Top quark properties

The top quark physics has entered the precision era. The CDF and D0 collaborations are finalizing their legacy results of the properties of the top quark after the shutdown of the Fermilab Tevatron three years ago. The ATLAS and CMS collaborations have been publishing results from the LHC Run I with 7 TeV and 8 TeV proton-proton collisions, with many more forthcoming. We present a selection of recent results produced by the Tevatron and LHC experiments.

Measurement of the inclusive leptonic asymmetry in top-quark pairs that decay to two charged leptons at CDF

We measure the inclusive forward-backward asymmetry of the charged-lepton pseudorapidities from top-quark pairs produced in proton-antiproton collisions, and decaying to final states that contain two charged leptons (electrons or muons), using data collected with the Collider Detector at Fermilab. With an integrated luminosity of 9.1 $rm{fb}^{-1}$, the leptonic forward-backward asymmetry, $A_{text{FB}}^{ell}$, is measured to be $0.072 pm 0.060$ and the leptonic pair forward-backward asymmetry, $A_{text{FB}}^{ellell}$, is measured to be $0.076 pm 0.082$, compared with the standard model predictions of $A_{text{FB}}^{ell} = 0.038 pm 0.003$ and $A_{text{FB}}^{ellell} = 0.048 pm 0.004$, respectively. Additionally, we combine the $A_{text{FB}}^{ell}$ result with a previous determination from a final state with a single lepton and hadronic jets and obtain $A_{text{FB}}^{ell} = 0.090^{+0.028}_{-0.026}$.

On the Forward-Backward Asymmetry of Leptonic Decays of $tbar{t}$ at the Fermilab Tevatron

We report on a study of the measurement techniques used to determine the leptonic forward-backward asymmetry of top anti-top quark pairs in Tevatron experiments with a proton anti-proton initial state. Recently it was shown that a fit of the differential asymmetry as a function of $q_{l}eta_{l}$ (where $q_{l}$ is the charge of the lepton from the cascade decay of the top quarks and $eta_{l}$ is the final pseudorapidity of the lepton in the detector frame) to a hyperbolic tangent function can be used to extrapolate to the full leptonic asymmetry. We find this empirical method to well reproduce the results from current experiments, and present arguments as to why this is the case. We also introduce two more models, based on Gaussian functions, that better model the $q_{l}eta_{l}$ distribution. With our better understanding, we find that the asymmetry is mainly determined by the shift of the mean of the $q_{l}eta_{l}$ distribution, the main contribution to the inclusive asymmetry comes from the region around $|q_{l}eta_{l}| = 1$, and the extrapolation from the detector-covered region to the inclusive asymmetry is stable via a multiplicative scale factor, giving us confidence in the previously reported experimental results.

Prospects for measuring the mass of heavy, long-lived neutral particles that decay to photons

We describe a novel way to measure the mass of heavy, long-lived neutral particles that decay to photons using collider experiments. We focus on a Light Neutralino and Gravitino model in a Gauge Mediated Supersymmetry Breaking scenario where the neutralino has a long-lifetime (O(ns)) as it is not excluded by current experiments. To illustrate our method and give sensitivity estimates we use recent CDF results and a production mechanism where sparticles are produced via $phi_{i} rightarrow widetilde{chi}^{1}_{0} widetilde{chi}^{1}_{0} rightarrow (gamma widetilde{G})(gamma widetilde{G})$ in which $phi_{i}$ indicates a neutral scalar boson, $widetilde{chi}^{1}_{0}$ is the lightest neutralino and $widetilde{G}$ is the gravitino, as a full set of background shapes and rates are available. Events can be observed in the exclusive photon plus Missing $E_{T}$ final state where one photon arrives at the detector with a delayed time of arrival. Surprisingly, a simple measurement of the slope of the delayed-time distribution with the full CDF dataset is largely insensitive to all but the $widetilde{chi}^{1}_{0}$ mass and allows for the possibility of determining its mass to approximately 25% of itself.