Transcript Document 7329715

D+K-p+p+:reconstruction and
perspectives for the
measurement of the elliptic
flow
Elena Bruna
University of Torino
Terzo Convegno Nazionale sulla Fisica di Alice, 12-14 Nov 2007
Outline
Exclusive reconstruction of D+ → K-p +p + in Pb-Pb
and pp collisions:
Selection strategy
Results
pp: Results with more statistics w.r.t. previous analysis
pp: A look at the systematic errors due to the
selection procedure
cut variables considered: distance primary-secondary
vertices, cosqpoint
Perspectives for the measurement of D+ azimuthal
distribution in Pb-Pb
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Selection strategy (1/4)
Cuts on single tracks ( pT, d0rf )
In Pb-Pb reduce the combinatorial background from 109 to 106 triplets
per event keeping ≈7% of the signal
pT cut different for K and p (identified by the charge sign: D+K-p+p+)
Cuts on Kp candidate pairs
K and p have opposite charge sign
Cut on the distance d between the vertex of the 2 tracks and the primary
vertex
primary
vertex
d K
Working point:
p
d>700 mm
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d (mm)
d cut (mm)
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Selection strategy (2/4)
Cuts on Kpp candidate triplets of tracks
selection based on the products of impact parameters
of the two Kp pairs  25% of BKG triplets rejected, ≈
all signal kept
Cuts on the secondary vertex quality:
BLACK: signal (Kpp from D+)
RED: BKG (Kpp combinatorics)
BLACK: signal (Kpp from D+)
RED: BKG (Kpp combinatorics)
σ (cm)
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Selection strategy (3/4)
Four selection variables:
cosqpoint
Distance between primary and secondary
vertex (dPS)
cosqpoint
Sum of squared impact parameters s =
d012+d022+d032
Max. pT among the 3 tracks
pM=Max{pT1,pT2,pT3}
BKG
SIG
Bruna
dElena
PS(mm)
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Selection strategy (4/4)
n-dimensional matrix procedure (n=4), i.e. one dimension
for each cut variable (dPS, pM, cosqpoint, s):
cosqpointCUT
Fill matrices Sijkl (signal) and Bijkl (background)
Each cell ijkl contains the number of triplets passing the set of cuts:
[dPS > cuti AND pM > cutj AND cosqpoint >cutk AND s > cutl ]
Advantages:
Significance
S/(S+B) for
2<pT<3 GeV/c
d CUT(mm)
Global maximization
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Easy to add new selection variables
(normalized to
107 PbPb events)
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Results: D+K-p+p+ in Pb-Pb (1)
Significance S/√S+B (normalized to 107 ev.) vs pT
Relative statistical error (sS/S =1/√S) on D± vs pT (in
107 ev.)
S/ev~10-3 , B/ev ~10-4
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Results: D+K-p+p+ in Pb-Pb (2)
Selection efficiency and pT spectra:
selected D 
 
D with 3 reconstructed daughters
pT integrated ε (D+) ≈ 1.5% (Ideal PID), 0.6% (Real PID), 1% (no
PID)
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Results: D+K-p+p+ in pp (1)
Signal statistics improved by a factor 2 w.r.t.
previous analysis
Significance and relative statistical error vs. D+ pT
S/ev~5 10-6 , B/ev ~5 10-6
Significance and relative statistical error (=1/S) normalized to 109
pp Minimum Bias events
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Results: D+K-p+p+ in pp (2)
Selection efficiency and pT spectra:
selected D 
 
D with 3 reconstructed daughters
Selection efficiency (pT integrated) ε~ 4% (NO PID)
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Results: Pb-Pb vs pp
(PID)
(no PID)
 sS/S the same for Pb-Pb
and pp for 2<pT<8 GeV/c
(PID)
(no PID)
S/√S+B in one year
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Studies on D+ Invariant Mass
Signal Inv. Mass distribution fitted with a
Gaussian function, in different D+ pT ranges
Signal normalized to 109 pp randomly
generated according to a Gaussian function
The same for the Bkg triplets, but a
polinomial function was used
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Invariant mass distributions
Statistics enhanced in order to match 1 year of data taking
2<pT<3 GeV/c
0<pT<2 GeV/c
pT>5 GeV/c
3<pT<5 GeV/c
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Systematic errors on pp events
Two samples of events:
1°) pp MB (85%) + pp with charm (14%) and beauty (1%)
+ events with cD+Kpp (divided in 4 subsamples of
pThard)
• extract the D+ number with Gaussian fit to the invariant mass
distribution: Nsel
2°) only events with cD+Kpp (divided in 4 subsamples
of pThard)
• extract the selection efficiency: ε
D+ reconstructed: Nrec=Nsel/
Nrec calculated for different values of the cut
variables
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Nsel vs d (norm to 109)
COUNT
From FIT
Cut on dprim-sec
2<pT<3 GeV/c
Nrec vs d (norm to 109)
COUNT
From FIT
 vs d
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Nsel vs cosqpoint (norm to 109)
COUNT
From FIT
Cut on cosqpoint
2<pT<3 GeV/c
Nrec vs cosqpoint (norm to 109)
COUNT
From FIT
 vs cosqpoint
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Perspectives for
flow
±
D
elliptic
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 (b=collision impact parameter)
v2 vs. pT in different centrality bins
HOW: fast simulation to generate:
ND+ (DpT,Db) with an angular distribution
dND/dj = v2D,in cos [2(j-YRP)]
For each D+: an event made of Ntracks is superimposed
dN/dj = v2ev,in cos [2(j-YRP)]
Inputs of the simulation: ND+ (DpT,Db) , v2D,in, Ntracks ,v2ev,in
Outputs: v2D measured.
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v2 vs. pT
6<b<9 fm
Large stat. errors on v2 of
D± → Kpp in 2·107 MB events
How to increase the statistics?

MB trigger
Sum D0→Kp and D±→Kpp
•Sufficient for v2 vs. centrality

Semi-peripheral trigger
•v2 vs. pT that would be obtained
from 2·107 semi-peripheral
events (e.g. 6<b<9 fm)
Semi-peripheral
trigger
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Summary
D+ in PbPb and in pp:
The results show that the D+ → K-p+p+ analysis in Pb-Pb and pp is
feasible with a pretty good Significance.
The selection strategy based on n-dimensional matrices has been
developed.
Recent updates:
Significance in pp improved with enhanced statistics
Invariant mass studies give the expected spectra in 1 year of
data taking
No evident systematic error on the selection procedure from a
first look: more statistics needed
D+ v2:
Large stat. errors on v2 of D± → Kpp in 2·107 MB events
How to increase the statistics?
Sum D0→Kp and D±→Kpp
Semi-peripheral trigger
Under study the methods for background subtraction
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Backup slides
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Initial Nsel statistics (vs dprim-sec)
2<pT<3 GeV/c
0<pT<2 GeV/c
3<pT<5 GeV/c
pT>5 GeV/c
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Initial Nsel statistics (vs cosqpoint)
0<pT<2 GeV/c
2<pT<3 GeV/c
3<pT<5 GeV/c
pT>5 GeV/c
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Initial Bkg statistics with |M-Minv|<1s (vs d)
2<pT<3 GeV/c
0<pT<2 GeV/c
3<pT<5 GeV/c
pT>5 GeV/c
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Results from RHIC
The Non-Photonic electrons
RAA shows interesting results
Suppression is very large when
compared to the expectation from
radiative energy loss
Other possible mechanisms?
•
Collisional EL, resonances, in
medium fragmentation…
First hint of strong charm v2
for pT<2 GeV/c
Very important, need to
disentangle charm from
bottom
PHENIX nucl-ex/0611018
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STAR nucl-ex/0607012
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Results scaled to a lower
multiplicity scenario for Pb-Pb
Results presented so far for Pb-Pb based on
dNch/dy=6’000
Ntracks=7000
BKG/ev=N(N-1)(N-2)/3! ~ 6x1010
Extrapolations from RHIC results seem to favour a lower
multiplicity scenario, dNch/dy=2’000
Ntracks~7000/3 ~2300
BKG/ev ~ 2x109
Let’s consider the D+ pT interval: 0<pT<2 GeV/c:
The results in the highest multiplicity scenario (dNch/dy=6’000) are
not satisfactory
BKG/ev is downscaled by a factor ~30;
SIG/ev not rescaled
S/√S+B (normalized to 107 ev.) ~ 10 in case of Real PID: it is
possible the study of the low-pT spectra
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Systematic uncertainities on D±
measurements
Acceptance, reconstruction and particle
identification efficiencies (~10%)
Centrality selection (~7%)
Parameters of the Woods-Saxon profile and nuclear
density (~5%)
Nucleon-nucleon inelastic cross section (~5%)
Branching ratio D+K-p+p+ (~3.6%)
Feed-down from beauty:
NcD± = ND± - NbB  D±
Contamination
K= NbB  D±/ NcD± =4%
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Feed-down from beauty
D+ from B are more displaced
The cut on distance between primary to secondary
vertex increases the fraction of selected D+ coming from
B decay
Contamination K vs d
d~1000mm  K =10%
Histograms normalized
to the same area
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