Top physics at LHC with ttbar events

The new CERN proton-proton collider, the LHC, is about to start in 2007 its data taking. Millions of top quarks will be available out of these data, allowing to perform a wide range of precision measurements and searches for new physics. An overview of the planned top physics program accessible with ttbar events is given for the ATLAS and CMS experiments. A particular emphasis is put on the precision measurements of the top mass, top polarization and searches for new physics in top production and decay.Comment: To appear in the proceedings of 41st Rencontres de Moriond on QCD and Hadronic Interactions, La Thuile, Italy, 18-25 Mar 200

Top physics at LHC with ttbar events

To appear in the proceedings of 41st Rencontres de Moriond on QCD and Hadronic Interactions, La Thuile, Italy, 18-25 Mar 2006.The new CERN proton-proton collider, the LHC, is about to start in 2007 its data taking. Millions of top quarks will be available out of these data, allowing to perform a wide range of precision measurements and searches for new physics. An overview of the planned top physics program accessible with ttbar events is given for the ATLAS and CMS experiments. A particular emphasis is put on the precision measurements of the top mass, top polarization and searches for new physics in top production and decay

Performance of the ATLAS electromagnetic calorimeter under beam tests

The physics program at LHC is highly demanding in terms of detector performance. In particular, the ATLAS electromagnetic calorimeter has to match challenging requirements for energy, position and time resolutions. Calorimeter prototype and production modules have been tested under electron beams at CERN during the last three years. Results are presented and compared to ATLAS requirements

Commissioning of the ATLAS electromagnetic calorimeter with minimum bias events

This note presents the potentiality to commission the ATLAS EM calorimeter during the (very) first days of data taking, prior to be able to trig and identify correctly electrons. For this purpose, a very simple analysis using the $\phi$ symmetry of the abundant minimum bias events and computing the energy accumulated in the EM calorimeter is proposed. No input from the Monte Carlo is necessary, and only information from the calorimeter is used. To cope with the high number of events, simulation and analysis are performed using the GRID technology. Adopting a simple energy reconstruction scheme based on the cosmic muon experience should allow to spot intrinsic problems of the EM calorimeter in a few days. It should also give first hints on the $\phi$ dispersion of the Inner Detector material, if the excess is sizeable >10\% X$_0$ in a region $\Delta \eta \times \Delta \phi=0.1\times 0.1$. Ultimately, a first flavor of systematics coming from the EM calorimeter $\phi$~non-uniformity and its positionning (in the ATLAS framework or with respect to the beam) could be possible

Comparison between full and fast simulations in top physics

All analyses performed in the top quark sector with a full simulation of the ATLAS detector show a good agreement with fast simulation results. The purpose of this note is to understand why detailed detector effects are not so crucial in these studies. A systematic comparison, based on a ``Rome'' Data Challenge fully simulated data sample and an ATLFAST simulated one, is done on all top event objects, electrons, muons, neutrinos, light and b-jets. A very good agreement on reconstructed top kinematics variables is shown. This assess the robustness of the results obtained with fast simulations in the top quark sector, and more particularly the detailed estimates of the related systematic uncertainties, which are the key points of most of these analyses

Signal reconstruction in the EM end-cap calorimeter and check with cosmic data in the region 0< eta <3.2

In 2007, the electromagnetic end-cap calorimeter joined the ATLAS commissioning effort. Since then, calibration and cosmic muon runs are taken regularly, allowing to set-up, debug and test in situ the signal reconstruction. These are the first data in the 1.4500 MeV) have been used to perform a systematic and quantitative comparison between data and predicted physics pulse shapes in a coherent way over the entire calorimeter coverage, 0< eta <3.2. This represents the first attempt to unify barrel and end-cap in situ data in a common analysis. Results are similar in the barrel and the end-cap, only slightly worse for the latter as expected from its more complex geometry. This is the first proof of the quality of an ATLAS-like signal reconstruction in the end-caps, despite its challenging aspect, and gives confidence that the energy reconstruction is in good control over the complete electromagnetic calorimeter coverage 0< eta <3.2

Effects of invisible particle emission on global inclusive variables at hadron colliders

We examine the effects of invisible particle emission in conjunction with QCD initial state radiation (ISR) on quantities designed to probe the mass scale of new physics at hadron colliders, which involve longitudinal as well as transverse final-state momenta. This is an extension of our previous treatment, arXiv:0903.2013, of the effects of ISR on global inclusive variables. We present resummed results on the visible invariant mass distribution and compare them to parton-level Monte Carlo results for top quark and gluino pair-production at the LHC. There is good agreement as long as the visible pseudorapidity interval is large enough (eta ~ 3). The effect of invisible particle emission is small in the case of top pair production but substantial for gluino pair production. This is due mainly to the larger mass of the intermediate particles in gluino decay (squarks rather than W-bosons). We also show Monte Carlo modelling of the effects of hadronization and the underlying event. The effect of the underlying event is large but may be approximately universal.Comment: 22 pages, expanded sections and other minor modifications. Version published in JHE

Top quark tensor couplings

We compute the real and imaginary parts of the one-loop electroweak contributions to the left and right tensorial anomalous couplings of the $tbW$ vertex in the Standard Model (SM). For both tensorial couplings we find that the real part of the electroweak SM correction is close to 10$$ of the leading contribution given by the QCD gluon exchange. We also find that the electroweak real and imaginary parts for the anomalous right coupling are almost of the same order of magnitude. The one loop SM prediction for the real part of the left coupling is close to the 3$\sigma$ discovery limit derived from $b\rightarrow s \gamma$. Besides, taking into account that the predictions of new physics interactions are also at the level of a few percents when compared with the one loop QCD gluon exchange, these electroweak corrections should be taken into account in order to disentangle new physics effects from the standard ones. These anomalous tensorial couplings of the top quark will be investigated at the LHC in the near future where sensitivity to these contributions may be achieved.Comment: 16 pages, 2 figure

Constraints on Randall-Sundrum model from top-antitop production at the LHC

We study the top pair production cross section at the LHC in the context of Randall-Sundrum model including the Kaluza-Klein (KK) excited gravitons. It is shown that the recent measurement of the cross section of this process at the LHC restricts the parameter space in Randall-Sundrum (RS) model considerably. We show that the coupling parameter ($\frac{k}{\bar{M}_{pl}}$) is excluded by this measurement from 0.03 to 0.22 depending on the mass of first KK excited graviton ($m_1$). We also study the effect of KK excitations on the spin correlation of the top pairs. It is shown that the spin asymmetry in $t\bar{t}$ events is sensitive to the RS model parameters with a reasonable choice of model parameters.Comment: 17 pages, 6 figure

Probing anomalous tbW couplings in single-top production using top polarization at the Large Hadron Collider

We study the sensitivity of the Large Hadron Collider (LHC) to anomalous tbW couplings in single-top production in association with a W^- boson followed by semileptonic decay of the top. We calculate top polarization and the effects of these anomalous couplings to it at two centre-of-mass (cm) energies of 7 TeV and 14 TeV. As a measure of top polarization, we look at various laboratory frame distributions of its decay products, viz., lepton angular and energy distributions and b-quark angular distributions, without requiring reconstruction of the rest frame of the top, and study the effect of anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to anomalous tbW couplings. We find that 1\sigma limits on real and imaginary parts of the dominant anomalous coupling Ref_{2R} which may be obtained by utilizing these asymmetries at the LHC with cm energy of 14 TeV and an integrated luminosity of 10 fb^{-1} will be significantly better than the expectations from direct measurements of cross sections and some other variables at the LHC and over an order of magnitude better than the indirect limits.Comment: 25 pages, 34 figure