ATLAS at CERN observes the excited Bc*+ meson for the first time, a new test of the strong nuclear force
The ATLAS collaboration reports in Physical Review Letters the direct observation of the excited Bc*+ meson, made of a charm quark and a bottom antiquark, a discovery that offers a precise input for theoretical models of heavy-quark masses.
- The ATLAS collaboration has directly observed the excited Bc*+ meson in proton-proton collisions for the first time.
- The meson pairs a charm quark with a bottom antiquark, a rare combination in nature.
- The result gives theorists a sharper benchmark for modelling heavy-quark hadron masses.
- The paper has been accepted by Physical Review Letters, with the preprint posted to arXiv.
The ATLAS collaboration at CERN, which brings together more than three thousand physicists from different institutes, reports the first direct observation of the excited Bc+ meson*. The paper has been accepted for publication in Physical Review Letters, and the preprint is available on the public arXiv server under the number 2605.16228.
The Bc*+ meson is the excited version of the already known Bc+ meson. Both are made of the same pair of quarks, a charm quark and a bottom antiquark, but Bc*+ has a higher mass and a different spin configuration, so it decays rapidly to the ground state by emitting a photon.
What a meson is
A meson is a kind of hadron, a particle made of a pair consisting of one quark and one antiquark, bound by the strong nuclear force. Compared with the best-known hadrons such as protons and neutrons, which have three quarks, mesons live extremely short lives and decay soon after they form.
The most typical mesons produced in the lab arise from light quarks, the up and the down. Mesons combining two different heavy quarks, like the Bc family, are extremely rare and have been observed only in the last two and a half decades. The first Bc+ meson was registered in 1998 at Fermilab in the United States. The excited state observed by ATLAS was predicted by theoretical models but had not been distinguishable above the statistical noise until now.
Observation methodology
ATLAS uses proton-proton collisions at a record energy of 13 and 13.6 teraelectronvolts, delivered by the Large Hadron Collider. Billions of events generate showers of secondary particles that the detector reconstructs in real time.
The identification strategy for the Bc*+ meson includes:
- searching for the characteristic decay into a Bc+ meson and a photon with a specific energy difference
- reconstructing the Bc+ candidate in its rare decay channels, primarily the channel into a J/psi meson and a pion
- statistical selection that eliminates background events with false particle combinations
- measuring the observed mass difference between Bc*+ and Bc+, which amounts to several tens of megaelectronvolts
The final signal stands above five standard deviations, the standard threshold for confirming an observation in particle physics.
What the discovery means for theory
This result provides a valuable new input for theoretical models that describe the masses of particles with heavy quarks and will help us improve our understanding of the strong nuclear force.
The ATLAS collaboration stated that measuring the mass difference between the excited and the ground state is a test of lattice chromodynamics, the modern branch of quantum chromodynamics tested numerically on computers. If the lattice predictions hold, the theory is in good shape. If deviations appear, that would be a hint of physics beyond the Standard Model.
What the study does NOT show
The observation of the Bc*+ meson does not mean the Standard Model is in crisis. This is an expected particle whose existence was predicted by theory but whose precise property had not been measurable until now. Like the observation of the Higgs boson in 2012, this confirms a prediction rather than overturning it.
A second limitation concerns statistics: the meson is rare, so a precise measurement of mass differences and decay widths requires more data-taking seasons. The next data will arrive during the LHC phase called Run 3, ending in 2025, with possible upgrades for Run 4 starting in 2030.
Link to Serbian physics
The discovery is of interest to the Institute of Physics at the University of Belgrade, which has researchers in the CMS detector at CERN, the sister collaboration to ATLAS. Local physicists coordinated by Petar Adzic work on similar analyses in the Bc sector, and comparable results will be available in the coming months.
Will rare Bc mesons become one of the key tests of physics beyond the Standard Model?
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