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Calibration of the COSY-TOF STT & pp Elastic Analysis

Sedigheh Jowzaee

IKP Group Talk11 July 2013

Outline

• COSY-TOF Spectrometer

• STT calibration goal

• Calibration comparison

• pp elastic analysis of data 2012

2

COSY-TOF Spectrometer

• Strangeness physics pp→ pK+ Λ pπ3

p

p

π-

K+

COSY-TOF STT• Installed in vacuum tank

• Consists of 2704 straw tubes with Ø=10 mm & 1050 mm length

• Organized in 13 double-layers

• Filled with Ar/CO2 gas at 1.2 bar overpressure

• Fixed in 3 orientations with angle 60˚ to each other for 3D track reconstruction

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Calibration Procedure

• Motivation for precise STT calibration• Reconstruction of events with STT at COSY-TOF • Event analysis based on the vertices reconstruction of the

charged final state particles (p, K, Λ p, π-)• Calibration steps

• TDC correction• Multiple hits removal• Signal width cut• Electronics offset correction

• Estimation of correlation between drift time and radius• Straw layers position correction

• pp elastic events measured in Fall 2012 at pbeam=2.95 GeV/c are analyzed for the calibration of the STT

• Data taking in November 2012 for 4 weeks

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TDC Correction

• First hit selection• Signal width cut

– 5ns width limit of readout electronics

Using the common-stop readout of the TDCs, higher values correspond to shorter drift times

raw TDC spectrum for 5.106 hits in the 3 double layers straw tubes

TDC spectrum after first hit selection and width cut

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Due to the positioning of tubes in dls (Time of flight)

TDC Correction

• Electronics offset correction• Different readout modules• Correction with fit method

• Ref. point=turning point of error function + 1σ

• Offset= 780 ns(arbitrary)-Ref. point

Turning point

σ

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Due to the positioning of readout board

Due to applying 3 racks of readout electronics

Self-Calibrating Method

• Main aim: determination of the correlation between the drift time and the isochrone radius

• Isochrone radius was calculated for each bins of drift time (homogeneous illumination assumption in whole straw)

Track

Straw Tube

Isochrone radius: cylinder of closest approach of the particle track to the wire

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Risochrone

min)()()( RN

NRRdttvtr

tot

iwiretubedrifti

Auto-Calibration Method• Track reconstruction with averaged r(t) curve of 3 groups of double layers

from self-calibrating method was done• Track parameters were analyzed to find the most probable correlation

between drift time and isochrone radius (track to wire distance)

distance to wire vs. drift time shift vs. isochrone radius

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STT Resolution• Residual=|d| – r d: track to wire distance, r: isochrone radius

• Spatial resolution: width of the Gaussian fit functions to the residual distribution as a function of drift time or radius

• The resolution at 0.25 cm averaged over all double layers is 142 ± 8 µm

residual vs. drift time resolution vs. isochron radius

Residual vs. isochron radius10

Residual Comparison

New calib.

Old calib.

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dl 5 dl 13

dl 5 dl 13

Resolution Comparison

New calibration

Old calibration

Resolution at 0.25 cm New: 142±8 µmOld : 174±18 µm

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pp Elastic Analysis

beampppyCoplanarit ˆ)ˆˆ( 21 )()( 21arg EEEEE ettbeammiss

13

pbeam ptarget

p1

p2

θ1

θ2

φGeometry of pp elastic events

pp Elastic Analysis

1)20

()5

( 22

MeV

E

mrad

C

selectioncircular

miss

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After coplanarity cut

After circular cut

Vertex Distribution

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• Dependent on the beam properties

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Vertex Distribution• Dependent on the target dimension

target dimension=5.17±0.03 mm

Closest Approach of Tracks

• Minimum distance of the two proton tracks of selected pp elastic scattering events

• Independent on the beam properties

• Dependent on the STT reconstruction precision

• Improvement of FWHM 7.6%

FWHM=1780 µm new calibration

FWHM=1920 µm old calibration

• Improvement in reconstruction accuracy

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Summary

• Signal width cut is effective to remove noise

• Electronics offset correction reduced the systematic error from different electronics modules and time of flight

• Improved spatial resolution 142 ±8 µm at 0.25 cm averaged over all double layers compared to the old calibration with same beam momentum (174 ±18 µm)

• The new calibration improved track reconstruction accuracy for pp elastic scattering events

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Mitg

lied

der H

elm

holtz-G

em

ein

schaft

Thank you

Mitg

lied

der H

elm

holtz-G

em

ein

schaft

Backup slides

21

Straw hits

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Drift time

• Corrected drift time spectra• Maximum drift time 145 ns

• Same drift time spectrum within each double layer

• Irregular shape and tail part in first 4double layers

• Improper recognition of first hits due to low sensitivity of their electronics

• Events mixing and tail pile-up

Monte Carlo Comparison

New calibration -2012

calibration-MC-2012

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Beam Direction

Beam Polarization

• distribution of asymmetry is fitted with the ā(θ*)cos(φ)

• The analyzing power A(θ*) is taken from the partial wave analysis Said

• calculated beam polarization

(69.9±10.0)%

Azimuthal asymmetry of elastic scattering events in

pppp

]60,45[

)()cos(

),()(

),(

)cos(

),(

)(

1

**

*

*

*

*

aconsta

AP

NN

NNa

a

AP

beam

beam

25