Physicists Discover Rare Particle Decay

After a quarter of a century of searching, physicists have discovered a rare particle decay that gives them an indirect way to test models of new physics.

Researchers with the CMS and LHCb collaborations at the Large Hadron Collider at CERN announced today (Friday, July 19) at the EPS-HEP Conference in Stockholm, Sweden, that their findings agreed closely with the Standard Model of particle physics, ruling out several models that predict new particles.

In this result, physicists showed for the first time enough evidence to declare the discovery of a decay of a particle made up of two kinds of quarks -- anti-bottom quarks and strange quarks -- into a pair of particles called muons.

The U.S. Department of Energy's Fermi National Accelerator Laboratory serves as the U.S. hub for more than 1,000 scientists and engineers -- including about a dozen Texas A&M University physicists -- who participate in the CMS experiment. DOE and the National Science Foundation support involvement by about 2,000 scientists and students from U.S. institutions in the LHC experiments CMS, ATLAS, LHCb and ALICE -- the vast majority participating at their home institutions via a powerful broadband network that ships data from CERN.

"This is a victory for the Standard Model," said CMS physicist Joel Butler of Fermi National Accelerator Laboratory. "But we know the Standard Model is incomplete, so we keep trying to find things that disagree with it."

The Standard Model predicts that the particle, called B-sub-s, will decay into two muons very rarely, only three times in every billion decays. However, the Standard Model assumes that the only particles involved in the decay are the ones physicists already know. If other, unknown particles exist, they might interfere, either making the decay happen more frequently than predicted or effectively canceling the decay out.

"This is the place to look for new physics," said LHCb physicist Sheldon Stone of Syracuse University. "Small deviations from the predicted rate would firmly establish the presence of new forces or particles."

What scientists found was a decay that followed the Standard Model's predictions almost to the letter. This spells trouble for several models, including a number of models within the theory of supersymmetry, which predicts that each known particle has an undiscovered partner particle.

But the hunters of new particles have not lost hope; the result leaves room for other models of physics beyond the Standard Model to be correct.

The analysis is a tour-de-force for the two LHC experiments, which needed to eliminate an enormous amount of background information generated by other particle decays that mimic the decay they were looking for. The latest results from searches at the ATLAS experiment at CERN and the CDF and Dzero experiments at Fermilab are consistent with the results from the LHCb and CMS experiments.

As much as scientists can learn from measuring this decay, they can learn even more if they compare it to the decay of another particle made of quarks: B-sub-d, which is made of an anti-bottom quark and a down quark. A B-sub-d particle should decay even more rarely into a pair of muons than a B-sub-s particle. Physicists did not have enough data to make a definitive statement about this decay in this analysis, but their work shows that they will be able to gather evidence of it after the LHC restarts in 2015 at higher energy.

EXPLORING THE TEXAS A&M TIES

Texas A&M University physicist Teruki Kamon describes today's finding as a moment he's been waiting for since 2002, when he co-authored a paper with fellow Texas A&M physicists Bhaskar Dutta and Richard Arnowitt proposing a powerful way to test a cosmologically consistent model of supersymmetry.

The Texas A&M trio, pioneers in combining theory and experiment at a single university, published those early findings in the international journal Physics Letters B, zeroing in on rare particle decays -- which had never been observed to date -- as their preferred technique and suggesting that the first evidence for supersymmetry might be observed through the Collider Detector at Fermilab (CDF) experiment, in which Kamon has been involved since 2002.

In 2011 Kamon and the CDF team published another paper detailing their recently completed analysis of 10 years of Tevatron-gleaned data that revealed a slight but tantalizing excess in the painstaking search for rare particle decay. While the result was not conclusive, Kamon says it was a definite conclusion.

"We searched for this at the Tevatron, hoping supersymmetry-related particles are light enough," Kamon said. "Now, they look heavier."

In switching to the CMS experiment, Kamon says he chose to pursue so-called direct searches instead of indirect because of the LHC's powerful capability and the conventional wisdom that new physics discoveries, such as supersymmetry, would come before actual particle observation.

"The LHC is truly the machine to probe new physics at Tera-electron volts," Kamon said. "But nature is tricky; the LHC discovered the Higgs boson instead. But this is actually encouraging, because supersymmetry needs the Higgs boson. So I just keep moving. This is not the end of the story nor of supersymmetry."

Kamon says the 2002 paper fits the theme of Texas A&M's present-day George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, which is uniquely suited to explore the interconnection between particle physics and cosmology, given its makeup of high-energy physicists and astronomers who are involved in many prominent international collaborations and experiments -- LHC, Fermilab and otherwise -- and well positioned to capitalize on the latest astrophysical observations regarding the precise determination of the content of the universe.

In addition to Kamon, fellow Mitchell Institute high-energy experimentalists Ricardo Eusebi, Alexei Safonov and David Toback are intensely involved with the CMS experiment at the LHC to discover the presence of new physics. At the same time, Mitchell Institute high-energy theorists Arnowitt and Dutta, along with Dimitri Nanopoulos, are utilizing LHC experiment results to understand the past, present and the future of the universe.

For more information on Texas A&M's Mitchell Institute, visit http://mitchell.physics.tamu.edu.

To learn more about high-energy physics research at Texas A&M, go to http://physics.tamu.edu/research/he.shtml or visit Texas A&M Collider Physics.