division. Quigg says that it may be possible to discover "supersymmetric" particles in the Tevatron. If supersymmetric particles materialize according to predictions, they may reveal new physical laws that radically change our understanding of the universe. "There is a sense in which the discovery of supersymmetry is like the discovery of imaginary numbers in math," said Quigg. "Even though we could go through our lives without ever knowing they exist, we'd be missing the answer to all kinds of problems without them." Among these problems is the mystery of dark energy in our universe and the details of electroweak forces, which explain the existence of atoms, chemistry and solid objects.

But in order to find the really exciting particles, the Tevatron needs to maximize luminosity. Since protons and antiprotons are made of distinct pieces, each collsion can produce a different amount energy depending on which pieces happen to hit each other first. "It's hard to say what we'll find until it happens," said Holmes. "But the more collisions we produce, the better chance we have of finding something rare." Something like the top quark, which is 100 times more massive than a proton, arises about once in 10 billion collisions. And for something even more massive than the top quark, like the yet to be found Higgs boson, the chance may be less than one in a trillion. Those are tough odds, but with several million collisions per second, the recent rise in integrated luminosity gives Fermilab physicists a much greater chance to unearth something rare. "We don't know exactly what we will find," said Quigg. "But it is possible that we will find something that changes our perception of the universe for good."

-Siri Steiner

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