Particle identification in ALICE

March 3, 2008 E. Scapparone INSTR08 1

Particle identification in ALICE

E. Scapparone(INFN/CERN) on behalf of the ALICE Collaboration INSTR08, March 3, 2008

Outline- Experimental layout and status of the main subdetectors;

- Detector performance;

- Example of ALICE phsycs potential.


ALICE: A Large Ion Collider Experiment at CERN-LHC

Size: 16 x 26 meters

Weight: 10,000 tonnes

p-p (L=1031 cm-2 s-1), Pb-Pb (L= 1027 cm-2 s-1)


Life in Pb-Pb interactionswas never easy….

NA49

At RHIC became harder…..

ALICE simulated central event !

..also more difficult @ RHIC

STAR


• 6 layers, three technologies (keep occupancy at a few % for max multiplicity)

• SPD: silicon pixels (0.2 m2, two layers, 9.8 M channels)

• SDD: silicon drift (1.3 m2, two layers, 133 k channels) • SSD: double-sided silicon strips (4.9 m2, two layers, 2.6 M channels)

Rout=43.6 cm

SPD

SSD

SDD

ITS(Inner Tracking System)

Lout=97.6 cm

Material Budget: ≤ 1% X0 per layer !


Challenge: track densities at r = 4 cm (1st layer): up to 100 / cm2

SPD (0.2 m2)Cell size (rz : 50 m x 425 µm space resolution(r) : 12 mspace resolution(z): 100 m

ITS

SDD (1.3 m2)cell size (r,z): 294 x 150 m2space resolution (r, z) : 35 mspace resolution(z): 23 m

SSD (4.9 m2)cell size (r,z): 95 x 4200 m2space resolution (r): 20 mspace resolution (z); 830 m

40

m m

75 mm42 m

m6 chips


Inner Tracking System~ 10 m2 Si detectors, 6 layers

Pixels, Drift, double sided Strips

Strips

SSD

Drift

SDD

Pixels

Inner Silicon Tracker

Pixels

SPD


TPC

SSD/SDD

SPD


• Optimized for dN/dη ≈ 8000• l = 5 m, Ø = 5.6m, 88 m3,

570,000 channels x 500 time bins, • up to 80 Mbytes/event (after 0

suppression)• Features:

• lightweight: 3% X0 total material for perpendicular tracks

• novel digital electronics (ALTRO)• highly integrated, digital shaping; tail

cancellation;0-suppression; Baseline restoration

• Powerful laser calibration system

TPC

Drift gas:Ne (86) / CO2 (9.5) / N2 (4.5)

σ(dE/dx) ≈ 5.5 to 6.5 %, depending on multiplicity (measurements and simulations)


ALICE TPC: 5 years construction


Cosmics recorded in TPC@ALICE

Commissioning with Cosmics and Laser tracksSpatial resolution and Electronics noise in experiment according to specifications

Status: installed and being commissioned


• e/ separation in central barrel p>1 GeV/c

• fast trigger for high momentum particles (hadrons, electrons)+ tracking

• 540 detectors ~ 760m2

• 18 super modules

• length: 7m

• X/X0 ~ 22 %

• 28 m3 Xe/CO2 (85/15)

• 1.2 million channels

Transition Radiation Detector (TRD)

Status: partially installed; being commissioned

TRD


TRD - Signal Generation & Processing

Ave

rag

e p

uls

e h

eig

th (

mV

)

e-) = 90 %


TRD Installation


TOF requirements:

Large array to cover the ALICE barrel (~150 m2)Time resolution σ < 100 ps High efficiency, ε > 95%High segmentation: few thousands of particles per unit of rapidity expected in Pb-Pb collision at s = 5.5 TeV/nucleon pair

ALICE choice: a detector based on MRPC, segmented in ~157,000 channels, (3.5 x 2.5) cm2 area

√


TOF

2 crates /side

157248 pads, total sensitive area: ~150 m2

18 Supermodules, each made of 5 modules: 2 external (19 strips), 2 intermediate (19 strips), 1 central (15 strips)

7.4 m

Readout performed bytwo VME Crates/Side


Cross section of a double-stack MRPC strip

2 rows of 48 readout pads (3.5 x 2.5 cm2)

Active strip length = 120 cm

Differential signal to FEA card

10 gaps 250 m


Test beam of single strips E

ffic

ien

cy

Tim

e re

solu

tio

n(

ps)ε > 99.9 %

average ~ 50 ps

Gas mixture: C2F4H2(90%) – SF6 (5%) – C4H10(5%)

Similar results obtained when strips are inserted inside the modules


M = p ((t2 c2 /L2 )-1)1/2

σTOF = 80 ps ; B = 0.5 T


10 SMs installed ; next 4 SMs to be installed next week; being

commissioned


December 2007 cosmic run

Event display of a muon crossing the TOF detector


• Proximity Focused RICH• 11 m2 active detector area• 7 modules• - 0.6< η < 0.6; Δφ = 58 deg • CsI Photocathode read by

16100 pads• Threshold momentum =

1.21* (Particle Mass)

• PID optimization for 1 GeV < p < 6 GeV

High Momentum Particle Identifier: HMPID

March 3, 2008 E. Scapparone INSTR08 2222HMPID (Sept ’06)

HMPID: Installed; being commissioned


Example of a high multiplicity event as seen by the HMPID

dN/d=6000


5 stations of high granularity pad tracking chambers, over 1 million channels

Complex absorber/small angle shield system (~10 I)

to minimize background(90 cm from vertex)

RPC Trigger Chambers

MUON spectrometer set-up

0.7 T, bending power 3 Tm 4 MW power, 800 tons World’s largest warm dipole

Trigger pt cut on single : ~1 GeV/c


Lepton Acceptance

ATLAS & CMS present a large lepton acceptance ||<2.4

ALICE combines muonic and electronic channels

- covers the low pT region (quarkonia)

- covers the forward region 2.5<<4.0


Charged Particle Identification

ITS

kaons

pions

protons

dE/d

x (M

IP u

nits

) TPC

All known PID techniques used in ALICE

dE/dx = 6.8% at dN/dy=8000 (5.5% for isolated tracks)

HADRON-ID 3 separation power

P(GeV/c)P(GeV/c)

dE/d

x (M

IP u

nits

)


Combined PID Performance

Efficiency/Contamination

Kaon PID (the most difficult case...)

ITSTPC

TOF(*)(120 ps)

p (GeV/c) p (GeV/c) p (GeV/c)

(C : CK : Cp = 0.75 : 0.15 : 0.1)

Higher efficiency & Lower contamination wrt individual detectors

p (GeV/c)

Combined PID ITS & TPC & TOF

(*) very conservative


Extension of PID to higher momenta combining the dE/dX of TRD and TPC

Combine TPC and TRD dE/dx capabilities (similar number of samples/track) to get statistical ID in the relativistic rise region

8<p<10GeV/c


Open Charm Detection in Hadronic DecaysOpen Charm Detection in Hadronic Decays

Mass 1.864 GeV/c2 c=124 m

~0.55 D0K-+ accepted/event important also for J/ normalization


SpectraSpectra


Finally …

• ALICE is becoming reality after almost 15 years in the making • 2nd Commissioning run is going on this week.• Further commissioning running until first beams, scheduled for

summer 2008• All systems on track for taking first pp collisions• ALICE is the general purpose Heavy Ion experiment, designed to

address the very rich, expected physics program: particle id will

play a central role in this challenging physics.


RISERVE


GIF results

No ageing effect measured at PS

Working voltage = 13 kV


GIF results: No current increase

Source on

Source off

Source on

Source off

I (A)Flow = 1.4 cm3/s

The gas going to the chamber 2 was bubbled through water for some

time of the exposure


Ageing study:

Total charge = 14 mC/cm2

Charge/event = 2 pC

7 *109 events /cm2 @ 50 Hz/cm2

T=1.4 108 sec = 1620 days Pb-Pb run

Time(min)

two MRPC strips irradiated at GIF


ALICE is taking shape All large structures are installed and integration

of detector subsystems will start in AugustThe ALICE Collaboration looks forward to first

operation of LHC for the expected and the unexpected…

ConclusionConclusion


PHOton Spectrometer: PHOS

• PbO4W- crystal calorimeter for Photons, neutral mesons, 1 to > 100GeV

• Crystals: 2.2*2.2 cm2 , 20 X0, APD R/O Operated at – 25 deg; 5 pe / MeV

• σ(E) ≈ 3%; σ(x,y) ≈ 4 mm; σ(t) ≈ 1 ns; at 1GeV

• - 0.12 < η < 0.12; Δφ = 100 deg; at R = 460 cm

• L0 trigger available at < 900 ns

• Module tested, calibrated in test beam; 1 or 2 modules to be installed in 04/08

• 3rd module during 2008; • Ultimately 5 modules with 18 000 crystals

expected by 2010


ElectroMagnetic Calorimeter: EMCAL

• Pb-Scintillator em calorimeter for• Jet physics in conjunction with

tracking and PID• Approximately opposite to PHOS• -0.7 <η < 0.7; Δφ = 107 deg• EM resolution σ(E) < 0.1/√E• 13 000 projective towers in ‘Shashlik’

geometry with APD R/O• L0 trigger in < 900 ns for high-pT jets,

photons, electrons

Status: project approved in Dec 2007 2 of 12 supermodules early 2009


Partial view of Muon chambers


TOF Standalone PID (80 ps res. as in the PPR)

Documents

Particle identification in ALICE