ATLAS 2013. 11. 28.آ  TLAS-CONF-2013-108 2013 ATLAS NOTE ATLAS-CONF-2013-108 November 28, 2013 ......

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    ATLAS NOTE ATLAS-CONF-2013-108

    November 28, 2013

    Evidence for Higgs Boson Decays to theτ+τ− Final State with the ATLAS Detector

    The ATLAS Collaboration

    Abstract

    A search for the Higgs boson with a mass of about 125 GeV decaying into a pair of τ leptons is performed with a data sample of proton-proton collisions, corresponding to an integrated luminosity ofL = 20.3 fb−1, collected with the ATLAS detector at the LHC at a centre-of-mass energy of

    √ s = 8 TeV. Final states in allτ decay combina-

    tions (both hadronic and leptonic) are examined. The observed (expected) deviation from the background-only hypothesis corresponds to a significance of 4.1 (3.2) standard devia- tions, and the measured signal strength isµ = 1.4+0.5−0.4. This is evidence for the existence of H → τ+τ− decays, consistent with the Standard Model expectation fora Higgs boson with mH = 125 GeV.

    c© Copyright 2013 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license.

  • 1 Introduction

    The observation of a new particle with a mass near 125 GeV by the ATLAS and CMS experiments [1, 2] in the search for the Standard Model (SM) Higgs boson [3–8] isa great success of the Large Hadron Collider (LHC) physics programme at the CERN laboratory. Todate, however, evidence for Higgs- boson decays into fermionic final states is not conclusive.

    With a branching ratio of 6.3% [9],H → τ+τ− is among the leading decay modes for a SM Higgs boson with a mass of 125 GeV. This decay mode can provide a direct measurement of the coupling of the Higgs boson to fermions, thereby testing an important prediction of the theory. The observation of theH → τ+τ− decay mode would be strong evidence that fermions acquire their mass through the Higgs mechanism.

    This note presents a search for the SM Higgs boson in theH → τ+lepτ − lep , H → τ

    + lepτ − had, and

    H → τ+hadτ − had final states

    1 with the ATLAS detector [10], using the full dataset collected in proton-proton (pp) collisions at

    √ s= 8 TeV in 2012 and corresponding to an integrated luminosity of L = 20.3 fb−1.

    The search is designed to be sensitive to the SM Higgs boson produced through gluon fusion (ggF) [11], vector boson fusion (VBF) [12], and associated production (VH) with V = W or Z de- caying hadronically. All these mechanisms can give rise to jet signatures, particularly in the VBF case, where two high-energy jets with a large pseudorapidity separation are produced.

    The results presented in this note supersede those of Ref. [13] which correspond to an analysis of part of the 2012 dataset with a luminosity ofL = 13.0 fb−1, and the full 2011 dataset. The most recent results from the CMS collaboration on the search for the SM Higgs boson in theτ+τ− channel can be found in Ref. [14].

    2 Analysis Strategy

    The ATLAS detector is described in Section 3, with details onthe data and simulated samples given in Section 4. Object identification cuts are applied as described in Section 5 and Section 6 details pre- selection cuts for all three channels:τlepτlep, τlepτhad and τhadτhad. The events are further classified into categoriesoptimized for the different SM Higgs production mechanisms, as described in Section 7. The category definitions depend to a certain extent on the channel because of the different background compositions.

    Following preselection and categorization, theH → τ+τ− signal is still overwhelmed by a variety of background sources. A boosted decision tree (BDT) multivariate analysis technique is used to discri- minate signal from background [15–17]. Separate BDTs are trained for each channel in each category as described in Section 8, based on input variables that havediffering distributions for signal and back- ground. The background estimates are described in Section 9. The BDT output distribution (BDT score) is then used as the final discriminant.

    Since a BDT is sensitive to the correlations among variables, it is necessary to demonstrate that these correlations are well modelled. For that reason, the BDT output is computed in several signal-depleted control regions, and the agreement between the data and the background model is confirmed, as described in Section 10.

    To ensure that analysis choices are not biased, discriminating variables which might on their own reveal the presence of a signal were blinded throughout the analysis optimization. This blinding was applied to variables such asmττ and the final BDT score in signal sensitive bins, but not to basic kinematic distributions such as the transverse momentum ofτ leptons.

    1τlep andτhad denote leptonically and hadronically decayingτ leptons, respectively. Charge-conjugate decay modes are implied. Throughout the remainder of this note, a simplifiednotation without the particle charges is used.

    1

  • Systematic uncertainties relevant to the analysis are mentioned in Section 11, and the signal extrac- tion procedure is detailed in Section 12. The results of thissearch are given in Section 13.

    3 The ATLAS Detector

    The ATLAS detector [10] is a cylindrical2 multi-purpose detector at the LHC. The detector subsystem closest to the interaction point, the Inner Detector (ID), provides precise position and momentum mea- surements of charged particles. It covers the pseudorapidity range|η| < 2.5 and provides full azimuthal coverage. It consists of three subdetectors arranged in a coaxial geometry around the beam axis: the silicon pixel detector, the silicon microstrip detector and the straw-tube transition-radiation tracker. A solenoid magnet generates a 2 T magnetic field in which the ID is immersed.

    Electromagnetic calorimetry in the region|η| < 3.2 is based on a high-granularity, lead/liquid-argon (LAr) sampling technology. Hadronic calorimetry uses a scintillating-tile/steel detector covering the re- gion |η| < 1.7 and a copper/LAr detector in the region 1.5 < |η| < 3.2. The most forward region of the de- tector 3.1 < |η| < 4.9 is equipped with a dedicated forward calorimeter, measuring both electromagnetic and hadronic energies using copper/LAr and tungsten/LAr modules.

    A large stand-alone Muon Spectrometer (MS) is the outermostpart of the detector. It consists of three large air-core superconducting toroidal magnet systems. The deflection of the muon trajectories in the magnetic field is measured in three layers of precision drifttube chambers for|η| < 2. In higherη regions (2.0 < |η| < 2.7), two layers of drift tube chambers are used in combinationwith one layer of cathode strip chambers in the innermost endcap wheels of the MS. Three layers of resistive plate chambers in the barrel (|η| < 1.05) and three layers of thin gap chambers in the endcaps (1.05 < |η| < 2.4) provide the muon trigger and also measure the muon trajectory in the non-bending plane of the spectrometer magnets.

    A three-level trigger system [18] is used to select events inreal time. A hardware-based Level-1 trigger uses a subset of detector information to reduce the event rate to a value of at most 75 kHz. The rate of accepted events is then reduced to about 300 Hz by two software-based trigger levels, Level-2 and the Event Filter.

    4 Data and Simulated Samples

    This search usespp collision data at √

    s = 8 TeV collected in 2012. After requiring that all detector systems are operational, the dataset used corresponds to anintegrated luminosity ofL = 20.3 fb−1. The triggers used by each channel are given in Table 1.

    The simulated event samples, based on Monte Carlo (MC) techniques and a full description of the the ATLAS detector [19] with GEANT4 [20], are listed below. These samples include the simulation of pile-up activity in the same or nearby bunch crossings. The MC samples are re-weighted to reproduce the observed distribution of the mean number of interactions per bunch crossing in the data.

    The simulation of signal events produced via the gluon-fusion and VBF-production mechanisms is performed using the POWHEG [21–23] event generator based onnext-to-leading order (NLO) QCD calculations. Soft-gluon resummation up to next-to-next-to-leading logarithm order [24] is adopted. The finite quark-mass effects are taken into account in POWHEG [25]. The parton shower, hadronization and underlying event simulations are provided by PYTHIA [26, 27]. The CT10 [28] parton distribution

    2ATLAS uses a right-handed coordinate system with its originat the nominal interaction point (IP) in the centre of the detector and thez-axis along the beam direction. Thex-axis points from the IP to the centre of the LHC ring, and they-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse (x, y) plane,φ being the azimuthal angle around the beam direction. The pseudorapidity is defined in terms of thepolar angleθ asη = − ln tan(θ/2). The distance∆R in theη − φ space is defined as∆R=

    (∆η)2 + (∆φ)2.

    2

  • Trigger pT threshold(s) [GeV] τlepτlep τlepτhad τhadτhad Electron 24 • • Muon 24 • Di-electron 12 ; 12 • Di-muon 18 ; 8 • Electron+Muon 12 ; 8 • Electron+ τhad 18 ; 20 • Muon+ τhad 15 ; 20 • Di-τhad 29 ; 20 •

    Table 1: Triggers used for each channel. When more than one trigger is used, a logical OR of the triggers is taken and the trigger efficiency is calculated accordingly. The electron+τhad and muon+τhad triggers ar