Status of DStatus of D++ analysis: analysis:

Vertex reconstruction and Vertex reconstruction and event productionevent 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

OutlineOutline

• Study of the exclusive decay D+ → K-π+π+ Physics motivation

• Simulation strategySignal and background productionPresent status of the GRID production

• Reconstruction of the secondary vertex in the ITSComparison of different algorithms of vertex

finding

• Future plans

Physics motivationPhysics motivation• GOAL: exclusive reconstruction of exclusive reconstruction of DD±± 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 of charmed meson

full reconstruction of decay products

Simulation strategySimulation strategy• Generate signalsignal events with only D± decaying in K:

Check the kinematics (done) Optimize the vertexing algorithm (in progress)

• Generate backgroundbackground 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 productionSignal production PYTHIA simulation: D+ forced to decay in K-++

D+ decays: both resonant via K0* (892) and non-resonantPYTHIA tuning: NLO from MNR calculations and CTEQ4L PDFs s1/2 = 5.5 TeVMagnetic Field = 0.5 T

1st , 2nd simulation tests on the Torino Farm (Feb. 2005)

AliRoot version: v4-01-Rev05v4-01-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-Rev00v4-02-Rev00

• 9100 D+ → K-++ 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 of D → K corresponding to 6X107 central Pb-Pb events

Kinematics: pKinematics: pTT distributions distributions

Plots refer to generated quantities

and from D+ non-resonant decayHIJING central (normalized)

Mean = 0.67

Mean = 0.50

pT (GeV/c)

Mean = 0.87

Mean = 0.65

pT (GeV/c)

Pions and kaons from D+ decay vs. Pions and Kaons in Hijing central event

Kinematics: Dalitz plotsKinematics: Dalitz plots

From reconstructed tracksFrom reconstructed tracks ( : the info given by the generation are taken into account)( : the info given by the generation are taken into account)

Non resonantNon resonant ResonantResonant

This is done as an internal cross-check procedure

Background productionBackground 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 productionTool 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.root ITS.RecPoints.root

< 2 GB per event Total Disk space ≈ 6TB• Steps:

1. Install AliRoot on the sites (done, but…)2. Test grid submission and data retrieval with short jobs (done)3. 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)

4. Submit 20K background events Problem with Hijing central events under Scientific Linux (to be solved)

Search for the secondary vertexSearch for the secondary vertexBased on the class AliITSVertexerTracks.h,.cxxAliITSVertexerTracks.h,.cxx (primary vertex finding and fitting algorithm in p-p)

Main steps:

1. AliESD* event (requirement)

2. An object AliITSVertexerTracks is created

3. The method FindVertexForCurrentEvent is called

4. The object AliESDVertex is ready

Ntrks > Min

FITTER

NUsedTrks < Min

PrepareTracks

Ntrks < Min

FINDER

NUsedTrks > Min

VERTEX is correctly found

All these steps work also for the secondary vertex, passing 3 tracks and the AliESD* event as input

Testing the vertexerTesting 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: resultsTesting the vertexer: resultsX (Finder) – X (MC)

Z (Finder) – Z (MC)

Y (Finder) – Y (MC)

FINDER X (Finder+Fitter) – X (MC)

Z (Finder+Fitter) – Z (MC)

Y (Finder+Fitter) – Y (MC)

Mean = -0.8 μm

RMS = 122.9 μm

Mean = 5.2 μmRMS = 118 μm

Mean = 1.4 μmRMS = 122.4 μm

Mean = -5.8 μmRMS = 63.2 μm

Mean = 5.5 μmRMS = 64.4 μm

Mean = 2.8 μmRMS = 83.1 μm

FINDER + FITTER

Testing the vertexer: finder vs. fitterTesting the vertexer: finder vs. fitter• Globally FITTER+FINDER has better resolution than FINDER alone

Finder better than fitter

X (Finder) – X (MC)

Fitter better than finder

X (Finder) – X (MC)

X (Finder+Fitter) – X (MC) X (Finder+Fitter) – X (MC)

The FITTER is not good The FITTER is good

… but in ~30% of the cases the FITTER worsens the result of the finder

Additional checks on the fitterAdditional 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 m, the FITTER is also affected• Improve the finder (see next slides)

Old vertex finderOld vertex finder• Based on the Straight Line Approximation (SLA) of a

track (helix) Developed to find the primary vertex in p-p

• Main steps1. The method receives N tracks as input2. Each track is aproximated by a straight line in the vicinity of

the primary vertex3. 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

4. The coordinates of secondary vertex are determined averaging among all the track pairs:

ijij

pairsfound x

Nx

1

ijij

pairsfound y

Ny

1 ij

ijpairs

found zN

z1

New vertex finderNew 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 steps1. The method receives N (N=3 in our case) tracks as input2. For each pair of tracks, the coordinates of the 2 points of closest

approach are calculated The method AliITStrackV2::PropagateToDCA() is used

3. 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

4. The coordinates of secondary vertex are determined averaging among all the track pairs:

ij

ijpairs

found xN

x1

ij

ijpairs

found yN

y1

ijij

pairsfound z

Nz

1

Compare different finders (I)Compare different finders (I)

FINDER (SLA) - MC

FINDER (DCA+w.mean)

- MC

FINDER (DCA) - MC

X coord Y coord Z coordRMS=179 μm

RMS=167 μm

RMS=165 μmRMS=182 μm

RMS=160 μmRMS=170 μm

RMS=165 μm RMS=152 μmRMS=169 μm

Compare different finders (II)Compare different finders (II)

RMS (x coordinate) vs. max. DCA of the 3 tracks

RMS (x coordinate) vs. minimum pT of the 3 tracks

Finder (SLA)Finder (DCA)

The other coordinates show the same behaviour

RM

SX

(μm

)

DCA max (μm)Pt min (GeV/c)

RM

SX

(μm

)

Compare different finders (III)Compare different finders (III)

RM

SX

(μm

)

decay l.(μm)

RM

SX

(μm

)

Finder (SLA)Finder (DCA)

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 anglePointing angle• Pointing angle = angle between the reconstructed p

of the D+ meson and the segment connecting primary to secondary vertex Cos(point) should be 1, but suffers from pT and vertex

resolution

Is there room to improve the finder?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=173 μm RMS=174 μmRMS=182 μm

RMS are larger than the one obtained without the weightsNo real improvement obtained introducing a further weight

Comparing the different FindersComparing 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 π π) 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.

Straight line approximationStraight line approximation• Geometrical calculations

x

y

DD

K

X’

y’

C

View on the transverse plane

PTK(GeV/c)

d (μm)

Decay dist (μm)

d (μm)

d is of the order of tens of nm

the SLA does not give rise to syst effects

d

d (μm) is the distance between the secondary vertex and the tangent line

Resolution of the different FindersResolution of the different Finders• With the DCA (new) Finders the residual distribution is ≈10 m

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 plansSummary 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 m) 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

Hadronic 3 charged-bodies decays of DHadronic 3 charged-bodies decays of D++

DD++KK--+++ + BR = 9.2 %BR = 9.2 %

N.B. The sum of these BRs is greater than 9.2% due to quantistic interference phenomena

D± I(JP) = ½ (0-)

m = 1869.4 MeV/c2

c = 311.8 m

(PDG ’04)

Non resonant D+→K-++ BR = 8.8 %

Resonant

D+→K*0(892)+→K-++ BR = 1.3 %

D+→K*0(1430)+→K-++ BR = 2.3 %

D+→K*0(1680)+→K-++ BR = 3.8·10-3 %