Diapositiva 1

1
Status of D analysis
Elena Bruna, Massimo Masera, Francesco
Prino University of Torino and INFN
ALICE Physics Forum, CERN 11 Oct 2006
2
Outline

• D? K-pp in Pb-Pb
• Motivations
• Selection strategies
• Preliminary results
• Results scaled to lower multiplicity scenario
• Preliminary results for D? K-pp in pp
• Perspectives for D azimuthal anisotropy
  measurements in Pb-Pb
• Conclusions

3

D in ALICE

• Physics motivations (for all the open charmed
  mesons)
• Measurement of D yield, together with D0 and Ds,
  and their relative yields, can provide
  information on the hadronization mechanism
• Necessary to reduce the systematic error on the
  absolute cross section
• Open charmed mesons as probes of the medium
  (RAA,v2,)
• Advantages
• D has a long mean proper length (ct 312mm
  compared to 123 mm of the D0)
• D fully reconstructable from a 3-charged body
  decay (Kpp)
• Possibility to exploit the resonant decay
  through Kbar0 to enhance S/B
• Drawbacks
• Kpp channel larger combinatorial background (3
  decay products instead of the 2 of the D0 ? K-p)
• Kpp channel smaller ltpTgt of the decay products
  ( 0.7 GeV/c compared to 1 GeV/c of the D0
  decay products)
• D less abundant than D0 (factor 3 due to D
  decays)

4
D in Pb-Pb
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Selection strategy

• General lines common for all the open charmed
  mesons in the barrel.
• Additional information given for the case D?Kpp

• Selection invariant mass analysis of fully
  reconstructed topologies coming from displaced
  vertices.
• Cuts on single tracks (pT, d0).
• D case cuts on Kp candidate pairs of tracks
  displaced from the primary vertex are also
  applied.
• Build triplets (or pairs or quadriplets) of
  tracks with the right combination of charge
  signs.
• Compute the vertex (i.e. the D.C.A. of the pairs
  for D0,). Require a good pointing of the
  reconstructed D momentum to the primary vertex.
• Use combined PID information (ITSTPCTRDTOFHMPI
  D)

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1st step single track cuts

• Cuts on pT and d0 of all tracks
• If PID information is used
• Reject p, deutons, e and m
• Different pT cut for p and K

Perfect PID is assumed
Working point (to preserve enough statistics at
low pT)
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2nd step combining Kp pairs

• K and p have opposite charge sign
• Cut on the distance between the DCA point of the
  2 tracks and the primary vertex
• Build the triplets starting from two selected Kp
  pairs

Accepted pairs with distgtdistcutMIN
distcutMIN (mm)
8
3rd step Combining the Triplets

• Single track cuts applied
• Cut on both the 2 Kp vertices in the triplet
  displaced from the primary vertex applied

Background
Signal
d0K x d0p2
d0K x d0p2
d0K x d0p1
d0K x d0p1
selection based on the products of impact
parameters of the two Kp pairs
When (d0K x d0p1)lt0 (d0K x d0p2)lt0 empty
region due to the kinematics of the decay
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4th step Secondary Vertex Finder on the Triplets
Cut on the quality of the Vertex. Sigma
BLACK signal RED BKG Kpp Triplets
track 3
track 1
d3
d1
d2
SecondaryVertex
track 2
Accepted triplets with s lt sMAX
ZOOM
BLACK signal RED BKG Kpp Triplets
BLACK signal RED BKG Kpp Triplets
sMAX (cm)
sMAX (cm)
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Possible cuts on the Triplets

• Quality of the vertex Sigma (prev. Slide)
• Distance between primary and secondary vertices

BLACK signal RED BKG Kpp Triplets
cos?point

• Cut on the cosine of the pointing angle defined
  by the PT of the D and the line connecting
  primary and secondary vertices

The signal is peaked at 1
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Starting point for the analysis

• Statistics used for this analysis
• SIGNAL 106 reconstructed triplets
• half of the sample produced on the grid
• BACKGROUND 1750 hijing events
• out of 20000 produced on the grid
• Analysis done for 2 different PID selections
• Perfect PID
• ? Combined PID in progress
• ? No PID in progress

12
Preliminary results in D pT intervals

• Multi-dimensional matrices SijkNsigcut-ijk and
  BijkNbkgcut-ijk counting triplets passing the
  cuts defined by each cell
• Selection variables in the matrix
• Distance prim-sec vertex
• Pointing angle
• min pT of the 3 tracks
• Easy to expand to new selection
  variables

Significance
2ltpTDlt3 GeV/c
cosqpoint
Significance
Max. Significance S/v(SB) normalized to 107 MB
events
Distance prim-sec (mm)
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Additional cut on s?i1,2,3 d0i2
s ?i1,2,3 d0i2 d0K2d0p12d0p22
SIGNAL triplets

• Already applied cuts
• single track cuts
• Kp displaced pairs
• quality of the vertices (?)

Accepted Kpp triplets
sgtsMIN (mm2)
BKG Kpp Triplets
BLACK signal RED BKG Kpp Triplets
sgtsMIN (mm2)
Cut on sMIN3.5x105 mm2 looks promising
Significance
sgtsMIN (mm2)
s (mm2)
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Preliminary estimate for dNch/dy 2000

• dNch/dy 6000 ? Ntracks7000 reconstructed
  tracks in the barrel
• S/ev0.001
• B/ev0.004
• dNch/dy 2000 ? Ntracks 1/3 x 70002500 doing a
  rough extrapolation, neglecting the different
  tracking efficiency
• S/ev 0.001
• B ? (Ntracks)3 ? B/ev 1/20 x 0.004 2x10-4
• S/B 5
• S/?(SB) 90

• Larger significance in a lower multiplicity
  scenario.
• Only BKG has been scaled
• Signal was not scaled not clear if the ccbar
  production is correlated with the multiplicity

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D in pp
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D in pp collisions

• Work in progress!
• MC EVENTS from pp production at ?s14 TeV with
  TPC parametrized
• produced in April 2006 on Italian grid (CNAF,
  LNL, TO)
• reconstructed in LNL in August 2006 with
  v4-04-Release
• GOALs
• Tune selection cuts for pp events
• Less combinatorial background with respect to
  Pb-Pb
• Worse resolution on the primary vertex due to low
  multiplicity
• Prepare analysis tools in view of PDC06 events

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Physics Run example (107 events)
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Statistics used in this analysis

• Pre-analysis
• 1.5 106 events in the physics run proportions
• PLUS
• 3 105 events with charm in the barrel forced to
  decay hadronically (4 bins of pthard)
• Used to increase the number of signal triplets to
  have enough statistics to tune some of the
  selection cuts
• CAVEAT the background triplets are biased by a
  larger contribution from charm events

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Analysis strategy

• First analysis apply same selection strategy
  developed for Pb-Pb events
• Single track cuts
• Cut on displaced Kp pairs
• Quality of the 3-track secondary vertex
• Cuts on the candidate triplets
• Differences with respect to Pb-Pb
• Primary vertex recalculated for each triplet
  excluding the 3 candidate D tracks

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1st step single track cuts

• Cuts on pT and d0 of all tracks
• For pT use the same values as Pb-Pb
• PT p gt 0.5 GeV/c
• PT K gt 0.7 GeV/c
• For impact parameter use a less severe cut than
  in Pb-Pb
• d0 gt 50 mm (instead of 95 mm used for Pb-Pb)
• No PID information
• parametrized PID could be introduced for these
  events

working point
21
2nd step combining Kp pairs

• K and p from D decay have opposite charge sign
• Cut on the distance between the intersection of
  the 2 tracks and the primary vertex
• Build the triplets starting from two selected Kp
  pairs
• Cut on the correlation between d0K x d0p1 and d0K
  x d0p2

cut value
Accepted pairs with distgtdistcutMIN
distcutMIN (mm)
22
3rd step Secondary Vertex Finder on the Triplets

• Cut on the quality of the vertex
• s track dispersion around the secondary vertex

Accepted triplets with s lt sMAX
cut value
sMAX (cm)
23
4th step cuts on the Triplets

• Selection variables
• Distance between primary and secondary vertices
• Cosine of the pointing angle

Signal
Background
cos ?point
dist. prim.-sec. (mm)
24
Significance

• Very preliminary result based on a physics run of
  1.5106 events (with correct proportions)
• Significance normalized to 109 pp events

Significance
cos ?point
dist. prim.-sec. (mm)
25
v2 of D mesons
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What to learn from v2 of D mesons?

• Low/interm. pT (lt 2-5 GeV/c)
• Flow is the dominant effect
• Test recombination scenario
• Degree of thermalization of charm in the medium

• Large pT (gt 5-10 GeV/c)
• Energy loss is the dominant effect
• Test path-length dependence of in-medium energy
  loss in an almond-shaped partonic system

? Armesto, Cacciari, Dainese, Salgado, Wiedemann,
hep-ph/0511257
? Greco, Ko, Rapp PLB 595 (2004) 202
other effects dominant
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Motivation and method

• GOAL Evaluate the statistical error bars for
  measurements of v2 for D mesons reconstructed
  from their Kpp decay in Pb-Pb collisions
• v2 vs. centrality (pT integrated)
• v2 vs. pT in different centrality bins
• TOOL fast simulation (ROOT 3 classes 1
  macro)
• Assume to have only signal
• Generate ND(Db, DpT) events with 1 D per event
• For each event
• Generate a random reaction plane (fixed YRP0)
• Get an event plane (according to a given event
  plane resolution)
• Generate the D azimuthal angle (fD) according to
  the probability distribution p(f) ? 1 2v2 cos
  2(f-YRP)
• Smear fD with the experimental resolution on D
  azimuthal angle
• Calculate v'2(D), event plane resolution and
  v2(D)

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D statistics

• Nevents for 2107 MB triggers
• Ncc number of c-cbar pairs
• MNR EKS98 shadowing
• Shadowing centrality dependence from Emelyakov et
  al., PRC 61, 044904
• D yield calculated from Ncc
• Fraction ND/Ncc 0.38
• Geometrical acceptance and reconstruction
  efficiency
• Extracted from 1 event with 20000 D in full
  phase space
• B. R. D? Kpp 9.2
• Selection efficiency
• No final analysis yet
• Assume e1.5 (same as D0)

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Event plane resolution scenario

• Event plane resolution depends on v2 and
  multiplicity

Ntrack number of p, K and p in AliESDs of
Hijing events with b ltbgt
Hadron integrated v2 input values (chosen 2 ?
RHIC v2)
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v2 vs. centrality
2107 MB events

• Error bars quite large
• Would be larger in a scenario with worse event
  plane resolution
• May prevent to draw conclusions in case of small
  anisotropy of D mesons

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v2 vs. pT
2107 MB events
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First conclusions about D v2

• Large stat. errors on v2 of D ? Kpp in 2107 MB
  events
• How to increase the statistics?
• Sum D0?Kp and D?Kpp
• Number of events roughly ?2 ? error bars on v2
  roughly /v2
• Sufficient for v2 vs. centrality (pT integrated)
• Semi-peripheral trigger
• v2 vs. pT that would be obtained from 2107
  semi-peripheral events (e.g. 6ltblt9 fm)

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Conclusions

• D in PbPb
• The preliminary results show that the D ? K-pp
  analysis in Pb-Pb is feasible with a pretty good
  Significance.
• The selection strategy based on multi-dimensional
  matrices has been developed.
• Results shown only for Perfect PID
• PID and NO PID case in progress
• D in pp
• First preliminary analysis on pp events with TPC
  parametrized
• Final tuning of the cuts and study in D pT bins
  require higher statistics
• To be done on PDG06 events
• D v2
• Large stat. errors on v2 of D ? Kpp in 2107 MB
  events
• How to increase the statistics?
• Sum D0?Kp and D?Kpp
• Semi-peripheral trigger

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Backup
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D statistics (I)

• ALICE baseline for charm cross-section and pT
  spectra
• NLO pQCD calculations (? Mangano, Nason, Ridolfi,
  NPB373 (1992) 295.)
• Theoretical uncertainty factor 2-3
• Average between cross-sections obtained with
  MRSTHO and CTEQ5M sets of PDF
• 20 difference in scc between MRST HO and
  CTEQ5M
• Binary scaling shadowing (EKS98) to extrapolate
  to p-Pb and Pb-Pb

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Worse resolution scenario

• Low multiplicity and low v2

Large contribution to error bar on v2 from event
plane resolution