Open charm reconstruction in the ALICE experiment

Open charm reconstruction in the ALICE experiment
Elena Bruna Supervisor: Prof. Massimo Masera Seminar for the end of 2nd year (XIX) – Torino, Dec 2nd 2005

Outline Physics motivations of open charm analysis in Heavy Ion Collisions D+ → K-π+π+ : overview of the kinematics Measurement of open charm in the ALICE experiment Exclusive reconstruction of D+ → K-π+π+: Event generation and reconstruction Reconstruction of the secondary vertex Selection strategy Perspectives for the measurement of D+ elliptic flow Summary and work plans Elena Bruna

Motivations for the Open Charm physics in Heavy Ion Collisions
Elena Bruna

Heavy quarks as probes of nuclear medium /1
charm, bottom produced at early stages of the collision (timescale ~ 1/mQ < QGP ~ 10 fm at LHC) Studies of initial state effects: nuclear shadowing Because of the very low x down to ~10-4 at LHC the so many gluons merge together, affecting the partons densities at low x w.r.t. protons partons ones. thermal production  The c quark might be produced in the plasma phase: mc (~ 1.2 GeV) comparable with predicted Tplasma (~ GeV) open QQ production (not Drell-Yan) natural normalization for QQ studies  Quarkonia enhancement at low PT and suppression at high PT. Elena Bruna

charm, bottom have long lifetime (> QGP ) and can probe the bulk, strongly interacting phase Studies of final state effects: 1) radiative energy loss Hard partons radiate gluons in the medium, lose energy and become quenched. Heavy quarks are expected to lose less energy than light quarks. High E  suppression of the produced particles (at high PT)  RAA≠1 Nuclear modification factor It depends on the properties of the medium (gluon density, temperature and volume), it provides information on such properties. Elena Bruna

Studies of final state effects: 2) anisotropic flow on the transverse plane Elliptic Flow = collective motion of particles (due to high pressure arising from compression and heating of nuclear matter) superimposed on top of the thermal motion Correlation between azimuthal angles  of outgoing particles and the direction of the impact parameter (REACTION PLANE RP) Elliptic flow coefficient High opacity of the medium (strongly interacting)  high anisotropic flow  high v2 v2 provides information on the opacity of the medium. Elena Bruna

Few experimental results from RHIC /1
radiative energy loss - RAA of the D mesons ( PT spectra of e+e- from D semileptonic decays ) q = 14 GeV2/fm q = 4 GeV2/fm q = 0 GeV2/fm dNg / dy = 1000 from QM05 from QM05 Charm is suppressed! Suppression is approximately the same as for hadrons. Challenge for energy loss models. Also pp and pA data are needed as reference! Elena Bruna

Few experimental results from RHIC /2
anisotropic flow – v2 of the D mesons ( f spectra of e+e- from D semileptonic decays ) from QM05 from QM05 Significant flow of charm quark as for light quarks  Strong coupling of charm quark to the medium Indication for reduction of v2 at pT > 2 GeV/c (PHENIX) Also pp and pA data are needed as reference! Elena Bruna

D+ → K-p+p+ : overview of the kinematics
Elena Bruna

Why D+ → K-p+p+ ? Advantages… …drawbacks
D+ has a “long” mean life (~311mm compared to ~123 mm of the D0) D+ → K-p+p+ is a 3-charge body decay  the most promising from an experimental point of view D+ → K-p+p+ has a relatively large branching ratio (BR=9.2% compared to 3.8% for D0 → K-p+). …drawbacks Combinatorial background for this 3-body channel is larger than for D0 → K-p+. The average PT of the decay product is softer (~ 0.7 GeV/c compared to ~ 1 GeV/c) Elena Bruna

Hadronic 3-charge-body decays of D+
D± I(JP) = ½ (0-) m = MeV/c2 c = m (PDG ’04) D+K-++ BR = 9.2 % D+→K-p+p+ Non Resonant BR = 8.8 % D+→K*0(892)p+→K-p+p+ Resonant BR = 1.3 % D+→K*0(1430)p+→K-p+p+ BR = 2.3 % D+→K*0(1680)p+→K-p+p+ BR = 3.8·10-3 % Elena Bruna

Kinematics (1) K PT distributions of the generated particles (ONLY PYTHIA generation, NO propagation and reconstruction in the detector) (nonresonant events) Mean = 0.87 GeV/c D Mean = 1.66 GeV/c  Mean = 0.67 GeV/c Knowledge of the PT shapes of the decay products important at the level of the selection strategy Elena Bruna

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

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

Measurement of open charm in the ALICE experiment
Elena Bruna

ALICE @ LHC setup HMPID TRD MUON SPECTR.. PHOS
Time Projection Chamber (TPC) Tracking, PID (dE/dx) -0.9<<0.9 LHC setup HMPID TRD MUON SPECTR.. PHOS Inner Tracking System (ITS): 6 SILICON layers (pixel, drift, strip) Vertices reconstruction, PID (dE/dx) -0.9<<0.9 Time Of Flight (TOF) Tracking, PID (time) -0.9<<0.9 Size: 16 x 26 m Weight: ~10,000 tons Elena Bruna

Track Impact Parameter d0
SIGMA (fit) expected d0 resolution (s) d0 – d0 sim MEAN (fit) 0.4<Pt<0.6 GeV/c Elena Bruna

Track Impact Parameter : d0 pull
SIGMA (fit) Calculate the pull MEAN (fit) Elena Bruna

Exclusive reconstruction of D+ → K-p+p+
Elena Bruna

Simulation strategy Our purpose: exclusive reconstruction of D± in the ALICE barrel (Inner Tracking System employed in the search for secondary vertexes) Too large statistics (108 events) would be required to study the signal!! Central Pb-Pb event (b<3.5 fm, dN/dy = 6000, √s=5.5 TeV) ~ 9 D+/D- in |y|<1 Signal and background events separately generated with the Italian GRID 5’000 signal events with only D± decaying in Kpp (using PYTHIA): Check the kinematics and the reconstruction Optimize the vertexing algorithm 20’000 background events (central Pb-Pb events using HIJING): cc pairs merged in addition in order to reproduce the charm yield predicted by NLO pQCD calculations (≈ 118 per event) Tune the cuts (impact parameter cut,…) on the tracks to be analyzed by the vertexing algorithm Evaluate the combinatorial background Elena Bruna

Reconstructed signal events: Dalitz Plots
From reconstructed tracks ( : the info given by the generation are taken into account) This is done as an internal cross-check procedure Non resonant Resonant Elena Bruna

Reconstructed signal events: D+ invariant mass
Mean Integrated over PT MEAN = GeV/c2 RMS = GeV/c2 this is not a complete reconstruction of the signal: tracks are grouped by means of info. stored at generation time. MINV Resolution (SIGMA of the gaussian fit) Knowledge of MINV resolution vs PT is important when selecting the signal candidates Elena Bruna

Reconstruction of the secondary vertex for D+ → K-p+p+
First idea: adapting and improving the method already written for the primary vertex finding and fitting in p-p Second idea: writing a new secondary vertex finder and comparing its performace with the previous ones Elena Bruna

Vertex finder Originally developed to find the primary vertex in p-p
Based on the Straight Line Approximation of a track (helix) Main steps The method receives N (N=3 in our case) tracks as input Each track is approximated 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 straight lines The coordinates of secondary vertex are determined averaging among all the track pairs: Elena Bruna

Improving the Straight Line Vertex Finder
Add a cut on the distance of closest approach (DCA) between the two straight lines A pair of tracks is not used for the vertex estimation if their distance of closest approach is > fDCAcut Use a weighted mean of the 2 DCA points In order to take into account the errors on the tracks parameters Calculate a parameter representing the dispersion of the vertices given by the track pairs (fSigma) Elena Bruna

DCA cut effect No DCAcut fDCAcut = 1.5 mm fDCAcut = 0.7 mm X coord
RMS=179 μm RMS=182 μm RMS=165 μm fDCAcut = 1.5 mm Finder- MC (mm) RMS=178 μm RMS=181 μm RMS=163 μm fDCAcut = 0.7 mm Finder- MC (mm) X coord RMS=179 μm Finder- MC (mm) Y coord RMS=183 μm Finder- MC (mm) Z coord RMS=166 μm Finder- MC (mm) Elena Bruna

Weighted mean effect Arithmetic mean Weighted mean X coord
RMS=179 μm RMS=183 μm RMS=160 μm Finder- MC (mm) X coord RMS=179 μm Finder- MC (mm) Improved resolution on Z Y coord RMS=183 μm Finder- MC (mm) Z coord RMS=166 μm Finder- MC (mm) Elena Bruna

Vertices dispersion Dispersion fSigma = standard deviation of the 3 vertex estimations obtained from each track pair fSigma (cm) The DCA cut (at 0.7 mm) reduces the dispersion Elena Bruna

Cutting on fSigma All events
RMS=700 μm Finder- MC (mm) Cutting on fSigma A cut fSigma < 0.4 cm cuts 0.5% of the events and ≈30% of the overflows and underflows (i.e. events for which the VertexFinder misses the true vertex by more than 1 mm) fSigma < 0.4 cm RMS=224 μm Finder- MC (mm) A cut fSigma < 0.07 cm (700 mm) cuts 6.4% of the events and gives a RMS of 151 mm (for X coordinate) fSigma < 0.07 cm RMS=151 μm Finder- MC (mm) Elena Bruna

Another improvement: Helix vertex finder
Based on the Distance of Closest Approach (DCA) between helices Does not use a Straight Line Approximation 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 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 coordinates of secondary vertex are determined averaging among all the track pairs: The dispersion of the vertices given by the track pairs is calculated Elena Bruna

Results from the helix finder
Straight Line Finder RMS=169 μm RMS=171 μm RMS=162 μm Helix Finder Finder- MC (mm) X coord RMS=179 μm Helix finder has better resolution and also a lower number of overflows and underflows (≈400 instead of ≈650) Finder- MC (mm) Y coord RMS=183 μm Finder- MC (mm) Z coord RMS=166 μm Finder- MC (mm) Elena Bruna

DCA cut effect on helix finders
fDCAcut=1 cm RMS=168 μm RMS=170 μm RMS=161 μm fDCAcut=1.5 mm Finder- MC (mm) RMS=167 μm RMS=169 μm RMS=158 μm fDCAcut=0.7 mm Finder- MC (mm) X coord X coord RMS=169 μm Finder- MC (mm) Y coord RMS=171 μm Finder- MC (mm) Z coord Z coord RMS=162 μm Finder- MC (mm) Elena Bruna

Weighted mean effect on helix finder
Arithmetic mean RMS=168 μm RMS=169 μm RMS=154 μm Weighted mean Finder- MC (mm) X coord RMS=169 μm Finder- MC (mm) Improved resolution on Z Y coord RMS=171 μm Finder- MC (mm) Z coord RMS=162 μm Finder- MC (mm) Elena Bruna

Vertices dispersion on Helix Finder
The DCA cut reduces the dispersion fSigma (cm) Same distribution as for Straight Line finder Elena Bruna

Cutting on fSigma All events
RMS=480 μm Finder- MC (mm) Cutting on fSigma A cut fSigma < 0.4 cm cuts 0.5% of the events and ≈35% of the overflows and underflows (i.e. events for which the VertexFinder misses the true vertex by more than 1 mm) fSigma < 0.4 cm RMS=209 μm Finder- MC (mm) A cut fSigma < 0.07 cm (700 mm) cuts 5.6% of the events and gives a RMS of 140 mm (for X coordinate) fSigma < 0.07 cm RMS=140 μm Finder- MC (mm) Elena Bruna

New secondary vertex finder
Straight Line Approximation used → analytic method Vertex coordinates (x0,y0,z0) from minimization of: Where: d1,d2,d3 are the distances (weighted with the errors on the tracks) of the vertex from the 3 tracks: P1 (x1,y1,z1) SecondaryVertex (x0,y0,z0) σx = σy d1 Elena Bruna

Resolution of the vertex finder
RMS x RMS y At high Pt of D+ (Pt>5-6 GeV/c), the RMS in the bending plane increases, instead of going down to ~15µm (spatial pixel resolution) as expected. RMS z Conclusion New method improves RMS of ~40μm for PtD+ ~ 2GeV/c for x, y and z with respect to previous Helix vertex finder based on DCA of pairs of tracks. Elena Bruna

Resolution at high Pt /1 Checks with events only made of pions show that the RMS on the bending plane: Decreases down to 50 µm if the 3 tracks have Pt ~ 2 GeV/c Reaches a value of ~20 µm (in agreement with spatial pixel resolution) if the 3 tracks have Pt =100 GeV/c 3 pion vertex: RMS in the bending plane vs. Pt Elena Bruna

Resolution at high Pt /2 In the signal events, as the Pt of the D+ increases, the “daughters” become more and more co-linear, resulting in a worse resolution along the D+ direction. x y x y π+ y’ x’ rotated π+ K- bending plane D+ Elena Bruna

Resolution in the rotated frame /1
Along the Pt of the D+ (x’ coord.) Orthogonal to the Pt of the D+ (y’ coord.) → Along the Pt of the D+: as Pt increases (for Pt>5-6 GeV/c) the angles between the decay tracks become smaller: in this coordinate the RMS increases → Orthogonal to the Pt of the D+: the RMS decreases as expected Elena Bruna

Resolution in the rotated frame /2
RMS along Pt RMS orthog Pt Ratios: RMS along Pt RMS z RMS orthog Pt RMS z Elena Bruna

Vertices dispersions/1
Δx = XVertex FOUND – XVertex MC Δx < 1000 μm 1000<Δx <3000 μm 3000<Δx <5000 μm Δx > 5000 μm fSigma bigger for bad vertices fSigma (cm) Elena Bruna

Vertices dispersions/2
Cut on fSigma (for X coordinate) Vertices taken / Vertices Tot (“True” vertices) “Fake” vertices (tracks coming from 3 different D+ vertices) RMS x (μm) Mean x (μm) fSigma < 0.7 cm cuts ~1% of the events and gives a RMS of 130 μm fSigma < 0.5 cm cuts ~6% of the events and gives a RMS of 110 μm Elena Bruna

Conclusions on the finders
The Straight Line vertex finder: DCA cut: negligible effect on the RMS of the residual distributions, slightly reduced number of overflows and underflows The use of a weighted mean: improves Z resolution by ≈6 mm Cutting on the dispersion fSigma: removes the events for which the VertexFinder misses the true vertex by more than 1 mm and improves the resolution The Helix vertex finder: Has better resolution w.r.t. Straight Line finder (by approximately 10 mm) Has less overflows and underflows w.r.t. Straight Line finder DRAWBACK: the DCA between helices is obtained by minimization DCA cut, weighted mean and fSigma cut: improve the resolution The Minimum Distance vertex finder: Has better resolution w.r.t. Helix finder (by approximately 30 mm) Has less overflows and underflows w.r.t. previous finders Is an analytic method Weighted mean and fSigma cut: improve the resolution Is presently THE candidate for first D+ analysis A cut on fSigma has to be tuned (it can be done at analysis level) Elena Bruna

D+ selection strategy Elena Bruna

Tuning the cuts GOAL: tune the cuts on both signal and background events and find the cuts giving the best S/B. (S/B = 11% was found for the D0K-p+) CUT TIPOLOGIES: On the single tracks used to “feed” the vertexer (Particle Identification, pT, track impact parameter)  reduce the number af all the possible combinations of track-triplets in a central Pb-Pb collision (~ 1010 without any initial cut!!). It MUST be cut by 4-5 orders of magnitude before using the more time-consuming vertexer. In progress. Once the triplets are combined, additional cuts (invariant mass and eventually pT, impact parameter) are mandatory before using the vertexer. These cuts are done on the triplets. To be done. The third kind of cuts is applied on the quality of the secondary vertices found (vertex dispersion-fSigma, pointing angle,…) Elena Bruna

MINIMUM Triplet BKG taken
Single track cuts /1 GOAL: find a compromise between the number of background triplets and the number of signals we want to take HOW: for each triplet (both signal and bkg) a loop on all the possible cuts (d0,Pt p,Pt K) is done % SIGNAL TAKEN Pt cut p (GeV/c) Pt cut K (GeV/c) d0 cut (mm) MINIMUM Triplet BKG taken 1 - 2 1,200 1,175 120 131 3 - 4 0,875 0,775 95 77.000 4 - 5 1,400 1,150 1,000 0,800 0,750 0,550 0,525 0,350 0,325 0,275 0,300 Cut on the track impact parameter (d0) Particle Id. given by the generation: initial approach The number of BKG triplets is reduced by a factor of ~100 when doing the cut on the Invariant Mass within 3s (see slide 22) Elena Bruna

Cut on d0  lower cuts on Pt (useful up to Bkg ~105)
Single track cuts /2 Triplet BKG Pt cut p (GeV/c) Pt cut K (GeV/c) d0 cut (mm) % MAX signal taken 1,325 1,200 105 0,9 0,900 0,800 85 3,1 1,225 1,000 6,0 0,975 0,775 11,0 0,750 0,600 19,4 1010 – 1011 0,000 100,0 The number of BKG triplets is reduced by a factor of ~100 when doing the cut on the Invariant Mass within 3s (see slide 22) Bkg=Triplets No cut on the track impact parameter (d0) Cut on d0  lower cuts on Pt (useful up to Bkg ~105) Particle Id. given by the generation: initial approach Elena Bruna

Tuning the single track cuts /2
When tuning a cut, one has to keep in mind how the Pt distribution of the D+ is modified Pt reconstructed D+ Mean=2.5 GeV/c Pt reconstructed D+ Pt cut (p) = 0.75 GeV/c Pt cut (K) = 0.6 GeV/c Mean=1.8 GeV/c Ratio: With cut / Wo cut Elena Bruna

Perspectives for the measurement of D+ elliptic flow
Elena Bruna

Measurement of v2 Elliptic Flow = correlation of particle emission angles with the reaction plane (i.e. w.r.t the impact parameter of the collision) Calculate the 2nd order coefficient of Fourier expansion of particle azimuthal distribution relative to the reaction plane The reaction plane is unknown. Estimate the reaction plane from particle azimuthal anisotropy: Yn = Event plane = estimator of the unknown reaction plane Calculate particle distribution relative to the event plane Correct for event plane resolution Resolution contains the unknown YRP Can be extracted from sub-events Event plane resolution Elena Bruna

Motivation and Method GOAL: Evaluate the statistical error bars for measurements of v2 for D± mesons decaying in Kpp v2 vs. centrality (pT integrated) v2 vs. pT in different centrality bins TOOL: fast simulation Assume to have only events with signal Generate ND±(Db, DpT) events with 1 D± per event For each event Generate a random reaction plane (fixed YRP=0) Get an event plane (with correct event plane resolution) Generate the D+ azimuthal angle (φD) according to the probability distribution p(φ)  1 + 2v2 cos [2(φ-YRP)] Smear φD with the experimental resolution on D± azimuthal angle Calculate v′2(D+), event plane resolution and v2(D+) Elena Bruna

D+ azimuthal angle resolution
MEAN RMS Average  resolution = 8 mrad = 0.47 degrees Elena Bruna

D+ statistics bmin-bmax (fm) s inel Pb-Pb (%) Nevents (106) Ncc per event D± yield 0-3 3.6 0.72 118 45.8 3-6 11 2.2 82 31.8 6-9 18 42 16.3 9-12 25.4 5.1 12.5 4.85 12-18 8.4 1.2 0.47 Nevents for 2·107 Minimum Bias triggers (without any requirement on the impact parameter of the collision) D+ selected after all the cuts is still missing: for the time being  e=1.5% (same as D0) ND±(Db, DpT) selected = e × D+ reconstructed  Total number of ND±(Db, DpT) selected Normalized to 2·107 Minimum Bias Events Elena Bruna

Results: v2 vs. centrality
2·107 Minimum Bias events bmin-bmax N(D±)selected s(v2) 0-3 1070 0.024 3-6 2270 0.015 6-9 1900 0.016 9-12 800 0.026 12-18 125 0.09 Error bars quite large Would be larger in a scenario with worse event plane resolution May prevent to draw conclusions in case of small anisotropy of D mesons Elena Bruna

Results: v2 vs. pT 2·107 MB events pT limits N(D±)sel s(v2) pT limits
0-0.5 140 0.06 0.5-1 280 0.04 1-1.5 390 1.5-2 360 2-3 535 0.03 3-4 250 0.05 4-8 265 8-15 50 0.11 pT limits N(D±)sel s(v2) 0-0.5 120 0.06 0.5-1 230 0.05 1-1.5 330 0.04 1.5-2 300 2-3 450 0.03 3-4 210 4-8 220 8-15 40 0.11 pT limits N(D±)sel s(v2) 0-0.5 50 0.10 0.5-1 100 0.07 1-1.5 140 0.06 1.5-2 125 2-3 190 0.05 3-4 90 4-8 95 8-15 20 0.15 Elena Bruna

Summary and work plans Preparatory checks on the kinematics and on the reconstructed signal events: completed Secondary Vertex: completed the method of the Minimum Distance of 3 tracks is presently THE candidate for first D+ analysis cuts on fSigma will be tuned at the analysis level D+ analysis cuts the work on the cuts on “single tracks” to feed the vertexer is in progress: Pt, impact parameter, PID. The work on the cuts in the “triplets” and on the secondary vertices has to be done. Analysis on D+ elliptic flow: in progress Elena Bruna

Open charm reconstruction in the ALICE experiment

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