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BNL : Y. Fisyak, V. Perevoztchikov

Detailed summary of the latest developments in D0 analysis utilizing full D0-vertex fitting techniques along with silicon information for improved accuracy and precision in the study of D0 decays.

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BNL : Y. Fisyak, V. Perevoztchikov

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  1. Update on D0 analysisusing full D0-vertex fit (µ-vertexing) and Silicon informationKENT :J. Bouchet, J. Joseph, S. Margetis, J. Vanfossen BNL : Y. Fisyak, V. Perevoztchikov STAR coll. Meeting, March 26-31, BNL

  2. Outline • Since last analysis meeting : • Full D0-vertex fit code (TCFIT, micro-vertex code) has been evaluated/debugged. • Redo HIJING+D0 simulation study • ‘real’ pT (not flat) spectra D0 • Central Hijing Au+Au event • Rapidity not  • Started looking on Embedding (CuCu + AuAu) • FULL Pass/Production of Au+Au (run7) data (~35 Million mbias events). STAR coll. Meeting, March 26-31, BNL

  3. Outline-II • To recall, our strategy is : • Step 1 : pure D0 (see how signal parameter behave) • Step 2 : simulation : Hijing+1D0 (background behavior) • Step 3 : real Data • Step 4 : embedding (corrections/x-section/physics) • for each step, confirm that the microvertex code works as expected • and find usable cuts STAR coll. Meeting, March 26-31, BNL

  4. Secondary vertex fit methods used • Linear fitabandoned. • Helix swimming to DCA of the two track helices (V0-like) using the global track parameters to reconstruct helices (StPhysicalHelix) not saved . • Helix swimming to DCA of the two track helices (V0-like) using the parameters from StDcaGeometry: save full track information (covariance matrix) inside the vacuum (center of beam pipe). • Full D0/Helix Fit (TCFIT) with vertex constraint and full errors • Also a full Kalman D0-fit was tried but not significant gains in time etc • The combined info from points 3, 4 will allow momentum dependent cut using the full track information. • Least square fit of the decay vertex. In 2 body decay, combination of 2 tracks + addition of constraint(s) to impose ‘external knowledge’ of a physic process and therefore force the fit to conform to physical principles. • 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). STAR coll. Meeting, March 26-31, BNL

  5. Long-ctau D0 evaluation • Each plot shows the correlation of the secondary vertex position from GEANT (y-axis) with 1 of the 3 methods investigated : TCFIT,global helix and DCA geometry helix (x-axis) for its 3 components. TCFIT GEANT DCA X Y Z J.Vanfossen RECO STAR coll. Meeting, March 26-31, BNL

  6. ‘normal’ c D0 evaluation • TCFit does a bit better job than either helix swimming method. • The scatter along the x axis of the swimming methods can be attributed to low pt D0’s and daughter tracks that are close to being parallel or anti parallel. X Y Z J.Vanfossen STAR coll. Meeting, March 26-31, BNL

  7. Resolution plots 230µm 230µm 190µm • TFCIT makes a slightly better job in terms of resolution 270µm 260µm 250µm 270µm 260µm 250µm J.Vanfossen STAR coll. Meeting, March 26-31, BNL

  8. Simulation • Previously we have shown results from simulation (Au+Au Hijing +1D0) but it was with flat pT D0 and flat in centrality • We have studied the significance of the signal (invariant mass peak) as a function of cuts. • We have redone the simulation with real pT spectrum etc. With proper scaling we have build realistic, expected inv. mass plots simulation Example M(D0) simulation L > 2 M(D0) J.Bouchet STAR coll. Meeting, March 26-31, BNL

  9. Real pT spectrum • Generate a pT distribution with a power law function : d2N/dpTdy = A[1 +pT/p0]n with <pT> = 2p0/(n-3) for a better extrapolation at pT0 Ref : • Manuel Calderon (thesis) • Transverse momentum spectra of charged particle in ppbar collisions at sqrt(s) = 630 GeV, Phys. Letter B 366(1996)434-440 • Pick randomly values in this distribution. • changing n from 10 to 14 does not really modify the shape (which is what we want) of the pT distribution. • Parameters chosen : n=10, <pT>=1 GeV/c. STAR coll. Meeting, March 26-31, BNL

  10. Check Generated function • Values (here 10k events) • randomly picked • Mean <pT> is 1 STAR coll. Meeting, March 26-31, BNL

  11. Simulation settings • Run the BFC chain with geometry y2007g • Use the slow simulator • Library : SL08f • Au+Au central collisions : b= 0-4.5 fm • Vertex position from Hijing event • D0 phase space : • 0 < pT < 5 GeV/C power law • -1 < y< 1 flat • 0 < phi < 6.28 flat Check the result from Starsim : if D0 in fz file is correct. STAR coll. Meeting, March 26-31, BNL

  12. Check .fz file • distributions look as expected STAR coll. Meeting, March 26-31, BNL

  13. NEXT with HIJING: • increase statistics (up to now 6.5k events) to make conclusion • verify cut selection • normalize signal -> (pseudo) estimate x-section, produce ‘realistic’ mass plots STAR coll. Meeting, March 26-31, BNL

  14. D0 embedding • Dataset : Au+Au@200GeV, run 7 • File:/eliza14/star/starprod/embedding/2007ProductionMinBias/D0_103_1234306453/P08ic.SL08f/*.MuDst.root (~ 4.8K Min-bias events) • The Embedded data contains SiliconHits, most of them have 0,1,and 2, but a very few has 3 SiHits. • The Multiplicity range goes from zero to ~1200 • The pT distribution of D0s goes from 0 to 5GeV/c • Rapidity goes +/-1 (Gaussian like) J.Joseph STAR coll. Meeting, March 26-31, BNL

  15. First look at Embedding with the microvertex code Cuts used : |PVz| < 10 TpcHits>20 Eta, +/-1 nSigmaPi|<2, |nSigmaK|<2 pTK > 0.1GeV/c, pTPi > 0.1GeV/c |cos(*)|<0.6, chargeK<0 ,chargePi>0, |etaD0|<1.85 Slength_tcfit < 1mm Requirement of silicon hits >0 No Signal S/N ~ 3. 7356 • Silicon hits are important • Goal is to get familiar with environment, see what is useful and formulate our request • More plots (QA) at : • http://www.star.bnl.gov/~jai2006/jjoseph/22ndMarch2010-EmbeddedDataAnalysis.htm J.Joseph STAR coll. Meeting, March 26-31, BNL

  16. Real Data • Run over run 7 data productionMinBias. • Sample is ~35 Million events/ ~55 vertices • QA plots done day by day : • http://drupal.star.bnl.gov/STAR/blog/bouchet/2010/feb/24/full-production-minbias-run7 • Cuts (see next slide) chosen to speed the code. STAR coll. Meeting, March 26-31, BNL

  17. Cuts used (real data) • EVENT level • triggerId : 200001, 200003, 200013 • 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>2 (tracks with sufficient DCA resolution) • Momentum of tracks p >.5GeV/c • Number of fitted TPC hits > 20 • Pseudo-rapidity :||<1(SSD acceptance) • dEdxTrackLength>40 cm • DCA to Primary vertex (transverse) DCAxy< .1 cm • DECAY FIT level • Probability of fit >0.1 && |sLength|<.1cm • Particle identification : ndEdx :|nK|<2, |nπ|<2 STAR coll. Meeting, March 26-31, BNL

  18. Multiple vertices • We (wrong) use all vertices in a given event. • As seen (plot left), events can have several vertices (with different positions and resolutions): event 31 has 8 vertices. • Issue (?) : we did not fill the rank of the vertex to make a selection on the best vertex. Event 31 • however there is a correlation btw the gRefMult (which we saved) and the rank of each vertices. • Cut is possible to select ‘only’ good (best) vertex. Possible offline cut STAR coll. Meeting, March 26-31, BNL

  19. D0 signal in 2007Production mbias • Cuts(offline): • 50µm< decaylength<400µm • trackDca<200 µm • dcaD0toPV<300µm • pTkaon>0.7GeV/c • pTpion>0.7GeV/c • Plot as a function of gRefMult J.Joseph STAR coll. Meeting, March 26-31, BNL

  20. 50<gRefMult S/N ~ 5.7 J.Joseph STAR coll. Meeting, March 26-31, BNL

  21. 50<gRefMult<500 S/N ~ 2.0181 J.Joseph STAR coll. Meeting, March 26-31, BNL

  22. 50<gRefMult<800 S/N ~ 4.9652 J.Joseph STAR coll. Meeting, March 26-31, BNL

  23. 50<gRefMult<1000 • D0 signal remains stable. Not the case with low statistics. • An increase in the significance when including high multiplicities events is observed. S/N ~ 5.6993 J.Joseph STAR coll. Meeting, March 26-31, BNL

  24. Outlook • Microvertex code gives consistent results with GEANT and helix swimming. • Run for the first time on the almost entire run 7 data with microvertex code. • D0 signal remains stable under various cuts (good). • D0 signal seems to increase when high multiplicities are included (good). • Next step : try pT bins and make spectra. • Simulation : work in progress to increase stats. of real pT D0 in Hijing to make conclusion. • Investigation (near future) of other decays. STAR coll. Meeting, March 26-31, BNL

  25. Back-up STAR coll. Meeting, March 26-31, BNL

  26. Constrained vertex fit • 2 = ∑(yi0 - yi(x*))TV-1(yi0 - yi(x*)) + F where : • x* : secondary vertex position • yi0 : track parameter of the original fit • y : track parameter after refit with knowledge of the secondary vertex • V : covariance matrix of the track parameter • i : sum over tracks • F : constraint  f • f : physical process to satisfy K- π+ 3d path length from primary vertex to decay particle vertex • The constraint(s) is(are) added to the total 2 via Lagrange multiplier  • The minimum of 2 is then calculated with respect to the fit parameters x and with respect to  because the condition ∂2/∂  =0 required for the minimum correspond the the constraint equation f STAR coll. Meeting, March 26-31, BNL

  27. Check the MuDst file for the simulation (real pT) All combinations K-π+ • after running the reconstruction, I have run MuKpi and checked the result • pT and the rapidity of reconstructed D0 look as the input All combinations K-π+ STAR coll. Meeting, March 26-31, BNL

  28. Invariant mass D0 after selected cuts • Look at the entire production done ~35M events • Cuts : • siliconHits ==4 • |cos*|<.6 • |yD0|<.5 • Slength > 200 µm • dcaXY(pion)*dcaXY(kaon)<0 • |cospointing|>.9 • Probability (of TCFIT) >75 • W/o any background substraction, a ‘signal’ can be seen J.Bouchet STAR coll. Meeting, March 26-31, BNL

  29. Resolution plots (Long) • There is no systematic shift in reconstructed quantities in all 3 directions. • The standard deviation of the distribution is flat at ~ 220 m , which is of the order of the resolution of (SSD+SVT). 220µm 210µm 200µm 210µm 230µm 210µm 200µm 230µm 220µm J.Vanfossen STAR coll. Meeting, March 26-31, BNL

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