Text: The World's No.1 Science & Technology News Service New sub-atomic particle confounds theory 13:38 01 May 03 NewScientist.com news service A sub-atomic particle predicted to exist by physicists has been detected for the first time in a particle accelerator in California - but its properties do not fit with theory. The particle, called Ds (2317), was discovered in the debris of collisions between other sub-atomic particles. But it has baffled and intrigued the 500 physicists working on the project. They think the particle belongs to a family of eight particles known as the charm-strange mesons. Four of them have been found so far, all precisely fulfilling the theorists' predictions. But the mass of the newly found Ds particle is significantly smaller than expected, casting doubt on current theories of the nature of matter. The discrepancy means physicists are confronted with a particle they predicted but cannot explain, says the University of Pisa's Marcello Giorgi, leader of the international BaBar project that made the discovery. "This has never happened before," he says. "It is very, very exciting". Charm and strange The predictions for the particle's mass were based on the theory of quark interactions. Quarks are the subatomic particles which make up, for example, protons and neutrons. These nuclear particles each contain three quarks, but mesons contain only two. Charm and strange are two different types of quark, and it is combinations of these and their antimatter counterparts that make up the charm-strange mesons. They are held together by special force-carrying particles called gluons. Modelling the strength of the force allows the mass of the quark conglomerates to be predicted. If the particle found is indeed the Ds - a charm quark paired with an anti-strange quark - it should have a mass of about 2500 mega electron-volts. But the particle they have found is 10 per cent lighter. Until now, says Giorgi "the theory has predicted everything very precisely, but this time it is way out". The result was described to an audience of physicists at the Stanford Linear Accelerator Centre in California this week. If it is accepted, the discovery will mean revising the current understanding of quarks. But it has been positively received by other researchers. Four-quark state Roy Briere, an experimental particle physicist at Carnegie Mellon University in Pittsburgh, told New Scientist: "It certainly looks real - the result is surprising, but they've done all the right things with their data." Briere is collaborating in an experiment called CLEO, which also collects information on charm quarks. "My experiment is in principle in a position to confirm this - there are several groups in the world that could, and we are all working on it" he says. While a failure of theory is the most likely explanation for the Ds particle's low mass, it is possible that the particle is in fact a combination of four quarks. But four-quark states have never been detected and the experimental evidence suggesting they might exist is far from convincing. Giorgi will be leading further experiments over the next few weeks to probe the new Ds particle by studying its decay products. He will also look for other particles that clash with the existing theory. Theorists are already pondering what revisions may be required to accommodate the light Ds particle, says Briere: "I've already heard hall-way talk of papers". He is looking forward to heated debate at the next big particle physics conference, which starts on 19 May in New York. Reference: Submitted to Physical Review Letters http://www.newscientist.com/news/news.jsp?id=ns99993687

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