NLC gg Backgrounds
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NLC - The Next Linear Collider Project
NLC BackgroundsTony Hill
Lawrence Livermore National LaboratoryThis 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
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NLC Backgrounds
Machine Backgrounds:•Synchrotron Radiation•Muon production at collimators•Direct Beam Loss
•Beam-Gas•Collimator edge scattering
•Neutron back-shine from Dump
IP Backgrounds:•Compton Backscatter•Coherent Interactions
•Disrupted primary beam•Beamstrahlung photons
•Incoherent Interactions•e+e- pair production•Radiative Bhabhas•Hadrons from interactions
comparable to e+e- backgrounds except for neutron flux from dump:
2x1011 hits/cm2/yr in VXD rad hard SVX electronics
Defines extraction aperture
Defines minimum beampipe envelopeDrives inner detector occupancyImpacts detector integration time
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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 rad
• ee eee+e- (Landau-Lifshitz)• e ee+e- (Bethe-Heitler)• e+e- (Breit-Wheeler)• hadrons
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• 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-,after backscatter – Beam profile nearly unaffected by backscatter– Beam not monochromatic after backscatter – Beam diverges after passing through oncoming beam
Nine colliders in one!
Interaction Region
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-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
Beam Energy Profiles
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Coherent Effects Define Extraction Line
• Most power exits as spent beams
• e+e- aperture too small for IR beam extraction
• Extraction line aperture is 8cm diameter at 4m from IP
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Displacement of Spent Beams
• 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
Interactions with faceplate cannot be shielded
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Deflection of Spent Beams
• 20 mrad extraction line aperture (8cm diameter/400cm from IP) accommodates 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
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e+e- Pairs Can Be Soft
• 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
• ee eee+e- (Landau-Lifshitz)• e ee+e- (Bethe-Heitler)• e+e- (Breit-Wheeler)
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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
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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
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Mirrors Require Larger Beampipe
• e+e- pairs from 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
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Particle Fluxes
Machine Parameters used:
1 TeV NLC-B1 x 1010 e-/bunch x 95 bunches/train @ 120 tps
IP Backgrounds:Disrupted BeamBeamstrahlung photons e+e- pairsRadiative Bhabhas
# particles/bunch2 x 1010
3 x 1010
1 x 105
3 x 105
E (GeV)46030
10.5370
VXD experts- “2 hits/mm2/train is acceptable”
on the edgemove VXD outintegrate fewer crossings
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Particle Fluxes
Machine Parameters used:
1 TeV NLC-B1 x 1010 e-/bunch x 95 bunches/train @ 120 tps
VXD experts- “2 hits/mm2/train is acceptable”
/e+e- on the edge - move VXD out - integrate fewer crossings
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• 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
Hadronic Beam Interactions
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CAIN Geometric Luminosity Files
- CAIN generates 9 geometric luminosity files, one for each type of collision
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Photons have Structure
• Three types of collisions
– Direct
– Once resolved
– Twice resolved
“”=0.99 + .01
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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 resolved17% once resolved15% all others
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Energy Flow of Background Events
• 95 bunches in 1 train
• Energy nearly split between barrel and endcaps– cos()<0.7 is barrel– cos()<0.9 is endcap
• This could be a problem
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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 experiment
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Conclusions
• backgrounds impose no new constraints on machine design– What works for e+e- will work for
• 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
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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