Status of D analysis:

1
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
2
Outline

• Study of the exclusive decay D ? K-pp
• 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

3
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

4
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)

5
Signal production

• PYTHIA simulation D forced to decay in K-pp
• 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) AliRoot version 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-pp per event, in the rapidity
  range ylt2 leading to a charged multiplicity
  dN/dy 6000
• 200 of such signal events produced in Torinos
  farm for preliminary studies.
• Goal to have 5K signal events with an amount of
  D ? Kpp corresponding to 6X107 central Pb-Pb
  events

6
Kinematics pT distributions
Pions and kaons from D decay vs. Pions and Kaons
in Hijing central event
7
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
8
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

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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.root ITS.RecPoints.root
• lt 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)

10
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 event
as input
11
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

12
Testing the vertexer results
X (FinderFitter) 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 (FinderFitter) 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 (FinderFitter) Z (MC)
Mean 1.4 µm RMS 122.4 µm
Mean 2.8 µm RMS 83.1 µm
13
Testing the vertexer finder vs. fitter

• Globally FITTERFINDER has better resolution than
  FINDER alone

14
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)

15
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 AliITSStrLineCross is used
• The coordinates of secondary vertex are
  determined averaging among all the track pairs

16
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 (N3 in our case) tracks as
  input
• For each pair of tracks, the coordinates of the 2
  points of closest approach are calculated
• The method AliITStrackV2PropagateToDCA() 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

17
Compare different finders (I)
X coord
Y coord
Z coord
RMS179 µm
RMS182 µm
RMS165 µm
RMS167 µm
RMS160 µm
RMS170 µm
RMS165 µm
RMS152 µm
RMS169 µm
18
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
19
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
20
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

21
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

22
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 p p) 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.

23
Straight line approximation

• Geometrical calculations

• d is of the order of tens of nm
• the SLA does not give rise to syst effects

24
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

25
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

26
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