NLC IR Layout and Background Estimates

1
NLC IR Layout and Background Estimates

• Tom Markiewicz/SLAC
• Snowmass 2001
• 05 July 2001

2
The Experts

• Takashi Maruyama (SLAC)
• Pairs and Neutron Backgrounds
• Jeff Gronberg (LLNL)
• Gamma-Gamma to Hadrons
• Stan Hertzbach (U. Mass)
• Synchrotron Radiation
• Lew Keller (SLAC)
• Muons
• Collimator Efficiency

3
Introduction

• At LCWS2000 background ESTIMATES were based on
• New (i.e. short) final focus
• L 4.3 m
• Large (ver.1) Detector
• NLC 500 GeV and 1 TeV B IP beam parameter sets
• Extraction line beginning at 6 m with 1 cm radius
  aperture
• This talk has
• Latest IP beam parameters
• 4x the luminosity with 190 bunches each with
  0.75E10 e-
• L 3.8m with LumMon _at_ 3.5m
• March 30, 2001 LD and SD detectors
• Same extraction line
• Neutrons from the dump
• Same Final Focus but newest shortest collimation
  scheme
• more muon backgrounds given similar halo
  assumptions

relative z location of calorimeters and L is
what matters
4
Large and Silicon Detectors(same scale)
3 Tesla
5 Tesla
5
LD and SD Detector Masking
30 mrad
32 mrad
6
Background Sources

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

Good, scale with luminosity 1) Transport them
away from IP 2) Shield sensitive detectors 3)
Detector Timing

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

Bad, get nothing in exchange 1) Dont make
them 2) Keep them from IP if you do
7
Beam-Beam InteractionSR photons from individual
particles in one bunch when in the electric field
of the opposing bunch

• Beams attracted to each other reduce effective
  spot size and increase luminosity
• HD 1.4-2.1
• Pinch makes beamstrahlung photons
• 0.9-1.6 g/e- with E3-9 E_beam
• Photons themselves go straight to dump
• Not a background problem, but angular dist. (1
  mrad) limits extraction line length
• Particles that lose a photon are off-energy
• Physics problem luminosity spectrum
• Extraction line problem
• NLC 1 TeV design has 77 kW of beam with Elt 50
  E_nom, 4kW lost (0.25 loss)
• 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
8
Energy Distributions
Tesla 500 GeV
NLC-1 TeV
9
NLC Extraction Line150 m long with chicane and
common g and e- dump
Problem Handling the large low E tail on the
disrupted beam cleanly enough to allow extraction
line diagnostics Working plan Ignore for now-
not a problem _at_ 500 GeV _at_ 1 TeV either measure
Pol, E upstream, steal undisrupted pulses for
diagnostics, calibrate other
4kW lost in EXT _at_ 1 TeV
10
e,e- pairs from beams. gg interactionsAt
NLC-1000 44K per bunch _at_ ltEgt10.5 GeV (0.85 W)
11
Direct Pairs

• PT of ee- from given bunch Sum of
• Pt from individual pair creation process
• small
• Pt from collective field of opposing bunch
• large
• limited by finite size of the bunch

12
Dead-Cone Formalismqmax from Dx, sx, sz
Tauchi, LC95
13
ee- Pair pT vs. theta Distribution
Hard edge from finite beam size
High pT inside cone
e,e- with high intrinsic pt can hit small radius
VXD
Low pt/high angle curl in field
50 mrad
14
Controlling e,e- Pair Background

• Direct Hits
• Increase detector solenoid field to wrap up pairs
  (3 Tesla adequate, 4 T better)
• Increase minimum beam pipe radius at VXD and stay
  out of pair dead cone
• Secondaries (e,e-, g,n)
• Remove point of first contact as far from IP/VXD
  as possible
• Increase L if possible
• Largest exit aperture possible to accept
  off-energy particles
• Keep extraneous instrumentation out of pair
  region
• Masks
• Instrumented conical dead cone protruding at
  least 60cm from face of luminosity monitor and
  8-10cm thick to protect against backscattered
  photons
• Low Z (Graphite, Be) 10-50cm wide disks covering
  area where pairs hit the low angle W/Si Pair
  Luminosity monitor

15
LCD-L2 (3T) with 3.8m L Optics
52 mrad Cal acceptance
32 mrad M1 acceptance
Calorimeter
SF1
QF1
M2
M1
SD0
QD0
Feedback BPM Kicker
6.3 mrad Lum-Mon acceptance
Low Z shield
Beampipe
Pair LumMon
1 mrad exit aperture
Support Tube
16
Pair Stay-Clear from Guinea-Pig Generator and
Geant
17
Pair hits at z 4 m
High momentum pairs mostly in exit beampipe
f2cm
Low momentum pairs trapped by detector solenoid
field
f4cm
18
Photons in the LD TPC _at_ 1 TeV Scoring plane _at_
r30 cm
SourceEither direct or secondary hits on the
beampipe Photon Distribution in Barrel Cal
similar
Positron annihilation peak
19
Photons in the Endcap CAL _at_ 1 TeVR18cm scoring
plane
LD
SD
20
LCD Hit Density/Train vs. Radius
Before conversion
21
Neutron BackgroundsThe closer to the IP a
particle is lost, the worse

• e/e- pairs and radiative Bhabhas hitting the
  Pair Lum-Mon, 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 NLC-LD-500 GeV
NLC-SD-500 GeV Beam-Beam pairs 1.8 x 109
hits/cm2/yr 0.5 x 109 hits/cm2/yr Radiative
Bhabhas 1.5 x 107 hits/cm2/yr no hits Beam
loss in extraction line 0.1 x 108
hits/cm2/year 0.1 x 108 hits/cm2/year Backshine
from dump 1.0 x 108 hits/cm2/yr 1.0 x 108
hits/cm2/yr TOTAL 1.9 x 109 hits/cm2/yr 0.6
x 109 hits/cm2/yr
Figure of merit is 3 x 109 for CCD VXD
22
Neutrons from Lost Pairs and Rad. Bhabhas
Neutrons which reach the IP are produced close to
the IP, mainly in the luminosity monitor
23
Neutrons from the Beam DumpControlled by
Shielding and Geometry
Geometric fall off of neutron flux passing 1 mrad
aperture parent distribution for
next slide
24
Dump-produced Neutron flux at z0 as a function
of radius

• 1.2E10 neutrons hit the beampipe within /-5cm at
  rgt1.0 cm
• 30 scatter into VXD
• Divide by area of VXD L1 to get quoted hit
  density 0.25E9/cm2/y
• Fall off for rgt1.0 cm due to limiting aperture of
  EXTRACTION LINE QUAD DOUBLET (currently 10-11 mm
  from L6-10.8 m from the IP SR concerns MAY
  require larger aperture)
• Fall off as r -gt 0cm comes from reduced solid
  angle view of the dump
• As r is reduced need to integrate more of this
  curve.

Limiting Aperture
25
Integrated Dump Neutron Flux vs. Radius

• Detector Group Constantly Asking why inner VXD
  radius cant be x2 SMALLER
• As Beampipe radius is reduced by x2
• Flux from dump up x10
• Hit density up by x40
• dump becomes equal to pairs as source of neutron
  hits
• SR issues (S. Hertzbach talk)

26
Control of Pair-Induced Neutrons
VXD Neutrons from Pairs with 10 cm Be Shield
Neutron Hit Density vs. Extraction Line Aperture
50 cm Be Shield is 3-4x better
27
Neutrons in the LD Barrel Cal _at_ from ee- pairs
at 1 TeV
In plot see contribution from z side
only Similar for SD
28
Summary LD _at_ 500 GeV
29
Summary SD _at_ 500 GeV
30
Summary LD _at_ 1 TeV
31
Summary SD _at_ 1 TeV
32
ee-? ee- gg ? ee- Hadrons

• NLC Analysis began Spring 2001 (Gronberg Hill
  / LLNL)
• CAIN simulation plus JETSET
• Need to integrate 190 bunches
• Doesnt appear to be a problem but one detector
  element with good time resolution will help if it
  is
• Analysis still young
• If we scale TESLAs event rate/BX by ng2 (50)
  and x 190 bunches get much larger numbers

33
ee-? ee- gg ? ee- HadronsEnergy Distribution
Barrel
Endcap
Mask
34
Synchrotron Radiation

• At SLD/SLC SR WAS a PROBLEM
• SR from triplet WOULD have directly hit beam-pipe
  and VXD
• Conical masks were installed to shadow the beam
  pipe inner radius and geometry set so that
  photons needed a minimum of TWO bounces to hit a
  detector
• Quantitative measurements of background rates
  could be fit by a flat halo model where it was
  assumed that between 0.1 and 1 (in the early
  days) of the beam filled the phase space allowed
  by the collimator setting.
• At NLC/TESLA
• Allow NO direct SR hits ANYWHERE near IP
• SR due to BEAM HALO in the final doublet, not the
  core of the beam
• Collimate halo before the linac AND after the
  linac
• Halo estimates are 10-6 of beam designing
  system to handle 10-3
• Optical solutions to handle halo under development

35
HALO Synchrotron Radiation Fans with Nominal 240
mrad x 1000 mrad Collimation
(Similar plots for TESLA)
36
Halo Collimators Potential Muon Source
Locations No Big Bend, Latest Collimation
Short FF
FF
Energy
Betatron
BetatronCleanup
37
Muon Backgrounds No Big Bend, Latest Collimation
Short FF
18m 9m Magnetized steel spoilers
If Halo 10-6, no need to do anything If Halo
10-3 and experiment requires lt1 muon per 1012 e-
add magnetized tunnel filling shielding Reality
probably in between
38
Muons Reaching z0 500 GeV/beamShows what
happens without spoiler
39
LD Muon Endcap Background e- Scraped to Make
1Muon
Bunch Train 1012
Engineer for 10-3 Halo
Efficiency of Collimator System is 105
Calculated Halo is 10-6
40
Muon Rates in LD per lost e-
41
Conclusions

• As we have pushed up luminosity x4, shrunken L
  from 4.3 to 3.5m, and reduced length of beam
  delivery system from 5km to 1km, backgrounds have
  risen in absolute terms to a level per train
  meriting attention
• Backgrounds/Unit of Luminosity constant before
  geometry mods
• Geometry adjustment always possible
• Nanosecond level detector timing would make
  everything except neutron-dominated VXD lifetime
  a non-issue
• Large detector Neutron damage lifetime needs more
  investigation
• Conclusion to all previous background talks was
  not a problem but now I am beginning to feel we
  need to start investigating detector response and
  optimizing detector design and performance with
  respect to these processes.