g****g
发帖数: 1828 | 1 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.
The BaBar Detector at SLAC. (Photo by Peter Ginter)
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 forces.
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 violation.
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 happen.
Two beam 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 mesons.
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 result.
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+ pairs.
A kicker 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. |
|
