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.