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Exotic relatives of protons and neutrons discovered by Carnegie Mellon

Carnegie Mellon Universtity : 14 March, 2007  (Technical Article)
The Department of Energy's Fermi National Accelerator Laboratory Collider Detector at Fermilab, a collaboration that includes researchers from the University of Pittsburgh and Carnegie Mellon University, announced yesterday the discovery of two rare types of particles, exotic relatives of the more common proton and neutron.
'These particles, named Sigma-b [Σb], are exotic composites, new ways of putting together the heavy and light quarks. We infer their existence from observing their decay products, and we're only seeing them now because even the decay products are pretty rare,' said Joseph Boudreau, associate professor of physics and astronomy at Pitt, who was involved in the research with Paul Shepard, professor of physics and astronomy at Pitt. Manfred Paulini, associate professor of physics at Carnegie Mellon, and James Russ, professor of physics at Carnegie Mellon, are also members of Fermilab's CDF collaboration; their research focuses on the study of particles containing heavy quarks.

'Ordinary matter consists of only two of the lightest quarks plus the lightest lepton, known as the electron,' Boudreau added. 'The heavier forms of matter exist only for short periods of time, but they're just as crucial to our understanding as the stable forms. You can't explain the proton, the neutron, or the electron without explaining the heavier members of the family.'

'The proton and neutron family tree has several different branches,' said Russ. 'These new particles are very similar to other relatives of the proton studied at Fermilab and elsewhere in a variety of experiments. The masses of these new states fit in beautifully with the pattern that we expect from those of charm baryons.'

Baryons, derived from the Greek word barys, meaning 'heavy,' are particles that contain three quarks, the most fundamental building blocks of matter. The CDF collaboration discovered two types of Sigma-b particles, each one about six times heavier than a proton.

There are six different types of quarks: up, down, strange, charm, bottom, and top (u, d, s, c, b, and t). The two types of baryons discovered by the CDF experiment are made of two up quarks and one bottom quark (u-u-b), and two down quarks and one bottom quark (d-d-b). For comparison, protons are u-u-d combinations, while neutrons are d-d-u. The new particles are extremely short-lived and decay within a tiny fraction of a second.

'These particles are like rare jewels that we mined out of our data,' said CDF spokesperson Jacobo Konigsberg of the University of Florida. 'Piece by piece, we are developing a better picture of how matter is built out of quarks. We learn more about the subatomic forces that hold quarks together and tear them apart. Our discovery helps complete the 'periodic table of baryons.''

Using Fermilab's Tevatron collider, the world's most powerful particle accelerator, physicists can recreate the conditions present in the early formation of the universe, reproducing the exotic matter that was abundant in the moments after the big bang. While the matter around us comprises only up and down quarks, exotic matter contains other quarks as well.

The Tevatron accelerates protons and antiprotons close to the speed of light and makes them collide. In the collisions, energy transforms into mass, according to Einstein's famous equation E=mc2. To beat the low odds of producing bottom quarks, which in turn transform into the Sigma-b, according to the laws of quantum physics, scientists take advantage of the billions of collisions produced by the Tevatron each second.

The CDF experiment identified 103 u-u-b particles, positively charged Sigma-b particles (Σ+b), and 134 d-d-b particles, negatively charged Sigma-b particles (Σ-b). In order to find this number of particles, scientists culled through more than 100 trillion high-energy proton-antiproton collisions produced by the Tevatron over the last five years.

Boudreau has contributed heavily to the reconstruction software for CDF; it follows charged particle trajectories through tracking detectors. 'The discovery of the Sigma-b is at the end of a long chain of reconstruction: Tracks are inferred from a few coordinates; heavier and less stable forms of matter are inferred from the tracks; and in this way we work our way back through a decay chain, generally towards heavier and more exotic forms of matter, and generally by putting together decay products,' said Boudreau.

Paul Shepard constructed the silicon vertex detector, a precise instrument that surrounds the particle interaction region and measures tiny amounts of ionization resulting from the passage of charged particles. 'The Sigma-b discovery would have been out of the question without that detector, which not only measures charged particle tracks but also selects them in real time from among an overwhelmingly large background,' said Boudreau.

James Russ was involved in an earlier CDF study of exotic baryons that tested many of the techniques of analysis used in the current study. Manfred Paulini, together with Carnegie Mellon postdoctoral researcher Soon Jun, are leading the CDF detector simulation project. 'Simulating particle decays using computer programs and tracing particle trajectories through a computer image of the CDF detector are essential for understanding where in the real detector we might have missed recording Sigma-b particles,' Paulini explained.

In a scientific presentation on Friday, Oct. 20, CDF physicist Petar Maksimovic, professor at Johns Hopkins University, presented the discovery to the particle physics community at Fermilab. He explained that the two types of Sigma-b particles are produced in two different spin combinations, J=1/2 and J=3/2, representing a ground state and an excited state, as predicted by theory.

Quark theory predicts six different types of baryons with one bottom quark and spin J=3/2 (see graphic at The CDF experiment now accounts for two of these baryons.

'Our data samples continue to increase, and I expect to see even more discoveries of this kind in the near future,' said Boudreau.

Also involved in the research from Pitt were postdoctoral research fellows Azizur Rahaman and Karen Gibson and graduate students Chunlei Liu and Mark Hartz.

CDF is an international experiment of 700 physicists from 61 institutions and 13 countries. It is supported by the Department of Energy, the National Science Foundation, and a number of international funding agencies. (The full list can be found at Using the Tevatron, the CDF and DZero collaborations at Fermilab discovered the top quark, the final and most massive quark, in 1995.

Fermilab is a national laboratory funded by the Office of Science of the U.S. Department of Energy, operated under contract by Universities Research Association, Inc.
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