Reconsidering the Standard Model

Change is coming quickly in the field of particle physics, and two Princeton researchers are helping to lead the way.

Traditionally, particle physics has been explained by two complementary theories, the Standard Model and general relativity, but physicists have been unable to combine the two into a single theory that can explain everything. There remain many kinks to be worked out in both models before they can be joined together. By testing the more theoretical aspects of these models, physicists hope to either confirm or deny postulates and place themselves on the right track.

"It's really the first time we're able to test certain aspects of the Standard Model and explore the frontier of knowledge in these ways," University physicist Dan Marlow said.

He and fellow University physicist Eric Prebys are part of a worldwide collaboration to test an aspect of the Standard Model — the quantum field theory that explains the forces that hold particles together. In particular, they are testing the exact degree of the lopsidedness between matter and anti-matter in nature.

They work with a team of 300 collaborators on the KEK project, the Japanese acronym for High-Energy Accelerator. The KEK project — which is based in Tsukuba, Japan, and is also known as Belle — is a B-factory that churns out certain subatomic particles — called Band anti-B-mesons — by the millions for data collection with a precision never before possible.

In particular, they are seeking to determine the difference in the decay amplitudes of these particles, which determine how quickly they will decay, Marlow said.

At this point, the research team is not ready to claim that it has definitively measured the degree of the asymmetry in the decay amplitudes of Band anti-B-mesons.

Because physicists have not yet been able to conclusively determine the exact difference in the decay amplitudes, they are uncertain as to whether the asymmetry is correctly predicted by the Standard Model, which also tries to explain how particles interact with one another.

More specifically, though the Standard Model can explain how particles hold together, it is at a complete loss in explaining gravity, which is better explained by Einstein's theory of general relativity. As of now, theoretical physicists have not been able to unify the Standard Model and general relativity, even though the two theories are complementary.

It was once believed that matter and anti-matter were exact complements, obeying all the same physical laws in exactly the same manner. The universe could be turned inside out and every particle turned into its anti-particle, and yet all the laws would still hold as before. Such a universe is said to be CP invariant.

The term CP refers to the fact that particles and anti-particles differ in charge and parity. For example, the electron carries a negative charge, but its anti-particle, the positron, carries a positive charge. Parity describes symmetry: Particles and anti-particles are mirror images of each other. CP invariance is the theory that particles and anti-particles obey all the same laws in the same way.

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But the concept of universal CP invariance raises a problem that goes back to the Big Bang. Fifteen billion years ago, the Big Bang released enormous amounts of energy and mass that gave birth to equal amounts of matter and anti-matter. Matter and anti-matter obliterate each other upon colliding and so should have left nothing in their wake, an empty universe.

But today's universe seems to be made up entirely of matter, and anti-matter has not been found, not even in galaxies light-years away. If there is much more matter than anti-matter in today's universe, there is clearly some difference, however tiny it may be, between matter and anti-matter — there must be CP violation.

The first clues to CP violation came in 1964, when two Princeton professors, Val Fitch and James Cronin, discovered that CP invariance does not hold for particles known as kaons. Kaons are unusual because they are part of a class of particles known as mesons, which are composed of a quark and an anti-quark. However, the two quarks do not annihilate each other because they are not the same types of quarks.

What Fitch and Cronin found was that kaons decay somewhat differently than anti-kaons do. They found that there was a very small difference in decay amplitudes: Anti-kaons decay about two-tenths of a percent faster than kaons. Because anti-kaons decay somewhat faster than kaons do, they leave kaons without a partner with which to annihilate. This small difference in their decay amplitudes can accumulate over a long time — so that eventually only kaons remain.

This startling finding was the first to show that matter and anti-matter do not always behave in the same ways — that CP invariance does not always hold. For this work, Fitch, now a professor emeritus, and Cronin, now at the University of Chicago, were awarded the Nobel Prize in physics.

This discovery was worked into the Standard Model, but for several reasons has proved to be problematic: There is currently insufficient quantitative knowledge to determine whether CP violation fits into the model. Scientists also have been unable to detect CP violation in other types of closely related particles.

If the Standard Model as it exists today is correct, CP violation should occur in other types of particles closely related to kaons, including B-mesons. The Standard Model, as it is, predicts the degree of difference of decay amplitudes between B-mesons and anti-B-mesons, a convenient target for particle physicists like Marlow and Prebys.

Belle, the project they work on, uses a high particle accelerator to smash together at very high energies electrons and their anti-particles, the positrons. In a small fraction of these collisions, B-mesons and anti-B-mesons are formed for perhaps one trillionth of a second. To be able to measure the life spans of these particles, the accelerator shoots out electrons at slightly different energies than positrons, creating a stream that flows at half the speed of light. The B-mesons and anti-B-mesons flow along this stream, and the researchers are able to measure the distance between the sites of birth and decay of the particles. Armed with the knowledge of this distance, they are able to determine how long the particles existed, and thus their decay amplitudes.

Because Belle works with such tiny values, the collaborators use blind analysis, an old scientific technique that is becoming increasingly common in experimental physics. It is a technique that disguises the data, hiding the answer until after data analysis is complete. While the experiments are running, the collaborators see only a stream of random numbers, a process that works to reduce bias. Marlow said that when working with such small values, "it is easy to influence the outcome to be in a certain way," thus distorting the process of data collection.

Using blind analysis forced the collaborators on Belle to include a set of data that gave a negative value, suggesting that it is matter, not anti-matter that decays more rapidly — a result so unexpected that Prebys said, "We would have come up with reasons not to include it." After debating whether to include the negative result, the collaborators decided to take an average of the sets of data, giving a lower, but still positive, value. The resulting lower positive value may turn out to be more accurate because it takes the margin of error into greater account.

Results are still preliminary, but the data as of now is not inconsistent with the Standard Model. There is some difference in the data collected by Belle and other B-factories, but none has a small enough margin of error to rule out the value suggested by the Standard Model. Marlow calls the results to date "ambiguous." The different B-factories need to collect more data before they can agree on whether the Standard Model is correct.

Since the accelerator for Belle is in Japan, Prebys and Marlow spend six to 10 weeks per year overseas to supervise the collaboration and meet with colleagues.

They started working on Belle in 1994, designing and constructing the detector to complement the accelerator. Marlow says that roughly speaking, the period from the summer of 1998 to 1999 was spent commissioning the accelerator and the detector, and the period from the summer of 1999 to the present has been spent accumulating data. He believes that by next summer they will have collected at least twice, and perhaps four to eight times, as much data as they currently have — enough to suggest with some authority the true difference between the decay amplitudes of Band anti-B-mesons.

Furthermore, Prebys is optimistic that they will be able to fine-tune the accelerator to increase the rate of collisions, thereby reducing statistical error in data collection.

Belle and the other B-factories are only part of an effort to better understand the intricate workings of the universe. There is also an ongoing effort to develop a new theory that will explain everything.

Theoretical physicists hope to overcome this obstacle in the next few decades and develop what grandiosely is called the Grand Unified Theory. There are currently several different GUTs floating around, and no one really knows which one, if any, is the best.

Prebys is skeptical of the current efforts to develop a theory to explain everything. He says that in the next few years, experimental physicists should be able to look for various particles theorized by the GUTs, but which have not been found yet. If they are able to find these particles, then there may be something to GUTs.

Marlow is more enthusiastic about GUTs, and he believes there will be something in place soon. When asked if he thought that there would be a GUT in place in the next few years, he said, "Oh yeah, absolutely."