D analysis: status and perspectives

1
D analysis status and perspectives
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
PWG Heavy Flavours and Quarkonia Alice Week
Feb 28th 2005
2
Outline

• Physics motivations of open charm in ALICE
• D ? K-pp overview of the kinematics
• Event generation and reconstruction
• Reconstruction of the secondary vertex in the
  ITS
• Summary and future plans

3
Motivations of open charm measurements in Heavy
Ion Collisions

• Probe the medium with heavy quarks, studying
  initial state effects (nuclear shadowing) and
  final state effects (radiative energy loss) on
  charm mesons
• Measure the nuclear modification effects for D
  mesons and for the charge hadrons
• Important reference to understand possible
  quarkonium suppression

4
Hadronic 3-charge-body decays of D
D?K-?? BR 9.2
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Decay channels used in this study
PYTHIA is the event generator used for this task
it can be forced to produce only processes of
interest
BR w.r.t Kpp decay
Nonresonant
77.5
22.5
Resonant
For our convenience forced to be K-pp
6
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 and the reconstruction (in
  progress)
• Optimize the vertexing algorithm
• Study the background on HIJING events from the
  last Data Challenge
• Add some cc pairs in order to reproduce the charm
  yield predicted by MNR calculations ( 100 per
  event)
• Tune the cuts (impact parameter cut,) on the
  tracks to be analyzed by the vertexing algorithm
• Evaluate the combinatorial background

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1st simulation test on the Torino Farm
AliRoot version v4-01-Rev05 (the one used in the
last Data Challenge) PYTHIA tuning NLO given by
MNR calculations (for correct cc distributions)
CTEQ4L (for correct PDFs)
s1/2 5.5 TeV Magnetic Field
0.5 T
8000 D ? K-pp (both resonant and nonresonant)
per event, in the rapidity range ylt2 leading to
a charged multiplicity dN/dy 5000
200 of such signal events produced in the Torino
farm. This is a first sample of what we expect to
produce. Goal to have 10000 signal events with
an amount of D ? Kpp corresponding to 5.6X107
central Pb-Pb events
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Problems on this sample of events

• Lower multiplicity with respect to Hijing central
  (cent1) events (6000 dN/dy) which will be used as
  background
• we need to increase the multiplicity because of a
  possible bias due to different tracking
  efficiency
• Systematic difference between generated PT and
  reconstructed PT

• Due to Magnetic Field setting.
• A constant field in the barrel must be set to
  have good reconstruction.

9
2nd simulation test on the Torino farm
AliRoot version v4-01-Rev05 (the one used in the
last Data Challenge) PYTHIA tuning NLO given by
MNR calculations (for correct ccbar
distributions) CTEQ4L
(for correct PDFs) s1/2 5.5
TeV Magnetic Field 0.5 T ? Constant field in
the barrel (field-gtSetL3ConstField(0))
9100 D ? K-pp (both resonant and nonresonant)
per event, in the rapidity range ylt2 leading to
a charged multiplicity dN/dy 6000
200 of such signal events produced in the Torino
farm. This is a first sample of what we expect to
produce. Goal to have 10000 signal events with
an amount of D ? Kpp corresponding to 5.6X107
central Pb-Pb events
Event generation and reconstruction completed on
Feb 24th 2005
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Event reconstruction (2nd set of events)
p
p
PT and F correctly reconstructed
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Kinematics (1)
K
Pseudorapidity distributions of the generated
particles
D
p
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Kinematics (2)
K
PT distributions of the generated
particles (nonresonant events)
Mean 0.87
D
Mean 1.66
?
Mean 0.67
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Kinematics (3)
K
Superimposed PT (GeV/c) distributions (log scale)
Mean 0.87 Mean 0.80
D
?
Mean 1.66 Mean 1.66
Mean 0.67 Mean 0.69
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Kinematics (4)
Ratio of the PT distributions PT (nonreson) / PT
(reson)
K
D
?
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Kinematics (5)
p
Mean 0.67 Mean 0.50
Comparing with HIJING central events PT
distributions
K
Mean 0.87 Mean 0.65
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Kinematics (6)
Impact parameter distributions for the generated
particles d0 in the transverse plane is
measured in µm
(nonresonant events)
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Invariant mass for the K0
generated
MEAN 895 MeV/c2 RMS 51 MeV/c2
using reconstructed tracks
this is not a complete
reconstruction of the signal tracks are paired
by means of info. stored at generation time.
MEAN 896 MeV/c2 RMS 54 MeV/c2
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Dalitz Plots
From Kinematics
Non resonant
Resonant
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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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?2 is given by the vertexer and reflects the
goodness of the algorithm
In all these cases the vertexer fails very few
times (less than 1/)
3 tracks from one true secondary vertex
3 tracks from 3 different vertices
3 tracks from 2 different vertices
dist distance between true sec. vertex position
(MC) and reconstructed sec. vertex position
(vertexer)
22
Residuals sec vertex (MC) sec vertex
(vertexer)
x
z
y
The RMS found are in agreement with the results
obtained in p-p for Ntracks 3
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Future Plans

• Increase the statistics of the signal events
• Use GRID tools to generate 10000 events, each
  consisting of 9100 D in y2
• Generate background events
• See next slide
• Study the possibility to use the resonant decays
  to improve the D reconstruction
• Try to optimize the vertexer algorithm to
  improve its secondary vertex finding efficiency.

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Background events

• Strategy
• Use central Pb-Pb events from Data Challenge 04
• Add to each event some charm
• 170 cc pairs should be added because HIJING does
  not reproduce the predicted amount of charm
• Present Status
• One HIJING cent1 event from PDC04 downloaded
• Still some technical problems with the merging
• Less ESD tracks in the merged event (with
  additional charm) than in the HIJING event.

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Invariant mass of D ? K-pp
MEAN 1.867 GeV/c2 RMS 0.019 eV/c2