Timing Counter analysis C.Voena - INFN Roma for the TC group MEG Review Meeting Feb 17th 2010

TC before insertion in COBRA

TC configuration in 2009 run Trigger thresholds lower than 2008: 20mV on single PMT, 70mV on sum of PMT(40/100mV in 2008) PMT gains re-equalized by adjusting HV Waveform digitization: - DRS3 for NIM pulse from Double Threshold Discriminator (DTD) for time measurement (like 2008) - DRS4 for PMT pulse Double-Threshold Discriminator thresholds revisited to optimize efficiency and time resolution 2009: 25/600 mV 2008:25/800mV

PMT gain equalization Use mean of log(QPMT1)/QPMT0) in cosmic ray data TC bar Use mean of log(QPMT1)/QPMT0) in cosmic ray data at “center” of the bar Change HV to have PMTs (inner vs outer) and bar equalization within 30% (changed 10 PMT HV with respect to 2008) PMT0 (inner) PMT1 (outer)

DTD thresholds scan Low threshold: as low as possible (first photoelectron) => Can improve time resolution Limited by noise No improvement seen lowering this threshold (tried 10-17 mV) => left at 25mV which was already optimal value! High threshold: select good pulses (tracks producing enough p.e., Landau peak) - Lowered to 600mV (was 800mV) to have more acceptance on positron with lower pulses 800-25 400-25 250-25 Eloss (a.u.)

TC time measurement amplitude of PMT signal effective velocity e+ z PMT1 L amplitude of PMT signal effective velocity t0,1= extracted with waveform template fits to NIM pulses from Double Threshold Discriminator for PMT0,1 T : time of positron at the impact point on first hit bar (connected to the positron track from DCH) z : impact point along bar length

TC calibrations Time walk correction for each PMT Improve single bar time resolution Michel nmult=3 Time offset between PMT of the same bar (z offset calibration) Michel (Cosmic in 2008) Effective velocity for each bar (z scale) DCH matched Michel positrons Relative time offset between bars (Boron in 2008) Absolute time offset between positron and photon _ Dalitz 0

Time Walk calibration As in 2008 TW corrections from triple-bars events in Michel Data On events with three adjacent hit bars (triples) minimize the differences (for all the bars) of: bar # e+ TC TA 1st bar TB |z| no TW with TW Control sample: double-bars events TA TB TA-TB (ns)

TC Time Resolution from double-bars events: upper limit on: TC intrinsic resolution + DRS resolution (~10ps) Upper limit on time resolution () in 70-100 ps range in 2009 (except bar 21) Slightly worse than 2008: under study. Still adequate for MEG performances 2009 resolution 2008 resolution

z-offset calibration Difference of PMTs electronic offsets - Needed for z measurement and to combine time measurement of adjacent hits - Use Michel matched positrons instead of previously used cosmic rays Obtained by aligning mean of ztrack-zTC ztrack= z predicted extrapolating track at TC zTC = z measured by TC The procedure assumes φ-symmetry

TC hit-map with z-offset calibration Good alignment of bars as can be seen from hit-map trigger “MEG” data before calibration trigger “MEG” data after calibration

z-scale:effective velocity Temporary calibration - Michel matched positrons Use ztrack as estimate of z at TC - Eventually use fibers for z mesurement veff = 14.8 cm/ns ztrack –zcenter (cm) t0-t1(ns)

Inter-bar time offset calibration Double-bar events in Michel data Tj LIB Ti LIB = Inter-Bar path, taken from MC LIB/c ~200ps Offsets for DS bars Offsets for US bars After that, Downstream bars are aligned with bar #0 and Upstream bars are aligned with bar#15 (there are not double-bar events which connect US and DS)

The Boron sample Boron events before calibration two photons: 4.4MeV (XEC) and 11.7MeV(TC) Cuts for cosmic ray rejection before calibration RMS of residual offsets (after calibration) is~70ps after calibration Mean of Tγγ (ns) Systematic effect of assumption on LIB/c (path between two bars) under study TC bar #

Monitoring TC stability Single bar time resolution - different colors correspond to different weeks Inter-bar time offset - Boron sample different colors correspond to Oct/Nov/Dec Mean of Tγγ (ns) Stability over time TC bar #

Absolute XEC-TC time offsets Dalitz 0 events Same topology as signal Worse resolution due to LH2 target Do not look at time resolution! Center of blinding window (for pre-selection) μ=24.9ns Teγ for reference bar - Dalitz data suffer from hardware problem on DCH side (DRS) that may affect resolution Teγ (ns)

TC-DCH match and Te+ algorithm DCH-TC match Extrapolated track at bar surface Reject bars with multiple hits Reject pairs TC-DCH with bad ztrack-zTC , rtrack-rTC and bad χ2 of match (multiple turns taken into account) Positron time : - If more than 1 TC hit in matched cluster: combine time measurement taking into account track length between bars Correct ad-hoc for Te+ correlation with ztrack-zTC Plans: do systematic studies of the algorithms (Monte Carlo and Dalitz sample)

Check on radiative decay peak Clearly see radiative decay peak in Eγ sideband Inter-bar calibration working well - Pre-selection window not well centered at this stage Temporary track and photon selection, kinematic cut (m2νν>0) Example : radiative peak for bar 19 Position of radiative decay peak vs TC bar# Mean of Teγ (ns) Teγ (ns) TC bar #

Summary TC bars very stable during 2009 run - Ready for the incoming long data-taking Calibration strategy applied successfully to 2009 data TC intrinsic time resolution < 70-100ps (a little worse than 2008, investigating) - one of the best Timing Counter detector Calibration methods and analysis algorithm constantly improving

Backup

Possible causes of worse resolution Electronic We checked the electronic contribution to the resolution by splitting PMT signal in two different electronic channels =>Same as 2008 Noise Same noise level as 2008 at DRS input for PMT signal (not sure of situation at DTD input) tin-tout PMT#

Possible causes of worse resolution Other possible causes: deterioration of PMT-bar coupling? Less scintillation light? 1) (relative) width of Landau peak: done but not conclusive, dominated by Eloss fluctuation 2) change of Λeff. Underway but may be not conclusive since we do not have precise measurement of veff 3) Tests in labs foreseen

Width of CR landau distr. 2008 2009

Applying z-calibration to doubles before calibration z-offset calibration from Michel data z-offset calibration from CR data z1 Mean of DZ=z2-z1 in double bar events (cm) z2 Second hit bar number

Boron: High energy photon High energy photon in XEC Low energy photon in XEC

CR backgound in Boron Sample Tγγ (ns) Black: boron data Red: CR data taken with same Boron trg

Resolution in Boron Sample Tγγ resolution (ns) vs TC bar: 2009, 2008 (July processing) Tγγ resolution vs Time

Multiple hits of TC bars Positron track TC hit position: dz = |z2|-|z1| Bar 1 Bar 2 * * 1st bar with multiple hit |z| dz>0 * * |z| dz< 0

Teγ vs ztrack-zTC on Dalitz No constraint for the track to come from muon target TC US Correlation is visible Correction of 20ps/cm (DZCorrection) Same as in 2008