The Timing Counter of the MEG experiment: calibration and performances

The Timing Counter of the MEG experiment: calibration and performances INFN Pavia On behalf of MEG TC group 12th seminar on IPRD Siena 1

MEG: signal and background background signal µ e γ physical e + µ γ + θeγ = 180 Ee = Eγ = 52.8 MeV Te = Tγ µ e γ ν ν (radiative decay) ν e+ µ+ γ ν Accidental background limits the sensitivity accidental µ e ν ν µ e γ ν ν ee γ γ ez ez γ ν e+ µ+ ν γ 2

MEG detection concept µ decay at rest + e+ θeγ = 180 µ + γ Stopped beam of 3x107 µ/sec in a 205 µm target Liquid Xenon calorimeter for γ detection (scintillation): Ee = Eγ = 52.8 MeV L iq. X e S c in tilla tio n D e te c to r L iq. X e S c in tilla tio n D e te c to r T h in S u p e rc o n d u c tin g C o il M u o n B e a m γ S to p p in g T a rg e t e+ γ T im in g C o u n te r e+ D rift C h a m b e r D rift C h a m b e r 1m Solenoid spectrometer (COBRA) & drift chambers for e+ momentum measurement Scintillation counters for e+ timing 3

Timing Counter APD F.E. Board APD APD Cooled Support Fibers PM Main Support Divider Board Scintillator Slab Scintillator Housing PM Scintillator Coupler 4

Timing Counter bar for φ Two sectors upstream and downstream the target Two layers of counters at right angle with each others Outer layers with scintillator bars Each sector has 15 bars read by 2 PMTs Full PMT waveforms digitized and read out for dedicated charge and time analysis For trigger and measurement of φ, z and t 5

Timing Counter fibres for z Inner layer with scintillation fibres: Each sector has 256 fibres 5 x 5 mm2 read by 1 APD Analog Signals from 16 fibres are used for trigger Each APD signal is discriminated and digital signals (on) are read out For trigger and measurement of z 6

Timing Counter Test Beam FWHM 100 ps Measurements of TC bars timing resolution in dedicated test beams at several positions and impact angles FWHM(T) = 91 ps (± 5%) MEG Goal 100 ps Impact angle 7

Readout electronics LLT DL LLth S_In 2 DLY 10ns D ½Sin 6dB DH HLth CK Q CK Q 3 Q\ 1 2 R 1 3 HLT Double Threshold Discriminator High threshold for efficiency Low threshold for time resolution Waveform digitizing for all channels (pile up rejection); PMT and NIM signals digitized at 1.6 GHz; 8

Digitization electronics Waveform digitizing for all channels (pile up rejection); Custom domino sampling chip (DRS) designed at PSI; 2.5 GHz sampling speed @ 40 ps timing resolution; Sampling depth 1024 bins; Readout similar to trigger; Trigger: Signals from TC bar are sampled at 100 MHz with separate ADC 9

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TC Monte Carlo Simulation: Pulse shape Optical geometry: dn/dt = (N0/2) Tmin/t2 with Tmin= nscd/c < t < Tmax = Tmin/ cos(θtr) 11

TC Monte Carlo Simulation: Attenuation Length Λeff The effective attenuation length Λeff combines the bulk Λband the surface reflection loss. 1/Λeff = 1/(Λb <cos Θ>) + 1/ΛR2 ΛR2= a/(ε<tan Θ>) where a is the bar size and ε the surface loss coeffcient Λeff depends on each bar through ε Measured and fed into the simulation 12

TC PMT time measurements Several approach to time measurement: PMT signal: Leading edge Constant Fraction Template fit NIM Signal Leading edge Template fit NIM template fit has highest precision!! 13

TC bar time measurements e+ PMT1 z PMT0 L t 0 =T+ z v +b 0 eff c 0 A 0 d 0 log A. t 1 =T+ 0 L z v +b 1 eff c 1 A 1 d log A amplitude of PMT signal effective velocity T : time of e+ at the impact point on bar z : impact point along bar length b01 PMT time offsets c01 d01 PMT Time Walk coefficients 1 14. 1

TC calibration & monitoring PMT time walk correction Michel e+, Template Inter PMT (same bar) time offset Cosmic rays PMT relative Gain Ratio Cosmic rays, Michel e+ ΛEff Attenuation length Cosmic rays, Michel e+ Interbar time offset Boron,Dalitz XEC TC time offset Dalitz Bar Veff Cosmic rays Time resolution Michel e+, 2 3 bar events 15

bar # Double and triple hit events e+ TC 1st bar On events with three adjacent hit bars (triples) minimize the differences (for all the bars) TA t TB 0C +t 2 0B t 0 A [ 1 2 TW 0C +TW 0B TW 0A ]. z On sample of two hit adjacent bars ( doubles ) test time walk correction (c01 and d01) DT=T T A B ns 16

Inter PMT time offsets Average t is PMT offset in cosmic runs MEG physics runs TC hitmap US Before t1 t0 [ns] After DS t1 t0 [ns] t1 t0 [ns] 17

Log Q1/Q0 vs z: PMT Gain Ratio and Λeff 18

Detector calibration: PMT Gain Ratio and Λeff Timing counter parameters: z= v t t PMT Gain charge response Effective velocity veff Effective attenuation length λeff eff 0 2 1 T= t +t 1 0 2 Q 2v eff G 2z 1 ln =ln Q G l λeff (cm) Bar L 1 0 PMT Gain 0 eff 40 % Spread Bar 19

Tc LXe calibration with 0 Dalitz π beam rather than µ Charge Exchange Production Dalitz Decay π p π0n π0 γ e+e Εε+ Εγ γ detected in Calorimeter e+ in TC π0 Εε E(γ) < 82.9 MeV Liquid Hydrogen Target 124 cm3 Cooled with liquid Helium 20

Inter bar offset extraction Boron 4.4MeV (XEC) and 11.7MeV(TC) DT = T gg XEC L XEC g c Tγγ [ns] relative to bar 17 0 2 1 L TC g c Sep 08/10 13/10 20/10 25/10 10/11 24/11 01/12 Bar17 signal bkg (rescaled) Tγγ[ns] t +t Stable in time! 21

Tc LXe calibration with Cockroft Walton accelerator Reactions induced by Cockroft Walton protons (E ~ 1 MeV) on Li and B targets Reactio n Li(p,γ)Be Peak energy 440 kev σ peak γ lines 5 mb (17.6, 14.6) MeV B(p,γ)C 163 kev 2 10 1 mb (4.4, 11.6, 16.1) MeV Boron target: Two gamma s emitted simultaneously tool for relative timing calibration Lithium spectrum on NaI 17.6 MeV line 14.6 MeV broad resonance >16.1 MeV >11.6 MeV 4.4 MeV 4.4 MeV ln Calorimeter 11.6 MeV in the TC 22

TC Effective velocity veff Effective velocity measurement (cm/ns) Z (cm) t (ns) Veff determined with < 1% precision by using bar and fibre information simultaneously Z (cm) t (ns) t (ns) 23

TC resolution with 'doubles' Timing resolution (timing difference between adjacent bars) σt 54 ps, close to project goal (100 ps FWHM) 24

MEG absolute time offset Dalitz π0 events π0 γ e+e DT= T Same topology as signal! Gamma/positron energy range (can be chosen) same as for signal Worse resolution due to LH2 target Comparison with signal is not exact XEC L XEC g c T L TC e c Centre of signal window Bar17 = 26.06±0.01ns σ = 267±10ps Control sample (flight length correction) ns 25

Track TC Match and Propagation time Need to correct for track propagation delay to precision of 50 ps track length to 1.5 cm Trajectory known from target plane through spectrometer to very good precision Projection to TIC complicated by material after spectrometer causing scattering, energy loss Currently, project to fixed φ of timing counter with using propagation of Kalman state vector No correction for mismatch with reconstructed position in timing counter Typical propagation distance is of order 1 m Systematic uncertainties in dr, dz seen, of order 1 cm Fully corrected photon positron timing difference currently at level of 150 ps in RD signal with photon energy above 40 MeV 26

Using DCH to Study Timing Counter Use DCH trigger data Require 4 hits in 5 contiguous chambers Run standard analysis, positron selection criteria Measure probability of having a TIC hit Loose matching criteria Tighter position match, timing criteria 27

TICZ readout tests After installation, test with pass thru cosmics All analog channels (8 APD in one ch.) show good signals! US TICZ digital readout working! DS TICZ performing poorly! Not used Digital electronics upgraded for 2010 run Commissioning on going US TICZ fiber # 28

Conclusions TC bar performance optimization requires careful calibration of many parameters All parameters are calibrated using Michel e+ or ad hoc signals (Cosmic rays,boron,dalitz) Fine tuning of the algorithm could improve performances TC fibre performance limited by malfunctioning digital electronics, adequate otherwise Reliable TC fibre will improve also TC bar calibration with precise z coordinate Commissioning detector for 2010 is ongoing 29

Backup slides 30

Background and Sensitivity Gamma energy % Goal Measured/ Simulated 5.0 Gamma Timing (ns) 0.15 Gamma Position (mm) 4.5 9.0 Gamma Efficiency (%) e Timing (ns) + e+ Momentum (%) e+ Angle (mrad) e+ Efficiency (%) Muon decay Point (mm) Muon Rate (108/s) Running Time (weeks) Single Event Sens (10 13) >40 0.1 0.8 10.5 65 2.1 0.3 100 0.5 Accidental Rate (10 13) 0.1 0.3 # Accidental Events 0.2 0.5 90% CL Limit (10 13) 1.7 1 week = 4 x 105 s Single Event Sensitivity Limited by Accidental Background hence Detector Performance BR(µ eγ) = (Rµ T Ω/4π εe εγ εsel ) 1 5 10 14 Prompt Physics Background (Radiative) BRpr < 3 10 15 Accidental Background BRacc Rµ Ee teγ ( Eγ )2 ( θeγ )2 3 10 14 Upper Limit at 90% C.L. for BR(µ eγ) 1 10 13 * The muon rate can be optimized to improve the limit 31

COBRA Spetrometer Solenoid with a gradient filed COnstant Bending RAdius Independent of emission angles e+ momentum easily used at trigger level reconstruction flat acceptance Michel e+ swept out quickly Reduce # of hits on the chamber 32

Timing Counter 3) TC after installation in MEG 33

Timing Counter (TC) 34

Waveform analysis Based on waveforms on 2 anode ends and 4 pads associated with each cell waveform noise limits resolution DRS voltage calibrated with on board constant voltage presented to input of DRS Waveform Analysis DRS time calibrated with off board sine wave of known frequency presented to each board. Bin by bin time calibration done for each DRS channel (~2x105 points) hardware improvements anticipated Improvement in noise level would significantly improve resolution 35

TC time resolution stability Runs 24xxx Runs 25xxx Runs 26xxx Runs 27xxx Runs 29xxx Runs 30xxx Runs 31xxx Same TW calibration constants Stable over time no need of different sets of constants 36

DCH TC match hitmap Extrapolate tracks from DCH to TC bars Dz=z DCH z bar Given a track and a TC bar hit matching efficiency is 91% Some data/mc discrepancy [cm] 37

Inter bar offsets monitoring Bar15: change in DRS board Sep 08/10 13/10 20/10 25/10 10/11 24/11 01/12 time Tγγ mean [ns] relative to bar 17 bar# Weekly monitoring (periodic DB updates) No clear trend vs time 38

TC time resolution s DT 2 Estimate of single bar time resolution Assuming the two bars to have the same intrinsic time resolution Upper limit on average time resolution (σ) in 60 90ps range Includes effect of DRS digitization (~10 ps) 39

Effective velocity with TICZ Pass thru cosmics, 2 hit fibers (clusters) expected Single cluster inefficiency: 27% due to cosmics geometrical inefficiency + dead/hot fibers (5%) Using Bar16 zbar zfiber (cm) z fiber = 1 2 v t t eff 1 0 40

Laser system Timing calibration device Providing 532 and 266 nm light at 50 Hz Optical fibres distributed to the centre of all TC bars (and XEC) 256 nm 50Hz @ 532nm 50 Hz 2 stages pulse amplifier Diode pumped Nd:YVO Acustic optical pulse selectors 3m cavity 48 MHz, 1064nm 41

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