U-M physicists play key role in discovery of new particle

Updated 2:20 PM July 5, 2007

U-M physicists play key role in discovery of new particle

By Carol Rabuck
Physics Department

U-M physicists have made significant contributions to the discovery of a new particle called the Cascade b (Xi-b) baryon, while working on the D0 (D-zero) experiment at Fermi National Accelerator Laboratory in Illinois.

This discovery serves as a road sign on a long journey to answer the question: Of what is the world made?

"My colleagues and I are delighted to have been involved with this discovery and to further confirm the extant model of particle physics in a domain where different generations of quarks combine to form a fundamental particle," says Homer Neal, U-M physics professor.

The world is made from a few fundamental building blocks. These and the forces holding them together are summarized by a theory called the Standard Model of particle physics. In this theory, the building blocks consist of three generations of quarks and leptons. Physicists study these building blocks through high-energy collisions at particle accelerators.

At Fermilab, protons and anti-protons collide at the world's highest laboratory energies in the Tevatron collider. These violent collisions occasionally produce heavy particles thanks in part to Einstein's famous equation E = mc², as matter and energy are convertible. Most of these heavy particles are short-lived and often decay, either directly or through other intermediate unstable particles, into lighter and long-lived particles. These stable particles are then detected using large sensitive instruments enclosing the interaction point.

The observation of the Xi-b baryon by the D0 experiment represents the first observance of a particle formed of quarks from all three known generations. The Xi-b baryon is made of a down, a strange, and a beauty (nickname for bottom) quark. It is the heaviest baryon discovered to date that decays through the weak interaction, a force responsible for most radioactive decays.

Sifting through more than a billion collisions recorded over the last five years, physicists from U-M, the University of Oklahoma and other colleagues found 19 events consistent with the decay of a new particle having approximately 6 times the proton mass. The odds of the observed signal due to background fluctuation are less than 3 x 10⁷.

Three Michigan physicists were centrally involved in this effort. Professor Jianming Qian, the current leader of the U-M D0 Group, and the recent coordinator of overall D0 physics, was the major coordinator of, and a contributor to, the ensuing analysis. Eduard de la Cruz Burelo, a postdoctoral research associate, first observed the signal in related data that he was processing at the request of his adviser, Neal, who founded the Michigan D0 group. Through analysis of this larger sample, evidence was found by Burelo for the new Xi-b particle.

After the original observation, the ensuing analysis was followed by the entire Michigan D0 Group. A paper titled "Direct observation of the strange b baryon Xi-b," has been submitted to the journal Physical Review Letters. This discovery fills a missing piece in the Standard Model and solidifies understanding of what makes up the world.

The Michigan D0 group also includes professor Bing Zhou, research fellows Alan Magerkurth and Jonas Strandberg, and graduate students Jim Degenhardt and Chun Xu.

D0 is an international experiment of approximately 600 physicists from 82 institutions of 19 countries. It is supported by the Department of Energy, National Science Foundation, and a number of international funding agencies.

U-M physicists play key role in discovery of new particle

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