Measurement of top quark properties using the ATLAS detector at the LHC

(1)

Simon Head

University of Birmingham

(on behalf of the ATLAS collaboration)

ICHEP 2014, Valencia

(2)

The Top Quark

•

Heaviest fundamental particle - large coupling to Higgs - special role in EWSB?

•

Short lifetime (~10

^-25

s) - decays before hadronisation

•

Spin information carried by final state particles - possibility to study a “bare” quark

•

Great tool to study the SM and search for New Physics

1

W helicity fractions - SM: F

0

~0.7,   F

L

~0.3, F

R

~0

Top quark polarisation - SM: ~0

Top quark spin correlation   - SM: correlated

Small amount of tt ̅ charge asymmetry

predicted by SM

Top quark charge   - SM: ±2/3

Flavour Changing Neutral Currents  

- SM ~0

(3)

Events / 0.06

0 20 40 60 80 100 120 140 160 180

200 µ+jets

Ldt = 2.05 fb-1

= 7 TeV s

combQ

Data 2011 t

tSingle top Multi-jets (DD) W+jets

Z+jets Diboson uncertainty exotic

ATLAS

Qcomb

-1 -0.5 0 0.5 1

Data/MC

0.5 1 1.5

!

•

Some models predict exotic quarks with a charge of -4/3e

•

Test whether electric charge is 2/3e (SM) or -4/3e

•

W charge from lepton, b-jet charge from weighted sum of associated tracks

!

•

Lepton and b-jet paired using invariant mass - m

_lb

•

Then correct for detector eﬀects…

!

•

Main systematic uncertainties - jet energy scale 8%, parton shower 8%

3

JHEP11 (2013) 031

Top Quark Charge

Q = 0.64 ± 0.02 (stat) ± 0.08 (syst) e Exclude models that propose -4/3e at 8𝜎

18 GeV, respectively. Events passing the trigger selection are required to contain exactly one reconstructed lepton, with E

_T

> 25 GeV for an electron or p

_T

> 20 GeV for a muon. At least four jets with transverse momenta p

_T

> 25 GeV and within the pseudorapidity range | η | < 2.5 are required. The missing transverse momentum, E

_T^miss

, has to exceed 35 GeV for the events with electrons, and 20 GeV for the events with muons. In addition, a primary vertex containing at least five charged particles with p

_T

> 0.4 GeV is required, and events containing jets with p

_T

> 20 GeV in poorly instrumented detector regions are removed.

The transverse mass of the leptonically decaying W boson in the event is reconstructed as m

_T

( W ) =

!

2 p

^ℓ_T

p

^ν_T

(1 − cos( φ

^ℓ

− φ

^ν

)), where the measured E

_T^miss

magnitude and direction provide the transverse momentum, p

^ν_T

, and azimuthal angle, φ

^ν

, of the neutrino, and the superscript ℓ stands for the e or µ . For events with electrons m

_T

( W ) has to exceed 25 GeV, while the sum of m

_T

( W ) and E

_T^miss

has to exceed 60 GeV for the events with muons.

Finally, at least one jet is required to be b-tagged using the b-tagging procedure described in Ref. [35]. The procedure combines an algorithm based on jet track impact parameters with respect to the primary vertex with an algorithm exploiting the topology of b- and c-hadron weak decays inside the jet. The combination of the two algorithms is based on artificial neural network techniques with MC-simulated training samples and variables describing the topology of the decay chain used as the neural network input [36]. The chosen b-tagging operating point corresponds to a 70% tagging efficiency for b-jets in simulated t t ¯ events, while light-flavour jets are suppressed by approximately a factor of 100.

These selection requirements, common to most of the ATLAS t t ¯ analyses (see e.g. [37]), are further referred to as the basic t t ¯ requirements. They are followed by requirements specific for reconstruction of the b-quark charge. In order to use the track charge weighting method (see section 5.1), the presence of a second b-tagged jet is required. Each of the two b-tagged jets has to contain at least two well-reconstructed tracks with transverse momenta above 1 GeV within the pseudorapidity range | η | < 2.5. A pairing criterion between the lepton and a b-jet is also applied (see section 5).

5 Top quark charge determination

The correlation between the top or exotic quark charge and the charges of their decay products can be used for the quark charge determination. In the SM the top quark is expected to decay according to

t

^(2/3)

→ b

⁽⁻^1/3)

+ W

⁽⁺¹⁾

, (5.1)

while the exotic quark (t

_X

) with charge –4/3 is assumed to decay according to

t

⁽_X⁻^4/3)

→ b

⁽⁻^1/3)

+ W

⁽⁻¹⁾

, (5.2) where the electric charges of the particles are indicated in parentheses. Considering the subsequent leptonic decay of the W bosons, W

^±

→ ℓ

^±

+ ν

ℓ

( ν ¯

ℓ

), the expectation for the SM case is that a positively charged lepton ℓ

⁺

is associated with the b-quark (Q

_b

= − 1/3) from the same top quark, while for the exotic case it is just the opposite: ℓ

⁻

is paired with the b-quark. In the SM case the

– 5 –

18 GeV, respectively. Events passing the trigger selection are required to contain exactly one reconstructed lepton, with E

_T

> 25 GeV for an electron or p

_T

> 20 GeV for a muon. At least four jets with transverse momenta p

_T

> 25 GeV and within the pseudorapidity range | η | < 2.5 are required. The missing transverse momentum, E

_T^miss

, has to exceed 35 GeV for the events with electrons, and 20 GeV for the events with muons. In addition, a primary vertex containing at least five charged particles with p

_T

> 0.4 GeV is required, and events containing jets with p

_T

> 20 GeV in poorly instrumented detector regions are removed.

The transverse mass of the leptonically decaying W boson in the event is reconstructed as m

_T

( W ) =

!

2 p

^ℓ_T

p

^ν_T

(1 − cos( φ

^ℓ

− φ

^ν

)), where the measured E

_T^miss

magnitude and direction provide the transverse momentum, p

^ν_T

, and azimuthal angle, φ

^ν

, of the neutrino, and the superscript ℓ stands for the e or µ . For events with electrons m

_T

( W ) has to exceed 25 GeV, while the sum of m

_T

( W ) and E

_T^miss

has to exceed 60 GeV for the events with muons.

Finally, at least one jet is required to be b-tagged using the b-tagging procedure described in Ref. [35]. The procedure combines an algorithm based on jet track impact parameters with respect to the primary vertex with an algorithm exploiting the topology of b- and c-hadron weak decays inside the jet. The combination of the two algorithms is based on artificial neural network techniques with MC-simulated training samples and variables describing the topology of the decay chain used as the neural network input [36]. The chosen b-tagging operating point corresponds to a 70% tagging efficiency for b-jets in simulated t t ¯ events, while light-flavour jets are suppressed by approximately a factor of 100.

These selection requirements, common to most of the ATLAS t t ¯ analyses (see e.g. [37]), are further referred to as the basic t t ¯ requirements. They are followed by requirements specific for reconstruction of the b-quark charge. In order to use the track charge weighting method (see sec- tion 5.1), the presence of a second b-tagged jet is required. Each of the two b-tagged jets has to contain at least two well-reconstructed tracks with transverse momenta above 1 GeV within the pseudorapidity range | η | < 2.5. A pairing criterion between the lepton and a b-jet is also applied (see section 5).

5 Top quark charge determination

The correlation between the top or exotic quark charge and the charges of their decay products can be used for the quark charge determination. In the SM the top quark is expected to decay according to

t

^(2/3)

→ b

⁽⁻^1/3)

+ W

⁽⁺¹⁾

, (5.1)

while the exotic quark (t

_X

) with charge –4/3 is assumed to decay according to

t

⁽_X⁻^4/3)

→ b

⁽⁻^1/3)

+ W

⁽⁻¹⁾

, (5.2) where the electric charges of the particles are indicated in parentheses. Considering the subsequent leptonic decay of the W bosons, W

^±

→ ℓ

^±

+ ν

ℓ

( ν ¯

ℓ

), the expectation for the SM case is that a positively charged lepton ℓ

⁺

is associated with the b-quark (Q

_b

= − 1/3) from the same top quark, while for the exotic case it is just the opposite: ℓ

⁻

is paired with the b-quark. In the SM case the

– 5 –

SM:

Exotic:

7 TeV 2.1 fb

^-1

tt: l+jets

2

product of charges of the top or anti-top quark decay products (Q

_ℓ⁺

× Q

_b

or Q

_ℓ⁻

× Q

b¯

) always has a negative sign while in the exotic case the sign is positive.

The charge of the W boson is taken from the charge of the high- p

_T

lepton in the event. The charge of the quark initiating the b-jet is estimated from a weighted average of the charges of the tracks in the jet (see section 5.1). A lepton–b-jet pairing criterion (hereafter referred to as ℓb- pairing) is then applied to match the W boson to the b-jet from the same top quark (see section 5.2).

5.1 Weighting procedure for b-jet charge calculation

For the determination of the effective b-jet charge a weighting technique [38 , 39] is applied in which the b-jet charge is defined as a weighted sum of the b-jet track charges,

Q

_b₋_jet

= ∑

i

Q

_i

| ⃗ j · ⃗ p

_i

|

^κ

∑

i

| ⃗ j · ⃗ p

_i

|

^κ

, (5.3)

where Q

_i

and ⃗ p

_i

are the charge and momentum of the i-th track, ⃗ j defines the b-jet axis direction, and κ is a parameter which was set to be 0.5 for the best separation between b- and ¯ b-jets mean charges using the standard MC@NLO t t ¯ simulated sample.

The calculation of the b-jet charge uses a maximum number of ten tracks with p

_T

> 1 GeV associated with the b-jet within a cone of ∆ R < 0.25. The b-jet tracks used in the calculation of the effective b-jet charge include not only the charged decay products of the b-hadron, but also b-fragmentation tracks, and can possibly also contain tracks from multiple interactions or pile-up.

The mean number of charged tracks within the b-jet cone is six for t t b-jets. If there are more ¯ than ten associated tracks, the highest- p

_T

tracks are chosen. The maximum number of tracks, the minimum track p

_T

and the value of ∆ R were optimized using the standard MC@NLO t t ¯ simulated sample. The optimization takes into account that the pile-up effect can be stronger for the high track multiplicity events and that low- p

_T

tracks, coming mainly from gluons, could dilute the jet charge.

The variable that is used to distinguish between the SM and exotic model scenarios is the combined lepton–b-jet charge (hereafter referred to as the combined charge) which is defined as

Q

_comb

= Q

^ℓ_b₋_jet

· Q

_ℓ

, (5.4)

where Q

^ℓ_b₋_jet

is the charge of the b-jet calculated with equation (5.3)

⁵

and Q

_ℓ

the charge of the lepton, the two being associated via the ℓb-pairing described below.

5.2 Lepton and b-jet pairing algorithm

The ℓb-pairing is based on the invariant mass distribution of the lepton and the b-jet, m(ℓ, b-jet).

If the assignment is correct, assuming an ideal invariant mass resolution, m(ℓ, b-jet) should not exceed the top quark mass provided that the decaying particle is the SM top quark. Otherwise, if the lepton and b-jet are not from the same decaying particle, there is no such restriction. This is shown in Figure 1, where the invariant mass distribution of a lepton and a b-jet in the signal MC sample is plotted for the correct pairing and the wrong pairing, for events fulfilling the basic t t ¯

5

The superscript ℓ is added to Q

_b₋_jet

to stress that the b-jet is paired with a lepton.

– 6 –

(4)

-|

|-| l l+

|

-3 -2 -1 0 1 2 3

Events / 0.5

0 100 200 300 400 500 600

2011 Data t t single top

µ Z µ Z dibosons fake leptons Norm. uncertainty

Preliminary ATLAS

L dt = 4.7 fb-1

µ - µ

t|

| - |y

|yt

-3 -2 -1 0 1 2 3

Entries/0.5

0 200 400 600 800 1000 1200 1400 1600 1800

2000 2011 Data

t t single top Z dibosons fake leptons Norm. uncertainty

Preliminary ATLAS

L dt = 4.7 fb-1

µ - e

tt Charge Asymmetry (Dileptonic Decays) ̅

4

ATLAS-CONF-2012-057

7 TeV 4.7 fb

^-1

tt: dil

3

A

c

= 0.057 ± 0.024 (stat) ± 0.015 (syst) A

cll

= 0.023 ± 0.012 (stat) ± 0.008 (syst)

• tt pair production has a small asymmetry under charge conjugation ̅ in the SM at the LHC - BSM physics could enhance this

1 Introduction

The measurement of the

tt

¯ production charge asymmetry represents an important test of quantum chromodynamics (QCD) at high energies and is also an ideal place to observe e↵ects of possible new physics processes beyond the Standard Model (BSM). Several BSM processes can alter this asymmetry [1–13], either with anomalous vector or axial–vector couplings (i.e. axigluons) or via interference with the Standard Model (SM). Di↵erent models also predict di↵erent asymmetries as a function of the invariant mass

m_t¯t

[14], the transverse momentum

p_T,t¯t

and the rapidity

|y_t¯t|

of the

tt–system.

¯

At leading order (LO),

tt

¯ production at hadron colliders is predicted to be symmetric under the exchange of top quark and antiquark. At next–to–leading order (NLO), the process

qq

¯

! ttg

¯ exhibits an asymmetry in the rapidity distributions of the top quark and antiquark, due to interference between initial– and final– state gluon emission. In addition, the

qq

¯

! t

¯

t

process itself possesses an asymmetry due to the interference between the Born and the NLO diagrams. The

qg

production process is also asymmetric, but its contribution is much smaller than the

qq

¯ one. The production of

t

¯

t

events by gluon fusion,

gg ! tt, is

¯ symmetric. At the Tevatron proton–antiproton collider, where

tt

¯ events are predominantly produced by

qq

¯ annihilation, top quarks are preferentially emitted in the direction of the incoming quark while the top antiquarks are emitted preferentially in the direction of the incoming antiquark [15–21]. The

tt

¯ asymmetry at the Tevatron is therefore measured as a forward–backward asymmetry,

A_FB

=

N

(

y >

0)

N

(

y <

0)

N

(

y >

0) +

N

(

y <

0)

,

where

y ⌘ y_t yt¯

is the di↵erence in rapidity between top quarks and antiquarks, and

N

represents the number of events with

y

being positive or negative. The interest in this measurement has grown after CDF and D0 collaborations reported

A_FB

measurements significantly larger than the SM predictions, in both the inclusive and di↵erential case as a function of

m_tt¯

and

|y_t¯t|

[22–26].

In proton–proton (pp) collisions at the LHC, the dominant mechanism for

t

¯

t

production is the

gg

fusion process, while production via

qq

¯ or

qg

interactions is small. Since the colliding beams are symmetric,

A_FB

is no longer a useful observable. However,

tt

¯ production via

qq

¯ or

qg

processes is asymmetric under top quark–antiquark exchange, and, in addition, the valence quarks carry, on average, a larger momentum fraction than antiquarks from the sea. Hence for

qq

¯ or

qg

production processes at the LHC, QCD predicts a small excess of centrally produced top antiquarks while top quarks are produced, on average, at higher absolute rapidities. Therefore, the

t

¯

t

production charge asymmetry

A_C

is defined as [1,

27]

A_C

=

N

(

|y| >

0)

N

(

|y| <

0)

N

(

|y| >

0) +

N

(

|y| <

0)

,

(1.1) where

|y| ⌘ |y_t| |y¯t|

is the di↵erence between the absolute value of the top quark rapidity

|y_t|

and the absolute value of the top antiquark rapidity

|y¯t|

.

The SM prediction for the

tt

¯ production charge asymmetry at the LHC is

A^SM_C

= 0.0123

±

0.0005 [21], computed at NLO in QCD including electroweak corrections. Recent

– 2 –

The t t-based charge asymmetry ¯ A

^t_C^t^¯

is defined as:

A

^t_C^t^¯

= N ( ∆ | y | > 0) − N ( ∆ | y | < 0)

N (∆ | y | > 0) + N (∆ | y | < 0) , (1) where ∆ | y | ≡ | y

_t

| − | y

t¯

| represents the di ﬀ erence of the absolute values of top and antitop rapidities ( | y

_t

| and | y

t¯

| ) and N is the number of events with ∆ | y | being positive or negative. The charge of the top or antitop quark is determined by the charge of the lepton.

The lepton-based asymmetry A

^ℓℓ_C

is defined as:

A

^ℓℓ_C

= N ( ∆ | η | > 0) − N ( ∆ | η | < 0)

N ( ∆ | η | > 0) + N ( ∆ | η | < 0) , (2) where ∆ | η | = | η

_l⁺

| − | η

_l⁻

| represents the di ﬀ erence of the absolute values of positively and negatively charged lepton pseudorapidities

¹

and N is the number of events with ∆ | η | being positive or negative. To allow comparisons with theory calculations, in both cases, the asymmetries are measured after background subtraction and after correction for acceptance and detector e ﬀ ects.

2 Data and Monte Carlo samples

In this note, data from LHC proton-proton collisions collected by the ATLAS detector in 2011 are used.

A detailed description of the detector can be found in [19]. The dataset corresponds to an integrated luminosity of 4.7 fb

⁻¹

.

Simulated top pair events are generated using the next-to-leading order (NLO) MC@NLO v.4.01 [20]

Monte Carlo (MC) generator with the NLO parton density (PDF) set CT10 [21]. Parton showering and underlying event are modeled using HERWIG [22] and JIMMY [23] with the AUET2-CT10 tuning [24].

This sample is generated assuming a top mass of 172.5 GeV and it is normalized to a cross-section of 166.8 pb obtained from the HATHOR tool which approximates the next-to-next-to leading order (NNLO) prediction [25]. Single top events are also generated using MC@NLO (A CER MC [26] for the t-channel) while the production of W / Z bosons in association with jets is simulated using the ALPGEN generator [27] interfaced to HERWIG and JIMMY. Diboson events (WW , WZ, ZZ ) are produced using HERWIG.

All Monte Carlo simulated samples are generated with multiple pp interactions (pile-up). These simulated events are re-weighted so that the distribution of the number of interactions per crossing in simulation matches that observed in the data. The samples are then processed through the GEANT4 [28]

simulation and the reconstruction software of the ATLAS detector [29].

3 Event selection

3.1 Object definition

The reconstruction of top quark pair events in the detector involves electrons, muons, jets and missing transverse momentum. Electron candidates are defined as energy deposits in the electromagnetic calorimeter with an associated well-measured track [30]. All electron candidates are required to have E

_T

> 25 GeV and | η

_cluster

| < 2.47, where η

_cluster

is the pseudorapidity of the electromagnetic cluster

1In the right-handed ATLAS coordinate system, the pseudorapidity η is defined as η = − ln[tan(θ/2)], where the polar angle θ is measured with respect to the LHC beamline. The azimuthal angle φ is measured with respect to the x-axis, which points towards the center of the LHC ring. The z-axis is parallel to the anti-clockwise beam viewed from above. Transverse momentum and energy are defined as p_T = psinθ and E_T = E sinθ, respectively.

2

The t t-based charge asymmetry ¯ A

^t_C^t^¯

is defined as:

A

^t_C^t^¯

= N ( ∆ | y | > 0) − N ( ∆ | y | < 0)

N (∆ | y | > 0) + N (∆ | y | < 0) , (1) where ∆ | y | ≡ | y

_t

| − | y

t¯

_t

| and | y

_t_¯

^ℓℓ_C

is defined as:

A

^ℓℓ_C

= N ( ∆ | η | > 0) − N ( ∆ | η | < 0)

N (∆ | η | > 0) + N (∆ | η | < 0) , (2) where ∆ | η | = | η

_l⁺

| − | η

_l⁻

¹

In this note, data from LHC proton-proton collisions collected by the ATLAS detector in 2011 are used.

⁻¹

.

This sample is generated assuming a top mass of 172.5 GeV and it is normalized to a cross-section of 166.8 pb obtained from the HATHOR tool which approximates the next-to-next-to leading order (NNLO) prediction [25]. Single top events are also generated using MC@NLO (A CER MC [26] for the t-channel) while the production of W / Z bosons in association with jets is simulated using the ALPGEN generator [27] interfaced to HERWIG and JIMMY. Diboson events ( WW , WZ , ZZ ) are produced using HERWIG.

All Monte Carlo simulated samples are generated with multiple pp interactions (pile-up). These simulated events are re-weighted so that the distribution of the number of interactions per crossing in simulation matches that observed in the data. The samples are then processed through the GEANT4 [28]

3 Event selection

_T

> 25 GeV and | η

_cluster

| < 2.47, where η

_cluster

1In the right-handed ATLAS coordinate system, the pseudorapidity η is defined as η = −ln[tan(θ/2)], where the polar angle θ is measured with respect to the LHC beamline. The azimuthal angle φ is measured with respect to the x-axis, which points towards the center of the LHC ring. The z-axis is parallel to the anti-clockwise beam viewed from above. Transverse momentum and energy are defined as p_T = psinθ and E_T = E sinθ, respectively.

2

The t t-based charge asymmetry ¯ A

^t_C^t^¯

is defined as:

A

^t_C^t^¯

= N ( ∆ | y | > 0) − N ( ∆ | y | < 0)

N ( ∆ | y | > 0) + N ( ∆ | y | < 0) , (1) where ∆ | y | ≡ | y

_t

| − | y

_t_¯

_t

| and | y

t¯

^ℓℓ_C

is defined as:

A

^ℓℓ_C

= N ( ∆ | η | > 0) − N ( ∆ | η | < 0)

N ( ∆ | η | > 0) + N ( ∆ | η | < 0) , (2) where ∆ | η | = | η

_l⁺

| − | η

_l⁻

¹

In this note, data from LHC proton-proton collisions collected by the ATLAS detector in 2011 are used.

⁻¹

.

This sample is generated assuming a top mass of 172.5 GeV and it is normalized to a cross-section of 166.8 pb obtained from the HATHOR tool which approximates the next-to-next-to leading order (NNLO) prediction [25]. Single top events are also generated using MC@NLO (A CER MC [26] for the t-channel) while the production of W /Z bosons in association with jets is simulated using the ALPGEN generator [27] interfaced to HERWIG and JIMMY. Diboson events (WW , WZ , ZZ ) are produced using HERWIG.

All Monte Carlo simulated samples are generated with multiple pp interactions (pile-up). These simulated events are re-weighted so that the distribution of the number of interactions per crossing in simulation matches that observed in the data. The samples are then processed through the GEANT4 [28]

3 Event selection

_T

> 25 GeV and | η

_cluster

| < 2.47, where η

_cluster

1In the right-handed ATLAS coordinate system, the pseudorapidity η is defined as η = −ln[tan(θ/2)], where the polar angle θ is measured with respect to the LHC beamline. The azimuthal angle φ is measured with respect to the x-axis, which points towards the center of the LHC ring. The z-axis is parallel to the anti-clockwise beam viewed from above. Transverse momentum and energy are defined as p_T = p sinθ and E_T = E sinθ, respectively.

2

lepton based asymmetry tt based asymmetry ̅

SM prediction: A

cll

= 0.004 ± 0.001

(5)

[GeV]

t

mt

0 100 200 300 400 500 600 700 800 900

CA

-0.1 -0.05

0 0.05 0.1 0.15

0.2 0.25

Data SM

Axigluon m=300 GeV Axigluon m=7000 GeV

ATLAS

= 7 TeV s

L dt = 4.7 fb-1

t|

|yt

0 0.2 0.4 0.6 0.8 1

CA

-0.1 -0.05

0 0.05 0.1 0.15

0.2 0.25

Data SM

Axigluon m=300 GeV Axigluon m=7000 GeV

ATLAS

= 7 TeV s

L dt = 4.7 fb-1

tt Charge Asymmetry (Single Lepton Decays) ̅

•

tt enriched sample of events with (e or μ), ̅ missing transverse momentum, and at least

four high p

_T

jets (one tagged as coming from a b-quark)

•

Results also presented as unfolded diﬀerential measurements in p

_T

, mass, rapidity of tt pairs ̅

5

JHEP02 (2014) 107

1 Introduction

The measurement of the t t ¯ production charge asymmetry represents an important test of quantum chromodynamics (QCD) at high energies and is also an ideal place to observe e↵ects of possible new physics processes beyond the Standard Model (BSM). Several BSM processes can alter this asymmetry [1–13], either with anomalous vector or axial–vector couplings (i.e. axigluons) or via interference with the Standard Model (SM). Di↵erent models also predict di↵erent asymmetries as a function of the invariant mass m

_t¯t

[14], the transverse momentum p

_T,tt¯

and the rapidity | y

_t¯t

| of the t ¯ t–system.

At leading order (LO), t ¯ t production at hadron colliders is predicted to be symmetric under the exchange of top quark and antiquark. At next–to–leading order (NLO), the process q q ¯ ! t ¯ tg exhibits an asymmetry in the rapidity distributions of the top quark and antiquark, due to interference between initial– and final– state gluon emission. In addition, the q q ¯ ! t t ¯ process itself possesses an asymmetry due to the interference between the Born and the NLO diagrams. The qg production process is also asymmetric, but its contribution is much smaller than the q q ¯ one. The production of t ¯ t events by gluon fusion, gg ! t ¯ t, is symmetric. At the Tevatron proton–antiproton collider, where t ¯ t events are predominantly produced by q q ¯ annihilation, top quarks are preferentially emitted in the direction of the incoming quark while the top antiquarks are emitted preferentially in the direction of the incoming antiquark [15–21]. The t ¯ t asymmetry at the Tevatron is therefore measured as a forward–backward asymmetry,

A

_FB

= N ( y > 0) N ( y < 0) N ( y > 0) + N ( y < 0) ,

where y ⌘ y

_t

y

¯t

is the di↵erence in rapidity between top quarks and antiquarks, and N represents the number of events with y being positive or negative. The interest in this measurement has grown after CDF and D0 collaborations reported A

_FB

measurements significantly larger than the SM predictions, in both the inclusive and di↵erential case as a function of m

_tt¯

and | y

_t¯t

| [22–26].

In proton–proton (pp) collisions at the LHC, the dominant mechanism for t t ¯ production is the gg fusion process, while production via q q ¯ or qg interactions is small. Since the colliding beams are symmetric, A

_FB

is no longer a useful observable. However, t ¯ t production via q q ¯ or qg processes is asymmetric under top quark–antiquark exchange, and, in addition, the valence quarks carry, on average, a larger momentum fraction than antiquarks from the sea. Hence for q q ¯ or qg production processes at the LHC, QCD predicts a small excess of centrally produced top antiquarks while top quarks are produced, on average, at higher absolute rapidities. Therefore, the t ¯ t production charge asymmetry A

_C

is defined as [1, 27]

A

_C

= N ( | y | > 0) N ( | y | < 0)

N ( | y | > 0) + N ( | y | < 0) , (1.1) where | y | ⌘ | y

_t

| | y

¯t

| is the di↵erence between the absolute value of the top quark rapidity | y

_t

| and the absolute value of the top antiquark rapidity | y

t¯

| .

The SM prediction for the t t ¯ production charge asymmetry at the LHC is A

^SM_C

= 0.0123 ± 0.0005 [21], computed at NLO in QCD including electroweak corrections. Recent

– 2 – Inclusive A

c

= 0.006 ± 0.010 (stat) ± 0.005 (syst)

7 TeV 4.7 fb

^-1

tt: l+jets

4

The t t-based charge asymmetry ¯ A

^t_C^t^¯

is defined as:

A

^t_C^t^¯

= N ( ∆ | y | > 0) − N ( ∆ | y | < 0)

N ( ∆ | y | > 0) + N ( ∆ | y | < 0) , (1) where ∆ | y | ≡ | y

_t

| − | y

t¯

| represents the diﬀerence of the absolute values of top and antitop rapidities ( | y

_t

| and | y

t¯

^ℓℓ_C

is defined as:

A

^ℓℓ_C

= N ( ∆ | η | > 0) − N ( ∆ | η | < 0)

N ( ∆ | η | > 0) + N ( ∆ | η | < 0) , (2) where ∆ | η | = | η

_l⁺

| − | η

_l⁻

| represents the diﬀerence of the absolute values of positively and negatively charged lepton pseudorapidities

¹

2 Data and Monte Carlo samples

In this note, data from LHC proton-proton collisions collected by the ATLAS detector in 2011 are used.

⁻¹

.

Monte Carlo (MC) generator with the NLO parton density (PDF) set CT10 [21]. Parton showering and underlying event are modeled using HERWIG [22] and JIMMY [23] with the AUET2-CT10 tuning [24].

This sample is generated assuming a top mass of 172.5 GeV and it is normalized to a cross-section of 166.8 pb obtained from the HATHOR tool which approximates the next-to-next-to leading order (NNLO) prediction [25]. Single top events are also generated using MC@NLO (A CER MC [26] for the t-channel) while the production of W / Z bosons in association with jets is simulated using the ALPGEN generator [27] interfaced to HERWIG and JIMMY. Diboson events (WW , WZ, ZZ ) are produced using HERWIG.

All Monte Carlo simulated samples are generated with multiple pp interactions (pile-up). These simulated events are re-weighted so that the distribution of the number of interactions per crossing in simulation matches that observed in the data. The samples are then processed through the GEANT4 [28]

3 Event selection

3.1 Object definition

_T

> 25 GeV and | η

_cluster

| < 2.47, where η

_cluster

1In the right-handed ATLAS coordinate system, the pseudorapidity η is defined as η = − ln[tan(θ/2)], where the polar angle θ is measured with respect to the LHC beamline. The azimuthal angle φ is measured with respect to the x-axis, which points towards the center of the LHC ring. The z-axis is parallel to the anti-clockwise beam viewed from above. Transverse momentum and energy are defined as p_T = psinθ and E_T = E sinθ, respectively.

2

(6)

• ATLAS and CMS results

combined results using BLUE

• Similar selection - exactly

one isolated lepton (e, mu), at least four jets, at least one b- tagged

• Combination consistent with prediction from the SM

6

A

C

-0.05 0 0.05

-2 4

ATLAS+CMS 0.005 ± 0.007 ± 0.006

ATLAS 0.006 ± 0.010 ± 0.005

CMS 0.004 ± 0.010 ± 0.011

[PLB 717 (2012) 129]

[JHEP 1402 (2014) 107]

0.0006 0.0115 ±

Theory (NLO+EW)

[JHEP 1201 (2012) 063]

(stat) (syst)

= 7 TeV s

ATLAS+CMS Preliminary

TOPLHCWG March 2014

ATLAS-CONF-2014-012 CMS TOP-14-006

red band: statistical

blue band: statistical + systematic

tt Charge Asymmetry (Combination) ̅

A

c

= 0.005 ± 0.007 (stat) ± 0.006 (syst)

7 TeV 4.7 - 5 fb

^-1

tt: l+jets

5

(7)

Top Quark Polarisation + tt Spin Correlation ̅

1. Reconstruct the t and ¯ t 4-vectors in the laboratory frame, which is the rest frame of the colliding hadrons.

2. Perform a rotation-free boost from the laboratory frame into the t t ¯ rest frame. Define the spin analysing vectors ˆ a and ˆ b, in this frame.

3. Perform a rotation-free boost from the t ¯ t rest frame to the rest frames of the t and ¯ t-quarks. Compute the direction ˆ q

₊

(ˆ q ) of the t (¯ t) decay product, l

⁺

(l ), in the t (¯ t) rest frame.

4. Compute the angles cos(✓

₊

) = ˆ a · q ˆ

₊

and cos(✓ ) = ˆ b · q ˆ .

A schematic overview of the procedure to measure cos(✓

₊

), for the top quark, is shown in Figure 3.1. The same procedure is repeated for the antitop quark in order to measure cos(✓ ).

top quark lepton neutrino

b-jet

lepton

neutrino

b-jet spin analysing basis

θ

boost to top quark rest frame spin analysing basis

Figure 3.1: Sketch of the procedure to measure the angle ✓

₊

in t ¯ t events.

The sketch on the left is in the t ¯ t rest frame and the sketch on the right is in the rest frame of the top quark. The same procedure can be applied to the antitop quark in order to obtain the angle ✓ .

28

in top quark rest frame

d

²

d cos(✓

₁

) cos(✓

₂

) / 1 + ↵

₁

P

³

cos(✓

₁

) + ↵

₂

P ¯

³

cos(✓

₂

) + ↵

₁

↵

₂

C

³³

cos(✓

₁

) cos(✓

₂

) polarisation

polarisation

spin correlation spin correlation

spin correlation spin analysing power: ~1 for charged leptons

6

•

Top quarks produced in pairs are almost unpolarised in SM

•

Spins of t and t in tt ̅ production are correlated ̅

•

New physics can aﬀect production and decay

•

Study the angular distributions of the decay product

Measurement of top quark polarization in top–antitop events from proton–proton collisions at p s = 7 TeV using the ATLAS detector

(The ATLAS Collaboration)

(Dated: September 9, 2013)

This Letter presents measurements of the polarization of the top quark in top–antitop quark pair events, using 4.7 fb ¹ of proton–proton collision data recorded with the ATLAS detector at the Large Hadron Collider at ^p^s ^{= 7 TeV}. Final states containing one or two isolated leptons (electrons or muons) and jets are considered. Two measurements of ↵_`P, the product of the leptonic spin-analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a CP conserving (CPC) or a CP violating (CPV) production process. The measurements obtained, ^↵_`^P_CPC ⁼ ^0.035 _± 0.014(stat) ± 0.037(syst) and ↵_`P_CPV = 0.020 ± 0.016(stat)^+0.013_0.017(syst), are in good agreement with the Standard Model prediction of negligible top quark polarization.

PACS numbers: 14.65.Ha,12.38.Qk

The short lifetime of the top quark [1–5] implies that it decays before hadronization takes place, allowing its spin state to be studied using the angular distributions of its de- cay products. In the Standard Model (SM), parity conser- vation in the strong production of top–antitop quark pairs ( t t ¯ ) in proton–proton ( pp ) collisions implies zero longitu- dinal polarization of the quarks. A negligible polarization (0.003) is generated by the weak interaction [6]. Physics beyond the SM can induce top quark polarization. For ex- ample, models that predict the top quark forward-backward production asymmetry to be larger than the SM prediction, as seen by the Tevatron experiments D0 [7, 8] and CDF [9], can generate non-zero polarization of top quarks [10–12].

A first study of polarization in t ¯ t events has been performed by the D0 collaboration [8], showing good agreement between the SM prediction and data.

In this Letter, measurements are presented of the polarization of the top quark in inclusive t t ¯ production in single charged lepton ( t t ¯ ! `⌫ q qb ¯ ¯ b ) and dilepton ( t t ¯ !

`

⁺

⌫` ⌫ ¯ b ¯ b ) events. The double differential distribution in polar angles, ✓ , of two of the final-state decay products, with respect to a given quantization axis is given by [13]

1 d

d cos ✓

₁

d cos ✓

₂

= 1

4 (1 + ↵

₁

P

₁

cos ✓

₁

+ ↵

₂

P

₂

cos ✓

₂

C cos ✓

₁

cos ✓

₂

) , (1) where ✓

₁

( ✓

₂

) is the angular distribution of the decay daughter particle of the top (antitop) quark. Here, C represents the t ¯ t spin correlation, P

₁

( P

₂

) represents the degree of polarization of the top (antitop) quark along the chosen quantization axis, and ↵

_i

is the spin-analyzing power of the final state object [14, 15], which is a measure of the sensitivity of the daughter particle to the spin state of the parent. At leading order, charged leptons and down- type quarks from W -boson decays are predicted to have the largest sensitivity to the spin state of the top quark with a spin-analyzing power of ↵ = 1 . The helicity basis is used, in which the momentum direction of the top quark in the t ¯ t center-of-mass frame is chosen as the quantization axis.

The cos ✓

_`

distributions of the charged leptons are used as

observables to extract a measurement of ↵

_`

P .

The analysis is based on the full 2011 dataset of pp collision events, collected at a center-of-mass energy of 7 TeV by the ATLAS detector [16], corresponding to an integrated luminosity of 4.66 ± 0.08 fb

¹

[17] after data quality requirements.

ATLAS includes an inner tracking detector, covering a pseudorapidity [18] range | ⌘ | < 2.5 , surrounded by a su- perconducting solenoid providing a 2 T magnetic field. A liquid argon (LAr) electromagnetic sampling calorimeter ( | ⌘ | < 3.2 ), an iron–scintillator tile hadronic calorimeter ( | ⌘ | < 1.7 ), a LAr hadronic calorimeter ( 1.4 < | ⌘ | < 3.2 ), and a LAr forward calorimeter ( 3.1 < | ⌘ | < 4.9 ) provide the energy measurements. The muon spectrometer con- sists of tracking chambers covering | ⌘ | < 2.7 , and trigger chambers covering | ⌘ | < 2.4 , in a toroidal magnetic field.

Events considered in this analysis are required to have one high-transverse-momentum ( p

_T

) electron or muon that passes requirements of the three-level trigger system.

Both data-driven techniques and Monte Carlo (MC) sim- ulations are used to estimate the sample composition of the data. For each MC sample, generated events are processed through a G

EANT

4 [19] simulation of the full AT- LAS detector [20], and the same reconstruction and analysis software is used for both the data and the MC events.

Signal t t ¯ events are simulated by the next-to-leading-order (NLO) generator MC@NLO 3.41 [21] with the NLO parton distribution function (PDF) set CT10 [22], assuming a top quark mass of 172.5 GeV. Parton showering is modeled with HERWIG 6.510 [23], and JIMMY 4.31 [24] is used for the underlying event. A t t ¯ production cross section of 167

⁺¹⁷₁₈

pb is used, calculated at approximate next-to-next- to-leading-order (NNLO) in QCD using H

ATHOR

1.2 [25].

Backgrounds are simulated using the MC@NLO, A

C

-

ER

MC [26], ALPGEN [27], and HERWIG generators, as

detailed in Ref. [28]. Each simulated signal or background

event is overlaid with additional pp collisions. The events

are given a weight such that the distribution of the average

number of events per beam crossing agrees with data. For

each sample the cross section is rescaled to the most up to

(8)

co s θ(ℓ⁺)

-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1

Events / 0.1

0 500 1000 1500 2000

2500 ATLAS

single lepton ∫ℒdt = 4.7 fb

−1

√s = 7 T e V

co sθ(ℓ⁻)

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Da ta αP = 0

Fit αP = +0.3

Bkgd . αP = −0.3

α_ℓP

C P C

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 Uncertainties

Statistical Total

Com bined Dilepton

µ µ eµ ee ℓ+ jets

+ jets µ

e + jets

ATLAS

∫ℒdt = 4.7 fb

−1

√s = 7 TeV

!

•

Extract 𝛼

_l

P from template fit to cos(θ

_l

) for data for two scenarios

•

CP Conserving (same 𝛼

_l

P for t and t ) ̅

•

CP Violating (opposite 𝛼

_l

P for t and t ) ̅

8

PRL 111, 232002 (2013)

Top Quark Polarisation

Combined fit to all channels:

𝛼

l

P

CPC

= -0.035 ± 0.014 (stat) ± 0.037 (syst) 𝛼

l

P

CPV

= 0.020 ± 0.016 (stat) +0.013 -0.017 (syst)

Main systematic uncertainty is jet reconstruction

7 TeV 4.7 fb

^-1

tt: l+jets, dil

7

CP Conserving scenario

CP Violating scenario

Polarisation measured to be

negligible in both scenarios,

as expected from SM

(9)

•

Top quarks produced in pairs have correlated spins

•

Directly observable in azimuthal opening angle of charged leptons in dilepton decays in the lab frame

•

Also via angles with respect to a basis vector in the top quark rest frame (requires reconstruction of both top quarks in the event)

9 [rad]

φ

∆

0 0.5 1 1.5 2 2.5 3

Events

0 200 400 600 800 1000 1200 1400 1600 1800

2000 _{fit result}

(A=SM) t

t

(A=0) t

t data

background

ATLAS Preliminary L dt = 4.6 fb-1

∫

^s^{= 7 TeV}

maximal -)

)cos(θ θ+

cos(

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Events / Bin Width

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

fit result (A=SM) t

t

(A=0) t

t data

background

ATLAS Preliminary L dt = 4.6 fb-1

∫

^s^{= 7 TeV}

ATLAS-CONF-2013-101

tt Spin Correlation ̅

7 TeV 4.6 fb

^-1

tt: dil

8

(10)

• Measured using several observables - diﬀering sensitivity to new physics in tt production and decay ̅

10

Standard model fraction

0 0.5 1 1.5 2

1.6 7.4

maximal basis

-) ) cos(θ θ+

cos( 0.83 ± 0.14 ± 0.17

helicity basis

-) ) cos(θ θ+

cos( 0.75 ± 0.19 ± 0.25

S-ratio 0.87 ± 0.11 ± 0.12

φ

∆ 1.19 ± 0.09 ± 0.15

ATLAS Preliminary

= 7 TeV s

-1, Ldt = 4.6 fb

∫

spin correlation measurements t

t

fSM ± (stat) ± (syst)

ATLAS-CONF-2013-101 Azimuthal angle between two

charged leptons in lab frame

tt Spin Correlation ̅

Measured spin correlation as a fraction of the SM expectation

Main systematic uncertainties: signal modelling and jet energy scale

7 TeV 4.6 fb

^-1

tt: dil

9

basis = top direction in tt rest ̅ frame

basis = use event kinematics to maximise for gg fusion

S = ( |M|

²RR

+ |M|

²LL

)

_corr

( |M|

²_RR

+ |M|

²_LL

)

_uncorr

(11)

W boson helicity fractions

-1 -0.5 0 0.5 1

ATLAS and CMS preliminary - 2.2 fb-1

=35 pb-1

= 7 TeV, Lint

s F_R F_L F₀

ATLAS 2011 (dilepton) CMS 2011 (single muon) ATLAS 2010 (single lepton) ATLAS 2011 (single lepton)

LHC combination NNLO QCD Combination

0)

L/F

R/F Data (F

•

Reconstruct W-boson and measure cos(θ*) - charged

lepton and reversed momentum of b-quark in the W-boson rest frame

•

Fit longitudinal, left and right polarisation coeﬃcients:

7 TeV 35 pb

^-1

-2.1 fb

^-1

tt: l+jets, dil

11

W-boson Polarisation

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

F_i

V_L=1 and g_R=g_L=V_R=0

cos θ*

F_L=Γ_L/Γ_tot

➟ ➟

^A⁺

F_R=Γ_R/Γ_tot (rescaled)

➟

A_-

F₀=Γ₀/Γ_tot

➟ ➟

A_FB

SM

ATLAS-CONF-2013-033 CMS PAS TOP-12-025

10

F

0

= 0.626 ± 0.034 (stat) ± 0.048 (syst) F

L

= 0.359 ± 0.021 (stat) ± 0.028 (syst)

F

0

+F

L

+F

R

= 1 gives F

R

= 0.015 ± 0.034 (stat + syst)

to the decay of the two produced W bosons. Each boson can decay either into a quark- antiquark pair or into a charged lepton and a neutrino. The single-lepton and dilepton topologies, both considered in the analyses presented in this paper, have one and two isolated charged leptons in the final state. Only electrons and muons, including those from τ decays, are considered here.

The W tb vertex is defined by the electroweak interaction and has a (V − A) structure where V and A are the vector and axial-vector contributions to the vertex. Since the W bosons are produced as real particles in top quark decays, their polarization can be longitudinal, left-handed or right-handed. The fractions of events with a particular polarization, F₀, F_L and F_R, are referred to as helicity fractions. They are predicted in next-to-next- to-leading-order (NNLO) QCD calculations to be F₀ = 0.687 ± 0.005, F_L = 0.311 ± 0.005, F_R = 0.0017 ± 0.0001 [4]. These fractions can be extracted from measurements of the angular distribution of the decay products of the top quark. The angle θ^∗ is defined as the angle between the momentum direction of the charged lepton from the decay of the W boson and the reversed momentum direction of the b-quark from the decay of the top quark, both boosted into the W boson rest frame [5]. The angular distribution is:

1 σ

dσ

d cosθ^∗ = 3 4

!1 − cos² θ^∗"

F₀ + 3

8 (1 − cos θ^∗)² F_L + 3

8 (1 + cosθ^∗)² F_R . (1.1) All previous measurements of the helicity fractions, performed by the CDF and DØ Col- laborations [6–8] at the Tevatron, are in agreement with Standard Model predictions.

Information about the polarization of the W bosons can also be obtained through complementary observables, such as the angular asymmetries, A₊ and A⁻, defined as:

A^± = N(cosθ^∗ > z) − N(cosθ^∗ < z)

N(cosθ^∗ > z) + N(cosθ^∗ < z) , (1.2) with z = ±(1−2^2/3) for A^±, allowing the dependence on F_L and F_R to cancel, respectively.

The asymmetries can be related to the helicity fractions by a simple system of equations [9, 10]. In the Standard Model, the NNLO values for these asymmetries are A₊ = 0.537±0.004 and A⁻ = −0.841 ± 0.006 [4].

In the presence of anomalous W tb couplings the helicity fractions and angular asymmetries depart from their Standard Model values [5, 10]. In eﬀective field theories, dimension- six operators can be introduced which modify the W tb vertex [11–13]. Coeﬃcients con- trolling the strength of these operators can be constrained by measurements of the helicity fractions or the angular asymmetries.

This paper describes measurements of the W boson polarization in top quark decays and the constraints on the W tb vertex structure based on a data set recorded with the ATLAS detector between March and June 2011 and corresponding to an integrated luminosity of 1.04 fb⁻¹. The helicity fractions were measured using two diﬀerent methods.

The first compares the observed cosθ^∗ distribution with templates for diﬀerent W boson helicity states obtained from simulation. The second method extracts angular asymmetries from an unfolded cosθ^∗ spectrum corrected for background contributions. Limits on anomalous couplings, generated by the aforementioned dimension-six operators, were set using the combined result from the two measurements.

– 2 –

to the decay of the two produced W bosons. Each boson can decay either into a quark- antiquark pair or into a charged lepton and a neutrino. The single-lepton and dilepton topologies, both considered in the analyses presented in this paper, have one and two isolated charged leptons in the final state. Only electrons and muons, including those from τ decays, are considered here.

The W tb vertex is defined by the electroweak interaction and has a (V − A) structure where V and A are the vector and axial-vector contributions to the vertex. Since the W bosons are produced as real particles in top quark decays, their polarization can be longitudinal, left-handed or right-handed. The fractions of events with a particular polarization, F₀, F_L and F_R, are referred to as helicity fractions. They are predicted in next-to-next- to-leading-order (NNLO) QCD calculations to be F₀ = 0.687 ± 0.005, F_L = 0.311 ± 0.005, F_R = 0.0017 ± 0.0001 [4]. These fractions can be extracted from measurements of the angular distribution of the decay products of the top quark. The angle θ^∗ is defined as the angle between the momentum direction of the charged lepton from the decay of the W boson and the reversed momentum direction of the b-quark from the decay of the top quark, both boosted into the W boson rest frame [5]. The angular distribution is:

1 σ

dσ

d cosθ^∗ = 3 4

!1 − cos² θ^∗"

F₀ + 3

8 (1 − cos θ^∗)² F_L + 3

8 (1 + cosθ^∗)² F_R . (1.1) All previous measurements of the helicity fractions, performed by the CDF and DØ Col- laborations [6–8] at the Tevatron, are in agreement with Standard Model predictions.

Information about the polarization of the W bosons can also be obtained through complementary o