Experiment of the Month

The CERN Connection

It is not often that a physics department is able to claim a connection to the famous accelerator laboratory, CERN, near Geneva, Switzerland. The story of Millersville's connection to an experiment at CERN is a pleasant surprise and requires some physics background. Please bear with us:

In ordinary life, we find mild surprises but not shocking surprises when we view the world in a mirror. For example if a left-handed pitcher throws a strike to a right-handed batter who hits the ball over the left field fence, we know what to expect to hear from someone who watched a reflection of the game in a mirror: A right-handed pitcher threw to a left handed batter who hit the ball over the right field fence. The picture is changed, but the physics of the ball's trajectory is not changed. The trajectory for the mirror home run is calculated in the same way as the direct home run.

This experience is expressed as a rule called "parity conservation:" If an experiment is possible, then the mirror image of the experiment will produce mirror-image results. The rule works almost perfectly. In the late 1950's C.S. Wu reported a measurement of the decay of radioactive cobalt 60 in which the rule failed. She and her coworkers were following theoretical suggestions made by T.D. Lee and C.N. Yang. Yang and Lee received Nobel Prizes for this work.

The cobalt nuclei were forced, with a downward magnetic field, to spin in a clockwise direction, as viewed from above the experiment. In their decay they emitted more electrons in the up direction than in the down direction. A mirror standing to the side of this experiment would show the cobalt nuclei spinning counter-clockwise but the decay electrons would still prefer to go up.

The counterclockwise spin is evidence that in the mirror, the magnetic field points up. Thus in the mirror image, electrons should be emitted preferentially down. If we create a new experiment which is the mirror image of the original, for Cobalt 60, the outcome is not the mirror image of the outcome. The fact that the mirror image experiment has a different outcome is said to be a violation of parity.

We are half way through our story.

It was found experimentally that every time parity conservation was violated, so was a rule called "charge conjugation." Charge conjugation is another way to create a new experiment from an existing experiment. In this construction all positive particles become negative and negative particles become positive. (Technically, the change is accomplished by changing each particle into its own antiparticle. This means that even neutral particles, such as neutrons, undergo a change in charge congugation.) After such a change, the electromagnetic forces between the particles are unchanged, so their motions are unchanged. Thus we expect the outcome of an experiment to be the same before and after charge conjugation.

This led to a new rule called CP conservation, stating that if parity (P) was violated then charge conjugation (C) was also. Stated positively, the combination of a parity plus a charge conservation transformation produces an experiment with no surprises. This rule held until 1964.

Then Christenson et al.(1) found that the decay of the long-lived neutral Kaon particle (more later on the short-lived neutral Kaon) included a surprise. 99.8% of the time, the K decays into 3 other particles, called pions. The surprise was that 0.2% of the time, it decays into just 2 pions. It turns out that CP conservation requires that the long-lived neutral Kaon cannot do both. For this evidence of CP violation, the senior investigators, Cronin and Fitch, received the Nobel prize.

This result triggered a search for other examples of CP violation. The most promising experiments were thought to be other Kaon decay modes, including the decay mode of a "short lived neutral Kaon." In 1971, a young graduate student at the State University of New York at Stony Brook published a calculation for the lifetime of the short lived kaon into (not pions but) a pair of gamma rays (photons). Then she waited for experimental physicists to measure that the corresponding decay rate.

And waited.

During the wait Ms. Uy became Dr. Uy and built a reputation as one of the excellent teachers in the MU physics department.

In 2000, the results arrived. In a paper in the journal Physics Letters, a group called the NA48 group at CERN published (2) their results of a measurement of the 2-gamma decay of the short-lived Kaon. It agrees with Dr. Uy's 1971 prediction.

We are pleased to be associated, through Dr. Uy, with one of the best physics labs in the world.

References:

1) Christenson et al., Phys. Rev. Lett. 13, 138 (1964).

2) Phys. Lett B 493, 29 (2000)

An online discussion of Kaon decay may be found at http://sist.fnal.gov/archive/96-topics/papers/Halton_Peters/Halton_Peters,_KTeV.htm