Lepton Photon 2003: A Week of Interactions

by Mike Perricone

Each day, they filled Wilson Hall's Ramsey Auditorium—some 800 representatives of long careers and future hopes in the global community of high-energy physics.

What they heard and saw at the 21st Lepton-Photon Symposium ranged from confirming results in top quark physics to conflicting results in B-physics; from the structure of hadrons to the structure of world-wide distributive computing; from SUSY to QCD (supersymmetry to quantum chromodynamics); from collider searches for exotic particles to explorations of the cosmic microwave background; from solar and accelerator neutrino experiments to dark matter and dark energy—all from early morning until well into the evening, from Monday, Aug. 11 until Saturday, Aug. 16.

"Particle physics is connecting with astrophysics," said Mikhail Danilov, director of the Institute for Theoretical and Experimental Physics (ITEP) in Russia. "What happens at the smallest distances affects what we see at the largest scales. We are becoming one science."

Still, the big picture is the sum of its details.

"[High-energy physics] is a very large and broad palette," said Jonathan Dorfan, director of Stanford Linear Accelerator Center (SLAC) in California. "But we still need those single ideas that transcend."

The B Factor
Some of those ideas might someday trace their origins back to this conference. If conflicting results are an indication, the discrepancies that have appeared in B-physics results might generate one of those ideas. One theorist has already said that he "cannot sleep" because of the differences reported out of the "B-factories:" the BELLE experiment at KEK, the High-Energy Accelerator Research Organization in Japan; and the BABAR experiment, at SLAC.

The experiments have reported differences in one of the decay modes of B-mesons, which are quark-antiquark pairs containing the bottom quark or its antimatter particle. BELLE has reported a decay path from the B-meson to a particle called phi K (short) that is larger than expected and with opposite sign from what is expected—presenting statistical differences from BABAR results. The BELLE results would contradict the Standard Model, the theoretical framework of particle physics for more than 30 years; the BABAR results are in agreement with the Standard Model.

"I'd say that's becoming the most interesting result in the field of B physics right now," said Nigel Lockyer of the University of Pennsylvania, who has a vested interest in the debate.

The BABAR and BELLE experiments have reported different results in the decay path of the B-meson into particles called pi (+) and pi (-), a situation that Lockyer believes the CDF experiment at Fermilab can address over the next year. Lockyer, co-spokesperson for the CDF collaboration, says the new secondary vertex trigger at CDF offers significant experimental reach in this area of B-physics—including additional important branching ratio information from Bs to k(+) and k(-), an area accessible only to the Tevatron. Together, the results from the B factories and CDF will provide new insights into CP violation this coming year.

"We have a whole program in B-physics that's unique to the Tevatron," Lockyer said. "It's been made possible by the SVT, which was developed by Luciano Ristori (of CDF and Italy's INFN-Pisa). With the SVT, we trigger on B-mesons, which has allowed us to find states that have never been seen before in the B-system. It will allow us to participate in the CP violation studies that the B-factories are so famous for now... "We will hopefully have enough data, and then we must analyze it. Of course, it depends on how well we do with that data."

Already, CDF and the Tevatron have established some of the best measurements in B-physics, with a prime goal of measuring mixing in the Bs sector.

"We're beginning to exploit high yields and upgraded detectors," said Kevin Pitts of the University of Illinois. "There's lots of work to do...this is a marathon, not a sprint."

Tevatron shows its reach
But either type of race begins with a single step, and results from the CDF and DZero experiments that showed up at Lepton-Photon demonstrate a significant step forward in the early days of Run II of the Tevatron. Since the onset of serious physics data just over a year ago, the upgraded Tevatron has generated more data to its significantly revamped detectors than was generated during all of Run I from 1992 to 1996.

Among the early results: DZero has established the most sensitive limit to date in the search for large extra dimensions, surpassing limits set during Run I, and by the Large Electron-Positron (LEP) collider at CERN. DZero has also reported the first results from its new silicon detector close to the interaction region. Again, DZero's Run II results are drawn from more data than was produced during all of Run I. LEP has been decommissioned to make way for the Large Hadron Collider.

The directors of seven particle physics laboratories met with the press (from left): Michael Witherell, Fermilab; Albrecht Wagner, DESY; Yoji Totsuka, KEK; Sergio Bertolucci, INFN; Jonathan Dorfan, SLAC; Luciano Maiani, CERN; Robert Aymar, Director General-Elect, CERN; Mikhail Danilov, ITEP.

"The 'discovery reach' of the Tevatron is taking us into uncharted territory, places where we haven't been before," said DZero physics coordinator Boaz Klima. "We already have twice as much data written on tape as in Run I. The quality of the data is better, because the detectors are better. The energy of the machine is up 10 percent, making a big impact in cross-section. Add it all together, and we've made very significant strides even before we know what physics we'll see."

The Tevatron has been establishing luminosity records with regularity, reaching 4.9E31 initial luminosity on Sunday, Aug. 10, the day before the opening of Lepton-Photon '03. It remains the world's highest-energy collider until LHC begins producing physics. The CDF and DZero detectors are offering combined results in top quark measurements that extend the understanding of the particle discovered at Fermilab in 1995.

Top quark measurements offer tools applicable to several other areas—for example, offering constraints in the electroweak sector, on the Higgs boson. Because the particle is so massive, top quark measurements also represent major areas to search for new physics; it may decay into as-yet unknown particles. In addition, the Tevatron is the only top quark "factory" until the LHC turns on.

So far, Run II results have agreed with Run I results on top mass, and close study of the top offers a whole area of new knowledge—as intense studies of strange, charm and bottom quarks have generated new knowledge and new directions. With top samples already larger than Run I, CDF, DZero and the Tevatron are positioned to answer questions approachable only at Fermilab's high-energy frontier: Is the top quark what it appears to be in the Standard Model? Or is there actually new physics lurking around this heaviest of the quarks?

"A very rich top physics program is underway," concluded Patrizia Azzi of INFN-Padova, in her presentation on top quark measurements. "Let's see what the top quark can do for us."

It will be at least 2007 before physicists can see what the LHC can do for them, but CERN Director-General Luciano Maiani declared that the new machine could be completed in late 2006, with commissioning and first beams injected in the spring of 2007. Maiani also predicted the first collisions by mid-2007 at LHC. He reported that the fiscal difficulties of 2001 have served to refocus the laboratory, with LHC progress now monitored by new control tools in addition to classical peer review committees. He described the production, installation, and integration of machine and detector components as reaching what he termed "cruising speed."

Good nu's and future views
With the worldwide "neutrino oscillation industry" soon to be augmented by results from Fermilab's short-baseline Mini BooNE experiment, and with the long-baseline MINOS detector now complete and taking cosmic ray data, theorist Alexei Smirnov of Italy's International Center for Theoretical Physics, and Moscow's Institute for Nuclear Research, commented on the "enormous progress" in determining neutrino masses and mixings, and in studies of the properties of the mass matrix. Still to come are experiments performing precision measurements of neutrino parameters.

"Apart from that," Smirnov concluded, "we will need results from non-neutrino experiments from astrophysics and cosmology, from searches for proton decay and rare decays, [and] from future high-energy colliders."

The issue of the future was on everyone's mind, and Berkeley theorist Hitoshi Murayama took on the challenge of envisioning what lies ahead for a field encompassing strings, supersymmetry, hadrons, dark matter, neutrinos, big machines and numerous other areas of focus—including politics—to be explored at the trillion-electron-volt level.

Stressing the synergy of search building on search, of result building on result, Murayama drew an array of Big Questions confronting the field, from the "horizontal" (such as: Why are there three generations of matter? What is the origin of the matter-antimatter asymmetry in the universe?) to the "vertical" (Why are there three unrelated gauge forces? Why is the strong interaction strong? Is there a unified description of the forces?). He described questions originating from The Heavens (What is dark matter? What is dark energy? Why is the universe so big?) and from Hell (What is the Higgs boson? Why does it have negative mass-squared? Is it elementary or composite?).

Murayama's conclusion: from any angle, the outlook for the next 20 years is a bright one. Many long careers and future hopes are dependent on that assessment.

Peter Rosen, the DOE Office of Science's Associate Director for High Energy & Nuclear Physics, made a point of offering an international welcome at the start of Lepton-Photon 2003 in Ramsey Auditorium. In fact, he made seven points—in English, Japanese, German, French, Italian, Russian and Spanish.

"This is a highly international field, and it has a long tradition of being a highly international field," Rosen said. "It's one of the features that is extremely important from the point of view of the success of the field. The achievements in the field have come about because we are able to get talented people from all parts of the world, and we are able to bring together resources from so many different parts of the world. That's something we must not lose at any time."

Rosen also stressed the Tevatron's pre-eminence.

"The Tevatron is, and will be for the next five years, the highest-energy collider in the world," he said. "Hadron physics has a tremendous variety of different processes and subfields of high-energy physics, and the Tevatron is right at the forefront of that variety of physics, right now."

Rosen wanted more time to weigh the early physics results from CDF and DZero in Run II, but he emphasized that data was invaluable to the discovery process: "You have to have the data in order to learn how best to extract the physics from it." The process is a dynamic one, he said, and people rising to a challenge can produce surprising results.

"The example that's most telling in this respect is the Main Injector," he said. "If you go back to the official documents when the Main Injector was first put into the budget in 1991 or '92, the reason for building it was for the increase in luminosity because, the words were, 'we're almost certain to see the first observation of the top quark.' However, even before the Main Injector was built, we discovered the top quark here at the Tevatron. That was because people discovered clever ways of extracting the physics. I think that's an important example of where people who are faced with a challenge of going after something will become very clever and invent new ideas, new ways of approaching the data."

Rosen, who staked out a front-row seat throughout L-P '03, said he was encouraged and "very impressed" by the number of young presenters.

"The quality of the talks was very high," he said. "Obviously there are a lot of young people out there in the field who are very talented not only as physicists but in terms of presenting their work."

"I haven't immersed myself in a conference like this for many years," Rosen added. "It's been a lot of fun."
—Mike Perricone