Transcript Document 7329510

Stato della preparazione
+
+
+
dell’analisi D → K p p
Elena Bruna, Massimo Masera,
Francesco Prino
Università di Torino e INFN
Secondo convegno nazionale sulla fisica di ALICE, 30 Maggio – 1 Giugno 06, Vietri sul Mare
Outline
 Physics motivations for D+ analysis
 Selection of the signal
1.
2.
3.
4.
Single track cuts
Secondary vertex finder
How to combine the triplets
Cuts on the triplets
 First preliminary results of the D+ feasibility
study on a sample of events:
1. Perfect PID (given by the simulation)
2. Experimental PID (combined)
3. NO PID
 Time-consumption problems, first ideas to avoid
these problems → new analysis strategy
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Why also D+ (and Ds…)?
Accurate measurement of the charm production
cross section
1. D0/D+ ratio: puzzle
• Expected to be 3.08 from spin degeneracy of D and D*
and decay BR
• Measured by ALEPH@LEP to be 2.32
2. Different selection strategies due to:
 D+ has a “longer” mean life (ct ~311mm compared to
~123 mm of the D0)
 D+ fully reconstructable from a 3-charged body decay
instead of the 2 (or 4) body decay of D0
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D+ → K-p+p+ vs D0 → K-p+
Advantages…
1. D+ has a “long” mean life (ct ~311mm compared to ~123
mm of the D0)
2. D+ → K-p+p+ has a relatively large branching ratio
(BR=9.2% compared to 3.8% for D0 → K-p+).
…drawbacks
1. Combinatorial background for this 3-body channel is
larger than for D0 → K-p+.
2. The average PT of the decay product is softer (~ 0.7
GeV/c compared to ~ 1 GeV/c for the D0)
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D± statistics
bmin-bmax
(fm)
s (%) Nevents Ncc / ev.
D±
yield/ev.
(106)
0-3
3.6
0.72
118
45.8
3-6
11
2.2
82
31.8
6-9
18
3.6
42
16.3
9-12
25.4
5.1
12.5
4.85
12-18
42
8.4
1.2
0.47
Nevents for 2·107 MB triggers
Ncc = number of c-cbar pairs
Includes shadowing EKS98
Shadowing centrality
dependence from Emelyakov et
al., PRC 61, 044904
D± yield calculated from Ncc
Fraction ND±/Ncc (≈0.38) from
tab. 6.7 in chapt. 6.5 of PPR
Geometrical acceptance and
reconstruction efficiency
Extracted from 1 event with
20000 D± in full phase space
B. R. D± Kpp = 9.2 %
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Simulation and analysis
strategy
Central Pb-Pb
event (b<3.5 fm,
dN/dy = 6000,
√s=5.5 TeV)
~ 9 D+/Din |y|<1
Too large statistics
(108 events) would
be required to
study the signal!!
Signal and background events separately
generated with the Italian GRID
PT of the decay tracks is “soft” ~0.7-0.8 GeV/c
The magnetic field is low - 0.5 T - to allow the reconstruction of soft
particles (~ 7000 in |y|<1)  huge combinatorial background
A dedicated trigger for D+ → K-p+p+ seems not possible.
Good secondary vertex reconstruction capability (ct (D+) ~ 300mm 
resolution of 200mm would be bad, 50mm would be a dream…)
Efficient system of cuts to discriminate the signal from the background
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st
1
step: Single track cuts
Cuts on PTp, PTK, track impact
parameter (d0) on all the tracks for
both signal and background events
Perfect PID
is assumed
Selection
S/event
B/event
S/B
No cuts
0.1
109
10-10
(8%)
0.008
106
10-8
Cuts:
PT p = 0.5
GeV/c
PT K =0.7
GeV/c
d0 = 95 mm
Not optimized cut: we want to
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preserve D+ down to PT~1 GeV/c
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2nd step: Combining K-p pairs
IDEA: start from BKG pairs of Kp
tracks (once the single track cut
have been performed) and cut on
the distance between the 2-track
vertex and the primary one
K
p
Ki
…
p1
p2…
pj
Kj
both Kp pairs are
required to pass the
cut
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3rd step: Combining the Triplets
 Single track cuts: applied
 Cut on the 2-tr vertex: applied
Kpp are combined together
according to the sign of their
impact parameter
Background
d0K x d0p2
d0K x d0p2
Signal
Cut on d0…
empty
d0K x d0p1
When (d0K x d0p1)<0 & (d0K x
d0p2)<0. Due to the kinematics
of the decay
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d0K x d0p1
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Secondary Vertex Finder on
the Triplets
Cut on the quality of the Vertex. Sigma:
2
2
2
sigma 2  D2  d1  d 2  d3
BLACK: signal
RED: BKG Kpp Triplets
P1 (x1,y1,z1)
d1
Secondary
Vertex
(x0,y0,z0)
ZOOM
BLACK: signal
RED: BKG Kpp Triplets
BLACK: signal
RED: BKG Kpp Triplets
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Cuts on the Triplets
1. Quality of the vertex:
Sigma (prev. Slide)
2. Distance between
primary and secondary
vertices
BLACK: signal
RED: BKG Kpp
Triplets
cosθpoint
The signal
is peaked
at 1
3. Cut on the cosine of the
pointing angle defined by
the PT of the D+ and the line
connecting primary and
secondary vertices
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First strategy adopted for the
analysis
For each variable (in the order Sigma, distance,
cosθpoint):
1. loop on all the triplets (both signal and background)
2. Keep the value of the cut with Max Significance S/√(S+B) in
the range |MINVKpp-MD+|<1s
3. Use this value to maximize the Significance for the next cut
variable
Different ranges of the reconstructed pT of the triplet
Perfect PID, PID and without PID
Statistics used in this preliminary analysis:
• BKG: 500 HIJING events
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• SIG: 8.5 X 105 reconstructed
signal triplets
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Perfect PID – pT integrated
Sigma
Significance
Significance
Distance
Chosen cuts:
Sigma=0.018 cm
Dist=1900 mm
Significance
The Significance
is normalized to
107 central events
cosθpoint
cosqpoint=0.995
Signif = 44±11
S/ev = 0.001
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B/ev = 0.004 13
SIGNAL – BKG at different cut
levels
Number of SIGNAL
triplets per event (full inv
mass range)
Number of BACKGROUND
triplets per event (full inv
mass range)
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PID
• Combined Bayesian PID
(ITS+TPC+TOF+TRD+HMPID) is used
• Prior probabilities used in input:
P(p)=0.055
P(K)=0.072
P(p)=0.864
P(e)=0.006
P(m)=0.003
• PID in the ITS done with:
 Clusters from all the 4 layers (2 SDD+2 SSD)
 Convoluted Landau-Gaussian fits to the response
functions in each layer
See AliITSpidESD2 implemented in AliRoot
• Track tagged as type i when the corresponding
combined Bayesian probability is P(i|track)>0.85
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Significance
PID – pT integrated
Significance
Sigma
Distance
Chosen cuts:
cosθpoint
Sigma=0.019 cm
Significance
Dist=2000 mm
cosqpoint=0.995
Signif = 40±15
S/ev = 0.0007
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B/ev = 0.002
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Without PID – pT integrated
Significance
Sigma
Significance
No selection of tracks based on the particle identity
Distance
Chosen cuts:
Significance
Sigma=0.019 cm
Dist=1800 mm
cosθpoint
cosqpoint=0.999
Signif = 39±12
S/ev = 8 X 10-4
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B/ev = 0.004
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Low pT under study:
1. Rebinning
2. Additional cuts
Significance
Preliminary global results
Analysis feasible also
without PID, but more
time consuming
pT integrated
results
Perfect PID
PID
No PID
Significance
44 ±11
40 ± 15
39 ± 12
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+
D
elliptic flow:
measurement perspectives
2·107 Minimum Bias events
Error bars quite large



Would be larger in a scenario with
worse event plane resolution (lower
dNch/dy or v2)
May prevent to draw conclusions in
case of small anisotropy of D
mesons
v2 vs pT requires a semi-peripheral
trigger
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Conclusions
The reconstruction of D+ → K-p+p+ is a
promising study.
The preliminary results show that the
analysis is feasible with a pretty good
Significance.
More statistics is mandatory for a more
accurate optimization of the cuts.
There still is room for optimization: work in
progress.
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Outlook
New cut strategy:
For each event fill 2 multi-dimesional matrices (one for Signal and
one for BKG), each cell containing the number of triplets
corresponding to all the possible combinations of cut variables. Es.
with 5 cut variables and 30 steps for each variable  305 cells
(~200MB per ev for Signal+BKG)
Sum all the multi-dimesional matrices (Signal and BKG) on all the
events
Maximize the multi-dim matrix of the Significance
Apply LDA
pp studies to come on the PDC06 events and on italian
production with parametrized TPC
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Backup slides
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Hadronic 3-charge-body decays of
D+
D±
I(JP) = ½ (0-)
m = 1869.4 MeV/c2
ct = 311.8 mm
(PDG ’04)
D+K-p+p+
BR = 9.2 %
D+→K-p+p+
Non Resonant BR = 8.8 %
D+→K*0(892)p+→K-p+p+
Resonant
BR = 1.3 %
D+→K*0(1430)p+→K-p+p+ Resonant
BR = 2.3 %
D+→K*0(1680)p+→K-p+p+ Resonant
BR = 3.8·10-3 %
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Kinematics (1)
PT distributions of the generated
particles (ONLY PYTHIA generation,
NO propagation and reconstruction in
the detector)
K
Mean = 0.87 GeV/c
(nonresonant events)
D
Mean = 1.66 GeV/c
p
Mean = 0.67 GeV/c
Knowledge of the PT shapes of the
decay products important at the level of
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the selection strategy
24
Dalitz Plots: Kinematics (2)
Non resonant
Resonant
Sharp borders due to PYTHIA cut
off on the tails of distributions
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Combining K-p pairs /2
Best Significance:
cut distance of the 2-tr vertex = 700 mm
Pt reconstructed D+
 63% Signal triplets pass the cut
Mean=2.66 GeV/c
• S/ev=0.006
 105 remaining BKG triplets per
event.
S/B = 6X10-8 still low…
Full invariant mass range
Low PT D+ still kept
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Display D+ decay (made with
the kinematics)
Primary vertex
Impact parameter:
K+
• Definition: segment of
minimum distance of the
(prolonged) track from the
primary vertex
p1-
• Sign:
• + : primary vertex in the
track “circle”
p2-
• - : primary vertex out of
the track “circle”
K+: d0>0
p1- : d0>0
p2- : d0>0
Secondary
vertex
Points on the 3 prolonged tracks
defining the impact parameter d0
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Measurement of v2
• Calculate the 2nd order coefficient of
Fourier expansion of particle azimuthal
distribution relative to the reaction plane
– The reaction plane is unknown.
• Estimate the reaction plane from particle
azimuthal anisotropy:
 Yn = Event plane = estimator of the
unknown reaction plane
• Calculate particle distribution relative to
the event plane
v2  cos(2(  YRP ))
Unknown reaction plane

w sin i
1
1   i
Yn  tan
  w cos 
n
i
i





v'2  cos(2(  Y2 ))
• Correct for event plane resolution
– Resolution contains the unknown YRP
– Can be extracted from sub-events
v'2
v2 
cos2Y2  YRP 
Event plane resolution
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Worse resolution scenario
• Low multiplicity and low v2
Large contribution to error bar on v2
from event plane resolution
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Semi-peripheral trigger
v2 vs. pT that would be obtained from 2·107 semi-peripheral events (
6<b<9 )
pT limits N(D±)sel
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sv2)
0-0.5
645
0.03
0.5-1
1290
0.02
1-1.5
1800
0.017
1.5-2
1650
0.018
2-3
2470
0.015
3-4
1160
0.02
4-8
1225
0.02
8-15
220
0.05
30