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Status of D + analysis: Vertex reconstruction and event production. Francesco Prino INFN Torino. People working on D + analysis: Torino: M.Masera, F.Prino, E.Bruna Bari: G.Bruno, D.Elia. PWG 3 meeting, Cern 30/05/05. Outline. Study of the exclusive decay D + → K - π + π +
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Status of D+ analysis: Vertex reconstruction and event production Francesco Prino INFN Torino People working on D+ analysis: Torino:M.Masera, F.Prino, E.Bruna Bari: G.Bruno, D.Elia PWG 3 meeting, Cern 30/05/05
Outline • Study of the exclusive decay D+ → K-π+π+ • Physics motivation • Simulation strategy • Signal and background production • Present status of the GRID production • Reconstruction of the secondary vertex in the ITS • Comparison of different algorithms of vertex finding • Future plans
Physics motivation • GOAL:exclusive reconstruction of D± in the ALICE barrel • ITS used in the reconstruction of the secondary vertex • Probe the medium created in the collision with heavy quarks • Initial state effects (nuclear shadowing) • Final state effects (radiative energy loss) • Reference for the study of quarkonia production • Charm elliptic flow • This measurement can: • Tell to what degree charm interacts and thermalize • Validate quark coalescence models • Constrain dynamical scenarios • To be studied in semi-central events • Need to know: pT and f of charmed meson full reconstruction of decay products
Simulation strategy • Generate signal events with only D± decaying in Kpp: • Check the kinematics (done) • Optimize the vertexing algorithm (in progress) • Generate background events with HIJING • Add some charmed mesons in order to reproduce the charm yield predicted by MNR calculations • ≈ 170 mesons to be added per event • Evaluate the combinatorial background • Tune the cuts for analysis: • On the tracks used to “feed” the vertexer (pT, impact parameter) • On the quality of the secondary vertex (DCA, pointing angle)
Signal production • PYTHIA simulation: D+ forced to decay in K-p+p+ • D+ decays: both resonant via K0* (892) and non-resonant • PYTHIA tuning: NLO from MNR calculations and CTEQ4L PDFs • s1/2= 5.5 TeV • Magnetic Field = 0.5 T 1st , 2nd simulation tests on the Torino Farm (Feb. 2005) AliRootversion:v4-01-Rev05(the one used in the last Data Challenge) (→ problems with merging for the background production: see following slides) 3rd simulation test on the Torino Farm (Apr. 2005) AliRoot version: v4-02-Rev00 • 9100 D+ →K-p+p+per event, in the rapidity range |y|<2 leading to a charged multiplicity dN/dy ~ 6000 • 200 of such signal events produced in Torino’s farm for preliminary studies. • Goal: to have 5K signal events with an amount ofD→Kpp corresponding to 6X107 central Pb-Pb events
K p Mean = 0.87 Mean = 0.65 Mean = 0.67 Mean = 0.50 pT (GeV/c) pT (GeV/c) pand K from D+ non-resonant decay Plots refer to generated quantities HIJING central (normalized) Kinematics: pT distributions Pions and kaons from D+ decay vs. Pions and Kaons in Hijing central event
Non resonant Resonant Kinematics: Dalitz plots From reconstructed tracks ( : the info given by the generation are taken into account) This is done as an internal cross-check procedure
Background production • Old strategy • Use central Pb-Pb events from Data Challenge 04 • AliRoot v4-01-Rev05 • Add to each event 170 charmed mesons • HIJING does not reproduce the predicted amount of charm • Status of art at the last Alice Week • One Hijing event from PDC04 downloaded • Technical problems (not solved) with merging • Less ESD tracks in the merged event (with additional charm) than in the HIJING event • New strategy • Use the last AliRoot tagged version (v4-02-Rev00) • Generate new background events • Switch off charm (and beauty) generation in Hijing • COCKTAIL: Hijing cent1 (without charm and beauty)+PYTHIA (230 charmed mesons + 9 beauty mesons) • GOAL: produce 20k of such events on the GRID
Tool used for the production • GRID production • gLite Resource Broker + LCG2 • LCG (LFC) file catalogue • Running on italian sites (CNAF, Torino, Bari, LNL) • Generate 5K signal events and 20K background events • Files stored (at CNAF) for each event: galice.root AliESDs.root Kinematics.root TrackRefs.rootITS.RecPoints.root • < 2 GB per event Total Disk space ≈ 6TB • Steps: • Install AliRoot on the sites (done, but…) • Test grid submission and data retrieval with short jobs (done) • Submit 5K signal events (in progress) • STATISTICS: ≈1000 jobs submitted (27/5 morning) • Done (success) 661 (=68%) • Aborted 72 (=8%) (gLite communication problems) • Done with error 230 (28%) (143 NFS crash, 55 disk space, 28 Aliroot crash) • Submit 20K background events • Problem with Hijing central events under Scientific Linux (to be solved)
PrepareTracks Ntrks < Min Ntrks > Min FINDER FITTER NUsedTrks < Min NUsedTrks > Min VERTEX is correctly found Search for the secondary vertex Based on the class AliITSVertexerTracks.h,.cxx (primary vertex finding and fitting algorithm in p-p) • Main steps: • AliESD* event (requirement) • An object AliITSVertexerTracks is created • The method FindVertexForCurrentEvent is called • The object AliESDVertex is ready All these steps work also for the secondary vertex, passing 3 tracks and the AliESD* eventas input
Testing the vertexer • AliITSVertexerTracks applied on a Hijing Pb-Pb central event to find the primary vertex • Sets of 10 reconstructed tracks are passed to the vertexer • Compare the result of the finder (1st step) with the known MC primary vertex • Compare the result of the fitter (2nd step) with the known MC primary vertex
Testing the vertexer: results X (Finder+Fitter) – X (MC) X (Finder) – X (MC) FINDER Mean = -5.8 μm RMS = 63.2 μm Mean = -0.8 μm RMS = 122.9 μm FINDER + FITTER Y (Finder+Fitter) – Y (MC) Y (Finder) – Y (MC) Mean = 5.2 μm RMS = 118 μm Mean = 5.5 μm RMS = 64.4 μm Z (Finder) – Z (MC) Z (Finder+Fitter) – Z (MC) Mean = 1.4 μm RMS = 122.4 μm Mean = 2.8 μm RMS = 83.1 μm
Fitter better than finder Finder better than fitter X (Finder) – X (MC) X (Finder) – X (MC) The FITTER is not good The FITTER is good X (Finder+Fitter) – X (MC) X (Finder+Fitter) – X (MC) … but in ~30% of the cases the FITTER worsens the result of the finder Testing the vertexer: finder vs. fitter • Globally FITTER+FINDER has better resolution than FINDER alone
Additional checks on the fitter • The FITTER was “fed” with an external starting guess for the vertex position (instead of using the FINDER) • If the input position is within ~100 μm from the MC vertex the RMS of the residual distribution is always ~60 m (80 m for Z), even when the position coincides with the MC vertex • Is there room to improve the fitter? • If the input position is far from the MC vertex (~500 μm )the residual distribution broadens • In the real algorithm, when the FINDER misses the true vertex position by more than ~300-500 mm, the FITTER is also affected • Improve the finder (see next slides)
Old vertex finder • Based on the Straight Line Approximation (SLA) of a track (helix) • Developed to find the primary vertex in p-p • Main steps • The method receives N tracks as input • Each track is aproximated by a straight line in the vicinity of the primary vertex • An estimation of the secondary vertex from each pair of tracks is obtained evaluating the crossing point between the 2 straght lines • The method AliITSStrLine::Cross is used • The coordinates of secondary vertex are determined averaging among all the track pairs:
New vertex finder • Based on the Distance of Closest Aprroach (DCA) between helices • Does not use a Straight Line Approximation (SLA) as the old one • Main steps • The method receives N (N=3 in our case) tracks as input • For each pair of tracks, the coordinates of the 2 points of closest approach are calculated • The method AliITStrackV2::PropagateToDCA() is used • An estimation of the secondary vertex from each pair of tracks is obtained averaging the coordinates of the points defining the DCA • Two different implemetations: arithmetic vs. wieghted mean The weighted mean is implemented in the same way as for the V0 in AliV0Vertex class • The coordinates of secondary vertex are determined averaging among all the track pairs:
FINDER (SLA) - MC FINDER (DCA+w.mean) - MC FINDER (DCA) - MC Compare different finders (I) X coord Y coord Z coord RMS=179 μm RMS=182 μm RMS=165 μm RMS=167 μm RMS=170 μm RMS=160 μm RMS=165 μm RMS=169 μm RMS=152 μm
Finder (SLA) Finder (DCA) RMSX (μm) Pt min (GeV/c) RMSX (μm) DCA max (μm) Compare different finders (II) RMS (x coordinate) vs. max. DCA of the 3 tracks RMS (x coordinate) vs. minimum pT of the 3 tracks The other coordinates show the same behaviour
Finder (SLA) Finder (DCA) RMSX (μm) RMSX (μm) decay l.(μm) Compare different finders (III) RMS (x coordinate) vs. decay length of D meson RMS (x coordinate) vs. pT of D meson The other coordinates show the same behaviour
Pointing angle • Pointing angle = angle between the reconstructed p of the D+ meson and the segment connecting primary to secondary vertex • Cos(qpoint) should be 1, but suffers from pT and vertex resolution
RMS=174 μm RMS=173 μm RMS=182 μm Is there room to improve the finder? • Introduce a weighted mean when averaging between the vertices found from eack pair of tracks • 1st guess: weight each pair of tracks by 1/DCA • IDEA: the larger the DCA of the 2 considered tracks, the worse the vertex definition • RMS are larger than the one obtained without the weights • No real improvement obtained introducing a further weight
Comparing the different Finders • Main differences between the original Finder (SLA) of AliITSVertexerTracks and the new Finders (DCA) • In the SLA algorithm: • Track parameters calculated by prolonging the tracks to the primary vertex • Straight Line Approximation (SLA) of the tracks on the distance between the primary and the secondary (D→K ππ) vertex (c = 311.8 m ) • In the DCA algorithm • Cut on the maximum DCA (set at 1.5 mm) of a pair of tracks to be used in the vertex determination.
View on the transverse plane d (μm) y y’ d d (μm)is the distance between the secondary vertex and the tangent line d (μm) C X’ PTK(GeV/c) x D K Decay dist (μm) Straight line approximation • Geometrical calculations • d is of the order of tens of nm • the SLA does not give rise to syst effects
Resolution of the different Finders • With the DCA (new) Finders the residual distribution is ≈10 mm narrower with respect to the old (SLA) Finder • Better performance of the new algorithm • The resolution is further improved (especially for the Z coordinate) if the average between 2 tracks is weighted with the Sigma of the track • Try to add a weighted mean also in the SLA (old) Finder • The introduction of a futher weight when averaging the vertices of the different pairs of tracks does not seem to help • The different performance of te Finders does not come from the Straight Line Approximation, but rather from different preselection of track pairs • The cut on the maximum DCA for a pair od tracks to be accepted must be studied and tuned • As expected, the RMS of the residual distribution: • Decreases when the minimum pT of the 3 tracks increases • Increases when the maximum of the DCA between track pairs increases
Summary and future plans • gLite production • The setup of the grid tools has been completed • The production of the 5K signal events started (662 good events already obtained) • Reconstruction problems with AliRoot v4-02-Rev00 to be fixed before starting the production of the 20K background events • Start tuning the D+ analysis cuts as soon as production events will be ready • Secondary Vertex • A new Vertex Finder algorithm has been tested on 200 signal events • Small improvement (≈10-15 mm) with respect to the standard one • Additional tests on the Finder to be done • Use AliGenBox to generate pions originating from the surface of a sphere and study the performance of the Finder as a function of the radius of the sphere and the pT of the pions • Investigate on the Fitter
D±I(JP) = ½ (0-) m = 1869.4 MeV/c2 c = 311.8 m (PDG ’04) Hadronic 3 charged-bodies decays of D+ D+K-++ BR = 9.2 % N.B. The sum of these BRs is greater than 9.2% due to quantistic interference phenomena