Precision timing with PbWO 4 crystals and prospects for a precision - - PowerPoint PPT Presentation

Precision timing with PbWO4 crystals and prospects for a precision timing upgrade of the CMS electromagnetic calorimeter at HL-LHC

Simone Pigazzini

• n 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 PbWO4 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 ET:

• 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.

n

σ /

eff

A

2

10

3

10

)[ns]

2

• t

1

(t σ

• 1

10 1

C 2 ⊕

n

σ /

eff

A N (t) = σ 2.0 ns ± N = 33.2 E in EB [GeV] 20 40 60 80100

CMS Preliminary - Run1 EB Z study

0.001 ns ± C = 0.154

• Timing resolution estimated from fit

to: tchannel 1 − tchannel 2.

• Take the two most energetic channel

for each electron cluster.

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

different readout units.

Channels in the same shower and same

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.
• Many crystals illuminated at the same

time, across different readout units.

• One crystal taken as reference (tref ),

timing resolution from fit to tcrystal − tref.

• Timing resolution of ∼ 40 ps measured,

regardless of same/different readout units.

• Clock distribution instabilities measured
• ver time (∼ 100 ps/days), between

different readout units.

• Instabilities occur after system resets.
• Impact of instabilities measured as shift in

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).

• Different solutions are under evaluation.
• Current ECAL electronics with faster shaping

time could satisfy the requirements.

Shorter signal Larger Amplitude/noise Better timing resolution.

Test beam: digitized APD signal

Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 7 / 12

PbWO4 intrinsic timing resolution: test beam

Test beam goals:

• Measure ultimate timing performance of PbWO4 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 PbWO4 crystal + APD.
• MCP, APD and SiPM signals sampled with a 5 GHz digitizer.

HODO MCP CRYSTAL Pair of SiPMs read-out separately with NINO chip

NINO NINO

VFE DIGITIZER

Standard APDs BEAM

t0 tSiPMs t APDs

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.
• Faster shaping time readout has almost ×2

A σnoise

(Signal amplitude/RMSnoise).

• Test beam custom electronics source of additional noise:

in CMS A/σnoise ∼ 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.

• Coincidence between the two SiPM

signals proves that SiPM has a ∼ 25 ps resolution (constant term).

• Fluctuation in the light production

depth affects timing from front face readout.

• Back face readout (previous

slide) not affected.

Energy [GeV] 50 100 150 200 250 [ps]

CT

σ 20 40 60 80 100 120 140 160 180 200

SiPM1 time wrt MCP SiPM2 time wrt MCP 2 intrinsic (SiPM1 time - SiPM2 time )/ C ⊕ E = A/

t

σ A = 191, C = 23 ps

Test Beam 2015 CMS ECAL preliminary

50 μm cell - MPPC

Simone Pigazzini Precision timing with PbWO crystals CALOR 2016 11 / 12

Summary

• Intrinsic timing resolution of PbWO4 + 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