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THE PHYSICS OF THE ALICE INNER TRACKING SYSTEM. Elena Bruna, for the ALICE Collaboration Yale University. 24 th Winter Workshop on Nuclear Dynamics, South Padre Island 5-12 April 2008. OUTLINE. The ALICE Inner Tracking System (ITS) Performance of the ITS Tracking - PowerPoint PPT Presentation
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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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E.Bruna
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
3
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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)