Precision timing with PbWO 4 crystals and prospects for a precision timing upgrade of the CMS electromagnetic calorimeter at HL-LHC Simone Pigazzini on behalf of the CMS collaboration CALOR 2016 Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 1 / 12
Outline • Precise time measurement of EM clusters as pileup mitigation techniques for HL-LHC. • ECAL timing performance during LHC operation at √ s = 8 TeV. • Test of PbWO 4 crystals timing resolution with electron beam. Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 2 / 12
Timing at HL-LHC • High number of concurrent interaction (PU) expected for HL-LHC spoiling the event reconstruction. • Precise time information of EM energy deposits provides a way to maintain the same performance of today. QCD event, photons � E T : • No pileup interactions (solid yellow). • 140 pileup interactions (solid black). CHS � track based charged hadrons cleaning. • 140 PU + Puppi cleaning (solid red). Puppi � probability based charged and neutral hadrons cleaning. • 140 PU + Puppi cleaning + timing (dashed red). Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 3 / 12
CMS ECAL current timing performance • Timing resolution of CMS ECAL better then 1 ns was not foreseen in the original design , despite this: � excellent timing resolution already achieved in 2012 (LHC collision @ 8 TeV) . Z → ee events. E in EB [GeV] 20 40 60 80100 1 )[ns] • Timing resolution estimated from fit CMS Preliminary - Run1 EB Z study 2 -t to: t channel 1 − t channel 2 . N σ ⊕ 1 (t) = 2 C (t σ A / σ eff n • Take the two most energetic channel ± N = 33.2 2.0 ns for each electron cluster. ± C = 0.154 0.001 ns 10 -1 2 3 10 10 σ A / eff n Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 4 / 12
CMS ECAL current timing performance • Timing resolution improves for channels of the same cluster. • Further gain when considering channels that belongs to the same readout unit. Channels in the same shower but Channels in the same shower and same different readout units. readout units. Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 5 / 12
Timing resolution: effect of clock distribution stability • Clock distribution checked using laser system. • Clock distribution instabilities measured • Many crystals illuminated at the same time, across different readout units . over time ( ∼ 100 ps/days), between different readout units . • One crystal taken as reference ( t ref ), timing resolution from fit to t crystal − t ref . • Instabilities occur after system resets. • Timing resolution of ∼ 40 ps measured, • Impact of instabilities measured as shift in regardless of same/different readout units. signal peak position. Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 6 / 12
CMS ECAL electronics for HL-LHC Improvements: • Noise from APD leakage current. � increased by long exposure to radiation. • Allow higher trigger rates. • Mitigate pileup from previous and following bunch crossings. • Mitigate signal contamination from concurrent interactions in the same bunch crossing (through timing). Test beam: digitized APD signal • Different solutions are under evaluation. • Current ECAL electronics with faster shaping time could satisfy the requirements. � Shorter signal � Larger Amplitude/noise � Better timing resolution. Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 7 / 12
PbWO 4 intrinsic timing resolution: test beam Test beam goals: • Measure ultimate timing performance of PbWO 4 crystal in response to electrons. • Timing resolution measured with external reference detector. • Study impact on timing of shower depth fluctuations : � Standard CMS ECAL APD readout. � Additional SiPM light collection from the front face (opposite to the APD). • Test different readout electronics configurations (different shaping times). Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 8 / 12
Test beam setup • Electron beam from CERN SPS, energies: 50, 100, 150, 200 GeV. • Multi-channel-plate (MCP) detector used to measure electron time of arrival: � reference time . • CMS ECAL barrel configuration: 23 cm PbWO 4 crystal + APD. • MCP, APD and SiPM signals sampled with a 5 GHz digitizer. HODO MCP CRYSTAL Standard APDs BEAM Pair of SiPMs read-out separately with NINO chip VFE NINO NINO t SiPMs t 0 t APDs DIGITIZER Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 9 / 12
Test beam results: timing resolution with APD • MCP resolution from independent measurement: 25 ps . A • Faster shaping time readout has almost × 2 (Signal amplitude/ RMS noise ) . σ noise • Test beam custom electronics source of additional noise: � in CMS A /σ noise ∼ 800 for a 50 GeV shower. Resolution vs Energy Resolution vs Amplitude/noise Test beam data: Current ECAL (43 ns) shaping time, shorter (21.5 ns) shaping time. Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 10 / 12
Impact of showers depth fluctuation on timing resolution • Comparison with reference MCP time yields to a timing resolution limited to 70-80 ps. CMS ECAL preliminary Test Beam 2015 200 [ps] 50 μ m cell - MPPC • Coincidence between the two SiPM SiPM1 time wrt MCP CT 180 SiPM2 time wrt MCP σ signals proves that SiPM has a intrinsic (SiPM1 time - SiPM2 time )/ 2 160 ∼ 25 ps resolution (constant term). σ = A/ E ⊕ C t 140 A = 191, C = 23 ps • Fluctuation in the light production 120 depth affects timing from front face 100 readout. 80 • Back face readout (previous 60 slide) not affected. 40 20 0 0 50 100 150 200 250 Energy [GeV] Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 11 / 12
Summary • Intrinsic timing resolution of PbWO 4 + APD system of the order of 50 ps can be achieved for shower above 25 GeV. • Clock stability needed to achieve excellent detector wide timing performance: � stability of ∼ 40 ps could be achieved with the current laser monitoring system. • First test beam results on timing with front face light collection show poor performance compared to APD (back) readout: � interpreted to be due to showers depth fluctuations. • Excellent ECAL timing resolution improves the event reconstruction at HL-LHC Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 12 / 12
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