Search for top squarks with the ATLAS detector Nicolas Khler Max - - PowerPoint PPT Presentation

SLIDE 1

Search for top squarks with the ATLAS detector

Nicolas Köhler

Max Planck Institute for Physics (Werner-Heisenberg-Institut)

Thursday 20th July, 2017

SLIDE 2

Why searching for Supersymmetry?

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 2/13

SLIDE 3

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Shortcomings of the Standard Model

Dark Matter candidate Neutrino masses Unification of gauge couplings Hierarchy problem Quantum Gravity (10d string theory)

Quarks u c t d s b e

µ τ νe νµ ντ

Leptons Gauge bosons Higgs bosons

γ

h, H0 Z W± g A H± Squarks

˜

u1,2 ˜ c1,2 ˜ t1,2

˜

d1,2 ˜ s1,2 ˜ b1,2

˜

e1,2 ˜

µ1,2 ˜ τ1,2 ˜ νe ˜ νµ ˜ ντ

Sleptons Gauginos

˜ χ0

1

˜ χ0

2

˜ χ0

3

˜ χ0

4

˜ χ±

1

˜ χ±

2

˜

g

→ Supersymmetry can explain all those questions

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 2/13

SLIDE 4

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Shortcomings of the Standard Model

Dark Matter candidate Neutrino masses Unification of gauge couplings Hierarchy problem Quantum Gravity (10d string theory)

Quarks u c t d s b e

µ τ νe νµ ντ

Leptons Gauge bosons Higgs bosons

γ

h, H0 Z W± g A H± Squarks

˜

u1,2 ˜ c1,2 ˜ t1,2

˜

d1,2 ˜ s1,2 ˜ b1,2

˜

e1,2 ˜

µ1,2 ˜ τ1,2 ˜ νe ˜ νµ ˜ ντ

Sleptons Gauginos

˜ χ0

1

˜ χ0

2

˜ χ0

3

˜ χ0

4

˜ χ±

1

˜ χ±

2

˜

g

→ Supersymmetry can explain all those questions

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 2/13

SLIDE 5

Why searching for top squarks at the LHC?

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 3/13

SLIDE 6

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Why searching for top squarks at the LHC?

The Higgs is unnaturally light! mH = 125 GeV with m2

H = (m2 H)bare + δm2 H

f ¯ f H H

∼ −λ2

f

˜ f H H

∼ λs → Fermionic and scalar contributions

cancel exactly if SUSY is unbroken Remaining contribution after SUSY breaking:

δm2

H ∼ −y2 t m2 stop ln

(

Λ

mstop

)

(one-loop-level) where Λ is the energy scale up to which the theory is renormalizable

→ mstop should be in the TeV range

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 3/13

SLIDE 7

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Simulated SUSY processes

The MSSM has 105 free parameters

→ Use simplified models where m˜

t,

m˜

χ0

1 and m˜

χ±

1 are in the TeV

range, but the other sparticles are decoupled with higher masses Stops described by these models are accessible to searches at the LHC Assume R-parity conservation Assume m˜

t2 ≫ m˜ t1

JHEP 1209 (2012) 035

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 4/13

SLIDE 8

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Top squark (˜ t1) decays

Assume only 2 decays are possible:

˜ t ¯ ˜ t p p ˜ χ0

1

t ˜ χ0

1

¯ t ˜ t ¯ ˜ t ˜ χ+

1

˜ χ−

1

p p b ˜ χ0

1

W + ¯ b ˜ χ0

1

W −

BR(˜

t → t + ˜

χ0

1) = 1 − BR(˜

t → b + ˜

χ±

1 )

→ Search for signatures with either 0, 1 or 2 Leptons

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 5/13

SLIDE 9

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Top squark searches in ATLAS during LHC Run 1

˜ t ¯ ˜ t p p ˜ χ0

1

t ˜ χ0

1

¯ t ˜ t ¯ ˜ t ˜ χ+

1

˜ χ−

1

p p b ˜ χ0

1

W + ¯ b ˜ χ0

1

W − ˜ t ˜ t ˜ χ±

1

b W ˜ χ∓

1

b W p p ˜ χ0

1

c ˜ χ0

1

c

;

[GeV]

1

t ~

m

200 300 400 500 600 700 800

[GeV]

1

χ ∼

m

50 100 150 200 250 300 350 400 450

1

χ ∼ t →

1

t ~

1

χ ∼ t →

1

t ~

1

χ ∼ /b f f’

1

χ ∼ W b →

1

t ~

1

χ ∼ W b →

1

t ~

1

χ ∼ c →

1

t ~

1

χ ∼ b f f’ →

1

t ~

1

χ ∼

,t) < m

1

t ~ m( ∆

W

+ m

b

) < m

1

χ ∼ ,

1

t ~ m( ∆ ) < 0

1

χ ∼ ,

1

t ~ m( ∆ 1

χ ∼ t →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ c →

1

t ~ /

1

χ ∼ b f f’ →

1

t ~ production,

1

t ~

1

t ~

ATLAS

1

χ ∼ W b

1

χ ∼ c

1

χ ∼ b f f’

Observed limits Expected limits All limits at 95% CL

• 1

=8 TeV, 20 fb s t0L/t1L combined t2L, SC WW t1L, t2L tc tc, t1L [GeV]

1

t ~

m 170 180 190 200 210 [GeV]

1

χ ∼

m 10 20 30 40 from https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/

CombinedSummaryPlots/SUSY/index.html

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 6/13

SLIDE 10

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

The full-hadronic decay channel (˜ t → bqq˜

χ0

1)

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q ˜ t ˜ t ˜ χ±

1

W ˜ χ∓

1

W p p b ˜ χ0

1

q q b ˜ χ0

1

q q Lepton veto leads to good background suppression Remaining backgrounds:

• - t¯

t+Z (Z → νν)

• - Z+jets (Z → νν)
• - t¯

t → bbqqℓν (missing the ℓ)

Full reconstruction of top quark due to full-hadronic W-decay

→ Both˜

t → t + ˜

χ0

1 and˜

t → b + ˜

χ±

1 are

accessible

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 7/13

SLIDE 11

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 12

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 13

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 14

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 15

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 16

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 17

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Select events of interest

Veto against leptons Large missing transverse energy (Emiss

T

> 250 GeV)

At least 4 jets (R = 0.4) with pT > (80, 80, 40, 40) GeV At least 2 b-jets

→ Reconstruct top quark + More discriminating variables

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 8/13

Events / 50 GeV 2000 4000 6000

Data SM Total t t Single Top +V t t W Z Diboson )=(600,300) GeV

1

χ ∼ ,

1

t ~ 20 x ( )=(1000,1) GeV

1

χ ∼ ,

1

t ~ 100 x (

ATLAS Preliminary

• 1

=13 TeV, 36.1 fb s >50 GeV

,min b T

m preselection +

[GeV]

,min b T

m

200 400 600 Data / SM 0.0 0.5 1.0 1.5 2.0

ATLAS-CONF-2017-020

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

SLIDE 18

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Reconstruct top quarks: Jet reclustering

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

http://www.quantumdiaries.org/author/ emily-thompson/

Higher centre-of-mass energy can lead to strongly boosted final state objects Full-hadronic top quark decay products can be collimated

→ Use large-R(adius) jets (R = 1.2)

[GeV]

=1.2 R jet,

m 200 400 600 800 [GeV]

1 =1.2 R jet,

m 200 400 600 800 Arbitrary units 0.00 0.02 0.04

ATLAS

Simulation Preliminary

= 13 TeV s ) = (1000,1) χ ∼ , t ~ (

TT TW T0

ATLAS-CONF-2017-020

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 9/13

SLIDE 19

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Reconstruct top quarks: Jet reclustering

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

http://www.quantumdiaries.org/author/ emily-thompson/

Higher centre-of-mass energy can lead to strongly boosted final state objects Full-hadronic top quark decay products can be collimated

→ Use large-R(adius) jets (R = 1.2)

[GeV]

=1.2 R jet,

m 200 400 600 800 [GeV]

1 =1.2 R jet,

m 200 400 600 800 Arbitrary units 0.00 0.02 0.04

ATLAS

Simulation Preliminary

= 13 TeV s ) = (1000,1) χ ∼ , t ~ (

TT TW T0

ATLAS-CONF-2017-020

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 9/13

SLIDE 20

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Reconstruct top quarks: χ2-method

Minimize

• mjj−mW±

mW±

• for all jet pairs jj

→ Define W± candidates

Minimize

• mbW±−mt

mt

• for all b-jet W± pairs

→ Define top quark candidates

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 10/13

SLIDE 21

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Reconstruct top quarks: χ2-method

Minimize

• mjj−mW±

mW±

• for all jet pairs jj

→ Define W± candidates

Minimize

• mbW±−mt

mt

• for all b-jet W± pairs

→ Define top quark candidates

Reconstruct top squarks:

Both ˜

χ0

1 of˜

t → t + ˜

χ0

1 decays contribute to Emiss

T

m2 T2 = min

q1+q2=Emiss

T

{

max

{

m2 T(q1, t), m2 T(q2,¯ t)

}}

splits Emiss

T

into 2 ˜

χ0

1

→ Top squark candidates are build

˜ t ˜ t t W t W p p ˜ χ0

1

b q q ˜ χ0

1

b q q

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 10/13

SLIDE 22

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

After applying all cuts to supress SM backgrounds

ATLAS-CONF-2017-020

Events / 50 GeV 5 10 15 Data SM Total Z t t Single Top +V t t W )=(1000,1) GeV

1

χ ∼ ,

1

t ~ (

ATLAS Preliminary

• 1

=13 TeV, 36.1 fb s SRA-T0

[GeV]

2

χ T2

m

600 800 1000 Data / SM 0.0 0.5 1.0 1.5 2.0

Out of >6 billion recorded events: Observed data events 18 Total SM 18.7± 5.40 Z+jets 9.8± 3.90 Single top 2.9± 3.10 t¯ t+W/Z 2.61± 0.64 W+jets 2.0± 0.96 t¯ t 1.31± 0.71 Multijets 0.12± 0.07

→ No excess over SM expectation found → Use data-driven

methods to estimate SM backgrounds

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 11/13

SLIDE 23

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Set exclusion limits

ATLAS-CONF-2017-020

SRD-high

Events 1 10

2

10

3

10 Data SM Total Z t t +V t t W Single Top Diboson Multijets ATLAS Preliminary

• 1

=13 TeV, 36.1 fb s

S R A

• T

T S R A

• T

W S R A

• T

S R B

• T

T S R B

• T

W S R B

• T

S R C 1 S R C 2 S R C 3 S R C 4 S R C 5 S R D

• l
• w

S R D

• h

i g h S R E

Data / SM 0.0 0.5 1.0 1.5 2.0

→ No excess over SM expectation

found

[GeV]

1

t ~

m 200 400 600 800 1000 1200 [GeV]

1

χ ∼

m 100 200 300 400 500 600 700 800 900 SRA+SRB+SRC

1

χ ∼

+ m

t

< m

1

t ~

m

1

χ ∼

+ m

b

+ m

W

< m

1

t ~

m

=8 TeV s ,

• 1

ATLAS 20 fb 1

χ ∼

(*)

t →

1

t ~ Top squark pair production,

• 1

=13 TeV, 36.1 fb s

ATLAS Preliminary

)

theory SUSY

σ 1 ± Observed limit ( )

exp

σ 1 ± Expected limit ( All limits at 95% CL

→ Exclude top squark masses up to

950 GeV (for m˜

χ0

1 < 300 GeV)

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 12/13

SLIDE 24

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Conclusions

Presented search for top squarks

0ℓ channel allows top reconstruction

Results published with 36.1fb−1of LHC Run 2 data 0 Lepton: ATLAS-CONF-2017-020 1 Lepton: ATLAS-CONF-2017-037 2 Lepton: ATLAS-CONF-2017-034

→ Top squark masses up to 950 GeV

(for m˜

χ0

1 < 300 GeV) excluded [GeV]

1

t ~

m

200 300 400 500 600 700 800 900 1000

[GeV]

1

χ ∼

m

100 200 300 400 500 600 700

1

χ ∼ W b →

1

t ~ /

1

χ ∼ t →

1

t ~

1

χ ∼ b f f' →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ t →

1

t ~

1

χ ∼ b f f' →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ t →

1

t ~

1

χ ∼ c →

1

t ~

• 1

=8 TeV, 20 fb s

t

) < m

1

χ ∼ ,

1

t ~ m ( ∆

W

+ m

b

) < m

1

χ ∼ ,

1

t ~ m( ∆ ) < 0

1

χ ∼ ,

1

t ~ m( ∆ 1

χ ∼ t →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ c →

1

t ~ /

1

χ ∼ b f f' →

1

t ~ production,

1

t ~

1

t ~

Status: May 2017

ATLAS Preliminary

1

χ ∼ W b

1

χ ∼ c

1

χ ∼ b f f'

Observed limits Expected limits All limits at 95% CL =13 TeV s [CONF-2017-020]

• 1

0L 36.1 fb [CONF-2017-037]

• 1

1L 36.1 fb [CONF-2017-034]

• 1

2L 36.1 fb [1604.07773]

• 1

Monojet 3.2 fb Run 1 [1506.08616]

from https://atlas.web.cern.ch/Atlas/GROUPS/

PHYSICS/CombinedSummaryPlots/SUSY/index.html

Thanks for your attention!

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 13/13

SLIDE 25

Introduction

˜

t → 0ℓ analysis Top reconstruction Results Summary

Conclusions

Presented search for top squarks

0ℓ channel allows top reconstruction

Results published with 36.1fb−1of LHC Run 2 data 0 Lepton: ATLAS-CONF-2017-020 1 Lepton: ATLAS-CONF-2017-037 2 Lepton: ATLAS-CONF-2017-034

→ Top squark masses up to 950 GeV

(for m˜

χ0

1 < 300 GeV) excluded [GeV]

1

t ~

m

200 300 400 500 600 700 800 900 1000

[GeV]

1

χ ∼

m

100 200 300 400 500 600 700

1

χ ∼ W b →

1

t ~ /

1

χ ∼ t →

1

t ~

1

χ ∼ b f f' →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ t →

1

t ~

1

χ ∼ b f f' →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ t →

1

t ~

1

χ ∼ c →

1

t ~

• 1

=8 TeV, 20 fb s

t

) < m

1

χ ∼ ,

1

t ~ m ( ∆

W

+ m

b

) < m

1

χ ∼ ,

1

t ~ m( ∆ ) < 0

1

χ ∼ ,

1

t ~ m( ∆ 1

χ ∼ t →

1

t ~ /

1

χ ∼ W b →

1

t ~ /

1

χ ∼ c →

1

t ~ /

1

χ ∼ b f f' →

1

t ~ production,

1

t ~

1

t ~

Status: May 2017

ATLAS Preliminary

1

χ ∼ W b

1

χ ∼ c

1

χ ∼ b f f'

Observed limits Expected limits All limits at 95% CL =13 TeV s [CONF-2017-020]

• 1

0L 36.1 fb [CONF-2017-037]

• 1

1L 36.1 fb [CONF-2017-034]

• 1

2L 36.1 fb [1604.07773]

• 1

Monojet 3.2 fb Run 1 [1506.08616]

from https://atlas.web.cern.ch/Atlas/GROUPS/

PHYSICS/CombinedSummaryPlots/SUSY/index.html

Thanks for your attention!

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS 13/13

SLIDE 26

BACKUP

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS

SLIDE 27

Backup

Signal Region TT TW T0 m0 jet,R=1.2

> 120 GeV

m1 jet,R=1.2

> 120 GeV [60, 120] GeV < 60 GeV

m

b,min

T

> 200 GeV

Nb−jet

≥ 2 τ-veto

yes

• ∆φ

(jet0,1,2, pmiss

T

)

• > 0.4

A m0 jet,R=0.8

> 60 GeV ∆R (b, b) > 1

• mχ2

T2

> 400 GeV > 400 GeV > 500 GeV

Emiss

T

> 400 GeV > 500 GeV > 550 GeV

B m

b,max

T

> 200 GeV ∆R (b, b) > 1.2

ATLAS-CONF-2017-020

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS B/U 1

SLIDE 28

Backup

Variable SRD-low SRD-high

• ∆φ

(jet0,1,2, pmiss

T

)

• > 0.4

Nb−jet

≥2 ∆R (b, b) > 0.8

p0,b T +p1,b T

> 300 GeV > 400 GeV τ-veto

yes p1 T

> 150 GeV

p3 T

> 100 GeV > 80 GeV

p4 T

> 60 GeV

m

b,min

T

> 250 GeV > 350 GeV

m

b,max

T

> 300 GeV > 450 GeV

Variable SRE

• ∆φ

(jet0,1,2, pmiss

T

)

• > 0.4

Nb−jet

≥2

m0 jet,R=0.8

> 120 GeV

m1 jet,R=0.8

> 80 GeV

mb,min T

> 200 GeV

Emiss

T

> 550 GeV

HT

> 800 GeV

Emiss

T

/√

HT

> 18 √ GeV

ATLAS-CONF-2017-020

07/20/2017 Nicolas Köhler - Search for top squarks with ATLAS B/U 2