Scientists from the CERN particle physics accelerator discovered a new subatomic particle last week, whose characteristics were predicted with great accuracy by an Israeli scientist some years ago. The new particle was seen for the first time last Thursday, during an experiment at the Large Hadron Collider particle accelerator outside Geneva.
The first unambiguous sighting of the new particle, the "doubled charmed Xi,” which for now is known by the unwieldy name of Xi-cc++ or the Xi cc for short, was officially announced at the EPS Conference on High Energy Physics in Venice last week.
The new particle is from the baryon family, which includes protons and neutrons (found in the nucleus of the atom) alongside much heavier, more exotic and less stable particles. At 3,621 million electron volts, it is almost four times heavier than a proton, but it isn't stable.
Prof. Marek Karliner of the School of Physics and Astronomy at Tel Aviv University predicted the characteristics of the new subatomic particle years ago, along with an American colleague. “It’s been known for years that all the possible combinations to bind three quarks are supposed to exist,” he says, “but because of the large mass and instability, it is hard to create them, and they tend to decay quickly. So it is much more challenging to find them in an experiment.”
“Finding a doubly heavy quark baryon is of great interest as it will provide a unique tool to further probe quantum chromodynamics, the theory that describes the strong interaction, one of the four fundamental forces,” said Giovanni Passaleva, a spokesman for the Large Hadron Collider collaboration. “Such particles will thus help us improve the predictive power of our theories,” he added.
The experiment was one of a series that looked at fundamental particles of matter to better understand the basic forces of physics. This can help scientists comprehend how matter was formed and various phenomena that exist in the universe.
The nucleus of an atom is made up of neutrons and protons, and is held together by what is known as the strong force. But both protons and neutrons are actually made up of small particles known as quarks, and these are divided into six types (three pairs), known as “flavors”: up, down, strange, charm, top and bottom.
The new particle is composed of two heavy charm quarks and one up quark. The difference in mass between the types of quarks is huge, from only 2.3 MeV for the up quark and 4.8 MeV for the down quark, the two lightest ones, to the massive 173,210 MeV top quark. Scientists have yet to come up with a theory to explain these huge differences in mass.
The proton, for example, is composed of two up quarks and one down quark, while the neutron is made up of one up quark and two down quarks.
Experiments conducted over the years have shown quarks cannot exist in isolation, but only when bound together, usually in one of two main types of particles: Baryons, made up of three quarks, and mesons, made up of a quark and an antiquark.
Baryons with up, down and strange quarks are seen all the time, and such particles have a mass just a little higher than the proton and neutron. Combinations of quarks with a single charm or bottom quark have been seen binding to lighter quarks, along with many other combinations. But this was the first unambiguous sighting of two charm quarks bound together, along with an up quark – something scientists have been searching for years.
The observation of the Xi cc in the Large Hadron Collider raises expectations to detect other members of the doubly heavy baryon family, and the search for them will now commence.
Three years ago, Karliner and his colleague Prof. Jonathan Rosner from the University of Chicago published an article in the scientific journal Physical Review, in which they predicted the characteristics of baryons with two heavy quarks, including the double charm identified last week. They estimated the Xi baryon with two charm quarks and one up quark would have a mass of 3,627 MeV, plus or minus 12 MeV. Last week’s results show their predictions were exactly on target.
"The importance of the discovery does not affect our day-to-day life,” says Karliner. “But the existence of the six quarks and the understanding of the strong forces acting between them are very relevant to understanding the history if the universe, to the understanding of the relative frequency of the various chemical elements, to understanding the stability of atoms and the solution to the question of why the universe contains so much matter and so little anti-matter.”
“In contrast to other baryons, in which the three quarks perform an elaborate dance around each other, a doubly heavy baryon is expected to act like a planetary system, where the two heavy quarks play the role of heavy stars orbiting one around the other, with the lighter quark orbiting around this binary system,” said Guy Wilkinson, the former spokesman of the collaboration.
In 1961, Israeli physicist and former Science Minister Yuval Ne’eman took the first – and critical – step toward understanding the structure of subatomic particles such as protons and neutrons, as American Jewish physicist Murray Gell-Mann also did independently. They proposed a classification structure later known as the Flavor SU(3), or the Eightfold Way quark model, which led directly to the first theory about quarks, proposed in 1964 by Gell-Mann. The same model was independently proposed by another Jewish American physicist, George Zweig. Gell-Mann won the Noble Prize in Physics in 1969 for his work on the theory of elementary particles.
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