Illustration: Benjamin Currie
CERN’s Large Hadron Collider-b (LHCb) experiment presented its latest discovery last week at a meeting of the European Physical Society Conference on High Energy Physics. Meet the double-charm tetraquark, the longest-lived exotic matter particle yet discovered.
Quarks are the building blocks of all matter; they’re subatomic particles that combine to form hadrons, the group that includes the familiar protons and neutrons. (In other words, quarks are smaller than small.) Protons and neutrons are both made up of three quarks, but the newly discovered hadron particle is made of four, making it a species of tetraquark. The first tetraquark was officially discovered in 2003.
Quarks have antimatter partners—evil twins, if you will. This new tetraquark is made up of two heavy quarks and two light antiquarks, stuck together into one particle.
Exotic particles like this can be created within accelerators like the Large Hadron Collider, but they pop into and out of existence extremely quickly. This new particle is considered to have a long lifespan before it decays, but “long” in this case is still so short it can hardly be measured in human terms. Its lifespan is probably a little longer than one-quintillionth of a second, said Patrick Koppenburg, physicist at the Dutch National Institute for Subatomic Physics and a member of the LHCb team at CERN.
“We will never be able to measure its lifetime directly,” Koppenburg wrote in an email to Gizmodo.
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Quarks differ from each other in mass and charge, giving them six distinct flavors: up, down, top, bottom, strange, and charm. The new tetraquark is the first exotic hadron that’s “doubly charmed,” meaning its two charm quarks are present alongside antiquarks that aren’t charmed. “Quarks can be seen as Lego bricks, so just discovering a new combination of four quarks that have not been previously observed is not necessarily exciting. What is interesting to study is HOW those particles are combined, because this can teach us how quarks stick together,” said Freya Blekman, an elementary particle physicist at Vrije Universiteit in Brussels who was not affiliated with the experiment, in an email. “I think it is a very exciting result.”
Like many other quark states, this double-charm tetraquark was found at LHCb using a method called bump hunting. Basically, the researchers fire up the particle accelerator and let particles collide, keeping their eyes peeled for an unexpected amount of energy or mass in the system. When they get results out of sync with the system’s basic noise after they’ve filtered out all irrelevant signals, the researchers have a clue they’ve stumbled across something new; it was bump hunting that revealed the Higgs boson in 2012. The 62 hadrons so far discovered at the Large Hadron Collider have basically been cajoled out of obscurity by the accelerator’s extreme physics and the vast team that labors over all the machinery and data.
The findings from particle colliders advance physicists’ understanding of how fundamental particles interact. “If you really want to know somebody, you put them in extreme situations. What we’re doing with these tetraquarks and pentaquarks is we’re putting the theory in extreme situations, which aren’t the run-of-the-mill that we have observed for the last 60 years,” Marek Karliner, a particle physicist at Tel Aviv University who was not affiliated with the recent research, explained in a video call. “It turned out that this particular design of the LHCb experiment is ideal for searching for new hadrons.”
The double-charm tetraquark (written scientifically as Tcc+) decays so slowly because it’s just slightly heavier than the particles it decays into. Its rare configuration puts it in a class of candidates for stable exotic hadronic states. Previous results from the LHCb allowed theoretical physicists to predict in 2017 that a similar tetraquark, called Tbb, could be entirely stable, meaning it would not decay at all through the force of strong interaction.
“It shall be a breakthrough in particle physics, if the discovery of a new type of tetraquark with two heavy quarks and two light antiquarks is proved,” said Rui-Lin Zhu, a theoretical physicist at Nanjing Normal University in China, in an email. “This discovery of doubly charm tetraquark in July 2021 is absolutely the triumph of theoretical predictions.”
Karliner, who co-authored the 2017 prediction, said that the experiments validated his earlier work and highlighted LHCb’s strengths as an instrument for particle hunting. “They are now ruling the hadron universe,” he said of the LHCb team. “They have cornered the market on exotic hadrons—not entirely—but in LHCb they have a much bigger market share.”
In other words, if anyone’s going to find something longer-lived than this double-charm tetraquark, it’ll probably be them. Producing the predicted particle Tbb, which the CERN Courier recently called Tcc+’s “beautiful cousin,” could be the next celebration on the horizon.
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