Transcript Document 7331024

Heavy Ion Physics with the
ATLAS Detector
Heavy Ion Physics working group:
K.Assamagan, B.Cole, J.Dolejsi, F.Gianotti, J.Nagle,
P.Nevski, A.Olszewski, L.Rosselet, H.Takai,
S.Tapprogge, S.White, R.Witt, B.Wosiek, K.Woźniak
Pavel Nevski, BNL
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Freiburg ATLAS week
Motivation:
High-pT Results from RHIC
Jet quenching observed in AuAu as predicted by pQCD:
(unquenching in dAu)
c
a
b
d
Hard processes:
excellent probes to test QCD!
PRL91, (2003)
Pavel Nevski, BNL
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Freiburg ATLAS week
From RHIC to LHC
sNN : 200 GeV
5,500 GeV
- 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
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ATLAS as a Heavy Ion Detector
1. Excellent Calorimetry
— Hermetic coverage up to || < 4.9
— Fine granularity (longitudinal and lateral segmentation)
— Very good jet resolution
High pT probes (jets, jet shapes, jet correlations, 0)
2. Large Acceptance Muon Spectrometer
— Coverage up to || < 2.7
Muons from , J/, Z0 decays
3. Inner Detector (Si Pixels and SCT)
— Large coverage up to || < 2.5
— High granularity pixel and strip detectors
— Good momentum resolution
Tracking particles with pT  0.5 GeV/c
2.+ 3. Heavy quarks(b), quarkonium suppression(J/ ,)
1.& 3. Global event characterization (dNch/dη, dET/dη, flow);
Jet quenching
Pavel Nevski, BNL
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Physics program
z
 Global variable measurement
dET/dη
dN/dη
elliptic flow
azimuthal distributions
y
 Jet measurement and jet quenching
x
 Quarkonia suppression
J/Ψ

Direct information
from QGP

p-A physics

Ultra-Peripheral Collisions (UPC)
Idea: take full advantage of the large calorimeter and μ-spectrometer
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Studies of the Detector Performance
 Constraint: No modifications to the detector.
 Simulations: HIJING event generator, dNch/dη = 3200
Full GEANT simulations of the detector response
 Large event samples:
|η|< 3.2
|η|< 5.1
Pavel Nevski, BNL
impact parameter range: b = 0 - 15fm (27,000 events)
impact parameter range: b = 10 - 30fm (5,000 events)
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Central Pb+Pb Collision (b<1fm)
 ~ 40,000 particles in ||  3
 CPU – 6 h per central event (800MHz)
Pavel Nevski, BNL
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Nch(|η|0.5)
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Detector Occupancies
Si detectors:
Pixels < 2%
SCT < 20%
b = 0 – 1fm
Calorimeters ( |η|< 3.2 )
0.10.1 Tower
TRT:
too high occupancy,
not used for these studies
~ 2 GeV/Tower
(limited usage for AA
collisions is under investigation)
0.10.1 Tower
~ .3 GeV/Tower
Muon Chambers:
0.3 – 0.9 hits/chamber
(<< pp at 1034 cm-2 s-1)
Pavel Nevski, BNL
Average ET
(uncalibrated):
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Global Event Characterization
Day-one measurements: Nch, dNch/d, ET, dET/d, b
 Constrain model prediction; indispensable for all physics analyses
Single Pb+Pb event, b =0-1fm
dNch/d=0 3200
Histogram – true Nch
Points – reconstructed Nch
measured
Points – reconstructed
— HIJING, no quenching
Nch(|| < 3)
Reconstruction errors ~5%
No track reconstruction used, only Nhits as calibrated in pp
Pavel Nevski, BNL
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Estimate of 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
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Track reconstruction
—Only Pixel and SCT detectors
—At least 10 hits out of 11 per track
—At most 1 shared hits
—For pT: 1 - 10 GeV/c:
efficiency > 70%
fake rate ~ 5%
pT-resolution ~3%
• 2000 reconstructed tracks per HIJING (b=0) event with pT > 1 GeV
and || < 2.5
• Fake rate at high pT can be reduced by matching with calorimeter data
Pavel Nevski, BNL
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Heavy Quark Production
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
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Jet quenching
Energy loss of fast partons by excitation and gluon radiation
larger in QGP
Suppression of high-z hadrons and increase of hadrons in jets.
Induced gluon radiation results in the modification of jet properties
like a broader angular distribution.
Could manifest itself as an increase in the jet cone size or an effective
suppression of the jet cross section within a fixed cone size.
Measuring jet profile is the most direct way to observe any change.
Pavel Nevski, BNL
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Freiburg ATLAS week
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 with ET > 50 GeV
~500 Z0() + jets with ET > 40 GeV
Pavel Nevski, BNL
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Jet Reconstruction
- Sliding window algorithm,    = 0.4  0.4,
after subtracting the pedestal (the average pedestal is 50 11 GeV)
- Accepted, if ET(window) > 40 GeV
Di-jet event from
PYTHIA in pp:
pThard=55GeV
Di-jet reconstructed
in PbPb
Di-jet embedded
in PbPb before
pedestal
subtraction
Di-jet embedded
in PbPb after
pedestal
subtraction
The used algorithm is not optimal, studies are ongoing.
Pavel Nevski, BNL
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Jet reconstruction efficiency
Pb-Pb collisions (b= 0 –1 fm)
Angular resolution
for 70 GeV jets
Efficiency
~2  resolution in pp
Fake rate
Pb-Pb
Energy resolution
p-p
• For ET > 75GeV: efficiency > 95%, fake < 5%
• Good energy and angular resolution
•Next: use tracking information to lower the threshold and reduce the fakes
Pavel Nevski, BNL
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ETcore Measurements
Jet Studies with Tracks
Fragmentation function
- Jets with ET = 100 GeV
- Cone radius of 0.4
- Track pT > 3 GeV
Energy deposited in a narrow cone:
R < 0.11 (HADCal), R < 0.07 (EMCal)
<ETcore> sensitive to
~10% change in ETjet
jet
track
z  pT
/ ET
The background has not been subtracted:
<ETcore>PbPb  <ETcore>pp
PbPb  HIJING-unquenched  pp
Promising, but a lot of additional work is needed!
Pavel Nevski, BNL
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Quarkonia Suppression
Color screening prevents various ψ, , χ states to be
formed when T→Tc for the phase transition to QGP
(color screening length < size of resonance)
QGP thermometer
Upsilon family
Binding energies (GeV)
Dissociation at the temperature
(1s)
(2s)
1.1
~2.5Tc
0.54
~0.9Tc
(3s)
0.2
~0.7Tc
Important to separate (1s) and (2s)!
Pavel Nevski, BNL
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Freiburg ATLAS week
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.
,: differences between ID and µ-spectrometer tracks
after back-extrapolation to the vertex for the best 2 association.
Pavel Nevski, BNL
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Upsilon Reconstruction
Barrel only (|| <1)
A compromise has to be found
between acceptance and mass
resolution to clearly separate
upsilon states.
|| <1
Acceptance 4.9%
+efficiency
Resolution 123 MeV
S/B
1.3
Purity
94-99%
|| <2.5
14.1%
147 MeV
0.5
91-95%
For a 106s run with Pb+Pb at L=41026 cm-2 s-1
we expect 104 events in |η| < 1.2 (6% acc+eff).
J/  +– - a study is under way (mass =53 MeV).
Pavel Nevski, BNL
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Freiburg ATLAS week
Trigger and DAQ
Interaction rate of 8 kHz for L = 1027 cm-2 s –1
Event size ~ 5 MB for central PbPb (a conservative estimate)
Data rate to storage the same as in pp: ~300 Hz  MB
Output rate (after HLT) ~ 50 Hz for central events
 Unbiased interaction trigger (LVL1) – use forward calorimeters (FCAL)
- Triggering on the total ET in FCAL with a Trigger Tower threshold
of 0.5 GeV selects 95% of the inelastic cross-section.
Pavel Nevski, BNL
ET cut
Rate [kHz]
Centrality
Fraction of tot
> 5.6 TeV
0.3
b < 3 fm
3%
> 4.3 TeV
0.8
b < 5 fm
10%
> 1.7 TeV
2.4
b < 9 fm
30%
> 0.3 TeV
5.6
b < 13 fm
70%
> 1 GeV
6.8
unbiased
85%
> 0.25 GeV
7.9
unbiased
99%
1<ET<30 GeV
0.9
b > 15 fm
11%
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Trigger and DAQ
 Triggering on events with b < 10 fm - use full ATLAS Calorimetry
 High Level Triggers (ATLAS T/DAQ) - jet trigger, di-muon trigger,...
Jet rate ~ 40 Hz for ET threshold = 50 GeV
~ 0.1 Hz for ET threshold = 100 GeV
Selection signatures
LVL1 signature
HLT signature
Physics coverage
random
random
Zero-bias sample
INT(FCAL)
int(FCAL)
Centrality/interaction
EM
e
Zee
EM

Photon production
2EM
2e
Zee
MU

Z ,  
2MU
2
Z ,  
nJ
nj
Jet production
Pavel Nevski, BNL
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Freiburg ATLAS week
Ultra-Peripheral Nuclear Collisions
High-energy - and -nucleon 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 x.
- 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
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Summary
 This work represents a just a first look into heavy-ion physics
with the ATLAS detector.
 The high granularity and large coverage of the calorimeter system,
the acceptance of the muon spectrometer and the tracking
capabilities of the inner detector allow for a comprehensive study
of high-pT phenomena and heavy quark production in heavy-ion
collisions.
 Studies of the detector and physics performance are ongoing:
- Optimization of algorithms for high-multiplicity environment
- The flow effects and its impact on the jet reconstruction
- pA collisions
These first results already demonstrate a very good potential of
the ATLAS experiment for heavy-ion studies and ensure
its valuable and significant contribution to the
LHC heavy-ion physics programme.
Pavel Nevski, BNL
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