1. Tony Hill Lawrence Livermore National Laboratory
NLC gg Backgrounds
Tony Hill
Lawrence Livermore National Laboratory
This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
July 6, 2001

2. NLC gg Backgrounds Machine Backgrounds: 2x1011 hits/cm2/yr in VXD
Synchrotron Radiation
Muon production at collimators
Direct Beam Loss
Beam-Gas
Collimator edge scattering
Neutron back-shine from Dump
comparable to e+e- backgrounds except for neutron flux from dump:
2x1011 hits/cm2/yr in VXD
 rad hard SVX electronics
IP Backgrounds:
Compton Backscatter
Coherent Interactions
Disrupted primary beam
Beamstrahlung photons
Incoherent Interactions
e+e- pair production
Radiative Bhabhas
Hadrons from gg interactions
Defines extraction aperture
Defines minimum beampipe envelope
Drives inner detector occupancy
Impacts detector integration time

3. Beam-Beam Interactions
Coherent (particle-field interactions) Disruption
Incoherent (particle-particle interactions) Processes
Beamstrahlung (energy)
Forward gamma- <E>=30 MeV
Pinch/Deflection (direction)
Transverse kick > 500 mrad
ee  eee+e- (Landau-Lifshitz)
eg  ee+e- (Bethe-Heitler)
gg  e+e- (Breit-Wheeler)
gg  hadrons

4. gg Interaction Region Nine colliders in one!
A little more going on than in the e+e- collisions…
CAIN simulates Compton backscattering and beamstrahlung/pinch (coherent effects) and electromagnetic incoherent effects
Beams consist of e+,e-,g after backscatter
Beam profile nearly unaffected by backscatter
Beam not monochromatic after backscatter
Beam diverges after passing through oncoming beam
Nine colliders in one!

5. Beam Energy Profiles -Not all electrons interact with laser
-Straggling caused mostly by backscatters
-Compton backscattered
photons not monoenergetic
-Oppositely charged particles produced primarily as photons converts in oncoming E-field

6. Coherent Effects Define Extraction Line
Most power exits as spent beams
e+e- aperture too small for ggIR beam extraction
Extraction line aperture is 8cm diameter at 4m from gg IP

7. Displacement of Spent Beams
Interactions with faceplate
cannot be shielded
Crossing angle responsible for downward trend of beam
~8 GeV is minimum energy of electron after Compton Backscatter
Less than 2% of spent beam particles have energy less than 25 GeV

8. Deflection of Spent Beams
20 mrad extraction line aperture (8cm diameter/400cm from IP) accommodates gg IR
Larger aperture exposes VXD to dump (2x1011 hits/cm2/yr)
Use rad hard VXD electronics
Some particles (low energy) are not pointing into the dump

9. e+e- Pairs Can Be Soft ee  eee+e- (Landau-Lifshitz)
eg  ee+e- (Bethe-Heitler)
gg  e+e- (Breit-Wheeler)
Disrupted beams and beamstrahlung photons exit through the extraction line
Not enough pt/pz to interact with anything near the IP
“soft” e+e- pairs curl in the detector’s solenoidal magnetic field
They can interact with objects near the IP

10. Curling sprays particles onto the front face of the magnets
Hard e+e- pairs
Travel down the beampipe away from the IP
Soft e+e- pairs
Curl in the magnetic field
Impact the front face of the final quad and other material
Neutrons from soft e+e- pairs is negligible compared to flux from dump through enlarged extraction aperture

11. e+e- Pairs Define the Beampipe Radius
e+e- pairs are focused/defocused by the beam particles
Curling in the solenoidal magnetic field forms a hard edge that the beam pipe must avoid.
Results nearly the same for both IRs
Large production pt events spray into detector

12. Mirrors Require Larger Beampipe
e+e- pairs from gg IP do not require larger beampipe
Larger diameter beampipe near masks required to accommodate final focus mirror
Opto-mechanical system for laser placed outside the e+e- aperture and does not induce extra backgrounds

13. Particle Fluxes IP Backgrounds: Disrupted Beam Beamstrahlung photons
e+e- pairs
Radiative Bhabhas
# particles/bunch
2 x 1010
3 x 1010
1 x 105
3 x 105
E (GeV)
460 30
10.5
370
VXD experts- “2 hits/mm2/train is acceptable”
gg on the edge
move VXD out
integrate fewer crossings
Machine Parameters used:
1 TeV NLC-B
1 x 1010 e-/bunch x tps

14. Particle Fluxes gg/e+e- on the edge - move VXD out
VXD experts-
“2 hits/mm2/train is acceptable”
gg/e+e- on the edge
- move VXD out
- integrate fewer crossings
Machine Parameters used:
1 TeV NLC-B
1 x 1010 e-/bunch x tps

15. Hadronic Beam Interactions
CAIN simulates Compton backscatter and electromagnetic beam-beam effects
Extraction line
Beampipe envelope
VXD placement/detector integration time
PYTHIA used to simulate hadronic backgrounds from discrete weak and strong interactions between beam particles
CAIN geometric luminosity files are multiplied by PYTHIA cross sections to calculate total luminosity distributions for each collision type
Total luminosity distributions are normalized to arrive at event probability distributions for event generation in PYTHIA

16. CAIN Geometric Luminosity Files
- CAIN generates 9 geometric luminosity files,
one for each type of collision

17. Photons have Structure
“g”=0.99 g r
Three types of gg collisions
Direct
Once resolved
Twice resolved

18. PYTHIA Parameterized Cross Sections
PYTHIA used to determine cross sections as a function of center of mass energy for each type of collision.
Resolved processes dominate
@500 GeV CM
68% twice resolved
17% once resolved
15% all others

19. Energy Flow of Background Events
95 bunches in 1 train
Energy nearly split between barrel and endcaps
cos(q)<0.7 is barrel
cos(q)<0.9 is endcap
This could be a problem

20. Higgs Impact 2-jet higgs analysis
Resolved photon backgrounds are embedded in signal events
Bias in mass (~10% per bunch)
Degrades width (~7% per bunch )
Complete simulation required to assess full impact
Pattern recognition
B-tagging
Resolved photon backgrounds will drive detector/electronics decisions for gg experiment

21. Conclusions gg backgrounds impose no new constraints on machine design
What works for e+e- will work for gg
Beam dump is primary source of neutrons after extraction aperture enlarged (VXD has direct line of sight to dump)
Rad hard electronics needed to handle the neutron flux
e+/e- pairs are the dominant source of “trackless” charged hits in VXD
Reduced integration time or increase placement radius
Twice resolved photons a large unwanted source of energy flow into detector
Reduce integration time to a few bunch crossings

22. Outlook
Once the machine parameters are “fixed” and the laser opto-mechanical system finalized, a full simulation and audit of the IR will need to be completed
Detector/electronics designed for reduced integration time will need to be studied
Full impact of backgrounds in detector must be understood
Full GEANT simulation
Complete pattern recognition and track reconstruction packages
Kalman fitter, vertexing and b-tagging packages