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NLC gg Backgrounds

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. NLC gg Backgrounds.

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NLC gg Backgrounds

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  1. 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: • 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. 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 gg Interaction Region 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 • 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

  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. 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 e+e- Pairs Can Be Soft • ee  eee+e- (Landau-Lifshitz) • eg ee+e- (Bethe-Heitler) • gg e+e- (Breit-Wheeler)

  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. Machine Parameters used: 1 TeV NLC-B 1 x 1010 e-/bunch x 95 bunches/train @ 120 tps 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

  14. Machine Parameters used: 1 TeV NLC-B 1 x 1010 e-/bunch x 95 bunches/train @ 120 tps Particle Fluxes VXD experts- “2 hits/mm2/train is acceptable” gg/e+e- on the edge - move VXD out - integrate fewer crossings

  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 • Three types of gg collisions • Direct • Once resolved • Twice resolved “g”=0.99 g + .01 r

  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

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