Production of Quarkonia and Heavy Flavor States in ATLAS

Two recent analyses of data collected with the ATLAS detector at the Large Hadron Collider are presented. In the first, a measurement of $b$ hadron pair production is presented, based on a data set corresponding to an integrated luminosity of 11.4~fb$^{-1}$ of proton-proton collisions recorded at $\sqrt{s}=8$ TeV. Events are selected in which a $b$ hadron is reconstructed in a decay channel containing a muon. Results are presented in a fiducial volume defined by kinematic requirements on three muons based on those used in the analysis. The fiducial cross section is measured to be $17.7 \pm 0.1({\rm stat})\pm 2.0({\rm syst.})$ nb. A number of normalized differential cross sections are also measured and compared to predictions from the {\sc Pythia8}, {\sc Herwig++}, {\sc MadGraph5\_aMC@NLO}+{\sc Pythia8}, and {\sc Sherpa} event generators, providing new constraints on heavy flavor production. In the second analysis, the modification of the production of $J/\psi$, $\psi(2S)$, and $\Upsilon(nS)~(n=1,2,3)$ in $p+$Pb collisions with respect to their production in $pp$ collisions has been studied. The $p+$Pb and $pp$ data sets correspond to integrated luminosities of 28~nb$^{-1}$ and 25~pb$^{-1}$ respectively, collected at a center-of-mass energy per nucleon pair of 5.02 TeV. The quarkonium states are reconstructed in the dimuon decay channel. The yields of $J/\psi$ and $\psi(2S)$ are separated into prompt and non-prompt sources. The measured quarkonium differential cross sections are presented as a function of rapidity and transverse momentum, as is the nuclear modification factor $R_{p{\rm Pb}}$ for the $J/\psi$ and $\Upsilon(nS)$. No significant modification of the $J/\psi$ production is observed, while $\Upsilon(nS)$ production is found to be suppressed at low transverse momentum in $p+$Pb collisions relative to $pp$ collisions. The production of excited charmonium and bottomonium states is found to be suppressed relative to that of the ground states in central $p+$Pb collisions.