BGV Vertex Resolution First Analysis Vertex Resolution as Function of Aperture
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Transcript of BGV Vertex Resolution First Analysis Vertex Resolution as Function of Aperture
LHC
BGV Vertex Resolution First Analysis
Vertex Resolution as Function of Aperture
M. Kuhn, P. Hopchev
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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LHC
Introductiono Actual transverse beam size for Gaussian shaped beams
is broadened by vertex resolution
o When : goal for final BGV!
o Motivation:
− Measurement at 7 TeV most challenging, as the beam is smallest
− Goal of initial BGV system: demonstrate the principle
• vertex resolution ~7 TeV beam (150-200 mm) would be sufficient
o Analyse vertex resolution as function of
− Beam pipe radius: 23 mm, 21 mm and 19 mm aperture
− Exit window tapering angle: 45º and 75º
Maria Kuhn - BGV Meeting #20 - 25.9.2013
aim at 0.2 aim at 10 %
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Input for MC Simulation and Vertex Reconstruction
o See talk in BGV meeting #17 (14/08/2013)
− Proton beam – neon gas target interactions simulated with HIJING
− Flat distribution of events in z-range [-500;1100] mm
− Process ~ 73000 events with large number of high multiplicity tracks
o 2 detector modules at z = 1591 mm and z = 2611 mm with 2 space point measurements per station
− Variable exit window thickness
• Thickness with R = 23 mm : 0.75 mm, thickness with R = 106 mm: 3.5 mm, linear interpolation in between
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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BGV sche-matic detector layout
LHC
o Vertex Resolution improves with track multiplicity (NTr)
− When measuring the beam profile, select events with high #tracks
o What is the expected rate, where we should cut on the NTr?
o Determine total inelastic interaction rate per bunch:
− Assuming: 2808 bunches per beam ()
• Ne gas flat over (gas target length)
• (0.45/7 TeV)
Cut on Track Multiplicity NTr (I)
4Maria Kuhn - BGV Meeting #20 - 25.9.2013
P. Hopchev
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Cut on Track Multiplicity NTr (II)
o Freduct indicates what fraction of the events is needed to retain in order to get 200/20000 events per 3 min
− i.e. to get 5/0.5 % statistical error on the beam profile fit (assuming Gaussian)
o Subsequently, we can tell what NTr can be reached
− Caveat: large uncertainty associated to the distribution tails (when a very small fraction of events is selected)
5Maria Kuhn - BGV Meeting #20 - 25.9.2013
P. Hopchev
LHC
Cut on Track Multiplicity NTr (III)o Guidelines for BGV vertex resolution study with toy MC:
− For detector layout with SciFi modules cut-out of 65 mm
− Choose only events with high track multiplicity
− For bunch measurements: NTr ≥ 11 (0.45 TeV); NTr ≥ 18 (7 TeV)
− For beam measurements: NTr ≥ 15 (0.45 TeV); NTr ≥ 25 (7 TeV)
o In the following:
− Estimates of the relative uncertainty on the beam width at 7 TeV
(always worse than at 450 GeV, because transverse beam size is smaller)
• As function of aperture
• And for different tapering angles
6Maria Kuhn - BGV Meeting #20 - 25.9.2013
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Vertex Resolution Definitiono Define and track true MC vertex xMC with BGV toy MC Simulation
− Sort events per number of tracks/vertex (NTr)
o Reconstruct primary vertex with LHCb algorithm xrec
o For each NTr plot histogram with residuals: xMC – xrec
o Fit histogram with 3 parameter Gauss Function to obtain s
Maria Kuhn - BGV Meeting #20 - 25.9.2013
= s vertex resolution
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Vertex Resolution versus NTr
o Divide original vertices from MC simulation in 4 z-position intervals
− [-500;-100], [-100;300], [300,700], [700,1100] mm
− Sort residuals accordinglyo Fit the x and y resolution with function:
− Fit convergences very sensitive to the initially given parameter values need to improve fitting method
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
Vertex Resolution versus NTr (II)
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Vertex Resolution versus NTr (III)
o Repeated vertex resolution calculation for all apertures with 75º tapering angle
− “Nominal” aperture (23mm)
− 2 mm reduced aperture (21 mm)
− 4 mm reduced aperture (19 mm)
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
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Weighted Vertex Resolution
o Choose events with NTr ≥ 18
o Weighted resolution:
− With and weights
o Vertex resolution improves when going closer to detector
− But less high-multiplicity events
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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detector
LHC
Summary Resolution for NTr ≥ 18o Comparison of weighted vertex resolution for different
apertures
− resolution error obtained from Gauss fit of histogram (statistical)
o Choose events with NTr ≥ 18
− Weighted vertex resolution for each NTr ≥ 18
− Results for x and y resolutions are similar, quoting the average of x and y
− To do: get resolution from curve fit and not from histogram
Maria Kuhn - BGV Meeting #20 - 25.9.2013
z range[mm]
Weighted resolution
“Nominal” aperture (23 mm)
2 mm reduced aperture (21 mm)
4 mm reduced aperture(19 mm)
700;1100 svtx [mm] 178 ± 6 164 ± 9 147 ± 10
300;700 svtx [mm] 192 ± 5 175 ± 5 165 ± 5
-100;300 svtx [mm] 192 ± 6 183 ± 5 175 ± 5
-500;-100 svtx [mm] 200 ± 6 187 ± 6 180 ± 5
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LHC
Conclusion - enorm = 1 mmo Reminder: with
o at 7 TeV for enorm = 1 mm & b = 170 m
Maria Kuhn - BGV Meeting #20 - 25.9.2013
z range[mm]
“Nominal” aperture (23 mm)
2 mm reduced aperture (21 mm)
4 mm reduced aperture(19 mm)
700;1100 0.141 0.119 0.096
300;700 0.163 0.136 0.120
-100;300 0.164 0.148 0.136
-500;-1000.176 0.154 0.144
Gain 10 – 15 % 20 – 30 %
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Conclusion - different enorm
o for enorm = 1.5 [2.,2.5] mm
− 7 TeV & b = 170 m
o Average over z range from [-500;700] mm (discard first z bin)
Maria Kuhn - BGV Meeting #20 - 25.9.2013
e [mm]
“Nominal” aperture (23 mm)
2 mm reduced aperture (21 mm)
4 mm reduced aperture(19 mm)
1.0 0.168 0.146 0.133
1.5 0.116 0.102 0.09
2.0 0.085 0.075 0.068
2.5 0.065 0.057 0.052
Gain 10 – 15 % 20 – 30 %
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
for Different Emittanceso Relative uncertainty on the transverse beam size
− See table on previous slide
o Better accuracy for larger transverse beam sizes
o Smaller relative beam size uncertainty for smaller aperture
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Vertex Resolution for 45º Tapering Angle
o Repeated these studies with different tapering angle
− 45º instead of 75º
− 23 mm aperture
− NTr ≥ 18
o Conclusions:
− For every z-position interval the vertex resolution is worse for the smaller tapering angle
− The relative uncertainty of the measured beam size improves by 9 % when choosing a larger tapering angle (75º)
• Again assuming
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
P. Hopchev
LHC
Summaryo The expected vertex resolution is estimated from the BGV toy
MC
− For 3 different apertures: 23 mm, 21 mm, 19 mm (75º tapering angle)
− And for 2 different tapering angles: 45º and 75º (23 mm aperture)
o The improvements on are
− 10 – 15 % better resolution for 2 mm reduced aperture
− 20 – 30 % better resolution for 4 mm reduced aperture
• With respect to 23 mm aperture
− 9 % better resolution for 75º tapering angle
• With respect to 45º
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
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Additional Slides
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
Vertex Resolution for 2 mm Reduced Aperture
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
Vertex Resolution for 2 mm Reduced Aperture
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
Vertex Resolution for 4 mm Reduced Aperture
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Maria Kuhn - BGV Meeting #20 - 25.9.2013
Vertex Resolution for 4 mm Reduced Aperture
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Vertex Resolution Nominal Aperture
o Scatter plot: vertex resolution per z and #tracks bin
− Colour represents vertex resolution for x and y position
− Circle size represents #events per bin (see previous slide)
o Best x and y vertex resolution for large #tracks!o Weighted resolution =
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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Nominal aperture = 23 mm
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2mm Reduced Apertureo Aperture = 21 mm
o Vertex resolution improves
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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4 mm Reduced Apertureo Aperture = 19 mm
o Vertex resolution even better
Maria Kuhn - BGV Meeting #20 - 25.9.2013
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NTr ≥ 25
Maria Kuhn - BGV Meeting #20 - 25.9.2013
Not very useful to analyse – not enough statistics in high track multiplicity bin
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