Search for Additional Heavy Neutral Higgs and Gauge Bosons with LHC Run 2 data in the ττ final state with the ATLAS detector Adam Bailey [email protected] CPAN 23/10/2017

Search for Additional Heavy Neutral Higgs and Gauge Bosons with LHC Run 2 data in the ττ final

state with the ATLAS detector

Adam Bailey

[email protected] CPAN 23/10/2017

1

Introduction

● Model independent results and MSSM (2HDM-type II)

● Two MSSM Higgs doublets, 5 Higgs:

● At tree level, can describe MSSM Higgs sector with m_A and tanβ (ratio of the v.e.v. of the two doublets)

● For large tanβ, couplings to down-type fermions are enhanced

○ Higher H → τ branching fraction

○ Enhanced production associated with b-quarks

● Also heavy Z’ appear in many BSM theories.

○ Z' with enhanced tau couplings might be related to heavy mass of 3rd generation or B-factory anomalies

○ Can decay to ττ, same signal as H/A

A

h H H

H

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Analysis Overview

3

● 36.1 fb^-1 of p-p collision data at centre-of-mass energy 13 TeV collected by the ATLAS detector in 2015/2016.

● Paper: https://arxiv.org/abs/1709.07242, and auxiliary material.

● Look for two back-to-back τ

● Two channels - τ_lepτ_had (~46%) and τ_hadτ_had (~42%)

● Split into b-tag and b-veto category for MSSM

The ATLAS Detector

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τ Decay Modes

5

● Neutrinos present - always some missing transverse momentum (MET)

● For hadronic taus:

○ Well collimated, low constituents multiplicity

○ Only 1 or 3 tracks

○ Use MVA to distinguish τ from QCD jets

τ Reconstruction

BDT discrimination against jets (1-track) τ trigger efficiency (1-track)

Also see conf. note on τ reconstruction: https://cds.cern.ch/record/2261772

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τ _had τ _had Selection

● Single-τ triggers: 80, 125, 160 GeV - depending on data taking period

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Lead τ

● Trigger matched

● p_T > [trigger p_T + 5 GeV]

● Pass medium jet BDT discrim

Sub-lead τ

● p_T > 65 GeV

● Pass loose jet BDT discrim

e μ

H

Δφ > 2.7

τ

τ

Medium ID: τ efficiency 75% (60%), QCD multijet rejection ~30 (30) for 1 (3) tracks Loose ID: τ efficiency 85% (75%), QCD multijet rejection ~50 (100) for 1 (3) tracks

τ → Lepton

● Isolated

● Trigger matched

τ _lep τ _had Selection

● Single-e and single-μ triggers, thresholds 20-140 GeV

● Lower trigger thresholds require tighter isolation τ → Hadrons

● Pass medium jet BDT discrim

● |η| < 2.3 (avoid mis-ID e backgrounds)

e μ

● To decrease W+jets background:

● To remove Z → ee peak, in e channel, reject events where lepton and τ_had have invariant mass 80 - 110 GeV.

H

Δφ > 2.4

τ

τ

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Backgrounds - τ _had τ _had

9

Z/γ* → ττ (2 real τ)

W → τν + jets (1 real tau, 1 jet faking tau) tt , single top (real τ, or fake lep/jets)

Other [W → lν + jets, Z/γ* → ll + jets, diboson]

QCD Multijet (fake τ) Data Driven

Monte Carlo, with

data-driven jet → τ fake rate measurement

Backgrounds - τ _lep τ _had

Z/γ* → ττ / tt (b-tag / b-veto) Z/γ* → ll

Diboson Single t

W + jets / tt (b-tag / b-veto)

Data Driven

Monte Carlo, with data-driven correction Real lepton, τ_had from jet

Multijet Lepton and τ_had from jets

e fakes τ_had Real τ

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● Usually the probability of faking τ/lepton not well-modelled in MC:

○ Requires data driven methods

● Use several control regions (CR) and fake factors

● CR with ID requirement inverted, weight those events by a fake factor f:

● Fake factor calculated in background-enriched region, defined as:

Fake Factors

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τ _had τ _had Multijet Background

● CR-1: Signal region, but sub-lead τ fail loose ID

● Estimate non-multijet backgrounds using MC, and subtract them.

● Fake factor, f_DJ calculated from region dominated by multijet events

● f_DJ = N τ₂ pass loose / N τ₂ fail loose

Fake factors show agreement in b-tag/veto/inclusive categories. Use b-inclusive, with extra stat uncertainty when using b-tag.

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τ _lep τ _had Data Driven Backgrounds

● Events with fake τ are mixture of W + jet and multijet, different fake factors

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Systematics Uncertainties

● Assess impact on cross section limits

● μ_stat⁹⁵ = limit with no systematics included

● μ_i⁹⁵ = limit including group of systematics i

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Results - Postfit Plots

15

Results - Cross Section Limits

● τ_lepτ_had more sensitive below m_H~0.6 TeV, τ_hadτ_had more sensitive above.

ggF b-assoc.

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● m_h^mod+ : benchmark where lightest Higgs matches that observed in ATLAS/CMS

● hMSSM: m_h = 125 GeV used to predict remaining masses/couplings

Results - tan β m _A Parameter Space

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m_h^mod+

hMSSM

Results

● 2D log likelihood scans for different Higgs masses

● Limits vs b-tag fraction

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Results - Z’ Models

● Limits on σxB for SSM Z’, for non-universal G(221) model use (m_Z’ ,sin²φ) plane

● φ = mixing angle between the heavy and light SU(2) gauge groups

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Conclusions

● Search for H/A or Z’ decaying to ττ in 2015/2016 ATLAS data

● No excess over the expected Standard Model background

● Upper limits set on σ x branching fraction to ττ for heavy Higgs and Z’

● Improved sensitivity over a larger mass range

● For hMSSM, exclude tan β > 1.0 for m_A = 0.25 TeV and tan β > 42 for m_A = 1.5 TeV at 95% CL

● SSM Z’ bosons with m_Z’ < 2.42 TeV excluded at 95% CL

● For non-universal G(221) model, m_Z’ < 2.25–2.60 TeV is excluded in the range 0.03 < sin² φ < 0.5

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- Backup -

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Backup - Object Reconstruction

Electrons:

Match EM calo deposit with inner detector p_T > 15 GeV

|η| < 2.47, exclude crack region

“Loose” likelihood working point

Muons:

Match inner detector track with muon spectrometer track

pT > 7 GeV

|η| < 2.5

“Loose” quality ID requirement.

Jets:

antiKt topo with R = 0.4 p_T > 20 |η| < 2.5

For p_T < 20 GeV, use MVA based on jet energy, vertexing and tracking to distinguish jets from pile-up

MET:

Track-based soft term algorithm (TST)

Jets, b-tag:

MVA based on tracks with large impact parameter, displaced secondary vertices, reconstructed paths of b/c hadrons

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Backup - Object Reconstruction

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Lepton in τ_lepτ_had: pT > 30 GeV

“Medium” ID requirement

Isolation Criteria - uses calorimeter and tracking info

Overlap Removal:

Jets within ΔR = 0.2 of τhad Jets within ΔR = 0.4 of e/μ τ_had within ΔR = 0.2 of e/μ e within ΔR = 0.2 of μ τ_had candidates:

Typically decays to 1 or 3 π^+/-, up to 2 π⁰ Seeded by antiKt4 jets, R = 0.4

p_T > 25 (45) GeV in τ_lep τ_had (τ_had τ _had)

|η| < 2.5, exclude crack region

n charged tracks == 1 or 3, |q| == 1

Use BDT to reject jet background, “loose“ and

“medium” ID requirements

Backup - τ Acceptance and b-Tag Fraction

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Backup - Non-multijet τ _had τ _had Backgrounds

● For non-multijet backgrounds (jets faking τ), estimate from simulation.

● Don’t apply τ ID to simulation - instead weight simulation by fake rate.

○ Ensures correct fake rate and uses all MC stats.

● Lead τ: N pass ID and τ trigger / N total

● Subleading τ: N pass ID / N total

● Use two regions, T-FR enriched in tt events, W-FR enriched in W+jets.

● For W + jets, W-FR: μ trigger (isolated) p_T > 55 GeV, τ₁ (no ID) p_T > 50 GeV, e-veto, Δφ > 2.4 m_T (p_T^μ, E_T^miss) > 40, no b-tagged jets.

● For tt, T-FR: Same as W-FR, but with at least 1 b-tagged jet

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Backup - Summary of Control Regions

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Backup τ _lep τ _had W+jets/tt Background

● Jet faking τ_had, with real lepton

● CR-1: Signal region, but τ_had fails medium ID.

● Subtract other backgrounds - real leptons estimated from MC, multijet from fake factor (see next slide).

● Fake factor, f_W calculated from region W-FR:

signal region, pass very loose τ_had ID, 70 (60) < m_T (p_T^l, E_T^miss) < 150 GeV in τ_eτ_had (τ_μτ_had)

● f_W = N pass medium τ_had ID / N fail medium τ_had ID

Events weighted by W+jets tau fake factor

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Backup - τ _lep τ _had Multijet Background

● CR-2: τ_had fails medium ID, and lepton fails isolation

● First weight by lepton isolation fake factor, f_L:

● f_L from region L-FR: l (trigger, selected), jet (selected), no τ_had, m_T (p_T^l, E_T^miss) < 30 GeV. f_L = N pass lepton isolation / N fail lepton isolation

● Then, apply multijet τ fake factor to the above:

● f_MJ from region MJ-FR: signal region, fail lepton isolation, pass very loose τ_had ID. f_MJ = N pass medium τ_had ID / N fail medium τ_had ID

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Backup - τ _lep τ _had Multijet Background

● Fake factors also have Δφ dependence due to τ_had ID dependence on energy response

● Due to limited size of CRs, extracted as sequential correction, only applied to b-veto channel.

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Backup - Systematic Uncertainties

● Integrated luminosity: 3.2%

● Data driven fake factors/rates, statistical uncertainty and MC subtraction

● Detector sim related ones affect: reco efficiency, ID, triggering, energy scale, resolution.

● On τ_had have:

○ ID measured from Zττ, 5-6%

○ Trigger efficiency unc. 3-14%

○ Energy scale 2-3%

○ P(e ID as τ_had) precision 3-14%

● Account for theoretical cross section uncertainties in MC samples, affects acceptance by:

○ ~5% on Z/γ* + jets

○ 10% on diboson

○ Top 6%

○ MSSM signals 1-4%

○ On Z’ is negligible

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Backup - Mass Variable

● Use binned likelihood function in m_T^tot, defined as:

● Worse peak resolution than other ττ mass reconstructions, but m_T^tot gave best separation from multijet backgrounds and better sensitivity.

● Signal/background predictions depend on systematics, parameterised as nuisance parameters and constrained using Gaussian probability density functions

● For τ_lepτ_had, use very pure top control region to constrain tt background

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Backup - Top Control Region for τ _lep τ _had

● For τ_lepτ_had, can define region enriched in tt events.

● CR-T: Signal region, but:

mT (p_T^l, E_T^miss) > 110 (100) GeV in τ_eτ_had (τ_μτ_had)

● Constrains the normalisation of the tt background.