Open charm reconstruction in ALICE: D Kp p

1
Open charm reconstruction in ALICE D ? K-pp
Elena Bruna, University of Torino, for the ALICE
Collaboration

• Open charm physics in Heavy-Ion collisions
• Measurement of D, D0 and Ds and their relative
  yields can provide information on the
  hadronization mechanism
• Accurate measurement of the charm absolute cross
  section in pp and Pb-Pb collisions
• Heavy quarks as probes of nuclear medium
• quark energy loss (by gluon radiation
  collisional ?)
• anisotropic flow on the transverse plane
• open Q(Q) production (not Drell-Yan) natural
  normalization for QQ studies

Measurement of open charm in the ALICE experiment

• Need for high precision vertex detector
• tracks from heavy flavour weak decays are
  typically displaced from primary vertex by
  100s µm
• resolutions of a typical heavy flavour apparatus
  10s µm

Primary vertex
D decay length
K
p
q
p
Secondary vertex
Flow collective motion of particles (due to
high pressure arising from compression and
heating of nuclear matter) superimposed on top of
the thermal motion
Track impact parameter
Inner Tracking System (ITS) 6 SILICON layers
(pixel, drift, strip) Vertices reconstruction,dE/d
x -0.9lt?lt0.9
Time Projection Chamber (TPC) Tracking,
dE/dx -0.9lt?lt0.9
Channel under study D ? K-pp
Advantages

• D has a long decay length (ct 311mm compared
  to 123 mm of the D0 ())
• D ? K-pp has a relatively large branching
  ratio (BR9.2 compared to 3.8 for D0 ? K-p).

B 0.5 T
drawbacks

• Combinatorial background for this 3-body channel
  is larger than for D0 ? K-p.
• The average PT of the decay product is softer (
  0.7 GeV/c compared to 1 GeV/c for the D0 ?
  K-p.)

Time Of Flight (TOF) Tracking, PID
(time) -0.9lt?lt0.9
Size 16 x 26 m Weight 10,000 tons
() D0 ? K-p 1st channel studied. A. Dainese,
Ph.D. Thesis, arXivnucl-ex/0311004. Poster by A.
Dainese presented at QM02
Analysis strategy
Pb-Pb
1st step single track cuts
2nd step combining Kp pairs
No cuts
both Kp pairs are required to pass the cut
Cut on the distance between the 2-track vertex
and the primary one
Cuts on PTp, PTK, track impact parameter (d0) of
all the tracks
Cuts PT p 0.5 GeV/c PT K 0.7 GeV/c d0 95 mm
dist gt distcutMIN
K
Primary vertex
p
dist
distcutMIN (mm)
4th step additional cuts on the triplets
3rd step combining Kpp triplets
track2
track3
track1
Cut on the quality of the Secondary Vertex
Primary vertex
d2
K
d3
Secondary Vertex
dist prim-sec
d1
?point
D reco
Secondary vertex resolution
d0K
p
Secondary vertex
BKG
p
cos?point
Accepted triplets with s lt sMAX
s d0K2d0p12d0p22
BLACK signal RED BKG Kpp Triplets
BLACK signal RED BKG Kpp Triplets
SIG
sMAX (cm)
Preliminary results
Max. Significance S/v(SB) normalized to 107 MB
events
dist prim-sec (mm)
s (mm2)
Conclusions
2ltpTDlt3 GeV/c
D in Pb-Pb

• D in Pb-Pb and pp
• The preliminary results show that the D ?
  K-pp analysis is feasible with a pretty good
  Significance.
• More statistics is mandatory for a more accurate
  optimization of the cuts.
• Perspectives for D v2 anisotropy measurements
• Evaluation of statistical error bars in
  centrality and pT bins for 2107 MB PbPb events
  (1 year of data taking) already done.
• Large stat. errors on v2 s(v2)/v220 for pT2
  GeV/c
• How to increase the statistics?
• Sum D0?Kp and D?Kpp
• Semi-peripheral trigger
• Methods for the background subtraction under
  study.

Significance
cosqpoint MIN
dist prim-sec MIN (mm)
cosqpoint gt cosqpointMIN distance prim-sec gt
dist prim-sec MIN
D in pp
cosqpoint MIN
Max. Significance S/v(SB) normalized to 109 MB
events
dist prim-sec MIN (mm)