Detector Backgrounds in a Muon Collider

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Detector Backgrounds in a Muon Collider

• Steve Kahn
• Muons Inc.
• Muon Collider Design Workshop
• Dec 11, 2008

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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)

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Parameters Used For Various Muon Collider
Scenarios
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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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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.

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Muon Decay Background

• Upper figure shows electron energy spectrum from
  decay of 2 TeV muons.
• 21012 Muons/bunch in each beam
• 2.6105 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.

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Strawman Detector Concept for a Muon Collider
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The Intersection Region as Modeled in Geant for
22 TeV Muon Collider
130 m Region from IP
Final Focus Quadrupoles
5 m
High Field Dipole Magnets to Sweep Upstream Decay
Electrons
20 m
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IP Region for 22 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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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.

5050 GeV case
250250 GeV case
Borated Polyethylene
W
Cu
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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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Electrons in the Intersection Region

• 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.

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IP Configuration Parameters
Parameter 5050 GeV 250250 GeV 22 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 7 m 8 m 6.5 m
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Bremsstrahlung Radiation

• 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.

ltE?gt500 MeV
Log(
)
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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 3104
  electron pairs per bunch crossing.
• The electron pairs have small transverse
  momentum, but the on-coming 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.

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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
• 5ltE?lt30 MeV
• Produce 1 neutron
• Quasi-Deuteron Region
• 30ltE?lt 150 MeV
• Produce 1 neutron
• Baryon Resonance Region
• 150 MeVltE?lt2 GeV
• Produce ? and nucleons
• Vector Dominance Region
• E?gt2 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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Gamma Nuclear Interaction Models
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Neutron Background
Generated Neutron Spectrum
Neutron Spectrum Seen in Detector
Log(
)
Log(
)
)
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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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Pion Background in the Detector
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Photon and Neutron Fluxes at Radial Planes
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Silicon Pad Occupancy as a Function of Radial
Position
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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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Bethe-Heitler Muon Trajectories for the 22 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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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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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.