Today, CERN, home of the world's large particle accelerator, announced that it had observed a new particle: the pentaquark.
One of the experiments on the Large Hadron Collider called LHCb (Large Hadron Collider beauty) reported that it had discovered the long-sought but until now elusive particle, which gets its name from the fact it's made up of five quarks, one of the most elementary particles.
"We've discovered for the first time unambiguous evidence for the existence of a particle containing five quarks, or more precisely four quarks and an antiquark," Guy Wilkinson, a University of Oxford particle physicist and spokesperson for the LHCb experiment, said in a phone interview. Antiquarks are the same as quarks but have the opposite charge.
"This so-called pentaquark is something that has been predicted for over 50 years," Wilkinson continued.
Pentaquarks, or particles like them, were first proposed by Murray Gell-Mann, an American physicist who won the Nobel Prize in physics in 1969 for his work on elementary particles. He proposed a key part of the quark model, which CERN writes "revolutionized" our understanding of the structure of matter.
Quarks combine to make up other particles, most notably protons and neutrons, which are each made of three quarks. Gell-Mann theorised the existence of particles made of five quarks, but until now we haven't found them. That's not for lack of trying, but previous reports of observations haven't proved conclusive.
"It has been very frustrating and maybe increasingly worrying that, after all this time, we've been unable to actually find this class of particles," said Wilkinson of the impact of his team's discovery. "So it tells us that quarks do bind together in the ways that he [Gell-Mann] said they should."
He hopes that the discovery of the pentaquark will help physicists better understand and make predictions regarding "strong interaction," the principle in the Standard Model that explains how quarks interact with each other. "Discovering this new type of particle and understanding the behaviour of this particle should tell us more about that," said Wilkinson.
Unlike protons and neutrons, pentaquarks aren't all around us on Earth today as they're unstable. To find them, the LHCb team had to use the immense power of the Large Hadron Collider. Their findings come from data taken from the first run of the LHC, which ended in 2013, and not the new run, which sees particles colliding at almost twice the energy (it takes a long time to sift through and analyse all that data).
"Now we know these creatures really do exist, there should be many more out there that we can hope to see."
They found the pentaquark in the decay of other particles during collisions. Wilkinson explained that they first noted a distinctive peak in their data which was a telltale sign of the new particle. They then looked at all the distributions of particles that went into it to be sure. "We checked that the hypothesis was compatible only with the pentaquark and could not be explained by more mundane things—a conspiracy of other established particles coming together to give this enhancement," explained Wilkinson.
The LHCb collaboration, which counts hundreds of scientists among it, published a paper on its findings on the Arxiv preprint server and has submitted it to the Physical Review of Letters.
Next, the group aims to study the properties of the pentaquark more precisely, such as how the quarks inside it are bound together—whether they're bonded together equally or not. "We could also look for other pentaquarks," said Wilkinson. "Now we know these creatures really do exist, there should be many more out there that we can hope to see."
The discovery adds another piece to the particle physics jigsaw that fits in with the Standard Model, the fundamental theory that describes the basic particles and interactions that make up matter.
The particle was confirmed using data from Run 1 of the Large Hadron Collider. Now that the collider is back at even greater energies, the physicists at CERN continue to hold out for more new discoveries—maybe even some that we're not expecting.
Watch more: Motherboard's new documentary on the reboot of the Large Hadron Collider and the search for new physics