A BRAND-NEW particle called the pentaquark has made its debut at labs in Japan and the US. Unlike the ordinary protons and neutrons in atomic nuclei, which consist of three quarks, the pentaquark contains five.
The result has delighted Russian physicists who had to weather a storm of scepticism when they predicted the mass of the particle in 1997. 鈥淚t was not an easy decision to publish our paper six years ago, but eventually we went ahead despite resistance in the community,鈥� says Maxim Polyakov, now at the Ruhr University in Bochum, Germany. 鈥淚t is a great pleasure that our theory seems to be correct.鈥�
Particles that contain quarks fall into two main categories. Baryons, such as protons and neutrons, contain three quarks. Short-lived mesons contain two, a quark and an anti-quark. Theory also permits the existence of a fleeting five-quark particle, so for more than 30 years scientists have been hunting for it in the debris of particle-smasher experiments.
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In 1997, Polyakov, Dmitri Diakonov and Victor Petrov at the Petersburg Nuclear Physics Institute in Russia predicted that one particular pentaquark 鈥� containing two 鈥渦p鈥� quarks, two 鈥渄own鈥� quarks and an 鈥渁nti-strange鈥� quark 鈥� should have about 1.5 times the mass of a proton.
Now scientists say they have spotted a particle with the right mass and all the hallmarks of a pentaquark (Physical Review Letters, vol 91, p 012002). A team led by Takashi Nakano of Osaka University in Japan and another led by Ken Hicks at the Thomas Jefferson National Accelerator Facility in Virginia made a high-energy gamma ray interact with a neutron to create a meson and a pentaquark. The pentaquark lived for only about 10鈭�20 seconds before decaying into a meson and a neutron (see Graphic).
The new particle may have been common in the universe just after the big bang, 14 billion years ago. And further studies of it could help patch up some holes in the theory of the strong force that glues quarks together. 鈥淭his discovery is not just getting another animal in the zoo,鈥� says Polyakov. 鈥淚t will seriously influence our understanding of what the ordinary proton and neutron are made of and how they work.鈥�
But so far physicists know little about the pentaquark. 鈥淲e haven鈥檛 had time to think about the implications,鈥� says Hicks. He hopes to find out all its basic qualities, such as its size and how densely the quarks inside clump together.
In 1979, two teams of physicists saw hints of another, heavier pentaquark in experiments with old-fashioned bubble chambers at CERN, near Geneva, and at Argonne National Laboratory in Illinois (快猫短视频, 31 January 1980, p 315). But confirming the discovery with the equipment available at the time would have meant almost a decade of work, analysing at least 5 million photos. 鈥淣one of us was signing up,鈥� remembers Peter Negus of the University of Glasgow in the UK, a member of the original CERN team. He adds that he is excited by the new results: 鈥淚t really takes me back.鈥�