Matter
| In near simultaneous discoveries was made by two collaborations: the Belle experiment at the Japanese High Energy Accelerator Research Organization (KEK) and BESIII experiment run by the Institute of High Energy Physics (IHEP) in China, a new form of matter, weighing more than a proton may have been found. |
It is possible that physicists working in China and Japan have found an exotic new form of matter.
The near simultaneous discovery was made by two collaborations: the Belle experiment at the Japanese High Energy Accelerator Research Organization (KEK) and BESIII experiment run by the Institute of High Energy Physics (IHEP) in China.
Both research teams were looking at a particle called Y(4260) that had been discovered in 2005. Smashing together electrons and their antiparticle, positrons, the experiments produced large numbers of Y(4260), which lives for only 10-23 seconds before falling apart into other particles.
In both cases, the data from the experiments had a peculiar hump around 3.9 gigaelectronvolts (GeV), an energy almost equal to four times the weight of a proton.
Research results from the Belle experiment that appeared in Physical Review Letters as well as second paper from BESIII appears in the same issue.
“Inspired by this discovery, we decided to further study the Y(4260) decay, which indeed did not disappoint us,” said particle physicist Zhiqing Liu, lead author of the study.
The teams have enough data to conclude they have discovered something new, a putative particle named Z(3900). But the scientists are still not entirely sure what to make of it. One possibility is that Z(3900) represents a subatomic structure made of four quarks, something that has never been solidly seen before.
There are six known quark types — named up, down, strange, charm, bottom, and top quarks – and they can be combined in various ways.
The matter most people are familiar with, namely protons and neutrons, is made when three quarks come together. Another class of particles can occur when two quarks are bound (technically, these are made from a quark and an antiquark). The most famous of these two-quark particles are kaons and pions. Though there have been hints of them in the past, no one has ever definitely discovered a particle with more than three quarks.
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| Image Source: APS/Alan Stonebraker |
The Y(4260) particle is thought to be a certain exotic type of particle with two quarks and an extra gluon, though its exact characteristics are still unknown.
Particle physicists seem to have a reasonably good understanding on the fundamental particles of the universe, but there are some glaring holes in this understanding.
Quarks are a good example of this according to physicist Eric Swanson, who examined the research in an article for Physical Review Letters.
"We know that all nuclear matter is made up of quarks, and we have a pretty good understanding of how two quarks interact at close range. But our quark theory cannot tell us which quark combinations will result in a bound particle or a stable nuclei. All we can go on is experience, and experience has shown that particles with four quarks do not exist. But the situation may have changed with the possible discovery of a new particle containing at least four quarks," Swanson writes.
The experiments have now produced more than 460 of these strange Z(3900) particles, suggesting that they are real phenomena and not simply a statistical fluke in the data. The new exotic particle appears to have an electric charge and contains at least a charm quark and an anti-charm quark. The simplest explanation for the rest of the particle’s properties is that it also contains an up and anti-down quark for a total of four quarks.
“We haven’t seen anything like that before and for that reason it’s exciting,” said Swanson.
There could still be other possible interpretations of the data. Scientists already know that two-quark particles exist. So what looks like four quarks bound together could actually turn out to be two two-quark particles interacting so strongly that they look like a four-quark particle. Such a finding would be known as a “hadron molecule” – another strange object speculated to exist in the subatomic world but never definitively seen. This is the explanation that Liu is leaning towards.
“The hadron molecule is just my personal preference,” he said. “But the real nature could also be something else.”
Swanson points out that there is another, more simple interpretation: that Z(3900) is composed of two two-quark particles interacting but not really strongly enough to stick together. This explanation would fit the data in an Occam's Razor manner - not as exciting, but still meaningful.
The next step for the researchers is to produce many new Z(3900) particles and watch how they decay, which should give some clues as to their properties. If the data shows they decay like ordinary, known particles, it could rule out the exotic interpretations. But if not, the scientists may have found something extremely interesting.
“We hope to reveal the nature of this particle in the following year,” said Liu.
SOURCES Wired, Eric Swanson
The experiments have now produced more than 460 of these strange Z(3900) particles, suggesting that they are real phenomena and not simply a statistical fluke in the data. The new exotic particle appears to have an electric charge and contains at least a charm quark and an anti-charm quark. The simplest explanation for the rest of the particle’s properties is that it also contains an up and anti-down quark for a total of four quarks.
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There could still be other possible interpretations of the data. Scientists already know that two-quark particles exist. So what looks like four quarks bound together could actually turn out to be two two-quark particles interacting so strongly that they look like a four-quark particle. Such a finding would be known as a “hadron molecule” – another strange object speculated to exist in the subatomic world but never definitively seen. This is the explanation that Liu is leaning towards.
“The hadron molecule is just my personal preference,” he said. “But the real nature could also be something else.”
Swanson points out that there is another, more simple interpretation: that Z(3900) is composed of two two-quark particles interacting but not really strongly enough to stick together. This explanation would fit the data in an Occam's Razor manner - not as exciting, but still meaningful.
The next step for the researchers is to produce many new Z(3900) particles and watch how they decay, which should give some clues as to their properties. If the data shows they decay like ordinary, known particles, it could rule out the exotic interpretations. But if not, the scientists may have found something extremely interesting.
“We hope to reveal the nature of this particle in the following year,” said Liu.
SOURCES Wired, Eric Swanson
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