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  08-07-2003: IDENTIFIED A NEW FORM OF MATTER: IT’S THETA+, A PARTICLE FORMED BY FIVE QUARKS. IT HAS BEEN SEARCHED IN VAIN FOR ABOUT 30 YEARS  
 COMPLETE LIST 

© Copyright 2002 INFN The use of photos is free of charge. Please request authorisation from the INFN Communication Office




The researchers have been waiting for more than 30 years, but the particle maneged to escape at every appointment. Finally in the last few months it has been observed in different laboratories throughout the world, in Japan, Russia, U.S., Germany (in the last three countries in particular in the experiments Diana, Clas and Hermes, all of them in collaboration with the Italian National Institute of Nuclear Physics, Infn). Its name is Theta+ and it’s an absolutely new particle, formed by five quarks, while all the other particles known up to now, are composed at most, by three quarks.

Before Theta+ all particles composed by quarks observed so far were barions (like protons and neutrons), formed by three quarks, or mesons, formed by a quark and an antiquark (the antiquark is a particle of antimatter identical to the corresponding particle of matter but provided with some opposite characteristics, like the electric charge). The Theta+ particle is instead formed by five quarks, two ‘down’ quarks and an antiquark ‘strange’. Its existence is not excluded by the Standard Model, the theory that today describes with success the matter particles and their interactions: for this reason physicists were waiting to see it appear in their experiments, but until now any research has been vain.

After been escaped to the chase since a long time, the elusive particle left the first traces last year, at the Leps experiment (Laser Electron Photon a Spring 8) in Osaka, Japan. Its mass is about 1.6 times the one of a proton. The announcement has urged the researchers of the Institute for Theoretical and Experimental Physics (Itep), in Moscow, to analyse again and with the new information a series of data raised more than 10 year ago by the experiment Diana, in which physicists of the National Laboratories of Frascati (Infn) were involved. And here comes a new surprise: just those data seemed to confirm the Japanese result. But the Japanese and Russian observations could only be considered the beginning, not sufficient in order to talk about a real discovery but such to rise ferment and curiosity in the scientific community.

In order to find a new confirm, starting from last March, the 1999 data have been studied again, at Jefferson Lab, in Usa, at Clas experiment, in which Infn researchers of the National Laboratories of Frascati and Genova section of Infn are involved. As in the Japanese and Russian experiments, Clas is not focalised on the pentaquark research, but on the study of proton and neutron instable states. But unlike the others, Clas is able to point out all the particles produced in the collision, among them the pentaquark. One more time from the data analysis comes out the observation of Theta+. The observation has been communicated on June 30th.

In the end the last confirm has been announced on July 2nd from Desy Laboratory in Hamburg, Germany. The data this time are those produced by Hermes experiment, in which Infn researchers are involved, in particular from National Laboratories of Frascati, from the Ferrara and Bari Infn sections and Roma-Sanità. Hermes neither was focalised on the pentaquark research, but on studies about the ‘quark’ structure and proton or neutron gluons.

What has been observed in four different laboratories seems to be in every respect a new form of matter. Now experiments focalised on the properties of pentaquarks are needed. A very high research project was already approved last month at Jefferson Lab and it will be soon operating.

What about the future? The Standard Model predicts other exotic forms of matter: six-quark-particle, quark and gluon particles (elementary particles that hold quarks inside protons and neutrons) or also glueballs, formed only by gluons.
It is a little bit like finding in far tropical forest a place in which big animals have many heads or a never-seen combination of limbs: it would be like an ecosystem to explore and full of surprises.


 RELATED SITES 
http://www.lnf.infn.it/esperimenti/aiace
http://arXiv.org/hep-ex/0307018

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