The LHC Discovery Potential of a Leptophilic Higgs

In this work, we examine a two-Higgs-doublet extension of the Standard Model in which one Higgs doublet is responsible for giving mass to both up- and down-type quarks, while a separate doublet is responsible for giving mass to leptons. We examine both the theoretical and experimental constraints on the model and show that large regions of parameter space are allowed by these constraints in which the effective couplings between the lightest neutral Higgs scalar and the Standard-Model leptons are substantially enhanced. We investigate the collider phenomenology of such a "leptophilic" two-Higgs-doublet model and show that in cases where the low-energy spectrum contains only one light, CP-even scalar, a variety of collider processes essentially irrelevant for the discovery of a Standard Model Higgs boson (specifically those in which the Higgs boson decays directly into a charged-lepton pair) can contribute significantly to the discovery potential of a light-to-intermediate-mass (m_h < 140 GeV) Higgs boson at the LHC.Comment: 25 pages, LaVTeX, 11 figures, 1 tabl

Reconstruction and identification of hadronic τ\tau decays in ATLAS

The identification of tau-jet from the hadronic tau-decay is crucial for the study of many physics channels like the production of Higgs bosons decaying to taus or SUSY. The tau-jet reconstruction in ATLAS is performed starting from the energy deposed in calorimeters or also combining measure from calorimeter and tracking with energy-flow technique. Quantities built both from the calorimeters and from the inner detector are used to identify tau-jets against other jets. In the talk we will review the performance of tau-jet reconstruction and identification, showing that the excellent tau-efficiency vs the jet-rejection obtained in ATLAS will allow the study of channels where the background from jets is potentially very large

Results from ATLAS Calorimeter Combined Test Beam

Beam tests of combinations of ATLAS calorimeters have been performed both for the barrel and end cap parts. During a combined test beam in summer 2004 a slice of the ATLAS barrel detector - including all detector sub systems from the inner tracker, the calorimetry to the muon system - was exposed to particle beams (electrons, pions, photons, muons) with different energies (1GeV to 350GeV). The aim was to study the combined performance of the different detector sub systems in ATLAS-like conditions. We will present the electronics calibration scheme of the electromagnetic calorimeter and its implementation. The following studies on the combined testbeam data have been performed and will be presented: performance of the electromagnetic calorimetry down to very low energies (> GeV), photon reconstruction including converted photons and position measurements using the very precise ATLAS tracker and the electromagnetic calorimeter. These measurements have been compared to Monte Carlo simulations showing the good description of the data. An end cap segment of the electromagnetic, hadronic and forward liquid argon calorimeters of ATLAS has been exposed to electron, pion and muon beams in the energy range from 10 to 200 GeV. The goal of this beamtest carried out at the CERN SPS is to obtain the calibration for the ATLAS calorimeter in a particular difficult transition region (2.4<3.8) between the various subdetectors, including the dead material effects. Results for electrons and pions will be presented and compared to MC simulation

2004 ATLAS Combined Testbeam : Computation and Validation of the Electronic Calibration Constants for the Electromagnetic Calorimeter

From July to November 2004, a full slice of the ATLAS barrel detector was studied in testbeam. A complete electromagnetic barrel module was used, read by the final electronics and operated by ATLAS TDAQ software. This note describes in details the electronic calibration procedure and the cell energy reconstruction: each step of the procedure and its associated software is explicitly described. The general calibration procedure is very similar to the one applied in previous barrel and endcap standalone testbeams. Emphasis is put on tools developed in the context of the combined testbeam which can be used for commissioning and operation of the calorimeters in ATLAS. Many validation studies were performed on each calibration constant. Previously unobserved effects such as the FEB temperature dependence of some constants were observed. Overall, the calibration performances are at the expected level

Energy Linearity and Resolution of the ATLAS Electromagnetic Barrel Calorimeter in an Electron Test-Beam

A module of the ATLAS electromagnetic barrel liquid argon calorimeter was exposed to the CERN electron test-beam at the H8 beam line upgraded for precision momentum measurement. The available energies of the electron beam ranged from 10 to 245 GeV. The electron beam impinged at one point corresponding to a pseudo-rapidity of eta=0.687 and an azimuthal angle of phi=0.28 in the ATLAS coordinate system. A detailed study of several effects biasing the electron energy measurement allowed an energy reconstruction procedure to be developed that ensures a good linearity and a good resolution. Use is made of detailed Monte Carlo simulations based on Geant which describe the longitudinal and transverse shower profiles as well as the energy distributions. For electron energies between 15 GeV and 180 GeV the deviation of the measured incident electron energy over the beam energy is within 0.1%. The systematic uncertainty of the measurement is about 0.1% at low energies and negligible at high energies. The energy resolution is found to be about 10% sqrt(E) for the sampling term and about 0.2% for the local constant term

Hunt for new phenomena using large jet multiplicities and missing transverse momentum with ATLAS in 4.7 fb−1 of s√=7TeV proton-proton collisions

Results are presented of a search for new particles decaying to large numbers of jets in association with missing transverse momentum, using 4.7 fb−1 of pp collision data at s√=7TeV collected by the ATLAS experiment at the Large Hadron Collider in 2011. The event selection requires missing transverse momentum, no isolated electrons or muons, and from ≥6 to ≥9 jets. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of a MSUGRA/CMSSM supersymmetric model, where, for large universal scalar mass m 0, gluino masses smaller than 840 GeV are excluded at the 95% confidence level, extending previously published limits. Within a simplified model containing only a gluino octet and a neutralino, gluino masses smaller than 870 GeV are similarly excluded for neutralino masses below 100 GeV

Measurement of inclusive two-particle angular correlations in pp collisions with the ATLAS detector at the LHC

We present a measurement of two-particle angular correlations in proton- proton collisions at s√=900 GeV and 7 TeV. The collision events were collected during 2009 and 2010 with the ATLAS detector at the Large Hadron Collider using a single-arm minimum bias trigger. Correlations are measured for charged particles produced in the kinematic range of transverse momentum p T  > 100 MeV and pseudorapidity |η| < 2.5. A complex structure in pseudorapidity and azimuth is observed at both collision energies. Results are compared to pythia 8 and herwig++ as well as to the AMBT2B, DW and Perugia 2011 tunes of pythia 6. The data are not satisfactorily described by any of these models

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Results of a search for H → τ τ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb−1 and 20.3 fb−1 at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV respectively. All combinations of leptonic (τ → `νν¯ with ` = e, µ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of µ = 1.43 +0.43 −0.37 is consistent with the predicted Yukawa coupling strength in the Standard Model

Measurement of the top pair production cross section in 8 TeV proton-proton collisions using kinematic information in the lepton plus jets final state with ATLAS

A measurement is presented of the $t\bar{t}$ inclusive production cross-section in $pp$ collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV using data collected by the ATLAS detector at the CERN Large Hadron Collider. The measurement was performed in the lepton+jets final state using a data set corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The cross-section was obtained using a likelihood discriminant fit and $b$-jet identification was used to improve the signal-to-background ratio. The inclusive $t\bar{t}$ production cross-section was measured to be $260\pm 1{\textrm{(stat.)}} ^{+22}_{-23} {\textrm{(syst.)}}\pm 8{\textrm{(lumi.)}}\pm 4{\mathrm{(beam)}}$ pb assuming a top-quark mass of 172.5 GeV, in good agreement with the theoretical prediction of $253^{+13}_{-15}$ pb. The $t\bar{t}\to (e,\mu)+{\mathrm{jets}}$ production cross-section in the fiducial region determined by the detector acceptance is also reported.Comment: Published version, 19 pages plus author list (35 pages total), 3 figures, 2 tables, all figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2013-06

Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H →γ γ, H → Z Z∗ →4l and H →W W∗ →lνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson