A heavy relative of the proton has been found at the world's largest particle accelerator. Physicists captured the elusive subatomic particle—sought for decades—in a fleeting moment during proton-collision experiments. Scientists said the discovery will help them understand the force that holds atomic nuclei together.
The European Organization for Nuclear Research (CERN) said on the 17th (local time) that "in experiments at the Large Hadron Collider (LHC) accelerating protons to near the speed of light and smashing them together, we discovered Xi-cc-plus (Ξcc⁺·Xicc+), a proton four times heavier." The proton is a particle with a +1 charge. Together with the uncharged neutron, it forms the atomic nucleus.
◇light up quark replaced with heavy charm quark
The LHC, located underground along the Swiss-French border, is the world's highest-energy particle accelerator, spanning a 27-kilometer ring. It is the largest scientific apparatus on Earth, recreating conditions of the Big Bang to reveal the origins of the universe and the secrets of matter. The discovery came from the LHCb experiment, an upgraded program at 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 at the University of Manchester by Ernest Rutherford," adding, "Rutherford's gold-foil experiment conducted in the basement of the University of Manchester completely changed our understanding of matter, and today's discovery follows in that legacy."
In 1911, Rutherford found that when alpha particles (helium nuclei) with a positive charge were fired at gold foil, some bounced back. He explained that positively charged atomic nuclei repelled the alpha particles. In 1919, he then observed that when alpha particles collided with nitrogen gas, positively charged hydrogen nuclei were emitted. He showed that this particle is a fundamental component of every element's nucleus, and in 1920 he named it the proton.
The proton discovered this time differs in composition from what Rutherford found. The standard model of physics describes 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 made of two up quarks, each with a 2/3 charge, and one down quark with a –1/3 charge. The research team said the new proton is four times heavier because, instead of up quarks, it contains charm quarks, which have the same charge but are heavier.
Scientists expected the heavy proton to help them understand the force that tightly binds the interior of the nucleus. Vincenzo Vagnoni, the LHCb Spokesperson, 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 describing the strong nuclear force (the strong force) that tightly binds protons and neutrons in the 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 found as particle accelerators are upgraded. Unlike ordinary protons made of light quarks, heavy quarks such as charm can also combine to form particles that make up the atomic nucleus.
This discovery is the second confirmation of that possibility. In 2017, the LHCb team discovered Xi-cc-plus-plus (Ξcc⁺⁺·Xicc++), composed of two charm quarks and one up quark. The heavy proton found this time is effectively a sister baryon appearing again after 10 years.
However, when heavy quarks combine, they become 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 far harder to detect. That raises the possibility of misobservation, but CERN ruled out an error. CERN said the discovery has statistical significance above 7 sigma.
In physics, a discrepancy must reach five times the uncertainty—5 sigma—for a result to count as a discovery. Generally, when an experiment's significance is 3 sigma (99.7%), it falls into the "hint" category, and 5 sigma (99.99994%) or above is recognized 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 below predictions and the significance was only 4.7 sigma. The heavy proton they were seeking has clearly emerged after 24 years. Seven sigma corresponds to a confidence level of 99.9999999999979%.
References
CERN (2026), https://home.cern/news/news/physics/lhcb-collaboration-discovers-new-proton-particle