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

This study focuses on the performance and tracking capabilities of silicon detectors in the STAR apparatus to enable microvertexing of rare decays in heavy ion collisions. The motivation is to gain insights into the properties of the produced medium and determine the nuclear modification factor. The method involves measurements of hadronic and semi-leptonic decays, as well as the use of decay chain fitting with a Kalman filter. Comparison with simulation and real data analysis, including cuts optimization, is also discussed.

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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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