Open charm: D

1
Open charm D
Elena Bruna University of Torino and I.N.F.N.
People working on D analysis Torino M.Masera,
F.Prino, E.Bruna Bari G.Bruno, D.Elia
Meeting ALICE Italia, Cagliari 4/05/05
2
Outline

• D ? K-pp
• Simulation strategy
• - signal and background production
• - tool used for the purpose
• Reconstruction of the secondary vertex in the
  ITS
• - comparison of different algorithms
  for the vertex finder
• Future plans

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Hadronic 3-charge-body decays of D
D?K-?? BR 9.2
4
Simulation strategy
Our purpose exclusive reconstruction of D in
the ALICE barrel (ITS employed in the search for
secondary vertexes)

• Generate signal events with only D decaying in
  Kpp
• Check the kinematics (done)
• Optimize the vertexing algorithm (in progress)
• Study the background on HIJING events from the
  last Data Challenge
• Add some charmed mesons in order to reproduce the
  charm yield predicted by MNR calculations ( 170
  per event)
• Tune the cuts
• on the tracks used to feed the vertexer (pT,
  impact parameter,)
• on secondary vertexes quality (DCAs, pointing
  angle, )
• Evaluate the combinatorial background

5
Signal production

• PYTHIA simulation D forced to decay in K-pp
  (both resonant via K0 (892) and nonresonant)
• PYTHIA tuning NLO given by MNR calculations
  (proper ccbar yield) 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 Torino.
• Goal to have 5K signal events with an amount of
  D ? Kpp corresponding to 6X107 central Pb-Pb
  events

6
Background production

• Old strategy
• Use central Pb-Pb events from Data Challenge 04
  (AliRoot v4-01-Rev05)
• Add to each event some charm because HIJING does
  not reproduce the predicted amount of charm
• Status of the art at the last Alice Week
  (March 05)
• One HIJING cent1 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 tagged version (AliRoot v4-02-00)
• Generate new background events cocktail of
  HIJING cent1 and additional charm made with
  PYTHIA
• Goal to have 20K of such events

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Tool used for the production
LHC Computing Grid Project -LCG2-
gLite RB node

• A job is submitted from the User Interface (UI)
  to the Workload Management System (WMS)
• The WMS looks for the best available Computing
  Element (CE) to execute the job
• Once the CE has been found the Users files are
  copied from the Resource Broker (RB) to the
  Working Node (WN) where the job is executed
• The output data produced by the job are made
  available on a Grid Storage Element (SE). The
  files are also copied to the CNAF MSS
• The output of the job can be retrieved by the user

UI
WMS
CE
Output Data available
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Getting started

• Obtain a certificate from a Certification
  Authority (CA) and join the Alice Virtual
  Organization (done)
• Prepare the submission tool (Job Wrapper JDL
  scripts) (done and tested the submission on gLite
  RB)
• Install AliRoot on the sites (to be done in few
  days)
• Submit the production, after testing the grid and
  data retrieval with few short jobs (starting next
  week)
• 5K signal events
• 20K background events
• files kept for each event galice.root,
  AliESDs.root, Kinematics.root, TracksRefs.root,
  ITS.RecPoints.root ? (lt2GB/event)
• Free space requested at CNAF (CASTOR) 6TB
• Data Catalogue choice LCG File Catalogue (LFC)

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Kinematics of the decay PT distributions
p
Mean 0.67 Mean 0.50
Comparing with HIJING central events
K
Mean 0.87 Mean 0.65
Plots refer to generated quantities
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Kinematics of the decay Dalitz Plots
From reconstructed tracks ( the info
given by the generation are taken into account)
This is done as an internal cross-check procedure
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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
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AliITSVertexerTracks.h,.cxx applied on a central
HIJING event to find the primary vertex passing
10 reconstructed primary tracks
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
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Result from the previous plots Globally the
FITTERFINDER resolution is better than the
FINDER stand-alone
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• Additional checks on the FITTER
• The FITTER was fed with an arbitrary starting
  guess for the vertex position (instead of using
  the FINDER)
• If the 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 ? (to be done)
• If the position is far from the MC vertex (500
  µm ) the residual distribution broadens. In
  practice, when the Finder finds a vertex far from
  the true one, the Fitter is also affected ?
  Investigate on the FINDER (in progress, see next
  slides)

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New Vertex Finder
Based on the distance of closest approach (DCA)
between two helices instead of using a Straight
Line Approximation (SLA) of the tracks as in the
old one

• Main steps
• The method receives 3 tracks as input
• For each pair of tracks, the coordinates of the 2
  points of closest approach are calculated (the
  method AliITStrackV2GetDCA is used).
• The coordinates of the secondary vertex are
  determined as

• In addition, a weighted mean on the coordinates
  of the 2 points of closest approach has been
  applied (this is implemented for the V0 finder in
  AliV0Vertex)
• The Secondary Vertex Finder was tested on the
  sample of 200 signal events produced in Torino

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Comparing the different Finders /1
X coord
Y coord
Z coord
RMS179 µm
RMS182 µm
RMS165 µm
RMS167 µm
RMS160 µm
RMS170 µm
RMS165 µm
RMS152 µm
RMS169 µm
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Comparing the different Finders /2
RMSX (µm)
Finder (SLA)
Finder (DCA)

• RMS (X coord) (µm) vs PT min of the 3 tracks
• RMS (X coord) (µm) vs DCA max of the 3 tracks
• RMS (X coord) (µm) vs decay length of D mesons

? the other coordinates show the same behaviour
Pt min (GeV/c)
RMSX (µm)
RMSX (µm)
DCA max (µm)
decay l.(µm)
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SLA Vertex Finder

• Possible problems applying the Finder of
  AliITSVertexerTracks to the secondary vertex D?K
  p p (c? 311.8 ?m )
• Straight Line Approximation (SLA) of the tracks
• Track Parameter calculated by prolonging the
  track to the primary vertex

d is of the order of tens of nm ? the SLA does
not give rise to syst effects
d (µm)
View on the transverse plane
y
y
X
C
PTK(GeV/c)
x
d (µm) is the distance between the secondary
vertex and the tangent line
d (µm)
D
K
d
Decay dist (µm)
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Comparing the different Finders /3

• With the new FINDER the residual distribution is
  10 µm narrower
• Adding the weighted mean method, the Z
  resolution is further improved
• The differences among the FINDERS are apparently
  not due to the Straight Line Approximation but
  rather to slightly different preselections of the
  tracks pairs
• As expected, the RMS of the residual
  distribution
• decreases with the increasing of the min PT of
  the tracks
• increases as the max DCA of the tracks increases

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Summary and Future Plans

• The production (5K signal and 20K background
  events) using the GRID tools is going to start
  (next week)
• A new secondary vertex finder algorithm has been
  tested on the signal events
• ? there is a small improvement w.r.t. the
  standard one 10 µm
• Additional tests on the finders are going to be
  done using AliGenBox to generate pions coming
  from a sphere whose radius can be tuned (next
  month)
• Investigate on the FITTER (to be done)
• Start with the tuning of the cuts (as soon as
  the events will be ready)