July 2019, ScientificAmerican.com 19CERN
PARTICLE PHYSICSLucky Charms
New evidence hints at what happened to the universe’s antimatter
We could have been living in an anti
matter universe, but we are not. Anti
matter is matter’s upsidedown twin—ev
ery matter particle has a matching anti
matter version with the opposite charge.
Physicists think the cosmos started out with
just as much antimatter as matter, but most
of the former got wiped out. Now they may
be one step closer to knowing why.
Researchers at the Large Hadron Collid
er Beauty (LHCb) experiment at CERN near
Geneva have discovered antimatter and
matter versions of “charm” quarks—one of
six types, or flavors, of a class of elementa
r y matter par ticles—acting differently from
one another. In a new study, which was
presented in March at the “Rencontres de
Moriond” particle physics conference in
La Thuile, Italy, the physicists found that
un stable particles called D^0 mesons (which
contain charm quarks) decayed into more
stable par ticles at a slightly different rate
than their anti matter counterparts. Such
differences could help explain how an
asymmetry arose between matter and
antimatter after the big bang, resulting in
a universe composed mostly of matter.
Matter and antimatter annihilate each
other on contact, and researchers believe
such collisions destroyed almost all of the
antimatter (and a large chunk of the matter)
that initially existed in the cosmos. But theydo not understand why a relatively small
excess of matter survived to become the
stars and planets and the rest of the cos
mos. Consequently, physicists have been
looking for a kind of matter that behaves so
differently from its antimatter version that
it would have had time to generate this
excess in the early universe.
The newly discovered mismatch in
decay rates between charm quarks and
antiquarks turns out to be too small to
account for the universe’s excess of matter.
The result, however, “does bring us closer
to finding the answer because it shows one
of the possible answers may not be the
right one,” says theoretical physicist Yuval
Gross man of Cornell University, who was
not involved in the new work. “I am also
excited because it’s the first time we’ve ever
seen this [phenomenon in charm quarks].”
Physicists previously found similar varia
tions in two other quark flavors, but those
were also too tiny to account for our mat
terdominated universe. Scientists are hold
ing out hope of finding much larger matter-
antimatter differences elsewhere, such as in
ghostly particles called neutrinos or reac
tions involving the Higgs boson—the parti
cle that gives others mass—says LHCb
team member Sheldon Stone of Syracuse
University: “There are lots of different
searches going on.” — Clara MoskowitzLarge Hadron Collider Beauty
(LHCb) experiment at CERN.The #1 In Value—Guided Tours Since 1952Grand Canyon; Go West! on a Fully Guided
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