Results of CERN particle accelerator: what have scientists found?

Physicists at the world’s largest atomic accelerator have observed three new exotic particles as they continue to search for clues about the mysterious forces that bind subatomic particles together, they said Tuesday.

The sighting of the particles – a new type of pentaquark and the first duo of tetraquarks – took place on a memorable day at CERN, the Geneva lab, which also restarted its Large Hadron Collider on Tuesday, after a three-year hiatus, with the the machine’s most powerful energy beam yet.

[Maintenance on the ATLAS Experiment. Image: CERN]The reboot, after meticulous maintenance and repairs, offers a worldwide network of scientists a new opportunity to search for the ‘strong force’ that holds the atomic nuclei together. The strong force is one of four forces that control the decay of particles, including the electromagnetic force, the gravitational force, and the “weak force.”

This week also marks the 10th anniversary of the confirmation of the Higgs boson, a milestone at CERN that helped solidify the so-called Standard Model, which forms the basis of particle physics. Named after the Nobel Prize-winning physicist who theorized it, the Higgs helps all matter give its mass, and is believed to have been present at the creation of the universe moments after the Big Bang 13.7 billion years ago.

But physicists still have questions about the Higgs, and they’re looking beyond the Standard Model. It’s the most comprehensive we have, but it doesn’t explain where gravity comes from, what dark matter is, or why there is so much more matter than antimatter in the universe.

Enter the LHC. Surrounding its gigantic underground ring, which stretches under 17 miles of landscape across the Swiss-French border, a series of superconducting magnets help propel subatomic particles close to the speed of light in order to smash them together as powerfully as possible.

Explosive collisions, powerful results

In particle accelerators like this, the slamming of high-energy protons can produce tiny fragments of the universe that aren’t normally seen. Like a car accident, where more explosive collisions result in smaller fragments, the higher the energy, the more likely the LHC’s detectors can help researchers pick up traces of the smallest, most exotic particles.

[The first collision of Run 3 of the ATLAS experiment on Tuesday. Image: CERN]Along with new magnets — which must be kept at a cool -271.3°C, just colder than space — CERN’s engineers have pushed proton beams to unprecedented energies to produce collision energies of 13.6 trillion electron volts, an increase from 13 TeV in the previous ride. A trillion electron volts is roughly the energy of a flying mosquito, but for a subatomic particle, that’s a lot of energy: CERN says the rays racing around the ring will eventually carry as much energy as a train traveling at 150 kilometers per hour.

The particle beams will also be denser, further increasing the probability of a collision, and the probability of producing Higgs bosons, finding new particles and perhaps new physics. One of the highly sought after prizes is the axion, a particle that is not thought to emit, absorb or reflect light. It is one of the main candidates for the composition of dark matter, the mysterious stuff that physicists believe makes up 85% of the matter in the universe.

The new particles

The very first pair of tetraquarks and the new pentaquark, discovered in torrents of data collected during previous research at the LHC, will help explain how subatomic particles are formed. These particles, like the protons and neutrons that make up the nuclei of atoms, are known as hadrons, and the more exotic hadrons are made up of two or three elementary particles called quarks.

[An illustration of the new pentaquark in its molecular state, which is the first found to contain a strange quark. Image: CERN]But six decades ago, physicists began to theorize that these combos could form four-quark and five-quark particles on rare occasions. Only in the past decade have these so-called tetraquarks and pentaquarks actually been observed by the LHC and other experiments. The new pentaquark is the first to contain a strange quark.

“We are witnessing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons was discovered and eventually led to the quark model of conventional hadrons in the 1960s,” physicist Niels Tuning said in a statement. “We are creating ‘particle zoo 2.0’.”

In particular, the new findings will help theorists develop a unified model of exotic hadrons and better understand conventional hadrons. While some theories describe exotic hadrons as “balls” of tightly bound quarks, other theoretical models present them as pairs of standard hadrons loosely bound in a molecular-like structure. More research is needed to know if one or both theories are true.

Researchers at CERN said the pentaquark’s finding has a whopping 15 standard deviations, well above the 5 standard deviations it takes for the physics community to claim a particle’s observation. The new tetraquarks, observed with a statistical significance of 6.5 and 8 standard deviations, respectively, are the first time a pair of tetraquarks has been observed.

An international effort

There is no greater laboratory for particle physics. More than 5,500 scientists from 245 institutes in more than 40 countries are working on the LHC’s largest experiment, ATLAS. Other new experiments at CERN investigating the nature of the universe will focus on high-energy ion collisions to better understand the plasma that was only present in the first microsecond after the Big Bang; probe the inside of protons; study cosmic rays; and look for the still hypothetical magnetic monopole, an insulated magnet with only one magnetic pole. Other detectors will help researchers study extremely bright and weakly interacting particles, such as neutrinos and dark matter.

[Managers and researchers celebrate the restart on Tuesday. Image: CERN]This week marks the start of “Run 3,” the third time the LHC has performed collisions since its launch in 2008, eclipsing the previous largest particle accelerator, the Tevatron at Fermilab, Illinois. This iteration will run until 2026, when the machines are upgraded again.

The multi-billion euro project is funded by contributions from each participating country. For fiscal year 2022, the Biden administration initially allocated only $40 million for future upgrades at the LHC, well below the approximately $90 million needed. The Department of Energy has subsequently diverted $25 million from several other physics projects, including a series of upgrades at Fermilab, and the Biden administration is now requesting $85 million, the American Institute of Physics reported.

During this run, the researchers behind LHC’s two main experiments will collide clusters of about 100 billion protons at a rate of 40 million collisions per second, each producing about 60 impacts, each of which will generate hundreds of particles.

“The more analyzes we do, the more species of exotic hadrons we find,” Tuning said.

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