Rubin-LSST

Abstract:

Using e+e- collision data corresponding to 5.88×106γ(3S) [25.9×106ψ(2S)] decays and acquired by the CLEO III [CLEO-c] detectors operating at the Cornell Electron Storage Ring, we study the single-pion transitions from γ(3S)[ψ(2S)] to the respective spin-singlet states hb[c]. Utilizing only the momentum of suitably selected transition-π0 candidates, we obtain the upper limit B(γ(3S)→π0h b)<1.2×10-3 at 90% confidence level, and measure B(ψ(2S)→π0hc)=(9.0±1.5±1.3) ×10-4. Signal sensitivities are enhanced by excluding very asymmetric π0→γγ candidates. © 2011 American Physical Society.

Abstract:

We present a statistical observational study of the tidal dwarf (TD) population in the nearby Universe, by exploiting a large, homogeneous catalogue of galaxy mergers compiled from the SDSS. 95% of TD-producing mergers involve two spiral progenitors, while most remaining systems have at least one spiral progenitor. The fraction of TD-producing mergers where both parents are early-type galaxies is <2%, suggesting that TDs are unlikely to form in such mergers. The bulk of TD-producing systems inhabit a field environment and have mass ratios greater than 1:7 (the median value is 1:2.5). TDs forming at the tidal-tail tips are ~4 times more massive than those forming at the base of the tails. TDs have stellar masses that are less than 10% of the stellar masses of their parents and typically lie within 15 optical half-light radii of their parent galaxies. The TD population is typically bluer than the parents, with a median offset of ~0.3 mag in the (g-r) colour and the TD colours are not affected by the presence of AGN activity in their parents. An analysis of their star formation histories indicates that TDs contain both newly formed stars (with a median age of ~30 Myr) and old stars drawn from the parent disks, each component probably contributing roughly equally to their stellar mass. Thus, TDs are not formed purely through gas condensation in tidal tails but host a significant component of old stars from the parent disks. Finally, an analysis of the TD contribution to the local dwarf-to-massive galaxy ratio indicates that ~6% of dwarfs in nearby clusters may have a tidal origin, if TD production rates in nearby mergers are representative of those in the high-redshift Universe. Even if TD production rates at high redshift were several factors higher, it seems unlikely that the entire dwarf galaxy population today is a result of merger activity over the lifetime of the Universe.

Abstract:

Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the Compact Muon Solenoid (CMS) detector at the LHC, using a data sample corresponding to an integrated luminosity of 6.7μb-1. Jets are reconstructed using the energy deposited in the CMS calorimeters and studied as a function of collision centrality. With increasing collision centrality, a striking imbalance in dijet transverse momentum is observed, consistent with jet quenching. The observed effect extends from the lower cutoff used in this study (jet pT=120 GeV/c) up to the statistical limit of the available data sample (jet pT≈210 GeV/c). Correlations of charged particle tracks with jets indicate that the momentum imbalance is accompanied by a softening of the fragmentation pattern of the second most energetic, away-side jet. The dijet momentum balance is recovered when integrating low transverse momentum particles distributed over a wide angular range relative to the direction of the away-side jet. © 2011 American Physical Society.

Abstract:

In this paper we report a measurement of the tt̄ production cross section in pp̄ collisions at √s=1.96TeV using data corresponding to an integrated luminosity of 2.2fb-1 collected with the CDF II detector at the Tevatron accelerator. We select events with significant missing transverse energy and high jet multiplicity. This measurement vetoes the presence of explicitly identified electrons and muons, thus enhancing the tau contribution of tt̄ decays. Signal events are discriminated from the background using a neural network, and heavy flavor jets are identified by a secondary-vertex tagging algorithm. We measure a tt̄ production cross section of 7.99±0.55(stat)±0.76(syst)±0.46(lumi) pb, assuming a top mass mtop=172.5GeV/c2, in agreement with previous measurements and standard model predictions. © 2011 American Physical Society.