The pentaquark, newly arrived matter
Physics: A discovery by Japanese and American physicists increases even more the mystery of quarks, elementary particles of the universe.
After thirty years of research, physicists seem to have at last put their hands on a rather strange particle composed of five quarks, a "pentaquark". If the interpretation of the experimental results obtained in Japan and in the United States is confirmed, it is the first time that this new, exotic form of matter has been observed. Usually, quarks are only found in groups of two or three inside particles that they compose. This unexpected discovery opens new doors for the understanding of the subatomic world, in which quarks have decidedly quite strange behavior.
Quarks are the fundamental particles that compose the core of ordinary matter of which we [ourselves] are composed, i.e. protons and neutrons, the heart of atomic nuclei. In the case of protons and neutrons, quarks are confined in groups of three. When a quark comes with an antiquark, this gives a particle called a meson. But in no case is a quark ever seen alone, and never before today have more than three quarks been seen in a single particle.
The object detected by the team of physicist Takashi Nakano, of the University of Osaka, thanks to the SPring-8 synchrotron, is made up of four quarks and an antiquark. The announcement of this first result at an international conference of particle and nuclear physics was received with skepticism by the community of specialists. "We have had rather negative reactions," says Takashi Nakano. "The majority of people thought that it was impossible." During the 1970s, some announcements of discovered exotic particles, such as baryonium, were consequently refuted and in fact were stray false signals in the background.
But some months after the Japanese announcement, similar experiments at Jefferson Laboratory in the United States and at the Institute for Theoretical and Experimental Physics (ITEP) in Moscow have just confirmed the results of the Japanese experiment. Takashi Nakano and his team have published their experimental results in the reputable journal Physical Review Letters . "According to the published data and those which will be published soon, the experimental signal no longer seems to be in doubt," explains Michel Garçon of Saclay. "On the other hand, the statistical precision is too weak to be certain of the interpretation that has been made. The experimentalists have only about 20 events, and need a good thousand events to have more invaluable information on the nature of the observed object. An experiment making it possible to reach this precision is in preparation at Jefferson Laboratory."
For the specialists, it is not yet certain that the five quarks are bound to each other in the same particle. It could be a question of a kind of molecule made up of two particles, bound together but each preserving its own identity. The difficulty of the physicists' work comes from the fact that they are unable to "see" the pentaquark itself. In fact, they detect its signature indirectly. The Japanese experiment consists of shooting a powerful beam of gamma rays on a target of carbon atoms. When a gamma ray photon hits a neutron or a proton of the carbon nucleus, it can produce a pentaquark. But this state is unstable and, at the end of an extremely short time, less than one hundredth of a billionth of a billionth of a second (a fraction of a second with more than 20 zeros in the denominator), the pentaquark dissociates into a K+ meson and a neutron which then hits the detector placed behind the target. Physicists then know that something has happened between the target and the detector because they measure a number less significant than envisaged of a certain particle, the K-.
In any case, this small deficit could very well have passed unnoticed because the initial experiment sought to measure another process of nuclear physics, which had nothing to do with the pentaquark. It is in fact the Russian theorist Dmitri Diakonov who suggested the idea to Nakano during informal discussions in 2000 over lunch at the Nordic Institute for Theoretical Physics (NORDITA) in Copenhagen, Denmark. In a theoretical article, Diakonov and his colleagues Polyakov and Petrov had predicted the existence of a form of pentaquark whose mass would be around 1.5 GeV (1.5 giga-electron volts is 1.5 billion electron volts). The Russian physicist suggested to his Japanese colleague to look in this low-energy domain, just above that of the proton, which was rather neglected by previous searches. Which Nakano did successfully!
According to Stavros Katsanevas, deputy manager of France's National Institute of Nuclear and Particle Physics (known as IN2P3) within the French National Science Research Center (CNRS), "this discovery opens the way for new avenues to fine-tune our as-yet partial understanding of quarks".
The secret of the quarks proves very difficult to reveal. As Frank Wilczek, pioneer of the study of quarks with MIT, explains in the Los Angeles Times, quarks can move around inside their subatomic bags but they can never leave. "When you have a really big nucleus, you lave lots and lots of protons and neutrons that can get as close together as they want to be. So why don't they share their quarks?" To this day, no "free" quark has ever been seen. We only know that, in certain cases, as in the split-second following the Big Bang for example, confinement can be broken and the quarks could then be found in a strange state, a soup of quarks dubbed the quark-gluon plasma by physicists. In studying a new system of five quarks, very different from the usual particles with two or three quarks, physicists have found a new means of understanding the secrets of quarks, key ingredients to the structure of matter.
 Physical Review Letters, vol 91, 4 July 2003.
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