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


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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


— HIJING, no quenching


— 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


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


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



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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


44Δη, ΔΦ=difference between ID and μ-spectrometer tracks after back-extrapolation to the vertex for the best χ2 association.

Single Upsilons

→ μ+ μ- using combined info from ID and μ-spectrometer:


45Δη, ΔΦ=difference between ID and μ-spectrometer tracks after back-extrapolation to the vertex for the best χ2 association.

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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heavy ion physics with the atlas detector

Documents

heavy quarks b

pt processes

highpt studiesatlas

highpt results

detector systems

heavy quarksb

jet shapes

jet correlations