Hyperon detection beyond the beam pipe

1
Hyperon detection beyond the beam pipe

• G.Lo Re, F.Riggi
• ALICE Week, June 2002

2
Strangeness detection

• First simulations of the reconstruction of K0s,
  ?, ?, ? already done
• K0s, ? extensive studies both by V1 and V2
  tracking versions
• ?, ? results reported by Y.Belikov at the last
  ITS Meeting
• One important aspect of such results is the low
  overall reconstruction efficiency (w.r.t. to the
  number of expected particles per event in HIJING)

3
Strangeness detection

• Sources of loss of such decaying particles

4
Strangeness detection

• Sources of loss of such decaying particles
• Branching ratios to charged particles
• Particle Decay B.R.()
  c? (cm)
• K0s ? ?- 68.6
  2.68
• ? p ?- 63.9
  7.89
• ?- ??- 99.9
  4.91
• ?- ? K- 67.8
  2.46

5
Strangeness detection

• Sources of loss of such decaying particles
• Branching ratios to charged particles
• Geometrical acceptance of TPC for daughter tracks

6
Strangeness detection

• Sources of loss of such decaying particles
• Branching ratios to charged particles K0s ?
  ? ?
• Geometrical acceptance of TPC for daughter
  tracks
• Tracking efficiency of TPC for daughter tracks
  (low momenta involved)

7
pT spectrum of K0s daughter tracks

K0s
8
pT spectrum of ? daughter tracks
p
?
9
Strangeness detection

• Sources of loss of such decaying particles
• Branching ratios to charged particles K0s ?
  ? ?
• Geometrical acceptance of TPC for daughter
  tracks
• Tracking efficiency of TPC for daughter tracks
  (low momenta involved)
• Tracking efficiency of ITS

10
Strangeness detection

• Sources of loss of such decaying particles
• Branching ratios to charged particles K0s ?
  ? ?
• Geometrical acceptance of TPC for daughter
  tracks
• Tracking efficiency of TPC for daughter tracks
  (low momenta involved)
• Tracking efficiency of ITS
• Secondary vertex finding (cuts,)

11
Working in the fiducial region
Due to these losses, expected number of found
strange particles per event are very
low Particle Hijing abundance
Old TP estimates Expected
(primary) K0s
1800
63 2.5 - 7 ?
400
8.3
0.1 - 1
Factors contributing to determine upper and lower
limits Use of full or parametrized Hijing
events Fast or slow ITS simulation Selection of
cuts and good to fake ratio within the invariant
mass spectrum
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How to improve strategies
These estimates have been made in a fiducial
region (0.5-2.5) cm, i.e. within the beam pipe,
since it would be more background free. However,
these values make it difficult to allow
correlation on an event-by-event basis. Need for
a better strategy Extend/Modify the
tracking strategies to allow for a better
secondary track finding Extend fiducial
region to check wether a substantial improvement
in the number of found particles can be
achieved without large worsening of other
parameters
13
Additional simulations
Additional simulations carried out by Aliroot
3.07 60 full Hijing events
Tracking V1 ITS full (slow) simulation

14
Extending the fiducial region K0s
Conservative approach fiducial region up to
the first pixel layer (0.5 - 4) cm
Sec. Vertex Resolution 350-400 ?m
15
Extending the fiducial region K0s
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Extending the fiducial region K0s
0.5
- 2.5 cm 0.5 - 4.0 cm
Gain factor Found particles
2.5 / event 4.2
/ event 1.7 RMS pT


1.3 RMS ?

6.5 mrad RMS ?

10 mrad Secondary vertex X Resolution

400 ?m Y Resolution

350 ?m Z Resolution
400 ?m
17
Extending the fiducial region ?
18
Extending the fiducial region ?
0.5
- 2.5 cm 0.5 - 4.0 cm
Gain factor Found particles
0.10 / event 0.30 /
event 3.0 RMS pT


2 RMS ?

4 mrad RMS ?
4
mrad Secondary vertex X Resolution

700 ?m Y Resolution

700 ?m Z Resolution
400 ?m
19
Transverse momentum spectrum of ?
?
?
(Within the TPC acceptance)
20
Radial distance distributions of sec. vertices
?
K0s
21
How much can we gain for Lambdas? Try to extend
the fiducial region beyond the first pixel layer?
In such a case a different way of selecting good
tracks in the ITS needed
22
Conclusions
Some improvements (by factors 2-4) may come from
extending the fiducial region beyond the beam
pipe and (possibly) even the first pixel layer
(to be tested). This effect is larger for ? than
for K0s. A substantial improvement (order of
magnitude) in the number of found hyperons may
only come from a different tracking strategy, to
recover the large fraction of low momentum
daughter tracks originating from decay.