Letter of Intent for Heavy I on Physics with the ATLAS Detector ATLAS Collaboration

Barbara Wosiek, IFJ PAN, Kraków LHCC, May 12, 20041

Letter of Intent for

Heavy Ion Physics with the ATLAS Detector

ATLAS Collaboration


Heavy Ion Physics at RHIC

Evidence that possibly two new forms of QCD matterhave been discovered:

sQGP – strongly coupled Quark-Gluon Plasma

CGC – a saturated gluon initial state (QGP-source)

• AuAu @ sNN = 200 GeV

• dN/dη in AuAu @ sNN = 200 GeV• high-pT suppression vs. y in dAu @ sNN = 200 GeV

Workshop organized by RIKEN BNL Research Center„New Discoveries at RHIC – The Strongly Interactive QGP”

May 14-15, 2004 at BNL


Empirical evidence for QGP at RHIC

Bulk matter collectivity

-hydrodynamic limit is reached-matter produced is equlibrated-consistent with QCD EoS (lattice)-matter behaves like an ideal fluid

Jet quenching in AuAu as predicted by pQCD(unquenching in dAu)

PRL91,PRL91, (2003)(2003)

Hard processes excellent probes forverifying QCD !


From RHIC to LHC

sNN : 200 GeV 5,500 GeVWhich of the RHIC concepts will still be valid at LHC??Super-hot QCD – Will we see a weakly coupled QGP??

- Initial state fully saturated (CGC)- Enormous increase of high-pT processes over RHIC - Plentiful heavy quarks (b,c)

LHC

RHICSPS


ATLAS A Superb Detector for High-pT Studies


ATLAS as a Heavy Ion Detector

— Hermetic coverage up to || < 4.9— Fine granularity (with longitudinal segmentation)

— Coverage up to || < 2.7

— Large coverage up to || < 2.5— Finely segmented pixel and strip detectors— Good momentum resolution

High pT probes (jets, jet shapes, jets correlations)

Muons from , J/, Z0 decays

Tracking particles with pT 1.0 GeV/c

2.+ 3. Heavy quarks(b), quarkonium suppression(J/ ,, ’)

1.& 3. Global event characterization (dNch/dη, dET/dη, flow), jets

1. High Resolution Calorimeters

3. Inner Detector (Si Tracker)

2. Large Acceptance Muon Spectrometer


Constraint: No modifications to the detector, with the exception of trigger software and forward instrumentation

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 |η|< 5.1 impact parameter range: b = 10 - 30fm


Central Pb+Pb Collision (b<1fm)

Nch(|η|0.5) About 75,000 stable particles ~ 40,000 particles in || 3 CPU – 6 h per central event (800MHz) Event size 50MB (without TRT)


Detector Occupanciesb = 0 – 1fm

Si detectors: Pixels < 2% SCT < 20%

TRT: too high, unusable (limited usage for PbPb collisions is under investigation)

Muon Chambers: 0.3 – 0.9 hits/chamber(<< pp at 1034 cm-2 s-1)

Calorimeters ( |η|< 3.2 )

In average:

~ 2 GeV/Tower

~ .3 GeV/Tower


Histogram – true Nch

Points – reconstructed Nch

Nch(|| < 3)

10% <3%

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 3200(HIJING, no quenching)

dNch/d|=0 6000(HIJING, with quenching)

Reconstruction errors ~5%


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.


Track Reconstruction

-Pixel and SCT detectors

-pT threshold of 1 GeV

-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)


Heavy Quark ProductionMotivation: Heavy quarks may radiate less energy in the dense medium (dead-cone effect) than light quarks.

Open beauty:-Use ppWHlbb events overlayed

on HIJING bckground-Search for a displaced vertex in the

Inner Detector

Rejection factor against u-jets ~ 100for b-tagging efficiency of 25%

Should be improved when combined

with soft-muon tagging in the Muon Spec.


Jet Reconstruction- Sliding window algorithm, = 0.4 0.4, after subtracting the average pedestal of (50 11) GeV- Accepted if ET(window) > 40 GeV

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


Reconstruction – 280 GeV jet

HIJING jet

HIJING jet(counted as fake)


Jet Reconstruction

Jet energy resolution

Correct plot is needed!Efficiency > 80% for ET >40GeVEnergy resolution xx% at ET= 40 GeV

Angular resolution for 70 GeV jets(~2 resolution in pp)


Jet Rates

For a 106s run with Pb+Pb at L=41026 cm-2 s-1 we expect in |η| < 2.5:

ET threshold Njets

50 GeV 40 106

100 GeV 1 105

150 GeV 2 105

200 GeV 2 104

And also: ~106 + jet events with ET > 50 GeV ~500 Z0() + jets with ET > 40 GeV


Jet StudiesGoal is to determine medium properties.

Jet quenching But, most energy is radiated INSIDE cone

vacuum

in medium

Salgado &Wiedemannhep-ph/0310079

Need to measure jet shapes:- Fragmentation function using tracking- Core ET and jet profile using calorimeters- Neutral leading hadrons using EM calorimeters


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 pp dN/djT broader in PbPb than in pp

Promising, but a lot of more work is needed!


ETcore Measurements

Energy deposited in a narrow cone around the jet axis:R < 0.11 (HADCal), R < 0.07 (EMCal)

pp

pp

<ETcore> sensitive to 10% change in ET

jet

ppPbPb

More work is needed to minimizeeffect of background fluctuations.


Isolated Neutral Hadrons

Select jets that have an isolated neutral cluster along the axis(~1% of the total jet sample)

pp

pp

Cluster ET sensitive to small change in ETjet

pp

Further studies of large samplesof jets embedded into PbPb events are needed.


Quarkonia SuppressionColor screening prevents various ψ, , χ states to be

formed when T→Tc for the PT to QGP

(color screening length < size of resonance)

Upsilon family (1s) (2s) (3s) Binding energies (GeV) 1.1 0.54 0.2Dissociation at the temperature ~2.5Tc ~0.9Tc ~0.7Tc

Important to separate (1s) and (2s)!

QGP thermometer


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.


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 decayed ’s

Acceptance & efficiency vs. |η| (used to estimate rate and S/B)


Upsilon ReconstructionBarrel only (|| <1) || <1 || <2.5

Acceptance 4.9% 14.1% +efficiency

Resolution 123 MeV (147 MeV)

S/B 1.3 (0.5)

Purity 94-99% (91-95%)

Are all these numbers correct???

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/ 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.


Trigger and DAQInteraction rate of 8 kHz for L = 1027 cm-2 s –1

Event size ~ 5 MB for central PbPbData rate to storage ~300 Hz MB

Output rate (after HLT) ~ 50 Hz for central events

Minimum-bias 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 inelatsic cross-section.

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


Ultra-Peripheral Nuclear CollisionsHigh-energy - and -nucleon collisions- Measurements of hadron structure at energies > HERA- Di-jet and heavy quark production- Tagging of UPC using Zero Degree Calorimeters (ongoing work on 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 will be available. L~1030 translates to about 1MHz interaction rate (compare to 40 MHz in pp)


Summary The Letter of Intent represents a first look into a heavy-ion physics with the ATLAS detector.

The high granularity and large coverage of the calorimeter system, external muon spectrometer and 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 performance are going on:

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’s heavy-ion physics programme.

- Optimization of algorithms for high-multiplicity environment- Study of the flow effects and its impact on the jet reconstruction- Study of pA collisions

Documents

Letter of Intent for Heavy I on Physics with the ATLAS Detector ATLAS Collaboration