The difference between the total mass of the Bc π π combination and the individual masses of the Bc and the two pions, as recorded in the 7 TeV data recorded by ATLAS in 2012. The peak in the distribution appears only for unlike-sign pions, as expected. (Image: ATLAS Experiment/CERN)

ATLAS has observed a particle state of mass and decay properties consistent with expectations for an excited state of the Bc meson. The discovery follows analysis of the full 7 TeV and 8 TeV proton-proton collision data sets from the LHC’s first run.

Mesons are subatomic particles composed of one quark and one antiquark. The Bc meson comprises two heavy quarks: beauty (a.k.a. bottom) and charm. The particle’s signature appears as a peak in the mass difference spectrum of its decays to the Bc ground state and a pair of charged pions. The position of this peak over the smooth background allows ATLAS to measure the particle’s mass to be 6842 ± 9 MeV, making it the heaviest meson state of two different quark flavours and the first member of the Bc family to be observed above the ground state.

This observation provides opportunities for testing predictions based on models of the strong force, quantum chromodynamics, and may contribute to extraction of the strong interaction potential.

The same m(Bc π)-m(Bc)-2m(π) mass difference distribution for 8 TeV data recorded by ATLAS in 2013. (Image: ATLAS Experiment/CERN)