The study of magnetic field for COMET experiment Weichao Yao - - PowerPoint PPT Presentation

The study of magnetic field for COMET experiment

Weichao Yao 12/23/2019 Year-End Presentation

Contents

• The magnet used in particle physics experiment
• Introduction to motion of charged particle in magnetic

field

• The optimization of muon target of COMET experiment
• Summary

Introduction to LHC & BESIII

• Main Dipoles
• there is a 27 km long

underground tunnel, the Main Bends(MBs, dipole magnets) fill more than 2/3

• f the ring
• Nominal magnetic filed

8.33T, ultimate field 9 T.

• ATLAS
• 1 Barrel Toroid, 2 End

Cap Toroids and 1 Central Solenoid

• 2T inner detector
• 1T for muon detectors
• 4.1T peak in winding

1T SC magnetic

The magnet used In COMET Experiment

1 1 2 3 2 3 4 4 5 5 6 6

Here will be introduced in this slide

5T 3T 3T 1T ~1.08T

The motion of charged particle in magnetic field

3.214

The distribution of magnetic field

• The total length of stopping target section is 5m
• The default position of Stopping target is 3.214 m
• Position? uniform field, slope field

Item Material aluminium Shape flat disk Radius 100 mm disk Thickness 200 μm Number of disks 17 Disk spacing 50 mm

• The period of motion will be changed with magnetic field
• The radius slightly larger, and the trajectory length became large
• the target design: the gap of start disks should be small and the gap of end disks

should be large

• If we set L =X∙r,
• qvB = mv2

r

T = 2πr v = 2π v mv qB = 2πm qB

v cos θ 2πm Y qB = X mv sin θ qB

vk T Y = X mv? qB

The motion of charged particle

• If there is no magnetic field, the initial angle larger than θ, the hit number of particles

is zero. The gap between disks more large more better.

• The energy loss of electron with 105MeV in Aluminium:
• βγ=105MeV/0.51=205.88, dE/dx≈-2.18 (MeV/g cm2), so Edep is 2.18×2.7= 5.86

MeV/cm: if the thickness is 0.2mm, then edep is 117.72 keV

• There magnetic field in this section, the motion of charged particle will be helix

θ

Energy deposit in target disks

The charged particle in gradient field: Mirror effect

• At the reverse point, the ν⫽=0.
• Magnetic moment is conserved
• Therefore
• Particles with appropriate angle will mirror in regions of higher B, If θ is too small, particle

does not mirror.

• Bmax =3T:
• If B=1.5T, θ=45 degree; if B=2.1T, then θ=56.8 degree; if B=2.5T, then θ=66 degree
• the particle within angle>θ could be reflected.

9

θ ν⫽ ν⊥ ν B

µ = mv2

0,⊥

2B0 = mv2

pitch,⊥

2Bpitch

B0 Bpitch = sin2(θ)

• the magnetic moment is considered the lowest-
• rder approximation of the adiabatic invariant

The calculation of mirror effect

• Because the change with cos(θ) is same,
• F(0-π/2) reflected, the total events is F(-π—π)
• From above, the calculation is larger than simulation.
• The distribution of particle is uniform with Cosθ,
• Because the too small velocity could not be reflected, The

calculation is a little different with simulation (about 1~2 %)

Rate = F0, π

2

F−π,π = 1 2 Z θ

π 2

d cos(θ) = 1 2 cos(θ)

Nreflected = Z θ

π 2

F(θ)d cos(θ)

Nreflected = F0− π

2

Z θ

π 2

d cos(θ)

Reflected rate of electron

11

velocity v axis θ

vz

• The definition of the angle θ
• Cos θ = vz / v
• The maximum vz

corresponding to to pitch angle

• if θ > pitch angle, the charged

particle could be reflected, however if the θ is too large, it also could not be reflected

mirror effect

θ>40 degree Downstream upstream, reflected

The charged particle motion w/wo gradient field

• The mirror efficiency will be effected by the gradient field
• If using the uniform field, the electron with small angle will be lost. The

gradient field is necessary

2T uniform 2T with slope field

• Purpose:
• Increasing the muons yield in stopping target
• improving the signal acceptance
• Method:
• Magnetic field option
• The parameters of Stopping target: Number disks,

Position, disks gap etc.

The study of the optimization

• f stopping target

The options of Magnetic field

• The default: center 1.8T, dB/dm=1.343
• Case1: center 2T, gradient < default
• Case2: center 2.1T, gradient< default
• Case3:center 2T, gradient ~> default
• Case4: center 2T, gradient > default
• Using the gradient field
• Improving the average field during

the Stopping target

• Changing the slope rate
• Changing the position

Number disks

• Purpose: increasing the muon yields

muon yields will be increased by a factor of 1.83 and 2.13 with 34 disks and 44 disks respectively

• However, with the change of the target, the spectrum of momentum of signal

electron will become broad, so this will effect the final signal acceptance , and when the number larger than 51, the muon yield the increase will become flat

hist

Entries 247890 Mean 104.9 RMS 0.765 Momentum (MeV) 100 101 102 103 104 105 106 1000 2000 3000 4000 5000

hist

Entries 247890 Mean 104.9 RMS 0.765 the distribution of Momentum entering Straw track with 17 disks

hist

Entries 246978 Mean 104.5 RMS 1.001 Momentum (MeV) 100 101 102 103 104 105 106 500 1000 1500 2000 2500

hist

Entries 246978 Mean 104.5 RMS 1.001

the distribution of Momentum entering Straw track with 34 disks

Window cut 104.2 MeV 83.2% efficiency of signal acceptance Window cut 104.2 MeV 67.8% efficiency of signal acceptance

Magnetic field

• Considering the muon yields and the signal capture, the

magnetic field of the center of the stopping target set as 1.84 T

• The value with gradient fiend is larger than uniform field: 12.6%

Uniform field Gradient field

w wo

Energy loss in stopping disk w/wo slope field

The design of Number disks

• The Number disks be set as 38, the Maximum signal

acceptance will be gained

Disks spacing

• The motion of charged particle in magnetic field has helix

trajectory, so the gap between disks will effect the hits number in disks. The gap need to be optimized to reduce the hits number

50mm

• Ex. the simulation of Muon beam

Summary

• The magnet is one of important parts in particle physics

experiment.

• In the muon target of COMET experiment, the magnetic field

is from 3T to 1 T, the center of target is 1.84T.

• The parameters of muon target have been optimized, Ex:

number disks is 38, the disks gap is 40~60mm.

• The stopping target set up to upstream for higher signal and

lower background.

• The thickness and the radius used the default design.