What is direct CP violation?
By Davide Castelvecchi
Direct and indirect CP violation are
two mechanisms that break the symmetry between the behavior of matter
and anti-matter. The BaBar experiment first discovered indirect CP
violation for B and anti-B (or B-bar) mesons in 2001. BaBar’s new result
is the first observation of direct CP violation for B/anti-B mesons.
Detector at SLAC. (Photo by
Historically, physicists believed
that in a mirror image of the world all laws of physics would be exactly
the same. In principle, physicists thought, observers watching an
experiment reflected in a mirror could be fooled into thinking that they
were seeing reality.
Physicists call this principle
parity invariance. (To be precise, “parity” refers to a mirror image
that also switches up with down and left with right.)
Most physical phenomena obey parity
invariance: The law applies to three of the four known fundamental
forces of nature – the strong, electro-magnetic and gravitational
In the mid-1950's however,
theoretical physicists Chen Ning Yang and Tsung-Dao Lee suggested that
the fourth force, called the weak force, might be an exception. Nuclear
physicist Chien Shiung Wu and her collaborators confirmed this
prediction at Columbia University in 1957, by observing parity violation
in radioactive decay.
Still, physicists believed that the
parity principle should hold true if, in addition to taking the mirror
image of the world, one also replaced each particle of matter with its
anti-matter correspondent, or anti-particle. Replacing matter with
anti-matter is called a charge reversal, so the new principle was called
charge-parity (CP) invariance.
That idyllic image was disrupted in
1964, when nuclear physicists James Cronin and Val Fitch of Brookhaven
National Laboratory discovered a slight anomaly in the decay of a
particle called the neutral K-meson, or neutral kaon. That anomaly was
the fingerprint of a failure of CP invariance – in other words, of CP
Nature, it turns out, is not
even-handed. Certain particles should behave as “mirror images” of each
other, but they don’t: In a perfectly symmetric world, that would never
pipes of the PEP-II Storage Ring at SLAC—the upper pipe carries
positrons, the lower pipe carries electrons. (Photo by Peter Ginter)
Cronin and Fitch knew they had
discovered CP violation from an indirect clue. The violation was not in
the observed decay, but in the state of the particle that was decaying –
what physicists call indirect CP violation.
Neutral Ks have the property that
they can spontaneously transform into their anti-particles. Now,
according to the principles of quantum mechanics, a system that can be
in two different states will not immediately pick one: It will
temporarily live in both states at the same time. Only the act of
measuring the state will force the particle to “decide” for one or the
other. Hence a neutral K that’s left alone will be simultaneously itself
and its anti-self; but when it hits a particle detector, it will only be
caught in either persona.
In a perfectly CP-invariant world,
the K would have no preference between the two personas. But Cronin and
Fitch’s data, coming from the quantum interference of the two states,
revealed that the K’s existence was unevenly split.
On the other hand, the effects of
the direct type of CP violation are extremely subtle to detect,
and weren’t discovered until 1999, in experiments on K mesons at CERN in
Geneva and at the Fermi National Laboratory in Illinois.
Physicists had long predicted that
CP violation should also appear in the weak-force phenomena involving B
Like the neutral Ks, neutral B
mesons have the ability to turn into their antiparticles, and they live
an existence that’s unevenly split between the two states. This fact
enabled BaBar – and the Belle experiment at the KEK laboratory in Japan
– to measure indirect CP violation for B mesons in a landmark 2001
BaBar has now discovered direct
CP violation for B mesons.
The BaBar team has studied the
thousands of gigabytes of data produced by the BaBar detector since it
started operations in 1999, looking at the decay patterns of millions of
neutral B and anti-B mesons. After they emerge from the collisions of
electrons and positrons, the mesons live for less than a millionth of a
second before decaying into other particles. Among the many ways the
mesons can decay, the scientists were looking for the rare events that
turned Bs into K+pi- pairs and anti-Bs into K-pi+
magnet on the PEP-II accelerator. (Photo by Peter Ginter)
According to theory, CP symmetry
would dictate that the two events have the same odds of happening.
Hence, by starting with equal numbers of Bs and anti-Bs one should end
with equal numbers of K+pi- and K-pi+
pairs. However, the BaBar collisions produced 910 K+pi-
pairs but only 696 K-pi+ pairs.
The situation can be compared to
rolling two dice – say, a blue die for a B and a red one for an anti-B.
To take the comparison further, suppose that getting K+pi-
from a B corresponded to the blue die landing on 1, while getting a K-pi+
from an anti-B were like getting the red die on 6. In a symmetric world,
by rolling both dice a million times one would expect to get 1s from the
blue as often as one gets 6s from the red.
But the weak force has a preference,
as if its dice were loaded.
As in the indirect case, direct CP
violation for B/anti-B (or K/anti-K) mesons is the result of a quantum
interference. In this case, however, the interference is not between two
states of the meson, but between two paths of decay, i.e., two different
sequences of intermediate decays that lead to the same end result.