The fact that the universe is made of matter is obvious enough. What bothers
fundamental physicists, though, is why that is. The best current theory
of particle physics, called the Standard Model, suggests that basically
equal amounts of matter and antimatter should have been produced in the
Big Bang. Famously, however, matter and antimatter annihilate on contact
and disappear in a puff of pure energy, so a primordial equity between
the two would have led to a lifeless, photon-filled universe. One
possible reason for the apparent imbalance is a phenomenon called charge
conjugation/parity violation (or CP violation for short).
If
charge conjugation and parity were conserved, nature would treat
particles and their antiversions, which carry the opposite electric
charge and opposite value of a property called spin, alike. The Standard
Model allows for a whiff of CP violation. But that is nowhere near
enough of it to explain matter's cosmic dominance.
Last November
an experiment called LHCb, which feeds off the world's biggest particle
accelerator, the Large Hadron Collider (LHC) near Geneva,
saw hints of excess CP violation beyond theorists' predictions. Now an experiment in America, known as CDF,
has spotted something similar. It announced its results at a meeting in La Thuile, in Italy.
Like
LHCb, CDF was looking at a subatomic species called mesons. These come
in a variety of types, or flavours, each containing a quark and an
antiquark. One flavour, known as D0-mesons, is composed of a charm quark
and an up antiquark. Its antimatter twin, known as D0-bar, consists of a
charm antiquark and an up quark. (A charm quark is a heavier cousin of
the up quark which, together with down quarks, makes up the ordinary
protons and neutrons found in atomic nuclei; lest things get too
straightforward, the down quark is not simply an up antiquark.)
D0s
and D0-bars are unstable and decay into other short-lived particles
called pions and kaons. According to the Standard Model, D0-mesons and
D0-bars should produce these in basically the same proportions. CDF has
confirmed that they do not.
The experiment has not been taking
new data since its source of particles, an accelerator called the
Tevatron at Fermilab, a big laboratory near Chicago, was
shut down last September.
But its boffins are busy crunching billions of as-yet-unanalysed
collisions and have now looked at all those remaining which involve
D0-mesons. They put the odds of the D0/D-bar discrepancy coming about by
pure chance, something physicists have to reckon with in the
probability-ruled world of quantum physics, at about one in 200. That is
less than LHCb's result of one in 2,000, and well below the exacting
standard of one in 3.5m required to claim discovery in particle physics.
But it lends credence to the idea that CP violation is indeed lurking
there.
It might yet emerge, however, that the findings do not in
fact contradict the Standard Model. That is because, for complicated
theoretical reasons that have to do with how the strong nuclear force
works, calculating the expected number of different D-meson decays is
notoriously tricky. Since until LHCb dropped its bombshell no one
expected to see much in the way of CP violation in charm quarks,
theorists have only recently got cracking on the difficult sums. They
need more time to reach a firm conclusion.
Experimentalists aren't sitting idly by. Researchers at LHCb and others
are looking for hints of CP violation
which are unequivocally at odds with the Standard Model. They know that
theory cannot be the last word in particle physics. Otherwise no one
would be here to ponder the matter/antimatter conundrum.
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