Material budget, energy losses and multiple scattering

Barrel tracking

Momenta resolution for low momenta tracks determined mainly by energy losses and multiple scattering

Left side – momentum resolution for pion Right side - proton

Energy loss between vertex and TPC

Left - rel. loss as a function of particle velocity Right – function of particle momenta

Energy losses (Bethe Bloch)

-particle velocity material density Z - atomic number of absorber A – mass number of absorber I – mean excitation energy – density effect correction factor –

material dependent and dependent

2)1(

2ln

1/ 2

2

22

21

I

cm

A

ZkdxdE e

Energy losses (Reconstruction)

- particle velocity material density K1 and K2 – Effective parameters

22

2

221 )1(ln

1/

kkdxdE

Energy loses correction

Left side - correction shift as function of particle velocity

Right side – correction shift as function of particle momenta (pion)

Energy loses correction

Left side - correction shift as function of particle momenta (kaon)

Right side – correction shift as function of particle momenta (proton)

Multiple scattering (Gaussian approximation)

-particle velocity material density P - particle momenta

0

2

2 *6.13

X

x

cp

MeV

Energy losses correction (Current) Material budget and radiation length

hardwired in the code Using symmetry of the detectors

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) Used to get information necessary for

energy loss calculation and multiple scattering Local information - in each point density,

radiation length, Z, A defined (mean excitation energy missing)

Mean query time ~ 15 s Mean number of queries

~15 – between 2 ITS layer ~15 – between 2 TRD layers

Geo modeler (1) Two option considered

1. Propagate track up to material boundary defined by modeler – get local material parameters

Time consuming - too many propagations 2. Calculate mean parameters between

start and end point <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

AliKalmanTrack::MeanMaterialBudget(Double_t *start, Double_t *end, Double_t *param)

First test Track references in inner volume of

the TPC – propagated to the vertex

TRD tracking

FollowProlongatioBackG implemented Using mean material budget 14 steps

Propagate to first plane Loop over TRD planes

Propagate and update in the sensitive layer Propagate to the next plane

Propagate to the outer volume of TRD

Energy loss estimate resolution

Left side - old propagation Right side – new propagation

Relative Pt resolution

Left side - old propagation Right side – new propagation

Relative Pt resolution

Left side - old propagation Right side – new propagation

Pt pulls

Left side - old propagation Right side – new propagation

Time pulls

Left side - old propagation Right side – new propagation

Conclusion First results in TRD tracking

Indication of improvements in the momentum and the time resolution

Test with propagation to the vertex using AliExternalParameter and GeoMedeler – better vertex position resolution Better interface required – without

user intervention

Conclusion

Default access to the TGeoManager required Currently loaded by hand

Better energy loss parameterization- options: 1. Mean Energy loss and multiple

scattering calculation using TGeoManager

2. Tuning 1 free parameter -