Search for Rare Particle Decay Comes Up Short

Aug 01, 2006 feature
BaBar detector image
BaBar detector image. Credit: Berkeley Lab

Based on work performed at a large particle detector known as the BaBaR experiment, scientists announced recently that they do not see evidence to support theoretical predictions that an extremely rare particle decay — the decay of the neutral “B” particle into a negative “tau” particle and its antiparticle, the positive tau — occurs often enough to be measured.

The search was initiated based on additions to the “Standard Model,” the broad physics theory that describes fundamental particles, how they interact, and how they bind together to form composite particles. The B particle, for example, is a member of a class of composite particles known as “mesons,” which consist of one quark — the “building block” of all other particles — and one antiquark. The tau, a fundamental, non-composite particle, can be thought of as a very heavy electron.

The original Standard Model allows the B meson to decay into the tau pair, but at a rate so low as to be extremely difficult to measure. However, recent proposed extensions to the model predict that the decay should occur at a measurable rate, perhaps through the addition of unknown forces or particles. Enter the BaBaR experiment.

Located at the Stanford Linear Accelerator Center (SLAC) near Stanford University, BaBaR was built, ultimately, to determine why matter dominates the present-day universe, with very little antimatter leftover from the Big Bang. The detector is positioned around one section of a very large particle accelerator, where a beam of electrons and a beam of anti-electrons — i.e. positrons — are directed in opposite circular paths at nearly the speed of light by a large ring of very powerful magnets.

The collisions produce B and anti-B mesons, which in turn decay into a shower of other particles. These particles pass into the BABAR detector, which records their trajectories, energies, and electric charge. From this data, scientists in the BABAR collaboration (about 600 physicists from more than 70 institutions worldwide) can work backward to determine how the B mesons decay.

“Our results show, at the 90 percent confidence level, that this decay occurs so infrequently as to be undetectable by BaBar. Our result is important because of what we did not find, since this places limits of possible new physics beyond the Standard Model,” said the experiment’s spokesperson, SLAC physicist David MacFarlane, to

BaBar physicists analyzed the collisions by calculating the “branching ratio” for this particular decay. In essence, the size of this number tells them how often the decay occurs versus other possible decay paths for the B meson. The branching ratio for the neutral B-to-tau/anti-tau decay was determined to be — at the largest — 0.0041.

This research was published in the June 23, 2006, online edition of Physical Review Letters.

On the Net:

By Laura Mgrdichian, Copyright 2006

Explore further: Linear accelerator could improve X-rays, particle colliders

add to favorites email to friend print save as pdf

Related Stories

On the front lines of the Higgs boson search

Oct 10, 2014

Finding the Higgs boson at CERN involved an exciting chain of events and sharing it with the wider public through the media was also a journey of discovery, Prof. Jon Butterworth told an audience at the IOP's ...

Detecting the flavors of neutrinos

Oct 07, 2014

Yesterday I talked about the weirdness of neutrinos, specifically that there three types of neutrinos (known as flavors), and they can oscillate between different flavors due to the quantum fuzziness of t ...

The evidence for pair-instability supernovae

Oct 06, 2014

A supernova is a stellar explosion. They can occur when a large star exhausts its ability to fuse hydrogen into higher elements, and its core collapses. The resulting rebound rips apart the outer layers of ...

Recommended for you

Cooling with molecules

14 hours ago

An international team of scientists have become the first ever researchers to successfully reach temperatures below minus 272.15 degrees Celsius – only just above absolute zero – using magnetic molecules. ...

Backpack physics: Smaller hikers carry heavier loads

Oct 21, 2014

Hikers are generally advised that the weight of the packs they carry should correspond to their own size, with smaller individuals carrying lighter loads. Although petite backpackers might appreciate the ...

User comments : 0