Run II Science: The Hunt is On

by Mike Perricone

Chris Tully made his first visit to Fermilab in 1988 as a high school student, representing the state of Virginia in the U.S. Department of Energy's national high school honors program. He learned to string wires for the muon chambers at the DZero detector.

He made his second visit on December 13, 2000 as a Fermilab Colloquium presenter, Princeton University physicist and CERN experimenter. This time, he was reporting on the "tantalizing hints" for the Higgs mechanism that had shown up at CERN's Large Electron Positron Collider, before the November 8 shutdown. This time, he was representing the worldwide state of anticipation over the beginning of Collider Run II of the Tevatron--and the search for the origin of mass.

"For eager Higgs hunters," Tully said, "the immediate focus will be on the Run II results from Fermilab as the next possible source for direct evidence for the Higgs mechanism. Now that evidence suggests a low mass Higgs, it might mean that Fermilab is in exactly the right place to observe a wealth of new physics."

The whole world is watching, and the Higgs is far from the only attraction as Fermilab opens Collider Run II of the Tevatron. In fact, Higgs candidates might not make an appearance for quite some time. CDF co-spokesperson Franco Bedeschi estimates that five years of Run II would produce about 3,000 Higgs candidates (out of 5x1014 proton-antiproton collisions) in the mass range of 115 GeV/c2 predicted by LEP results and other data.

So what else is new? Almost everything: new particles, new dimensions, new top quark measurements and production channels, new CP violation results in B physics. New physics. New excitement. News.

"There is the possibility of something definitive early on," said theorist Chris Hill. "For example, it's possible we will uncover a new layer of physics with new strong dynamics. That could show up in the first inverse femtobarn." (See box: The broad side of a femtobarn.)

Right near the top of the Run II wish list is the top quark. Discovered at the Tevatron in 1995, the top is due for a step up in precision and a new production mode. Called single top production, the process starts with an up quark annihilating against a down quark (within the Tevatron's proton-antiproton collisions). Out pops a "virtual" W boson, which quickly decays into a top and an antibottom.

"Single top production has never been observed before," Hill said. He called it a "new window into the top," allowing views of how it couples to the W boson. It also provides tests of the Standard Model and background for Higgs production."

Precision measurements of the top mass (down to around ±0.6 GeV) and the W (±20 MeV) also serve as constraints on the Higgs mass, Hill said.

"These precision electroweak tests use the top mass and the W mass in combination with other measurements to predict the Higgs mass," he continued. "You then have the potential to define precisely where the Higgs ought to be, and check it with a discovery."

In Run I, Fermilab produced the grand total of 150 top quarks. Run II, however, will yield thousands.The top is also a route into super-symmetry, the theory that all Standard Model particles have "superpartners." But it's a route with a twist.

"It seems to work in reverse," Hill explained. "Because the top is heavy, many people expect its superpartner [the `stop'] to be light. The production of `stop' and `antistop' are possibilities, although the decay modes are very model-dependent: you have to determine what they're decaying into. There are many possible channels, but `stop' production is something people might expect in Run II."

It won't stop there. New physics goes from top to bottom.

Fermilab discovered the bottom quark in 1977. The accelerating field of B physics measures the behavior of particles containing bottom quarks, known as B mesons. The decays of B mesons and their antimatter counterparts (anti-B mesons) produce subtle differences that could go a long way toward explaining the universe's preferential treatment of matter of antimatter, leading to Life As We Know It.

Here, the key quantity differentiating the decays is sin2b, and the goal is measuring that quantity as accurately as possible. Fermilab's CDF collaboration set a new standard in sin2b measurement with data from Collider Run I, establishing a value of 0.79±0.4 which is consistent with Standard Model predictions of a large positive CP-violating asymmetry in this decay mode. In other words, a big gap between the behavior of matter and antimatter.

Then along came BABAR, the electron-positron collider at Stanford Linear Accelerator Center. BABAR raised the bar with its recently-announced sin2b measurement of 0.34 ± 0.20, "which is about twice as accurate as previously published values," as stated in the paper submitted to Physical Review Letters.

"We measured the sin2b CP violation parameter with an error of about 0.40, compared with the BABAR error of 0.21," said CDF's Bedeschi. "We are aiming for the Summer of 2002 to match the BABAR resolution, but it's an aggressive goal. In the meantime BABAR will be taking even more data, so they will still be ahead. With a good luminosity profile over the next few years, however, we should in principle surpass them."

"CDF will be very competitive with BABAR," Hill added. "That's borne out by recent BABAR and BELLE measurements with errors on them comparable to what CDF had a couple of years ago. So when CDF is back up to speed, they'll be well able to address CP Violation in the B system."

Fermilab has a long history of offering up something extra, and extra dimensions may be a bonus for Run II.

"These are the `K-K' or Kalusza-Klein modes," Hill explained. "These are carbon copies of Standard Model particles but at much higher masses. For example, a KK mode of the gluon would be a heavy gluon, and the mass of the heavy gluon would measure the size of the extra dimension. Personally, I think extra dimensions are beyond the reach of the Tevatron, but hope springs eternal."

And hope springs from luminosity, the number of collisions the Tevatron can produce over the course of its run to light up the field with new discoveriesóHiggs and otherwise.

"The big question is can we get the integrated luminosity," said veteran CDF experimenter Henry Frisch of the University of Chicago. "If we make enough Higgs candidates, the newly-upgraded detectors will definitely be capable of seeing them."

Making enough candidates applies to the entire range of Run II science. That puts the focus on Fermilab's Beams Division, performing the intricate tasks of creating antiprotons, "cooling" them into intense beams, and colliding them with proton beams. As Frisch pointed out, the Beams Division has a long and distinguished history of exceeding its goals.

For example, the original design goal for the Tevatron collider luminosity was 1030 cm-2/sec, which corresponds to about 50,000 collisions per second witnessed at each detector. The collisions are inelastic collisions, violent collisions that break up both the proton and the antiproton, sending lots of stuff flying all over the place.

The Beams Division took that goal and exceeded it by a factor of about 16. They got to 1.6x1031, which corresponds to inelastic collisions occurring at a rate of about 800,000 per second.

"Now we're talking on the order of 10 to 20 times that numberóas many as 10 to 15 million collisions per second," Frisch said.

Luminosity holds the key to discoveriesóspecifically, integrated luminosity, or the number of total collisions over the course of the run. Frisch explained that the Higgs has an extremely small cross-sectionóphysics-speak for the probability that a proton would actually make a Higgs particle, or any specific particle under investigation. The equation in question is simple:

(luminosity) x (cross section) = collision rate, or number of events per second

As Frisch pointed out, a small cross section requires lots of luminosity to produce a significant number of observable events.

"The people in the Beams Division have always had wonderful ideas to get the luminosity up," Frisch said. "We're not yet running up against a `brick-wall' limit set by physical law. Clever ideas, new techniques and a lot of hard work may well get us what we need."

All this against the background of pushing forward with the neutrino experiments, MINOS and MiniBooNE; and of the lab's continuing support effort for the LHC and the Compact Muon Solenoid at CERN.

"Looking from the outside," Chris Tully said, perhaps wistfully, "the prospects for Run II at Fermilab are very promising if new physics is sitting just beyond what LEP was able to explore."

Just beyond LEP, and just within reach of the Tevatron? The whole world is watching.