Particle oddball surprises physicists
Scientists of the CDF experiment at the Department of Energy's Fermilab near Chicago have found evidence of an unexpected particle whose curious characteristics may reveal new ways that quarks can combine to form matter. The CDF physicists have called the particle Y(4140), based on how the particle is produced and how much it weighs. The particle now joins the handful of X and Y particles previously discovered at other laboratories, all of which flout nature's known rules for fitting quarks and antiquarks together.
"It must be trying to tell us something," said CDF cospokesperson Jacobo Konigsberg of the University of Florida. "So far, we're not sure what that is, but rest assured we'll keep on listening."
Matter as we know it comprises building blocks called quarks. Quarks fit together in various well-established ways to build other particles: mesons, made of a quark-antiquark pair, and baryons, made of three quarks. So far, it's not clear exactly what Y(4140) is made of. But scientists know that its mass is 4140 Mega-electron volts.
Atoms such as hydrogen and oxygen can combine to form molecules such as water, or H2O. Some theorists think that X and Y particles are the subatomic equivalent of molecular structures, perhaps two mesons bound together for a short period of time. While physicists have an excellent theory (known as Quantum Chromodynamics) that describes the formation of individual mesons and baryons, the forces that govern the extended quark structures underlying the X and Y particles are not well understood, said Fermilab theorist Chris Hill.
The X and Y particle discoveries are not related to the disputed pentaquark observations, which seemed to indicate the existence of particles containing five quarks. Today, scientists reject the notion of pentaquarks since several follow-up experiments failed to confirm initial observations.
In contrast, the discoveries of various X and Y particles have been confirmed by independent observations. For example, the Belle collaboration at the KEK laboratory in Japan discovered the X(3872) particle in 2003, and the CDF collaboration at Fermilab as well as the BaBar collaboration at DOE's SLAC National Accelerator Laboratory in California confirmed the particle's existence. In 2005, the BaBar collaboration was the first to see the Y(4260) particle.
Last fall, CDF physicist Tommaso Dorigo summarized in his blog the status of precision measurements of the mass of the X(3872) particle, showing that some of these particles are now well-measured but poorly understood.
The Y(4140) particle reported by the CDF collaboration decays into a pair of particles, the J/psi and the phi, suggesting to physicists that it might be a composition of charm and anticharm quarks (also known as charmonium). However, the characteristics of this decay do not fit the conventional expectations for such a make-up, said Kai Yi, of the University of Iowa, who presented the Y(4140) result at a seminar at Fermilab yesterday. Other possible interpretations beyond a simple quark-antiquark structure are hybrid particles that also contain gluons, or even four-quark combinations.
The CDF scientists observed Y(4140) particles in the decay of a much more commonly produced particle containing a bottom quark, the B+ meson. Sifting through trillions of proton-antiproton collisions from Fermilab's Tevatron, CDF scientists identified a small sampling of B+ mesons that decayed in an unexpected pattern. Further analysis showed that the B+ mesons were decaying into Y(4140). The 600 scientists of the CDF collaboration submitted a paper on the Y(4140) observation to Physical Review Letters this week.
The Y(4140) particle is the newest member of the family of X-and-Y particles of similar unusual characteristics observed in the last several years by experimenters at Fermilab's Tevatron as well as at KEK laboratory and at SLAC National Accelerator Laboratory. The Tevatron experiments used collisions of protons and antiprotons to produce the new particles, while the experiments at KEK and SLAC relied on electron-positron collisions.
"We congratulate CDF on the first evidence for a new unexpected Y state that decays to J/psi and phi," said Japanese physicist Masanori Yamauchi, a cospokesperson of KEK's Belle experiment. "This state may be related to the Y(3940) state discovered by Belle and might be another example of an exotic hadron containing charm quarks. We will try to confirm this state in our own Belle data."
Theoretical physicists are trying to decode the true nature of these exotic combinations of quarks that fall outside our current understanding of mesons and baryons. Meanwhile experimentalists happily continue to search for more such particles.
"We're building upon our knowledge piece by piece," said CDF cospokesperson Rob Roser of Fermilab, "and with enough pieces, we'll understand how this puzzle fits together."