Lepton Polarisation at HERA A brief overview of lepton polarisation - its use and measurement at HERA, including testbeam analysis of the performance.
Download ReportTranscript Lepton Polarisation at HERA A brief overview of lepton polarisation - its use and measurement at HERA, including testbeam analysis of the performance.
Lepton Polarisation at HERA A brief overview of lepton polarisation - its use and measurement at HERA, including testbeam analysis of the performance of the upgraded Transverse Polarimeter (TPOL). Chris Collins-Tooth (ZEUS, IC-London) Outline Lepton Beam Polarisation - how, why? Measurement of Polarisation - currently, and in the future. Test Beam setup of the Telescope and Transverse Polarimeter What is the Telescope and what does it do? What data was gathered? Analysis of the data Multiple Coulomb Scattering, Beam spread, and Telescope resolution Relative rotations between parts of the Telescope between the Telescope and the TPOL silicon What can be done about these factors? What does this tell us about the TPOL silicon - resolution,efficiency? Conclusions Relativistic e+/- emit synchrotron radiation in curved portions of a storage ring. Emission can cause spin flip. and flip rates differ. e- become polarised antiparallel to the guide field, e+ become polarised parallel to field. P(t) =Pst [1-exp(-t/Tst)] = (N -N )/(N +N ) Pst was ~0.51 at HERA Tst=time-constant ~20min P(%) Lepton Beam Polarisation 60 50 40 30 20 10 0 -10 t(min) Why is polarisation important? More accurate knowledge of polarisation state of beam will allow ‘new physics’ to be investigated e.g.: Standard Model: no right handed Charged Currents (W + - L / W R ). Plotting s obs(P) allows direct measurement of right-handed W-R mass. - present limit is 720GeV set in 10/2000 by D0. Neutral Current (Z ,g ) cross sections split into 4 at high Q2 if Leptons polarised. This allows measurement light quark Neutral Current couplings v v a a . (complimentary to LEP b,c quarks). CC 0 u d u d How is polarisation currently measured? Transversely polarised leptons collide with circularly polarised laser light to give angular asymmetry. Angular asymmetry translates to spatial asymmetry. Compton-scattered photons enter calorimeter. Calorimeter is in two halves to measure up-down energy asymm. Polarisations measured to + 6% (photon position measured to within 1000 mm) Upgrade will improve accuracy. What does the prototype TPOL look like? Only changes to calorimeter section of TPOL. New 1cm2 Si strip detector in front of calorimeter (80 mm pitch for horizontal strips). Due to small beam spot radiation damage may occur. Movable scintillating fibre to calibrate Si response over ~5yr lifetime. Production TPOL has 6x6cm2 Si strip detector, with horizontal and vertical strips (just tested at CERN). Test Beam setup Telescope TPOL 6 GeV e- beam enters from left e- beam passes through Telescope then moves into the TPOL Telescope mounted as close to TPOL as possible on movable table Telescope has 3 position sensitive detectors T1,T2 and T3 (Td was ‘dead’ material being used for a second experiment) The Telescope Previously, degree of polarisation estimated using energy asymmetry in calorimeter (Calorimeter resolution ~1000 mm) Now measure polarisation using 80 mm pitch Si Something more accurate needed to probe TPOL silicon resolution - the Telescope. 3 planes of 50 mm pitch Si, with horizontal and vertical strips. T1,T2,T3 detectors roughly 3cm × 3cm (TPOL silicon ~1 cm2) The data T1,T2,T3 used to predict Si strip to fire. As expected, fitted line has slope =1 0.01 Offset simply due to T1,2,3 being physically larger than TPOL Silicon Width of data about fitted line gives indication of TPOL resolution The width Width =132.15 ± 2.93 mm TPOL Silicon strip pitch =80 mm Intrinsic resolution ~80/12 mm -800 -400 00 400 800 Analysis of the data Observed width does not relate directly to the TPOL resolution Multiple Coulomb Scattering (MCS) of e- beam at T1,Td,T2,T3 and TPOL Aluminium Box Finite resolution of the Telescope e- beam not 100% collimated Misalignments of the Telescope detectors T1,T2,T3 Misalignments of the Telescope (as a whole) and the TPOL Telescope internal misalignment T1,T2 and T3 could all be misaligned with respect to each other. Rotations would produce systematic shifts of ‘predicted minus actual’ strip firing from left-to-right Most important are rotations about beam-axis (pictured). A 0.08o rotation would cause a shift of 1 strip across breadth of detector ‘Predicted minus Actual’ shifts for T3,T2 and T1 from LR T3 T2 T1 Using T1,T2 to predict T3 (left) we observe a shift from left to right of approximately 50 microns Correction of misalignment Iterative process invoked Rotating T3 by 0.27o flattened off all the plots (to within errors) TPOL misalignment The TPOL silicon had no vertical strips Track through T1,T2,T3 used to predict vertical strip to fire to give indication of horizontal position of impact No discernable shift observed before or after T3 rotation applied Correction for rotations caused no discernable reduction in observed ‘width’ MCS, Telescope resolution and beam collimation Simple Monte-Carlo simulation using PDG formula for MCS with gaussian width: s=(13.6MeV/cp) (x/Xo) (1+0.038 n [x/ Xo]) Telescope resolution and beam collimation are small factors in comparison to MCS Together, all these factors contribute ~102 mm to the width Subtracting in quadrature, the TPOL resolution obtained is (1322-1022) 83 mm But - MCS is not actually gaussian. Attempted to use GEANT to improve estimate GEANT v3.13 Geant v3.13 used to simulate MCS through 5 surfaces Gives width of 138 mm ± 1.5 cf Observed width 132 mm ± 3 There are other errors on this simulation, 1.5 mm only statistical error. Consistent with good performance of TPOL Silicon. Error propagation T1 Td T2 T3 m Al m3 T4 (TPOL) m3+3 3 +z Alternative approach to Monte-Carlo Use errors introduced by MCS etc., and propagate them to the TPOL silicon T4=T3+Z4m3+Z43+(Z4-ZAl)Al Var(T4)=Var(T3)+Z42Var(m3)+Z42Var(3)+(Z4-ZAl)2Var(Al)+Z4Cov(T3,m3) T3=T1-Z1m3-Zdd-Z22 Var(m3)=(1/Z12)(T1-T3-Zdd-Z22) Variances are calculated, (e.g. Var(T1,T2,T3)=s2T1,T2,T3 =208 mm) sT4=[Var(T4)]=120 mm= Error on TPOL Si due to uncertainties in Telescope Resolution of TPOL Si = [1322-1202]=55 mm TPOL Resolution Obviously other effects (eg MCS) mask the resolution, so we can only say that TPOL Si results are consistent with theory. From Monte-Carlo Simulation with PDG formula we obtain RTPOL 83 mm From Error Propagation we obtain RTPOL 55 mm TPOL Silicon Efficiency Ratio of hits NOT registering in TPOL Telescope used to predict TPOL events Efficiency is the ratio of: Horizontal Position (mm) events with TPOL Silicon signal events predicted by Telescope TPOL Silicon edges found by looking at ratio of hits not registering in the TPOL as a function of position Log plot reveals edges where Telescope predicts TPOL hits but TPOL does not register Horizontal edges at 13000 & 20000 mm Vertical edges at 11000 & 18000 mm Consistent with active area of Silicon Vertical Position (mm) Efficiency cuts Using the edges from previous slide, we must remove predicted events from efficiency calculations where they miss the boundaries of the TPOL Si Figures show events predicted by the Telescope and registered by the TPOL (black) For effect, events in red are added. They are predicted events which had no TPOL response, but the event was inside the opposite direction boundary, and so should have registered. Clearly, the boundaries look correct. Final Efficiency With the positional cuts made, the efficiency of the TPOL silicon can be calculated This is the ratio of : events with a TPOL silicon response all predicted events inside the boundary The efficiency is uniform across the detector, at ~97.8% Conclusions Upgrade should improve accuracy of asymmetry measurement. Currently aiming for better than 1% error on P(t) Misalignments, Coulomb Scattering and other factors contribute significantly to observed width of 132 mm. TPOL Silicon resolution TBA but preliminary studies suggest 55 and 83 mm TPOL Silicon efficiency spatially uniform at 97.8% More accurate knowledge of polarisation state of beam will allow ‘new physics’ to be investigated e.g.: Standard Model: no right handed Charged Currents (W + - L / W R ). Plotting s obs(P) allows direct measurement of right-handed W-R mass. - present limit is 720GeV set in 10/2000 by D0. Neutral Current (Z ,g ) cross sections split into 4 at high Q2 if Leptons polarised. This allows measurement light quark Neutral Current couplings v v a a . (complimentary to LEP b,c quarks). CC 0 u d d u