11

Tracking for lepton physicsTracking for lepton physics

Tetyana GalatyukGoethe-Universität, Frankfurt

low - momentum

low - momentum

22

The challenge…The challenge…

No electron identification in front of tracking

Background due to material budget of the STS

Reduction of background by reconstructing pairs from conversion and 0 Dalitz decay

Intr

oduct

ion

Intr

oduct

ion Radial vs. z position (eγ) and By

along the beam axis

33

Characteristic of the eCharacteristic of the e+ + and eand e--

Invariant e+e- spectrum in 25 AGeV Au+Au collisions (b = 0, full phase space)

Intr

oduct

ion

Intr

oduct

ion Momentum distribution

Opening angle distribution

1 signal decay / 400 1 signal decay / 400 background eventsbackground events

44

Combinatorial background topologyCombinatorial background topology

Small (moderate) opening angle “close pairs” if there is no field Asymmetric laboratory momenta

ee 0

eemedium

Intr

oduct

ion

Intr

oduct

ion

55

Momentum correlation plotMomentum correlation plot

plab identified e- vs. plab identified e+ plab identified e- vs. plab nrec e+

Intr

oduct

ion

Intr

oduct

ion

Identified e+/-

Track Segment

Identified e+/-

ee 0

Track Fragment

66

Wish N1 : Increase the size of the tracking detectorsWish N1 : Increase the size of the tracking detectors

x vs. y position of the extrapolated tracks

STS1STS2

STS2 STS3

STS3 STS4

Done!Done!

By now standard geometry

By now standard geometry

Wis

h N

1W

ish N

1

77

Wish N2 : The choice of the proper magnetic fieldWish N2 : The choice of the proper magnetic field

Trade: o Acceptance vs. Resolution

Currently a scaling of the filed is used. Our choice is 70% of the nominal field value

Wis

h N

2W

ish N

2 p as a function of the magnetic field value

88

Wish N3 : Good tracking performanceWish N3 : Good tracking performance

Momentum resolutionReconstruction efficiency

Wis

h N

3W

ish N

3

Reconstruction efficiency ~93% (p < 1 GeV/c) Momentum resolution < 2% Number of primary tracks with momentum < 500 MeV/c increased by 26%

99

Wish N4 : Field free region between the target and 1Wish N4 : Field free region between the target and 1stst MVD MVD

Excellent double-hit resolution (<100m) provides substantial close pair rejection capability

A realistic concept to suppress the field between the target and first MVD station has to be worked out

With magnetic field = 30% of its nominal value

Wis

h N

4W

ish N

4 Distance to closest neighbor hit in the 1st MVD station

1010

~ 7 kG

target ~ few kG RICH ~ 0 kG

Wish N5 : Refined field profile in the target regionWish N5 : Refined field profile in the target region

Vertical magnetic field strength (i.e. By) along the beam axis

Magnetic field map was developed (E. Litvinenko)

Problems with the tracking, most likely do to the strong field gradient at around 10 cm downstream of the target?

Trade:o Suppression of delta-electrons vs. opening of close pairs

Wis

h N

5W

ish N

5

1111

SummarySummary

Past:o Optimize tracking performance for tracks with momentum < 1 GeV/co Changes to the the detector setup

Present:o Proposed STS geometry is “standards” by nowo High track reconstruction efficiency for low-momentum tracks

Future:o Tracking within the field free region between the target and MVD 1o Track reconstruction with MVD (using the 3/4 MVD setup)o Reconstruction of the conversion on the detector material (using KF article)

Thank Thank youyou