Search for top squark pair production in final states with one isolated lepton, jets, and missing transverse momentum in $\sqrt s =$8 TeV $pp$ collisions with the ATLAS detector

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)

CERN-PH-EP-2014-143

Submitted to: JHEP

Search for top squark pair production in final states with one

isolated lepton, jets, and missing transverse momentum in

√

s

= 8 TeV pp

collisions with the ATLAS detector

The ATLAS Collaboration

Abstract

The results of a search for top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The analysis is performed with proton–proton collision data at√s = 8 TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of20 fb−1. The lightest supersymmetric particle (LSP) is taken to be the lightest neutralino which only interacts weakly and is assumed to be stable. The stop decay modes consid-ered are those to a top quark and the LSP as well as to a bottom quark and the lightest chargino, where the chargino decays to the LSP by emitting aW boson. A wide range of scenarios with differ-ent mass splittings between the stop, the lightest neutralino and the lightest chargino are considered, including cases where the W bosons or the top quarks are off-shell. Decay modes involving the heavier charginos and neutralinos are addressed using a set of phenomenological models of super-symmetry. No significant excess over the Standard Model prediction is observed. A stop with a mass between 210 and 640 GeV decaying directly to a top quark and a massless LSP is excluded at 95% confidence level, and in models where the mass of the lightest chargino is twice that of the LSP, stops are excluded at95% confidence level up to a mass of 500 GeV for an LSP mass in the range of 100 to 150 GeV. Stringent exclusion limits are also derived for all other stop decay modes considered, and model-independent upper limits are set on the visible cross-section for processes beyond the

Prepared for submission to JHEP

Search for top squark pair production in final states

with one isolated lepton, jets, and missing transverse

momentum in

√

s

= 8 TeV pp collisions with the

ATLAS detector

The ATLAS Collaboration

Abstract: The results of a search for top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The analysis is performed with proton–proton collision data at √s = 8 TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20 fb−1. The lightest supersymmetric particle (LSP) is taken to be the lightest neutralino which only interacts weakly and is assumed to be stable. The stop decay modes considered are those to a top quark and the LSP as well as to a bottom quark and the lightest chargino, where the chargino decays to the LSP by emitting a W boson. A wide range of scenarios with different mass splittings between the stop, the lightest neutralino and the lightest chargino are considered, including cases where the W bosons or the top quarks are off-shell. Decay modes involving the heavier charginos and neutralinos are addressed using a set of phenomenological models of supersymmetry. No significant excess over the Standard Model prediction is observed. A stop with a mass between 210 and 640 GeV decaying directly to a top quark and a massless LSP is excluded at 95% confidence level, and in models where the mass of the lightest chargino is twice that of the LSP, stops are excluded at 95% confidence level up to a mass of 500 GeV for an LSP mass in the range of 100 to 150 GeV. Stringent exclusion limits are also derived for all other stop decay modes considered, and model-independent upper limits are set on the visible cross-section for processes beyond the Standard Model.

Contents

1 Introduction 2

2 Analysis strategy 4

3 The ATLAS detector 5

4 Trigger and data collection 6

5 Simulated samples 7

5.1 Background samples 8

5.2 Signal samples 8

6 Physics object reconstruction and discriminating variables 10

6.1 Physics object reconstruction 10

6.2 Tools to discriminate signal from background 14

7 Signal selections 16

7.1 Event preselection 17

7.2 Selections for the ˜t₁ → t ˜χ0_{1 decay 18}

7.3 Selections for the ˜t₁ → b ˜χ±_{1 decay 21}

7.4 Selections for the mixed, three- and four-body decays 27

8 Background estimates 28

8.1 Control regions 29

8.2 Validation 32

9 Systematic uncertainties 36

10 Results 39

11 Summary and conclusions 45

12 Acknowledgements 47

A Detailed description of the discriminating variables 62

1 Introduction

The hierarchy problem [1–4] has gained additional attention with the observation of a new particle consistent with the Standard Model (SM) Higgs boson [5,6] at the LHC [7]. Super-symmetry (SUSY) [8–16], which extends the SM by introducing supersymmetric partners for all SM particles, provides an elegant solution to the hierarchy problem. The partner particles have identical quantum numbers except for a half-unit difference in spin. The superpartners of the left- and right-handed top quarks, ˜t_L and ˜t_R, mix to form the two mass eigenstates ˜t₁ and ˜t₂, where ˜t₁ (top squark or stop) is the lighter one. If the super-symmetric partners of the top quarks have masses . 1 TeV, loop diagrams involving top quarks, which are the dominant contribution to the divergence of the Higgs boson mass, can be largely cancelled [17–24]. Significant mass splitting between ˜t₁ and ˜t₂ is possible due to the large top Yukawa coupling1_{. Furthermore, effects of the renormalisation group}

equations are strong for the third generation squarks, usually driving their masses signifi-cantly lower than those of the other generations. These considerations suggest a light stop which, together with the stringent LHC limits excluding other coloured supersymmetric particles up to masses at the TeV level, motivates dedicated stop searches.

SUSY models can violate the conservation of baryon number and lepton number, re-sulting in a proton lifetime shorter than current experimental limits [25]. This is commonly solved by introducing a multiplicative quantum number called R-parity, which is 1 and−1 for all SM and SUSY particles, respectively. A generic R-parity-conserving minimal su-persymmetric extension of the SM (MSSM) [17, 26–29] predicts pair production of SUSY particles and the existence of a stable lightest supersymmetric particle (LSP). In a large variety of SUSY models, the lightest neutralino2 _{( ˜χ}0

1) is the LSP, which is also the

assump-tion throughout this paper. Since the ˜χ01 interacts only weakly it is a candidate for dark

matter.

The stop can decay into a variety of final states, depending amongst other things on the SUSY particle mass spectrum, in particular on the masses of the stop and the lightest neutralino. Figure1illustrates the simplest decay modes as a function of the stop and LSP masses. In the rightmost wedge, the stop mass is greater than the sum of the top quark and the LSP masses, hence the decay ˜t₁ → t ˜χ0_{1 is kinematically allowed. A lighter stop} can undergo a three-body decay ˜t_{1→ bW ˜}χ01 if the stop mass is still above the b + W + ˜χ01

mass. For an even lighter stop, the decay proceeds via a four-body process ˜t_{1 → bff}0χ˜0₁, where f and f0 are two distinct fermions, or flavour-changing neutral current (FCNC) processes, such as the loop-suppressed ˜t_{1 → c ˜}χ01. If supersymmetric particles other than

the ˜χ01 are lighter than the stop, then additional decay modes can open up. The stop

decay to a bottom quark and the lightest chargino (˜t₁ → b ˜χ±_{1) is an important example,}

1_{The masses of the ˜t}

1 and ˜t2 are given by the eigenvalues of the stop mass matrix. The stop mass

matrix involves the top-quark Yukawa coupling in the off-diagonal elements, which typically induces a large mass splitting. The stop mass matrix is diagonalised by the stop mixing matrix, which gives the ˜tLand ˜tR

components of the mass eigenstates ˜t1and ˜t2. 2_{The charginos ˜χ}±

1,2and neutralinos ˜χ01,2,3,4are the mass eigenstates formed from the linear superposition

of the charged and neutral SUSY partners of the Higgs and electroweak gauge bosons (higgsinos, winos and binos).

[GeV] [GeV] 0 100 200 300 100 m(t) Δm > m(t) m(W)+m(b) Δm > m(W)+m(b)

1 Introduction

m = m(˜t₁) m( ˜0₁) m( ˜01) m(˜t₁) ˜t₁! t + ˜01 ˜t_{1! b + ˜}± 1 ˜t₁! b + W + ˜01 ˜t₁! b + W⇤_{+ ˜}0 1 ˜t₁! c + ˜01 1

1 Introduction

m = m(˜t₁) m( ˜01) m( ˜01) m(˜t₁) ˜t₁! t + ˜01 ˜t₁! b + ˜±1 ˜t₁! b + W + ˜01 ˜t₁! b + W⇤_{+ ˜}0₁ ˜t₁! c + ˜01 1

1 Introduction

m = m(˜t₁) m( ˜01) m( ˜01) m(˜t₁) ˜t₁! t + ˜01 ˜t₁! b + ˜±1 ˜t_{1! b + W + ˜}01 ˜t₁! b + W⇤_{+ ˜}0₁ ˜t₁! c + ˜01

1 Introduction

m(˜t₁) < m( ˜01) m = m(˜t₁) m( ˜01) m( ˜01) m(˜t₁) ˜t₁ ! t + ˜01 ˜t_{1 ! b + ˜}± 1 ˜t₁ ! b + W + ˜01 ˜t_{1 ! b + W}⇤_{+ ˜}0₁ ˜t₁ ! c + ˜01 0

F

igu

re

1.

Ill

us

tr

at

ion

of

top

sq

uar

k

de

ca

y

m

od

es

in

th

e

m

as

s

pl

an

e

sp

an

ne

d

by

th

e

top

sq

uar

k

(˜t

1

)

an

d

ligh

te

st

ne

ut

ral

in

o

(˜

0 1

),

w

he

re

th

e

lat

te

r

is

as

su

m

ed

to

be

th

e

ligh

te

st

su

pe

rs

ym

m

et

ric

par

tic

le

.

le

ft/r

igh

t

st

op

m

ix

in

g

an

d

th

e

ne

ut

ral

in

o/c

har

gi

no

se

ct

or

.

Not

e

th

at

th

e

LE

P

lo

we

r

lim

it

1

on

th

e

ligh

te

st

ch

ar

gi

no

m

as

s

(m

(˜

±₁

)

>

103

.5

G

eV

at

95%

con

fid

en

ce

le

ve

l [

25

])

im

plies

2

th

at

th

e

˜t

1

!

b

˜

±₁

de

ca

y

is

ki

ne

m

at

ic

al

ly

op

en

on

ly

if

th

e

st

op

m

as

s

is

ab

ov

e

th

is

lim

it.

3

LHC

se

ar

ch

es

for

el

ec

tr

ow

eak

pr

od

uc

tion

of

ch

ar

gi

no

an

d

ne

ut

ral

in

o

pai

rs

(˜

±₁

˜

0 2

) c

an

pu

sh

4

th

e

m

as

s

lim

it

up

to

ab

ou

t

700

G

eV

(s

ee

for

ex

am

pl

e

R

ef

. [

26

]).

Ho

we

ve

r,

th

es

e

st

rin

ge

nt

5

lim

its

de

pe

nd

up

on

as

su

m

pt

ion

s

w

hi

ch

m

ax

im

ise

th

e

se

ar

ch

se

ns

iti

vi

ty

,

su

ch

as

a

de

ca

y

6

m

od

e

vi

a

sle

pt

on

s,

a

lo

w

-m

as

s

˜

01

, an

d

ne

ar

ly

m

as

s-

de

ge

ne

rat

e

˜

±1

˜

02

st

at

es

w

ith

th

e

fie

ld

7

con

te

nt

se

t

to

yi

el

d

a

hi

gh

pr

od

uc

tion

cr

os

s

se

ct

ion

.

8

T

hi

s

ar

tic

le

pr

es

en

ts

a

se

ar

ch

for

di

re

ct

˜t

1

pai

r

pr

od

uc

tion

in

fin

al

st

at

es

w

ith

on

e

9

isol

at

ed

le

pt

on

(e

le

ct

ron

or

m

uon

1

),

se

ve

ral

je

ts

, an

d

a

sign

ifi

can

t am

ou

nt

of

m

iss

in

g

tr

an

s-10

ve

rs

e

m

om

en

tu

m

(t

he

m

agn

itu

de

of

w

hi

ch

is

re

fe

rr

ed

to

as

E

miss T

).

T

he

pot

en

tial

ly

lar

ge

11

E

miss T

is

ge

ne

rat

ed

by

th

e

tw

o

un

de

te

ct

ed

LS

Ps

an

d

ne

ut

rin

o(

s)

.

Al

l

top

sq

uar

k

de

ca

y

12

sc

en

ar

ios

de

sc

rib

ed

ab

ov

e

ex

ce

pt

for

th

e

fla

vou

r-

ch

an

gi

ng

m

od

es

ar

e

con

sid

er

ed

.

He

av

y

13

fla

vou

r

taggi

ng

in

for

m

at

ion

is

ut

ili

se

d

in

th

e

ev

en

t

se

le

ct

ion

s,

an

d

for

con

st

ru

ct

in

g

ki

ne

-14

m

at

ic

var

iab

le

s.

T

he

se

ar

ch

for

a

he

av

y

st

op

ex

pl

oi

ts

a

de

di

cat

ed

te

ch

ni

qu

e

of

lar

ge

-r

ad

iu

s

15

(lar

ge

-R

)

je

ts

.

Fu

rt

he

rm

or

e,

lo

w

-m

om

en

tu

m

le

pt

on

s

(r

ef

er

re

d

to

as

sof

t-le

pt

on

s)

ar

e

re

-16

con

st

ru

ct

ed

an

d

id

en

tifi

ed

to

en

han

ce

th

e

se

ns

iti

vi

ty

to

se

ar

ch

for

˜t

1

!

b

˜

±₁

deca

ys

with

17

ne

ar

ly

m

as

s-

de

ge

ne

rat

e

˜

01

an

d

˜

±₁

st

at

es

.

18

Se

ar

ch

es

for

di

re

ct

˜t

1

pai

r p

ro

du

ct

ion

ha

ve

pr

ev

iou

sly

be

en

re

por

te

d

by

th

e A

T

LAS

[

27

–

19

32

] an

d

C

M

S

[

33

–

36

] c

ol

lab

or

at

ion

s,

as

we

ll

as

by

th

e

C

D

F

an

d

D

Ø

col

lab

or

at

ion

ss

as

-20

suming

di

↵e

re

nt

SUS

Y

m

as

s

sp

ec

tr

a

an

d

de

ca

y

m

od

es

(s

ee

for

ex

am

pl

e

R

ef

s.

[

37

,

38

]).

21

In

di

re

ct

se

ar

ch

es

for

st

op

s,

m

ed

iat

ed

by

gl

ui

no

pai

r p

ro

du

ct

ion

, h

av

e

be

en

re

por

te

d

by

th

e

22

AT

LAS

[

39

–

43

] an

d

C

M

S

[

44

] c

ol

lab

or

at

ion

s.

23

˜t

1

!

t˜

0 1 1

˜t

1

!

b

˜

±₁ 2 1El ectro nsan dm uons from tau deca ysare incl uded .

–

3

–

˜t

1

!

bW

˜

01 3

˜t

1

!

bf

f

0

˜

01 4

˜t

1

!

c

˜

0 1 5

2

T

he

A

T

LA

S

de

te

ct

or

and

da

ta

sa

m

pl

es

6

T

he

AT

LAS

ex

pe

rim

en

t

[

45

] i

s

a

m

ul

ti-

pu

rp

os

e

par

tic

le

ph

ys

ic

s

de

te

ct

or

w

ith

ne

ar

ly

4⇡

7

co

ve

rage

in

sol

id

an

gl

e.

It

con

sis

ts

of

in

ne

r

tr

ac

ki

ng

de

vi

ce

s

(I

D

),

el

ec

tr

om

agn

et

ic

an

d

8

had

ron

ic

cal

or

im

et

rie

s

an

d

a

m

uon

sp

ec

tr

om

et

er

(M

S)

w

ith

fou

r

su

pe

rc

on

du

ct

in

g

m

agn

et

9

sy

st

em

s,

w

hi

ch

com

pr

ise

a

th

in

sol

en

oi

d

su

rr

ou

nd

in

g

th

e

ID

, an

d

a

bar

re

l an

d

tw

o

en

d-

cap

10

tor

oi

ds

pr

ov

id

in

g

th

e

m

agn

et

ic

fie

ld

for

th

e

m

uon

sp

ec

tr

om

et

er

. T

he

ID

con

sis

ts

of

a

sil

ic

on

11

pi

xe

l d

et

ec

tor

, a

sil

ic

on

m

ic

ros

tr

ip

de

te

ct

or

an

d

a

st

ra

w

tu

be

tr

ac

ke

r (

for

a

ps

eu

dor

ap

id

ity

2 12

of

|⌘|

<

1.

9)

.

It

pr

ov

id

es

pr

ec

isi

on

tr

ac

ki

ng

an

d

m

om

en

tu

m

m

eas

ur

em

en

t

of

ch

ar

ge

d

13

par

tic

le

s f

or

|⌘|

<

2.

5

an

d

al

lo

w

s e

ffi

cien

t b

-je

t i

de

nt

ifi

cat

ion

. Hi

gh

-gr

an

ul

ar

ity

liq

ui

d-

ar

gon

14

(LAr

)

sam

pl

in

g

el

ec

tr

om

agn

et

ic

cal

or

im

et

er

s

co

ve

r

in

th

e

ps

eu

dor

ap

id

ity

ran

ge

|⌘|

<

3.

2.

15

T

he

had

ron

ic

cal

or

im

et

er

sy

st

em

is

bas

ed

on

tw

o

di

↵e

re

nt

te

ch

nol

ogi

es

,

sc

in

til

lat

in

g-til

e

16

sam

pl

in

g

cal

or

im

et

er

(|⌘

|

<

1.

7)

an

d

LAr

sam

pl

in

g

cal

or

im

et

er

(1

.5

<

|⌘|

<

4.

9)

.

T

he

17

m

uon

sp

ec

tr

om

et

er

has

se

par

at

e

tr

igge

r

an

d

hi

gh

-p

re

ci

sion

tr

ac

ki

ng

ch

am

be

rs

, t

he

lat

te

r

18

pr

ov

id

in

g

m

uon

id

en

tifi

cat

ion

an

d

m

om

en

tu

m

m

eas

ur

em

en

t

for

|⌘|

<

2.

7.

19

T

he

dat

a

sam

pl

e

us

ed

in

th

is

an

al

ys

is

was

tak

en

du

rin

g

th

e

pe

rio

d

from

M

ar

ch

to

20

D

ec

em

be

r

2012

w

ith

th

e

LHC

op

er

at

in

g

at

a

pp

ce

nt

re

-of

-m

as

s

en

er

gy

of

p s

=

8

Te

V

.

21

Af

te

r

ap

pl

ic

at

ion

of

be

am

,

de

te

ct

or

an

d

dat

a-

qu

al

ity

re

qu

ire

m

en

ts

,

th

e

tot

al

in

te

gr

at

ed

22

lu

m

in

os

ity

is

20.

3 f

b

1

w

ith

an

un

ce

rt

ai

nt

y

of

±

2.

8%

.

T

he

un

ce

rt

ai

nt

y

is

de

riv

ed

fol

lo

w

in

g

23

th

e

sam

e

m

et

ho

dol

ogy

as

th

at

de

tai

le

d

in

R

ef

. [

46

],

from

a

pr

el

im

in

ar

y

cal

ib

rat

ion

of

th

e

24

lu

m

in

os

ity

sc

al

e

de

riv

ed

from

be

am

-s

ep

ar

at

ion

sc

an

s

pe

rfor

m

ed

in

No

ve

m

be

r

2012.

25

T

he

dat

as

et

us

ed

for

th

is

an

al

ys

is

was

re

cor

de

d

us

in

g

th

ee

di

↵e

re

nt

ty

pe

s

of

tr

igge

rs

26

bas

ed

on

re

qu

iri

ng

a

sin

gl

e-el

ec

tr

on

, a

sin

gl

e-m

uon

, or

lar

ge

E

missT

. C

an

di

dat

e

ev

en

ts

in

th

e

27

el

ec

tr

on

(m

uon

) c

han

ne

l ar

e

col

le

ct

ed

us

in

g

a

logi

cal

-O

R

com

bi

nat

ion

of

th

e

sin

gl

e-el

ec

tr

on

28

(s

in

gl

e-m

uon

)

an

d

E

miss T

tr

igge

rs

.

T

he

sin

gl

e-

le

pt

on

tr

igge

rs

ha

ve

a

tr

an

sv

er

se

m

om

en

tu

m

29

(p

T

)

th

re

sh

ol

d

of

24

G

eV

. A

se

con

d

se

t

of

com

pl

em

en

tar

y

sin

gl

e-le

pt

on

tr

igge

rs

w

ith

p

T 30

th

re

sh

ol

ds

at

60

G

eV

(36

G

eV

)

for

th

e

el

ec

tr

on

(m

uon

)

cas

e

re

co

ve

rs

sm

al

l

in

effi

ciencies

31

as

so

ci

at

ed

w

ith

th

e

lo

we

r

th

re

sh

ol

d

sin

gl

e-

le

pt

on

tr

igge

rs

du

e

to

le

pt

on

isol

at

ion

cr

ite

ria.

32

The

E

missT

tr

igge

r

has

a

th

re

sh

ol

d

of

80

G

eV

an

d

re

ac

he

s

fu

ll

effi

ci

en

cy

for

ev

en

ts

w

ith

33

o✏

ine

E

missT

ab

ov

e

150

G

eV

. D

ur

in

g

th

e

be

gi

nn

in

g

of

2012

dat

a

tak

in

g,

th

e

E

missT

tr

igge

r

34

was

par

tly

ke

pt

di

sab

le

d,

cau

sin

g

a

los

s

of

ab

ou

t

2 p

b

1

in

in

te

gr

at

ed

lu

m

in

os

ity

.

T

he

35

com

bi

ne

d

tr

igge

r

effi

ciency

is

>

98%

for

th

e

le

pt

on

an

d

E

missT

se

le

ct

ion

cr

ite

ria

ap

pl

ie

d

1

in

th

e

an

al

ys

is.

For

th

e

sof

t-le

pt

on

se

le

ct

ion

s,

th

e

sin

gl

e-

le

pt

on

tr

igge

r

th

re

sh

ol

ds

ar

e

to

o

2

lar

ge

re

nd

er

in

g

th

es

e

tr

igge

rs

us

el

es

s.

Al

l c

an

di

dat

e

ev

en

ts

ar

e

th

er

ef

or

e

re

cor

de

d

us

in

g

3 2 AT LA Sus esa right -han ded coord inat esy stem with its origi nat the nom inal inte ract ion poin tin the ntre of th ede tect oran dth e z-axis along the beam pipe . C ylin drica l coo rdin ates (r, )are used inth e vers epl ane, bein gth eaz imut hal a ngle aro und the beam pipe . T he pseu dora pidi ty ⌘ isdefi ned in ofth epo lar angl e✓ by ⌘= lntan (✓/2), and R= p ( ⌘) 2 + ( )2

–

4

–

˜t

1

!

bW

˜

01 3

˜t

1

!

bf

f

0

˜

01 4

˜t

1

!

c

˜

0 1 5

2

T

he

A

T

LA

S

de

te

ct

or

and

da

ta

sa

m

pl

es

6

T

he

AT

LAS

ex

pe

rim

en

t

[

45

] i

s

a

m

ul

ti-

pu

rp

os

e

par

tic

le

ph

ys

ic

s

de

te

ct

or

w

ith

ne

ar

ly

4⇡

7

co

ve

rage

in

sol

id

an

gl

e.

It

con

sis

ts

of

in

ne

r

tr

ac

ki

ng

de

vi

ce

s

(I

D

),

el

ec

tr

om

agn

et

ic

an

d

8

had

ron

ic

cal

or

im

et

rie

s

an

d

a

m

uon

sp

ec

tr

om

et

er

(M

S)

w

ith

fou

r

su

pe

rc

on

du

ct

in

g

m

agn

et

9

sy

st

em

s,

w

hi

ch

com

pr

ise

a

th

in

sol

en

oi

d

su

rr

ou

nd

in

g

th

e

ID

, an

d

a

bar

re

l an

d

tw

o

en

d-

cap

10

tor

oi

ds

pr

ov

id

in

g

th

e

m

agn

et

ic

fie

ld

for

th

e

m

uon

sp

ec

tr

om

et

er

. T

he

ID

con

sis

ts

of

a

sil

ic

on

11

pi

xe

l d

et

ec

tor

, a

sil

ic

on

m

ic

ros

tr

ip

de

te

ct

or

an

d

a

st

ra

w

tu

be

tr

ac

ke

r (

for

a

ps

eu

dor

ap

id

ity

2 12

of

|⌘|

<

1.

9)

.

It

pr

ov

id

es

pr

ec

isi

on

tr

ac

ki

ng

an

d

m

om

en

tu

m

m

eas

ur

em

en

t

of

ch

ar

ge

d

13

par

tic

le

s f

or

|⌘|

<

2.

5

an

d

al

lo

w

s e

ffi

cien

t b

-je

t i

de

nt

ifi

cat

ion

. Hi

gh

-gr

an

ul

ar

ity

liq

ui

d-

ar

gon

14

(LAr

)

sam

pl

in

g

el

ec

tr

om

agn

et

ic

cal

or

im

et

er

s

co

ve

r

in

th

e

ps

eu

dor

ap

id

ity

ran

ge

|⌘|

<

3.

2.

15

T

he

had

ron

ic

cal

or

im

et

er

sy

st

em

is

bas

ed

on

tw

o

di

↵e

re

nt

te

ch

nol

ogi

es

,

sc

in

til

lat

in

g-til

e

16

sam

pl

in

g

cal

or

im

et

er

(|⌘

|

<

1.

7)

an

d

LAr

sam

pl

in

g

cal

or

im

et

er

(1

.5

<

|⌘|

<

4.

9)

.

T

he

17

m

uon

sp

ec

tr

om

et

er

has

se

par

at

e

tr

igge

r

an

d

hi

gh

-p

re

ci

sion

tr

ac

ki

ng

ch

am

be

rs

, t

he

lat

te

r

18

pr

ov

id

in

g

m

uon

id

en

tifi

cat

ion

an

d

m

om

en

tu

m

m

eas

ur

em

en

t

for

|⌘|

<

2.

7.

19

T

he

dat

a

sam

pl

e

us

ed

in

th

is

an

al

ys

is

was

tak

en

du

rin

g

th

e

pe

rio

d

from

M

ar

ch

to

20

D

ec

em

be

r

2012

w

ith

th

e

LHC

op

er

at

in

g

at

a

pp

ce

nt

re

-of

-m

as

s

en

er

gy

of

p s

=

8

Te

V

.

21

Af

te

r

ap

pl

ic

at

ion

of

be

am

,

de

te

ct

or

an

d

dat

a-

qu

al

ity

re

qu

ire

m

en

ts

,

th

e

tot

al

in

te

gr

at

ed

22

lu

m

in

os

ity

is

20.

3 f

b

1

w

ith

an

un

ce

rt

ai

nt

y

of

±

2.

8%

.

T

he

un

ce

rt

ai

nt

y

is

de

riv

ed

fol

lo

w

in

g

23

th

e

sam

e

m

et

ho

dol

ogy

as

th

at

de

tai

le

d

in

R

ef

. [

46

],

from

a

pr

el

im

in

ar

y

cal

ib

rat

ion

of

th

e

24

lu

m

in

os

ity

sc

al

e

de

riv

ed

from

be

am

-s

ep

ar

at

ion

sc

an

s

pe

rfor

m

ed

in

No

ve

m

be

r

2012.

25

T

he

dat

as

et

us

ed

for

th

is

an

al

ys

is

was

re

cor

de

d

us

in

g

th

ee

di

↵e

re

nt

ty

pe

s

of

tr

igge

rs

26

bas

ed

on

re

qu

iri

ng

a

sin

gl

e-el

ec

tr

on

, a

sin

gl

e-m

uon

, or

lar

ge

E

missT

. C

an

di

dat

e

ev

en

ts

in

th

e

27

el

ec

tr

on

(m

uon

) c

han

ne

l ar

e

col

le

ct

ed

us

in

g

a

logi

cal

-O

R

com

bi

nat

ion

of

th

e

sin

gl

e-el

ec

tr

on

28

(s

in

gl

e-m

uon

)

an

d

E

miss T

tr

igge

rs

.

T

he

sin

gl

e-

le

pt

on

tr

igge

rs

ha

ve

a

tr

an

sv

er

se

m

om

en

tu

m

29

(p

T

)

th

re

sh

ol

d

of

24

G

eV

. A

se

con

d

se

t

of

com

pl

em

en

tar

y

sin

gl

e-le

pt

on

tr

igge

rs

w

ith

p

T 30

th

re

sh

ol

ds

at

60

G

eV

(36

G

eV

)

for

th

e

el

ec

tr

on

(m

uon

)

cas

e

re

co

ve

rs

sm

al

l

in

effi

ciencies

31

as

so

ci

at

ed

w

ith

th

e

lo

we

r

th

re

sh

ol

d

sin

gl

e-

le

pt

on

tr

igge

rs

du

e

to

le

pt

on

isol

at

ion

cr

ite

ria.

32

The

E

missT

tr

igge

r

has

a

th

re

sh

ol

d

of

80

G

eV

an

d

re

ac

he

s

fu

ll

effi

ci

en

cy

for

ev

en

ts

w

ith

33

o✏

ine

E

missT

ab

ov

e

150

G

eV

. D

ur

in

g

th

e

be

gi

nn

in

g

of

2012

dat

a

tak

in

g,

th

e

E

missT

tr

igge

r

34

was

par

tly

ke

pt

di

sab

le

d,

cau

sin

g

a

los

s

of

ab

ou

t

2 p

b

1

in

in

te

gr

at

ed

lu

m

in

os

ity

.

T

he

35

com

bi

ne

d

tr

igge

r

effi

ciency

is

>

98%

for

th

e

le

pt

on

an

d

E

missT

se

le

ct

ion

cr

ite

ria

ap

pl

ie

d

1

in

th

e

an

al

ys

is.

For

th

e

sof

t-le

pt

on

se

le

ct

ion

s,

th

e

sin

gl

e-le

pt

on

tr

igge

r

th

re

sh

ol

ds

ar

e

to

o

2

lar

ge

re

nd

er

in

g

th

es

e

tr

igge

rs

us

el

es

s.

Al

l c

an

di

dat

e

ev

en

ts

ar

e

th

er

ef

or

e

re

cor

de

d

us

in

g

2 AT LAS uses arig ht-h ande dco ord inat esy stem with its origi nat the nom inal inte ract ion poin tin the ofth ede tect or an dth e z-axis along the beam pipe . C ylin drica l coo rdin ates (r, )are used inth e epl ane, bein gth eaz imut hal a ngle aro und the beam pipe . T heps eudo rapi dity ⌘is defi ned in pola ran gle ✓by ⌘= lntan (✓/2), and R= p ( ⌘) 2 + ( )2

–

4

–

˜t

1

!

bW

˜

01 3

˜t

1

!

bf

f

0

˜

01 4

˜t

1

!

c

˜

0 1 5

2

T

he

A

T

LA

S

de

te

ct

or

and

da

ta

sa

m

pl

es

6

T

he

AT

LAS

ex

pe

rim

en

t

[

45

] i

s

a

m

ul

ti-

pu

rp

os

e

par

tic

le

ph

ys

ic

s

de

te

ct

or

w

ith

ne

ar

ly

4⇡

7

co

ve

rage

in

sol

id

an

gl

e.

It

con

sis

ts

of

in

ne

r

tr

ac

ki

ng

de

vi

ce

s

(I

D

),

el

ec

tr

om

agn

et

ic

an

d

8

had

ron

ic

cal

or

im

et

rie

s

an

d

a

m

uon

sp

ec

tr

om

et

er

(M

S)

w

ith

fou

r

su

pe

rc

on

du

ct

in

g

m

agn

et

9

sy

st

em

s,

w

hi

ch

com

pr

ise

a

th

in

sol

en

oi

d

su

rr

ou

nd

in

g

th

e

ID

, an

d

a

bar

re

l an

d

tw

o

en

d-

cap

10

tor

oi

ds

pr

ov

id

in

g

th

e

m

agn

et

ic

fie

ld

for

th

e

m

uon

sp

ec

tr

om

et

er

. T

he

ID

con

sis

ts

of

a

sil

ic

on

11

pi

xe

l d

et

ec

tor

, a

sil

ic

on

m

ic

ros

tr

ip

de

te

ct

or

an

d

a

st

ra

w

tu

be

tr

ac

ke

r (

for

a

ps

eu

dor

ap

id

ity

2 12

of

|⌘|

<

1.

9)

.

It

pr

ov

id

es

pr

ec

isi

on

tr

ac

ki

ng

an

d

m

om

en

tu

m

m

eas

ur

em

en

t

of

ch

ar

ge

d

13

par

tic

le

s f

or

|⌘|

<

2.

5

an

d

al

lo

w

s e

ffi

cien

t b

-je

t i

de

nt

ifi

cat

ion

. Hi

gh

-gr

an

ul

ar

ity

liq

ui

d-

ar

gon

14

(LAr

)

sam

pl

in

g

el

ec

tr

om

agn

et

ic

cal

or

im

et

er

s

co

ve

r

in

th

e

ps

eu

dor

ap

id

ity

ran

ge

|⌘|

<

3.

2.

15

T

he

had

ron

ic

cal

or

im

et

er

sy

st

em

is

bas

ed

on

tw

o

di

↵e

re

nt

te

ch

nol

ogi

es

,

sc

in

til

lat

in

g-til

e

16

sam

pl

in

g

cal

or

im

et

er

(|⌘

|

<

1.

7)

an

d

LAr

sam

pl

in

g

cal

or

im

et

er

(1

.5

<

|⌘|

<

4.

9)

.

T

he

17

m

uon

sp

ec

tr

om

et

er

has

se

par

at

e

tr

igge

r

an

d

hi

gh

-p

re

ci

sion

tr

ac

ki

ng

ch

am

be

rs

, t

he

lat

te

r

18

pr

ov

id

in

g

m

uon

id

en

tifi

cat

ion

an

d

m

om

en

tu

m

m

eas

ur

em

en

t

for

|⌘|

<

2.

7.

19

T

he

dat

a

sam

pl

e

us

ed

in

th

is

an

al

ys

is

was

tak

en

du

rin

g

th

e

pe

rio

d

from

M

ar

ch

to

20

D

ec

em

be

r

2012

w

ith

th

e

LHC

op

er

at

in

g

at

a

pp

ce

nt

re

-of

-m

as

s

en

er

gy

of

p s

=

8

Te

V

.

21

Af

te

r

ap

pl

ic

at

ion

of

be

am

,

de

te

ct

or

an

d

dat

a-qu

al

ity

re

qu

ire

m

en

ts

,

th

e

tot

al

in

te

gr

at

ed

22

lu

m

in

os

ity

is

20.

3 f

b

1

w

ith

an

un

ce

rt

ai

nt

y

of

±

2.

8%

.

T

he

un

ce

rt

ai

nt

y

is

de

riv

ed

fol

lo

w

in

g

23

th

e

sam

e

m

et

ho

dol

ogy

as

th

at

de

tai

le

d

in

R

ef

. [

46

],

from

a

pr

el

im

in

ar

y

cal

ib

rat

ion

of

th

e

24

lu

m

in

os

ity

sc

al

e

de

riv

ed

from

be

am

-s

ep

ar

at

ion

sc

an

s

pe

rfor

m

ed

in

No

ve

m

be

r

2012.

25

T

he

dat

as

et

us

ed

for

th

is

an

al

ys

is

was

re

cor

de

d

us

in

g

th

ee

di

↵e

re

nt

ty

pe

s

of

tr

igge

rs

26

bas

ed

on

re

qu

iri

ng

a

sin

gl

e-el

ec

tr

on

, a

sin

gl

e-m

uon

, or

lar

ge

E

missT

. C

an

di

dat

e

ev

en

ts

in

th

e

27

el

ec

tr

on

(m

uon

) c

han

ne

l ar

e

col

le

ct

ed

us

in

g

a

logi

cal

-O

R

com

bi

nat

ion

of

th

e

sin

gl

e-el

ec

tr

on

28

(s

in

gl

e-m

uon

)

an

d

E

miss T

tr

igge

rs

.

T

he

sin

gl

e-

le

pt

on

tr

igge

rs

ha

ve

a

tr

an

sv

er

se

m

om

en

tu

m

29

(p

T

)

th

re

sh

ol

d

of

24

G

eV

. A

se

con

d

se

t

of

com

pl

em

en

tar

y

sin

gl

e-le

pt

on

tr

igge

rs

w

ith

p

T 30

th

re

sh

ol

ds

at

60

G

eV

(36

G

eV

)

for

th

e

el

ec

tr

on

(m

uon

)

cas

e

re

co

ve

rs

sm

al

l

in

effi

ciencies

31

as

so

ci

at

ed

w

ith

th

e

lo

we

r

th

re

sh

ol

d

sin

gl

e-

le

pt

on

tr

igge

rs

du

e

to

le

pt

on

isol

at

ion

cr

ite

ria.

32

The

E

missT

tr

igge

r

has

a

th

re

sh

ol

d

of

80

G

eV

an

d

re

ac

he

s

fu

ll

effi

ci

en

cy

for

ev

en

ts

w

ith

33

o✏

ine

E

missT

ab

ov

e

150

G

eV

. D

ur

in

g

th

e

be

gi

nn

in

g

of

2012

dat

a

tak

in

g,

th

e

E

missT

tr

igge

r

34

was

par

tly

ke

pt

di

sab

le

d,

cau

sin

g

a

los

s

of

ab

ou

t

2 p

b

1

in

in

te

gr

at

ed

lu

m

in

os

ity

.

T

he

35

com

bi

ne

d

tr

igge

r

effi

ciency

is

>

98%

for

th

e

le

pt

on

an

d

E

missT

se

le

ct

ion

cr

ite

ria

ap

pl

ie

d

1

in

th

e

an

al

ys

is.

For

th

e

sof

t-le

pt

on

se

le

ct

ion

s,

th

e

sin

gl

e-

le

pt

on

tr

igge

r

th

re

sh

ol

ds

ar

e

to

o

2

lar

ge

re

nd

er

in

g

th

es

e

tr

igge

rs

us

el

es

s.

Al

l c

an

di

dat

e

ev

en

ts

ar

e

th

er

ef

or

e

re

cor

de

d

us

in

g

3 2 AT LAS uses arig ht-h ande dco ord inat esy stem with its origi nat the nom inal inte ract ion poin tin the cent reof the dete ctor and the z-ax isal ong the beam pipe . C ylin drica l coo rdin ates (r, )are used inth e trans vers epl ane, bein gth eaz imut hal a ngle aroun dth ebe am pipe . T he pseu dora pidi ty⌘ is defi ned in rms ofth epo lar angl e✓ by ⌘= lntan (✓/2), and R= p ( ⌘) 2 + ( )2

–

4

–

Figure 1. Illustration of stop decay modes in the plane spanned by the masses of the stop (˜t1)

and the lightest neutralino ( ˜χ01), where the latter is assumed to be the lightest supersymmetric

particle. Stop decays to supersymmetric particles other than the lightest supersymmetric particle are not displayed.

where the ˜χ±1 can decay to the lightest neutralino by emitting an on- or off-shell W boson

( ˜χ±_{1 → W}(∗)_χ˜0

1). The ˜t₁ → b ˜χ±1 decay is considered for a stop mass above around 100 GeV

since the LEP limit on the lightest chargino is m_χ˜±

1 > 103.5 GeV [30].

This article presents a search for direct ˜t₁ pair production in final states with exactly one isolated charged lepton (electron or muon,3 henceforth referred to simply as ‘leptons’), several jets, and a significant amount of missing transverse momentum, the magnitude of which is referred to as E_Tmiss. The lepton arises from the decay of either a real or a virtual W boson, and the potentially large E_Tmiss is generated by the two undetected LSPs and neutrino(s). All stop decay modes described above except for the FCNC modes are considered, as illustrated in figure 2. With several decay modes kinematically available, the ˜t₁ decay branching ratio is determined by factors including the stop mixing matrix and the field content of the neutralino/chargino sector. Results are mainly based on simplified models that have 100% branching ratio to one or a pair of these specific decay chains. In addition, phenomenological MSSM (pMSSM) [31] models are used to study the sensitivity to realistic scenarios where more complex decay chains are present alongside the simpler ones.

Searches for direct ˜t₁pair production have previously been reported by the ATLAS [32– 38] and CMS [39–43] collaborations, as well as by the CDF and DØ collaborations (for ex-ample refs. [44,45]) and the LEP collaborations [46]. Indirect searches for stops, mediated by gluino pair production, have been reported by the ATLAS [47–50] and CMS [39,40,51– 55] collaborations.

˜

t

1

˜

t

1

t

W

t

W

p

p

˜

χ

0₁

b

`

ν

˜

χ

0₁

b

q

q

(a)

˜

t

1

˜

t

1

W

W

p

p

˜

χ

0₁

b

`

ν

˜

χ

0₁

b

q

q

(b)

˜

t

1

˜

t

1

p

p

˜

χ

0₁

b

`

ν

˜

χ

0₁

b

q

q

(c)

˜

t

1

˜

t

1

˜

χ

±₁

W

(∗)

˜

χ

∓₁

W

(∗)

p

p

b

˜

χ

0₁

`

ν

b

˜

χ

0₁

q

q

(d)

Figure 2. Diagrams illustrating the considered signal scenarios, which are referred to as (a) ˜t1→

t ˜χ01, (b) ˜t₁→ bW ˜χ01(three-body), (c) ˜t₁→ bff0χ˜01(four-body), (d) ˜t₁→ b ˜χ±1. Furthermore, a

non-symmetric decay mode where each ˜t1can decay via either ˜t1→ t ˜χ

0

1 or ˜t1→ b ˜χ

±

1 is considered (not

shown). In these diagrams, the charge-conjugate symbols are omitted for simplicity; all scenarios begin with a top squark–antisquark pair. The three-body and four-body decays are assumed to proceed through an off-shell top quark, and an off-shell top quark followed by an off-shell W boson, respectively.

2 Analysis strategy

Searching for ˜t₁ pair production in the various decay modes and over a wide range of stop masses requires different analysis approaches. The ˜t₁ pair production cross-section falls rapidly with increasing stop mass m˜t₁: for the range targeted by this search, m˜t₁ ∼ 100–

700 GeV, the cross-section at √s = 8 TeV proton–proton (pp) collisions decreases from 560 pb to 8 fb. While the various ˜t₁ decay modes considered all have identical final state objects — one electron or muon accompanied by one neutrino (or more for a leptonic τ decay), two jets originating from bottom quarks (b-jets), two light-flavour jets, and two LSPs – their kinematic properties change significantly for the different decay modes and as a function of the masses of the stop, LSP, and lightest chargino (if present). The search presented in this paper is based on 15 dedicated analyses that target the various

scenarios. The identification of b-jets (b-tagging) is utilised in the event selections and for constructing kinematic variables. The search for a heavy stop exploits a specialised technique, which reconstructs several decay products in a single large-radius (large-R) jet. Low-momentum leptons (referred to as soft leptons) are reconstructed and identified to enhance the sensitivity for ˜t₁ → b ˜χ±_{1 decays where the ˜}χ0_{1 and ˜}χ±_{1 states are close in mass.} These and other tools and variables to discriminate signal from background, described in section 6, are used to design sets of requirements for the event selection. Each of these sets of requirements is referred to as a signal region (SR), and is optimised to target one or more signal scenarios. Furthermore, two different analysis techniques are employed, which are referred to as ‘cut-and-count’ and ‘shape-fit’. The former is based on counting events in a single region of phase space (bin), while the latter employs several bins. By utilising different signal-to-background ratios in the various bins, shape-fits enhance the search sensitivity in challenging scenarios, where it is particularly difficult to separate signal from background. All SRs are described in section 7.

The dominant background in most SRs arises from top quark pair production (t¯t) where both W bosons decay leptonically (dileptonic t¯t) but one of the leptons is not identified, is outside the detector acceptance, or is a hadronically decaying τ lepton. The sub-leading background for most SRs stems from W +jets production. As part of each analysis, the t¯t and W +jets backgrounds are estimated using dedicated control regions (CRs), making the analysis more robust against potential mis-modelling effects in simulated events and reducing the uncertainties on the background estimates. Other small backgrounds are estimated using simulation only. Dedicated samples are used to validate the background predictions. The background estimation including the definition of all CRs is detailed in section 8.

The analysis results are based on maximum likelihood fits, which include the CRs to simultaneously normalise the t¯t and W +jets backgrounds. Systematic uncertainties due to theoretical and experimental effects are considered for all background and signal processes, and are described in section 9. The final results and interpretations, both in terms of model-dependent exclusion limits on the masses of relevant SUSY particles and model-independent upper limits on the number of beyond-SM events, are presented in section 10.

3 The ATLAS detector

The ATLAS experiment [56] is a multi-purpose particle physics detector with nearly 4π stera-dian coverage in solid angle. It consists of an inner detector of tracking devices surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer in a toroidal magnetic field. The inner detector, in combination with the 2 T axial field from the solenoid, provides precision tracking and momentum measurement of charged particles up to |η| = 2.5 and allows efficient b-jet identification.4 _{It consists}

4_{ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the}

centre of the detector and the z-axis along the beam pipe. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity η is defined in

of a silicon pixel detector, a semiconductor microstrip detector and a straw-tube tracker which also provides transition radiation measurements for electron identification. High-granularity liquid-argon (LAr) sampling electromagnetic calorimeters cover the pseudora-pidity range_{|η| < 3.2. The hadronic calorimeter system is based on two different} technolo-gies, a scintillator-tile sampling calorimeter (|η| < 1.7) and a LAr sampling calorimeter (1.5 < _{|η| < 3.2). LAr calorimeters in the most forward region (3.1 < |η| < 4.9) provide} electromagnetic and hadronic measurements. The muon spectrometer has separate trigger and high-precision tracking chambers, the former provide trigger coverage up to|η| = 2.4 while the latter provide muon identification and momentum measurements for _{|η| < 2.7.} Events are selected by a three-level trigger system [57], the first level (L1) is implemented in customised hardware while the two high-level triggers (HLT) are software-based.

4 Trigger and data collection

The data used in this analysis were collected from March to December 2012 with the LHC operating at a pp centre-of-mass energy of √s = 8 TeV. After application of beam, detector and data quality requirements, the total integrated luminosity is 20.3 fb−1 with an uncertainty of 2.8%. The uncertainty is derived, following the methodology detailed in ref. [58], from a preliminary calibration of the luminosity scale from beam-separation scans performed in November 2012.

The dataset was recorded using three different types of triggers based on requiring either an electron, a muon, or large Emiss

T . The single-electron trigger identifies electrons

based on the presence of an energy cluster in the electromagnetic calorimeter with a shower shape consistent with that of an electron, low hadronic leakage, and a matching track in the inner detector. The HLT threshold5 _{on the energy deposit transverse to the beam}

(ET) is 24 GeV. An electron isolation criterion at the HLT requires the scalar sum of the

transverse momenta (pT) of tracks within a cone of radius ∆R = 0.2 around the electron

(excluding the electron itself) to be less than 10% of the electron ET. The single-muon

trigger identifies muons using tracks reconstructed in the muon spectrometer and inner detector. The pT threshold at the HLT is 24 GeV. An isolation criterion at the HLT

requires the scalar sum of the pT of tracks within a cone of radius ∆R = 0.2 around the

muon (excluding the muon itself) to be less than 12% of the muon pT. To recover some of

the small efficiency loss for high-pTleptons, events were also collected using complementary

single-lepton triggers. These triggers have less stringent shower-shape requirements and no hadronic leakage criterion for electrons, and no isolation criteria, but have an increased ET (pT) threshold of 60 GeV (36 GeV) for electrons (muons). Corrections are applied to

the simulated samples to account for small differences between data and simulation in the lepton trigger efficiencies.

terms of the polar angle θ by η =_{− ln tan(θ/2), and the angular separation ∆R in the η–φ space is defined} as ∆R =p(∆η)2_{+ (∆φ)}2_.

5_{The trigger thresholds refer to lower requirements on the given quantity, and the HLT thresholds are}

always more stringent than the corresponding L1 thresholds.