July 15, 2005  
 

 

BABAR Finds New Massive Particle

By Heather Rock Woods

On Thursday, June 30, at the international Lepton-Photon symposium in Uppsala, Sweden, the BABAR collaboration announced the discovery of a new massive particle with curious behavior.

The particle is produced in collisions between electrons and their antimatter counterparts, called positrons. For the moment the particle has been named Y(4260), reflecting the measured mass of 4260 Mega-electron volts but the unknown nature of the state. The discovery is the subject of a paper recently submitted to Physical Review Letters.

A plot showing the resonance of Y(4260), the curious new massive particle whose discovery was recently announced by BABAR.
(Image courtesy of BABAR)

“This is a very beautiful result,” said Persis Drell, Director of Particle and Particle Astrophysics. “The techniques used to extract the signal take full advantage of the enormous BABAR data set. We don’t yet know what this new discovery is telling us... but we know enough to be surprised. This isn’t what we expected!”

One unusual aspect of the discovery is the Y(4260) is seen in events where the annihilating electron-positron pair are accompanied by an energetic photon emitted before the collision, and nothing else. This allows BABAR physicists to establish the particle’s quantum numbers—which define a particle’s intrinsic properties such as spin and charge. However, they have yet to learn which combinations of quarks and gluons—the universe’s indivisible bricks and mortar—make up the particle.

The most likely scenario is that Y(4260) is part of a large family of particles, known as psi mesons. These are particles composed of a charm quark and an anti-charm quark tightly bound by the strong force. Although they have the same basic composition, psi mesons exist with different masses, in just the same way as an electron bound to a proton in a hydrogen atom can only have certain allowed energies according to quantum mechanics.

There are problems with this explanation. What intrigues the research team is the implied pattern of Y(4260) decays into other particles. BABAR has only observed one set of decay products, a J/psi, pi+ and pi-. If this new particle is a type of psi meson—and it does have the same quantum numbers—it ought to decay much more often into particles containing a charm quark and a non-charm quark. Other measurements of total electron-positron annihilation rate suggest that this cannot be the case.

“We’re seeing hints that the particle doesn’t decay the way you would expect if it was part of the psi family,” said BABAR spokesman David MacFarlane. “It’s mysterious. Either we don’t understand the theory that explains how the strong force works in these bound states or the particle is more exotic than a simple charm anti-charm particle.”

The exotic possibilities include bound diquarks, particles with two quarks and two antiquarks, and hybrid mesons, particles with a quark, antiquark and bound gluon.

The discovery of the Y(4260) adds to the growing list of exotic new particles that have been seen in recent years at BABAR and Belle, the experiment at the KEK laboratory in Tsukuba, Japan. Some of these, such as the DSJ(2317) and DSJ(2458), refine our understanding of how quarks are bound into particles; others, such as the X(3872) and (3940) also defy conventional explanations and challenge our understanding of nature.

In addition to announcing Y(4260), the BABAR collaboration presented 65 papers at the Lepton-Photon symposium. The experiment is actively pursuing hints that matter-antimatter asymmetries in certain types of decays, called penguin modes, may be influenced by non-standard physics such as supersymmetry. This possible new symmetry of spacetime, where each of the presently known fundamental particles would be partnered by a very massive ‘superpartners.’ Existence of this or other new physics could impact penguin modes, which are particularly sensitive to the resulting effects. Two of the conference papers present data on two new types of penguin decays, one that follows standard physics and one that adds to the growing set of hints from BABAR and Belle that non-standard physics such as supersymmetry might be influencing these decays.

“We expect to double our data set by July 2006,” said MacFarlane, “which will possibly give us enough statistical significance to convincingly show that new physics is occurring in the penguin modes. It’s an exciting time.”

Some 600 scientists and engineers from 75 institutions in Canada, China, France, Germany, Italy, the Netherlands, Norway, Russia, Spain the United Kingdom and the United States are working on BABAR.

 

The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Friday July 15, 2005 by Topher White