alice offline week, cern, 22.10.08 andrea dainese 1 primary vertex with tpc-only tracks andrea...

ALICE Offline Week, CERN, 22.10.08 Andrea Dainese 1

Primary vertexPrimary vertexwith TPC-only trackswith TPC-only tracks

Andrea Dainese

INFN Legnaro

Motivation: TPC stand-alone analyses (multiplicity, spectra)


Barrel Tracking & Vertex Recons.

ITS local reconstruction (➔ RecPoints)

Vertex (x,y,z) from SPD tracklets

Barrel Tracking

Vertex with tracks

Compute diamond transverse profile, as mean and RMS of vertices in high-multiplicity events of each “LHC fill”

Repeat zvtx with SPD starting from mean (x,y)

important, to have zvtx for all events (even with one tracklet), w/o bias

Barrel Tracking

Vertex with tracks, using info on diamond profilefinal (x,y) resolution better than diamond RMS for ALL events

1st p

ass

on

sub

sam

ple

2nd p

ass

full

stat

istic

s

(twice: ITS+TPC, TPC only; both stored in ESD)

(twice: ITS+TPC, TPC only; both stored in ESD)

(twice: ITS+TPC, TPC only)


Vertex reconstruction with tracks AliVertexerTracks

Three main “tasks” implemented:TRACKS SELECTION try reject secondary tracks

VERTEX FINDINGgive first estimate of vtx position

VERTEX FITTING:give optimal estimate of the position of the vertexgive vertex covariance matrix

give a (vertex quality)


Primary vertex reco with tracksAliVertexerTracks::FindPrimaryVertex()

Primary vertex reconstruction: iterative procedureSelect tracks

track points to fiducual cylinder (R<3 cm, |z|<30 cm) for ITS+TPC: kITSrefit and >4 ITS clusters for TPC: kTPCin, >50 cls and |tg|<1

0th iteration: VERTEX FINDING, using track pairs vtx0

1st iteration: TRACKS SELECTION: |d0

3D(vtx0)| < 0.5 cm for ITS+TPC < 2.5 cm for TPC

VERTEX FINDING VERTEX FITTING vtx1

2nd iteration: TRACKS SELECTION: |d0

r(vtx1)| < 3 d0

VERTEX FINDING VERTEX FITTING vtx2 (final estimate)


Impact parameter distr. of TPC tracks

r z

reasonable pulls


Diamond constraintWhy diamond constraint?

vertex also for events with 1 “primary” trackbetter rejection of secondaries(x,y) resolution always better than diamond size

Exercise on CAF (as it could be done on the data)use 10k triggered events, pp 900 GeVselect vertices with > 10 tracksweighted average of (x,y,z)diamond:

centre: weighted average (tried also with plain average: compatible results)

sigma: error of weighted average (20-30 m) diamond rms from machine (200 m @ 900 GeV)

use this diamond as constraint and re-evaluate vertices (2nd reconstruction pass)


TPC Vertex efficiencyEfficiency: events with vertex / triggered (MB1=SPD||V0A||V0C)

Different normalization: events with vertex / events triggered and with >0

TPC tracks pointing to the beam pipe

no constr. (1st pass) with constr. (2nd pass)PYTHIA 900 0.5T 73% 85%PHOJET 900 0.5T 81% 91%PHOJET 900 0T 78% 91%

no constr. (1st pass) with constr. (2nd pass)PYTHIA 900 0.5T 83% 98%PHOJET 900 0.5T 87% 98%PHOJET 900 0T 83% 97%


TPC Vertex efficiency

PHOJET 900 0T PHOJET 900 0T

PHOJET 900 0.5T PHOJET 900 0.5T


First Physics test production

(TPC misaligned, miscalibrated)

Ideal simulation and reconstructionno constraint

no constraint

with constraint

with constraint

Resolutions and pulls (PHOJET 900 0T)closed markers: RMS; open markers: fit


CodeAliVertexerTracks can take an array (TObjArray) of AliExternalTrackParam tracks

Separate “MeanVertex” objects in OCDB for ITS+TPC and TPC-only

Selection criteria for AliVertexerTracks kept in new class AliGRPRecoParam (from rec.C or OCDB)

separate criteria (incl. constraint flag) for ITS and TPC “mode”

performance optimisation without touching compiled code

In AliReconstruction: AliVertexerTracks tVertexer;

// TPC + ITS primary vertex tVertexer.SetVtxStart(fDiamondProfile); // from OCDBtVertexer.SetITSMode(grpRecoParam->GetITSCuts());... find vertex ESD// TPC-only primary vertex tVertexer.SetVtxStart(fDiamondProfileTPC); // from the OCDBtVertexer.SetTPCMode(grpRecoParam->GetTPCCuts());... find vertex ESD


NextThe extraction of the mean vertex and storage in the OCDB is run manually on CAFOK for first data at 900 GeV

check stability of diamond position vs time, run#, ...

Later the procedure will have to become automatic and run on ESDs accessed on grid

how to identify the run/event/time at which the diamond moves?


Comparison with MC vertex: shift

First Physics test production

(TPC misaligned, miscalibrated)

Ideal simulation and reconstruction


Tracks SelectionIdea: use “primary” tracks for vertex reconstruction

Only tracks with 5 or 6 points in ITS

Select primary tracks on the basis of their d0

Cut |d0| < nd0(pt)

d0(pt) = vtx track(pt)

track

vertex x

yfrom track’s covariance matrix

need anestimate of the vertexand its cov. matrix


Vertex Fitter“Fast Vertex Fitting”, CMS Note 1997/051

Tracks are propagated to the point given by the vtx finder

Each track gives an independent measurement of the vertex position: a 2 is written as the sum of the single track

2s w.r.t. a generic vertex position rvtx:

where ri is the (x,y,z) position of the ith track and Vi = Wi-1 is

the cov matrix of ri, extracted from the track’s cov matrix

The solution that minimizes this 2 is analytic:

)rr( W)rr()r()r( 22ivtxi

T

iivtx

ivtxivtx

ii

ii

ivtx rWWr1

1

WV

ii

vertexvertex

covariancematrix


Easy to add “beam constraint”

Same procedure as before + constraint:

iii

iivtx

rWrVWVr b

1-b

11

b

11

b WVV

ii

vertex

vertexcovariance

matrix

zz

yyxy

xyxx

z

y

x

wgtd

wgtd

wgtd

00

0

0

V ;r bbwhere:

alice offline week, cern, 22.10.08 andrea dainese 1 primary vertex with tpc-only tracks andrea...

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