Heavy Ion Physics with the ATLAS Detector
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Transcript of Heavy Ion Physics with the ATLAS Detector
Pavel Nevski, BNL Los AlamosMarch 13, 2005
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Heavy Ion Physics with the ATLAS Detector
Pavel NevskiATLAS Heavy Ion Physics working group
Pavel Nevski, BNL Los AlamosMarch 13, 2005
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Jet quenching observed in AuAu as predicted by pQCD:(unquenching in dAu)
PRL91, (2003)PRL91, (2003)
Hard processes:excellent probes to test QCD!
c
d
a b
Motivation: High-pT Results from RHIC
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From RHIC to LHC sNN : 200 GeV 5,500 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
RHICSPS
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ATLAS A Superb Detector for High-pT Studies
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ATLAS as a Heavy Ion Detector
— Hermetic coverage up to || < 4.9— Fine granularity (with longitudinal segmentation)— Very good jet resolution
— Coverage up to || < 2.7
— Large coverage up to || < 2.5— High granularity pixel and strip detectors— Good momentum resolution
High pT probes (jets, jet shapes, jet correlations, 0)
Muons from , J/, Z0 decays
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
1. Excellent Calorimetry
3. Inner Detector (Si Pixels and SCT)
2. Large Acceptance Muon Spectrometer
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Physics program
Global variable measurement
dN/dη dET/dη elliptic flow
azimuthal distributions
Jet measurement and jet quenching
Quarkonia suppression
J/Ψ
p-A physics
Ultra-Peripheral Collisions (UPC)
Idea: take full advantage of the large calorimeter and μ-spectrometer
Direct information from QGP
x
z
y
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Constraint: No modifications to the detector, with the exception of trigger software and probably very forward region
Studies of the Detector Performance
Simulations: HIJING event generator, dNch/dη = 3200 Full (GEANT-3) simulations of the detector response
Large event samples: |η|< 3.2 impact parameter range: b = 0 - 15fm (27,000 events) |η|< 5.1 impact parameter range: b = 10 - 30fm (5,000 events)
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Central Pb+Pb Collision (b<1fm)
Nch(|η|0.5) About 70,000 stable particles ~ 40,000 particles in || 3 CPU – 6 h per central event (800MHz) Event size 50MB (without TRT and with no zero-suppression)
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Detector Occupanciesb = 0 – 1fm
Si detectors: Pixels < 2% SCT < 20%
TRT: too high, not used for these studies (limited usage for AA collisions is under investigation)
Muon Chambers: 0.3 – 0.9 hits/chamber(<< pp at 1034 cm-2 s-1)
Calorimeters ( |η|< 3.2 )
Average ET
(uncalibrated):
~ 2 GeV/Tower
~ .3 GeV/Tower
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Global Event CharacterizationDay-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 3200Points – reconstructed
— HIJING, no quenching
Reconstruction errors ~5%
Nch(|| < 3)
Histogram – true Nch
Points – reconstructed Nch
measured
No track reconstruction used, only Nhits as calibrated in pp
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Collision Centrality
Use 3 detector systems to obtain impact parameter:
Pixel&SCT, EM-Cal HAD-Cal
Resolution of the estimated impact parameter ~1fmfor all three systems.
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Track Reconstruction
-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: 1 - 10 GeV/c: efficiency ~ 70 % fake rate ~ 5%Momentum resolution ~ 3%(2% - barrel, 4-5% end-caps)
Standard ATLAS reconstruction for ppis used, not optimized for PbPb.
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Heavy Quark ProductionMotivation: Heavy quarks may radiate less energy in the dense medium (dead-cone effect) than light quarks.
Rejection factor against u-jets ~ 100for b-tagging efficiency of 25%
Should be improved by optimized algorithmsand with soft muon tagging in the Muon Spec.
b-tagging capabilities offer additional tool to understand quenching.
To evaluate b-tagging performance:- ppWHlbb events overlayed on HIJING background have been used.- A displaced vertex in the Inner Detector has been searched for.
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Jet RatesFor a 106s run with Pb+Pb at L=41026 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
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Jet quenchingEnergy 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.
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Jet StudiesGoal is to determine medium properties.
Jet quenching But, most energy is radiated INSIDE conevacuum
in medium
Salgado &Wiedemannhep-ph/0310079
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
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Reconstruction – 280 GeV jet
HIJING jet
HIJING jet(counted as fake)
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• For ET > 75GeV: efficiency > 95%, fake < 5%• Good energy and angular resolution •Next: use tracking information to lower the threshold and reduce the fakes
Jet reconstruction efficiencyAngular resolution
for 70 GeV jets
~2 resolution in pp
Pb-Pb collisions (b= 0 –1 fm)
EfficiencyFake rate
Energy resolutionPb-Pb
p-p
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Jet Studies with Tracks- Jets with ET = 100 GeV- Cone radius of 0.4- Track pT > 3 GeV
Fragmentation function Momentum componentperpendicular to jet axisjet
TtrackT E/pz
PbPb HIJING-unquenched ppdN/djT broader in PbPb than in pp
(background fluctuations)Promising, but a lot of additional work is needed!
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ETcore Measurements
Energy deposited in a narrow cone around the jet axis:R < 0.11 (HADCal), R < 0.07 (EMCal)
<ETcore> sensitive to
~10% change in ETjet
ppPbPb
More work is needed to minimizeeffect of background fluctuations.The background
has not been subtracted: <ET
core>PbPb <ETcore>pp
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Isolated Neutral HadronsSelect jets that have an isolated neutral cluster along the axis
(~1% of the total jet sample)
Cluster ET sensitive to small change in ETjet
pp
Further studies of large samples of PbPb events with jets are needed.
Gauss =4.5%
Jets with ET >75GeV embedded into central PbPb events:
Relative error on the measurementof the neutral cluster ET.
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Quarkonia suppressionColor 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 plasmaUpsilon family (1s) (2s) (3s) Binding energies (GeV) 1.1 0.54 0.2Dissociation at the temperature ~2.5Ttrans ~0.9Ttrans ~0.7Ttrans
=>Important to separate (1s) and (2s)
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Upsilon Reconstruction- Overlay decays on top of HIJING events.- Use combined info from ID and μ-Spectrometer
+–
Single Upsilons
HIJING backgroundHalf ’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.
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Upsilon Reconstruction
A compromise has to be found between acceptance (overall rate of events) and mass resolution to clearly separate upsilon states.
Mass resolution vs |η| of decay ’s.
Deterioration of the mass resolutionat larger |η| is due to material effects.
Integrated Acceptance & efficiency as a function of |η|.
Drop in the reconstruction efficiencyfor +PbPb events is now understood(software technical problem) andsignal and background are beingre-evaluated.
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Upsilon ReconstructionBarrel only (|| <1)
|| <1 || <2.5Acceptance 4.9% 14.1% +efficiencyResolution 123 MeV 147 MeV
S/B 1.3 0.5
Purity 94-99% 91-95%
For a 106s run with Pb+Pb at L=41026 cm-2 s-1 we expect 104 events in |η| < 1.2 (6% acc+eff).
J/ +– - a study is under way.
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J/ Reconstruction- a study is under way
- Large cross-section- σmass=53 MeV
=>easy separation of J/ψ and ψ’- good for J/ψ with pT > 4-5 GeV
J/ +–
Full pT analysis is only possible in forward and backward regions, but there the background is largest.
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Trigger and DAQInteraction 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.
ET cut Rate [kHz] Centrality Fraction of tot
> 5.6 TeV 0.3 b < 3 fm 3%> 4.3 TeV 0.8 b < 3 fm 10%> 1.7 TeV 2.4 b < 3 fm 30%> 0.3 TeV 5.6 b < 3 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 signaturesLVL1 signature HLT signature Physics coverage
random random Zero-bias sampleINT(FCAL) int(FCAL) Centrality/interaction
EM e ZeeEM Photon production
2EM 2e ZeeMU Z ,
2MU 2 Z ,
nJ nj Jet production
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Ultra-Peripheral Nuclear CollisionsHigh-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 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)
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Summary The Letter of Intent represents 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:
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.
- Optimization of algorithms for high-multiplicity environment- The flow effects and its impact on the jet reconstruction- pA collisions
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Back-up slides
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ATLAS Calorimeters
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Time (h)
Tuning time
0.2
0.4
0.6
0.8
1<L>/L0
0 5 10 15 200ALICE PPR CERN/LHCC 2003-049
Yves Schutz, QM’2004
Luminosity Issues
* = 2 - 0.5 mIO = 108 ions/bunch
1
2
3
Experiments
20% reduction for 3 experiments as opposed to 2 experiments
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Global Event CharacterizationCMS
Bolek Wyslouch, QM’2004Single Pb+Pb event, b =0-1fm
dNch/d=0 3200Points – reconstructed
— HIJING, no quenching
dNch/d=0 6000Points – reconstructed
— HIJING, with quenching
Reconstruction errors ~5%
ATLAS
Comparable performance
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Tracking Efficiency & Fake RateCMS
Bolek Wyslouch, QM’2004ATLAS
Slightly lower efficiency as compared to CMS, but reconstruction algorithm is not yet optimized for PbPb.
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Momentum ResolutionCMS
Bolek Wyslouch, QM’2004ATLAS
ALICESlightly worse resolutionas compared to CMS,but reconstruction algorithmis not optimized for PbPb andcurrent study is limited to pT < 20 GeV.
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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 GeVThe used algorithm is not optimal, studies are ongoing.
Di-jet event from PYTHIA in pp: pT
hard=55GeV
Di-jet embedded in PbPb before pedestalsubtraction
Di-jet embedded in PbPb after pedestalsubtraction
Di-jet reconstructed in PbPb
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Jet ReconstructionATLAS CMS
Bolek Wyslouch, QM’2004
ALICESarah Blyth, QM’2004
Using US EMCal(under discussion)
ATLAS and CMS comparable (ATLAS has better efficiency at 40-50 GeV), both better than ALICE with the proposed EMCal
Jet energy resolution
Efficiency
Fraction of fake jets
Pb-Pb
p-p
Pb-Pb collisions (b= 0 –1 fm)
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Jet Pointing ResolutionATLAS
ALICESarah Blyth, QM’2004
Using US EMCal(under discussion)
ATLAS better performance than in ALICE with the proposed EMCal.
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Fragmentation FunctionATLAS CMS
Bolek Wyslouch, QM’2004ALICE
Sarah Blyth, QM’2004
Using US EMCal(under discussion)
jetT
trackT E/pz
Comparable performance for ATLAS, CMS and ALICE with the proposed EMCal
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Fragmentation FunctionATLAS CMS
Bolek Wyslouch, QM’2004 ALICESarah Blyth, QM’2004
Using US EMCal(under discussion)
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Comparison between experiments for +-
Exp. Ref. L used for the signal estimation (cm-2 s-1)
S S for same L and
running time
Mass resol.
S/B
ATLAS LoI 4x1026 10000 for 106 s
10000 125 MeV (53 MeV for J/+- )
1.3
CMS Hep-ph/0311048
4x1026 18000 for 1.3x106 s(cf 24000 J/+- )
14000 46-60 MeV
0.9
ALICE Forward spectrometer
Hep-ph/0311048
5x1026 1800 for 106 s
(cf 230000 J/+- )
1400 100 MeV (70 MeV for J/+- )
7.1
ALICE can also measure 2600 e+e- in the central spectrometer per 106 s if B is increased from 0.2 to 0.4 T
Upsilon Reconstruction
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Jet studies
Reconstruction: sliding window algorithm ΔΦxΔη =0.4x0.4 with splitting/merging after background energy subtraction (average and local)
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 algorithm is not fully optimized yet
PYTHIA jets embedded with central Pb-Pb HIJING events
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Single Upsilons
→ μ+ μ- using combined info from ID and μ-spectrometer:
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Single Upsilons
HIJING background
Half μ’s from c, b decays, half from π, K decays for pT>3 GeV.
Background rejection based on χ2 cut, geometrical Δη x ΔΦ cut and pT cut.
→ μ+ μ- using combined info from ID and μ-spectrometer:
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|| <1 || <2.5Acceptance 4.9% 14.3% +efficiencyResolution 126 MeV 152 MeVS/B 1.3 0.5Purity 94-99% 91-95%
E.g. for |η| < 1.2 (6% acc+eff) we expect 104 /month of 106s at L=41026 cm-2 s-1
J/ +– - a study is under way (mass =53 MeV).
A compromise has to be found between acceptance and mass resolution to clearly separate states with maximum statistics.
+- reconstruction
Barrel only (|| <1)
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Ultra-Peripheral Collisions (UPC)
b > 2R only electromagnetic interactions γ–γ
γ–N
γ–Ρ with/without nucleus diffraction
γ–W
σ(γ–γ)~Z4 W γ–γ < 2γħc/RA =200 GeV for Pb
σ(γ–γ)~Z4 W γ–γ < 2γħc/RA =200 GeV for Pb
η
η
Φ
Φ
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