Niklaus Berger

(1)

Searching for Lepton Flavour Violation with the

Mu3e Experiment

Niklaus Berger

Institut für Kernphysik, Johannes-Gutenberg Universität Mainz PSI 2016

October 2016

(2)

Niklaus Berger – PSI, October 2016 – Slide 2

LFV Muon Decays: Experimental Situation

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

(3)

LFV Muon Decays: Experimental Situation

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

MEG (PSI) SINDRUM II (PSI) SINDRUM (PSI)

B(μ

⁺

→ e

⁺

γ) < 4.2 ∙ 10

^-13

(2016) B(μ

^-

Au → e

^-

Au) < 7 ∙ 10

^-13

(2006) B(μ

⁺

→ e

⁺

e

^-

e

⁺

) < 1.0 ∙ 10

^-12

(1988)

(4)

LFV Muon Decays: Experimental Situation

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

MEG (PSI) SINDRUM II (PSI) SINDRUM (PSI)

B(μ

⁺

→ e

⁺

γ) < 4.2 ∙ 10

^-13

(2016) B(μ

^-

Au → e

^-

Au) < 7 ∙ 10

^-13

(2006) B(μ

⁺

→ e

⁺

e

^-

e

⁺

) < 1.0 ∙ 10

^-12

(1988)

upgrading Mu2e/Comet Mu3e

(5)

LFV Muon Decays: Experimental signatures

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Kinematics

• 2-body decay

• Monoenergetic e⁺, γ

• Back-to-back

Kinematics

• Quasi 2-body decay

• Monoenergetic e^-

• Single particle detected

Kinematics

• 3-body decay

• Invariant mass constraint

• Σ p_i = 0

(6)

LFV Muon Decays: Experimental signatures

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Kinematics

• 2-body decay

• Back-to-back Background

• Accidental background

Kinematics

• Single particle detected Background

• Decay in orbit

• Antiprotons, pions, cosmics

Kinematics

• 3-body decay

• Σ p_i = 0 Background

• Radiative decay

(7)

LFV Muon Decays: Experimental signatures

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Kinematics

• 2-body decay

• Back-to-back Background

Kinematics

• Single particle detected Background

• Decay in orbit

• Antiprotons, pions

Kinematics

• 3-body decay

• Σ p_i = 0 Background

• Radiative decay

Con tinuous Be am

Con tinuous Be am Pul sed Be

am

(8)

e ⁺

e ⁺ e ^-

• μ⁺ → e⁺e^-e⁺

• Two positrons, one electron

• From same vertex

• Same time

• Sum of 4-momenta corresponds to muon at rest

• Maximum momentum: ½ m_μ = 53 MeV/c

The signal

(9)

• Combination of positrons from ordinary muon decay with electrons from:

- photon conversion, - Bhabha scattering, - Mis-reconstruction

• Need very good timing, vertex and momentum resolution

Accidental Background

e

⁺

e

⁺

e

^-

(10)

• Allowed radiative decay with internal conversion:

μ

⁺

→ e

⁺

e

^-

e

⁺

νν

• Only distinguishing feature:

Missing momentum carried by neutrinos

Internal conversion background

• Need excellent

momentum resolution

• New: NLO available from Matteo Fael and Signer et al. - now 10-20% easier

Branching Ratio

10^-12

10^-16 10^-18 10^-14

e⁺e^-e⁺ mass (MeV/c²)

105 106 104

103 102

101

Internal conversion background

Signal

(11)

Building the

Mu3e Experiment

aiming for a branching ratio sensitivity of 10

^-16

(12)

• Apply magnetic field (e.g. 1 Tesla)

• Measure curvature of particles in field

• Limited by detector resolution and scattering in detector

Momentum measurement

(13)

2 Billion Muon Decays/s

50 ns, 1 Tesla field

(14)

High voltage monolithic active pixel sensors - Ivan Perić

• Use a high voltage commercial process (automotive industry)

Fast and thin sensors: HV-MAPS

P-substrate

N-well E field

(15)

• Small active region, fast charge collection via drift

Fast and thin sensors: HV-MAPS

P-substrate N-well

Particle

E field

(16)

• Small active region, fast charge collection via drift

Fast and thin sensors: HV-MAPS

P-substrate N-well

Particle E field

• Implement logic directly in N-well in the pixel - smart diode array

• Can be thinned down to < 50 μm

(I.Perić, P. Fischer et al., NIM A 582 (2007) 876 )

(17)

Submitting large (2×1 cm²) prototype in next weeks

Performance

row-axis [mm]

0 0.5 1 1.5 2 2.5 3

column-axis [mm]

0 0.5 1 1.5 2 2.5 3

efficiency_pixeluv

Entries 900390

Mean x 1.557

Mean y 1.803

RMS x 0.922

RMS y 0.8324

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

efficiency_pixeluv

Entries 900390

Mean x 1.557

Mean y 1.803

RMS x 0.922

RMS y 0.8324

Mupix7, 735 mV threshold, HV = -85 V

Threshold [V]

0.68 0.7 0.72 0.74 0.76 0.78

Efficiency

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1

Efficiency Noise

99 %

Noiserate per pixel [1/s]

1 10 102

103

(18)

• 50 μm silicon

• 25 μm Kapton™ flexprint with aluminium traces

• 25 μm Kapton™ frame as support

• About 1‰ of a radiation length per layer

Mechanics

(19)

(20)

• Add no material:

Cool with gaseous Helium (low scattering, high mobility)

• ~ 250 mW/cm² - total ~3 kW

• Simulations: Need ~ several m/s flow

Cooling

• Full scale heatable prototype built

• 36 cm active length

• Vibrations studied using Michelson-Interferometer

• Can keep temperature below 70°C

(21)

• 1 T magnetic field

• Resolution dominated by multiple scattering

• Momentum resolution to first order:

Σ

_P

/P ~ θ

^MS

/Ω

• Precision requires large lever arm (large bending angle Ω) and low multiple scattering θ_MS

Momentum measurement

Ω MS

θ

_MS

B

(22)

Precision vs. Acceptance

50 MeV/c 25 MeV/c 12 MeV/c B^→

33 cm

(23)

Precision vs. Acceptance

(24)

Precision vs. Acceptance

(25)

Precision vs. Acceptance

(26)

Precision vs. Acceptance

Ω ~ π MS

θ_MS

B

(27)

Detector Design

muon beam

target

(28)

Detector Design

muon beam

target

(29)

Detector Design

muon beam

target

inner pixel layers

(30)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers

(31)

Detector Design

outer pixel layers

muon beam

target inner pixel layers recurl pixel

layers

scintillating ﬁbres

(32)

2

] [MeV/c m

rec

96 98 100 102 104 106 108 110

2

Events per 0.2 MeV/c

−3

10

−2

10

−1

10 1 10 10

2

at 10

-12

→ eee µ

at 10

-13

→ eee µ

at 10

-14

→ eee µ

at 10

-15

→ eee µ

ν ν

→ eee µ

muons/s muon stops at 10

8

10

15

Performance Simulations: Mass reconstruction

Work in progress

Poster by Alexandr Kozlinskiy on track reconstruction

(33)

Need suppression of accidental background:

Timing

(34)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers

(35)

Detector Design

outer pixel layers

muon beam

layers

recurl pixel layers

Scintillating tiles

(36)

• 3 layers of 250 μm scintillating fibres

• Read-out by silicon photomultipliers (SiPMs) and custom ASIC (MuTRiG)

• Timing resolution O(0.5 - 1 ns)

(See posters by Giada Rutar, Angela Papa)

Timing Detector: Scintillating Fibres

Single photon Efficiency > 98%

(≥ 2 photons)

(37)

Timing Detector: Scintillating tiles

• ~ 0.5 cm³ scintillating tiles

• Read-out by silicon photomultipliers (SiPMs) and custom ASIC (STiC)

Scin ator Tiles

SiPM Readout

Electronics

Time Difference [ps]

-7500 -500 -250 0 250 500 750

2000 4000 6000 8000 10000

σ = 79.2 ps

• Test beam with tiles, SiPMs and readout ASIC

• Timing resolution ~ 80 ps

(38)

Sensitivity

Phase IA: Starting 2018 2∙10⁷ μ/s

Target Inner pixel layers

Outer pixel layers μ Beam

(39)

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

Sensitivity

Phase IB: 2019+

1∙10⁸ μ/s

(40)

Sensitivity

Phase II: 2021+

New Beam Line 2∙10⁹ μ/s

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

(41)

• Mu3e aims for μ → eee at the 10^-16 level

• First large scale use of HV-MAPS

• Build detector layers thinner than a hair

• Timing at the 100 ps level

• Reconstruct 2 billion tracks/s in 1 Tbit/s on ~50 GPUs

• Start data taking in 2018

• 2 billion muons/s not before 2021

Conclusion

1940 1960 1980 2000 2020

Year

90%–CL bound

10–14 10^–12 10^–10 10^–8 10–6 10–4 10^–2 10⁰

μ eγ

μ 3e

μN eN

τ μγ

τ 3μ

10^–16

SINDRUM SINDRUM II MEG

MEG plan Mu3e Phase I

Mu3e Phase II

(42)

Backup Material

(43)

New physics in μ

⁺

→ e

⁺

e

^-

e

⁺

Tree diagrams

• Higgs triplet model

• Extra heavy vector bosons (Z’)

• Extra dimensions (Kaluza-Klein tower) Loop diagrams

• Supersymmetry

• Little Higgs models

• Seesaw models

• GUT models (leptoquarks)

• and much more...

(44)

New physics in μ

⁺

→ e

⁺

e

^-

e

⁺

Muon decays at the 10

^-16

level sensitive to new physics

at O (1000 TeV) scale for O (1) couplings!

(45)

MUPIX electronics

(46)

A general effective Lagrangian

Tensor terms (dipole)

L

_{μ → eee}

= 2 G

_F

( m

_μ

A

_R

μ

_R

σ

^μν

e

_L

F

_μν

+ m

_μ

A

_L

μ

_L

σ

^μν

e

_R

F

_μν

+ g

₁

(μ

_R

e

_L

) (e

_R

e

_L

) + g

₂

(μ

_L

e

_R

) (e

_L

e

_R

)

+ g

₃

(μ

_R

γ

^μ

e

_R

) (e

_R

γ

^μ

e

_R

) + g

₄

(μ

_L

γ

^μ

e

_L

) (e

_L

γ

^μ

e

_L

)

+ g

₅

(μ

_R

γ

^μ

e

_R

) (e

_L

γ

^μ

e

_L

) + g

₆

(μ

_L

γ

^μ

e

_L

) (e

_R

γ

^μ

e

_R

) + H. C. )

e.g. supersymmetry

Four-fermion terms scalar

vector

e.g. Z’

(Y. Kuno, Y. Okada,

Rev.Mod.Phys. 73 (2001) 151)

(47)

Comparison with μ

⁺

→ e

⁺

γ

L

_LFV

= A m

_μ _R

μ

_R

σ

^μν

e

_L

F

_μν

+ (μ

_L

γ

^μ

e

_L

) (e

_L

γ

^μ

e

_L

) (κ+1)Λ

²

κ (κ+1)Λ

²

• One loop term and one contact term

• Ratio κ between them

• Common mass scale Λ

• Allows for sensitivity comparisons between μ → eee and μ → eγ

• In case of dominating dipole couplings (κ = 0):

B(μ → eee) = 0.006 (essentially α_em) B(μ → eγ)

(48)

Detector Design

outer pixel layers

muon beam

layers

recurl pixel layers

Scintillating tiles

(49)

Muons from PSI

DC muon beams at PSI:

• πE5 beamline: ~ 10⁸ muons/s

(MEG experiment, Mu3e phase I)

• Surface muons, p = 29.7 MeV/c Stopped in < 1 mm of plastic

(50)

Muons from PSI

• The μ → eee experiment (final stage) requires 2 × 10⁹ muons/s focused and collimated on a ~2 cm spot

(51)

Muons from PSI

• The μ → eee experiment (final stage) requires 2 × 10⁹ muons/s focused and collimated on a ~2 cm spot

• More than ~ 10¹¹ muons/s are produced;

bring magnetic elements closer to cap- ture them:

High intensity muon beamline (HiMB) study currently ongoing

(52)

HV-MAPS

3 m m

(53)

HV-MAPS

3 m m

Pixels with amplifier

40 x 32 pixels

80 x 103 μm pixel size

(54)

HV-MAPS

3 m m

Pixels with amplifier

40 x 32 pixels

80 x 103 μm pixel size

Comparator and digital pixel logic

(55)

Tests done at

• CERN 250 GeV pions

• DESY 5 GeV electrons

• PSI 250 MeV pions

• Mainz 1.5 GeV electrons

• Thanks for all the beam time and support!

Beam tests

(56)

Introduction

Y

• X

(57)

Introduction

Y

• X

(58)

Introduction

Y

• X

(59)

Introduction

Y

• X

(60)

(61)

(62)

Timing Detector: Scintillating tiles

• Test beam with tiles, SiPMs and readout ASIC

• Timing resolution ~ 80 ps

Time Difference [ps]

-7500 -500 -250 0 250 500 750

2000 4000 6000 8000 10000

σ = 79.2 ps

Front

Back

3.5 cm

(63)

Data Acquisition

(64)

• 280 Million pixels (+ fibres and tiles)

• No trigger

• ~ 1 Tbit/s

• FPGA-based switching network

• O(50) PCs with GPUs

Data Acquisition

2928 Pixel Sensors

up to 36 1.25 Gbit/s links

FPGA FPGA FPGA

...

86 FPGAs

1 6 Gbit/s link each

GPU

PC GPU

PC

GPU 12 PCs PC

12 10 Gbit/s links per

8 Inputs each

~ 3072 Fibre Readout Channels

FPGA FPGA

...

48 FPGAs

~ 3500 Tiles

FPGA FPGA

...

48 FPGAs

Data Collection

Server

Mass Storage Gbit Ethernet

Switching

Board Switching

Board

Front-end(inside magnet)

Switching Board

(65)

• 280 Million pixels (+ fibres and tiles)

• No trigger

• ~ 1 Tbit/s

• Need to find and fit billions of tracks/s

Online reconstruction

(66)

• PCs with Graphics Processing Units (GPUs)

• Online track and event reconstruction

• 10⁹ 3D track fits/s achieved

• Data reduction by factor ~1000

• Data to tape < 100 Mbyte/s

Online filter farm

(67)

Introduction

Y

• X

(68)

Cooling tests

Global helium stream

Local helium stream

(69)

• Limited by detector resolution and scattering in detector