1. Heavy Ion Physics with the ATLAS Detector
Pavel Nevski
Brookhaven National Laboratory
On behalf of the ATLAS Collaboration
Pavel Nevski, BNL
QM2005

2. From RHIC to LHC Super-hot QCD: Will we see a weakly coupled QGP??
sNN : 200 GeV ,500 GeV
Super-hot QCD: Will we see a weakly coupled QGP??
Initial state fully saturated (CGC)
Enormous increase of high-pT processes over RHIC
Plenty of heavy quarks (b,c)
Weakly interacting probes become available (Z0, W)
LHC
RHIC
SPS
Pavel Nevski, BNL
QM2005

3. ATLAS as a Heavy Ion Detector is:
An Excellent Calorimetry
A large acceptance Inner Tracker
A hermetic Muon Spectrometer
Pavel Nevski, BNL
QM2005

4. ATLAS as a Heavy Ion Detector
1. Excellent Calorimetry
Hermetic coverage up to || < 4.9
High granularity (.025x.025 electromagnetic, .1x.1 hadronic) with fine longitudinal segmentation (~7 sections)
Very good jet energy resolution (50%/sqrt(E) in pp)
High pT probes (jets, jet shapes, jet correlations, 0)
Large Acceptance Muon Spectrometer
Coverage up to || < 2.7
Muons from , J/, Z0 decays
Inner Detector (Si Pixels and Strips, no TRT used)
Large coverage up to || < 2.5
High granularity pixel and strip detectors (o~1%,10%)
Good momentum resolution (dpT/pT~3% up to 15GeV)
Tracking particles with pT  0.5 GeV/c
1.& 3.
Global event characterization (dNch/dη, dET/dη, flow);
Jet quenching study
2.+ 3.
Heavy quarks(b), quarkonium suppression(J/ ,)
Pavel Nevski, BNL
QM2005

5. And it is coming SOON !
Pavel Nevski, BNL
QM2005

6. Studies of the Detector Performance
Constraint: No modifications to the detector, except for trigger
and probably very forward region
Simulations: HIJING event generator, dNch/dη = 3200
Full GEANT simulations of the detector response
Large event samples:
|η|< impact parameter range: b = fm (27,000 events)
|η|< impact parameter range: b = fm (5,000 events)
Pavel Nevski, BNL
QM2005

7. Predicted Detector Occupancies
b = 0 – 1fm
Calorimeters ( |η|< 3.2 )
Si detectors:
Pixels < 2%
SCT < 20%
TRT:
- High occupancy for tracking
- Still visible TR signal for electrons
-> Limited usage for AA
collisions is under investigation
Will be fully useful for pA
Muon Chambers:
0.3 – 0.9 hits/chamber
(<< pp at 1034 cm-2 s-1)
Average ET
(uncalibrated):
~ 2 GeV/Tower
~ .3 GeV/Tower
Pavel Nevski, BNL
QM2005

8. Day One Physics with ATLAS
Pavel Nevski, BNL
QM2005

9. Global Event Characterization
Day-one measurements: Nch, dNch/d, b, ET, dET/d
Single Pb+Pb event, b =0-1fm
Nch(|| < 3)
Histogram – true Nch
Points – reconstructed Nch
dNch/d|=0 3200
(HIJING, no quenching)
10% <3%
No track reconstruction, only Nhits calibrated with pp
Pavel Nevski, BNL
QM2005

10. Correlation of Signals with Flow
Data Type
V2 (R) 
Hit clusters, Pixel layer 1
0.042
Hit clusters, Pixel layer 2
0.036
Hit clusters, Pixel layer 3
0.032
EM Barrel Calo
0.029
EM EndCap Calo
0.031
EM FCAL Calo
HAD FCAL Calo
0.025
V2=0.042
Same properties of Φ production assumed in my simulations.
Some properties of Φ generator used may slightly differ: shape of rapidity (or pseudorapidity?) distribution, vertex Z distribution! (strongly affects final rates)
Used latest magnetic field and PR01 geometry (fdor AuAu studies).
-R
v2Truth = 0.05
Distribution of azimuthal angle  (v2) vs true reaction plane position, R
Pavel Nevski, BNL
QM2005

11. Jet Physics
Pavel Nevski, BNL
QM2005

12. For a 106s run with Pb+Pb at L=41026 cm-2 s-1
Jet Rates
For a 106s run with Pb+Pb at L=41026 cm-2 s-1
we expect in |η| < 2.5:
ET threshold
Njets
50 GeV
30  106
100 GeV
1.5  105
150 GeV
1.9  105
200 GeV
4.4  104
And also: ~106  + jet events
~500 Z0() + jets with ET > 40 GeV
Pavel Nevski, BNL
QM2005

13. Jet reconstruction efficiency
Pb-Pb collisions (b= 0 –1 fm)
Angular resolution
for 70 GeV jets
Efficiency
Fake rate
~2  resolution in pp
Pb-Pb
Energy resolution
p-p
Two jet finder algorithms testet up to now - Sliding Window and Cone Fit
For ET > 75GeV: efficiency > 95%, fake < 5%
Good energy and angular resolution
Pavel Nevski, BNL
QM2005

14. Jet Quenching Studies
To determine medium properties we need to measure jet shapes
3 methods explored so far:
Fragmentation function using tracking
Core ET and jet profile using calorimeters
Neutral leading hadrons using EM calorimeters
vacuum
in medium
Most of energy is radiated inside a narrow cone
(Salgado &
Wiedemann
hep-ph/ )
Quenching may depend on quark flavor:
- Tagging of b-jets using impact parameter
Pavel Nevski, BNL
QM2005

15. Jet Studies with Tracks dN/djT broader in PbPb than in pp
Jets with ET = 100 GeV
Cone radius of 0.4
Track pT > 3 GeV
Momentum component
perpendicular to jet axis
Fragmentation function
dN/djT broader in PbPb than in pp
(background fluctuations)
PbPb  HIJING-unquenched  pp
Pavel Nevski, BNL
QM2005

16. Jet ETcore Measurements
Energy deposited in a narrow cone around the jet axis:
R < 0.11 (HADCal), R < 0.07 (EMCal) pp
PbPb
<ETcore> sensitive to
~10% change in ETjet
More work is needed to minimize
effect of background fluctuations.
The background
has not been subtracted:
<ETcore>PbPb  <ETcore>pp
Pavel Nevski, BNL
QM2005

17. Isolated Neutral Hadrons
Select jets that have an isolated neutral cluster along the axis
(~1% of the total jet sample)
Jets with ET >75GeV embedded
into central PbPb events: pp
Gauss =4.5%
Relative error on the measurement
of the neutral cluster ET.
Cluster ET sensitive to small change in ETjet
Pavel Nevski, BNL
QM2005

18. b-quark Jet Tagging
Motivation: Heavy quarks may radiate less energy in the dense
medium (dead-cone effect) than light quarks.
b-tagging capabilities offer additional tool to understand quenching.
To evaluate b-tagging performance:
ppWHlbb events overlayed
on HIJING background have been used.
A displaced vertex in the Inner Detector
has been searched for.
Rejection factor against u-jets ~ 100
for b-tagging efficiency of 25%
Should be improved by optimized algorithms
and with soft muon tagging in the Muon Spec.
Pavel Nevski, BNL
QM2005

19. Physics with
Muon Spectrometer
Pavel Nevski, BNL
QM2005

20. +- reconstruction
  +–
Muons momenta measured by ID tracks
tagged by coincidence with track segment in -spectrometer
pT >3 GeV
||  1
||  2
||  2.5
Acceptance + efficiency
4.7%
12.5%
17.5%
Resolution
123 MeV
145 MeV
159 MeV
S/B
0.3
0.2
S/√ S+B 37 46 55
Rate/month
15,000
||  2
For |η| < 2 (12.5% acc+eff) we expect
15K /month of 106s at L=41026 cm-2 s-1
Pavel Nevski, BNL
QM2005

21. J/+- reconstruction
||  2.5, pT  1.5 GeV
||  2.5
pT > 3 GeV
pT > 1.5 GeV
Acceptance + efficiency
0.055%
0.530%
Resolution
68 MeV
S/B
0.4
0.15
S/√ S+B 56
113
Rate/month
11,000
104,000
We expect 8K to 100K J/+-
per month of 106s at L=41026 cm-2 s-1
If a trigger is possible forward with a muon pT >1.5 GeV, we gain a factor 4 in statistics…A solution might be to reduce the toroidal field for HI runs
Pavel Nevski, BNL
QM2005

22. Other issues
Pavel Nevski, BNL
QM2005

23. Ultra-Peripheral Nuclear Collisions Proton-Nucleus Collisions
High-energy - and -nucleus collisions
Measurements of hadron structure at high energies (above HERA)
Di-jet and heavy quark production
Tagging of UPC requires a Zero Degree Calorimeter
Ongoing work on ZDC design and integration
with the accelerator instrumentation:
Proton-Nucleus Collisions
Link between pp and AA physics
Study of the nuclear modification of the gluon distribution at low xF.
Study of the jet fragmentation function modification
Full detector capabilities (including TRT) will be available.
L~1030 translates to about 1MHz interaction rate (compare to 40 MHz in pp)
Pavel Nevski, BNL
QM2005

24. CONCLUSION
ATLAS has a very good potential for making a valuable and significant contribution to the LHC’s heavy-ion physics programme: x z y
Global variable measurement on Day1
dN/dη, dET/dη, elliptic flow.
Jet measurement and jet quenching
Quarkonia suppression (J/Ψ, )
p-A physics
Ultra-Peripheral Collisions (UPC)
More will come
Direct information
from QGP
Pavel Nevski, BNL
QM2005

25. Back-up material
Pavel Nevski, BNL
QM2005

26. Heavy Ion Physics with the ATLAS Detector
Pavel Nevski
Brookhaven National Laboratory
On behalf of the ATLAS Collaboration
Pavel Nevski, BNL
QM2005

27. ATLAS as a Heavy Ion Detector :
Good Tracker
Excellent Calorimetry
Large Acceptance Muon Spectrometer
Pavel Nevski, BNL
QM2005

28. Central Pb+Pb Collision (b<1fm)
Nch(|η|0.5)
Scenario with 3,200 charged particle per rapidity unit:
About 70,000 stable particles, ~ 40,000 particles in ||  3
CPU for simulations – 6 h per central event (800MHz,G3)
Occupancy ~2% in pixels, 20% in strips,
Calorimeter in 0.1x0.1: 2 GeV e.m. But only 0.3 GeV in Tile
Event size 50MB (without TRT and with no zero-suppression)
Pavel Nevski, BNL
QM2005

29. Tracking Performance For pT: 1 - 10 GeV/c: efficiency ~ 70 %
Standard ATLAS reconstruction for pp
is used, not optimized for PbPb.
Pixel and SCT detectors
pT threshold of 1 GeV
(used in this preliminary studies)
tracking cuts:
At least 10 hits out of 11(13) available
in the barrel (end-caps)
All three pixel hits
At most 1 shared hits
2/dof < 4
For pT: GeV/c:
efficiency ~ 70 %
fake rate ~ 5%
Momentum resolution ~ 3%
(2% - barrel, 4-5% end-caps)
Pavel Nevski, BNL
QM2005

30. Electron-pion separation in TRT
Pion fraction vs electron efficiency
In central Pb-PB collisions (3200 ch.particle per rapidity unit) factor 20 in pion rejection can be achieved by selecting a TR threshold corresponding to 50% electron efficiency
Pavel Nevski, BNL
QM2005

31. Day-one measurements: Nch, dNch/d, ET, dET/d, b
Global Measurements
Day-one measurements:
Nch, dNch/d, ET, dET/d, b
Constrain model prediction
Indispensable for all physics analyses
Predictions for Pb+Pb central collisions at LHC
(dNch/d)0
Model/data
~ HIJING:with quenching, no shadowing
~ HIJING:with quenching, with shadowing
~ HIJING:no quenching, no shadowing
~ Saturation Model (Kharzeev & Nardi)
~ Extrapolation from lower energy data
Pavel Nevski, BNL
QM2005

32. Use 3 detector systems to obtain impact parameter:
Collision Centrality
Use 3 detector systems to
obtain impact parameter:
Pixel&SCT,
EM-Cal
HAD-Cal
Resolution of the estimated
impact parameter ~1fm
for all three systems.
Pavel Nevski, BNL
QM2005

33. ATLAS Calorimeters
Pavel Nevski, BNL
QM2005

34. Jet studies PYTHIA jets embedded with central Pb-Pb HIJING events
Reconstruction:
- sliding window algorithm ΔΦxΔη =0.4x0.4 with splitting/merging
after background energy subtraction (average and local)
- ET fitting with Gaussian+constant background
Average background 50 ±11 GeV (Pb-Pb Hijing, b=0-1 fm)
=> threshold for jet reconstruction ~30-40 GeV in calorimeter
cf p-p ~ 15 GeV
Studies of algorithm optimization is underway
Pavel Nevski, BNL
QM2005

35. Quarkonia suppression
Color screening prevents various ψ, , χ states to be formed when T→Ttrans to QGP (color screening length < size of resonance)
Modification of the potential can be studied by a systematic measurement of heavy quarkonia states characterized by different binding energies and dissociation temperatures
~thermometer for the plasma
Upsilon family (1s) (2s) (3s)
Binding energies (GeV)
Dissociation at the temperature ~2.5Ttrans ~0.9Ttrans ~0.7Ttrans
=>Important to separate (1s) and (2s)
Pavel Nevski, BNL
QM2005

36. Upsilon Reconstruction
  +–
Overlay  decays on top of HIJING events.
Use combined info from ID and μ-Spectrometer
Single Upsilons
HIJING background
(Half ’s from c, b decays, half from π, K decays for pT>3 GeV)
Background rejection:
2 cut
geometrical    cut
pT cut.
Momentum resolution is worse at higher rapidity due to material
,: differences between ID and µ-spectrometer tracks
after back-extrapolation to the vertex for the best 2 association.
Pavel Nevski, BNL
QM2005

37. Summary
Global observables, including elliptic flow, should be accessible from day-one, even with a very low luminosity (early scheme)
Jet physics (jet quenching) is very promising, jet reconstruction is possible despite the additional background, possibility to study separately light and heavy q-jets
Heavy-quarkonia physics (suppression in dense matter) well accessible, capability to measure and separate  and ’, to measure the J/ using a specially developed  tagging method
Ongoing studies:
- Optimization of algorithms for high-multiplicity environment
- The flow effects and its impact on the jet reconstruction
- pA collisions
ATLAS has a very good potential for making a valuable and significant contribution to the LHC’s heavy-ion physics programme
Pavel Nevski, BNL
QM2005