A heavy relative of the proton has been found at the world's largest particle accelerator. The subatomic particle, which physicists have searched for over decades, was captured for a split second in proton collision experiments. Scientists called it a discovery that will help them understand the force that holds atomic nuclei together.
The European Organization for Nuclear Research (CERN) said on the 17th that "in an experiment at the Large Hadron Collider (LHC) accelerating protons to near the speed of light and colliding them, we discovered Xicc+ (Ξcc⁺), a proton four times heavier," in local time. Protons carry a +1 electric charge. Together with neutrons, which carry no charge, they form the atomic nucleus.
◇ light up quarks replaced by heavy charm quarks
The LHC, an underground particle accelerator straddling the Swiss-French border, is the world's highest-energy accelerator, with a circumference of 27 kilometers. It is the largest scientific apparatus on Earth, recreating the moment of the Big Bang to unravel the origins of the universe and the secrets of matter. The discovery came from the LHCb experiment, an upgrade of the LHC. It is an international project involving about 1,000 scientists from 20 countries.
Chris Parkes of the University of Manchester, who led LHCb, said, "The newly observed proton is a heavier relative of the proton first discovered by Ernest Rutherford at the University of Manchester," adding, "Rutherford's gold foil experiment, conducted in the University of Manchester basement, completely changed our understanding of matter, and today's discovery carries on that legacy."
In 1911, Rutherford found that when he shot alpha particles (helium nuclei) with a positive charge at gold foil, some bounced back. He explained that the atomic nucleus, which carries the same positive charge, repelled the alpha particles. Later, in 1919, by colliding alpha particles with nitrogen gas, he observed positively charged hydrogen nuclei being ejected. He showed that this particle is a basic component of the nuclei of all elements, and in 1920 he named it the proton.
The proton discovered this time differs in composition from the one Rutherford found. The Standard Model of physics explains all matter in terms of 16 fundamental particles—six quarks, six leptons, and four force carriers—plus the Higgs that gives them mass, for a total of 17. A proton is composed of two up quarks with a charge of 2/3 and one down quark with a charge of –1/3. The research team said the new proton is four times heavier in mass because, instead of up quarks, it contains charm quarks, which have the same charge but are heavier.
The scientific community expected the heavy proton to help in understanding the force that tightly binds the interior of atomic nuclei. Vincenzo Vagnoni, the Spokesperson for the LHCb team, said, "This new proton is the first new particle confirmed since the LHCb detector upgrade completed in 2023," adding, "It will help theoretical physicists test quantum chromodynamics (QCD) models."
Quantum chromodynamics is the theory that describes the strong nuclear force that binds protons and neutrons tightly in the atomic nucleus. According to the Standard Model of physics, fundamental particles interact through electromagnetism, the weak nuclear force, the strong nuclear force, and gravity.
◇ exists only for a millionth of a second
Scientists say more new protons could be discovered as particle accelerators are upgraded. Unlike ordinary protons made of light quarks, heavy quarks such as charm quarks can also combine to form particles that make up atomic nuclei.
This discovery is the second proof of that possibility. In 2017, the LHCb team found Xicc++ (Ξcc++), composed of two charm quarks and one up quark. The heavy proton found this time is, in effect, a sister baryon appearing again after 10 years.
However, when heavy quarks combine, they are more unstable and quickly decay into other particles. The heavy proton discovered this time existed for less than a trillionth of a second. CERN said the new proton's lifetime is only one-sixth that of the heavy proton reported in 2017, making it much harder to detect. Although that raises the chance of a misobservation, CERN ruled out the possibility of error. CERN said the discovery has a statistical significance above 7 sigma.
For a finding to count as a new discovery in physics, the level of discrepancy must be five times the uncertainty, or 5 sigma. Generally, when an experiment's confidence level is 3 sigma (99.7%), it falls into the category of a "hint," and 5 sigma (99.99994%) or higher is accepted as a "discovery." In 2002, scientists at the Fermi National Accelerator Laboratory in the United States also found a particle very similar to this proton, but its mass was far lower than predicted and its confidence level was only 4.7 sigma. The heavy proton they were looking for has now clearly appeared after 24 years. A 7-sigma result corresponds to a confidence level of 99.9999999999979%.
References
CERN (2026), https://home.cern/news/news/physics/lhcb-collaboration-discovers-new-proton-particle