Luminosity Measurement questions

1
Luminosity Measurement questions
calorimeter-related
Stewart Takashi Boogert and David John Miller
Department of Physics and Astronomy University
College London sboogert_at_hep.ucl.ac.uk,
djm_at_hep.ucl.ac.uk
1. First order optimism (duplicates MDI session
talk). 2. Forward tracking plus endcap
calorimetry. 3. Smaller angle measurements. 4.
Work planned.
2
First order optimism
First order studies suggested can
be measured to the required energy
precision 1 in 103 for top-antitop
threshold 1 in 104 for WW threshold On that
basis we have advertised
10s of MeV (Martinez, St. Malo)
(TESLA TDR)
Frary and Miller DESY 92-123A,
http//www.hep.ucl.ac.uk/lc/documents/frarymiller.
pdf. Y.Kurihara, talk at Munich workshop
1993. D.Cinabro, LCWS Sitges, proceedings
p249, http//www.desy.de/lcnotes/sitges/D.Cinabro
-10.ps.gz K.Moenig, LC-PHSM-2000-60-TESLA
http//www.desy.de/lcnotes/2000/060/beamspec.ps.g
z
3
Sources of Energy Spread
Spike gives mass sensit- ivity
zoom
To make polarised positrons in TESLA, get ?p/p
0.15 for e-, lt 0.05 for e (what is NLC
spread?). Maybe tolerable for top threshold, if
spread can be well measured. For W mass at ?s
161 GeV How to unfold 1 in 10 4 from spread of
1.5 in 10 3? Alternative e source?
4
Bhabha acollinearity
Reference process should 1. be based on real
events, to truly represent the physics samples.
2. have better statistics than the physics
channels. 3. have energy resolution to match
the mass resolution required. Measurement of
Bhabha acollinearity in the endcap region (
milliradians) appears to
meet all 3 criteria. Nothing else does (?? rate
physics rates Z? not so precise etc.
etc.) Basic Principle
, small ?A
Where but (we discuss
questions about how beam effects can louse-up
correlation between ?A and ?s in MDI session)
p
p-
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Endcap best?
For small ?A and Gaussian errors,
, so with
fixed angular resolution the error on ?s
blows up at small Bhabha scattering angle ?.
But the Bhabha rate for
millirads is already gt 400 times the ??
rate. And above 100 milliradians there is good
forward-tracking (7 layers of pixels/Si in
TDR) with calorimetric backing (CALICE endcap)
HCAL
CALICE
TPC
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For and against endcap
Is tracker angular resolution good enough?
0.02 mrad (but/sin ?
for ) - fine for top threshold - just OK
for WW? But forward trackers are never
100 efficient. Rely on calorimeter to measure
efficiency of tracker. Is calorimeter angular
res- olution good enough? - just OK for
top - not for WW
.25mrad
CALICE in TDR (endcaps as good?)
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Need FTD plus CALICE
Calorimeter, assumed to have 100 efficiency for
high energy electrons. It surveys efficiency of
tracker. Tracker has the resolution needed for
acollinearity, but needs calorimeter to measure
its efficiency. Tracker must have good internal
survey, to 25 microns over 3 metres. Clean
electron tracks used to survey calorimeter
position scale (c.f. OPAL). BUT TPC cage and
ends make electrons shower - may
move calorimeter cluster position, spoil purely
calorimetric acollinearity measurement (study
starting)
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What about LAT and LCAL?
LAT is SiW 28 to 83 millirad. (Zeuthen et al
studying potential) Similar angular resolution
to CALICE? No TPC structures in front to shower,
but /sin ? gives worse . Very high
rates (needed at GigaZ). Theoretical Bhabha
cross sections more reliable at small angles
than in endcap. LAT favoured for absolute
luminosity measurement (c.f. LEP and SLC). LCAL
may be PbWO 5 to 27 millirad. Serious
backgrounds from pairs and gammas. Not a
precision luminometer, but good instantaneous
rates for tuning. Background asymmetries reflect
bunch misalignment.
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Work Planned
A. Simulate realistic data samples of Bhabhas
measured in forward tracker and endcap
calorimeters (including machine pathologies
mentioned in MDI talk). B. Reconstruct, and
extract measured luminosity spectra (using
unfolding - c.f. Blair and Poirer?). C. Compare
with Monte Carlo truth from Guinea-Pig. D.
Investigate effects on mt and MW . Please tell
us what else we should be worrying about!