Especially strange: Physicists have discovered for the first time a quark made of two charming heavy quarks and two lighter antiquarks – the first representative of an entirely new class of four-quark particles. These doubly heavy tetraquarks are candidates for stable, and possibly stable, representatives of strange quark combinations. The discovery of one of these particles at the Large Hadron Collider (LHC) at CERN opens new opportunities for research into this strange form of matter.
In ordinary matter, quarks typically occur in groups of three, as do protons and neutrons in an atomic nucleus. There are also short-lived mesons made of quarks and antiquarks. So much for the classic inventory. However, in recent years physicists have discovered other strange groups of quarks in particle accelerators, including particles with fourAnd five or even six quarks.
The first double quark heavy quark
There is now growth in the particle zoo: physicists in Large Hadron Collider (LHC) of the research center CERN discovered the first representative of a new class of four quarks – the heavy double quark (Tcc)+). These particles, made up of two heavy quarks and two light quarks, were hypothesized as early as the 1980s, but have not been detected since. They are considered the best candidates for stable hadrons despite their strange mixture of quarks.
This is Tcc+Tetraquark has now been discovered by CERN researchers. It arises from the collision of protons in the LHC particle accelerator and was discovered on the basis of a noticeable “hump” in the mass distribution of the particles captured in the LHCb detector. “The tetraquark manifests itself as a narrow peak in the D meson mass spectrum,” physicists describe the signal.
According to the researchers, the statistical significance is more than 20 deviations – and so far above the five sigma threshold, from which one speaks of a discovery in particle physics.
The newly discovered tetraquark consists of two charm quarks, an up antiquark and a down antiquark. This means that it’s not only the first double-weighted quaternary quark – it’s also the first tetrahedron to contain two charm quarks without charm antiquarks. Physicists refer to such a particle as possessing a double “open magic” – a double dose of magic, so to speak.
For a long time, it was not clear whether these particles exist and whether they, like other quarks, are bound together by the strong nuclear force. “The much-anticipated discovery this week now shows that such a Tcc+-The tetraquark exists – and its signature can be experimentally proven,” the LHCb collaboration physicists reported on July 29, 2021 at the European Society’s High Energy Physics Conference.
More durable than all other exotics
Interestingly enough, the new tetraquark is the longest-lived of all known exotic hadrons, physicists report. Hadrons are all particles that pass through strong nuclear power together. The mass of the tetraquark is slightly less than the value of the decay products – two D mesons each consisting of a heavy charm quark and a light antiquark.
Thus, the energy in the bound quaternary state is slightly higher than that of the decay products and this is an indication of a relatively high stability. “This means that this quaternion still decays into a D mesonion through the strong interaction, but that it happens more slowly and less intensely than it does with other exotic hadrons,” explains LHCb researcher Ivan Polyakov of Syracuse University. The new particle thus confirms that the double-heavy tetraquark is in fact particularly stable.
New insights into alien hadrons
The discovery of the new tetraquark now allows the first conclusions to be drawn about the next, and possibly more stable, representative of a double-heavy tetraquark – a particle made up of two bottom quarks with two light antiquarks. However, this heavier hadron cannot yet be detected with the current performance of the LHC and is extremely rare, Polyakov explains.
Most useful is that its lighter cousin, the double “wizard” Tetraquark, is easy to spot and therefore searchable as well. “New Tcc+-Tetraquark is a tempting topic for further research. “Because its decay includes only easily detectable particles,” the LHCb team says. “This allows for high accuracy in mass determination and rigorous testing of existing theoretical models.” (European Physical Society Conference on High Energy Physics)EPS-HEP 2021))
Quelle: CERN, LHCb Collaboration
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