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 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 Elena Bruna, Yale University

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) 3 R out =43.6 cm L out =97.6 cm SPD SSD SDD Elena Bruna, Yale University SPD SSD SDD

Elena Bruna, Yale University 4 TRD PHOS HMPID MUON SPECTR... Size: 16 x 26 m Weight: ~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 LHC setup TPC TOF

STATUS OF THE ITS The ITS was put inside the TPC in March-April Elena Bruna, Yale University 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

6 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 Elena Bruna, Yale University 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 7 Elena Bruna, Yale University

ITS PERFORMANCE: TRACKING (3 OF 3) 8 impact parameter resolution (on the bending plane) vs p T Elena Bruna, Yale University

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 tracks  better 9 Pixels from ITS Elena Bruna, Yale University Tracks from ITS+TPC (w. beam line constraint) # tracklets

ITS PERFORMANCE : SECONDARY VERTEX 10 Charmed mesons: c  ~  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 Z found -Z true Y found -Y true X found -X true Elena Bruna, Yale University (my thesis work)

Elena Bruna Along P t D + coord Orthog P t D + coord z coord SECONDARY VERTEX FINDER Elena Bruna, Yale University bending plane rotated y K-K- π+π+ π+π+ D+D+ y’ x’ x y

ITS PERFORMANCE : PID 12 Elena Bruna, Yale University 12 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 13 Elena Bruna, Yale University

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 14 Elena Bruna, Yale University

D0 K-Π+D0 K-Π+ S/B ≈ 10% Significance for 1 month Pb-Pb run: S/√(S+B) ≈ Elena Bruna, Yale University statistical. systematic.

D +  K - Π + Π + (1) Analysis strategy (1) Cuts on single tracks (p T, transverse impact parameter) Cuts on pairs: distance primary vertex-K π vertex: δ Product of impact parameters: 16 Elena Bruna, Yale University K  primary vertex  SignalBackground When (d 0 K x d 0  1 )<0 & (d 0 K x d 0  2 )<0: empty region kinematically not allowed  selection based on the products of impact parameters of the two K  pairs: 25% of BKG triplets rejected d 0 K X d 0 π 2 d 0 K X d 0 π 1 d 0 K X d 0 π 2 d 0 K X d 0 π 1

17  point D +  K - Π + Π + (2) Cuts on the triplets K ππ : 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 p M =Max{p T1,p T2,p T3 } cos ϑ point s=d d d Elena Bruna, Yale University Results for Pb-Pb p T 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

18 Elena Bruna, Yale University D +  K - Π + Π + (3) Results for pp (No PID) 0<p T <2 GeV/c

19 Elena Bruna, Yale University DS+K-K+Π+DS+K-K+Π+ Significance p T (GeV/c) Significance p T (GeV/c) D s  K 0 *K  KKΠ

20 Elena Bruna, Yale University 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. 21 Elena Bruna, Yale University Thank you

BACKUP SLIDES 22 Elena Bruna, Yale University

ITS ALIGNMENT 23 Elena Bruna, Yale University 250  m xy

Elen a Bru na RESULTS FOR 3< P T (D + )<5 GEV/C cos  point d s=  d 0 2 cos  point Significance normalized to 10 9 pp MB events

25 ITS DETECTOR RESOLUTIONS SPD (r = 4 & 7 cm) SDD (r = 14 & 24 cm) SSD (r = 39 & 44 cm) spatial resolutions Rφ  z [  m 3 ] 12   2020  830 Two-track resolution (rφ) [  m] Two-track resolution (z) [  m]

ITS DIMENSIONS layertypeR (cm)± z (cm) 1Pixel pixel drift drift strip37.8/ strip42.8/ Elena Bruna, Yale University

E.Br una SECONDARY VERTEX FINDER Tracks (helices) approximated with Straight Lines: analytic method Vertex coordinates (x 0,y 0,z 0 ) from minimization of:  xi,  yi,  zi are the errors on the track parameters Quality of the vertexer (not weighted): where: Secondary Vertex (x 0,y 0,z 0 ) d1d1 track 1 d2d2 track 2 d3d3 track 3

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  primary vertex Working point:  700  m 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) Working point:  < 200  m (optimized in p T ranges of D + ) Selected SIG=50% Selected BKG=1% S/B~3 x : still too small

E.Br una 30 Selection efficiency and p T spectra: p T integrated ε (D +) ≈ 1.5% (Ideal PID), 0.6% (Real PID), 1% (no PID) RESULTS: D +  K -  +  + IN PB- PB

E.Br una 31 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 K d~1000  m  K =10% K Contamination K vs d

Elena Bruna 32 D +  K -  +  + BR = 9.2 % D ± I(J P ) = ½ (0 - ) m = MeV/c 2 c  =  m (PDG ’04) D + →K -  +  + Non ResonantBR = 8.8 % D + →K *0 (892)  + →K -  +  + ResonantBR = 1.3 % D + →K *0 (1430)  + →K -  +  + ResonantBR = 2.3 % D + →K *0( 1680)  + →K -  +  + ResonantBR = 3.8·10 -3 % Hadronic 3-charge-body decays of D +

Elena Bruna 33 D K  P T P T 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) Knowledge of the P T shapes of the decay products important at the level of the selection strategy

Elena Bruna 34 K  nonresonant D + decay HIJING central (normalized) Comparing with Pb-Pb central events (ONLY HIJING generation, NO propagation and reconstruction in the detector): P T P T distributions: Mean = 0.67 GeV/c Mean = 0.50 GeV/c Mean = 0.87 GeV/c Mean = 0.65 GeV/c Kinematics (2) K and  from D + are harder than K and  produced in a Pb-Pb event

Elena Bruna 35 Non resonant Resonant Sharp borders due to PYTHIA cut off on the tails of distributions Dalitz Plots: Kinematics (3)