VOLUME 12, NUMBER 17 • MARCH 9, 2001

Rockefeller High-energy Physics Lab to Participate in New Fermilab Collider Project

"Our goal is to understand everything about our universe and be able to describe it with a single equation that can fit on a business card," says Professor Konstantin Goulianos, head of the Laboratory of Experimental High Energy Physics at Rockefeller University, describing the lab’s involvement in the Collider Detector at Fermilab (CDF) in Batavia, Ill. The Goulianos team has participated in the CDF project since its beginning, nearly 20 years ago, sharing important discoveries with 500 or so physicists from 50 collaborating institutions.

Members of the Goulianos lab work with one of two "MiniPlug" particle detectors built here at Rockefeller for the Collider Detector at Fermilab. From left to right: (standing) Michele Gallinaro and Vadim Sherman, (seated) AndreaBocci, Stefano Lami, Andrei Solodsky, Kenichi Hatakeyama and Konstantin Goulianos.

Most prominent of these has been the 1995 discovery of the "top quark," the last and heaviest of the six-member set of the fundamental constituents of matter whimsically named "quarks." This discovery provided additional support for the already extremely successful "Standard Model," the prevailing theory of particles and forces that determine the structure of matter. This theory, first formulated in the 1960s, unifies the seemingly unrelated interactions responsible for radioactivity and electromagnetism and explains the composition of matter, and by extension, the make up of the world around us. However, it is also known to have some weaknesses, such as its inability to account for the effects of gravity.

"At the end of the 19th century," Goulianos continued, "people thought that they knew everything about the laws of nature. But then came quantum mechanics and the world changed. Now, at the click of a mouse billions of electrons around the globe perform a quantum dance that brings information via computers, laser printers, cell phones, MRI machines and thousands of other technologies unimaginable in the pre-quantum era." Detailing the variety of applications of quantum mechanics, he then added: "What is really amazing is that the more compact the equations of our theory, the more we can squeeze out of them. As successful as the Standard Model may be, it doesn’t incorporate gravity, nor does it explain some very intriguing phenomena, such as the apparent abundance of ‘dark matter’ in our universe. Perhaps the root of the trouble with the Standard Model is that it cannot fit on a business card. It just has too many parameters that are not determined by the theory, as for example the masses of the quarks. So, there must be something out there beyond it."

A step towards understanding the origin of mass is the "Higgs force," named after its proponent Peter Higgs. The main goal in the next CDF run is to search for the Higgs, the quantum associated with this force. The next run is scheduled to begin this month with an upgraded Fermilab particle accelerator and an upgraded CDF detector. Within the next two years the accelerator will deliver 20 times more proton-antiproton collisions than in all previous runs. These collisions will occur at a rate of several million per second, and at the end of the run, according to estimates, a handful of them may produce a Higgs particle.

The Higgs decays instantly into b and anti-b quarks, which in turn decay into "jets" of particles. To positively identify the Higgs, one must measure the directions and energies of all these particles and determine the Higgs’s mass. This will be done off-line. But how does one pick out the few Higgs candidate events from the trillions of "common" events expected in this run? Certainly, one cannot study a trillion events.

"If you are investigating a robbery in New York, you don’t go out and interrogate all the residents of all five boroughs. You have to have information leading to someone; you need a suspect." Goulianos used this edgy analogy to explain the process by which the Higgs candidate events will be selected. As in the past run, there will be a three- level, on-line event selection, performed by sophisticated electronics and software designed exclusively for CDF. At each level events are "interrogated" for increasingly strict requirements that must be satisfied to pass on to the next level. The requirements depend on the type of study being conducted. The Higgs suspects are rounded up simultaneously with the suspects of all the other studies. This process leads to a few events per second passing all three levels of selection. For each such event a "trigger" is then generated, prompting a readout of the over one million electronic channels of all CDF detector components. Among the most sophisticated of these components is the silicon vertex detector, which itself employs about a million channels and is capable of reconstructing the event vertex with a resolution of a few microns. Another high- tech component, designed and constructed in the Goulianos lab, is the "shower maximum detector," which employs optical fiber technology to measure the position of particle showers initiated by electrons or gamma rays in a "calorimeter," a device that measures particle energies. The information provided by all detector components is used in the off-line event analysis.

Accurate reconstruction of the energy of the jets of particles emanating from Higgs decays is essential to a Higgs discovery in the upcoming run. Assistant Professor Stefano Lami, working with Graduate Fellow Andrea Bocci, has devised a new technique to improve the jet energy measurement, resulting in a more precise determination of the Higgs mass in the presence of the 100 or so extraneous particles produced in a typical event. The improvement achieved so far will more than double the Higgs discovery potential.

The top quark discovered in the last run, which confirmed the Standard Model expectation of the existence of six quarks, will be used in the next run to answer some important questions. Assistant Professor Luc Demortier, who was instrumental in the top quark’s discovery, will study single top production–events in which a top quark is produced without an anti-top partner. The single top production rate can tell us about the possible existence of quarks heavier than the top and provide information about various theoretical schemes for physics beyond the Standard Model.

Another important topic that will be investigated is the question of whether the quarks themselves have structure. The answer will be sought in a study of events in which two high-energy jets of particles emerge from a proton-antiproton collision at angles close to 90 degrees. This happens when (anti)proton constituents, such as quarks, scatter off like two hard balls and then decay into particle jets. "We can calculate precisely what to expect from quark scattering, and we are looking for deviations that can be attributed to scattering of quark constituents," says Goulianos. This topic is part of a larger study of Quantum Chromodynamics, the Standard Model theory of the "strong" force. The effort is led by Assistant Professor Anwar Bhatti, working closely with Research Assistant Christina Mesropian.

Goulianos’s team is also working on a study of events in which no particles are emitted from the collision in certain directions. As Goulianos describes, "if you think of an event as a pie of particles distributed all around the collision point, these events are missing a piece of the pie." How does such an incredible thing happen? "It is probably an interference effect resulting from the quantum mechanical character of the quarks and gluons that make up the (anti)proton. These studies can show us how quarks and gluons are grouped together and perhaps explain why they are confined within the (anti)proton." This work is done almost exclusively by the Rockefeller team under the leadership of Goulianos, working closely with Research Assistants Michele Gallinaro and Koji Terashi and Graduate Fellows Kenichi Hatakeyama and Andrei Solodsky. The project requires two calorimeter-type detectors to measure the directions and energies of particles produced at small angles with respect to the proton and antiproton beams. With the expert help of instrument maker Vadim Sherman, the Goulianos team built these detectors, called "MiniPlugs," in their Weiss Research building machine shop using a unique optical fiber based on a design invented by the team. "The entire lab contributed to this effort," says Goulianos, "including our secretary, Christina Ferraro."

A Higgs discovery in the upcoming run will be an extremely significant contribution to the Standard Model, even as the world is increasingly interested in what lies beyond it. "This doesn’t make it a bad theory," says Goulianos. "Newton’s laws of motion, for example, aren’t the whole story, but we still use them. They work, we just know there’s more out there. Reaching beyond Newton’s mechanics was our ticket to a magnificent quantum show that changed our vision of the universe and transformed our lives in a way we could never imagine a mere 100 years ago. One may try to imagine what lies beyond the Standard Model, but judging from the past, the truth may once again defy the imagination."

Goulianos is head of the Laboratory of Experimental High Energy Physics. The work of Goulianos’s lab at Rockefeller and Fermilab in Illinois is supported by the Department of Energy of the U.S. federal government