At a Future Linear Collider (Some) Aspects of Polarimetry At a Future Linear Collider M. Woods SLAC Polarimetry Requirements precision electroweak: measurements of ALR for weak mixing angle determinations from Giga-Z, Bhabha/Moller scattering, fixed target Moller scattering W-pair asymmetry and other Standard Model Asymmetries background estimations Polarimetry for the SLD Experiment at SLC design and systematic errors Polarimeter Design Issues at NLC

Improved Weak Mixing angle measurement at Giga-Z (positrons unpolarized)

Fixed Target Polarized Moller Scattering at NLC from SLAC-PUB-8725, L. Keller et al. (2001) Moller measurements at NLC (requires 0.3% polarization measurement)

Polarized Moller Scattering with e-e- Collisions From F. Cuypers and P. Gambino, Phys. Lett. B388: 211-218, 1996, Measure 3 asymmetries: They considered to determine: With 250 fb-1, could expect to achieve

SM Asymmetries in e+e- Polarimetry to 0.5% or better desirable From Snowmass ‘96 study, Consider, Final State #events ALR W+W - 560K 100% q q 250K 45% 0.005 l+l - 120K 10% 0.032 Polarimetry to 0.5% or better desirable

Other Considerations for Precision of Polarimetry Background suppression of W pairs in e+e- most important is to achieve high polarization; increasing P from 80% to 90% allows for a factor 2 further background reduction need more precise polarimetry as P increases An example P =90% Observe 400 events -- after analysis cuts, but no polarization cut Observe 40 events -- after additional requirement on polarization state 0 2.4% 1% 2.6% 2% 3.1% An excess of 20 events is observed above the expected W pair background. Would like 1% polarimetry in order to achieve a 4s signal.

Polarimetry Options at NLC/TESLA Mott Polarimeter at Source for commissioning Compton Polarimeter before IP after IP + after energy collimation for fixed target Blondel scheme if both beams polarized W-pair asymmetry - with only electrons polarized - better with both beams polarized

Polarimetry using W-pair asymmetries Can we use asymmetry in forward W pairs as a polarimeter? Yes, if can achieve backgrounds below 1%. (This level of backgrounds is achieved for LEP200 W mass measurements, if require one W to decay to ee or mm.) If positron beam is also polarized, can use Blondel-type scheme to fit for beam polarizations as well as physics asymmetry and eliminate sensitivity to backgrounds advantage wrt Compton polarimetry is that any depolarization in beam-beam interaction is properly accounted for; also need to be above W-pair threshold disadvantage wrt Compton polarimetry is Compton can achieve 1% accuracy in a few minutes e- e+ W- W+ n

Blondel scheme with electron and positron beams both polarized Can also use ‘Blondel scheme’ to determine beam polarizations directly: - using Blondel technique, just need Compton polarimeters for measuring polarization differences between L,R states; and only need to spend approx. 10% of running time in NRR, NLL states - this technique directly measures lum-wted polarizations (any depolarization effect properly taken into account)

Polarized Positrons? Need to understand relative detector efficiency for ‘RL’ and ‘LR’ modes at level of 10-4 Need to measure polarization difference, PR+-PL+, at level of 10-3 This will be difficult unless can measure these modes simultaneously, ie. can switch positron polarization randomly pulse-to-pulse, as is done for electrons. Note: even if positrons are nominally unpolarized, need to verify this! For d(ALR)=4 x 10-4, want d(P+)<2x10-4. SLD’s ‘posipol’ measurement achieved d(P+)=7x10-4. (This is relevant for electron-only ALR measurement, which has a goal a factor 5 better than SLD’s result.)

Compton Polarimetry at SLD

Features of Compton polarimetry Physics well understood and radiative corrections <0.1% - no atomic or nuclear physics corrections (ex. Levchuk effect in Moller polarimetry) Easy to measure backgrounds with laser off pulses Polarimetry data taken parasitic to physics data Scattering rate is high and can achieve small statistical errors in short amount of time (1% in a few minutes or less is possible) Easy to reverse laser polarization quickly Laser polarization can be determined to 0.1% With Polarimeter after IP, can measure beam-beam depolarization effects by comparing polarization with and without collisions

Compton Scattering Kinematics The cross section for Compton scattering is where The measured asymmetry in a channel is Where the analyzing power is calculated from the Compton Cross section and the channel response function, Ri.

Raw Data from CKV Detector

Laser Polarization Systematic error Abox PD (adc counts) CP Voltage (Volts) Ability to extinguish laser light after Helicity filter determines polarization purity Residuals (adc counts) CP Voltage (Volts) CP Voltage (Volts) CKV7 Raw Asymmetry Monitor phase shifts in laser polarization with frequent Pockels cell scans Laser Polarization at Compton IP (%) 0.1% systematic error

Analyzing Power Systematic Error Estimate CKV7 systematic error in analyzing power at 0.3% from table scan data that determines location of Compton edge and accuracy of modeling detector response function Results from cross-check Polarimeters 0.4% systematic error

Linearity Systematic error Measurement of electronics linearity with pulser Study measured Compton asymmetry as a Function of background level. 0.2% linearity systematic

Goals for Compton Polarimetry Systematics At NLC (mainly ‘Giga-Z’) Uncertainty SLD dP/P NLC Goal Laser Polarization 0.1% Analyzing Power 0.4% 0.2% Linearity Electronic Noise 0.05% TOTAL 0.50% 0.25% Improve linearity by: - higher resolution adcs (16-bit rather than 11-bit) - blue diode laser calibration and bias system (provide large stable background) - use of Compton laser power scans Improve analyzing power by: - more segmentation of channels near Compton edge in CKV detector - careful design and simulation of spectrometer and detector (original design goal for SLD was 1%) - detailed systematics data taking - cross-checks (if possible): Compton gamma detectors, W-pair asymmetries, polarized Moller asymmetries Electronic Noise includes crosstalk and laser pickup: reduce by careful setup and accurate measurements

Compton Kinematics and Cross Sections at NLC/TESLA can have a large separation between the kinematic endpoint energy the beam energy - large Compton asymmetry at high energy Spectrum of Compton-scattered electrons (500 GeV electrons, 1.165 eV photons)

Depolarization Effects at NLC (see SLAC PUB 8716, K. Thompson Jan. 2001) Lum-wted depolarization is approx. 25% of average depolarization Depolarization can be large for beam particles that lose significant energy to beamstrahlung Depolarization can be large for large vertical offset (may have large effect for first bunches in train)

Depolarization Results from SLAC PUB 8716, K. Thompson Jan. 2001 BMT: spin precession effects ST: Sokolov-Ternov spin flip effects

Extraction Line Design at NLC (Y. Nosochkov) Location for Compton IP (also for wire scanner to measure beam energy distribution) - secondary focus with 20mm dispersion Detailed design for polarimeter still to be done

Additional comments on Beam Delivery Issues for E158’ at NLC New strained super-lattice photocathodes will hopefully reach 90% polarization and with less (x3?) anisotropy wrt linear polarization of incident laser light (good for minimizing false beam asymmetries) Damped Beams eliminate any helicity-dependent position asymmetries; should only have charge asymmetries after DR Intensity and Position Asymmetries. Will want to zero charge asym after DR. Beam losses in extraction line will introduce intensity jitter and significant charge asymmetry due to this jitter (expect 1ppm over run). Do not want charge asym in Linac; leads to energy asym due to beam loading and position asym due to residual Linac dispersion and any wakefield amplification. Momentum Slits Will have (1-2)% slits in extraction line before target. During colliding beam running, will only accept 70% of extracted beam from IP. - intensity jitter may be large; also position and energy jitter? - will need 0.1% linearity for detectors and toroids (1% for E158) - want polarimeter downstream of slits; additional chicane for Compton IP? Spotsize Jitter and Tails Spotsize jitter can cause target density fluctuations. In E158, taking advantage of SR emittance growth in A-line to mitigate effects. Sensitivity to Linac emittance and beam-beam effects on emittance may be significant Depolarization in Target? - Compton IP and spectrometer/detector after target to measure effects?

Intensity-Position Correlation Observed during E158 engineering run (April-May 2001) Stripline BPM in S30 Stripline BPM in S30 Head of pulse Tail of pulse 45 GeV 2.3e11 in 130ns train 0.3ns microbunch spacing (low intensity NLC beam!) rf bpm in front of E158 target 2 bands related to K02 problem Position-intensity correlation believed to be due to beam loading (5%) and dispersion in Linac, with possible wakefield amplification We minimize position-intensity correlation with careful steering in injector, but corrections vary in time. Entire pulse average