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Mesons violate Bell’s
inequality
6 November 2003

The famous Bell's inequality of quantum mechanics has been tested in
a high-energy particle physics experiment for the first time. The
inequality was violated by three standard deviations in experiments with B
mesons at the KEK laboratory in Japan - yet again confirming the
predictions of quantum theory (arxiv.org/abs/quant-ph/0310192; J. Mod.
Optics to be published). Previously most Bell's inequality experiments
have been performed with photons or ions.

Experiments to test Bell's inequality involve measuring the properties
of pairs of particles that are space-like separated in the sense of
special relativity: in other words, there is no time for a light signal to
travel between them within the duration of the experiment. In a typical
Bell's inequality experiment the polarizations of a pair of photons are
measured as the relative angle between the axes of polarizers making the
measurements is varied.

Belle experiment

Quantum mechanics predicts that "non-local" correlations can exist
between the particles. This means that if one photon is polarized in, say,
the vertical direction, the other will always be polarized in the
horizontal direction, no matter how far away it is. However, some
physicists argue that this cannot be true and that quantum particles must
have local values - known as "hidden variables" - that we cannot measure.

Bell and others showed that it was possible to distinguish between
quantum mechanics and these hidden-variable theories in a certain type of
experiment that measure a parameter known as S. Put simply, the local
theories predict that S will always be less than two, whereas the quantum
prediction is S = 2√2. When S is greater than two, Bell's inequality is
said to be violated.

Apollo Go of the National Central University in Taiwan and co-workers
in the Belle collaboration performed the experiment at the KEK B-factory.
At this accelerator beams of electrons and positrons are collided to
produce pairs of B mesons and their antiparticles, which then decay into
lighter particles. The meson pairs behave like photon pairs, but instead
of analyzing correlations between directions of polarization, the Belle
team study particle-antiparticle correlations using a technique known as
"flavour tagging". Go and colleagues calculated that S = 2.725, with error
bars that mean that the inequality is violated by three standard
deviations.

"If quantum mechanics is the fundamental description of nature, then
non-local correlations should be found with any quantum number," Go told
PhysicsWeb. "In this experiment we are testing a quantum number
that has never been tested before. Moreover, the particle-antiparticle
quantum number is a very fundamental quantity in particle physics and the
results might have implications in this area - I am waiting for comments
from other particle theorists!"

The team now plans to study particle-antiparticle correlations in more
detail and probe the boundary between classical and quantum mechanics.

Author
Peter Rodgers is Editor of
Physics World

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Related links

Apollo Go
homepage

The Belle
collaboration

Restricted
links

arxiv.org/abs/quant-ph/0310192

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Author

Peter
Rodgers

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