BaBar Data Hints at New Physics


It might not have the name recognition of Fermilab’s late, lamented Tevatron, or the Large Hadron Collider at CERN, but results from the BaBar experiment at the SLAC National Accelerator Laboratory continue to yield interesting results hinting at potential new physics.

No, this is not the iconic cartoon elephant beloved of children over generations. BaBar is a particle accelerator, so named because it was designed to measure the decay of B-mesons and their antiparticles, known as B-bar mesons. The international collaboration is especially interested in the question of why there is matter but little antimatter in our universe, among other questions.

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Antimatter is the same as regular matter except that each particle has an opposite charge. So whereas an electron has a negative charge, its antimatter counterpart, a positron, has a positive charge and they annihilate each other when they get too close.

A long time ago, when our universe was still in its earliest birthing throes, matter and antimatter were colliding and annihilating each other out of existence constantly.

This process slowed down as our universe gradually cooled, but there should have been equal parts matter and antimatter — and there weren’t. Instead, there were slightly more matter particles than antimatter.

We know this because we can see the remnants of the survivors of that cosmological massacre all around us: every bit of matter in our observable universe, from galaxies to dust mites and everything in between, exists because matter won that long-ago war of attrition.

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Measuring the precise differences between matter and antimatter was one of the original objectives of BaBar. From 1999 until 2008, BaBar recorded the collisions of more than 9 billion pairs of electrons and positrons. But physicists continue to analyze the data, searching for further clues to some of the most challenging puzzles in particle physics.

The pinnacle of BaBar’s scientific achievement was the confirmation of a 40-year-old theory explaining the matter/antimatter asymmetry. That data and measurement resulted in the awarding of the 2008 Nobel Prize in Physics to theorists Makoto Kobayashi and Toshihide Maskawa, who first developed the theory.

Their work has been incorporated into the Standard Model of physics, the theoretical framework that includes all known particles — only the Higgs boson remains undiscovered — and three of the four fundamental forces. But there’s a problem: the theory vastly underestimates the degree of asymmetry in the universe, which is why experiments like BaBar continue to search for new sources of asymmetry in particle interactions or decay patterns.

Physicists are only able to recognize the heavier particles produced in accelerator collisions by the electronic signatures they leave behind. These “signatures” are nuclear decay patterns. B-bar mesons only exist for fractions of a second before they decay into other secondary particles.

Decay patterns are like branching generations in the family tree, and every bit as complicated. In the case of the B-bar meson, the particle decays into a D meson, an anti-neutrino, and a tau lepton, so that is the signature BaBar’s detectors look for.

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According to the Standard Model, this particular decay pattern should occur only once in every 100 times a B meson is produced in the collision. But the latest results indicate that it occurs more frequently than predicted.

It is not yet statistically significant enough to claim a bona fide discovery: the level of certainly of the excess is around 3.4 sigma, while a 5 sigma result is required to claim discovery. Most 3-sigma results ultimately go away as further data is added to the analytical mix.

Nonetheless, this is an intriguing indication that the Standard Model might be showing signs of cracks. It builds on an earlier result from BaBar announced in February that found an unusual imbalance in the decay of charged B mesons — a potential source of the matter/antimatter asymmetry.

Negatively charged B mesons decayed into the expected three charged K mesons 30 percent more often than positively charged B mesons. It’s another 3-sigma result (2.8 sigma, to be precise), but it’s one more chink in the Standard Model’s armor.

Researchers are now looking to their colleagues in the Belle collaboration, another experiment studying the same kind of particle collisions. If Belle observes similar asymmetries in those crucial decays, that may comprise sufficiently compelling evidence that there is physics beyond the Standard Model.

Images: (top) A technician works on the BaBar detector. Source: SLAC. (middle) BaBar collaboration, SLAC. (bottom) Conceptual illustration of B-bar meson decay. Source: Greg Stewart, SLAC.

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