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

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

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

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

Background Light in Potential Sites for the ANTARES Undersea Neutrino Telescope

The ANTARES collaboration has performed a series of {\em in situ} measurements to study the background light for a planned undersea neutrino telescope. Such background can be caused by $^{40}$K decays or by biological activity. We report on measurements at two sites in the Mediterranean Sea at depths of 2400~m and 2700~m, respectively. Three photomultiplier tubes were used to measure single counting rates and coincidence rates for pairs of tubes at various distances. The background rate is seen to consist of three components: a constant rate due to $^{40}$K decays, a continuum rate that varies on a time scale of several hours simultaneously over distances up to at least 40~m, and random bursts a few seconds long that are only correlated in time over distances of the order of a meter. A trigger requiring coincidences between nearby photomultiplier tubes should reduce the trigger rate for a neutrino telescope to a manageable level with only a small loss in efficiency.Comment: 18 pages, 8 figures, accepted for publication in Astroparticle Physic

Position resolution and particle identification with the ATLAS EM calorimeter

In the years between 2000 and 2002 several pre-series and series modules of the ATLAS EM barrel and end-cap calorimeter were exposed to electron, photon and pion beams. The performance of the calorimeter with respect to its finely segmented first sampling has been studied. The polar angle resolution has been found to be in the range 50-60 mrad/sqrt(E (GeV)). The neutral pion rejection has been measured to be about 3.5 for 90% photon selection efficiency at pT=50 GeV/c. Electron-pion separation studies have indicated that a pion fake rate of (0.07-0.5)% can be achieved while maintaining 90% electron identification efficiency for energies up to 40 GeV.Comment: 32 pages, 22 figures, to be published in NIM

Light Higgs boson discovery from fermion mixing

We evaluate the LHC discovery potential for a light Higgs boson in t tbar H (-> l nu b bbar b bbar jj) production, within the Standard Model and if a new Q=2/3 quark singlet T with a moderate mass exists. In the latter case, T pair production with decays T Tbar -> W+ b H tbar / H t W- bbar -> W+ b W- bbar H provides an important additional source of Higgs bosons giving the same experimental signature, and other decay modes T Tbar -> H t H tbar -> W+ b W- bbar H H, T Tbar -> Z t H tbar / H t Z tbar -> W+ b W- bbar H Z further enhance this signal. Both analyses are carried out with particle-level simulations of signals and backgrounds, including t tbar plus n=0...5 jets which constitute the main background by far. Our estimate for SM Higgs discovery in t tbar H production, 0.4 sigma significance for M_H = 115 GeV and an integrated luminosity of 30 fb^-1, is similar to the most recent ones by CMS which also include the full t tbar nj background. We show that, if a quark singlet with a mass m_T = 500 GeV exists, the luminosity required for Higgs discovery in this final state is reduced by more than two orders of magnitude, and 5 sigma significance can be achieved already with 8 fb^-1. This new Higgs signal will not be seen unless we look for it: with this aim, a new specific final state reconstruction method is presented. Finally, we consider the sensitivity to search for Q=2/3 singlets. The combination of these three decay modes allows to discover a 500 GeV quark with 7 fb^-1 of luminosity.Comment: LaTeX, 37 pages, 57 PS figures. Many improvements in the analysis. Final version to appear in JHE

In situ commissioning of the ATLAS electromagnetic calorimeter with cosmic muons

In 2006, ATLAS entered the {\it in situ} commissioning phase. The primary goal of this phase is to verify the detector operation and performance with cosmic muons. Using a dedicated cosmic muon trigger from the hadronic Tile calorimeter, a sample of approximately $120\,000$ events was collected in several modules of the barrel electromagnetic (EM) calorimeter between August 2006 and March 2007. As cosmic events are generally non-projective and arrive asynchronously with respect to the trigger clock, methods to improve the standard signal reconstruction for this situation are presented. Various selection criteria for projective muons and clustering algorithms have been tested, leading to preliminary results on calorimeter uniformity in $\eta$ and timing performance