• Keine Ergebnisse gefunden

2020 CERN for the benefit of the ATLAS Collaboration

N/A
N/A
Protected

Academic year: 2021

Aktie "2020 CERN for the benefit of the ATLAS Collaboration"

Copied!
28
0
0

Wird geladen.... (Jetzt Volltext ansehen)

Volltext

(1)

ATLAS-CONF-2020-043 14August2020

ATLAS CONF Note

ATLAS-CONF-2020-043

28th July 2020

Search for heavy resonances decaying into a Z boson and a Higgs boson in final states with leptons

and√ b-jets in 139 fb1 of p p collisions at s = 13 TeV with the ATLAS detector

The ATLAS Collaboration

This note presents a search for new resonances decaying into aZ boson and a 125 GeV Higgs boson in theννb¯ b¯and`+`bb¯final states, where`=e±orµ±, inppcollisions at

s=13 TeV.

The data used correspond to a total integrated luminosity of 139 fb1collected by the ATLAS detector during Run-2 of the Large Hadron Collider. The search is conducted by examining the reconstructed invariant or transverse mass distributions ofZ hcandidates for evidence of a localised excess in the mass range from 300 GeV to 5 TeV. No significant excess is observed and 95% confidence level upper limits between 1 pb and 0.4 fb are placed on the production cross-section times branching fraction ofZ0resonances in heavy-vector-triplet models and the CP-odd scalar bosonAin two-Higgs-doublet models.

© 2020 CERN for the benefit of the ATLAS Collaboration.

Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license.

(2)

1 Introduction

Following the discovery of the Higgs boson [1,2], measurements of its properties so far indicate consistency with the Standard Model (SM) predictions. Nevertheless, several questions related to electroweak symmetry breaking remain open, in particular how the Higgs boson mass is protected against large radiative corrections (the naturalness problem [3–5]) and whether the Higgs boson is part of an extended scalar sector.

Various models with dynamical electroweak symmetry breaking scenarios attempt to solve the naturalness problem by assuming new strong interactions at a higher energy scale. These models generally predict the existence of new vector resonances that naturally decay into a vector boson and a Higgs boson, for example in Minimal Walking Technicolour [6–8], Little Higgs [9] or composite Higgs models [10,11]. Resonance searches are typically not sensitive to all free parameters of the underlying theory, thus generally simplified models are used, such as the Heavy Vector Triplet (HVT) parameterized Lagrangian [12,13], which adds an additionalSU(2)L field to the SM and provides a restricted number of new couplings.

A second class of models extend the scalar sector by including additional Higgs singlets or doublets [14].

Examples of models with an extended scalar sector are the minimal supersymmetric SM [15–19], axion models [20] or baryogenesis models [21]. Again, instead of targeting any of these specific theories, a generic two-Higgs-doublet model (2HDM) with a CP-conserving Higgs potential is probed. The scalar sector of the theory consists of five Higgs bosons: two charged (H±), two neutral CP-even (h,H) and one neutral CP-odd (A).

This note presents a search for the production of new heavy vector bosons, denoted hereafter byZ0, and a search for a heavy CP-odd scalar bosonAthat decay into aZ boson and the SM-like Higgs bosonh. The note targets leptonic decays of theZboson and decays of the Higgs boson into ab-quark pair. This results in two search channels`+`bb¯ andννb¯ b¯. Previous searches in the same final states have been performed on a partial Run-2 data set by CMS [22] and ATLAS [23]. Results have also been published in other final states: ATLAS has performed a search in the fully hadronic final state [24] on the full Run-2 data set and CMS has published results on partial data sets in the fully hadronic final state [25] and in theτ+τ`+` final state [26].

Apart from the significantly larger data set, several improvements have been implemented with respect to the previous publication [23], including improvedb-tagging, lepton isolation and jet reconstruction, a reoptimization of the lepton selection in the`+`bb¯ channel and a new selection based on theEmiss

T

significance in theννb¯ b¯ channel (see Sections4and5).

The search presented in this note is performed by looking for a localised excess in the distribution of the reconstructed mass of the ννb¯ b¯ and`+`bb¯ systems in a range from 300 GeV to 5 TeV. At low transverse momenta, the decay products of the Higgs boson are reconstructed as individual jets, while at high transverse momenta they are reconstructed as a single large radius jet. Results are extracted by binned maximum-likelihood fits to the data distribution of the reconstructed resonance candidate mass.

The results from the HVTZ0search are interpreted in two benchmark models. In the first model, referred to asModel A, the branching fractions to fermions and gauge bosons are comparable, as in some models with an extended gauge symmetry [27]. ForModel B, fermionic couplings are suppressed, as in strong dynamical models such as the minimal composite Higgs model [28]. The search focuses on high resonance masses, from 300 GeV up to 5 TeV. The search forAZ huses a smaller mass window from 300 GeV to 2 TeV, as for the class of models targeted in this note, the Higgs potential becomes unstable for too largeA boson masses.

(3)

The search performs selections of signal regions (SR) and of background-dominated control regions (CR) based on requirements on kinematic properties of final-state particles and event-level quantities. The statistical interpretation is performed by a profile likelihood fit on the final discriminant, a channel-dependent closest proxy to the reconstructed resonance mass, using the signal and control regions. The backgrounds are modelled using simulations. The normalisation of the major backgrounds is constrained by the fit.

Additional control regions are defined to improve the modelling ofW/Z+jets events.

This note is structured as follows. Sections2and 3provide a brief description of the ATLAS experiment and the data and simulated event samples. The event reconstruction and selections are discussed in Sections4and5. The background estimation and systematic uncertainties are described in Sections6 and7. Finally, Sections8and9detail the statistical analysis and provide a discussion of the results and concluding remarks.

2 ATLAS detector

The ATLAS detector [29] is a general-purpose particle detector used to investigate a broad range of physics processes. It includes an inner tracking detector (ID) surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters and a muon spectrometer (MS) incorporating three large superconducting toroid magnets with eight coils each. The ID consists of fine-granularity silicon pixel and microstrip detectors, and a straw-tube tracker. The silicon pixel detector includes an insertable B-Layer [30], which was installed before the start of Run-2. The ID is immersed in a 2 T axial magnetic field produced by the solenoid and provides precision tracking for charged particles in the range|< 2.5, whereηis the pseudorapidity of the particle1. The straw-tube detector also provides transition radiation measurements for electron identification. The calorimeter system covers the pseudorapidity range|< 4.9.

It is composed of sampling calorimeters with either liquid argon (LAr) or scintillator tiles as the active medium, and lead, steel, copper, or tungsten as the absorber material. The MS provides muon identification and momentum measurements for|η| <2.7. The ATLAS detector has a two-level trigger system to select events for further analysis [31].

3 Data and Monte Carlo samples

The data used in this analysis were recorded with the ATLAS detector between 2015 and 2018 in pp- collisions at

s =13 TeV and correspond to a total integrated luminosity of 139 fb1[32]. The data are required to satisfy criteria that ensure that the detector was in good operating condition. Monte Carlo (MC) simulation samples are used to model background and signal processes.

For the HVT interpretations of the ATLAS data, the quark-antiquark annihilation induced production of Z0bosons was generated with MadGraph5 2.3.3 [33] at leading-order (LO) accuracy, interfaced with Pythia 8.186 [34] using the A14 tune [35] and the NNPDF 2.3 LO parton density function (PDF) set [36].

1ATLAS 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 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). Angular distance is measured in units of

∆R q

(∆η)2+(∆φ)2.

(4)

Events were generated for a range of resonance masses from 300 to 5000 GeV, assuming a zero natural width and using benchmarkModel Aand Higgs-boson decays intobb¯ orcc¯.

For the 2HDM interpretations of the ATLAS data, events for the gluon–gluon fusion production of Abosons (ggA) were generated based on the narrow-width approximation and the 2HDM FeynRules model with MadGraph5 2.3.3 at LO accuracy, interfaced with Pythia 8.186 using the A14 tune and the NNPDF 2.3 LO PDF set. Events were generated for a range of Aboson masses from 300 to 2000 GeV assuming a zero natural width. For theAboson signals, only decays of the Higgs boson into abb¯pair were generated.

The production ofW and Z bosons in association with jets is simulated with the Sherpa v2.2.1 [37]

generator using next-to-leading order (NLO) matrix elements (ME) for up to two partons, and LO matrix elements for up to four partons calculated with the Comix [38] and OpenLoops [39–41] libraries. They are matched with the Sherpa parton shower [42] using the MEPS@NLO prescription [43–46] using the set of tuned parameters developed by the Sherpa authors. The NNPDF 3.0 NLO set of PDFs [47] is used and the samples are normalised to a next-to-next-to-leading order (NNLO) prediction [48].

The production oftt events is modelled using the PowhegBox v2 [49–52] generator at NLO with the NNPDF 3.0 NLO PDF set and theh

damp parameter2 set to 1.5m

top [53]. The events are interfaced to Pythia 8.230 [54] to model the parton shower (PS), hadronisation, and underlying event, with parameters set according to the A14 tune and using the NNPDF 2.3 LO set of PDFs. The cross section was calculated at NNLO accuracy including the resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms with Top++2.0 [55–61].

The associated production of top quarks withW bosons (Wt) as well as s- and t-channel production of top quarks are modelled using the PowhegBox v2 generator at NLO in QCD using the five-flavour scheme and the NNPDF 3.0 NLO set of PDFs. The diagram removal scheme [62] is used to remove interference and overlap withtt production. The events are interfaced to Pythia 8.230 using the A14 tune and the NNPDF 2.3 LO set of PDFs. Thett +hsamples were generated at NLO accuracy using the PowhegBox v2 generator with the NNPDF 3.0 NLO PDF set and interfaced to Pythia 8.230 with the A14 tune and the NNPDF 2.3 LO PDF set. The production oftt+V events is modelled using the MadGraph5_aMC@NLO v2.3.3 generator at NLO with the NNPDF 2.3 LO PDF set. The events are interfaced to Pythia 8.230 using the A14 tune and the NNPDF 2.3 LO PDF set.

Diboson events (WW,W Z, Z Z) were simulated using Sherpa 2.1.1 with the NNPDF 3.0 NNLO PDF set, including off-shell effects and Higgs-boson contributions, where appropriate. Diagrams with up to one additional emission are calculated with NLO precision in QCD, while diagrams with up to 3 parton emissions are described at LO accuracy [63]. They are merged and matched using the MEPS@NLO prescription. Loop-induced diboson processes that are initiated via theggproduction mode are simulated at LO in QCD for diagrams with up to one additional parton emission at the matrix element using OpenLoops in Sherpa 2.2.2, using the NNPDF 3.0 NNLO PDF set.

Finally, the production of the SM-like Higgs boson in association with a vector boson is simulated using Powheg [51] and interfaced with Pythia 8.212 for parton shower and non-perturbative effects. The Powheg prediction is accurate to next-to-leading order for theV h boson plus one jet production. The loop-inducedggZ hprocess is generated separately at leading order. The PDF4LHC15 PDF set [64]

2Thehdampparameter is a resummation damping factor and one of the parameters that controls the matching of Powheg matrix elements to the parton shower and thus effectively regulates the high-pTradiation against which thettsystem recoils.

(5)

Table 1: Summary of the Monte-Carlo generators used to produce the various background processes. The column

“Prediction order” gives the precision in QCD of the inclusive production cross sectionσprodapplied to the respective process. The order at which the corresponding matrix elements are calculated in the Monte Carlo is not necessarily the same as for the cross section.

Process Generator Prediction order ofσ

prod

W,Z``,Zνν Sherpa 2.2.1 NNLO

tt Powheg + Pythia8 NNLO+NNLL

single top (s/t/Wt-channel) Powheg + Pythia8 NLO

tt+h MG5_aMC@NLO + Pythia8 NLO (QCD) and NLO (EW)

tt+V MG5_aMC@NLO + Pythia8 NLO

qg/qq¯ VV ``/`ν/νν+qq¯ Sherpa 2.2.1 NLO

ggVV ``/`ν/νν+qq¯ Sherpa 2.2.2 NLO

qg/qq¯ ``νν Sherpa 2.2.2 NLO

qqW h+bb¯ Powheg + Pythia8 NNLO (QCD) and NLO (EW)

qqZ h``/νν+bb¯ Powheg + Pythia8 NNLO (QCD) and NLO (EW)

ggZ h``/νν+bb¯ Powheg + Pythia8 NLO+NLL

and the AZNLO tune [65] of Pythia 8.212 are used. TheggZ hproduction cross section was calculated at NLO accuracy including the resummation of next-to-leading logarithmic (NLL) soft gluon terms [66].

A summary of event generators used for the simulation of background processes is provided in Table1.

All simulated event samples include the effect of multipleppinteractions in the same and neighbouring bunch crossings (pile-up) by overlaying simulated minimum-bias events on each generated signal or background event. The minimum-bias events were simulated with the single-, double- and non-diffractive ppprocesses of Pythia 8.186 using the A3 tune [67] and the NNPDF 2.3 LO PDF. For all MadGraph and Powheg samples, the EvtGen v1.6.0 program [68] was used for the bottom and charm hadron decays. The generated samples were processed using the Geant-based ATLAS detector simulation [69,70] and the same event reconstruction algorithms were used as for the data.

Simulated events are corrected to compensate for differences between data and simulations regarding the energy (or momentum) scale and resolution of leptons and jets, the efficiencies for the reconstruction, identification, isolation and triggering of leptons as well as the tagging efficiency for heavy-flavour jets.

4 Event reconstruction

Collision vertices are reconstructed from at least two ID tracks with transverse momentum pT > 500 MeV [71]. Among all vertices, the one with the highest p2

T sum of all associated tracks is chosen to be the primary vertex (PV) of the event.

Electrons are reconstructed from ID tracks that are matched to clusters in the electromagnetic calorimeter and which come from the PV. The latter condition is satisfied by requirements on the transverse impact parameter significance,|d

0|/σ(d

0)< 5.0, and on the longitudinal impact parameter,|z

0sin(θ)| <0.5 mm.

(6)

Electrons must satisfy requirements for the electromagnetic shower shape, track quality, and track–cluster matching, using a likelihood-based approach with a “Tight” working point [72]. They must also be isolated:

both the calorimeter energy sum and the transverse momentum sum of all ID tracks within a variable cone around the electron have to be smaller than 0.06 times the electronET. The maximum cone size is

∆R=0.2, shrinking for largerE

T [72].

Muons are identified by matching tracks found in the ID to either full tracks or track segments reconstructed in the muon spectrometer (“combined muons”), or by stand-alone tracks in the muon spectrometer. They have to fullfil similar impact-parameter equirements: |d

0|/σ(d

0)<3.0 and|z

0sin(θ)| <0.5 mm. Muons are required to pass “Tight” identification requirements based on quality requirements applied to the ID and muon spectrometer tracks. They must also be isolated: thepTsum within a variable-radius cone in the ID system around the combined track has to be smaller than 0.06 times the muon transverse momentum.

The maximum cone size is∆R=0.3, shrinking for larger muonp

T[73].

Electron and muon candidates are required to have a minimumpTof 7 GeV and to lie within|< 2.5 for muons and|η| < 1.37 or 1.52< |η| < 2.47 for electrons. The “Tight” identification is relaxed to a “Loose”

identification [72,73] to perform the electron and muon veto in the 0-lepton channel.

Three jet types are reconstructed, using the anti-kt[74] algorithm implemented in the FastJet package [75].

Small-Rjets are reconstructed from noise-suppressed topological clusters in the calorimeter [76,77] using a distance parameters ofR=0.4. They are required to have apT > 20 GeV for central jets (|η| <2.5) and pT> 30 GeV for forward jets (2.5< |< 4.5). To suppress central jets from pile-up interactions, they are required to pass the jet vertex tagger [78] selection, with an efficiency of about 90%, if they are in the range pT< 60 GeV and|η| <2.4.

Large-Rjets are used to reconstruct Higgs boson candidates with high momenta for which theb-quarks are emitted close to each other. These jets are built using a distance parameters ofR=1.0 and track-calo clusters (TCCs) [79] as inputs. The TTCs are formed by combining information from the calorimeter and the ID. Trimming [80] is applied to remove the energy of clusters that originate from initial-state radiation, pile-up interactions or the underlying event. This is done by reclustering the constituents of the initial jet, using thekt algorithm [81,82], into smallerRsub=0.2 subjets and then removing any subjet that has apT less than 5% of thep

T of the parent jet [83]. The Large-Rjets are required to havep

T > 250 GeV and

|η| < 2.0. The momenta of both the Large-Rand Small-Rjets are corrected for energy losses in passive material and for the non-compensating response of the calorimeter. Small-Rjets are also corrected for the average additional energy due to pile-up interactions [84,85]. The third type of jets are clustered from ID tracks using a variable radius (VR) that shrinks with increasingp

Tof the studied proto jet [86]. VR track jets are used in this analysis for the identification ofb-jets from decays of boosted Higgs bosons.

They must contain at least two ID tracks compatible with the primary vertex and are required to have a transverse momentum above 10 GeV as well as|η| < 2.5. In this analysis, only Large-Rjets with at least two ghost-associated [87] VR track jets are retained.

Small-R jets and VR track jets containing b-hadrons are identified using the MV2c10 b-tagging al- gorithm [88–91], with an operating point that corresponds to a selection efficiency forb-jets of 70%, as measured in simulatedttevents. Applying theb-tagging algorithm reduces the number of light-flavour and gluon jets, jets containing hadronically decayingτ-leptons andc-quark jets by a factor of 300, 36 and 8.9, respectively [88].

Hadronically decayingτ-lepton candidates [92,93] are used in theννb¯ b¯channel to reject backgrounds with real hadronic τ-leptons. They are reconstructed using calorimeter based R = 0.4 jets. They are required to have either one or three associated tracks,p

T > 20 GeV and|η| < 1.37 or 1.52< |η| < 2.5.

(7)

They are identified using a multivariate tagging algorithm, which is 55% (40%) efficient for one-track (three-track)τcandidates [93].

The missing transverse momentum Emiss

T is calculated from calibrated calorimeter cells belonging to identified high-pTobjects, but also from the track-based soft term, i.e. all tracks compatible with the primary vertex and not associated to any object used in theEmiss

T calculation [94,95]. In addition, a track-based missing transverse momentum estimatorpmiss

T is built as the negative vectorial sum of the transverse momenta of all tracks from the primary vertex. In order to decrease backgrounds from mis-measurements of jet and lepton energies, an object-basedEmiss

T significance [96] is exploited. This observable takes the resolutions of objects entering theEmiss

T calculations into account and quantifies how likely it is that there is significantETmissfrom undetectable particles in the event.

Leptons and jets are reconstructed and identified independently. When those objects are spatially close, these algorithms can lead to ambiguous identifications. An overlap removal procedure [97] is therefore applied to uniquely identify these objects.

To improve the mass resolution of the reconstructed Higgs boson candidates, dedicated energy corrections are applied to bothb-tagged Small-Rand Large-Rjets to account for semileptonicb-hadron decays. The four-vector of the closest muon in∆R with p

T larger than 5 GeV inside the jet cone is added to the jet four-vector after removing the energy contribution deposited by the muon in the calorimeter [98]

(refered to as muon-in-jet correction). For this correction, muons are not required to pass any isolation requirements. For Small-Rjets including muons, an additional p

T-dependent correction is applied to the jet four-momentum to account for biases in the response ofb-jets, improving the resolution of the dijet mass. This correction is determined from simulated SMV h(hbb)¯ events by calculating the ratio of the pTof the generator levelb-jets from the Higgs boson decay to thep

Tof the reconstructedb-tagged jets after the muon-in-jet correction.

5 Analysis strategy and event selection

The search for theZ0andAbosons in theZ hννb¯ b¯andZ h`+`bb¯decay modes uses event selections wherein the number of reconstructed charged leptons is exactly zero or two (0-lepton and 2-lepton channels).

In the 0-lepton channel, largeEmiss

T signals the presence of two neutrinos.

For the 0-lepton channel, ETmiss triggers with a thresholds of 70110 GeV are used for the various data periods, corresponding to the increasing instantaneous luminosity. Events are required to have Emiss

T > 150 GeV, corresponding to a trigger efficiency of above 80%. In the 2-lepton channel, events were recorded using a combination of single-lepton triggers with isolation requirements. The lowest pT thresholds range from 24 26 GeV in the electron channel and from 20 26 GeV in the muon channel. Additional triggers without an isolation requirement are used to recover efficiency for leptons withpT >60(50)GeV in the electron (muon) channel. The lepton that satisfied the trigger is required to match a reconstructed electron (muon) withp

T > 27 GeV. The trigger efficiencies for the combined single electron and combined single muon triggers are larger than 90%.

The Higgs-boson candidate is reconstructed from the 4-vectors of its decay products, theb-quarks. When the Higgs boson has relatively lowp

T, theb-quarks can be reconstructed as two Small-Rjets (“resolved”

category ). As the momentum of the Higgs boson increases, the twob-quarks become more collimated and a selection using a single Large-Rjet becomes more efficient (“merged” category).

(8)

For the resolved signal region, two central Small-Rjets are required to have an invariant massmj jin the range 110 –140 GeV for the 0-lepton channel and in the range 100 –145 GeV for the 2-lepton channel. The latter selection is relaxed to take advantage of the smaller backgrounds in this channel. This dijet is defined by the twob-tagged Small-Rjets when twob-tagged jets are present in the event. In the case where only oneb-tagged jet is present, the dijet pair is defined by theb-tagged jet and the leading Small-Rjet in the remaining set. The leading jet in the pair must havep

T >45 GeV. For the merged signal region, a Large-R jet is required with massmJin the range 75–145 GeV and at least one associatedb-tagged track-jet. Events which satisfy the selection requirements of both the resolved and merged categories are assigned to the resolved one due to its better dijet mass resolution and lower background contamination.

Higgs-boson candidates with one or two b-tagged jets define the “1 b-tag” or “2 b-tag” categories, respectively. For the merged selection, only one or two leading track-jets associated with the Large-Rjet are considered in this counting. Events with ab-tagged track-jet that is not ghost-associated to the Large-R jet are rejected from the merged event categories. Events with more than 2b-tags are excluded from the signal selection for this note.

The calculation of the reconstructed resonance mass depends on the decay channel. In the 0-lepton channel, where it is not possible to reconstruct the Z h system fully due to the presence of two neutrinos from the Z boson decay, the transverse mass is used as the final discriminant, defined as:

mT,V h = r

Eh,T+ETmiss 2

p®h,T+E®Tmiss 2

, where Eh,T is the transverse energy of the Higgs boson candidate, whileE®miss

T andp®h,

Tare vectors of the missing transverse momentum and Higgs boson transverse momentum, respectively. For the 2-lepton channel, the final discriminant is the invariant masss of theZ h system,mV h. The mass resolution in the signal region is improved by scaling the four-momentum of the dimuon system by 91.2 GeV /mµµ. In resolved event topologies, also the four-momentum of the dijet system is scaled by 125 GeV/mj j.

An event cleaning procedure is applied to all lepton channels. Events are removed if they contain overlaps between one of the VR track jets used forb-tagging and at least one VR track jet with ap

Tabove 5 GeV and with at least two associated tracks, in order to prevent the selection of jets with an ambiguous association of tracks from heavy flavour decays. Additional selections are applied for each lepton channel, as outlined below, to reduce the main backgrounds and enhance the signal sensitivity.

The 0-lepton channel vetoes not only electrons and muons but also hadronically decayingτ-lepton candidates (τ

had). The multijet background is reduced by requiring a minimum missing transverse momentum of Emiss

T > 150(200)GeV in the resolved (merged) category, a minimumEmiss

T significance of 9.0–13.6, increasing as a function ofmV h, a minimum missing transverse track momentum, pmissT > 60 GeV, a maximum angular separation ofEmiss

T andpmiss

T ,∆φ(Emiss

T ,pmiss

T )< π/2, a minimum angular separation of Emiss

T and the Higgs-boson candidate,φ(ETmiss,h) > 2π/3 and a minimum angular separation ofEmiss

T

and the jets that are used in the event, min[∆φ(ETmiss,jets)]> π/9(π/6)for events with up to three (more than three) jets. In the resolved channel the angular separation between the two Higgs-boson candidate jets is required to beφ(j,j) <7π/9, and the scalar sum of all jetp

Tvalues must followH

T > 120(150)GeV if the event contains two (more than two) central jets3. Finally, the resolved channel requires at least two Small-Rjets and the merged channel at least one Large-Rjet.

In the 2-lepton channel, same-flavour leptons (ee or µµ) are selected wih p

T > 27 GeV for the leading lepton and pT > 20 (25) GeV for the subleading lepton in the resolved (merged) category.

3In the 2-lepton channel,H

Tis calculated as the scalar sum of thep

Tof the leptons and the Small-Rjets in the event.

(9)

Three kinematic selections are optimised as a function of the resonance mass to reduce the tt and Z+ jets backgrounds. Selections on the mass of the dilepton system, max[40 GeV, 87 GeV0.030·mV h]

<m`` <97 GeV+0.013·mV h, and onEmiss

T /p

(1 GeV) · H

T <1.15+8×103·mV h/ (1 GeV)are relaxed for higher-mass resonances to account for resolution effects and smaller backgrounds. The momentum of the dilepton system (p

T,``) is required to be greater than 20 GeV+9 GeV ·p

mV h/(1 GeV) −320 for mV hgreater than 320 GeV. In the resolved dimuon category, an opposite-charge requirement is applied to further reduce diboson backgrounds and since the probability to mis-reconstruct the charge of individual muons is very low.

All signal-region selections are summarised in Table 2. The product of kinematic acceptance and reconstruction efficiency forZ0 Z h(hbb/c¯ c)¯ and forggAZ h(hbb)¯ is shown in Figure1as a function of the resonance mass. ThemV hresolution is expected to be in the range of 6% (16%) to 12%

(25%) for the resolved (merged)`+`bb¯channel, while them

T,V hresolution is expected to range from 8%

(16%) to about 40% (30%) for the resolved (merged) ννb¯ b¯ channel. The resolution of the (transverse) mass degrades in the resolved (merged) categories with increasing (decreasing) resonance mass. In both lepton channels, the resolution is determined as the full width at half maximum.

In addition to the signal selection, dedicated control regions are used to improve the modelling of major backgrounds. These control regions are constructed by inverting one of the selection requirements of the signal region: in the 2-lepton resolved category,tt control regions are created by selecting events with different-flavour leptonseµ, oppositely charged leptons, and without theEmiss

T /p

HT requirement. The ttpurity of this selection is greater than 90%. These events are used to constrain thett normalization in the final fit. Dedicatedmj j sideband regions are defined in the resolved and merged 0-lepton regions by requiringmj j (mJ) to be within 50 GeV to 200 GeV but outside the respective signal region windows, defined above. They are used to improve the modelling of theW/Z+jets kinematics and to constrain the normalisation of thett andW/Z+jets backgrounds. In the 2-lepton channel,mj j sidebands are only used as validation regions, in which the kinematics of theZ+ jets background are corrected (see Section6). The 2-leptonmj j sideband regions are not included into the likelihood fit as theZ+ jets backgrounds are by far the most dominant contributions to the 2-lepton signal region such that no additional control region is required to constrain theZ+ jets backgrounds.

(10)

Table 2: Topological and kinematic selections for each channel and category as described in the text. The various signal regions are divided into “1b-tag” or “2b-tags” categories depending on the multiplicity ofb-tagged jets in the event.(∗)Applies in the case of only two central jets.

Variable Resolved Merged

Common selection Number of jets

2 central Small-Rjets (0, 2-lep.) 1 large-Rjet

2 VR track jets (matched to leading large-Rjet)

Leading jetpT[GeV] >45 >250

mj j [GeV] 110–140 (0-lep.), 100–145 (2-lep.) 75–145

0-lepton selection

ETmiss[GeV] >150 >200

HT[GeV] >150(120)

∆φbb </9

pmissT [GeV] >60

∆φ( ®ETmiss,p®Tmiss) < π/2

∆φ( ®ETmiss,h) >2π/3 min

h∆φ( ®ETmiss,Small-Rjet)i

> π/9 (2 or 3 jets),> π/6 (4 jets) Nτ

had 0

ETmisssignificance

>9 ifmV h<400 GeV,

>6.6+0.01·mV h/(1 GeV) if 400 GeV<mV h<700 GeV,

>13.6 ifmV h>700 GeV, 2-lepton selection

Leading leptonpT[GeV] >27 >27

Sub-leading leptonpT[GeV] >20 >25

ETmiss/p HT[

GeV] <1.15 +8×103·mV h/(1 GeV)

pT,``[GeV] >20+9·p

mV h/(1 GeV) −320 formV h> 320 GeV m``[GeV] [max[40, 870.030·mV h/(1 GeV)], 97+0.013·mV h/(1 GeV)]

(11)

1000 2000 3000 4000 5000 [GeV]

mZ'

0 0.2 0.4 0.6 efficiency×Acceptance 0.8

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2 Preliminary

ATLAS

= 13 TeV, 139 fb-1

s

νcc ν νbb, ν

Zh 0L channel, HVT Z'

(a)

400 600 800 1000 1200 1400 1600 1800 2000

[GeV]

mA

0 0.2 0.4 efficiency×Acceptance 0.6

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2 Preliminary

ATLAS

= 13 TeV, 139 fb-1

s

νbb ν

Zh

A 0L channel, gg

(b)

1000 2000 3000 4000 5000

[GeV]

mZ'

0 0.2 0.4 0.6

efficiency×Acceptance

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2 Preliminary

ATLAS

= 13 TeV, 139 fb-1

s

llbb, llcc

Zh 2L channel, HVT Z'

(c)

400 600 800 1000 1200 1400 1600 1800 2000 [GeV]

mA

0 0.2 0.4

efficiency×Acceptance

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2

All signal regions -tag, resolved b

1

-tags, resolved b

2

-tag, merged b

1

-tags, merged b

2 Preliminary

ATLAS

= 13 TeV, 139 fb-1

s

llbb

Zh

A 2L channel, gg

(d)

Figure 1: Product of acceptance and efficiency for theZ0 Z hννb¯ b, ν¯ νc¯ c¯(a),gg A Z hννb¯ b¯(b), Z0 Z h`+`bb, `¯ +`cc¯(c), andgg A Z h`+`bb¯ (d) as a function of the resonance mass for the 0-lepton SR (a,b) and for the 2-lepton SR (c,d). The figures show the total product of acceptance and efficiency and the separate values for the various SR.

6 Background estimation

The background composition in the signal region depends on the charged-lepton andb-jet multiplicities:

In the 0-lepton channel, the dominant background sources areZ+ jets andtt events with a significant contribution fromW+ jets, whereas in the 2-lepton channel,Z+ jets production is the dominant background followed bytt production. Contributions from diboson, SMV h,tt+h, andtt +V production is small in the two channels. The multijet background, due to semileptonic hadron decays or misidentified jets, is found to be negligible in both channels after applying the event selections described in Section 5.

Background distributions are estimated from the samples of simulated events with normalisations of the main backgrounds estimated from the data. The simulatedW/Z+jets samples are reweighted as a function ofpbb

T , based on fits to the data/simulation distribution in themj j sideband regions, defined in Section5.

Thepbb

T correction functions are derived after differences in the overall normalisation between data and the simulations have been accounted for.

(12)

Simulated event samples ofW/Z+jets are split into different flavour components. In the resolved category, the samples are split according to the generated flavour of the two Small-Rjets forming the Higgs-boson candidate. In the merged category, they are split according to the generated flavour of the one or two leading track-jets associated with the large-Rjet. The generated jet flavour is determined by counting generated heavy-flavour hadrons withp

T >5 GeV ghost-associated to the reconstructed jet. If ab-hadron is found, the jet is labelled as ab-jet, otherwise if ac-hadron is found the jet is labelled as ac-jet. If neither ab-hadron nor ac-hadron is associated with the reconstructed jet, it is labelled as a light jet. Based on this association scheme, theW/Z+jets simulated event samples are split into three components:W/Z+hf (W/Z+bb,W/Z+bcandW/Z+cc),W/Z+hl (W/Z+blandW/Z+cl) andW/Z+l ; in this notationlrefers to a light jet. In the statistical analysis described in Section8, the global normalisations of theZ+hf and Z+hl components are determined via a fit to data.

The processestt¯, single top,t+Vandtt¯+hare combined into one single component, which is referred to as top quark backgrounds.

The normalisation of the top quark andW/Z+jets backgrounds is determined from fits to data separately for the 0- and 2-lepton channels. In the 0-lepton channel, themj j sideband regions are used to constrain the W/Z+jets and top quark backgrounds. In the 2-lepton channel, aneµcontrol region is used to constrain the normalisation of the combined top quark backgrounds. Themj j sideband regions of the 0-lepton channel are defined for both resolved and merged event topologies, while thecontrol region exists only for resolved event topologies. All control regions are detailed in Section5.

Table 3: Post-fit normalisation scale factors (NF) and their uncertainties obtained from a combined background-only fit to the various signal and control regions of the 0- and 2-lepton channels. Numbers are presented for the background components that are floating in the likelihood fit. The top quark background is normalised with separate NFs for the 0-lepton and 2-lepton regions, while the NFs for the theZ+hf andZ+hl backgrounds are correlated between the two channels.

N Ftop(0lep) N Ftop(2lep) N FZ+hf N FZ+hl 0.85±0.05 1.02±0.02 1.21±0.07 1.12±0.06

7 Systematic uncertainties

The distributions ofmV handmT,V hare affected by both experimental and modelling uncertainties, which enter the final fits as nuisance parameters. Uncertainties on the modelling of physics objects are correlated over signal and background processes, channels, kinematic regions and distributions of observables.

The largest experimental systematic uncertainties are associated with the calibration and resolution of the Small-Rand Large-Rjet energy and of the Large-Rjet mass. Further dominant uncertainties are related to the determination of the jetb-tagging efficiency and misidentification rate. The uncertainties in the Small-Rjet energy scale have contributions fromin situcalibration studies, from the dependency on the pile-up activity, and from the flavour composition of jets [84,85]. An additional uncertainty in the energy calibration ofb- andc-jets is also used. The uncertainty in the scale and resolution of Large-Rjet energy and mass is estimated by comparing the ratio of calorimeter-based to track-based measurements in dijet data and simulation [83,99]. Differences in theb-tagging efficiency measured in data and simulation results

Abbildung

Table 1: Summary of the Monte-Carlo generators used to produce the various background processes
Table 2: Topological and kinematic selections for each channel and category as described in the text
Figure 1: Product of acceptance and efficiency for the Z 0 → Z h → ν νb ¯ b, ν ¯ νc ¯ c ¯ (a), gg → A → Z h → ν νb ¯ b ¯ (b), Z 0 → Z h → ` + ` − b b, `¯ + ` − c c ¯ (c), and gg → A → Z h → ` + ` − b b¯ (d) as a function of the resonance mass for the 0-lep
Table 4: Relative systematic uncertainties in the normalisation, cross-region extrapolation, and shape of the signal and background processes included in the fits described in the text
+7

Referenzen

ÄHNLICHE DOKUMENTE

In order to address this issue, we developed a data infrastructure for sci- entific research that actively supports the domain expert in tasks that usually require IT knowledge

The data points which scatter within our experimental resolution are in excellent agreement with the above results obtained from calculated particle images and show the same

Several differential fiducial cross sections are measured for observables sensitive to the Higgs-boson production and decay, including kinematic distributions of the jets produced

58(a) Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei; (b) Institute of Frontier

For the 0 and 2 lepton channels it was verified, using simulated samples and data-driven studies respectively, that the multi-jet background made up less than 1% of the total

The dotted lines show the e ff ect on the observed limit when varying the signal cross section by ±1σ of the theoretical uncertainty.. The green band represents the LEP limit [22–26]

In the muon channel, for all values of dilepton invariant mass, the multijet background was estimated from data by inverting the isolation cut.. The resulting contamination was found

Monte Carlo (MC) simulation is used to estimate the SUSY signal yields, as well as SM background contributions from processes leading to two real taus in the final state, such as