CEPC MDI Beam

Backgrounds StudyHaoyu SHI, Wei XU, Hongbo ZHU, Ke LI

IHEP, CASOn behalf of the CEPC MDI BG Study Group,

ICHEP 2020, Accelerator: Physics, Performance, and R&D for Future Facilities,

Virtual Conference, 2020

Outline

ØMotivation & WorkflowØBackground Study Status in detailØSummary & Outlook

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Introduction

• Circular Electron-Positron Collider(CEPC) is a future leptoncollider on design phase• The Conceptual Design Report(CDR) was published in Nov. 2018

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J. GAO

X.C. LOU

Motivation & Workflow

ØBackgrounds may impact IR components, especially detectors in several ways, so that they are important inputs to the detector(also accelerator) design, such as radiation tolerance, detector occupancy…

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Simulation

Benchmark

Design

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Simulation

Benchmark

Design

Interaction Region Layout

• Based on CDR Design and Parameters for both Detector & Accelerator• Higgs Mode, One IP, One Beam• Impact on Vertex Detector

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Workflow

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Simulation

Benchmark

Design

Source Analysis

•Effects•Single Beam• Touschek Scattering• Beam Gas Scattering• Beam Thermal Photon Scattering• Synchrotron Radiation

• Luminosity Related• Beamstrahlung• Radiative Bhabha Scattering

• Injection

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Source Analysis

•Effects•Single Beam• Touschek Scattering• Beam Gas Scattering• Beam Thermal Photon Scattering• Synchrotron Radiation

• Luminosity Related• Beamstrahlung• Radiative Bhabha Scattering

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Off Energy Beam

Particles

Photons

Workflow

• Simulation workflow:

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Generation

• Using the existing generators• Building our own generators

Tracking

• Let the particles move alongside the pipe• Record the lost particles within the IR

DetectorSimulation

• Quantify Impacts using hit density, TID, and NIEL• Adopted the method ATLAS used(ATL-GEN-2005-001) for background estimation

Cross Check

Basic SimulationFor Photon BGs and Beam BGsUsing its own Generator.Using SAD or BDSim as tracking tool.Using Geant4 to perform full detector simulation

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Source Selection

•Effects•Single Beam• Touschek Scattering• Beam Gas Scattering• Beam Thermal Photon Scattering• Synchrotron Radiation

• Luminosity Related• Beamstrahlung(Pair Production)• Radiative Bhabha Scattering

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Off Energy Beam

Particles

Photons

Synchrotron Radiation• Synchrotron radiation were emitted by magnets when bending

beams, sometimes would be critical at circular machines• Using BDsim&Geant4 as the tool to transport beam particles

from the last dipole to the interaction region and record the photons hitting the central beryllium pipe.

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Background Type Hit Density(𝒄𝒎!𝟐 ⋅ 𝑩𝑿!𝟏) TID(𝒌𝒓𝒂𝒅 ⋅ 𝒚𝒓!𝟏)Synchrotron Radiation ~18.2 ~584.0

Pair Production

• Charged Particles attract by the opposite beam emit photons(beamstrahlung), followed by an electron-positron pair production.• Using Gienea-pig++ as the generator and implementing the

external magnetic field by code updating.

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Source Selection

•Effects•Single Beam• Touschek Scattering• Beam Gas Scattering• Beam Thermal Photon Scattering• Synchrotron Radiation

• Luminosity Related• Beamstrahlung• Radiative Bhabha Scattering

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Off Energy Beam

Particles

Photons

CEPC Beam Lifetime

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Beam Lifetime Others

Touschek effect >1000 h

Beam Gas Coulomb >400 hCO, 10!" Pa

Beam Gas Bremsstrahlung 63.8 h

Beam Thermal Photon Scattering 50.7 h

Radiative bhabhaScattering 74 min

Beamstrahlung 80 min

S. BAI

Beamstrahlung

• Beamstrahlung is the radiation background from one beam of charged particles in storage rings or linear colliders caused by its interaction with the electromagnetic field of the other beam• Generated by formula(PyBS)• Code Written by Dr. Yue

• Checked with Guinea-Pig++

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Hist1Entries 4e+08Mean 117.6Std Dev 0.4002

0 20 40 60 80 100 1200

2

4

6

8

10

12

14

16

610× Hist1Entries 4e+08Mean 117.6Std Dev 0.4002

The Energy Spectrum of BTH inputBS

Radiative Bhabha

• Radiative bhabha scatteringrepresents the progress thatone electron/position interactwith the other and emitphotons. The electron/positioncould loss energy, and mightbecome background.• Generated by bbbrem• Could also generated by

formula(Py_RBB)• Code Written by Dr. Yue

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Beam Thermal Photon

• Beam Particle interact with the photon emitted by beam pipe• Important on high energy

machine• Generated by PyBTH:• Written by Dr. Yue• The scattering was generated

with 200 meters upstream ofthe IP• Tracking whole ring for many

turns

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Beam Gas• Beam Particle interact with

residual gas• Beam-Gas Inelastic Scattering:• With nuclei• With electron• 𝜎 doesn’t relevant with energy,

has strong impact on both high and low energy machine

• Generated by PyBGS2• Written by Dr. Yue• Residual Gas CO, with 10!"𝑃𝑎• The scattering was generated

with 200 meters upstream ofthe IP• Tracking whole ring for many

turns

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Lost Distribution without Collimators

• 4 types of Backgrounds

• Normalized to loss rate in MHz(one beam)

• BS contributes the most

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Ways to MitigateUsing Shielding/Mask/CollimatorGenerated whole ring scattering for BGS/BTH

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SR Mask

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±20 cm from IP

SR Mask Effects

• With masks located at -4.2, -1.93 and -1.51m along the beam pipe to shield the central beam pipe.• Detector hit numbers could

be decreased from 7.73×10#down to 111 per bunch.

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Collimator• Now we put 2 sets of horizontal collimators, but sure to need

more in future.(Trying 5th Horizontal one upstream of IR)• We assume the collimator were “ideal”.

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Name Location Distance to IP

APTX1 D1I.1897 2139.06

APTX2 D1I.1894 2207.63

APTX3 D1O.10 1832.52

APTX4 D1O.14 1901.09

S. BAI

Collimator Effects

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Lost Distribution with Collimators

• Including Radiative Bhabha, Beam-Gas, Beam Thermal Photon. Almost No Beamstrahlung.

• Normalized to loss power in mW/10cm(one beam)

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Combine Results

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Background TypeHit

Density(𝒄𝒎!𝟐 ⋅𝑩𝑿!𝟏)

TID(𝒌𝒓𝒂𝒅 ⋅𝒚𝒓!𝟏)

1 MeV equivalentneutron fluence

(𝒏𝒆𝒒 ⋅ 𝒄𝒎!𝟐 ⋅ 𝒚𝒓!𝟏)

Pair production 2.26 591.14 1.11×10'(

Synchrotron Radiation 0.026 15.65

Radiative Bhabha 0.34 592.66 1.44×10'(

Beam Gas 9.025 9775.78 23.6×10'(

Beam Thermal Photon 0.32 318.12 0.75×10'(

Total 11.971 11233.15 26.9×10'(

Higgs Backgrounds on 1st layer of Vertex. With a safety factor of 10.

Off-energy Background Study Status

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Effects Beam BG Photon BGTouschek Maybe -

Beam Gas Coulomb Doing To DoBeam Gas Brem Done DoingBeam Thermal

PhotonDone To Do

SR - DoneRBB Done DoingBS Done Done

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Simulation

Benchmark

Design

Benchmark - Motivation

• Important to validate the modellings and Monte CarloSimulation codes for the CEPC beam background simulationwith real data where they are applicable• BEPC II/BES III, SuperKEKB/Belle II, LEP I/II…

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What can be learnt from BEPCII/BESIII

• Basic Principles• Single beam mode: three dominant contributions from

Touschek, beam-gas and electronics noise & cosmic rays.• 𝑂!"#$%& = 𝑂'()! + 𝑂$*! + 𝑂#("!&+, =

𝑆' % 𝐷(𝜎-#) %𝐼' % 𝐼.𝜎-𝜎/𝜎0

+ 𝑆$ % 𝐼' % 𝑃(𝐼') + 𝑆&

• Double beam mode: additional contributions from luminosityrelated backgrounds, mainly radiative Bhabha scattering• 𝑂'('*% = 𝑂&$ + 𝑂&% + 𝑂ℒ

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Machine Studies Last Summer

• Two hours of machine time allocated to background studies lastsummer.• Recorded parameters: bunch size, beam current, beam lifetime,

vacuum pressure, MDC/EMC cluster counts.

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We may need one more round

• Simulations results DO NOT match the measurements

• Let the accelerator and detector configured properly beforestudy• Most importantly, longer time, more data.

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H.C. ShiH.C. Shi

Summary

• Background Study Checklist

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Category Tasks Status

DesignIR Design Optimization To Do

Collimator Design Doing

Simulation

BG Simulation Doing

Collimator Simulation To Do

Detector Simulation Doing

IR Components Simulation To Do

BenchmarkBEPCII/BESIII Machine Study Doing

Literature Benchmark Doing

Outlook

• Improve & Optimize our study• Code Effeiciency• Benchmark

• New Baseline.• New baseline design as a new start.

• Other Mode. • Z & W

• The impact on other sub detectors / accelerator components in the IR

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Thank You

Backup

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Off Energy Beam

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BeamstrahlungIP1

IP3

Beam Lost ParticlesEnergy Loss > 1.5%(energy acceptance)

Radiative BhabhaScattering

242 bunchesRevolution frequency: 2997Hz𝟏. 𝟓×𝟏𝟎𝟏𝟏 particles/Bunch

L: 𝟑×𝟏𝟎𝟑𝟒𝒄𝒎$𝟐𝒔$𝟏

Beam-Gas BremScattering

Beam-Thermal photon Scattering

Energy Spectrum

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Collimator Effects - BS

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Collimator Effects - RBB

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Collimator Effects - BTH

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Collimator Effects – BGS2

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Lost Distribution with Collimators

• Including Radiative Bhabha, Beam-Gas, Beam Thermal Photon. Almost No Beamstrahlung.

• Normalized to loss power in mW/10cm(one beam)

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Add One More Collimator - Position

• Red Line and Black Lineare different runs with 4Collimator Sets.• Orange Line shows one

more collimator locatedat -30m.• Aqua Line shows one

more collimator locatedat -6m.

• Far is safe. Chose -31m.

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Conclusion of this initial study• Add one more set of collimator after the last bending magnet

can help cut down the lost within the IR• For now, -30.92m could be one choice.• However, this collimator here must be thicker than others.• For now, 10cm Copper + 4cm Tungsten seems to be Ok.• Initial Results, more study need to be done in future.• No Neutrons survive at -6m.• Other issues, including heat deposition, photons hitting the

magnets, instability of the beam, other particles might come out from the collimator, etc., would be done in future.

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Collimators Hit Density TID NIEL4 Co 36.8372 39901.139 9.65e+135 Co 7.797 8354.393 2.02e+13Ratio 4.72 4.77 4.77