Search for the Standard Model Higgs boson in H → τ

ATLAS-CONF-2012-160 14November2012

ATLAS NOTE

ATLAS-CONF-2012-160

November 13, 2012

Search for the Standard Model Higgs boson in H → τ

⁺

τ

⁻

decays in proton-proton collisions with the ATLAS detector

The ATLAS Collaboration

Abstract

A search for the Standard Model (SM) Higgs boson decaying into a pair ofτleptons is reported. The analysis, exploiting each of the τ_lepτ_lep, τ_lepτ_had andτ_hadτ_had final states, is based on data samples of proton-proton collisions collected by the ATLAS experiment at the LHC and corresponding to integrated luminosities of 4.6 fb⁻¹and 13.0 fb⁻¹at centre-of- mass energies of √

s=7 TeV and 8 TeV, respectively. The observed (expected) upper limit at 95% CL on the cross-section times the branching ratio for SM H → τ⁺τ⁻ is found to be 1.9 (1.2) times the SM prediction for a Higgs boson with massmH=125 GeV. For this mass, the observed (expected) deviation from the background-only hypothesis corresponds to a local significance of 1.1 (1.7) standard deviations.

c

Copyright 2012 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 of about 125 GeV by the ATLAS and CMS experiments [1, 2] in the search for the Standard Model (SM) Higgs boson [3–8] is a great success of the Large Hadron Collider (LHC) physics programme and the beginning of a new era in particle physics.

For a Higgs boson with a mass of 125 GeV, the

H→

τ

⁺

τ

⁻

channel, with a branching ratio of 6.3% [9]

is one of the leading decay modes, that may provide a measurement of the coupling of the Higgs to fermions and test an important prediction of the Standard Model. The study of the

H →

τ

⁺

τ

⁻

channel across various production processes will ultimately lead to measurements of a number of important Higgs couplings [10].

The process with the largest SM Higgs-boson production cross-section at the LHC is gluon fusion, gg

→ H, with a cross-section of 15.3 pb (19.5 pb) at √

s =

7(8) TeV for

m_H =

125 GeV [9]. Higgs- boson production via

W

or

Z

vector-boson fusion

qq → qqH

(denoted VBF, with cross-sections of 1.22 and 1.57 pb at 7 and 8 TeV, respectively) and via Higgs-strahlung

qq → V H

(denoted VH), in association with a hadronically decaying vector boson (V

= W

or

Z), are highly relevant as well. This

is due to the resulting additional jets in the final state, which provide distinct experimental signatures.

In particular, the VBF topology of two high-energy jets with a large rapidity separation offers a good discrimination against background processes. For each of the aforementioned production processes, one can exploit the boost of the Higgs boson in the transverse plane by requiring additional high-p

_T

jets in the event. Typically, such boosted topologies exhibit larger transverse momentum of the τ decay products and thus facilitate the measurement of the Higgs resonance signal and the discrimination of the signal from background processes.

This note presents SM Higgs boson searches in the

H→

τ

⁺_lep

τ

⁻_lep

,

H →

τ

⁺_lep

τ

⁻_had

, and

H →

τ

⁺_had

τ

⁻_had

channels,

¹

where τ

_lep

and τ

_had

denote leptonically and hadronically decaying τ leptons, respectively. The analyses use proton-proton (pp) collision data collected by the ATLAS experiment corresponding to integrated luminosities of 4.6 fb

⁻¹

at

√

s=

7 TeV for data taken in 2011 and 13.0 fb

⁻¹

at

√

s=

8 TeV for data recorded during 2012 between April and September. The event selection used in this analysis has been optimized significantly compared to the one applied in the recently published ATLAS search that used 2011 data only [11]. In order to enhance the sensitivity of the search, the selected events are analysed in several separate categories according to the number and kinematic properties of reconstructed jets. The search results using the shape of the reconstructed ττ mass distribution from these various categories and two di

ff

erent data-taking periods are statistically combined according to production mode and finally for all categories. A profile likelihood ratio is used to extract measurements of the signal strength parameters, µ

_ggF

and µ

_VBF+VH

, defined as the ratios of measured and SM production cross-sections for gg

→H

and VBF

+

VH processes.

2 Data and Monte Carlo simulated samples

The ATLAS experiment utilizes a multipurpose detector with a forward-backward symmetric cylindrical geometry and nearly 4π coverage in solid angle [12]. It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and an external muon spectrometer incorporating three large superconducting air-core toroid magnets.

Electrons, muons, τ leptons, jets and neutrinos (through the presence of missing transverse energy) can be reconstructed, identified, and measured with high precision in the ATLAS detector.

²

Only data

1Charge-conjugated decay modes are implied. Throughout the remainder of this note, a simplified notation without the particle charges is used.

2ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and thez-axis along the beam pipe. Thex-axis points from the IP to the centre of the LHC ring, and they-axis points

taken with all sub-systems relevant to this analysis operational are used.

The signal contributions considered here include the gluon fusion production process, the VBF pro- duction process, and the Higgs-strahlung

V H

production in association with a hadronically-decaying vec- tor boson. For the decay of the Higgs boson, the

H→

ττ mode is considered. Next-to-next-to-leading- order (NNLO) Quantum Chromodynamics (QCD) corrections, soft-gluon resummations calculated in the next-to-next-to-leading-log-approximation and next-to-leading-order (NLO) electroweak (EW) cor- rections are applied to the signal cross-sections for the gg

→ H

production process [13–24]. The cross- sections of the VBF process are calculated with full NLO QCD and electroweak corrections [25–27], and approximate NNLO QCD corrections [28]. The

V H

processes are calculated with NNLO QCD cor- rections [29, 30] and NLO EW radiative corrections are applied [31]. The gg

→ H

and VBF processes are modelled using the POWHEG [32, 33] Monte Carlo (MC) generator, interfaced to PYTHIA [34] for showering and hadronisation. In the gg

→ H

process, the Higgs boson

p_T

spectrum is reweighted to agree with the prediction of the HqT program [35]. The associated

V H

production process is modelled using PYTHIA. The simulated samples are normalised to the cross-sections discussed above; the Higgs boson decay branching ratios are calculated using HDECAY [36].

ALPGEN [37], interfaced to HERWIG [38], with the MLM matching scheme [39] is used to model the production of single

W

and

Z/γ^∗

bosons decaying to charged leptons in association with jets.

MC@NLO [40] is used to model

t¯t, WW

,

WZ

and

ZZ

production processes, using HERWIG for the parton shower and hadronisation and JIMMY [41] for the underlying event modelling. AcerMC [42] is used to model single top-quark production for all three production channels (

s-channel, t-channel, and Wt

production).

The TAUOLA [43] and PHOTOS [44] programs are used to model the decay of τ leptons and the Quantum Electrodynamics (QED) radiation of photons, respectively.

The set of parton distribution functions (PDF) CT10 [45] is used for the MC@NLO samples, CTEQ6L1 [46] for the ALPGEN samples, and MRST2007 [47] for the PYTHIA and HERWIG samples.

Acceptances and e

ffi

ciencies are based on simulations of the ATLAS detector using GEANT4 [48, 49].

Since the data are a

ff

ected by the detector response to multiple interactions (pileup) occurring in the same or nearby bunch crossings, the simulated events are re-weighted so that the distribution of the number of minimum-bias interactions per bunch crossing agrees with data.

The

Z/γ^∗→

ττ background processes are modelled with a

Z/γ^∗→

µµ data sample where the muons have been replaced by simulated τ lepton decays as described in Section 6.

3 Selection and reconstruction of physics objects

Electron candidates are formed from energy deposits in the electromagnetic calorimeter associated to tracks measured in the inner detector. They are selected if they have a transverse energy

E_T

> 15 GeV, lie within

|η|

< 2.47 but outside the transition region between the barrel and end-cap calorimeters (1.37 <

|η|

< 1.52), and meet quality requirements based on the expected shower shape [50].

Muon candidates are formed from tracks measured in the inner detector linked to tracks in the muon spectrometer [51]. They are required to have a transverse momentum

p_T

> 10 GeV and to lie within

|η|

< 2.5. Additionally, the difference between the

z-position of the point of closest approach of the inner

detector track of the muon to the beam-line and the

z-coordinate of the primary vertex is required to be

less than 1 cm.

³

This requirement reduces the contamination due to cosmic-ray muons and beam-induced backgrounds. Muon quality criteria based on e.g. inner detector hit requirements are applied in order to achieve a precise measurement of the muon momentum and reduce the misidentification rate.

upwards. Cylindrical coordinates (r, φ) are used in the transverse plane,φbeing the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angleθasη=−ln tan(θ/2).

3The primary vertex is defined as the reconstructed vertex with the largestP

p²_T of the associated tracks.

Identified electrons and muons are required to be isolated in the calorimeters and tracking system:

the additional transverse energy in the electromagnetic and hadronic calorimeters in small cones of

∆R= p

(∆ η)

²+

(∆ φ)

²

around lepton directions must be less than a certain fraction of the electron transverse energy (or muon transverse momentum) and the sum of the transverse momenta of all tracks with

p_T

above 1 GeV located within a small cone around the lepton direction and originating from the same primary vertex must be less than a certain fraction of the electron transverse energy (or muon transverse momentum). The cone sizes and fraction of isolation energy (momentum) vary depending on the channel, lepton type, and 7 TeV or 8 TeV data in the range

∆R∼0.2–0.4 and 4–14%, respectively.

Jets are reconstructed using the anti-k

t

algorithm [52] with a distance parameter value of

R =

0.4.

Reconstructed, noise-suppressed clusters of calorimeter cells are used by the algorithm. Reconstructed jets with

p_T

> 25 GeV and within

|η|

< 4.5 are selected. The reconstructed jet energy is calibrated using

p_T

- and η-dependent correction factors based on MC simulation and validated with data [53].

Events are discarded if a jet is associated with out-of-time activity or calorimeter noise. A jet-vertex fraction (JVF) is used to reduce the number of jets in the event due to pile-up activity in the same or nearby bunch-crossings for the event of interest. The JVF is defined as the ratio between the sum of the transverse momentum of the tracks in the jet associated to the primary vertex and the sum of the transverse momentum of the tracks in the jet associated to any vertex in the event. Conventionally, JVF

= −1 is assigned to jets with no associated tracks. To suppress contributions from pileup, jets

with

|η|

< 2.4 are required to have

|JVF|

> 0.75 for the 7 TeV dataset; the requirement is relaxed to

|JVF|

> 0.50 for the 8 TeV dataset due to the increased pileup.

In the pseudorapidity range

|η|

< 2.5,

b-tagged jets are identified using a tagging algorithm based

on the impact parameter information and on the reconstruction of the displaced vertices of the hadron decays inside the jets [54]. For the working point used in this analysis, the

b-tagging algorithm has an

average e

ffi

ciency of 60–70% for

b-tagged jets with pT

> 15 GeV in simulated

tt

events [55], with an improved version of the algorithm applied to the data collected at 8 TeV. The corresponding light-quark jet misidentification probability is 0.1–0.5%, depending on the jet

p_T

and η [56].

Hadronic decays of τ leptons are characterised by the presence of one or three charged hadrons accompanied by a neutrino and possibly neutral hadrons, which result in collimated shower profiles in the calorimeters with only a few associated tracks. The visible decay products are combined into τ

had

candidates. These candidates are reconstructed as jets, which are re-calibrated to account for the di

ff

erent calorimeter response to hadronic tau-decays as compared to hadronic jets. The four-momenta of the τ

_had

candidates are reconstructed from the energy deposits in the calorimeters and the rejection of jets misidentified as hadronic τ-lepton decays is performed by a multivariate discriminator based on a boosted decision tree [57] that uses both tracking and calorimeter information. The identification is optimised for three different working points (“loose”, “medium” and “tight”) corresponding to approximate efficiencies of 60%, 50% and 30%, with the improved algorithm used for the 2012 data giving the best performance, while the jet misidentification probability is kept below 1%. A τ

_had

candidate must lie within

|η|

< 2.5, have a transverse momentum greater than 20 GeV, one or three associated tracks

⁴

(with

p_T

> 1 GeV) within

∆R

< 0.2 around the τ

_had

direction and a total charge of

±1 computed from the associated tracks.

Dedicated electron and muon veto algorithms are used [58].

When different objects selected according to the above criteria overlap with each other geometrically (within

∆R

< 0.2), only one of them is considered for further analysis. The overlap is resolved by selecting muon, electron, τ

_had

and jet candidates in this order of priority.

The magnitude of the missing transverse momentum [59] (E

^miss_T

) is reconstructed including con- tributions from muon tracks and energy deposits in the calorimeters. Calorimeter cells belonging to three-dimensional noise-suppressed clusters are used, calibrated taking into account the reconstructed

4As is discussed in Section 6.3,τhadcandidates with a track multiplicity different from one or three tracks are used to obtain the normalization of the multi-jet background.

physics object to which they belong. To mitigate the impact of pile-up on

E^miss_T

in the 8 TeV data, each jet with

|η|

< 2.4 is weighted with its JVF and the energy contributions not associated to physics objects are scaled using the so-called soft-term-vertex-fraction. This fraction is the ratio of the transverse mo- mentum sum of the tracks unmatched to physics objects that are associated to the primary vertex and the transverse momentum sum of the tracks unmatched to physics objects that are associated to any vertex in the event [60].

Differences in electron, muon and hadronic tau reconstruction and identification efficiencies between data and simulation are taken into account, as are the di

ff

erences in the energy and momentum scales and resolutions. In particular, the correction for the efficiencies for hadronic τ-lepton decays is less than 5%. The efficiencies of the electron, muon and hadronic tau triggers are measured using collision data and are used to correct the simulation.

4 Preselection

An initial selection of events is performed by requiring a vertex from the primary

pp

collisions that is consistent with the beam spot position, with at least three associated tracks, each with

p_T

> 500 MeV.

Overall quality criteria are applied to suppress events with fake

E^miss_T

, produced by non-collision activity such as cosmic ray muons, beam-related backgrounds, or noise in the calorimeters.

The τ

_lep

τ

_lep

, τ

_lep

τ

_had

and τ

_had

τ

_had

final states considered in this search are defined in a mutually exclusive way: a requirement of exactly two, one, or zero electrons or muons is imposed, respectively.

4.1 H → τ

lep

τ

lep

Signal events in this channel are selected by requiring exactly two isolated and oppositely-charged light leptons (electrons and

/

or muons). Single lepton and di-lepton triggers are used to select the data, in order to maximize the lepton acceptance. The identification requirements for single-object triggers were tightened during the data-taking period to cope with increasing instantaneous luminosity. The triggers and corresponding o

ffl

ine thresholds are summarized in Table 1. Di

ff

erent combinations of single- and di-lepton triggers are applied for the different combination of lepton flavours as follows:

•

7 TeV analysis:

E_T

> 22 GeV is required if an electron satisfies only the single electron trigger.

The

E_T

requirement is increased to 24 GeV when the trigger threshold is 22 GeV. The o

ffl

ine muon

pT

is required to be 1-2 GeV above the corresponding muon trigger.

•

8 TeV analysis: For events in the

ee

and

eµ

channels that contain an electron with

E_T

> 25 GeV, that electron is required to be spatially matched to the single electron trigger. If both electrons in

ee

events have

E_T

< 25 GeV, the electrons are required to be matched to the di-electron trigger.

If the electron in the

eµ

channel has

E_T

< 25 GeV, the leptons are required to be matched to the

eµ

di-lepton trigger. In the µµ channel, the muon associated with the higher threshold leg of the di-muon trigger is required to have

p_T

> 20 GeV.

To correct the di

ff

erence in trigger e

ffi

ciencies (as a function of

p_T

and η) between the MC simulation and the data, the MC e

ffi

ciency is scaled by a factor which is obtained from

Z →ll

data using a tag-and- probe method.

To search for a Higgs within the range 100 GeV <

m_H

< 150 GeV, the di-lepton invariant mass is

required to be in the range 30 GeV <

m_``

< 100 GeV for the

eµ

channel, whereas for the

ee

and µµ

channels 30 GeV <

m_``

< 75 GeV is required, to reduce the contamination from

Z/γ^∗→

`` .

Table 1: The triggers used in the various channels, along with the corresponding trigger and offline

p_T

thresholds on the reconstructed objects. Ranges indicate that the threshold varied during the data-taking between the given values. The label τ

_had-vis

denotes specific observables based entirely on the visible products of the τ lepton decay.

Channel Trigger Trigger

p_T

Threshold (GeV) Offline

p_T

Threshold (GeV) 7 TeV

H→

τ

_lep

τ

_lep

single electron

p_T^e

> 20

−

22

electron

pT

2 GeV above trigger threshold

p_T^µ

> 10

single muon

p_T^µ

> 18

p_T^µ

> 20

p_T^e

> 15 di-electron

p_T^e1

> 12

p_T^e2

> 12

p_T^e1

> 15

p_T^e2

> 15 di-muon

p_T^µ1

> 15

p_T^µ2

> 10

p_T^µ1

> 16

p_T^µ2

> 10

e−

µ combined

p_T^e

> 10

p_T^µ

> 6

p_T^e

> 15

p_T^µ

> 10

H→

τ

_lep

τ

_had

single electron

p_T^e

> 20

−

22

p_T^e

> 25

–

p_T^τhad-vis

> 20

single muon

p_T^µ

> 18

p_T^µ

> 25

–

p_T^τhad-vis

> 20

combined

e+

τ

_had-vis p_T^e

> 15 17 <

p_T^e

< 25

p_T^τhad-vis

> 16

−

20

p_T^τhad-vis

> 25

H→

τ

_had

τ

_had

combined two τ

_had p_T^τhad-vis

> 29

p_T^τhad-vis

> 40

p_T^τhad-vis

> 20

p_T^τhad-vis

> 25

8 TeV

H→

τ

_lep

τ

_lep

single electron

p_T^e

> 24

p_T^e

> 25

p_T^µ

> 10 di-electron

p_T^e1

> 12

p_T^e2

> 12

p_T^e1

> 15

p_T^e2

> 15 di-muon

p_T^µ1

> 18

p_T^µ2

> 8

p_T^µ1

> 20

p_T^µ2

> 10

e−

µ combined

p_T^e

> 12

p_T^µ

> 8

p_T^e

> 15

p_T^µ

> 10

H→

τ

_lep

τ

_had

single electron

p_T^e

> 24

p_T^e

> 26

–

p_T^τhad-vis

> 20

single muon

p_T^µ

> 24

p_T^µ

> 26

–

p_T^τhad-vis

> 20

combined

e+

τ

had-vis p_T^e

> 18 20 <

p_T^e

< 26

p_T^τhad-vis

> 20

p_T^τhad-vis

> 25

combined µ

+

τ

_had-vis p_T^µ

> 15 17 <

p_T^µ

< 26

p_T^τhad-vis

> 20

p_T^τhad-vis

> 25

H→

τ

_had

τ

_had

combined two τ

_had p_T^τhad-vis

> 29

p_T^τhad-vis

> 40

p_T^τhad-vis

> 20

p_T^τhad-vis

> 25

4.2 H → τ

_lep

τ

_had

Signal events in this channel are characterised by exactly one isolated light lepton, a τ

_had

candidate, and large

E_T^miss

due to the undetected neutrinos. For the τ

e

τ

_had

(τ

_µ

τ

_had

) final states, events are preselected using either a single electron (muon) trigger, or a combined τ

_had-vis

and lepton trigger. For the 7 TeV data, a combined τ

_had-vis

and electron trigger is used, while for the 8 TeV data a combined τ

_had-vis

and muon trigger is used as well. The triggers and corresponding thresholds are documented in Table 1. Due to the lepton threshold in the combined trigger being consistently lower than the equivalent threshold for the single lepton triggers, such a mixture increases the signal yield.

Exactly one electron or muon passing the offline

p_T

requirement listed in Table 1 is required. Events with more than one electron or muon candidate are rejected to suppress events from

Z/γ^∗→

`

⁺

`

⁻

decays and from

t¯t

or single top-quark production. For the purpose of vetoing additional light leptons, the isolation requirements are removed, and a looser identification requirement is used for electrons, while the transverse momentum (energy) threshold for muons (electrons) is lowered to 10 GeV (15 GeV).

Exactly one τ

_had

candidate passing at least the “medium” identification requirements is required, with a transverse energy of at least 20 or 25 GeV, for events triggered by the single lepton or the combined trigger respectively. Finally, the charges of the selected lepton and τ

_had

candidate are required to be opposite.

4.3 H → τ

_had

τ

_had

Signal events in this channel are characterised by two identified hadronic τ lepton decays (τ

_had

) and large

E^miss_T

from the undetected neutrinos.

There are several preselection criteria which are identical for the 7 TeV and 8 TeV datasets. In each case, the event selection starts with a double hadronic τ trigger, where the

p_T

thresholds are 29 GeV and 20 GeV for the leading and sub-leading τ

_had

candidates, respectively. The hadronic τ identification algorithm used in the trigger is di

ff

erent in the two datasets, leading to di

ff

erent treatments in the analysis.

In either case, the trigger algorithm is designed to detect τ

_had

candidates with high e

ffi

ciency [61]. These trigger conditions lead to minimum

p_T

thresholds in the analysis of

p_T

> 40 GeV and

p_T

> 25 GeV on the leading and the sub-leading τ

_had

candidates, respectively. Additional selections common to the 7 TeV and 8 TeV datasets include: the two τ

_had

candidates are required to be of opposite charge, a requirement of exactly zero charged light leptons is imposed (as defined in Section 3), and the missing transverse momentum is required to be at least 20 GeV.

For the 7 TeV data, both τ

_had

candidates are required to pass the “medium” quality requirements and they must be separated by

∆R(τ₁

, τ

₂

) < 3.2. For the 8 TeV data both τ

_had

candidates are required to pass medium quality requirements and at least one must also pass “tight” quality requirements. Further, the separation criteria have been tightened to 0.8 <

∆R

< 2.8 and

∆

η < 1.5.

5 Analysis categories

The selected event samples are split into several categories according to the number and kinematic prop-

erties of reconstructed jets. The sensitivity of the search is usually higher for categories where the

presence of one or more jets is required, as discussed in Section 1. Events without any reconstructed

high-p

_T

jets are also considered in some cases. In particular, all three channels in this analysis exploit

a VBF topology [62–64] of two high-energy jets with a large rapidity separation, which o

ff

ers a good

discrimination against background processes. Events with this clean and unique signature are analysed

in a dedicated event category. The analysis also exploits the fact that the ττ invariant mass resolution for

Z →

ττ background and signal events is significantly improved if the ττ system is boosted in the trans-

verse plane. Better mass resolution implies greater separation between signal and the dominant

Z→

ττ

background. The

H →

τ

_lep

τ

_lep

and

H →

τ

_lep

τ

_had

channels define such an analysis category (Boosted) by requiring large transverse momenta of the

l+l⁰+E^miss_T

or

l+

τ

_had+E_T^miss

systems. The

H→

τ

_had

τ

_had

channel defines the Boosted category by requiring the presence of a high-p

T

jet in the event. The fol- lowing sub-sections provide a detailed description of the analysis categories and corresponding selection requirements for all three channels. In many cases, the event selection requirements are the same for the 7 TeV and 8 TeV data, hence only one cut value is provided. In all channels, the event selection requirements were optimized for the best exclusion potential at

m_H=125 GeV.

The invariant mass is the final discriminating observable used for all categories, and is reconstructed by means of the Missing Mass Calculator (MMC) [65] (except for 7 TeV data in the

H →

τ

_lep

τ

_lep

channel). This technique provides a reconstruction of event kinematics in the ττ final state with >99%

e

ffi

ciency and 13–20% resolution in

m_ττ

, depending on the event topology and final state (better resolu- tion is obtained for events with high-

p_T

jets). Conceptually, the MMC is a more sophisticated version of the collinear approximation [66]. The main improvement comes from requiring that relative orientations of the neutrinos and other decay products are consistent with the mass and kinematics of a τ lepton decay.

This is achieved by maximising a probability defined in the kinematically allowed phase space region.

In the 7 TeV analysis in the

H →

τ

_lep

τ

_lep

channel, the collinear approximation was used to recon- struct the mass of the ττ system in all categories with at least one jet. For τ

_lep

τ

_lep

events with no jets, the invariant mass of the di-lepton and

E_T^miss

system, referred to as e

ff

ective mass

m^e_ττ^ff

, was used be- cause the performance of the collinear approximation is not optimal in events where τ-decay products are back-to-back in the transverse plane.

The background processes considered in this search are the production of QCD jets,

W

and

Z

bosons in association with jets (W/Z+jets), pairs of top quarks (t¯

t), single top quarks, and pairs of electroweak

gauge bosons (WW,

WZ,ZZ). In particular, the Z/γ^∗ →

ττ background is the dominant one for this search. A detailed description of how these processes are modeled can be found in Section 6.

5.1 H → τ

_lep

τ

_lep

Five mutually exclusive categories that are characterized by their jet multiplicity and kinematics are defined for this channel: 2-jet VBF, Boosted, 2-jet VH, 1-jet and 0-jet. In all categories with jets (2-jet VBF, Boosted, 2-jet

V H

and 1-jet), the following requirements are made on top of the requirements defined in Section 4.1:

•

The presence of a hadronic jet with a transverse momentum

p_T

> 40 GeV is required and, to suppress the

t¯t

background, the event is rejected if any jet with

p_T

> 25 GeV and

|η|

< 2.5 is identified as a

b-tagged jet.

•

For the 2-jet VBF and VH categories, a subleading jet with

p_T

> 25 GeV is required as well.

• E^miss_T

> 40 GeV (E

_T^miss

> 20 GeV) for the

ee, µµ

(eµ) channels. An additional requirement on

H_T^miss

> 40 GeV, which is the missing transverse energy defined by high-p

T

objects only

⁵

is also applied for

ee, µµ

events in the 8 TeVanalysis.

•

In the collinear approximation technique it is assumed that the neutrinos from the τ decay are aligned with the visible decay products. With this assumption the mass of the di-tau system can be computed if the

E_T^miss

in an event is solely due to undetected neutrinos from the τ decays. In this computation the variables

x₁

and

x₂

are used, which denote the visible momentum fraction of the τ leptons carried by the leading and sub-leading charged leptons respectively:

5These objects (identified electrons, muons and jets) must satisfy requirements described in Sections 3 and 4.1

x_1,2= |p_vis1,2|

|(p_vis1,2+p_mis1,2

)| . (1)

Here,

p_vis1,2

denotes the momenta of the leptons while

p_mis1,2

denotes the invisible decay products of the tau leptons inferred by the collinear approximation. Events that do not satisfy 0.1 <

x₁

,

x₂

<

1.0 are rejected.

•

0.5 <

∆

φ

``

< 2.5 (to suppress the

Z/γ^∗→

`` and top backgrounds).

Since not all categories are orthogonal by their selection requirements alone, an order of preference is applied when sorting events into categories. Events that are not selected in one category will be considered by the next category in the order. This order, together with the requirements unique to each category, is given below:

1.

2-jet VBF:

An absolute pseudorapidity difference between the two selected jets

∆

η

j j =|η_j1−η_j2|

>

3 and a di-jet invariant mass

mj j

> 400 GeV are required. Finally, the event is only selected in the 2-jet VBF category if no additional jet with

p_T

> 25 GeV and

|η|

< 2.4 is found in the pseudorapidity range between the two leading jets, which is called the Central Jet Veto (CJV). The leptons are also required to lie between the two leading jets in η (lepton centrality).

2.

Boosted:

Events that do not fulfill the requirements of the 2-jet VBF selection can be selected in the Boosted category if they satisfy

p_T,ττ

> 100 GeV, where

p_T,ττ

is the scalar

p_T

of the di-lepton and

E

~

^miss_T

system, defined as:

p_T,ττ=|~p^`,1_T +

~

p^`,2_T +E

~

_T^miss|.

(2) 3.

2-jet VH:

Events that do not qualify for the 2-jet VBF and Boosted categories but contain a second jet with

p_T

> 25 GeV can be selected in the

H+

2-jet

V H

category. The requirements on the pseudorapidity separation of the jets and on the di-jet invariant mass are

∆

η

j j

< 2 and 30 GeV <

m_{j j}

< 160 GeV.

4.

1-jet:

Events failing the cuts for the three categories defined above are considered in the 1-jet cat- egory. For the 1-jet category, the invariant mass of the two τ leptons and the leading jet is required to fulfill

m_ττj

> 225 GeV, where the τ momenta are taken from the collinear approximation. The Higgs boson production mechanism mainly contributing to this category is the gg

→H

process.

5.

0-jet:

This category uses an inclusive selection to collect part of the signal not selected by the categories with jets. Only the

eµ

final state is considered because of the overwhelming

Z/γ^∗→

``

background in the

ee

and µµ final states. In order to reduce the

t¯t

background, it is required that the di-lepton azimuthal opening angle be

∆

φ

_``

> 2.5 . This category was not included in the 8 TeV search, as differences in modelling of kinematic distributions of the embedded τ lepton decays in the

Z/γ^∗→

ττ background estimate for this particular category was found to bias the MMC mass distributions. The 0-jet also has the lowest signal-to-background ratio among the 5 categories.

A summary of all categories and event selection requirements can be found in Table 2. Across all cat-

egories and production modes the product of signal acceptance and selection e

ffi

ciency is 5.7% for the

7 TeV analysis, and 1.6% for the 8 TeV analysis, for

mH =

125 GeV. Here, the acceptance times effi-

ciency for the 7 TeV analysis is significantly larger due to the inclusion of the 0-jet category. Without this

category the acceptance times e

ffi

ciency amounts to 1.5%, which is comparable to the number obtained

for the 8 TeV analysis.

Table 2: The categorization of the

H →

τ

_lep

τ

_lep

analysis. The JVF cut is

|JV F|

> 0.75 for 7 TeV data, the lepton centrality is not applied for 7 TeV analysis, and the 0-jet category is not used for 8 TeV data analysis.

2-jet VBF Boosted 2-jet VH 1-jet

Pre-selection: exactly two leptons with opposite charges 30 GeV <

m_``

< 75 GeV (30 GeV <

m_``

< 100 GeV)

for same-flavor (different-flavor) leptons, and

p_{T,`1</sub>+ p<sub>T,`2}

> 35 GeV At least one jet with

pT

> 40 GeV (|JV F

jet|

> 0.5 if

|η_jet|

< 2.4)

E_T^miss

> 40 GeV (E

^miss_T

> 20 GeV) for same-flavor (different-flavor) leptons

H_T^miss

> 40 GeV for same-flavor leptons 0.1 <

x_1,2

< 1

0.5 <

∆

φ

_``

< 2.5

p_T,j2

> 25 GeV (JVF) excluding 2-jet VBF

p_T,j2

> 25 GeV (JVF) excluding 2-jet VBF, Boosted and 2-jet VH

∆

η

j j

> 3.0

p_T,ττ

> 100 GeV excluding Boosted

m_ττj

> 225 GeV

mj j

> 400 GeV

b-tagged jet veto ∆

η

j j

< 2.0

b-tagged jet veto b-tagged jet veto

– 30 GeV <

mj j

< 160 GeV

Lepton centrality and CJV

b-tagged jet veto

–

0-jet (7 TeV only)

Pre-selection: exactly two leptons with opposite charges

Different-flavor leptons with 30 GeV <

m_``

< 100 GeV and

p_{T,`1</sub>+ p<sub>T,`2}

> 35 GeV

∆

φ

_``

> 2.5

b-tagged jet veto

5.2 H → τ

_lep

τ

_had

Events passing the pre-selection requirements described in Section 4.2 are split into the following mutu- ally exclusive analysis categories:

1.

VBF:

This category includes all selected events with

p_T^τhad-vis

> 30 GeV,

E^miss_T

> 20 GeV and at least two jets (

j1,j2) with p_T

> 40 GeV (for the 8 TeV analysis, the threshold is reduced to 30 GeV for the second jet only). The two leading jets are required to be in the forward and backward halves of the detector (η

_j1×

η

_j2

< 0), be separated in pseudorapidity by

∆

η

j j

> 3.0, and have an invariant mass

m_{j j}

> 500 GeV. Both the lepton and τ

_had

candidates are required to be found in the pseudorapidity interval between the two leading jets, i.e. min(η

_j1

, η

_j2

) < η

l

, η

_τhad-vis

<

max(η

_j1

, η

_j2

); the latter condition will be referred to as the centrality requirement for the lepton and τ

_had-vis

. In addition, the quantity

p_T^Total

, defined as

p_T^Total=|~p^`_T+

~

p^τ_T^had-vis +

~

p_T^j1+

~

p_T^j2+E

~

^miss_T |,

(3) is required to be at most 40 (30) GeV in the 7 (8) TeV analysis. Finally, for the 8 TeV analysis, the lepton

p_T

is required to be greater than 26 GeV. The VBF category combines the τ

e

τ

had

and τ

µ

τ

_had

final states.

2.

Boosted:

This category includes all events failing the VBF selection and satisfying the following criteria:

p_T^H

> 100 GeV,

E^miss_T

> 20 GeV, 0 <

x₁

< 1 and 0.2 <

x₂

< 1.2, where

p_T^H=|~p^`_T+

~

p^τ_T^had-vis +E

~

_T^miss|,

(4)

and

x₁

,

x₂

are respectively the energy fraction of the leptonic and the hadronic visible τ decay products in the collinear approximation, defined in Section 5.1. Additionally, for the 8 TeV analysis only,

p_T^τhad-vis

is required to be at least 30 GeV. The Boosted category also combines the τ

e

τ

_had

and τ

_µ

τ

_had

final states.

3.

1-jet:

This category includes all pre-selected events that fail the requirements for the above two categories, have

E^miss_T

> 20 GeV and at least one jet with

p_T

> 25 (30) GeV in the 7 (8) TeV analysis. The τ

e

τ

_had

and τ

µ

τ

_had

final states are considered separately.

4.

0-jet:

This category includes all pre-selected events failing the requirements for previous cate- gories, have

E_T^miss

> 20 GeV and have no jets with

p_T

> 25 (30) GeV in the 7 (8) TeV analysis.

The τ

e

τ

_had

and τ

_µ

τ

_had

final states are considered separately.

The dominant background in this analysis is due to

Z/γ^∗→

ττ production, which has the same final

state and similar event kinematics as the signal, and is therefore largely irreducible. Another important

background comes from QCD multijet processes, where one of the jets is misidentified as a hadronic τ

lepton decay and another jet as an electron or muon (leptons can also come from semi-leptonic decays

of

B

and

D

hadrons).

W

boson production in association with jets also provides a significant source of

background due to its relatively large cross-section and the combination of a charged lepton and

E^miss_T

from the leptonic decay of the

W

boson in the final state, while hadronic jets accompanying the

W

boson can be misidentified as hadronic τ decays.

Z/γ^∗ →

`` is important primarily in the 0- and 1-jet

categories, especially when the lighter lepton is misidentified as a τ

_had

candidate, since the invariant mass

peaks approximately where the signal might be expected. The production of

t¯t

events is a considerable

background, particularly in the Boosted and VBF categories. Finally, diboson and single-top production

are also sources of background, although less important.

After the event classification, additional requirements are applied to reduce these backgrounds. The transverse mass,

m_T

, of the lepton and

E_T^miss

system e

ff

ectively discriminates signal (low

m_T

) and back- grounds with

W →e/µ+

ν (large

m_T

). It is defined by:

m_T = q

2p

^`_TE^miss_T

(1

−

cos

∆

φ) , (5)

where

∆

φ denotes the angle between the lepton and

E^miss_T

in the plane perpendicular to the beam di- rection. Additional suppression of the

W+

jets background is provided by the

P∆

φ variable defined as:

X∆

φ

=|φ`−

φ

_E^miss

T |+|φτ−

φ

_E^miss

T |

. (6)

In

H→

ττ events, the

E^miss_T

vector usually points between the visible τ-lepton decay products, leading to

P∆

φ < π. This is often not the case for the

W+

jets background, where

P∆

φ can be significantly larger than π.

The angular separation of the two τ-leptons from

H→

ττ depends on the boost of the Higgs boson in the transverse plane. This correlation is di

ff

erent for ττ resonances like Higgs and

Z

bosons and for

l−

τ

_had-vis

pairs from non-resonant backgrounds. The dependence of the average

∆R^lτhad-vis

on

p^lτ_T^had-vis

in signal is parametrized by a Landau function. For each value of

p^lτ_T^had-vis

a prediction,

∆R^lτ_pred^had-vis

, can then be obtained from this parametrisation.

∆(∆R), defined as∆(∆R)=|∆R^lτhad-vis −∆R^lτ_pred^had-vis

(p

^lτ_T^had-vis

)|, is then used to further suppress non-resonant backgrounds.

Further suppression of multijet and

W+

jets backgrounds is achieved in the 0-jet category by a cut on the

p_T

difference between the two τ decay products (p

^`_T− p^τ_T^had-vis

< 0), which is expected to be peaked at negative values for signal and the

Z →

ττ background, due to the presence of two neutrinos in the leptonic τ decay. Dedicated cuts are also applied on the τ

_had

candidates aimed at reducing the rate of lepton misidentification and thereby the

Z →

`` background in the 0 and 1 jet categories where this background is significant. Finally, in the 8 TeV analysis, a veto on

b-tagged jets is applied for the VBF

and Boosted categories, to reduce the top-quark background. The actual requirements applied depend on the event category, and were tuned separately for the 7 and 8 TeV analyses. The categorization and event requirements in this channel are summarized in Table 3.

Across all categories and production modes the signal acceptance and selection e

ffi

ciency is 1.7%

for the 7 TeV analysis, and 2.4% for the 8 TeV analysis, for

mH=

125 GeV. The acceptance times signal selection e

ffi

ciency in the 8 TeV analysis is higher than that in the 7 TeV one primarily because of a lower threshold on

p_T

(µ): i.e.,

p_T

(µ) >25 GeV for 7 TeV and

p_T

(µ) >17 GeV for 8 TeV (see Table 1).

5.3 H → τ

_had

τ

_had

In the

H →

τ

_had

τ

_had

channel, separately optimized selections are applied in two mutually exclusive categories: VBF and Boosted. The categorization is done in a cascading manner, considering all events for the VBF category and only those which fail VBF selection for the Boosted category.

For the analysis of the 8 TeV dataset, additional preselection stages are defined specific to each category, based on jet multiplicity:

1. VBF preselection: at least two tagging jets (

j₁

,

j₂

)

2. Boosted preselection: at least one tagging jet, fails VBF signal region selection

where tagging jets are defined as those with

p_T

> 30 GeV in the central region (|η| < 2.4) and

p_T

>

35 GeV in the forward region (

|η|

> 2.4). Additional cuts on the system of the two τ

_had

candidates are

also applied at this stage and are detailed in Table 4. These preselected datasets are used to perform

Table 3: Event requirements applied in the different categories of the

H →

τ

_lep

τ

_had

analysis. Require- ments marked with a triangle (.) are categorization requirements, meaning that if an event fails that requirement it is still considered for the remaining categories. Requirements marked with a bullet (

•

) are only applied to events passing all categorization requirements in a category; events failing such require- ments are discarded.

7 TeV 8 TeV

VBF Category Boosted Category VBF Category Boosted Category

.

p_T^τhad-vis

>30 GeV – .

p_T^τhad-vis

>30 GeV .

p_T^τhad-vis

>30 GeV

.

E^miss_T

>20 GeV .

E^miss_T

>20 GeV .

E^miss_T

>20 GeV .

E^miss_T

>20 GeV .

≥

2 jets .

p^H_T

> 100 GeV .

≥

2 jets .

p^H_T

> 100 GeV .

p_T ^j1

,

p_T ^j2

> 40 GeV . 0 <

x₁

< 1 .

p_T ^j1

> 40,

p_T^j2

>30 GeV . 0 <

x₁

< 1 .

∆

η

j j

> 3.0 . 0.2 <

x₂

< 1.2 .

∆

η

j j

> 3.0 . 0.2 <

x₂

< 1.2 .

mj j

> 500 GeV . Fails VBF .

mj j

> 500 GeV . Fails VBF

. centrality req. – . centrality req. –

. η

_j1×

η

_j2

< 0 – . η

_j1×

η

_j2

< 0 –

.

p_T^Total

< 40 GeV – .

p_T^Total

< 30 GeV –

– – .

p_T^`

>26 GeV –

•m_T

<50 GeV

•m_T

<50 GeV

•m_T

<50 GeV

•m_T

<50 GeV

•∆

(

∆R)

< 0.8

•∆

(

∆R)

< 0.8

•∆

(

∆R)

< 0.8

•∆

(

∆R)

< 0.8

• P∆

φ < 3.5

• P∆

φ < 1.6

• P∆

φ < 2.8 –

– –

•b-tagged jet veto •b-tagged jet veto

1 Jet Category 0 Jet Category 1 Jet Category 0 Jet Category

.

≥

1 jet,

p_T

>25 GeV . 0 jets

p_T

>25 GeV .

≥

1 jet,

p_T

>30 GeV . 0 jets

p_T

>30 GeV .

E^miss_T

>20 GeV .

E^miss_T

>20 GeV .

E^miss_T

>20 GeV .

E^miss_T

>20 GeV . Fails VBF, Boosted . Fails Boosted . Fails VBF, Boosted . Fails Boosted

•m_T

<50 GeV

•m_T

<30 GeV

•m_T

<50 GeV

•m_T

<30 GeV

•∆

(

∆R)

< 0.6

•∆

(

∆R)

< 0.5

•∆

(

∆R)

< 0.6

•∆

(

∆R)

< 0.5

• P∆

φ < 3.5

• P∆

φ < 3.5

• P∆

φ < 3.5

• P∆

φ < 3.5

–

•p^`_T− p^τ_T

< 0 –

• p^`_T− p^τ_T

< 0

background normalisation and data-to-model validation, described in Section 6.3. The analysis of the 7 TeV dataset is instead performed using a single common preselection.

The two dominant backgrounds in this channel are

Z/γ^∗ →

ττ and multi-jet production. The two signal regions are defined through a series of cuts designed to minimize these backgrounds:

1.

VBF:

This signal region is designed as a tight selection optimized for the vector boson fusion Higgs production mode. The selection is common to the 7 TeV and 8 TeV datasets. At least two tagging jets are required and the leading tagging jet should have

p_T

> 50 GeV. The two leading tagging jets need to be in opposite hemispheres, η

_j1×

η

_j2

< 0 and

∆

η

j j

> 2.6, and have a combined invariant mass

mj j

> 350 GeV. Finally, the two τ

_had

candidates need to be in between the two leading tagging jets in pseudorapidity, and

E^miss_T

> 20 GeV is required.

2.

Boosted:

This signal region is intended to accept signal events which are produced mainly by the gluon fusion Higgs production mode and are boosted by recoiling against an additional high-

p_T

jet. The category is defined by events failing the VBF selection and having at least one tagging jet with

p_T

> 70 GeV (8 TeV dataset) or

p_T

> 50 GeV (7 TeV dataset). Furthermore, the separation of the two τ

_had

candidates is required to be

∆R(τ₁

, τ

₂

) < 1.9. Finally, there is a requirement

E^miss_T

> 20 GeV, and if the

E^miss_T

vector is not pointing in between the two τ

_had

candidates, min

n

∆

φ(E

^miss_T

, τ

₁

),

∆

φ(E

_T^miss

, τ

₂

)

o

< 0.1π must hold.

Table 4 summarizes the selection criteria for the

H →

τ

_had

τ

_had

channel. Across all categories and production modes the signal acceptance times selection e

ffi

ciency ranges from 0.2-0.3% for both the 7 TeV and 8 TeV analyses, for

mH=

125 GeV.

Table 4: Summary of the event selection and categories for the

H→

τ

_had

τ

_had

channel.

Cut Description

Preselection No muons or electrons in the event

Exactly 2 mediumτ_hadcandidates matched with the trigger objects At least 1 of theτ_hadcandidates identified as tight

Bothτ_hadcandidates are from the same primary vertex

Leadingτ_had-vis pT>40 GeV and sub-leadingτ_had-vis pT>25 GeV,|η|<2.5 τ_hadcandidates have opposite charge and 1- or 3-tracks

0.8<∆R(τ₁, τ₂)<2.8

∆η(τ, τ)<1.5

ifE_T^missvector is not pointing in between the two taus, minn

∆φ(E_T^miss, τ₁),∆φ(E_T^miss, τ₂)o

<0.2π VBF At least two tagging jets, j₁,j₂, leading tagging jet withp_T >50 GeV

η_j1×η_j2<0,∆ηj j>2.6 and invariant massm_{j j}>350 GeV min(η_j1, η_j2)< ητ1, ητ2<max(η_j1, η_j2)

E^miss_T >20 GeV Boosted Fails VBF

At least one tagging jet withp_T >70(50) GeV in the 8(7) TeV dataset

∆R(τ₁, τ₂)<1.9 E^miss_T >20 GeV

ifE_T^missvector is not pointing in between the two taus, minn

∆φ(E_T^miss, τ₁),∆φ(E_T^miss, τ₂)o

<0.1π.

6 Background estimation and modelling

The background composition and normalisation are determined using data-driven methods and the sim-

ulated event samples described in Section 2.

[GeV]

miss

ET

0 10 20 30 40 50 60

Arbitrary Units

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Embedded µ

µ Z→

Data µ µ Z→

ATLAS Preliminary = 8 TeV s

L dt = 13.0 fb-1

∫

Selection µ µ Z→

(a) E_T^miss inZ/γ^∗→µµ data

[GeV]

τ

mτ

MMC mass

0 50 100 150 200 250 300

Arbitrary Units

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Embedded τ

τ

→ Z

Alpgen MC τ

τ

→ Z Emb. syst.

ATLASPreliminary = 8 TeV s

L dt = 13.0 fb-1

∫

Preselection µ

µ µ + + e e e

(b) Invariant massmττinτlepτlepchannel

[GeV]

τ

mτ

MMC mass

50 100 150 200 250

Arbitrary Units

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Embedded τ

τ

→ Z

Alpgen MC τ

τ

→ Z Emb. syst.

ATLAS Preliminary = 8 TeV s

L dt = 13.0 fb-1

∫

Preselection τhad

µ

had + τ e

(c) Invariant massmττinτlepτhadchannel

[GeV]

τ

mτ

MMC mass

0 50 100 150 200 250

Arbitrary Units

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24

Embedded τ τ Z→

Alpgen MC τ τ Z→ Emb. syst.

ATLASPreliminary = 8 TeV s

L dt = 13.0 fb-1

∫

Preselection τhad

τhad

(d) Invariant massmττinτhadτhadchannel

Figure 1: (a)

E_T^miss

distributions for the

Z/γ^∗ →

µµ data before and after muon embedding and (b,c,d) MMC mass distributions (defined in Section 5) for the τ-embedded

Z/γ^∗ →

µµ data and simulated

Z/γ^∗ →

ττ events in τ

_lep

τ

_lep

(b), τ

_lep

τ

_had

(c) and τ

_had

τ

_had

(d) events, respectively. Plot (a) is made after requiring two muons and plots (b,c,d) are made after preselection requirements. For (a) only statistical uncertainties are shown; (b,c,d) also include systematic uncertainties associated with the embedding procedure as discussed in Section 7.

The main background to the Higgs boson signal in all selected final states is due to the largely irre-

ducible

Z/γ^∗ →

ττ process. While it is not possible to select a signal-free

Z/γ^∗ →

ττ sample directly

from the data, this background can be modelled in a data-driven way by choosing a control sample

where the expected signal contamination is negligible. In a sample of selected

Z/γ^∗ →

µµ data events,

the muon tracks and associated calorimeter cells are replaced by τ leptons from a simulated

Z/γ^∗→

ττ

decay with the same kinematics, where the τ polarisation and spin correlations are modelled with the

TAUOLA program and processed by the full ATLAS detector simulation, digitisation, and reconstruc-

tion. These simulated τ decays are then merged with the initial data event. Thus, only the τ decays and