April 2006 LC-ABD Manchester ILC IR pair background simulations – status and prospects Tony Hartin FONT collaboration  P Burrows, G Christian, C Clarke, C Swinson, H Dabiri Khah, G White, S Molloy

April 2006 LC-ABD Manchester Outline Feedback BPM background hits Feedback BPM background hits effect of noise on the BPM effect of noise on the BPM low energy pair hits and GEANT code modification low energy pair hits and GEANT code modification ESA tests and simulation support ESA tests and simulation support potential electromagnetic field solvers potential electromagnetic field solvers Geant simulations Geant simulations Origin of BPM spray Origin of BPM spray Variation with Solenoid and DiD magnetic field Variation with Solenoid and DiD magnetic field Variation of background hits with the parameter space Variation of background hits with the parameter space Theoretical sources of backgrounds Theoretical sources of backgrounds The background generators: GUINEA-PIG and CAIN The background generators: GUINEA-PIG and CAIN New, non-linear sources of e+e- backgrounds at the IP New, non-linear sources of e+e- backgrounds at the IP Prospects for experimental detection Prospects for experimental detection

April 2006 LC-ABD Manchester BPM noise from backgrounds Noise form factor sinc(π.f.T) Secondary emission down to 100 eV needs to be considered Geant3 minimum transport is 10 keV! 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!

April 2006 LC-ABD Manchester Geant3/4 mod for Low Energy Transport Geant3 X-section parametrization wrong below 10kev cut Recode Geant3 to parametrize real data from NEA site

April 2006 LC-ABD Manchester Geant3 code changes Modified source and linux library available from www-pnp.physics.ox.ac.uk/~hartin/FONT Work directory Sub directory “data” GEANT3 source Sub directory “gphys” Gcompi.f Gcompi_mod.f Gbrge.f Gbrge_mod.fComptonBremstr.

April 2006 LC-ABD Manchester 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) ~ 10 5 hits per strip per bc

April 2006 LC-ABD Manchester Electromagnetic Field Solvers 2D ‘particle in cell’ Plasma dynamics $1000 per 2 year research license 2D/3D/Post processor plasma dynamics $20,000 per 12 month license PDP2 (Los Alamos) Free 2D plasma dynamics Allows for collisions with electrodes and secondary emission Coupled to external circuit

April 2006 LC-ABD Manchester 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. BPM 93443

April 2006 LC-ABD Manchester 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 B

April 2006 LC-ABD Manchester The impact of a Detector Integrated Dipole(20mrad) (Seryi & Parker, Phys Rev ST Accel Beam 8, (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

April 2006 LC-ABD Manchester GEANT simulation parameter space 8 independent parameters, 2688 possible combinations Each run ~12 hours Parameter Best case Worst Case 14 schemes 500 GeV LowQ 1 TeV High Lumi Solenoid field lowhigh BPM z position Midway bet. Mask & QFEX1 Close to the mask, between extraction quads L* minimal variation QFEX1 z position minimal variation Minimal variation Crossing angle 20mrad 2 mrad Mask hole radius largesmall BPM radius 0.4 cm 2 cm

April 2006 LC-ABD Manchester 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 1 st 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 2 nd order non-linear sources of background pairs? High LumiScheme14 Low PScheme13 Large YScheme12 Low QScheme11 NominalScheme10 USSCScheme9 1 TeV TESLAScheme 8 High LumiScheme7 Low PScheme6 Large YScheme5 Low QScheme4 NominalScheme3 USSCScheme2 500 GeV TESLA Scheme 1

April 2006 LC-ABD Manchester 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/E c ~0.3. Scheme1 has Y=0.054, Scheme14 has Y= nd order process rather than 1 st order Rules for onset are different Calculation is complicated, but simplified when the photons are co-linear    ee nk k b     ee kk bb 21 Unknown: Multiphoton Breit-Wheeler Unknown: Multiphoton Breit-Wheeler 2 b k 1 b k e-e+

April 2006 LC-ABD Manchester Orders of the non-linear processes 1 st order nonlinear process Suppressed by limiting time travelling through bunch field 2 nd order nonlinear process Beamsstrahlung photon Resonance occurs in the virtual particle exchange

April 2006 LC-ABD Manchester Resonances in multiphoton B-W Multiphoton Breit-Wheeler Resonances Multiphoton Bremstrahlung (non-resonant) Ordinary Breit-Wheeler )1()( 2 22  m nk q Pairs created in intense e-m field have a quasi-level structure and resonant transitions can occur (Zeldovich, 1967) 2 nd order IFQED x-section can exceed normal x-sections by orders of magnitude (Oleinik, JETP 25(4) 697, 1967) 2 nd order Breit-Wheeler process in CIRCULARLY POLARISED field shows the same feature hepwww.ph.qmul.ac.uk/~hartin/thesis

April 2006 LC-ABD Manchester 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 idttMefU fi ti   )(       2 )()( exp ee AipAid 

April 2006 LC-ABD Manchester Calculation of Resonance widths The Electron Self Energy must be included in the Multiphoton Breit-Wheeler process This is a 2 nd 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

April 2006 LC-ABD Manchester Where do the resonances occur? Beamsstrahlung photon E S >> MeV Beam photon E B < 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) ~ E S /E B Resonance PeakResonance Width

April 2006 LC-ABD Manchester Notes on the cross-section calculation Full trace contains ~ 100,000 terms 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 1 k 2 qqr k 0 +-

April 2006 LC-ABD Manchester Experimental evidence for the IFQED processes 1 st order: One photon pair production Experiment E144 SLAC. 46 GeV beam with Nd:glass laser peak intensity 0.5x10 18 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 2 b k 1 b k e-e+ 1b k e-e+ 2 nd order: Substantial theoretical studies but no experimental efforts yet! BUT potentially more detectable because of resonances

April 2006 LC-ABD Manchester 3 GeV electrons Focussed Tabletop TeraWatt Laser low spec laser Synchrotron Experimental detection of Stimulated Breit- Wheeler resonances

April 2006 LC-ABD Manchester 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/ ) Monte-Carlo doesn’t fit measurements at low P t This is precisely what is predicted by resonance calculations of stimulated Breit-Wheeler

April 2006 LC-ABD Manchester 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 2 nd 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 1 st 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