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ILC IR pair background simulations – status and prospects

Tony Hartin. ILC IR pair background simulations – status and prospects. FONT collaboration P Burrows, G Christian, C Clarke, C Swinson, H Dabiri Khah, G White, S Molloy. Feedback BPM background hits effect of noise on the BPM low energy pair hits and GEANT code modification

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ILC IR pair background simulations – status and prospects

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  1. Tony Hartin ILC IR pair background simulations – status and prospects • FONT collaboration • P Burrows, G Christian, C Clarke, C Swinson, H Dabiri Khah, G White, S Molloy

  2. Feedback BPM background hits effect of noise on the BPM low energy pair hits and GEANT code modification ESA tests and simulation support potential electromagnetic field solvers Geant simulations Origin of BPM spray Variation with Solenoid and DiD magnetic field Variation of background hits with the parameter space Theoretical sources of backgrounds The background generators: GUINEA-PIG and CAIN New, non-linear sources of e+e- backgrounds at the IP Prospects for experimental detection Outline

  3. Copper strips with 1 mm gap to wall Noise from secondary charges crossing strip-wall gap ~1pm error for each charge absorbed or emitted. 1e6 hits per b.c. would be a problem! Noise form factor sinc(π.f.T) Secondary emission down to 100 eV needs to be considered Geant3 minimum transport is 10 keV! BPM noise from backgrounds

  4. Geant3/4 mod for Low Energy Transport • Geant3 X-section parametrization wrong below 10kev cut • Recode Geant3 to parametrize real data from NEA site

  5. Geant3 code changes Work directory Sub directory “data” • Modified source and linux library available from www-pnp.physics.ox.ac.uk/~hartin/FONT GEANT3 source Sub directory “gphys” Gcompi.fGcompi_mod.f Gbrge.fGbrge_mod.f Compton Bremstr.

  6. Geant3 total BPM hits and emitted charges • LowE mods reveal significant increase in BPM hits at 100eV cut. • Generally Factor of 5 increase in BPM compared to default Geant cut of 1 MeV, and factor of 2 increase against Geant default minimum cut • Can define Geant areas (ROIs) around BPMS which are tuned for Low Energy particles • Worst case scenario (scheme14 in the 20mrad case) ~ 105 hits per strip per bc

  7. 2D ‘particle in cell’ Plasma dynamics $1000 per 2 year research license 2D/3D/Post processor plasma dynamics $20,000 per 12 month license Electromagnetic Field Solvers • PDP2 (Los Alamos) • Free • 2D plasma dynamics • Allows for collisions with electrodes and secondary emission • Coupled to external circuit

  8. BPM Geant model of 20mrad design • Background primaries are traced through from the IP • Optimal position for the BPM can be established • BPM hits vary with parameter sets, geometry and magnetic fields. 93443

  9. B Where does BPM spray originate from? (20mrad) • X,Y view at z=3.12m • E+ and E- have different trajectories • spray originates from annulus around extraction line

  10. The impact of a Detector Integrated Dipole(20mrad)(Seryi & Parker, Phys Rev ST Accel Beam 8, 041001 (2005) • Introduced to offset Y displacement at the IP due to solenoid field • Steers more primaries into Lumi Cal (K. Busser). Extra BPM spray sources • 30% more spray hits delivered to the BPM

  11. GEANT simulation parameter space • 8 independent parameters, 2688 possible combinations • Each run ~12 hours

  12. Scheme 1 500 GeV TESLA Scheme2 USSC Scheme3 Nominal Scheme4 Low Q Scheme5 Large Y Scheme6 Low P Scheme7 High Lumi Scheme 8 1 TeV TESLA Scheme9 USSC Scheme10 Nominal Scheme11 Low Q Scheme12 Large Y Scheme13 Low P Scheme14 High Lumi Background Primaries – ILC parameter Sets • High Lumi sets produce the greatest numbers of background pairs • Guinea-Pig and Cain include L-L, B-H and B-W processes • ILC parameters chosen so that 1st order non-linear processes are suppressed • Beam field is treated linearly i.e. equivalent photon method scaled by the number density of beam field photons • BUT.. The beam fields are intense – 10% of Schwinger critical field • What about 2nd order non-linear sources of background pairs?

  13.     k nk e e b       k k nk e e Unknown: Multiphoton Breit-Wheeler 1 2 b b k k 2 b b 1 Is there another non-linear source of pairs at the IP? • Known: multiphoton pair production • rate described by Sokolov-Ternov and onset governed by beam parameter Y=E/Ec~0.3. • Scheme1 has Y=0.054, Scheme14 has Y=0.376 • 2nd order process rather than 1st order • Rules for onset are different • Calculation is complicated, but simplified when the photons are co-linear e- e+

  14. Orders of the non-linear processes Beamsstrahlung photon Suppressed by limiting time travelling through bunch field 1st order nonlinear process Resonance occurs in the virtual particle exchange 2nd order nonlinear process

  15.    2 2 2 ( q nk ) m ( 1 ) Resonances in multiphoton B-W Pairs created in intense e-m field have a quasi-level structure and resonant transitions can occur (Zeldovich, 1967) 2nd order IFQED x-section can exceed normal x-sections by orders of magnitude (Oleinik, JETP 25(4) 697, 1967) 2nd order Breit-Wheeler process in CIRCULARLY POLARISED field shows the same feature Multiphoton Breit-Wheeler Resonances Multiphoton Bremstrahlung (non-resonant) Ordinary Breit-Wheeler hepwww.ph.qmul.ac.uk/~hartin/thesis

  16. Including the external field in IFQED calculations • ‘Operator Method’: quantum interaction of electron and external field photons but electron trajectory is considered classical. Due to Baier et al (JETP 28(4) p.807, 1969) • Full quantum treatment: Horrendously complex but potentially doable with Vermaseren’s FORM • ‘Semi-classical method’: Dirac equation is solved exactly for interaction with a classical plane-wave e-m field. Most common method. Used originally by Narozhnyii, Nikishov and Ritus in the mid 1960s ò w = i t U f e M ( t ) dt i fi fi { } [ ] exp ò f - e e 2 d i ( A p ) i ( A )

  17. Calculation of Resonance widths • The Electron Self Energy must be included in the Multiphoton Breit-Wheeler process • This is a 2nd order IFQED process in its own right. • Renormalization/Regularization reduces to that of the non-external field case • The Electron Self Energy in external CIRCULARLY POLARISED e-m field originally due to Becker & Mitter 1975 for low field intensity parameter =(ea/m)2. Has been recalculated for general  • ESE in external CONSTANT CROSSED field is due to Ritus, 1972 • Optical theorem: the imaginary part of the ESE is the same form as the Sokolov-Ternov equations

  18. Where do the resonances occur? • Beamsstrahlung photon ES >> 0.511 MeV • Beam photon EB < 0.511 MeV • Processes which give/take energy to/from the field are allowed and mass shell can be reached for physical values • For collinear beamstrahlung photons, resonance condition is r (external field photons) ~ ES/EB Resonance Peak Resonance Width

  19. Notes on the cross-section calculation • Full trace contains ~ 100,000 terms • Dramatically simplified by • Special “centre of mass-like” reference frame • Assume beamsstrahlung photons and beam field photons are collinear • Only insert Imaginary part of self energy to get resonance width k k q q r k 0 1 2 + -

  20. k b 1 2nd order: Substantial theoretical studies but no experimental efforts yet! BUT potentially more detectable because of resonances k k 2 b b 1 Experimental evidence for the IFQED processes e- e+ • 1st order: One photon pair production • Experiment E144 SLAC. 46 GeV beam with Nd:glass laser peak intensity 0.5x1018 Wcm-2. Up to 4 photons contributed to each non-linear event • Meyerhofer et al (1996) other non-linear phenomena such as electron mass shift observed e- e+

  21. Experimental detection of Stimulated Breit-Wheeler resonances low spec laser Focussed Tabletop TeraWatt Laser Synchrotron 3 GeV electrons

  22. Unexplained resonances in heavy ion collisions – a test for the theory? • Intense field QED processes present in Au-Au collisions at RHIC • Meissner & Morozov performed the calculation with leading order QED (arXiv:nucl-ex/0307006) • Monte-Carlo doesn’t fit measurements at low Pt • This is precisely what is predicted by resonance calculations of stimulated Breit-Wheeler

  23. Summary • GEANT tweaking and simulations revealed that BPM hits are within a factor of 10 of problem levels. More detailed studies are needed. • ESA tests are being planned. GEANT simulation support required and detailed electromagnetic studies in progress • 2nd order, nonlinear interactions of beamsstrahlung photons with the beam fields should be taken into account because the cross-sections are potentially resonant and can exceed 1st order and “linear” ordinary cross-sections • Preliminary calculations of the Stimulated Breit-Wheeler process (simplified case) suggests that this will be an issue at the ILC • Possible experimental confirmation in RHIC collisions – will be investigated theoretically

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