NLC IP Backgrounds

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NLC IP Backgrounds

• Jeff Gronberg/LLNL
• MAC Collaboration Meeting
• June 1, 2000

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Detector Backgrounds
Standard HEP 4p detectors

• Charged particles in tracking volume
• 1 hit/mm2/train in SVX-1
• Neutrons in the Detectors
• 3 x 109 hits/cm2/yr

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Backgrounds Sources

• Machine Backgrounds
• Synchrotron Radiation
• Muons Production at collimators
• Direct Beam Loss
• Beam-Gas
• Collimator edge scattering
• Neutron back-shine from Dump
• Extraction Line Losses

Bad, get nothing in exchange 1) Dont make
them 2) Keep them from IP if you do
Good, scale with luminosity 1) Transport them
away from IP 2) Shield sensitive detectors 3)
Timing

• IP Backgrounds
• Beam-Beam Interaction
• Disrupted primary beam
• Beamstrahlung photons
• e,e- pairs from beams. gg interactions
• Hadrons from beams. gg interactions
• Radiative Bhabhas

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Beam-Beam InteractionSR photons from individual
particles in one bunch when in the electric field
of the opposing bunch
Pinch makes beamstrahlung photons 1.5E10 per
bunch _at_ ltEgt30.3 GeV (0.83 Mw)

• And Disrupts the Beam Particles
• Photons and Disrupted beam particles go straight
  to the dump
• Not a background problem, but angular dist. (1
  mrad) limits extraction line length
• Photons interact with opposing e,g to produce
  e,e- pairs and hadrons

gg ? ee- (Breit-Wheeler) eg ? eee-
(Bethe-Heitler) ee ?eeee- (Landau-Lifshitz)
gg ? hadrons
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Beam - Beam Simulation

• Guinea - Pig is used to simulate
• Disrupted Beam
• Beamstrahlung Photons
• e/e- pairs
• Radiative Bhabhas

• Machine Parameters used
• 1 TeV NLC-B
• 1 x 1010 e-/bunch x 95 bunches/train _at_ 120 tps

IP Backgrounds Disrupted Beam Beamstrahlung
photons ee- pairs Hadrons from gg Radiative
Bhabhas
particles/bunch 2 x 1010 3 x 1010 88K 250K
E (GeV) 460 30 10.5 370
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Detector and Beamline Simulation

• Full Geometry simulated
• Large and small detectors
• Masks
• Incoming beamline Z lt 200m
• Complete Extraction Line
• GEANT3
• e/e- and photon backgrounds
• FLUKA98
• Neutron backgrounds

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e,e- pairs from beams. gg interactions44K per
bunch _at_ ltEgt10.5 GeV (0.85 W)

• Disrupted beam and beamstrahlung photons exit
  through the outgoing beamline
• They travel far enough away from the IP that they
  are not a significant background
• e/e- pairs curl in the detector solenoidal
  magnetic field
• They can interact with objects close to the IP

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ee- pairs define the beampipe radius

• e/e- pairs are focused by the beam particles
• They curl in the solenoidal magnetic field
  forming a hard edge that the beam pipe must avoid.

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Curling sprays particles onto the front face of
the magnets

• High momentum pairs
• Travel down the beampipe away from the IP
• Low momentum pairs
• Curl in the magnetic field
• Impact the front face of the final quad and other
  material

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New Masking
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1.2 cm VXD L1 in BOTH L S Detectors
Black Layer 1 Turquoise Layer 2 Green Layer
3 Blue Layer 4 Red Layer 5
3.5 x more Layer 1hits at 3 Tesla
Hits / bunch
With few backscattered hits, LCD group currently
feels aggressive 1.2 cm VXD is also possible for
Large Detector (3-4 T) detector
B (Tesla)
2.0 hits/mm2/train 84 from multiple hits by
primary pair electrons
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New Large Detector model

• Update the Large detector model
• Change VXD to same design as the small detector.
• Assume 4T B-field
• Move inner edge of M1 out to respect the pair
  edge.
• Add a second ring to shield SVX layer 2

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SVX Backgrounds for new LCDs
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Neutron backgrounds

• Neutrons are produced where the pairs hit matter
  and shower
• Magnets
• Masks
• Beampipe

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Beamline magnets provide shielding

• Neutrons which reach the IP are produced close to
  the IP, mainly in the luminosity monitor

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Neutrons Leakage around the quads

• Neutrons produced farther away can leak into the
  calorimeters and muon systems

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Total VXD Neutron Backgrounds

• e/e- pairs and radiative Bhabhas hitting
  beam-pipe and magnets in the extraction line.
• Disrupted beam lost in the extraction line.
• 0.25 beam loss in recent redesign
• Disrupted beam and beamstrahlung photons in the
  dump

Neutron hit density in VXD Beam-Beam pairs (small
det.) 1.9 x 109 hits/cm2/yr Beam-Beam pairs
(large det.) 4.4 x 109 hits/cm2/yr Radiative
Bhabhas 0.01 x 109 hits/cm2/yr Beam loss in
extraction line 0.01 x 109 hits/cm2/year Backshine
from dump 0.25 x 109 hits/cm2/yr TOTAL 2.2-4.7
x 109 hits/cm2/yr
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Conclusions

• e/e- pairs are the dominant source of charged
  hits and neutron backgrounds.
• VXD occupancy is as good as it can for this
  B-field and VXD radius.
• Neutron backgrounds are acceptable as is but will
  improve if we go to L 4 meters