Physicists have made significant advancements with the ATLAS Collaboration at CERN’s Large Hadron Collider (LHC), observing the excited state of the Bc*+ Meson. This unique meson consists of a charm quark paired with a bottom antiquark.
Protons and neutrons, fundamental components of matter, belong to a larger class of particles known as hadrons. Hadrons are composite particles formed from quarks held together by the strong force.
These particles are classified into two main groups: baryons, which are composed of three quarks (e.g., protons and neutrons), and mesons, which consist of a quark-antiquark pair.
Despite years of research, many phenomena associated with the strong force still remain elusive, particularly the interactions among quarks in hadrons.
Heavy quark mesons, such as those containing charm and bottom quarks, serve as essential testbeds for evaluating theoretical models regarding these interactions.
Particularly noteworthy is the Bc+ meson, which contains both charm quarks and bottom antiquarks.
ATLAS physicists created an excited form of the Bc+ meson through high-energy proton-proton collisions at the LHC.
Following these collisions, the Bc*+ quickly decays into Bc+ mesons accompanied by photons.
Detecting these photons, along with the decay products of Bc+, provides critical evidence confirming the existence of the Bc*+ meson.
However, researchers face a challenge as the expected mass of Bc+ mesons is only marginally greater than that of Bc+ mesons, resulting in photons with very low energy that are challenging to detect using traditional methods.
Instead of standard photon identification techniques, scientists looked for photons that transformed into electron-positron pairs in the ATLAS tracking detector, leaving behind a trail of densely charged particles emerging from a common origin distinct from the initial proton-proton collision.
The lateral momentum of these tracks is around 100 MeV, significantly lower than typical values analyzed in ATLAS studies.
Consequently, the team had to implement a specialized trajectory reconstruction method to successfully identify the photons and confirm the existence of the Bc*+ meson.
The measured mass difference between the Bc*+ meson and the Bc+ meson stands at 64.5 ± 1.4 MeV.
According to the physicists, “This is within the range of available theoretical predictions, though it slightly diverges from the latest high-precision calculations.”
These findings will significantly contribute to theoretical models explaining the mass of particles with heavier quarks and enhance our understanding of the strong nuclear force.
The team’s research will soon be published in the journal Physical Review Letters.
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Collaboration with ATLAS. 2026. Observation of Bc*+ Mesons using the ATLAS detector. Physical Review Letters, in press. arXiv: 2605.16228
Source: www.sci.news
