Download Material budget, energy losses and multiple scattering

Material budget, energy losses
and multiple scattering
Barrel tracking

Momenta resolution for low momenta tracks determined mainly
by energy losses and multiple scattering
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Left side – momentum resolution for pion
Right side - proton
Energy loss between vertex
and TPC
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Left - rel. loss as a function of particle velocity
Right – function of particle momenta
Energy losses (Bethe Bloch)
Z 1
dE / dx  k1
A b2
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  2me c 2 b 2 

d
2


r  ln 
b
2 

I
(
1
b
)
2




b -particle velocity
r - material density
Z - atomic number of absorber
A – mass number of absorber
I – mean excitation energy
d – density effect correction factor – material
dependent and b dependent
Energy losses (Reconstruction)
2
 


b
2


dE / dx  k1 2 r  ln  k 2
b

b   (1 - b 2 ) 

1
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b - particle velocity
r - material density
K1 and K2 – Effective parameters
Energy loses correction
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Left side - correction shift as function of
particle velocity
Right side – correction shift as function of
particle momenta (pion)
Energy loses correction
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Left side - correction shift as function of
particle momenta (kaon)
Right side – correction shift as function of
particle momenta (proton)
Multiple scattering (Gaussian
approximation)
 13.6MeV
  
 bcp
2
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b -particle velocity
r - material density
P - particle momenta
2

x
 *
 X0
Energy losses correction
(Current)
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Material budget and radiation length
hardwired in the code
Using symmetry of the detectors
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Correction layer by layer during propagation
Intervals in y and z in the local coordinate frames
Fast access
Difficult to describe non symmetric parts (big
problem in TRD)
Geo modeler (0)
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Used to get information necessary for energy
loss calculation and multiple scattering
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Local information - in each point density, radiation
length, Z, A defined (mean excitation energy
missing)
Mean query time ~ 15 ms
Mean number of queries
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~15 – between 2 ITS layer
~15 – between 2 TRD layers
Geo modeler (1)
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Two option considered
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1. Propagate track up to material boundary
defined by modeler – get local material
parameters
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Time consuming - too many propagations
2. Calculate mean parameters between start and
end point
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<density>, <density*Z/A>, <radiation length>
Faster (only one propagation), reusable in the case of
parallel hypothesis (ITS), not big changes in the tracking
Implementation
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AliKalmanTrack::MeanMaterialBudget(D
ouble_t *start, Double_t *end,
Double_t *param)
First test
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Track references in inner volume of the
TPC – propagated to the vertex
TRD tracking
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FollowProlongatioBackG implemented
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Using mean material budget
14 steps
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Propagate to first plane
Loop over TRD planes
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Propagate and update in the sensitive layer
Propagate to the next plane
Propagate to the outer volume of TRD
Energy loss estimate
resolution
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Left side - old propagation
Right side – new propagation
Relative Pt resolution
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Left side - old propagation
Right side – new propagation
Relative Pt resolution
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Left side - old propagation
Right side – new propagation
Pt pulls
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Left side - old propagation
Right side – new propagation
Time pulls
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Left side - old propagation
Right side – new propagation
Conclusion
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First results in TRD tracking
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Indication of improvements in the
momentum and the time resolution
Test with propagation to the vertex
using AliExternalParameter and
GeoMedeler – better vertex position
resolution
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Better interface required – without user
intervention
Conclusion
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Default access to the TGeoManager
required
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Currently loaded by hand
Better energy loss parameterizationoptions:
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1. Mean Energy loss and multiple
scattering calculation using TGeoManager
2. Tuning 1 free parameter -