Many extensions of the Standard Model posit the existence of heavy particles with long lifetimes. In this Letter, results are presented of a search for such particles, which decay at a significant distance from their production point, using a final state containing charged hadrons and an associated muon. This analysis uses a data sample of proton-proton collisions at sqrt(s)= 7 TeV corresponding to an integrated luminosity of 4.4 fb-1 collected in 2011 by the ATLAS detector operating at the Large Hadron Collider. Results are interpreted in the context of R-parity violating supersymmetric scenarios. No events in the signal region are observed and limits are set on the production cross section for supersymmetric particles, multiplied by the square of the branching ratio for a neutralino to decay to charged hadrons and a muon, as a function of the neutralino lifetime. To allow these limits to be used in a variety of models, they are presented for a range of squark and neutralino masses.
The inclusive top quark pair ($t\bar{t}$) production cross-section $\sigma_{t\bar{t}}$ has been measured in $pp$ collisions at $\sqrt{s}=7$ TeV and $\sqrt{s}=8$ TeV with the ATLAS experiment at the LHC, using $t\bar{t}$ events with an opposite-charge $e\mu$ pair in the final state. The measurement was performed with the 2011 7 TeV dataset corresponding to an integrated luminosity of 4.6 fb$^{-1}$ and the 2012 8 TeV dataset of 20.3 fb$^{-1}$. The cross-section was measured to be: $\sigma_{t\bar{t}}=182.9\pm 3.1\pm 4.2\pm 3.6 \pm 3.3$ pb ($\sqrt{s}=7$ TeV) and $\sigma_{t\bar{t}}=242.9\pm 1.7\pm 5.5\pm 5.1\pm 4.2$ pb ($\sqrt{s}=8$ TeV, updated as described in the Addendum), where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the knowledge of the integrated luminosity and of the LHC beam energy. The results are consistent with recent theoretical QCD calculations at next-to-next-to-leading order. Fiducial measurements corresponding to the experimental acceptance of the leptons are also reported, together with the ratio of cross-sections measured at the two centre-of-mass energies. The inclusive cross-section results were used to determine the top quark pole mass via the dependence of the theoretically-predicted cross-section on $m_t^{\rm pole}$, giving a result of $m_t^{\rm pole}=172.9^{+2.5}_{-2.6}$ GeV. By looking for an excess of $t\bar{t}$ production with respect to the QCD prediction, the results were also used to place limits on the pair-production of supersymmetric top squarks $\tilde{t}_1$ with masses close to the top quark mass decaying via $\tilde{t}_1\rightarrow t\tilde{\chi}^0_1$ to predominantly right-handed top quarks and a light neutralino $\tilde{\chi}_0^1$, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 177 GeV are excluded at the 95% confidence level.
Measurements of four-lepton (4$\ell$, $\ell=e,\mu$) production cross sections at the $Z$ resonance in $pp$ collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass region $m_{\ell^+\ell^-} > 5$ GeV and $80 < m_{4\ell} < 100$ GeV, the measured cross sections are $76 \pm 18 \text { (stat) } \pm 4 \text { (syst) } \pm 1.4 \text { (lumi) }$ fb and $107 \pm 9 \text{ (stat) } \pm 4 \text{ (syst) } \pm 3.0 \text { (lumi) }$ fb at $\sqrt s$ = 7 and 8 TeV, respectively. By subtracting the non-resonant 4$\ell$ production contributions and normalizing with $Z\rightarrow \mu^+\mu^-$ events, the branching fraction for the $Z$ boson decay to $4\ell$ is determined to be $\left( 3.20 \pm 0.25\text{ (stat)} \pm 0.13\text{ (syst)} \right) \times 10^{-6}$, consistent with the Standard Model prediction.
The production rates of prompt and non-prompt $J/\psi$ and $\psi(2\mathrm{S})$ mesons are measured using 2.1 $fb^{-1}$ and 11.4 $fb^{-1}$ of data collected with the ATLAS experiment at the LHC, in proton-proton collisions at $\sqrt{s}=7$ and 8 TeV respectively. Production cross-sections for both prompt and non-prompt production sources, ratios of $\psi(2\mathrm{S})$ to $J/\psi$ production, and fractions of non-prompt to inclusive production for $J/\psi$ and $\psi(2\mathrm{S})$ are measured double-differentially as a function of meson $p_{T}$ and rapidity. These measurements are made in a restricted fiducial volume and also corrected for geometrical acceptance after which they are compared to a variety of theoretical predictions.
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