For almost five decades, scientists have theorized that a significant particle may exist beyond the realms of our current knowledge of subatomic physics, but proving this has been difficult.

This is no longer the case, as scientists now say that they have found the much sought-after pentaquark, first predicted to exist in the 1960s. After 50 years of searching, the particle has turned up at the LHCb experiment at CERN’s Large Hadron Collider.

Most significantly of all, the discovery indicates that there may be a new type of matter – albeit one that lives for one billion trillion trillionth of a second before it decays.

“The pentaquark is not just any new particle,” LHCb spokesperson Guy Wilkinson said in a statement. “Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”

The pentaquark is made of five quarks – which are the smallest particles that we know to exist. In different combinations they produce larger particles. For example, groups of three quarks are known as baryons, which include things like protons.

Last year, scientists at CERN announced that they had found the first four-quark particle, a tetraquark, named Z(4430). This latest discovery eclipses that, and could open up new realms of physics.

“It is an important result in that it shows that there is a new state of matter,” Professor Sheldon Stone from Syracuse University, who did the physics analysis for the result with his colleagues, told IFLScience. “Although pentaquark states were thought possible from the dawn of the quark model, the theory that explains the structure of baryons like the proton, they had never been seen before.”

The discovery was made at CERN’s LHCb experiment. CERN.

The team behind the discovery has submitted the research to the journal Physical Review Letters, but has been extremely careful in confirming the finding first. Previous “discoveries” of the pentaquark, such as one in 2005, were subsequently proven to be false.

“Benefitting from the large data set provided by the LHC, and the excellent precision of our detector, we have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states,” LHCb physicist Tomasz Skwarnicki, also of Syracuse University, said in a statement.

Pentaquarks were found by examining the decay of a baryon called Lambda b into three other particles: J-psi, a proton and a charged kaon. They were found to be made of four quarks and an antiquark, which is the antimatter version of a regular quark with the same mass but opposite charge.

There are six types of quarks, known as “flavors,” which denote their mass, charge and spin. This last term does not refer to rotation, but instead dictates the magnetic field and other properties. The flavors are Up, Down, Charm, Strange, Top and Bottom. The pentaquark was found to be made of two up quarks, a down quark, a charm quark and one anticharm quark.

The discovery was made from data gathered in the first run of the LHC. Now that it can achieve higher energies in its second run, the researchers will search for more data on pentaquark and how they interact with other subatomic particles.


The Large Hadron Collider, famous for finding the Higgs boson, has now revealed another new and rather unusual particle. Teams at the LHC, the world’s largest particle accelerator, recently began a second run of experiments using far more energy than the ones that found the Higgs particle back in 2012. But another of the groups, LHCb, have also been sifting through its data from the billions of particle collisions of the first run of the LHC, and now think they’ve spotted something new: pentaquarks.

Pentaquarks are an exotic form of matter first predicted back in 1979. Everything around us is made of atoms, which are mode of a cloud of electrons orbiting a heavy nucleus made of protons and neutrons. But since the 1960s, we’ve also known that protons and neutrons are made up of even smaller particles named “quarks”, held together by something called the “strong force”, the strongest known force in nature in fact.

Experiments in 1968 provided the evidence for the quark model. If protons are hit hard enough, the strong force can be overcome and the proton smashed apart. The quark model actually explains the existence of more than 100 particles, all known as “hadrons” (as in Large Hadron Collider) and made up of different combinations of quarks. For example the proton is made of three quarks.

All hadrons seem to be made up of combinations of either two or three quarks, but there is no obvious reason more quarks could not stick together to form other types of hadron. Enter the pentaquark: five quarks bound together to form a new type of particle. But until now, nobody knew for sure if pentaquarks actually existed – and, although there have been several discoveries claimed in the last 20 years, none has stood the test of time.


The intricate dance of the J/psi and the proton CERN

Pentaquarks are incredibly difficult to see; they are very rare and very unstable. This means that if it is possible to stick five quarks together, they won’t stay together for very long. The team on the LHCb experiment made their discovery by looking in detail at other exotic hadrons produced in the collisions and they way these break apart. In particular, they looked for the Lambdab particle, which can decay into thee other hadrons: a Kaon, a J/psi, and a proton.

The J/psi is made of two quarks and the proton is made of three. The scientists discovered that for a short period of time these five quarks were bound together in a single particle: a pentaquark. In fact, through detailed analysis of the data, they actually discovered two pentaquarks and have given them the catchy names Pc(4450)+ and Pc(4380)+.

Why Is This Important?

The discovery answers a decades-old question in particle physics and highlights another part of the mission of the LHC. Discoveries of new fundamental particles such as the Higgs boson tell us something completely new about the universe. But discoveries like pentaquarks give us a more complete understanding of the rich possibilities that lie in the universe we already know.

By developing this understanding, we may get some hints about how the universe developed after the Big Bang and how we’ve ended up with protons and neutrons instead of pentaquarks making up everyday matter.

With the LHC now colliding protons at almost twice the energy, scientists are ready to tackle some of the other open questions in particle physics. One of the main targets with the new data is Dark Matter, a strange particle which seems to be all around the universe, but has never been seen. Testing the current understanding of quarks, the strong force and all the known particles at this new energy is an essential step towards making such discoveries.

The Conversation

Gavin Hesketh is Lecturer in Particle Physics at UCL.

This article was originally published on The Conversation. Read the original article.

The universe isn’t just expanding, it’s accelerating. And no one knows why. Dark energy was proposed to fill in this knowledge gap: a sort of “repulsive gravity” that pushes matter and space-time away instead of dragging it closer. Rather than gathering around regions of dense matter (such as stars or galaxies), dark energy prefers to hang out in the most isolated neighborhoods of the universe in the vast regions of empty space.

But what sort of matter or energy field would act in this reclusive way? Clearly, if a particle was responsible for this, it would be unlike anything even cutting-edge particle physicists had ever seen before. 


It was these strange properties that gave physicists the idea of the chameleon field. “The chameleon theory introduces a new ‘fifth’ force into our understanding of physics,” Clare Burrage, a physicist from the University of Nottingham, explained to IFLScience. This force’s strength varies depending on how much matter is in the vicinity. The force gets weaker as the amount of matter gets denser, so it wouldn’t be easily detectable on Earth. However, in the empty voids of space, the force extends to a massive and powerful range, pushing the matter in the universe apart – the opposite effect of gravity.

If correct, then the chameleon field would permeate the universe and be associated with dark energy particles, which the team termed chameleon particles, much like the electromagnetic field is linked to light particles (photons). Chameleon particles are unusual in that they don’t have a fixed mass, unlike electrons or protons that have well-defined masses. The force of the chameleon field is inversely proportional to the mass of the particle. In the vicinity of matter, the chameleon force is weak and the particles are heavy. Conversely, in a vacuum, the force is strong but the particle is light.

The theory may already sound preposterous: How can there be an additional force acting on matter that we’ve never detected? Co-author Holger Müller from the University of California, Berkeley, told IFLScience, “If you invent a new particle, you would expect that it interacts with normal matter. And if it does that then it might contradict previous experiments because we think we understand all the forces that interact with normal matter and we’ve never found an additional force.”

Well, this is where the name of the chameleon field starts to make sense. Like a chameleon changes its color to blend in with its environment, the chameleon field’s properties vary depending on the density of a nearby object. This is why it’s so difficult to detect on Earth: The force gets so small when it’s around matter that it’s effects are barely detectable. The further away from matter the chameleon field is, the stronger the force gets and in the deep, empty recesses of space, it will expand for many light-years, pushing matter apart. It was given this unusual property of varying repulsion in order to abide by all the longstanding theories of physics.

“Unlike gravity, the force effectively switches off when the object gets sufficiently large and sufficiently dense,” Burrage explained to IFLScience.

When it comes to a candidate for dark energy, “The chameleon particle is a particle that has all the required properties: It can explain the cosmological observations, and unlike many other theories it doesn’t contradict existing theories,” Müller summarized to IFLScience. The results of the experiment have been published in Science, and a pre-print is also available on arXiv.

Left is a diagram of the inteferometer experiment (credit: Simca Bouma); the atoms (violet dots) probe the vacuum in search of dark energy. To the right is a photograph of the actual equipment (credit: Holger Müller).

An experiment devised to catch an elusive chameleon involves spotting an inconsistency from the predictions of gravity. Sadly, we can’t just trap one with a net. Instead, the experiment takes place in the vacuum chamber of an atom interferometer, replicating the conditions of the vacuum of space. An aluminum sphere, with a diameter of 2.5 centimeters (1 inch), sits impressively in the chamber. The scientists then created a cloud of cesium atoms on top of the aluminum sphere and just let them fall. If there is no chameleon field, then the only force felt by the atoms is Earth’s gravity. However, if there is a chameleon, then they would experience Earth’s gravity plus the chameleon force.

“The sphere acts as an extra source of the chameleon force, such that atoms should fall a little faster in its presence (if the chameleon exists),” co-author Justin Khoury, from the University of Pennsylvania and one of the original founders of the chameleon theory, said to IFLScience.

Unfortunately, this experiment didn’t rustle up any unusual results. Everything was in accordance with the force of gravity and no other forces were detected. Chameleons are difficult to detect “because they were designed to hide from our experiments!” exclaimed Burrage.

However, this isn’t the end of the hunt for the evasive chameleon. “Now, obviously, Earth’s gravity is a million times stronger than the force we’re looking for,” explained Müller. So, the chameleon force would be much easier to spot if gravity just wasn’t there.

Ignoring gravity is hard on the surface of Earth, but there is somewhere the experiment can be performed where gravity’s effects are negated. And it whips around the Earth every 92 minutes: the International Space Station (ISS).

International Space Station. NASA

IFLScience spoke with Mark Lee, a senior scientist from NASA, who is working on the prospect of sending and getting astronauts to perform this experiment on the ISS. He cautiously announced that “it’s going to go up within a year and a half,” however this schedule may be revised.

The equipment that would be sent up, the atom interferometer, is a versatile tool and can be used for many experiments, so it’s impossible to say when it would be used to detect chameleons.

Müller said this floating environment would be “ten thousand times better” for the test. Unlike on Earth where the cesium atoms fall very quickly, in space this time is increased. “In space you can observe [the atom] for maybe up to ten seconds,” said Lee.

This extra time and precision will be the make-or-break test for the chameleon theory. At this level of accuracy, “we’d either completely eliminate them, or we’d discover them,” commented Müller. “It can’t hide from detection forever!”