Luminosity Measurement at the International Linear Collider
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Transcript of Luminosity Measurement at the International Linear Collider
Luminosity Measurement at the International Linear Collider
Iftach Sadeh
Tel Aviv University
( On behalf of the FCAL collaboration )
December 28th 2008http://alzt.tau.ac.il/~sadeh/
Overview
1.The International Linear Collider (ILC).
2.Luminosity measurement at the ILC.
3.Design and performance of the luminosity calorimeter (LumiCal).
4.A clustering algorithm for LumiCal.
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The ILC & The ILD detector3
LumiCal Performance requirements
1. Required precision is:
2. Measure luminosity by counting the number of Bhabha events (NB) in a well defined angular and energetic range:
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3 6
3 6
10 , GigaZ (hadronic Z decays) 10 / year
~10 , 10 / year
~10 , 10 / year
L
LL
e e W WLL
e e q qL
3
1Bd
d
max
min
, rec genB B
B B gen
N NN NLL
L N N
RIGH LEFTTΔθ - θθLEFTθ
RIGHTθ
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LumiCal Design parameters
3. Layers:
Number of layers - 30
Tungsten Thickness - 3.5 mm
Silicon Thickness - 0.3 mm
Elec. Space - 0.1 mm
Support Thickness - 0.6 mm
1. Placement:
2270 mm from the IP
Inner Radius - 80 mm
Outer Radius - 190 mm
2. Segmentation:
48 azimuthal & 64 radial divisions:
Azimuthal Cell Size - 131 mrad
Radial Cell Size - 0.8 mrad
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ares
σ(θ)
Intrinsic parameters of LumiCal
Relative energy resolution:
Position reconstruction (polar angle):
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4
min
21.5 10
L
L
Topology of Bhabha scattering Bhabha scattering with √s = 500 GeV
θ Φ
Energy
Separation between photons and leptons, as a function of the energy of the low-energy-particle.
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Clustering Algorithm
Phase I:Near-neighbor clustering in a single layer.
Phase II:Cluster-merging in a single layer.
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Phase III:Global (multi-layer) clustering.
Overlap of multiple showers
Profile of the energy of a single 250 GeV shower.
Profile of the energy of two showers (230 and 20 GeV) separated by one Moliere radius (indicated by the circles).
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Clustering - Results (event-by-event)
Event-by-event comparison of the energy and position of showers (GEN) and clusters (REC) as a function of EGen.
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(θGen - θRec)/ θGen(ΦGen - ΦRec)/ ΦGen
(EGen - ERec)/ EGen
Summary1. Intrinsic properties of LumiCal:
Energy resolution: ares ≈ 0.21 √GeV.
Relative error in the luminosity measurement:ΔL/L = e(rec) e(stat) = 1.5 · 10-4 4 · 10-5 (at 500 fb-1).
- The reconstruction error, e(rec), is due to the error in reconstruction of the polar angle.
- The statistical error, e(stat), is due to fluctuations in the expected number of Bhabha events, ΔNB/NB, for an integrated luminosity of 500 fb-1.
- The theoretical uncertainty is expected to be ~ 2 · 10-4.
2. Partial event reconstruction (counting of radiative photons):
Merging-cuts need to be made on the minimal energy of a cluster and on the separation between any pair of clusters.
The algorithm performs with high efficiency and purity. The number of radiative photons within a well defined phase-space may be counted with an acceptable uncertainty.
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AuxiliarySlides
The ILC & The LDC detector13
Physics Background Four-fermion processes are the main background, dominated by two-photon
events (bottom right diagram).
The cuts reduce the background to the level of 10-4
ffeeeeFour-fermion processes
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Selection of Bhabha events
Right side signal
Left
sid
e si
gnal
deg5
rad310
LRLR RERE ,min1.0
Simulation distribution
Distribution after acceptance and energy balance selection
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RIGH LEFTTΔθ - θθLEFTθ
RIGHTθ
Detector signal
Distribution of the deposited energy spectrum of a MIP (using 250 GeV muons):MPV of distribution = 89 keV ~ 3.9 fC.
Distributions of the charge in a single cell for 250 GeV electron showers.
The influence of digitization on the energy resolution and on the polar bias.
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ares
σ(θ)
ares
Digitization
σ(θ)
Δθ
Thickness of the tungsten layers (dlayer)
σ(θ)Δθ
ares
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Number of radial divisions
σ(θ) Δθ
min
2
L
L
-1
5
500GeV
1.23nb
500 fb
4 10
s
L
N
N
Dependence of the polar resolution, bias and subsequent error in the luminosity measurement on the angular cell-size, lθ.
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Inner and outer radii
Beamstrahlung spectrum on the face of LumiCal (14 mrad crossing angle): For the antiDID case Rmin must be larger than 7cm.
-1500 fbL
63.2 10 rad
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MIP (muon) Detection in LumiCal Many physics studies demand the ability to detect muons (or the lack thereof) in
the Forward Region.
Example: Discrimination between super-symmetry (SUSY) and the universal extra dimensions (UED) theories may be done by measuring the smuon-pair production process. The observable in the figure, θμ, denotes the scattering angle of the two final state muons.
“Contrasting Supersymmetry and Universal Extra Dimensions at Colliders” – M. Battaglia et al. (http://arxiv.org/pdf/hep-ph/0507284)
1 1 1 1
2UED:
~ 1 coscos
e e
d
d
0 01 1
2SUSY:
~ 1 coscos
e e
d
d
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MIP (muon) Detection in LumiCal
Multiple hits for the same radius (non-zero cell size).
After averaging and fitting, an extrapolation to the IP (z = 0) can be made.
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1. High beam-beam field (~kT) results in energy loss in the form of synchrotron radiation (beamstrahlung).
2. Bunches are deformed by electromagnetic attraction: each beam acting as a focusing lens on the other.
Change in the final state polar angle due to deflection by the opposite bunch, as a function of the production polar angle.
Beam-Beam effects at the ILC
Since the beamstrahlung emissions occur asymmetrically between e+ and e-, the acolinearity is increased resulting in a bias in the counting rate.
“Impact of beam-beam effects on precision luminosity measurements at the ILC”
– C. Rimbault et al. (http://www.iop.org/EJ/abstract/1748-0221/2/09/P09001/)
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Effective layer-radius, reff(l) & Moliere Radius, RM
Dependence of the layer-radius, r(l), on the layer number, l.
RM
r(l)
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Distribution of the Moliere radius, RM.
Clustering - Algorithm
Phase I:Near-neighbor clustering in a single layer.
Phase II:Cluster-merging in a single layer.
Longitudinal shower shape.
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Clustering Algorithm26
Phase III:Global (multi-layer) clustering.
Clustering - Energy density corrections
Event-by-event comparison of the energy of showers (GEN) and clusters (REC).
Before After
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Clustering - Results28
(2→1): Two showers were merged into one cluster.
(1→2): One shower was split into two clusters.
The Moliere radius is RM (= 14mm), dpair is the distance between a pair of showers, and Elow is the energy of the low-energy shower.
Clustering - Geometry dependence29
96 div
48 div
24 div
Clustering - Results (measurable distributions)
Energyhigh
θhigh θlow
Energylow
Δθhigh,low Energy and polar angle (θ) of high and low-energy clusters/showers.
Difference in θ between the high and low-energy clusters/showers.
Merging-cuts:Elow ≥ 20 GeV , dpair ≥ RM
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Clustering - Results (relative errors) Dependence on the merging-cuts of the errors in counting the number of
single showers which were reconstructed as two clusters (N1→2), and the number of showers pairs which were reconstructed as single clusters, (N2→1).
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