THE pentaquark, an oversized subatomic particle that has challenged physicists since its reported discovery two years ago, does not exist after all, according to a sensitive experiment designed to look for it.
The existence of pentaquarks was hypothesised in 1997 by Russian researchers at the Petersburg Nuclear Physics Institute in St Petersburg. Unlike the ordinary protons and neutrons in atomic nuclei, which consist of three of the elementary particles called quarks, the pentaquark contains five. The Russians convinced a team of experimental physicists at a Japanese particle accelerator to look for the particle debris expected when pentaquarks decay, and in 2003 the team reported finding them (New Scientist, 12 July 2003, p 16).
That surprise discovery led other researchers to scour their archived data for evidence of the pentaquark. Within months, a dozen teams announced seeing it, with a couple of groups even claiming to have discovered two additional types of pentaquark. But about a dozen other teams failed to find the particles in their data, prompting theorists to wonder whether pentaquarks could only arise under certain experimental conditions.
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However, even when experiments did detect a pentaquark, different groups reported slightly different values for its mass. More alarmingly, all the measurements suggested the particle took about 100 times longer to decay than other particles of its mass, which is about 1.5 times the mass of a proton. 鈥淭he theory community, myself included, became rather troubled about the particle,鈥� says Bob Jaffe, a theoretical physicist at the Massachusetts Institute of Technology.
Now an international group called the CLAS collaboration has used the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, to search for pentaquarks by shooting energetic photons at liquid hydrogen. The experiment found no evidence for pentaquarks, even though its design was similar to a German experiment called SAPHIR that had previously produced positive results.
CLAS was 50 times as precise as SAPHIR, collecting more than 10 times as many readings at the relevant energy levels. 鈥淭hey put together a sophisticated experiment with high statistics,鈥� says Jaffe. 鈥淭hey watched for a long time and didn鈥檛 see it,鈥� and if the pentaquark had been there they should have seen it, he says.
Tom Cohen, a theoretical physicist at the University of Maryland in College Park, agrees the new result is impressive. He says previous experiments were not specifically designed to hunt for the pentaquark and so provided limited results, which were then trimmed further because researchers had to make assumptions about specific scenarios of pentaquarks decaying.
This had the effect of lowering the signal-to-noise ratio, meaning that a few random particles in the right energy range could look like pentaquark decay products, says Cohen.
CLAS team member Raffaella De Vita of Italy鈥檚 National Institute of Nuclear Physics presented the latest results at a meeting of the American Physical Society in Tampa, Florida, on 16 April. The results from another pentaquark-hunting experiment at the Jefferson lab are expected later this year. 鈥淲e need to push the limits of our experimental abilities. The question is still open,鈥� says Ken Hicks, who is running the Jefferson lab experiment.
