Text: New particle is double trouble for physicists 17:05 17 November 03 NewScientist.com news service A mysterious sub-atomic particle has been revealed that does not to fit any of the models currently used by physicists. The discovery either suggests that a new family of molecule-like sub- atomic particles exists, or that theorists must substantially re- think their theory of the masses of sub-atomic particles. "If the molecular interpretation is correct, then it would be one of the first compelling cases for a new type of matter - it opens up a whole new realm of study," says Gerry Bauer, at the Massachusetts Institute of Technology, one of the team involved in confirming the existence of the new particle. There would also probably be more molecule-like particles to find, he adds. Adapting the quantum chromodynamic (QCD) theory used to predict the masses of sub-atomic particles is a less dramatic possibility, but still significant. The current theory would need "a lot of work", says Steve Olsen, of the University of Hawaii, and part of the Belle Collaboration that discovered the new particle. Charm and anti-charm Particles that contain quarks fall into two main categories. "Baryons", such as stable protons and neutrons in atomic nuclei, contain three of the six known quarks. "Mesons" contain two, a quark and an anti-quark, but they are never stable and vanish in a split second. A type of meson, made up of one charm and one anti-charm quark, is called a charmonium. It was the search for a charmonium called psi2 that led to the discovery of the new particle, dubbed X(3872). It was created in the KEK electron-positron collider in Tsukuba, Japan. The data was studied by the Belle collaboration in Hawaii and is now due to be published in Physical Review Letters. Initially, the Belle team thought they had found psi2. But they found the particle was 60 MeV heavier than the mass predicted by QCD. And a closer analysis revealed two more problems. Firstly the particle did not decay by either of two predicted modes. Secondly, the way that the energy was distributed within the observed decay mode did not fit the theory for psi2. But then the researchers noticed a strange coincidence - the X particle had exactly the same mass as the sum of two completely different particles, both types of D mesons. D mesons contain either a charm quark or an anti-charm quark but not both. Smoking gun The coincidence suggests that, instead being just one charmonium particle, the X particle was two D mesons joined together, rather like how atoms can combine to form a molecule. "Maybe the QCD model is screwed up, but what is suspicious is that X happens to be right next to that mass," says Bauer, who is part of the CDF collaboration that studies the Tevatron collisions at Fermilab in Illinois. His team confirmed that a particle X exists at this mass. Theorists have speculated that a double-meson particle might exist since the 1970s, but this has never been experimentally confirmed. The best indication until now was a lighter particle called Ds2317, revealed in April at the Stanford Linear Accelerator Centre. "People have used [a double-meson] particle to solve theoretical problems, but there has been no smoking gun," Olsen told New Scientist. To confirm the smoking gun, Olsen says more work is needed on the data and possibly more particle-smashing experiments. But he is also busy looking for particles that could, with adjustments to the QCD theory, explain the new X particle. Celeste Biever http://www.newscientist.com/news/news.jsp?id=ns99994389

See Also:

Source: