Charged particle spectra and nuclear modification factor in lead-lead - - PowerPoint PPT Presentation

SLIDE 1

Charged particle spectra and nuclear modification factor in lead-lead collisions at √sNN = 2.76 TeV with the ATLAS detector at the LHC

Petr Balek

for the ATLAS collaboration

16 August 2012 IPNP, Charles University in Prague

Petr Balek 16 August 2012 1 / 19

SLIDE 2

introduction

why to study charged particle spectra?

◮ to understand properties of hot dense matter created in HI collisions ◮ to understand the mechanism of energy loss of partons

study ratio of central (C) and peripheral (P) spectra: RCP =

1 Ncoll(C)·Nevents,C d2NC/dηdpT 1 Ncoll(P)·Nevents,P d2NP/dηdpT

as a peripheral bin we take 60–80% previously measured RCP and spectra up to 30 GeV high luminosity collected by ATLAS in 2011 allow us to extend previous measurements

Petr Balek 16 August 2012 2 / 19

SLIDE 3

ATLAS detector – Inner detector

pT > 0.5 GeV |η| < 2.5 full φ coverage

Petr Balek 16 August 2012 3 / 19

SLIDE 4

determination of centrality

centrality based on energy deposited in Forward Calorimeter – 3.1 < |η| < 4.9 model – based on Glauber calculation convoluted with p+p data

Petr Balek 16 August 2012 4 / 19

SLIDE 5

event selection

used data 2010 and 2011: minimum bias 2010 – 7 µb−1, 50.7M events

◮ MBTS or ZDC

minimum bias 2011 – 7 µb−1, 50.7M events

◮ total energy > 50 GeV ◮ or signal from ZDC+track

hard probes 2011 – 0.14 nb−1, 998M sampled events

◮ unprescaled jet trigger; anti-kT, ET > 20 GeV Petr Balek 16 August 2012 5 / 19

SLIDE 6

track selection

each track is required to have: at least 2 Pixel hits, one in the innermost layer at least 7 SCT hits no Pixel or SCT holes

η

• 2
• 1

1 2

• n track

〉 Pixel hits 〈 3.0 3.5 4.0 4.5 Preliminary ATLAS Pb+Pb = 2.76 TeV

NN

s

• 1

b µ = 7

int

L

η

• 2
• 1

1 2

• n track

〉 SCT Hits 〈 7.0 7.5 8.0 8.5 9.0 9.5

η

• 2
• 1

1 2

• n track

〉 SCT Holes 〈 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 2010 Data 0-5% 2010 MC 0-5% 2010 Data 60-80% 2010 MC 60-80% Petr Balek 16 August 2012 6 / 19

SLIDE 7

track selection

[mm] d

• 4
• 3
• 2
• 1

1 2 3 4 [GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10

60-80% 2010-11MB a) 0-5% 2010-11MB b) 0-5% 2011HP c) 0-5% 2010-11MB ATLAS Pb+Pb = 2.76 TeV

NN

s

• 1

= 15nb

int

L d)

]

• 1

mm

• 1

)) [GeV d(d

T

/ (dp

track

Z axis: dN

• 1

10 1 10

2

10

3

10

4

10

5

10

6

10

7

10

8

10

9

10

Preliminary

Pixel measure d0 and z0 sin θ precisely due to high occupancy, SCT measure pT of some tracks incorrectly this caused populating area of high pT and high d0 significance cuts on d0 and z0 sin θ solve this problem

Petr Balek 16 August 2012 7 / 19

SLIDE 8

raw spectra

standard jets 0.6 0.8 1.0 1.2 standard reconstruction 1 10

2

10

3

10

[GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3

2

1 10

Preliminary ATLAS = 2.76 TeV

NN

s Pb+Pb

• 1

= 0.15 nb

int

Data 2010-11 L Centrality 0-5%

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3

HP jet-matched/HP HP jet-matched/MB HP jet-matched/MB (10-80%)

2

1 10

MB HP Centrality 60-80%

tracking cuts able to deal with signal/background up to 1/100 need matching to jets from ∼60 GeV for spectra and RCP, muon and electrons (and positrons) are subtracted – these are µ± and e± from W decay

Petr Balek 16 August 2012 8 / 19

SLIDE 9

fakes and secondaries correction

low-pT tracks are corrected for fakes and secondary particle based on PYTHIA events embedded into HIJING

Fraction

0.90 0.95 1.00

[GeV]

T

p 2 4 [GeV]

T

p 2 4 [GeV]

T

p 2 4

30-40% 10-20% 5-10% 0-5% |<1.00 η | ATLAS Preliminary =2.76TeV

NN

s Pb+Pb simulation |<1.75 η 1.50<| |<2.50 η 2.25<| Petr Balek 16 August 2012 9 / 19

SLIDE 10

efficiency correction

efficiency estimated with PYTHIA events embedded into HIJING (2010) or data (2011) evaluate efficiencies in 7 η bins and 9 centrality bins used bin-by-bin unfolding to correct spectra for momentum resolution we use iterative approach - once fully corrected spectra are obtained, MC spectrum is reweighed to follow data

Efficiency

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 [GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10

2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

|<1. η | 60-80% 30-40% 10-20% 0-5%

2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

|<1.75 η 1.5<|

2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

|<2.5 η 2.25<| ATLAS = 2.76 TeV

NN

s Pb+Pb simulation Preliminary Petr Balek 16 August 2012 10 / 19

SLIDE 11

systematic uncertainties

Source Spectra RCP Tracking cuts 4% 3% PV pointing cuts 1-4% 1-3% Truth association 1% 2% Efficiency 3-20% 5% Feed down 0-3% – Material budget 2-5% – Momentum scale 0-6% – Trigger efficiency 2% 1% Electroweak decays 1% 1% Ncoll ratios – 3.8-11.7% in uncertainty of efficiency is hidden all unfolding uncertainties

Petr Balek 16 August 2012 11 / 19

SLIDE 12

spectra

use minimum bias events for pT < 30 GeV and hard probes events for pT > 30 GeV

]

• 2

) [GeV

T

dp η /(d

ch

) dN

T

p π 1/(2

• 10

10

• 6

10

• 2

10

2

10 [GeV]

T

p

1 10

2

10

[GeV]

T

p

1 10

2

10

[GeV]

T

p

1 10

2

10

|<1.0 η |

ATLAS Preliminary =2.76TeV

NN

s Pb+Pb Data 2010 + 2011

• 1

= 0.15nb

int

L

|<2.0 η 1.0<| |<2.5 η 2.0<|

(0-5)% (30-40)% (50-60)% (60-80)% Petr Balek 16 August 2012 12 / 19

SLIDE 13

RCP

CP

R

1 0.2

[GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10

|<1.0 η |

ATLAS Preliminary =2.76TeV

NN

s Pb+Pb Data 2010 + 2011

• 1

= 0.15nb

int

L

|<2.0 η 1.0<| |<2.5 η 2.0<|

(50-60)% / (60-80)% (30-40)% / (60-80)% (0-5)% / (60-80)%

RCP has no η dependency

Petr Balek 16 August 2012 13 / 19

SLIDE 14

RCP

CP

R

1 0.2

[GeV]

T

p 1 10

2

10

|<2.5 η |

ATLAS Preliminary =2.76TeV

NN

s Pb+Pb Data 2010 + 2011

• 1

= 0.15nb

int

L (50-60)% / (60-80)% (30-40)% / (60-80)% (0-5)% / (60-80)%

Petr Balek 16 August 2012 14 / 19

SLIDE 15

RCP

CP

R

1 0.2

[GeV]

T

p 1 10

2

10 |<2.5 η |

ATLAS Preliminary =2.76TeV

NN

s Pb+Pb Data 2010 + 2011

• 1

= 0.15nb

int

L (50-60)% / (60-80)% (30-40)% / (60-80)% (0-5)% / (60-80)%

[GeV]

T

p

40 60 80 100 120 140 160 180 200 0.5 1 0 - 10 % 0.5 1 10 - 20 % 0.5 1 30 - 40 % CP

R

0.5 1 1.5 2 50 - 60 % = 2.76 TeV

NN

s Pb+Pb

• 1

b µ = 7 L dt

∫

ATLAS

= 0.2 R

t

k anti-

compatible with jet measurement

Petr Balek 16 August 2012 15 / 19

SLIDE 16

RCP

CP

R

1 0.2

[GeV]

T

p 1 10

2

10

|<2.5 η |

ATLAS Preliminary =2.76TeV

NN

s Pb+Pb Data 2010 + 2011

• 1

= 0.15nb

int

L (50-60)% / (60-80)% (30-40)% / (60-80)% (0-5)% / (60-80)%

0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1 1.2

(GeV/c)

T

p

1 2 3 4 5 67 10 20 30 100

0-5% / 50-90%

CMS Collaboration, Eur. Phys. J. C72 (2012) 1945 CP

R

0.2 0.4 0.6 0.8 1

CMS has different peripheral bin ATLAS used lower threshold for jet trigger → better statistics at high pT

Petr Balek 16 August 2012 16 / 19

SLIDE 17

conclusion

measured spectra and RCP within |η| < 2.5 and 0.5 GeV < pT < 150 GeV minimum seen at around 7 GeV; above show rise up to high pT very good statistics up to ∼70 GeV; higher pT limited by statistics of peripheral bin compatible with level of suppression measured in jets

Petr Balek 16 August 2012 17 / 19

SLIDE 18

backup

Petr Balek 16 August 2012 18 / 19

SLIDE 19

raw spectra

]

• 1

[GeV

T

/dp

track

dN

events

1/N

• 8

10

• 4

10 1

2

10

[GeV]

T

p 1 10

2

10 [GeV]

T

p 1 10

2

10

Preliminary ATLAS = 2.76 TeV

NN

s Pb+Pb Data 2010-11

• 1

= 0.15 nb

int

L Centrality 0-5% MB reconstruction HP reconstruction MB standard HP standard HP standard, in jets

±

µ HP standard, Centrality 60-80%

good agreement between MB and HP at high pT

Petr Balek 16 August 2012 19 / 19