A new particle identification method with the Belle II ...

Savino Longo ([email protected]) ICHEP 2018

Belle1

A new particle identification method with the Belle II calorimeter using pulse shape

discrimination in CsI(Tl)

S. Longoa, J. M. Roneyab, C. Cecchic, T. Ferberd, A. Fodore, C. Heartybf, A. Kuzming, F. Meierh, K. Miyabayashii, I. Nakamuraj, M. Remnevg, V.

Shebaling, A. Sibidanova, Y. Unnok, V. Zhulanovg

on behalf of the Belle II Calorimeter Group

University of Victoriaa, Institute of Particle Physicsb, INFNc, DESYd, McGill Universitye, University of British Columbiaf, BINPg, University of Sydneyh, Nara Women's Universityi, KEKj, Hanyang Universityk

Savino Longo ([email protected]) ICHEP 20182Image from: http://belle2.desy.de/e103206/

• Belle II is a B-factory operating at the SuperKEKB e+e- collider in Tsukuba, Japan.

• Planned to be operating at world record luminosities, Belle II will conduct searches for new physics in the flavour sector of the Standard Model.

• This talk discusses Pulse Shape Discrimination (PSD) which is a new technique using the CsI(Tl) calorimeter crystals to improve particle identification at Belle II.

• Enabled by new, higher bandwidth pulse digitization on each crystal, Belle II is the first e+e- collider experiment to use CsI(Tl) PSD.

The Belle II Experiment


CsI(Tl) Pulse Shape Discrimination• When a particle deposits energy in a CsI(Tl) crystal a scintillation pulse is emitted.

• The shape of a CsI(Tl) scintillation pulse changes depending on the dE/dx of the particle depositing energy in the crystal.

• Below shows pulse comparison for low energy photon and alpha particles.

• PSD is widely applied in nuclear physics at lower energies (E<10 MeV) for charged particle ID.

• We look to extend this technique to higher energies (E ~ 0.1 - 5 GeV) for application at e+e- colliders such as Belle II.

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K0L ?

� ?

Improving Neutral Hadron Identification with PSD• The goal of this study is to improve Particle IDentification (PID) of neutral

hadrons such as neutral kaon-longs (K0L) and neutrons by distinguishing

them from photons.

• K0L identification critical for numerous physics studies at Belle II.

• Eg: B0 ! K0L+J/ used for studying Charge-Parity violation of B-meson

system.

• KL0 identification is critical for numerous physics studies at Belle II.

• Example is used for Charge-Parity violation measurements.

Neutral Hadron vs Photon Detector Signature

• KL0 and photon detector signature is similar - no track in drift chamber and energy cluster in calorimeter.

• We explore using PSD with CsI(Tl) to distinguish photon and neutral hadron clusters.


TRIUMF Test beam Neutron Data• Test beams were completed at the TRIUMF Proton and Neutron Irradiation Facility (PIF/

NIF) to study CsI(Tl) response to neutrons and protons.

• Standard method to characterize CsI(Tl) pulse shape is with a charge ratio, RPSD.

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• RPSD vs Qlong plot allows for the variety of pulse shapes in the neutron data sample to be observed.

• Bands at high charge ratios correspond to secondary charged hadrons produced in inelastic neutron scatters in the CsI(Tl) crystal.

TRIUMF Neutron Data

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TRIUMF Test beam Neutron Data• Test beams were completed at the TRIUMF Proton and Neutron Irradiation Facility (PIF/

NIF) to study CsI(Tl) response to neutrons and protons.

• Standard method to characterize CsI(Tl) pulse shape is with a charge ratio, RPSD.

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1. Photons 2. Cosmic muons (MIP) 3. Single protons from neutron scatter 4. Two protons from neutron scatter 5. Alpha from neutron scatter 6. Multiple hadrons from neutron

scatter 7. High energy proton from neutron

scatter

TRIUMF Neutron Data

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Hadron Scintillation Component• Using the variety of pulses in the neutron data we studied the pulse shape differences

between a template photon pulse and the various hadron pulse shapes.

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• After the photon component is subtracted from the hadron pulses, we are left with a third scintillation component with decay time measured to be 630±10 ns.

muon pulse after photon component subtraction

proton pulse after photon component subtraction

alpha pulse after photon component subtraction


Applying the Model to the Neutron Data• The hadron scintillation component defines a new model for CsI(Tl) emission:

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• All shape parameters are fixed except the photon and hadron component energies.

• By fitting the pulses we measure NHadron and plot NHadron vs LTotal.

• The pulse shape bands are now understood as pulses with different hadron component light output.

TRIUMF Neutron Data


Hadron Component Model - Simulation Methods• Simulation methods to compute the hadron component light output were also

developed and validated with the test beam data

• We show that the hadron component scintillation light output can be computed from the instantaneous ionization dE/dx of the particles in a shower.

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Simulation Truth: neutrons 1/E Energy Spectrum


Proton Distributions

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• Further simulation validation was completed with proton beam data collected.

• Comparison of proton response in data with GEANT4 simulations and numerical computation validate simulation methods developed.

67 MeV Proton TRIUMF Data

67 MeV Proton Simulation Truth

Two primary protons from beam


Predictions for Neutral Hadron Performance

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• Predictions for expected improvement in neutral hadron PID performance are made using GEANT4 simulations incorporating pulse shape simulations.

K0L/�

• We study the fraction of clusters with hadron component light output above a threshold value.

• With this simple identification algorithm these simulations predict high efficiency and purities for neutral hadron vs photon identification using only PSD.

• 5x5 CsI(Tl) crystal cluster is simulated in GEANT4.


The Belle II Calorimeter

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• The Belle II calorimeter uses 8736 CsI(Tl) unique scintillation crystals.

• Calorimeter electronics upgrade from Belle to Belle II enable online digitization (18 bit @ 1.76 MHz) and waveform analysis.

• Online fitting algorithm uses photon template fit to measure time and energy.

• To allow for PSD, all waveforms with online energy greater than 30 MeV are saved for offline shape analysis.


CsI(Tl) Pulse Shape Discrimination at Belle II

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• Offline waveforms are characterized with photon+hadron component fit.

• Hadron response for all crystals is calibrated using Fourier analysis to compute the impulse response for each calorimeter crystals signal chain.

CsI(Tl) Diodes Preamp ShaperDSP

RHadron(t) = LHadron(t) ⇤ IShaperDSP(t)IShaperDSP(t) = IFT�FT (R�(t))FT (L�(t))

�FT = Fourier Transform IFT = Inverse Fourier Transform

Sample Fit of Hadron Pulse in Collision Data

Belle II Phase 2 data – preliminary


First Beam = First Hadrons Observed!

• When SuperKEKB beams started circulating (April 2018) hadronic backgrounds in ECL allowed for hadronic pulse shapes to be observed at Belle II.

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HER = 0.15 A LER < 0.001 A

ECL Data all barrel crystals

HER = 0A LER = 0A

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ECL Data all barrel crystals

Observing Hadronic Pulse Shapes with Belle II Calorimeter

electromagnetic pulse shapes

proton bands

alphas Belle II Phase 2 data – 1st glimpse-preliminary

Belle II Phase 2 data – 1st glimpse-preliminary

SuperKEKB info: http://www-linac.kek.jp/skekb/snapshot/ring/dailysnap-20180404-0001.png

http://www-linac.kek.jp/skekb/snapshot/ring/dailysnap-20180404-0001.png


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At least one photon passes tight hadron selection with PSD

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• Work is now ongoing to incorporate PSD into neutral and charged particle ID at Belle II. • The preliminary result below illustrates hadronic cluster selection at Belle II with PSD.

Identifying Hadronic Showers with Pulse Shape Discrimination

No pi0 peak when tight neutral hadron selection with PSD is applied.

• Expect no pi0’s from hadronic neutral clusters.


Conclusions

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• CsI(Tl) pulse shape discrimination offers a new method to improve particle identification for high energy physics experiments.

• Using test beam data collected at the TRIUMF PIF the hadron component model was developed for characterizing CsI(Tl) pulses.

• Simulation methods for computing the magnitude of the hadron scintillation emission from the dE/dx of the interacting particles in a shower were also developed and validated with test beam data.

• Results and methods developed from the TRIUMF test beam study were recently published in the Journal of Instrumentation:

S. Longo and J. M. Roney 2018 JINST 13 P03018

arXiv:1801.07774

• Work is now ongoing to apply these techniques at the Belle II Experiment.

• Belle II is the first e+e- collider experiment to use pulse shape discrimination for improving particle identification.

A new particle identification method with the Belle II ...

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