A team of scientists, including four at Johns Hopkins,
has discovered two new subatomic particles, rare but
important relatives of the familiar, commonplace proton and
Named "Sigma-sub-b" particles, the two exotic and
incredibly quick to decompose particles are like rare
jewels mined from mountains of data, said team leader Petar
Maksimovic, assistant professor of physics and astronomy
in the university's Krieger School of Arts and Sciences.
"These particles are members of what we call the
'baryonic' family, so-called for the Greek word barysâ
which means heavy," Maksimovic said. "Baryons are particles
that contain three quarks, which are the fundamental
building blocks of matter."
The simplest baryons are the proton and neutron, which
make up the nuclei of atoms of ordinary matter.
"These newest members of that family are unstable and
ephemeral, but they help us to understand the forces that
bind quarks together into matter," Maksimovic said.
Containing the second-heaviest quark — called
"the bottom quark" — the new particles are the
heaviest baryons found yet, heavier even than a complete
helium atom, which has two protons, though lighter than a
lithium atom, which has three.
How rare is Sigma-sub-b? The team combed through a
hundred trillion proton-antiproton collisions at the
Tevatron, the world's most powerful particle accelerator,
to find about 240 Sigma-sub-b candidates, Maksimovic said.
The new particles are extremely short-lived, decaying
within a tiny fraction of a second.
"Little by little, we are compiling an ever-clearer
picture of how quarks build matter and how subatomic forces
hold quarks together and tear them apart," said Maksimovic,
who noted that the discovery — confirming the
expectation of theorists that Sigma-sub-b particles exist
— helps complete the so-called "periodic table of
There are six different types of quarks: up, down,
strange, charm, bottom and top (u, d, s, c, b and t). One
of the new baryons discovered by the CDF experiment is made
of two up quarks and one bottom quark (u-u-b), the other of
two down quarks and a bottom quark (d-d-b). For comparison,
protons are u-u-d combinations, while neutrons are
The Tevatron collider helped the team of physicists
re-create 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 is constructed with only up and down
quarks, exotic matter contains other quarks as well,
according to Maksimovic.
The Tevatron is located at the Department of Energy's
Fermi National Accelerator Laboratory, also known as
Fermilab, in Batavia, Ill. Led by Maksimovic, the team
included Johns Hopkins graduate student Jennifer Pursley,
former undergraduate student Michael Schmidt and
postdoctoral fellow Matthew Martin, along with five other
scientists from Fermilab and the University of New Mexico.
All are members of the collaboration of 700 physicists
working on the CDF detector at Fermilab.
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. The odds of producing
bottom quarks — which in turn transform into the
Sigma-sub-b, according to the laws of quantum physics
— are extremely low. But scientists were able to beat
the low odds by producing billions of collisions in the
Tevatron each second.
CDF co-spokesman Rob Roser, of Fermilab, said, "It's
amazing that scientists can build a particle accelerator
that produces this many collisions, and equally amazing
that the CDF collaboration was able to develop a particle
detector that can measure them all. We are confident," he
said, "that our data hold the secret to even more
discoveries that we will find with time."
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. Using the
Tevatron, the CDF and DZero collaborations at Fermilab
discovered the top quark, the final and most massive quark,
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.