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Boffins five quarks short of a sub-atomic particle

No joy in pentaquark hunt

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A theoretical sub-atomic particle reportedly discovered in 2003 doesn't exist after all, scientists have discovered. The elusive pentaquark - comprising five quarks - failed to put in an appearance during experiments at Thomas Jefferson National Accelerator Facility in Virginia specifically carried out to search for the little blighter.

The pentaquark was first mooted in 1997 by Russian researchers who "convinced an experimental team at a Japanese particle accelerator to look for the products expected when pentaquarks decay", New Scientist reports. The Japanese duly announced in 2003 that they had identified the particle.

The alleged discovery led to a bit of a pentaquark landrush, as other teams scoured previous data for evidence of the five-quark package. Shortly thereafter, "a dozen teams announced seeing the particle, with a couple of groups even claiming to have discovered two additional types of pentaquark".

Now, however, a team called the CLAS colloboration has been unable to find evidence for the pentaquark. It fired energetic photons at liquid hydrogen and - despite the experiment resembling a similar effort by German outfit SAPHIR which came up trumps - CLAS turned up a blank.

Indeed, there had been a certain amount of scepticism regarding the pentaquark before the latest findings. Various groups who claimed pentaquark-spotting success reported different masses for the particle. More alarmingly, as the NS puts it, "all of the measurements suggested the particle took about 100 times longer to decay than other particles of its mass - about 1.5 times the mass of a proton".

MIT theoretical physicist, Bob Jaffe, said: "The theory community - myself included - became rather troubled about the particle." Jaffe applauded CLAS's methodology, stating: "They put together a sophisticated experiment with high statistics. They watched for a long time and didn't see it - and they should have, if the pentaquark had been there."

Jaffe - who with his colleague Frank Wilczek won the 2004 Nobel Prize for Physics for their work on quarks - further admitted: "I'm actually delighted." The reason for his joy is it now appears that while the force which binds quarks together in mesons (two quarks) and protons and neutrons (three quarks) is so strong that quarks are "never found alone in nature", it's not enough to bind five quarks - a fact which should keep the theoretical boffins at their blackboards for a while. ®

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