THE PHYSICS OF THE ALICE INNER

TRACKING SYSTEMElena Bruna, for the ALICE Collaboration

Yale University

24th Winter Workshop on Nuclear Dynamics, South Padre Island 5-12 April 2008

OUTLINE The ALICE Inner Tracking System (ITS) Performance of the ITS

Tracking Primary vertex reconstruction Secondary vertex reconstruction (from heavy flavor

decays) Particle identification

Physics analyses with the ITS Hadronic and semi-leptonic decays of heavy-flavor

particles Multiplicity studies

Conclusions 2

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6 layers of Silicon detectors:

Pixel Chambers (SPD): 2 innermost layers

Drift Chambers (SDD): 2 intermediate layers

Double-sided Strip Chambers (SSD): 2 outermost layers

THE INNER TRACKING SYSTEM (ITS)

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Rout=43.6 cmLout=97.6 cm

SPD

SSD

SDD

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SPD

SSD

SDD

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TRD

PHOS

HMPID

MUON SPECTR..

Size: 16 x 26 mWeight: ~10,000 tons

Inner Tracking System (ITS):

6 SILICON layers (pixel, drift, strip)

Vertices reconstruction, PID (dE/dx)

-0.9<<0.9

B = 0.5 T

ALICE @ LHCsetup

TPC

TOF

STATUS OF THE ITS

The ITS was put inside the TPC in March-April 2007

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First cosmics seen in February!

• Tracking worked in the SPD

• Aligned clusters seen in SPD and SDD

The ITS is ready to collect the first pp collisions!

SPD+SDD

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ITS PERFORMANCE: TRACKING (1 of 3)

Tracking is the major challenge in ALICE: ~7000 tracks in a central HIJING Pb-Pb event at 5.5 TeV in the ITS + TPC acceptance

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TOF

TRD ITS

TPC

PHOS

RICH

Tracking strategy:• from TPC ‘seeds’, tracks are extrapolated towards the ITS with the Kalman filter technique and then backpropagated to the outer detectors

ITS PERFORMANCE: TRACKING (2 OF 3)

Tracking Stand-alone in the ITS:• used in the reconstruction software • tracks not reconstructed by the TPC • first day physics (track multiplicity

and PID), both in pp and Pb-Pb• useful in case of initial alignment

problems with the TPC

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ITS PERFORMANCE: TRACKING (3 OF 3)

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impact parameter resolution (on the bending plane) vs pT

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ITS PERFORMANCE: PRIMARY VERTEX

Primary vertex reconstruction: more problematic in pp than in Pb-Pb

Pb-Pb: primary vertex resolution dominated by the mis-alignment

pp: 2 steps for primary vertex finding: Before tracking: using ITS pixels (“tracklets”) After tracking: using tracksbetter

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Pixels from ITS

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Tracks from ITS+TPC

(w. beam line constraint)

# tracklets# tracklets

ITS PERFORMANCE: SECONDARY VERTEX

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Charmed mesons: c ~ 100-300 m A good tracking system is required to

separate primary and secondary vertex

Good resolution on primary and secondary vertices

RMS ~ 120 m: good to measure vertices displaced of 300 m

Zfound-ZtrueYfound-Ytrue

Xfound-Xtrue

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(my thesis work)

Elena Bruna

Along Pt D+ coordOrthog Pt D+ coord

z coord

SECONDARY VERTEX FINDER

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bending planerotated

y

K-

π+

π+

D+

y’x’

x

y

ITS PERFORMANCE: PID

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Protons

Kaons

Pions

electrons

0.45<p<0.48 GeV/c

Based on specific ionization (dE/dx) in the SDD and SSD (4 Silicon layers)

Add information to the PID given by the TPC (combined-Bayesian PID)

Identify tracks not reconstructed by the TPC: Low momentum Out of TPC acceptance Dead zones of TPC (between sectors)

dE/dx in the ITS,

full tracking

PHYSICS ANALYSES RELATED WITH THE ITS

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CHARGED PARTICLE MULTIPLICITY WITH THE SPD

Why multiplicity: first measurement in pp collisions for ALICE global observable characterizing the event comparison with results obtained at lower energies

Why multiplicity with pixels: available in a short time advantages over reconstructed tracks (ITS+TPC)

larger acceptance coverage only alignment of the two pixel layers required

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D0 K-Π+

S/B ≈ 10% Significance for 1

month Pb-Pb run: S/√(S+B) ≈ 40

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statistical.

systematic.

D+K-Π+Π+ (1) Analysis strategy (1)

Cuts on single tracks (pT, transverse impact parameter) Cuts on pairs:

distance primary vertex-Kπ vertex:δ

Product of impact parameters:

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K

primaryvertex

Signal Background

When (d0K x d01)<0 & (d0K x d02)<0: empty region kinematically not allowed

selection based on the products of impact parameters of the two K pairs: 25% of BKG triplets rejected

d0K

X d

0π2

d0K X d0π1

d0K

X d

0π2

d0K X d0π1

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point

D+K-Π+Π+ (2) Cuts on the triplets Kππ :

Quality of the secondary vertices Global optimization on a hyper-surface of:

Distance between prim and sec vertices Maximum transverse momentum among the 3 tracks pM=Max{pT1,pT2,pT3} cosϑpoint

s=d012+d02

2+d032

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Results for Pb-Pb

pT D+

D+ sel (Id. PID)D+ sel (Real PID)

D+ sel (No PID)

3 dau in acceptance

3 dau reconstructed

D+K-π+π+ produced in 4π

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D+K-Π+Π+ (3)Results for pp (No PID)

0<pT<2 GeV/c

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DS+K-K+Π+

Significance

pT(GeV/c)

Significance

pT(GeV/c)

DsK0*KKKΠ

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B MESONS VIA B e e X Inclusive measurement of electrons coming

from semi-electronic decay of beauty hadrons need good electron identification: combined PID

in TPC (dE/dx) + TRD (+EMCal in future) good measurement of the track impact

parameter

SUMMARY

Interesting analyses will be possible with the ITS, thanks to its excellent vertexing and tracking capabilities and PID: Heavy flavor physics:

Hadronic and semi-leptonic decays of charm and beauty particles Charged multiplicity, the “day one” measurement

ALICE is looking forward to collecting wonderful data.

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Thank you

BACKUP SLIDES

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ITS ALIGNMENT

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250 m

xy

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RESULTS FOR 3< PT(D+)<5 GEV/C

cos

po

int

d

s=d

02

cospoint

Significance normalized to 109 pp MB events

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ITS DETECTOR RESOLUTIONS

SPD(r = 4 & 7 cm)

SDD(r = 14 & 24 cm)

SSD(r = 39 & 44 cm)

spatial resolutions

Rφ z [m3]12 120 38 20 20 830

Two-track resolution (rφ) [m] 100 200 300

Two-track resolution (z) [m] 850 200 300

ITS DIMENSIONSlayer type R (cm) ± z (cm)

1 Pixel 3.9 14.1

2 pixel 7.6 14.1

3 drift 15.0 22.2

4 drift 23.9 29.7

5 strip 37.8/38.4 43.1

6 strip 42.8/43.4 48.9

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SECONDARY VERTEX FINDER

Tracks (helices) approximated with Straight Lines: analytic method

Vertex coordinates (x0,y0,z0) from minimization of:

xi, yi, zi are the errors on the track parameters Quality of the vertexer (not weighted):

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2 dddD

2

0

2

0

2

02

zi

i

yi

i

xi

ii

zzyyxxd

where:

Secondary Vertex (x0,y0,z0)

d1

track

1

d2tra

ck

2

d3

track

3

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2 ddd

COMBINING K PAIRS

K and have opposite charge sign Cut on the distance between the vertex of

the 2 tracks and the primary vertex

K

primaryvertex

• Working point: 700m

• Selected SIG=67%

• Selected BKG=5%

(m)

SECONDARY VERTEX FINDER ON THE TRIPLETS

Secondary vertex resolution: ~120 m Cut on the quality of the Vertex:

BLACK: signal

RED: BKG K Triplets

(cm)

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2 ddd

• Working point: < 200 m

(optimized in pT ranges of D+)• Selected SIG=50%• Selected BKG=1%• S/B~3 x 10-4: still too small

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Selection efficiency and pT spectra:

pT integrated ε (D+) ≈ 1.5% (Ideal PID), 0.6% (Real PID), 1% (no PID)

RESULTS: D+K-++ IN PB-PB

daughterstedreconstrucwithD

Dselected

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FEED-DOWN FROM BEAUTY

D+ from B are more displaced The cut on distance between primary to secondary

vertex increases the fraction of selected D+ coming from B decay

Histograms normalized to the same area

d~1000m KK =10%

Contamination K K vs d

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DD++KK--+++ + BR = 9.2 %BR = 9.2 %

D± I(JP) = ½ (0-)

m = 1869.4 MeV/c2

c = 311.8 m

(PDG ’04)

D+→K-++ Non Resonant BR = 8.8 %

D+→K*0(892)+→K-++ Resonant BR = 1.3 %

D+→K*0(1430)+→K-++ Resonant BR = 2.3 %

D+→K*0(1680)+→K-++ Resonant BR = 3.8·10-3 %

Hadronic 3-charge-body decays of Hadronic 3-charge-body decays of DD++

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D

K

PPTT distributions of the generated particles (ONLY PYTHIA generation, NO propagation and reconstruction in

the detector)

(nonresonant events)

Mean = 1.66 GeV/c

Mean = 0.87 GeV/c

Mean = 0.67 GeV/c

Kinematics (1)Kinematics (1)

Knowledge of the PT shapes of the decay products important at the level of the selection strategy

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K

nonresonant D+ decay

HIJING central (normalized)

Comparing with Pb-Pb central events (ONLY HIJING generation, NO propagation and reconstruction

in the detector):

PPTT distributions:

Mean = 0.67 GeV/c

Mean = 0.50 GeV/c

Mean = 0.87 GeV/c

Mean = 0.65 GeV/c

Kinematics (2)Kinematics (2)

K and from D+ are harder than K and produced in a Pb-Pb event

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Non resonantNon resonant ResonantResonant

Sharp borders due to PYTHIA cut off on the tails of distributions

Dalitz Plots: Kinematics (3)Dalitz Plots: Kinematics (3)