The big bang created a lot of matter—along with the same amount of
antimatter, which wiped out everything and brought the universe to an
untimely end. That's what accepted theoretical physics tell us—though
things clearly didn't work out that way. Now, results from a U.S.
particle smasher are providing new evidence for a subtle difference in
the properties of matter and antimatter that may explain how the early
universe survived.
The first evidence of a difference between matter and antimatter was
found in the 1960s in the decay of particles called neutral kaons, which
led to the awarding of a Nobel Prize in physics. In 2001, accelerators
in the United States and Japan found more evidence for a difference in
particles called B mesons. Then last year at CERN's Large Hadron
Collider (LHC) near Geneva, Switzerland, evidence was found in a third
system, D mesons, but there wasn't enough data to rule out a statistical
fluke. The new results—which come from the Collider Detector at
Fermilab (CDF) experiment near Chicago—are still not conclusive
evidence, but they bring the chances of a fluke down to about one in
10,000. "I'm sure in a few days everyone in the field will feel much
more confident that this is actually real," says Giovanni Punzi,
spokesperson of the CDF experiment.
Physicists have long suspected that a difference in the properties of
matter and antimatter is key to the early universe's survival. Such a
difference—technically known as charge-parity (CP) violation—would have
allowed normal matter to prevail over antimatter so that normal matter
could go on to form all of the stuff we see in the universe today.
To witness CP violation, physicists study particles to see if there is
any difference in the rate of decay between normal particles and their
antiparticles. The accepted theory of elementary particles, the standard
model, allows for a low level of CP violation—including that revealed
in the discoveries of the 1960s and 2000s—but not enough to explain the
prevalence of normal matter. So researchers have been trying to find
cases in which CP violation is higher.
The LHCb detector at CERN, and CDF at Fermilab, are two such
experiments. They trace the paths of D0 meson particles and their
antiparticles. These can decay into pairs of either pions or kaons, and
by tallying these decay products, the LHCb and CDF teams can calculate
the difference in decay rates between the D0 particles and
antiparticles.
In November, the LHCb team reported that the decay rates differed by
0.8%—some eight times the amount the standard model is generally
expected to allow, and perhaps enough to help explain the origin of
matter's prevalence over antimatter. Unfortunately, the measurement was
not very precise: The statistical significance was about 3 sigma,
meaning there was about one chance in a 100 that it was a random blip in
the data.
The latest CDF results—announced earlier today at a meeting in La
Thuile, Italy—drastically decreased the odds of a fluke. They point to
CP violation at the level of 0.6%, with a statistical significance of
2.7 sigma. Combined with the previous LHCb results, the CDF results
bring the significance to about 3.8 sigma—or about one chance in 10,000
that the CP violation is a random blip.
The results cannot be claimed as a bona fide discovery, which requires a
statistical significance of 5 sigma—or the chance of it being random at
less than one in a million. Still, particle physicists are excited. "We
cannot yet say for sure it is CP violation," says Angelo Carbone, a
member of the LHCb collaboration. "But it's close."
Paul Harrison, an experimental particle physicist at the University of
Warwick in the United Kingdom, says the 5-sigma standard is important
because it helps avoid biases that arise in lopsided statistical
distributions. But he thinks it is reassuring that the results come from
two independent experiments. "I wouldn't be expecting a mistake in the
experiments at this point," he says. "These guys are serious people. ...
They've been at it a long time, and they know what they're doing."
To
see whether the statistical significance can be improved toward 5
sigma, onlookers will have to wait until later this year, when the LHCb
team examines the rest of its data. But even if the CP violation turns
out to be real, there is the question of whether it is "new physics"—in
other words, whether the current standard model can explain it.
Particle theorist Sebastian Jaeger at the University of Sussex in the
United Kingdom thinks the answer is uncertain because no one is sure how
far the standard model can be pushed. "The main issue is that CP
[violation] is difficult to quantify—it's rather challenging, from a
theoretical point of view, to make a prediction for it. ... So even if
the significance becomes 5 or 10 sigma, the standard model may still not
be ruled out."
