Kahn

Report
Detector Backgrounds in a Muon Collider
Steve Kahn
Muons Inc.
Muon Collider Design Workshop
Dec 11, 2008
Dec 11, 2008
S. Kahn -- Muon Collider Detector
Backgrounds
1
Introduction
•
•
•
This talk is a review of previous presentations on muon collider detector
backgrounds. Nothing presented here is new. A large fraction of the the
detector background studies was performed by Iuliu Stumer and Nikolai
Mokhov.
I will try to convince you that you can do physics at a Muon Collider.
– The backgrounds encountered are certainly worse than an ee– collider,
but they are no worse and probably better than that expected at the LHC
and the LHC will produce physics in that environment!
References:
– Snowmas 1996 Feasibility Study
– Status Report published in Phys. Rev. AB(1999)
– Highest Energy Muon Collider Workshop (Montauk, 1999)
– Rosario Muon Collider Workshop (May 1997)
– UCLA Workshop (July 1997)
–  Collider Conference, San Francisco (Dec 1997)
Dec 11, 2008
S. Kahn -- Muon Collider Detector
Backgrounds
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Parameters Used For Various Muon
Collider Scenarios
Parameter
COM Energy
Energy
Luminosity
Bunches/Fill
Muons/Bunch
Rep Rate
p/p rms
6D Emiitance
,N
*
R
Z

Nturns
Collider
Circumference
Ave Bend
Field
Dec 11, 2008
Units
Demonstration
m
Higgs
Factory
2001
0.1
0.050.05
1031
2
41012
15
0.003
1.710-9
290
14.1
294
14.1
0.015
450
350
1996
0.5
0.250.25
1033
4
21012
15
0.14
1.710-9
50
2.6
26
2.6
0.044
450
1000
High
Energy
1996
4
22
1035
4
21012
15
0.16
1.710-9
50
0.3
3.2
0.3
0.044
450
6000
Tesla
3
4.7
5.2
TeV
TeVTeV
cm2sec1
Hz
%
( m)3
 mm mrad
cm
m
cm
S. Kahn -- Muon Collider Detector
Backgrounds
HEMC
LEMC
2006
2006
1.5
1.5
0.750.75 0.750.75
1034
2.71034
1
10
1011
21012
13
65
0.001
0.01
4.410-11 1.51012
25
2.1
1
0.5
1
3124
0.5
0.06
2500
2270
6
10
3
Background Sources
•
•
•
Muon Decay Background
– Electron Showers from high energy electrons.
• Lepto-production of hadrons not included in studies.
– Not important for 22 TeV or smaller colliders.
– Bremsstrahlung Radiation for decay electrons in magnetic fields.
– Photonuclear Interactions
• Source of hadrons background.
– Bethe-Heitler muon production.
Beam Halo
– Beam Scraping at 180° from IP to reduce halo. Could it cause some?
– Collider sources such as magnet misalignments.
Beam-Beam Interactions.
– Believed to be small.
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Backgrounds
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Muon Decay Backgrounds
•
•
•
Muon decay backgrounds are expected to be high (see table)
The effort to minimize the backgrounds will have strong influence on
– Design of the Detector
– Design of the Final Focus for the IR
– The IR design itself
If the  per bunch can be reduced as we believe can be done for the LEMC, the detector
backgrounds will also be reduced.
– An order of magnitude reduction is a blessing.
– Most of the numbers presented in this talk will refer to the earlier designs with
larger numbers of muons per bunch. The results should be scaleable.
Collider
50  50 GeV
250  250 GeV
2  2 TeV
2.5  2.5 TeV LEMC
Dec 11, 2008
 per bunch
4  1012
2  1012
2  1012
1.6  1011
S. Kahn -- Muon Collider Detector
Backgrounds
Decays/meter
2.6  107
2.6  106
3.2  105
2.0  104
5
Muon Decay Background
•
•
Dec 11, 2008
Upper figure shows electron energy
spectrum from decay of 2 TeV
muons.
– 2×1012 Muons/bunch in each
beam
– 2.6×105 decays/meter
– Mean Decay Electron energy =
700 GeV
Lower figure shows trajectories of
decay electrons.
– Electron decay angles are of
the order of ~10 microradians.
– In the final focus section, the
decay electrons tend to stay in
the beam pipe until they see
the final focus quad fields.
S. Kahn -- Muon Collider Detector
Backgrounds
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Strawman Detector Concept for a Muon
Collider
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Backgrounds
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The Intersection Region as Modeled in
Geant for 2×2 TeV Muon Collider
130 m Region from IP
Final Focus Quadrupoles
5m
High Field Dipole Magnets
to Sweep Upstream Decay
Electrons
20 m
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Backgrounds
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IP Region for 2×2 TeV
(Similar Diagrams for other Energies)
Tracker Region
Vertex Detector
Borated Polyethylene for
neutron capture
20º Tungsten Cone For
electromagnetic shielding
Last final focus quadrupole
The figure represents 10 meters
around the IP
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Backgrounds
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Interior Design of the Tungsten
Shielding
• The tungsten shielding is
designed so that the detector is
not connected by a straight line
with any surface surface hit by
a decay electron in forward or
backward direction.
50×50 GeV case
250×250 GeV case
Borated Polyethylene
W
Cu
Dec 11, 2008
S. Kahn -- Muon Collider Detector
Backgrounds
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Summarizing Shielding Configuration to
Reduce Backgrounds
• 20 degree conical tungsten shield in forward/backward direction.
• Expanding inner cone from minimum aperture point is set at 4  beam
size.
• Inverse cone between IP and minimum aperture point is set to 4 
beam divergence.
– Designed so detector does not see surfaces struck by incident
electrons.
• Inner surface of each shield shaped into collimating steps and slopes to
maximize absorption of electron showers.
– Reduces low energy electrons in beam pipe.
• High field sweeping dipole magnets placed upstream of first
quadrupole. These dipoles have collimators inside to sweep decay
electrons in advance of final collimation.
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Backgrounds
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Electrons in the Intersection Region
•
•
Dec 11, 2008
Top figure shows the expanded
view of the region near the IP.
– The lines represent electrons
from a random sample of
muon decays.
– Electrons are removed by
interior collimation surfaces.
The bottom figure shows a detailed
view of the IR.
– Electrons from a random set of
muon decays.
– Electrons do not make it into
the detector region.
S. Kahn -- Muon Collider Detector
Backgrounds
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IP Configuration Parameters
Parameter
50×50 GeV
250×250 GeV
2×2 TeV
Shield Angle
20º
20º
20º
Open Space to
IP
6 cm
3 cm
3 cm
Min Aperture
Point
80 cm
1.1 m
1.1 m
Riris
0.8 cm
0.5 cm
Distance to First
Quad
7m
8m
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S. Kahn -- Muon Collider Detector
Backgrounds
6.5 m
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Bremsstrahlung Radiation
•
<E>=500 MeV
•
•
Log(
Dec 11, 2008
)
The decay electrons radiate
synchrotron photons as they
propagate through the fields in the
final focus region, losing on the
average about 20% of their energy.
Each electron radiates on the
average 300 synchrotron photons.
– The synchrotron photons carry
small energy and do not point
to small opening at the
intersection region.
The resulting background,
however, in the detector region is
small compared to the other
backgrounds because of the design
of the shielding as previously
described.
S. Kahn -- Muon Collider Detector
Backgrounds
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Incoherent Pair Production
•
Incoherent pair production from
ee can be significant for
high energy muon colliders.
– Estimated cross section of 10 mb
giving 3×104 electron pairs per
bunch crossing.
– The electron pairs have small
transverse momentum, but the oncoming beam can deflect them
towards the detector.
– Figures show examples of electron
pairs tracked near the detector in
the presence of the detector
solenoid field.
– With a 2 Tesla field, only 10% of
electrons make it 10 cm into the
detector. With 4 Tesla field no
electrons reach 10 cm.
Dec 11, 2008
S. Kahn -- Muon Collider Detector
Backgrounds
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Photonuclear Interactions
•
•
•
This is the primary source of hadron background.
The probability for photo production is small relative to other processes.
– Large numbers of photons released per crossing make this an important
background.
Different mechanisms in different energy bands:
– Giant Dipole Resonance Region
• 5<E<30 MeV
• Produce ~1 neutron
– Quasi-Deuteron Region
• 30<E< 150 MeV
• Produce ~1 neutron
– Baryon Resonance Region
• 150 MeV<E<2 GeV
• Produce  and nucleons
– Vector Dominance Region
• E>2 GeV
• Produce 0 that decay to .
•
GEANT 3.2.1 had to be modified to include photonuclear production. (I think that
GEANT 4 includes these.)
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Backgrounds
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Gamma Nuclear Interaction Models
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Backgrounds
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Neutron Background
Generated Neutron Spectrum
Log(
Dec 11, 2008
Neutron Spectrum Seen in
Detector
)
Log(
S. Kahn -- Muon Collider Detector
Backgrounds
)
)
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Time Distribution of Neutron
Background
• The top distribution shows the
time distribution of the neutron
background generated.
• The lower distribution shows
the time distribution of the
neutron background that is
seen in the tracker.
• The neutron flux has fallen by
two orders of magnitude before
the next bunch crossing (10 s
later).
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Backgrounds
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Pion Background in the Detector
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Photon and Neutron Fluxes at Radial
Planes
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Backgrounds
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Silicon Pad Occupancy as a Function of
Radial Position
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Backgrounds
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Backgrounds
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Bethe-Heitler Muons
• Electrons interacting with the beam pipe wall or tungsten shielding can
produce muon pairs. We call these muon pairs Bethe-Heitler Muons.
• These ’s can penetrate the shielding to reach the detector.
• Some Bethe-Heitler ’s will cross the calorimeter and produce
catastrophic bremsstrahlung losses that could put spikes in the energy
distribution.
• Time-of-Flight information:
– Fast timing can remove B-H ’s in the central calorimeter.
– Significant number of B-H ’s in for forward calorimeter are
likely to be in time with the signal.
• Fine Segmentation in both longitudinal and transverse directions will
be necessary to distinguish B-H background from signal.
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S. Kahn -- Muon Collider Detector
Backgrounds
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Bethe-Heitler Muon Trajectories for the
2×2 TeV Collider
Muon pair production at beam pipe
for example
NN
eNeN
(electrons are more likely to hit beam pipe).
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Backgrounds
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Effect of Timing on Bethe-Heitler
Muons
Muon pair production at
beam pipe for example
NN
50 ps could be
attainable now. This is
a significant
improvement over the
last decade
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Backgrounds
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Backgrounds
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Backgrounds
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Future Tasks:
What We Need to Plan to Do
•
We need to start to examine beam related backgrounds produced by currently
in vogue IP designs.
– This is expected to take a fair amount of work.
– We would have to optimize the current IP design as previously done to
reduce backgrounds.
• Compare to previous designs.
•
We need to reexamine the forward/backward shielding.
– Can we reduce the 20º blind cone angle by instrumenting the cone to
identify electromagnetic punch-through background so that it can be
ignored.
– Can we instrument the core to identify muons. This would help
enormously in identifying multi-lepton channels produced by SUSY.
– Can we instrument the low beta forward-backward regions.
• Mary Anne will tell us more about that.
Dec 11, 2008
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Backgrounds
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