Since the Federal Reserve signaled plans to gradually pare down its holdings of Treasuries and agency debt and mortgage-backed securities (MBS) accumulated in the aftermath of the 2008 financial ...crisis, investors have expressed concern over how the removal of the Fed as an active buyer would impact the government-sponsored enterprise MBS market. In addition, expectations for increased MBS issuance fueled by a strong labor market have added to worries about rising supply in the face of declining demand. It has been more than one year since the Fed began to shrink its balance sheet. This article examines how market conditions over the past year have affected the speed of Fed balance sheet normalization relative to expectations, as well as ongoing developments in the agency MBS market, and finds that feared market imbalances have largely failed to materialize.
The large rate of multiple simultaneous proton-proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse ...effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 Formula: see text data sample collected at a centre-of-mass energy of Formula: see text. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.
Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, Formula: see text system and event-level kinematic observables in ...proton-proton collisions at a centre-of-mass energy of Formula: see text. The observables have been chosen to emphasize the Formula: see text production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fbFormula: see text, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a
-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.
Distributions of transverse momentum Formula: see text and the related angular variable Formula: see text of Drell–Yan lepton pairs are measured in 20.3 fbFormula: see text of proton–proton ...collisions at Formula: see text TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton–proton collisions at Formula: see text TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the
-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of Formula: see text the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of Formula: see text this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of Formula: see text while the fixed-order prediction of Dynnlo falls below the data at high values of Formula: see text. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the Formula: see text and Formula: see text distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.
The production rates of prompt and non-prompt Formula: see text and Formula: see text mesons in their dimuon decay modes are measured using 2.1 and 11.4 fbFormula: see text of data collected with the ...ATLAS experiment at the Large Hadron Collider, in proton-proton collisions at Formula: see text and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of Formula: see text to Formula: see text production, and the fractions of non-prompt production for Formula: see text and Formula: see text are measured as a function of meson transverse momentum and rapidity. The measurements are compared to theoretical predictions.
This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of Formula: see text. A method utilising the production of Formula: see text leptons via Formula: ...see text decays is used. This method is applied to the sample of 20.3 fbFormula: see text of
collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the Formula: see text lepton branching fraction into three muons, Formula: see text, is Formula: see text (Formula: see text) at 90 % confidence level.
Results of a search for new phenomena in events with at least three photons are reported. Data from proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated ...luminosity of 20.3 fbFormula: see text, were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay Formula: see text, as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (
), limits are found to be Formula: see text for 10 GeV Formula: see text 62 GeV. Limits are also presented for Higgs boson-like scalars (
) for Formula: see text 125 GeV, and for a Formula: see text decaying to three photons via Formula: see text. Additionally, the observed limit on the branching ratio of the
boson decay to three photons is found to be BRFormula: see text, a result five times stronger than the previous result from LEP.
The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 Formula: see textbFormula: see text of proton-lead collisions at a ...nucleon-nucleon centre-of-mass energy of Formula: see text Formula: see text using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The Formula: see text collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the Formula: see text collision have been carried out using the Glauber model as well as two Glauber-Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon-nucleon collisions in the modelling of the initial state of Formula: see text collisions.
This paper reports a detailed study of techniques for identifying boosted, hadronically decaying
bosons using 20.3 fbFormula: see text of proton-proton collision data collected by the ATLAS detector ...at the LHC at a centre-of-mass energy Formula: see text. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying
bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2-4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.
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