Observation of the χc1(1P) and χc2(1P) charmonium states in √s = 7 TeV pp collisions at the ATLAS experiment

We present the observation of the χc1 (1P) and χc2 (1P) charmonium states in χc → J/ψ γ decays in ATLAS using pp collision data at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 39 pb−1. J/ψ candidates are reconstructed from the decay J/ψ → μ+μ− while photons are reconstructed with a calorimetric measurements. χc candidates are observed in the kinematic range pT(χc) > 10 GeV and rapidity |y(χc)| < 2.4. An extended unbinned maximum likelihood fit is performed to the invariant mass difference of the μ+μ− and μ+μ−γ systems to yield 2960 ± 120 (stat.) ± 90 (syst.) χc1 and χc2 candidates.