By Rob Enslin
Physicists
in the College of Arts and Sciences (A&S) have confirmed that matter and
antimatter decay differently for elementary particles containing charmed
quarks.
Distinguished
Professor Sheldon Stone says the findings are a first, although
matter-antimatter asymmetry has been observed before in particles with strange
quarks or beauty quarks.
Quarks
are elementary particles that are the building blocks of matter.
Stone
and members of the College’s High-Energy Physics (HEP) research group have
measured, for the first time and with 99.999-percent certainty, a difference in
the way D0 mesons and anti-D0 mesons transform into
more stable byproducts.
Mesons
are subatomic particles composed of one quark and one antiquark, bound together
by strong interactions.
"There
have been many attempts to measure matter-antimatter asymmetry, but, until now,
no one has succeeded,” says Stone, who collaborates on the Large Hadron
Collider beauty (LHCb) experiment at the CERN laboratory in Geneva , Switzerland
The
findings may also indicate new physics beyond the Standard Model, which describes
how fundamental particles interact with one another. "Till then, we need
to await theoretical attempts to explain the observation in less esoteric
means," he adds.
Every
particle of matter has a corresponding antiparticle, identical in every way, but
with an opposite charge. Precision studies of hydrogen and anti-hydrogen atoms,
for example, reveal similarities to beyond the billionth decimal place.
When
matter and antimatter particles come into contact, they annihilate each other
in a burst of energy—similar to what happened in the Big Bang, some 14 billion
years ago. “That’s why there is so little naturally occurring antimatter in the
Universe around us,” says Stone, a Fellow of the American Physical Society,
which has awarded him this year's W.K.H. Panofsky Prize in Experimental
Particle Physics.
The
question on Stone's mind involves the equal-but-opposite nature of matter and
antimatter. “If the same amount of matter and antimatter exploded into
existence at the birth of the Universe, there should have been nothing left
behind but pure energy. Obviously, that didn’t happen,” he says in a whiff of
understatement.
Thus,
Stone and his LHCb colleagues have been searching for subtle differences in
matter and antimatter to understand why matter is so prevalent.
The
answer may lie at CERN, where scientists create antimatter by smashing
protons together in the Large Hadron Collider (LHC), the world’s biggest, most
powerful particular accelerator. The more energy the LHC produces, the more
massive are the particles—and antiparticles—formed during collision.
It
is in the debris of these collisions that scientists such as Ivan Polyakov, a
postdoc in Syracuse
“We
don’t see antimatter in our world, so we have to artificially produce it,"
he says. "The data from these collisions enables us to map the decay and
transformation of unstable particles into more stable byproducts."
HEP
is renowned for its pioneering research into quarks, of which there are six
types, or flavors. Scientists usually talk about them in pairs: up/down,
charmed/strange and top/bottom. Each pair has a corresponding mass and
fractional electronic charge.
In
addition to the beauty quark (the "b" in "LHCb"), HEP is
interested in the charmed quark. Despite its relatively high mass, a charmed
quark lives a fleeting existence before decaying into something more
stable.
Recently,
HEP studied two versions of the same particle. One version contained a charmed
quark and an antimatter version of an up quark, called the anti-up quark. The
other version had an anti-charm quark and an up quark.
Using
LHC data, they identified both versions of the particle, well into the tens of
millions, and counted the number of times each particle decayed into new
byproducts.
“The
ratio of the two possible outcomes should have been identical for both sets of
particles, but we found that the ratios differed by about a tenth of a
percent," Stone says. "This proves that charmed matter and antimatter
particles are not totally interchangeable.”
Adds
Polyakov, “Particles might look the same on the outside, but they behave
differently on the inside. That is the puzzle of antimatter.”
The
idea that matter and antimatter behaves differently is not new. Previous
studies of particles with strange quarks and bottom quarks have confirmed as
such.
What
makes this study unique, Stone concludes, is that it is the first time anyone
has witnessed particles with charmed quarks being asymmetrical: "It's one
for the history books."
HEP's
work is supported by the National Science Foundation.
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