Measurement of Hard Probes of the Quark-Gluon Plasma with the ATLAS Experiment at the LHC

Measurement of Hard Probes of the Quark-Gluon Plasma

with the ATLAS Experiment at the LHC

Barbara Wosiek, for the ATLAS Collaboration Institute of Nuclear Physics PAS, Kraków, Poland

Probing the medium

High transverse momentum probes in Pb+Pb, p+Pb

ch. hadrons suppressed Z⁰, W, γ unchanged

quarkonia dissociated jets quenched

talk by Marisilvia Donadelli

How to measure if a probe is affected by the medium?

Yields in Pb+Pb (p+Pb) are expected to scale as the p+p cross-section times T_AB - the mean nuclear thickness function (incoming nucleon/parton flux)

1 N_evt

d²N_jet^cent

dp_Tdy = T_{AB cent}× d²σjet pp

dp_Tdy R_AB =

1 N_evt

d²N^cent_jet dp_Tdy T_{AB cent} d²σ^pp_jet

dp_Tdy

R_AB=1

probe is unaffected

The ATLAS detector

Three main subsystems

with a full coverage in azimuth:

•  Inner Detector – charged particle tracking |η|<2.5

•  Calorimetry – jets, photons, electrons |η|<4.9

•  Muon Spectrometer – muons, |η|<2.7

Jet production in Pb+Pb and p+Pb

§  Pb+Pb – jet energy loss in the hot, dense medium

•  Indicated by the energy imbalance in dijet events and the suppression of yields in central as compared to peripheral collisions ATLAS Phys.Rev.Lett.105(2010)252303, Phys. Lett. B719(2013)220

§  p+Pb – no energy loss expected due to much shorter transverse path length than in Pb+Pb

§  The knowledge of the expected geometric

enhancement – T

_AB

– is critical for understanding rates of hard probes in collisions involving nuclei

§  Geometry from the penetrating probes – γ/W/Z

Centrality dependence of γ production

Pb+Pb photon yield normalized by T_AA

agrees with p+p cross sections from JETPHOX

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5 2

|<2.37 40-80%, 1.52<|η

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5 2

|<2.37 20-40%, 1.52<|η

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5 2

|<2.37 10-20%, 1.52<|η

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5 2

|<2.37 0-10%, 1.52<|η [GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5 2

|<1.37 40-80%, |η

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5

2 JETPHOX PDF+scale err.

pp JETPHOX Pb+Pb/

& err.

pp JETPHOX EPS09/

|<1.37 20-40%, |η

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5 2

|<1.37 10-20%, |η

[GeV]

pT

photon 102

)ppRatio to JETPHOX (

0 0.5 1 1.5

2 ATLAS Preliminary

=2.76 TeV sNN

Pb+Pb

= 0.14 nb-1

Lint

|<1.37 0-10%, |η

ATLAS-CONF-2014-026

Centrality dependence of W yields

N_coll normalized Pb+Pb data compared to NLO

Pb+Pb yields:

•  described by NLO calculations

•  scale with N_coll

•  The precise check on centrality (better than 10%)

N_coll = T_AA × σ_NN

Jet R_AA significantly smaller than 1

! a clear evidence for jet quenching

part 〉

〈 N 0 50 100 150 200 250 300 350 400

eventsNN 〉 coll N〈

9 10

0 5 10 15 20 25 30 35 40

(Data) W±

± NLO W

± LO*

W

(Data) W+

+ NLO W

(Data) W-

- NLO W

0.14-0.15 nb-1

Ldt ≈

∫

Pb+Pb s_NN = 2.76 TeV

ATLAS Preliminary

ATLAS-CONF-2014-023

ATLAS measurements of jets

Data samples

System √s_NN [TeV] When Integrated L

Pb+Pb 2.76 2011 0.14 nb^-1

p+p 2.76 2013 4 pb^-1

p+Pb 5.02 2013 28 nb^-1

Jet reconstruction:

•  Anti-kt, R=0.4 calorimeter jets

•  Underlying event estimated and subtracted on the jet-by- jet basis

•  Selected by online high-level jet triggers

•  PYTHIA p+p dijets overlaid onto Pb+Pb/p+Pb real data to evaluate the response and perform unfolding

[GeV]

pT

[ nb/GeV ] yd Tpd

jetN2 d 〉 AAT〈 evtN1

10-10

10-8

10-6

10-4

10-2

1 102

104

106

108

1010

1012

1014

1016

40 60 100 200 400

8 ) 10

× 0 - 10 % (

6 )

× 10 10 - 20 % (

4 )

× 10 20 - 30 % (

2 )

× 10 30 - 40 % (

0 ) 10

× 40 - 50 % (

-2 )

× 10 50 - 60 % (

-4 )

× 10 60 - 70 % (

-6 )

× 10 70 - 80 % (

Preliminary ATLAS

= 2.76 TeV sNN

= 0.4 jets,

tR k anti-

= 0.14 nb-1

Lint

2011 Pb+Pb data, = 4.0 pb-1

Lint

data, pp 2013

| < 2.1 y

|

Pb+Pb jet yields – centrality evolution

Fixed rapidity |y|<2.1 Pb+Pb jet yield/T_AA

Dashed lines – σ_pp reference

Centrality-dependent jet suppression

ATLAS-CONF-2014-025

Jet spectra measured as a function of centrality

over the kinematic range:

32 < p_T < 500 GeV, |y| < 2.1

Jet R _AA vs. p _T

three rapidity intervals three centrality classes

Large (~2x suppression) in the most central events Weak but significant rise with p_T, in the more

central bins

AAR

0 0.5 1

| < 2.1 y

|

Preliminary

ATLAS anti-k_t R = 0.4 jets = 2.76 TeV sNN

= 0.14 nb-1

Lint

2011 Pb+Pb data,

= 4.0 pb-1

Lint

data, pp 2013

AAR

0 0.5 1

| < 0.8 y 0.3 < |

0 - 10 % 30 - 40 % 60 - 80 %

[GeV]

pT

AAR

0 0.5 1

40 60 100 200 400

40 60 100 200 400

40 60 100 200 400

| < 2.1 y 1.2 < |

0 - 10 % 30 - 40 % 60 - 80 %

ATLAS-CONF-2014-025

Jet R _AA vs. rapidity

No significant rapidity dependence

AAR

0 0.5 1

< 80 GeV pT

63 <

Preliminary ATLAS

= 0.4 jets

tR k anti-

= 2.76 TeV

sNN 0 - 10 % 30 - 40 % 60 - 80 %

< 100 GeV pT

80 <

| y

|

0 0.5 1 1.5 2

AAR

0 0.5 1

< 126 GeV pT

100 <

= 0.14 nb-1

Lint

2011 Pb+Pb data, = 4.0 pb-1

Lint

data, pp 2013

| y

|

0 0.5 1 1.5 2

< 158 GeV pT

126 <

ATLAS-CONF-2014-025

Jet R _AA vs. N _part

Smooth evolution with centrality

Maximal suppression of 0.4 in 0-1% most central collisions

AAR

0 0.5 1

< 80 GeV pT

63 < | y | < 2.1

Preliminary ATLAS

= 0.4 jets

tR k anti-

= 2.76 TeV sNN

< 100 GeV pT

80 < | y | < 2.1

part 〉

〈 N

0 50 100 150 200 250 300 350 400

AAR

0 0.5 1

< 126 GeV pT

100 < | y | < 2.1

= 0.14 nb-1

Lint

2011 Pb+Pb data, = 4.0 pb-1

Lint

data, pp 2013

part 〉

〈 N

0 50 100 150 200 250 300 350 400

< 158 GeV pT

126 < | y | < 2.1

ATLAS-CONF-2014-025

Fragmentation of quenched jets

A significant modification of fragmentation seen in central collisions:

•  enhancement in fragment yields for z < 0.04

•  reduction for 0.04 < z < 0.25

•  enhancement for z > 0.25.

R p cos

z p_jet

T had

T Δ

≡

z

10-1 1

D(z)R

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

0-10%/60-80%

>100 GeV

jet

pT

R=0.4 anti-kT

ATLAS

=2.76 TeV sNN

Pb+Pb 0.14 nb-1

z

10-1 1

D(z)R

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

30-40%/60-80%

ATLAS

=2.76 TeV sNN

Pb+Pb 0.14 nb-1

z

10-1 1

D(z)R

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Data

Systematic Uncertainty 10-20%/60-80%

ATLAS

=2.76 TeV sNN

Pb+Pb 0.14 nb-1

z

10-1 1

D(z)R

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

40-50%/60-80%

ATLAS

=2.76 TeV sNN

Pb+Pb 0.14 nb-1

z

10-1 1

D(z)R

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

20-30%/60-80%

ATLAS

=2.76 TeV sNN

Pb+Pb 0.14 nb-1

z

10-1 1

D(z)R

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

50-60%/60-80%

ATLAS

=2.76 TeV sNN

Pb+Pb 0.14 nb-1

p_T^had>2 GeV

ATLAS:arXiv:1406.2979

R_D(z)=D(z)^cent/D(z)^periph

Jet measurements in p+Pb

Jets measured over a wide kinematic range:

50 < p_T <1000 GeV -2.1 < y* < 2.8

[GeV]

p ]-1 *) [GeVyd TpN/d2 )(d evt(1/N

10-12

10-10

10-8

10-6

10-4

10-2

1 102

104

106

108 ₁

+3.6 < y* < +4.4, × 101

+2.8 < y* < +3.6, × 102

× +2.1 < y* < +2.8,

103

+1.2 < y* < +2.1, × 104

+0.8 < y* < +1.2, ×

105

+0.3 < y* < +0.8, × 106

-0.3 < y* < +0.3, × 107

× -0.8 < y* < -0.3,

108

-1.2 < y* < -0.8, × 109

-2.1 < y* < -1.2, ×

= 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

∫Ldt = 27.8 nb^-1 0-90%

ATLAS

Preliminary

20 100 1000

ATLAS-CONF-2014-024

y* rapidity in the CMS frame:

y* > 0 – p-going direction y* < 0 – Pb-going direction

Inclusive jet R _pPb

2013 p+p data set

interpolated to 5.02 TeV Different y* ranges

0-90% centrality range

No suppression is seen at all rapidities

Results consistent with pQCD/EPS09 calculations

[GeV]

pT

pPb R

+2.1 < y* < +2.8

ATLAS Preliminary

[GeV]

T p

pPbR

+0.8 < y* < +1.2

[GeV]

pT

pPbR

-0.3 < y* < +0.3

[GeV]

T p

pPbR

-1.2 < y* < -0.8

= 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

[GeV]

pT

pPbR

+1.2 < y* < +2.1

+Pb, 0-90%

p

EPS09 calculation

[GeV]

T p

pPbR

+0.3 < y* < +0.8

[GeV]

pT

pPbR

-0.8 < y* < -0.3

[GeV]

T p

pPbR

-2.1 < y* < -1.2

∫L^pPbdt = 27.8 nb^-1

∫L^ppdt = 4.0 pb^-1

1.6

1.0 0.4

1.6

1.0 0.4

1.6

1.0

0.4

1.6 1.0

0.4

pPbR

40 100 1000 40 100 1000

[GeV]

pT ATLAS-CONF-2014-024

Centrality dependence of R _pPb

Three different centrality ranges

Jets are enhanced in peripheral collisions and suppressed in central collisions at forward rapidities

At low p_T and y* < 0, R_pPb≈1 What is the origin of this rapidity dependence??

[GeV]

pT pPb R +2.1 < y* < +2.8

[GeV]

T p pPb R +0.8 < y* < +1.2

[GeV]

pT pPb R -0.3 < y* < +0.3

ATLAS

Preliminary

pPb R -1.2 < y* < -0.8

= 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

[GeV]

pT pPb R +1.2 < y* < +2.1

0-10%

20-30%

60-90%

[GeV]

T p pPb R +0.3 < y* < +0.8

[GeV]

pT pPb R -0.8 < y* < -0.3

pPb R -2.1 < y* < -1.2

∫L^pPbdt = 27.8 nb^-1

∫L^ppdt = 4.0 pb^-1

1.6

1.0 0.4

1.6

1.0 0.4

1.6

1.0

0.4

1.6 1.0

0.4

pPbR

40 100 1000 40 100 1000

ATLAS-CONF-2014-024

p+Pb: Rapidity dependence of R

_CP

Ratio of yields in central to peripheral collisions each normalized by <T_pPb>_cent

Increasing suppression

at more forward y* and high p_T Unexpected centrality and y*

dependence of the relative suppression

[GeV]

pT CP R +3.6 < y* < +4.4

ATLAS

Preliminary

[GeV]

T

p

CP R +2.1 < y* < +2.8

[GeV]

T

p

CP R +0.8 < y* < +1.2

[GeV]

pT CP R -0.3 < y* < +0.3

[GeV]

T

p

CP R -1.2 < y* < -0.8

∫Ldt = 27.8 nb^-1 = 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

[GeV]

pT CP R +2.8 < y* < +3.6

[GeV]

T

p

CP R +1.2 < y* < +2.1

[GeV]

T

p

CP R +0.3 < y* < +0.8

[GeV]

pT CP R -0.8 < y* < -0.3

[GeV]

T

p

CP R -2.1 < y* < -1.2

0-10%/60-90%

20-30%/60-90%

40-60%/60-90%

1.4

1.0

0.4

1.4

1.0

0.4

1.4 1.0

0.4

1.4 1.0

0.4

1.4 1.0

0.4

CPR

20 100 800 20 100 800

[GeV]

pT

ATLAS-CONF-2014-024

p+Pb: Scaling of R _CP vs. p

Replot the R

_CP

versus p = p

_T

cosh(y*)

Forward y*>0 Backward y*< 0

R_CP(p) follows a single trend at y*>0, but not at y*<0

A correlation of the jet kinematics with a centrality variable?

∫ Ldt = 27.8 nb^-1 = 5.02 TeV sNN

+Pb p

0-10%

=0.4 R

t, k anti-

ATLAS

Preliminary

+3.6 < y* < +4.4 +2.8 < y* < +3.6 +2.1 < y* < +2.8 +1.2 < y* < +2.1 +0.8 < y* < +1.2

+0.3 < y* < +0.8 -0.3 < y* < +0.3 -0.8 < y* < -0.3 -1.2 < y* < -0.8 -2.1 < y* < -1.2

1.4

1.0

0.4

CPR

40 100 1000 40 100 1000

cosh(<y*>) [GeV]

T × ATLAS-CONF-2014-024 p

Summary

•  ATLAS results on jet studies in Pb+Pb and p+Pb collisions provide valuable

constraints on the energy loss models

•  A strong jet suppression is observed in Pb+Pb collisions, as expected due to the parton energy loss in the medium

•  p+Pb jets show unexpected behaviour

–  Inclusive jets are not suppressed

–  Non-trivial dependence on centrality and rapidity –  Very different of what is seen in Pb+Pb

Backup slides

Pb+Pb and p+Pb centrality

Measured using forward calorimeters For T_AA, N_coll, N_part:

Pb+Pb – usual Glauber MC

p+Pb – Glauber MC and Glauber-Gribov colour-fluctuation model

[ nb/GeV ] yd Tpd

jetN2 d 〉 AAT〈 evtN1

10-5

10-4

10-3

10-2

10-1

1 10 102

103

104

105

106

107

108

109

1010

1011

[GeV]

pT

[ nb/GeV ] yd Tpd

jetN2 d 〉 AAT〈 evtN1

10-5

10-4

10-3

10-2

10-1

1 10 102

103

104

105

106

107

108

109

1010

1011

40 60 100 200 400

0 - 10 %

6 )

× 10

| < 0.3 ( y

|

4 )

× 10 | < 0.8 ( y

≤ | 0.3

2 )

× 10 | < 1.2 ( y

≤ | 0.8

0 ) 10

× | < 2.1 ( y

|

≤ 1.2

| < 2.1 y

|

6 ) 10

× 0 - 10 % (

4 )

× 10 20 - 30 % (

2 ) 10

× 40 - 50 % (

0 )

× 10 60 - 80 % (

Preliminary ATLAS

= 2.76 TeV sNN

= 0.4 jets

tR k anti-

= 0.14 nb-1

Lint

2011 Pb+Pb data, = 4.0 pb-1

Lint

data, pp 2013

Jet spectra: p+p and Pb+Pb

Jet spectra measured as a function of centrality

over the kinematic range: 32 < p_T < 500 GeV, |y| < 2.1

p+p jet cross section

Pb+Pb jet T_AA-normalized yield

Pb+Pb points fall below p+p Rapidity dependence – fixed Pb+Pb centrality

[GeV]

p [ nb/GeV ] yd Tpdσ2 d

10-5

10-4

10-3

10-2

10-1

1 10 102

103

104

105

106

107

108

109

1010

1011

40 60 100 200 400

8 )

× 10

| < 2.1 ( y

|

6 )

× 10

| < 0.3 ( y

|

4 ) 10

× | < 0.8 ( y |

≤ 0.3

2 )

× 10 | < 1.2 ( y

≤ | 0.8

0 )

× 10 | < 2.1 ( y

≤ | 1.2

Preliminary ATLAS

= 2.76 TeV s

=0.4,

tR k anti-

= 4.0 pb-1

Lint

data, pp 2013

Correlated nearby jets in Pb+Pb

•  1^st step to study the internal structure of parton showers

•  Conditional yield of nearby jets is

suppressed

ATLAS

=0.14 nb-1

Lint

Preliminary

= 2.76 TeV sNN

Pb+Pb 2011

d=0.2 kT

anti-

<1.6

∆R 0.5<

>80 GeV

test

ET

ATLAS

=0.14 nb-1

Lint

Preliminary

= 2.76 TeV sNN

Pb+Pb 2011

d=0.2 kT

anti-

<1.6

∆R 0.5<

>80 GeV

test

ET

[GeV]

nbr

ET

40 60 80 100 120

]-1 [GeVnbr TE/dR ∆Rd ^-4₁₀

10-3 ATLAS

=0.14 nb-1

Lint

Preliminary

= 2.76 TeV sNN

Pb+Pb 2011

d=0.2 kT

anti-

<1.6

∆R 0.5<

>80 GeV

test

ET

ATLASPreliminary

d=0.2 kT

anti-

<1.6

∆R 0.5<

>90 GeV

test

ET

ATLASPreliminary

d=0.2 kT

anti-

<1.6

∆R 0.5<

>90 GeV

test

ET

[GeV]

nbr

ET

40 60 80 100 120

]-1 [GeVnbr TE/dR ∆Rd _-4

10 10-3

ATLASPreliminary

d=0.2 kT

anti-

<1.6

∆R 0.5<

>90 GeV

test

ET

ATLASPreliminary

d=0.2 kT

anti-

<1.6

∆R 0.5<

>110 GeV

test

ET

0-10%

10-20%

20-40%

40-80%

ATLASPreliminary

d=0.2 kT

anti-

<1.6

∆R 0.5<

>110 GeV

test

ET

0-10%

10-20%

20-40%

40-80%

[GeV]

nbr

ET

40 60 80 100 120

]-1 [GeVnbr TE/dR ∆Rd

10-4

10-3

ATLASPreliminary

d=0.2 kT

anti-

<1.6

∆R 0.5<

>110 GeV

test

ET

0-10%

10-20%

20-40%

40-80%

ATLAS-CONF-2014-028

Charged hadron suppression in Pb+Pb

For central collisions:

•  A pronounced minimum at p_T=6-7 GeV where R_CP≈0.2

•  At higher p_T R_CP rises

•  Suppression at high p_T at the same level as jet suppression

ATLAS-CONF-2012-120

Charged hadron suppression in p+Pb

[GeV]

pT

10 10²

pPbR

0.5 1 1.5 2

60 - 90 % Preliminary

ATLAS

=25 nb-1

p+Pb Lint

=5.02 TeV sNN

-2<y*<1.5

[GeV]

pT

10 10²

pPbR

0.5 1 1.5 2

20 - 30 %

[GeV]

pT

10 10²

pPbR

0.5 1 1.5 2

0 - 10 %

[GeV]

pT

10 10²

pPbR

0.8 1 1.2 1.4 1.6 1.8 2 2.2

Preliminary ATLAS

=25 nb-1

p+Pb Lint

=5.02 TeV sNN

0-90%

-1<y*<1

ATLAS-CONF-2014-029

Jet R

_CP

comparison

[GeV]

pT

CPR

0 0.5 1

| < 2.1 y

| 0 - 10 %

b-1

= 7 µ Lint

sys. error, 2010 stat. ⊕

b-1

= 140 µ Lint

sys. error, 2011 stat. ⊕

[GeV]

pT

CPR

0 0.5 1

40 60 100 200 400 800

= 0.4 HI jets

t R k anti-

= 2.76 TeV sNN

30 - 40 %

Preliminary ATLAS

ATLAS Phys. Lett. B719 (2013)220 vs.

ATLAS-CONF-2014-025