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Performances studies of the silicon detectors in STAR towards microvertexing of rare decays

Performances studies of the silicon detectors in STAR towards microvertexing of rare decays. Motivations STAR : detectors apparatus Method Summary. Jonathan Bouchet, for the STAR Collaboration KENT STATE UNIVERSITY. Charmed mesons in Heavy Ion collisions.

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Performances studies of the silicon detectors in STAR towards microvertexing of rare decays

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  1. Performances studies of the silicon detectors in STAR towards microvertexing of rare decays Motivations STAR : detectors apparatus Method Summary Jonathan Bouchet, for the STAR Collaboration KENT STATE UNIVERSITY APS meeting, Feb. 13-17 2010

  2. Charmed mesons in Heavy Ion collisions • Heavy flavor is supposed to be produced at the earlier stages of the collision via gluon fusion: • give insights of the medium created. • If dense medium created, RAA = nuclear modification factor of charged hadrons : • =1 means no nuclear effect. • <1 means suppression. APS meeting, Feb. 13-17 2010

  3. Measurement : how to • Direct method • Measurement of hadronic decays through decay topological reconstruction • D0 (D0)K-+(K+-)BR : 3.8 % • D+/-K BR : 9.2% • indirect method • Measurement of semi-leptonic decays of B,D mesons • D0e++X , BR : 6.9 % • D+/- e+/-+X BR : 17.2% RAA of non photonic electrons in AuAu STAR Phys. Rev. Lett. 98 (2007) 192301 D0 reconstruction d+Au dAu : Phys. Rev. Lett. 94 (2005) AuAu : STAR arXiv:0805.0364 CuCu: preliminary • In order to disentangle between the b and c quarks contribution of the semi-leptonic measurement, a full topological reconstruction of the decaying particle is needed. • Challenging for charmed particles because of the small decay length (c (D0) ~ 124 µm). APS meeting, Feb. 13-17 2010

  4. STAR detector (in 2007) SSD TPC SVT • The tracking system consisted in : • Time Projection Chamber (TPC) provides measurements of : • track momentum, particle identification (pId) • 2 silicon detectors : • 1 layer of silicon strip detectors (SSD) and 3 layers of silicon drift detectors (SVT) • High spatial resolution.  Note : the silicon tracker wasn’t designed originally (in terms of distance and layer thickness) for charmed particle detection APS meeting, Feb. 13-17 2010

  5. Decay fitting Least square fit of the decay vertex [1] : In 2 body decay, combination of 2 tracks + addition of the constraint that they’re coming from a common point. The Kalman fitter machinery allows the knowledge with high precision of tracks near the primary vertex (by taking into account the MCS due to the silicon layers) Method : 1. Form pair of tracks and use TPC pId’s 2 .Compute the position of the secondary vertex associated to each pair. decay tree Vertex position 2 =∑ (qi-h(x,pi))T V-1i (qi-h(x,pi)) tracks i Parametrisation of the track (helix) covariance matrix of measured track parameters measured track parameters 3d path length from primary vertex to decay particle vertex [1]Decay Chain Fitting with a Kalman Filter,W. D. Hulsbergen (arxiv:physics,0503191) APS meeting, Feb. 13-17 2010

  6. Comparison with simulation Reco vs. MC [cm] Reco - MC [cm] • Left : correlation between reconstructed path length and MC • Right : • There is no systematic sift (red crosses = mean) in reconstructed quantities. • The standard deviation (blue crosses) of the distribution (reco-MC) is flat at ~ 250 m , which is of the order of the resolution of (SSD+SVT). MC [cm] MC [cm] APS meeting, Feb. 13-17 2010

  7. Real data:Scan of DCA* resolution *: Distance of Closest Approach • run 7 Au+Au@200GeV (MinBias trigger) • Cuts • |zvertex |<5 cm : to have a clean sample • TPC hits> 15 • || in SSD acceptance (||<1.2) • pT >0.1 • Fit function : • Reflect the (detector+alignment) resolution and Multiple Coulomb Scattering. • Fit done for 0.2 < 1/P < 5 • resolution@1GeV (for tracks with N=2,3,4) vs day number to study stability of DCA resolution. Real data Real data Including the silicon detectors in the tracking improves the pointing resolution : with 4silicon hits, pointing resolution of tracks to the interaction point ~ 220 m at P = 1GeV/c APS meeting, Feb. 13-17 2010

  8. *: Distance of Closest Approach Real data APS meeting, Feb. 13-17 2010

  9. Example of cuts • The significance of the path length, defined as  = sLength/dsLength, where : • sLength = path from primary vertex to decay particle vertex • dsLength = error associated, can be used to select physical decays. • From simulation, an enhancement of the D0 invariant mass is observed by applying a cut at  > 2. simulation M(D0) simulation  > 2 M(D0) APS meeting, Feb. 13-17 2010

  10. In real life … • AuAu@200GeV : 1M events. • A cut at pT > 1.5 GeV/cof daughter tracks of each D0 candidates is applied.  > 2 No cut Work is in progress APS meeting, Feb. 13-17 2010

  11. Summary • We presented a method using full track information to obtain high precision of decay vertex. • agreed with simulation. • Work on going for cuts optimization. • Method is baseline for analysis involving the future upgrade in STAR : Heavy Flavor Tracker. Low mass detector designed to identify mid-rapidity Charm and Beauty mesons and baryons through direct reconstruction and measurement of the displaced vertex with unprecedented pointing resolution. APS meeting, Feb. 13-17 2010

  12. MTD EMC barrel MRPC ToF barrel 100% ready for run 10 EMC End Cap FMS BBC FPD TPC FHC PMD Completed DAQ1000 HLT Ongoing R&D HFT FGT STAR detector (current) APS meeting, Feb. 13-17 2010

  13. Cuts used in reconstruction code EVENT level Primary vertex position along the beam axis : |zvertex| < 10 cm Resolution of the primary vertex position along the beam axis: |zvertex|< 200µm TRACKS level Number of hits in the vertex detectors :SiliconHits>1 (we want tracks with good DCA resolution) Momentum of tracks p >.3GeV/c Pseudo-rapidity :||<1.2 (SSD acceptance) dEdxTrackLength>40 cm DCA to Primary vertex (transverse) DCAxy< .1 cm DECAY FIT level Probability of fit >0.01 && |sLength|<.1cm Particle identification : ndEdx :|nK|<2.5, |nπ|<2.5 APS meeting, Feb. 13-17 2010

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