Challenges for Polarimetry at the ILC Moritz Beckmann, Anthony - PowerPoint PPT Presentation

Challenges for Polarimetry at the ILC Moritz Beckmann, Anthony Hartin, Jenny List DESY - FLC EUCARD Workshop “Spin optimization at Lepton accelerators” Mainz, Germany February 12, 2014

Outline • Introduction • Polarimetry in the ILC beam delivery system • Spin transport • Results • Beamline simulation • Collision effects • Polarization measurement at the disrupted beam • Beamline design in view of the polarization measurement • Impact of the laser spot size at the downstream polarimeter Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 2 / 26

ILC Beam Delivery System (BDS) Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 3 / 26

Laser Compton Polarimeter (Principle) dipole magnets polarized laser e ± beam detector scattered e ± e ± γ → e ± γ • Laser: Compton scattering • Scattering cross section depends on P z • Magnet chicane separates scattered e ± from beam • P z is determined from scattered electrons Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 4 / 26

Polarimetry at the ILC upstream downstream IP polarimeter polarimeter e ± beamlines 150 m ~1 650 m • Two Compton polarimeters per beam to measure P z • Upstream polarimeter undisturbed by collision effects • Downstream polarimeter assesses collision effects • 0.25 % systematic uncertainty (goal) • What do these measurements tell us about the longitudinal polarization at the IP? → spin transport simulation • Aim to understand spin transport to 0.1 % Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 5 / 26

Spin Precession • Spin precession in electromagnetic fields: T-BMT equation • For � B ⊥ only: ϑ spin = b(E) · ϑ orbit b ( E ) = a γ + 1 = g − 2 · E m + 1 2 ≈ 568 for 250 GeV-electrons B • Dipole magnets, no beam energy spread: spin vectors precess uniformly, | � P| conserved Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 6 / 26

Spin Fan-Out in Quadrupole Magnets quadrupole magnets P' P P For illustration purposes, the second quadrupole is stronger. Two-dimensional betatron oscillations are not taken into account here. • Different precession angles after first quadrupole ⇒ polarization | � P| decreases • | � P| recovered by second quadrupole Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 7 / 26

Beam-Beam Collision Effects + _ e e Pairs Beamstrahlung A. Vogel Bunches focus each other by their electromagnetic fields: • Spin fan-out (like in quadrupole magnets) • Spin flip by emission of beamstrahlung (Sokolov-Ternov effect) Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 8 / 26

Spin Transport Simulation Framework beam parameters beamline layout particle / spin transport along the BDS Bmad UP IP DP polarimeter beam-beam collision polarimeter measurement Guinea-Pig++ measurement data analysis • Developed a beamline simulation (based on Bmad) • Simulate 40 000 (macro)particles per bunch, generated from beam parameters at the beginning of the BDS • Interfaced directly to the simulation of the collisions (Guinea-Pig++) Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 9 / 26

Results • √ s = 500 GeV • Beam parameters according to Reference Design Report (RDR, 2007) • Collision effects also for beam parameters according to Technical Design Report (TDR, 2013) Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 10 / 26

Spin Transport in the BDS: Basic Configuration polarization 0.8 0.6 0.4 - e 0.2 P z 0 | P | -0.2 -0.4 -0.6 UP IP DP -0.8 0 500 1000 1500 2000 2500 3000 3500 distance s along BDS [m] UP/DP: up-/downstream polarimeter Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 11 / 26

Spin Transport in the BDS: Basic Configuration polarization ] 0.8 0 -3 relative change [10 - e -0.5 extraction 0.7995 P z line | P | quadrupoles -1 dipole chicanes 0.799 IP DP 3400 3450 3500 3550 distance s along BDS [m] DP: downstream-polarimeter • Quadrupoles cause spin fan-out • Changes in P z well below 0 . 1 % without collisions Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 12 / 26

Factors affecting the spin transport (without collisions) contribution uncertainty [10 − 3 ] Beam and polarization alignment 0 . 72 (∆ ϑ bunch = 50 µ rad, ∆ ϑ pol = 25 mrad) Random misalignments (10 µ m) 0 . 43 Variation in beam parameters (few %) 0 . 03 Bunch rotation (crab cavities) < 0 . 01 Detector solenoid 0 . 01 Synchrotron radiation 0 . 005 Total (quadratic sum) 0 . 85 Now: e + e − beam collisions Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 13 / 26

Spin Transport after Collision ] z long. polarization P 0.8 0 -3 relative change [10 0.79 -20 0.78 no collision no collision lumi-weighted lumi-weighted after collision after collision 0.77 measurable measurable -40 - e 0.76 IP DP 3400 3450 3500 3550 distance s along BDS [m] DP: downstream-polarimeter • Luminosity-weighted ( • ): P z of the colliding particles • Larger angular divergence / energy spread after collision • Large spin fan-out in extraction line quadrupoles Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 14 / 26

Spin Transport after Collision ] z long. polarization P 0.8 0 -3 relative change [10 0.79 -20 0.78 no collision no collision lumi-weighted lumi-weighted after collision after collision 0.77 measurable measurable -40 - e 0.76 IP DP 3400 3450 3500 3550 distance s along BDS [m] DP: downstream-polarimeter • Extraction line design: restore luminosity-weighted P z ( • ) at the downstream polarimeter • Employ spin fan-out : focus beam at downstream polarimeter with half divergence angle w. r. t. the IP Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 15 / 26

Spin Transport after Collision ] z long. polarization P 0.8 0 -3 relative change [10 0.79 -20 0.78 no collision no collision lumi-weighted lumi-weighted after collision after collision 0.77 measurable measurable -40 - e 0.76 IP DP 3400 3450 3500 3550 distance s along BDS [m] DP: downstream-polarimeter 2 θ x ≫ θ y ⇒ ∆ P z ∝ θ x � 2 � θ x ∆ P lum ≈ 1 4 ∆ P z ∝ z 2 P lum = P DP Idea: | R 22 (IP → DP) | = 0.5 ⇒ z z Further reading: SLAC-PUB-4692, SLAC-PUB-8397 Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 16 / 26

Laser and Particle Bunch at the Downstream Polarimeter without after collision collision ~mm ~cm 0.04mm laser spot ~cm e ± beam • Without collision: entire beam exposed to laser • After collision: center of beam exposed to laser sample of scattered electrons representative? Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 17 / 26

Downstream Measurement • Downstream polarimeter located in magnet chicane ⇒ particle position correlated with energy ( dispersion ) vert. particle position y [mm] 0.015 6 10 Laser spot size 0.01 10 5 10 0.005 4 10 0 0 3 10 -0.005 2 10 -0.01 -10 -0.015 10 -0.02 -20 1 -0.4 160 -0.3 180 200 -0.2 220 -0.1 240 0 particle energy [GeV] • Laser spot size at Compton-IP only ∼ 0.1 - 1 mm Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 18 / 26

Downstream Measurement • Beamstrahlung correlates energy and P z ⇒ P z correlated with particle position ⇒ Selective measurement, measurement bias z 0.8 long. polarization P 0.79 0.78 0.77 laser spot size 0.76 0.75 -0.015 -15 -0.01 -10 -0.005 -5 0 0 vert. particle position y [mm] • Measurable longitudinal polarization := average P z of particles within a given (laser spot) radius Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 19 / 26

Spin Transport for Different Beam Parameters no energy spread/loss RDR TDR TDR* RDR TDR TDR* ] z 0.8 0 -3 long. polarisation P relative change [10 -5 0.795 -10 0.79 before collision lumi-weighted -15 after collision 0 20 40 60 DP DP measurable (r=0.1/0.2/0.5/1.0 mm) DP: downstream polarimeter • No energy spread/loss: no discrepancy between measurement ( � ) and average P z ( � ) at downstream polarimeter • RDR → TDR: stronger focussing ⇒ higher collision intensity ⇒ larger spin fan-out in collision and afterwards Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 20 / 26

Spin Transport for Different Beam Parameters no energy spread/loss RDR TDR TDR* RDR TDR TDR* ] z 0.8 0 -3 long. polarisation P relative change [10 -5 0.795 -10 0.79 before collision lumi-weighted -15 after collision 0 20 40 60 DP DP measurable (r=0.1/0.2/0.5/1.0 mm) DP: downstream polarimeter ( • ) at downstream pol. ( � ) Extraction line design: restore P lum z • Design ( | R 22 | = 0 . 5) assumes D x ≪ 1 D RDR D TDR = 0 . 17 = 0 . 3 x x • More beamstrahlung (not accounted for by design) Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 21 / 26

Spin Transport for Different Spin Configurations DP IP R 22 =-0.5 after collision TDR T-BMT assuming θ spin = (aγ+1)·θ orbit at the beginning P' P TDR* T-BMT assuming θ spin = (aγ+1)·θ orbit at the beginning P P' For illustration only. All angles exaggerated. Beamstrahlung effects neglected. Moritz Beckmann DESY-FLC EUCARD Workshop Mainz 2014 22 / 26