Transcript Perspectives for the measurement of Ds mesons via the Ds

Analysis of the
D+s K+K -π+ channel in the
ALICE experiment
(4050 m. asl)
Serhiy Senyukov
Università & INFN di Torino
29/08/2008
ALICE Italia 2008
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Open charm and QGP
π
D0
A
K
c
c-
QGP
π
Ds
A
K
K
Primary
production
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Interaction with
QGP
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Decay of
D mesons
Detection
of
products
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Results for D0 and D+
Pb-Pb @ 5.5TeV
p-p @ 14 TeV
2<pT<3 GeV/c
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Why Ds?
Question:
We know that D0 and D+ can be reconstructed with high
significance. Why do we need also Ds?
Answer:
1. To measure charm yield more precisely we need to
measure as many channels as we can
2. Study of different ways of hadronization:
• String fragmentation:
Ds+ (cs) / D+ (cd) ~ 0.6
it should be easier to take a light meson from a
vacuum than the strange one
• Recombination:
Ds+ (cs) / D+ (cd) ~ N(s)/N(d) (~ 1 at LHC?)
recombination occurs inside the medium.
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Inner Tracking
System:
ALICE setup
Vertexing,
Tracking,PID
Time Projection
Chamber:
Tracking, PID
Time-of-Flight
detector:
PID
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DsKKp
Selection strategy
invariant-mass analysis of fullyreconstructed topologies
originating from displaced
vertices
 build triplets of tracks with
correct combination of charge
signs and large impact
parameters
 particle identification to tag the
decay products
 calculate the vertex (DCA point)
of the tracks
 good pointing of reconstructed D
momentum to the primary vertex
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Ds flight line
p
Primary Vertex
K
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Secondary vertex
K
Ds vs. D+ & D0
D0
D+
Ds
Combinations *
106 pairs
109 triplets
108 triplets
cτ
123 μm
312 μm
150 μm
Hadronic
branching ratio
3.8%
9.2%
4.3%
Abundance**
30
9
5
* - without cuts, with Ideal PID at dη/dy=6000
** - PbPb (0-5% central), NLO pQCD + Binary Scaling + EKS98, in central acceptance
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Cut tuning scheme
o
Signal – 2500 PYTHIA events
with full simulation each
containing 9100* Ds generated in
the barrel acceptance
(-0.9<η<0.9) forced to decay into
hadronic channels:
o
o
o
D+sK+K0*K+K -π+
D+sφπ+K+K-π+
Signal events
Background
events
Cuts alghoritms
Cuts alghoritms
Normalization to
107 events
Normalization to
107 events
Background – 15000 HIJING
(Number of signal is negligible)
o
Significance – calculated with
respect to 107 central events (1
year)
* Number is tuned in order to have the same
performance of the detector as in BKG event
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Significance calculation
Signif=S/(S+B)1/2
Significance
normalized
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Single track selection
3 variables:
•
Pt(K) – Transverse
momentum of Kaon*
•
Pt(π) – Transverse
momentum of Pion*
•
d0 – impact parameter of
track
Cut: Pt>0.5 GeV/c
Cut: Pt>0.5 GeV/c
Cut: d0>35 μm
Before cuts
After cuts
Pt of Ds (GeV/c)
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* Ideal PID is assumed
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Pt of Ds (GeV/c)
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Combining tracks to pairs
•
•
•
Single tracks are combined to pairs with opposite sign (K+K- & K+π-) or
(K-K+ & K-π+)
Crossing point is found using vertexing algorithm
Two values are calculated:
2
•
•
First cut is made on dispersion: σ<300 μm
Second cut is made on the distance: d(pair)<300 μm
2
2
2
2
– Dispersion of tracks around the cross-point:    ( xk  xo )  ( yk  yo )  ( zk  zo ) 
k 1
– Distance from cross-point to primary vertex
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Triplet selection
• Triplet of 3 tracks is
formed
• Secondary vertex is
calculated
• Dispersion of tracks
around the vertex is found
• Cuts are tuned in for Pt
bins:
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Resonances separation
• Ds decays via resonances
• Calculation of Inv Mass for KK and Kπ pairs
Inv Mass (Kπ)
Inv Mass (Kπ)
• Accept triplet if
|M(KK)-m(φ)|<Δm1 OR |M(Kπ)-m(K0*)|<Δm2
Inv Mass (KK)
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Inv Mass (KK)
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Final multicut
Variables considered:
1.
Cos Θpoint
2.
Cos Φopening
3.
Distance between primary and secondary vertex
4.
Sum of impact parameters squared
5.
Dispersion of secondary vertex (additional tuning)
π+
D +s
Θpoint
Pt(Ds)
Φopening
K+
K-
Cos Point
φ-like BKG
Signal Ds via φ
Cos Opening
Signal Ds via
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K0*-like BKG
K0*
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Results for Ideal PID
φ
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K0*
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Conclusion and To-Do
• Monte-Carlo simulation shows that reconstruction of the DsKKπ
is feasible in the range of Pt down to 3 GeV/c.
3-5 GeV/c >5 GeV/c
φ channel
5.1±0.4
19±5
K0*
channel
2.0±0.7
9±3
• Study of Real PID is in progress. Reduction of significance by factor
of ~2 is expected
• Bigger statistics for background events is needed to reduce the
error and better tuning of cuts.
• Case of pp collisions is under study. Some preliminary results without
multicut are present.
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