GE1/1-III GEM GE1/1-III GEM Cluster Size Cluster Size and and ResolutionResolution Studies with the FNAL Beam Test DataStudies with the FNAL Beam Test Data

Aiwu Zhang, Vallary Bhopatkar, Marcus Hohlmann

Florida Institute of Technology

CMS GEM workshop IX14-18/07/2014

OutlineOutline• Current data analysis status from the FNAL

beam test• Cluster size (number of strips in a cluster)

studies• Resolution results• Summary and next plans

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FNAL beam test data analysis statusFNAL beam test data analysis status

• Finished basic characteristics: cluster charge distributions, gain curve, detection efficiency, etc.

• Implemented spatial resolution study method in polar coordinates.

• This talk presents the cluster size studies and resolution results, with HV scan data in middle-sector 5 and position scan data in middle position of each sector. Note analog pulse information are decoded from APV chips.

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Mean cluster sizes Mean cluster sizes with different thresholdswith different thresholds

HV scan in middle sector 5 Position scan in middle sectors at 3250V

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Fraction of different cluster sizes Fraction of different cluster sizes – HV scan in middle-sector 5– HV scan in middle-sector 5

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3σ threshold 4σ threshold

5σ threshold

Fraction of different cluster sizes Fraction of different cluster sizes – HV scan in middle-sector 5– HV scan in middle-sector 5

N=1 N=2 N=3

N=4 N>=5

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Fraction of different cluster sizes – Fraction of different cluster sizes – middle-sector position scan (3250V)middle-sector position scan (3250V)

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3σ threshold 4σ threshold

5σ threshold

• Majority of the events have cluster size 2 and 3.

Fraction for different cluster sizes – Fraction for different cluster sizes – middle-sector position scan (3250V)middle-sector position scan (3250V)

N=1 N=2 N=3

N=4 N>=5

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Spatial resolution study -- methodSpatial resolution study -- method• We study resolution in polar coordinates (φ)

due to the trapezoidal shape and radial strips. We use the vertex as origin and align the trackers by finding the X, Y offsets. (rotation of the GEM is also taken into account, not shown here.)

X offset

Eta5vertex

9.94°

tracker

Step 1. Fix an X offset, loop over Y offset with a step 0.2mm; we get a pair of (X,Y) by requiring residual mean equals to 0.

Step 2. Fix a Y offset, loop over X offset with a step 1mm; we find another pair of (X,Y) from the minimal of the parabola of residual sigma.

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We assume that any misalignment will shift peak of residual distribution away from zero and give larger residual width (or make distribution wider)

GE1/1-III GEM Residual distributionsGE1/1-III GEM Residual distributions

σ=138μrad σ=110μrad

Exclusive residual

Inclusive residual

Exclusive (inclusive) residuals at 3250V in sector 5 (from HV scan data)Cluster size in GEM and trackers all >0.

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Resolution results – behavior in detailResolution results – behavior in detail

Resolution vs. HV in middle-sector 5 Resolution vs. sector at 3250V

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Strip non-linear response study -- motivationStrip non-linear response study -- motivation

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• From the centroid position distributions of fixed cluster sizes N=2 and 3, we observe that they have apparent bumps around each strip. For N=2, the positions in the middle of two neighboring strips; for N=3 the positions in the strip. (note we define the position of a strip in its center.)

• This brings us to study the non-linear strip response of charge distribution on position reconstruction, and hence to flat these distributions.

• Centroid position distribution from COG method (in middle-sector 5).

N=2 N=3

Strip non-linear response study -- methodStrip non-linear response study -- method

h(η2) distribution h(η3)distribution

Correction function for 2-strip events

Correction function for 3-strip events

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Cluster position distributions Cluster position distributions before and after correctionbefore and after correction

N=2

N=3

HV scan data @ 3250V

Before correction After correction

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Strip response functionsStrip response functions

N=2N=4N=3

HV scan on middle sector 5

N=2

N=4N=3Position scanat 3250V

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Resolution results (HV scan)Resolution results (HV scan)before and after position correctionbefore and after position correction

Resolution is not improved after position correction with HV scan data!

All valid events with cluster sizes 2,3,4

N=2

N=3 N=4

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Resolution results (position scan)Resolution results (position scan)before and after position correctionbefore and after position correction

Resolution is not improved after position correction with position scan data!

All valid events with cluster sizes 2,3 and 4

N=2

N=3N=4

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SummarySummary Most of the clusters have sizes no more than 5. Spatial resolution is on the order of 120μrad. We don’t observe improvement on resolution after applying cluster

position correction.

• What else can be done with these data (HV scan, position scan in middle sectors):

Study details of cluster size distributions using VFAT-like strip hits. Apply position correction a second time to flatten position

distributions even more and to see if resolution could be improved. Study resolution with different thresholds.

• What needs to be done with other data: We also performed two position scans with the detector tilted by

7degrees from normal incidence. The effect of the inclination needs to be studied.

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