1. for the ALICE Collaboration
RAA in ALICE
Michael Linus Knichel
for the ALICE Collaboration
High-pT Probes of High-Density QCD at the LHC, École Polytechnique, Palaiseau
May 31st, 2011
Michael L. Knichel
HPHD 1

2. Content ALICE detector (subsystems used for the RAA analysis)
Tracking, high pT capabilities
pp measurements – reference for RAA
Pb-Pb spectra
RAA for charged particles and Centrality dependence
Comparison to CMS/RHIC
Comparison to models
RAA vs. Reaction plane angle ϕ
Particle Identification at high pT via dE/dx in the TPC
Charged pion spectra and RAA
RAA of Λ and K0s
Summary
Michael L. Knichel
HPHD 2

3. Motivation
• Heavy Ion Physics ↔ Study of nuclear matter at extreme conditions
• RHIC : “The production of charged hadrons at high pT is suppressed in central Au-Au collision compared to the superposition of nucleon-nucleon collisions”
• Dominant production mechanism for high pT charged hadrons
– Fragmentation of high pT partons from hard scatterings
at an early stage of nuclear collisions
• Suppression is probably due to energy loss of partons,
traversing a hot, dense QCD medium → the Quark-Gluon-Plasma (QGP)
• Understanding the suppression goes towards understanding
the medium properties of the QGP
→ What happens at LHC energies ?
Michael L. Knichel
HPHD 3

4. Introduction
ALICE Collaboration, Phys. Lett. B696 (2011)
Supression of high pT particles in Pb-Pb collisions at LHC was one of the first ALICE results
Showing strongest suppression in central collisions at pT around 7 GeV
RAA increases for pT above 7 GeV/c
σINELPP = 64 mb
Nuclear overlap functionTAA from Glauber model
Michael L. Knichel
HPHD 4

5. ALICE detector Central Barrel Detectors:
Inner Tracking System (ITS) 6 layers of Silicon detectors:
Pixel: 2 layers (tracking, vertex, trigger)
Each 2 layers of Drift and Strip (tracking, PID via dE/dx)
Forward/Trigger detector:
V0 scintillator hodoscopes: Trigger, centrality determination
ZDC scintillating fibre calorimeter (+-116 m from IP): centrality, timing
Michael L. Knichel
HPHD 5

6. ALICE detector EMCal Pt resolution:
In the 0.5 T magnetic field a track with 100 GeV/c is almost straight
The deviation from a straight line is only 5 mm over the track length of 2.5 m
Very high space point precision needed, achieved with many measured points
PHOS
Central Barrel Detectors:
Time Projection Chamber (TPC): tracking, PID
Time of Flight detector (TOF): PID
Transition radiation detector (TRD): e/π separation, possibility to improve pT resolution at high pT
Michael L. Knichel
HPHD 6

7. Event and Track selection
Use of combined tracking Information from TPC and ITS (Silicon detectors)
Reconstructed primary vertex within 10 cm of nominal position
Centrality quality cuts (for Pb-Pb)
Select long tracks in the TPC, two hits in the ITS, one in the Pixels
Tracks must point to the primary vertex (7σ)
Tracks must pass some additional quality cuts
All Events taken with Minimum Bias Trigger
Total Numbers: ~15M Events in 2.76 ATeV Pb-Pb (2010)
~65M Events in 2.76 TeV pp (2011)
(after offline trigger and event selection)
Michael L. Knichel
HPHD 7

8. Transverse momentum resolution
Analysis with combined tracking information from ITS and TPC
pT resolution looks very similiar in pp and Pb-Pb
σ(pT)/pT ~ 20% at 100 GeV/c
Resolution estimated from covariance matrix of the Kalman Filter algorithm used for tracking
Veryfied with invariant mass analysis of K0s -> π+ π - decays up to 12 GeV/c
Systematic uncertaintiy on the resolution ±20% (estimated from K0 decay, relative uncertainty constant with pT)
Factor 2 improvement expected soon (better alignment/calibration)
Further improvement with TRD
Michael L. Knichel
HPHD 8

9. Proton-Proton charged particle spectra
Corrections from Simulations
Differences between data and MC: main source of systematics
Pb-Pb very similiar to 7 TeV, 900 GeV
2.76 TeV data sample analyzed was taken without SDD (Drift) detector
Lower efficiency, larger systematics for the 2.76 TeV data
Small differences pos/neg
Contamination of secondary tracks form weak decays of K0s and Λ derived from data
Track cuts remove most secondaries
Remaining second. <1% above 4 GeV
Michael L. Knichel
HPHD 9

10. Proton-Proton charged particle spectra
pT spectra at √s = 0.9, 2.76, 7 TeV
7 TeV: up to 100 GeV/c
2.76 TeV: up to 35 GeV/c
For pT > 50 GeV/c: systematics from pt resolution dominant
For 2.76 TeV uncertainties ~10% + 3% (normalization)
Michael L. Knichel
HPHD 10

11. Proton-Proton 2.76 TeV
Sufficient statistics only up to ~35 GeV/c in pT
Phythia8 describes the data very well above 1GeV/c
As reference for RAA we are interested in an extended pT range of up to 100 GeV/c
Michael L. Knichel
HPHD 11

12. A Reference for Pb-Pb
For RAA we use the measured yields bin-by-bin up to 5 GeV/c, above 5 GeV/c we fit a modified Hagedorn function to the data
For 5-35 GeV we use this fit as a reference to avoid fluctuations in the reference
Above 35 GeV we extrapolate using the fit
Systematic uncertatinties includes using different function (powelaw) and fit ranges
Good agreement with Pythia8 and alternative references
CMS: arXiv: v1
Michael L. Knichel
HPHD 12

13. Centrality
VZERO Amplitude used as centrality estimator, SPD for systematics
outliers in the multiplicity correlations, of SPD vs V0, TPC vs V0, ZDC vs V0
σ = 64 mb
A. Dainese
Michael L. Knichel
HPHD 13

14. Spectra in Pb-Pb
Text
Charged particle spectra measured up to 100 GeV/c versus collision centrality
Visible change of the pT shape from central to peripheral collisions
Michael L. Knichel
HPHD 14

15. RAA compared to published
new analysis:
new pp reference based on 2.76 TeeV pp data
all Pb-Pb statistics
in publication:
pp reference (interpolation between AliCE data TeV using power law)
20% of Pb-Pb statistics
Michael L. Knichel
HPHD 15

16. RAA: Comparison to RHIC
Similar shape
Stronger suppresion at LHC
Indicates denser medium
rise above 7 GeV measured with ALICE
Michael L. Knichel
HPHD 16

17. RAA vs. Centrality Charged particles: • RAA shown in pT range
(0.15 – 100 GeV/c)
• Different suppression
pattern depending on
collision centrality
• RAA has minimum at pT
= 6-7 GeV/c in all
Centralities
• Hint of leveling off
above 50 GeV/c
Michael L. Knichel
HPHD 17

18. RAA in bins of pT low pT: approximate scaling
with multiplicity density,
matching also RHIC
results
Michael L. Knichel
HPHD 18

19. RAA in bins of pT high pT: weak suppression, no significant centrality
dependence
low pT:
approximate scaling
with multiplicity density,
matching also RHIC
results
Michael L. Knichel
HPHD 19

20. RAA: Comparison to CMS
QM2011 talk by Andre Yoon
Agreement up to 50 GeV/c within the uncertainties
Different systematic uncertainties above 50 GeV/c
Some tension at highest pt, but agreement within the (presently large) systematic uncertainties
Comparison of Pb-Pb spectra CMS/ALICE ongoing
Michael L. Knichel
HPHD 20

21. RAA: Comparison to models
Bass et al, PRC79,
pronounced pT dependence
of RAA at LHC
sensitivity to details of the
energy loss distribution
Michael L. Knichel
HPHD 21

22. RAA vs. Reaction plane angle
Can be build from RAA and v2
Different path length in the medium: in plane vs. out of plane
Michael L. Knichel
HPHD 22

23. Particle Identification via dE/dx
almost constant separation of pions to K, p above p = 3 GeV/c
Track-by-track PID possible in the region, where the bands are clearly separated (low momentum)
At high momentum, the differences of energy loss in the relativistc rise can be used for statistical PID
Michael L. Knichel
HPHD 23

24. Particle Identification in the relativistic rise
from TPC dE/dx
Michael L. Knichel
HPHD 24

25. Charged pion fraction
Meaure the ratio pions/charged as a function of pt
(both in pp and Pb-Pb)
Michael L. Knichel
HPHD 25

26. Charged pion spectra
Multiply by the charged particle spectra to get pion spectra for pp and Pb-Pb
Michael L. Knichel
HPHD 26

27. Charged pion RAA Charged pion RAA up to 20 GeV/c in pT
agrees with charged particle RAA
- in peripheral events
- for pT > 6 GeV/c
is smaller than charged particle RAA for pT < 6 GeV/c
Michael L. Knichel
HPHD 27

28. Measurement of Λ and K0s Recontruction via weak decays K0S  π+ π -
Λ  π - p
Invariant mass analysis
Extraction of yields
Michael L. Knichel
HPHD 28

29. RCP of Λ and K0s K0s - RAA very similar to that of charged particles:
strong suppression of K0s at
high pT
Λ - RAA significantly larger than
charged at intermediate pT:
enhanced hyperon production
counteracting suppression
for pT > 8 GeV/c, Λ and K0s - RAA
similar to charged particle RAA:
strong high-pT suppression also
of Λ
Michael L. Knichel
HPHD 29

30. RCP of Λ and K0s K0s - RAA very similar to that of charged particles:
strong suppression of K0s at
high pT
Λ - RAA significantly larger than
charged at intermediate pT:
enhanced hyperon production
counteracting suppression
above 8 GeV/c, Λ and K0s - RAA
are similar to charged:
strong high-pT suppression also
of Λ
Λ enhancement reaches out
to higher pT than at RHIC
magnitude of RCP compatible
Michael L. Knichel
HPHD 30

31. RAA of Λ and K0s
high-pT: suppression of Λ and K0s similar to charged particles
intermediate pT: moderate enhancement in peripheral collisions
 central RAA similar to RCP
Michael L. Knichel
HPHD 31

32. RAA of Λ and K0s
strong nuclear enhancement of Λ observed at RHIC not seen at the LHC
Michael L. Knichel
HPHD 32

33. All ALICE RAA
Michael L. Knichel
HPHD 33

34. Summary
charged particle pT spectra in Pb-Pb at √sNN = 2.76 TeV measured
with ALICE at the LHC
new reference pT spectrum derived from pp collisions at √s = 2.76 TeV
strong suppression charged particle production observed at pT < 50 GeV/c
comparison to RHIC data suggests that suppression scales with the
charged particle density
at pT > 50 GeV/c, no strong centrality dependence of charged particle
production is observed
results on identified particles will allow to disentangle the interplay
between quark and gluon energy loss, and recombination mechanisms
at intermediate pT
Michael L. Knichel
HPHD 34