Niklaus Berger

(1)

Searching for Lepton Flavour Violation with the

Mu3e Experiment

Niklaus Berger

Institut für Kernphysik, Johannes-Gutenberg Universität Mainz Physics Colloquium

Heidelberg, May 2015

(2)

Particle Physics:

What are the fundamental constituents of matter

and how do they interact?

(3)

The Standard Model of Elementary Particles

(4)

Niklaus Berger – Heidelberg, May 2015 – Slide 4

Hugely successful

Magnetic moment of the electron:

• Theory:

g_e = -2.002 319 304 363 56 (154)

(Aoyama et al., PRL 109, 111807 (2012))

• Experiment:

g_e = - 2.002 319 304 361 53 (53)

(Hanneke et al. PRL 100, 120801 (2008))

(5)

Open Questions?

(6)

Dark Matter

NASA: HST and Chandra

(7)

Dark Matter

NASA: HST and Chandra

(8)

Matter-Antimatter Asymmetry

10’000’000’000 Antimatter

10’000’000’001

Matter

(9)

Matter-Antimatter Asymmetry

1 Radiation Us

(10)

Gravity

(11)

The Structure of the Standard Model

(12)

The Structure of the Standard Model

τ

1 eV 1 KeV 1 MeV 1 GeV 1 TeV 10

³

TeV 10

⁶

TeV 10

⁹

TeV 10

¹²

TeV 10

¹⁵

TeV 10

¹⁸

TeV 1 meV

1 μeV

Neutrinos e u

d s

μ c

b W Z H t

Planck-Scale

(Gravity)

(13)

The Structure of the Standard Model

τ

1 eV 1 KeV 1 MeV 1 GeV 1 TeV 10³ TeV 10⁶ TeV 10⁹ TeV 10¹² TeV 10¹⁵ TeV 10¹⁸ TeV 1 meV

1 μeV

Neutrinos e u

d s μ c

b WZ Ht

Planck-Scale (Gravity)

(14)

The Structure of the Standard Model

τ

1 eV 1 KeV 1 MeV 1 GeV 1 TeV 10³ TeV 10⁶ TeV 10⁹ TeV 10¹² TeV 10¹⁵ TeV 10¹⁸ TeV 1 meV

1 μeV

Neutrinos e u

d s μ c

b WZ Ht

Planck-Scale (Gravity)

h h

t

(15)

The Structure of the Standard Model

τ

1 eV 1 KeV 1 MeV 1 GeV 1 TeV 10

³

TeV 10

⁶

TeV 10

⁹

TeV 10

¹²

TeV 10

¹⁵

TeV 10

¹⁸

TeV 1 meV

1 μeV

Neutrinos e u

d s

μ c

b W Z H t

Planck-Scale (Gravity)

LHC ? ?

(16)

Direct production

(17)

Indirect effects in quantum loops

(18)

Indirect effects in quantum loops

Large discovery reach if:

• Many incoming particles

• Long lifetime

• Little Standard Model background

(19)

Look at muons

Leptons

Large discovery reach if:

• Many incoming particles (10⁸/s)

• Long lifetime (2.2 μs)

• Little Standard Model background

(20)

Lepton Flavour

(21)

Lepton Flavour

(22)

Lepton Flavour Violation!

(23)

Charged Lepton Flavour Violation?

(24)

Heavily suppressed in the SM by (Δm

_ν²

/m

_W²

)

²

Branching fraction < 10

^-54

Charged Lepton Flavour Violation?

(25)

This

(charged lepton flavour violation) has never been seen

and not because we have not looked

(26)

History of LFV experiments

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 Comet/Mu2e

(Updated from W.J. Marciano, T. Mori and J.M. Roney,

Ann.Rev.Nucl.Part.Sci. 58, 315 (2008))

(27)

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...

(28)

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!

(29)

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!

τ

1 eV 1 KeV 1 MeV 1 GeV 1 TeV 10

³

TeV 10

⁶

TeV 10

⁹

TeV 10

¹²

TeV 10

¹⁵

TeV 10

¹⁸

TeV 1 meV

1 μeV

Neutrinos e u

d s

μ c

b W Z H t

Planck-Scale

(Gravity)

(30)

Searching for

μ

⁺

→ e

⁺

e

^-

e

⁺

at the 10

^-16

level

(31)

• We want to find or exclude μ → eee at the 10^-16 level

• 10^-15 in phase I (existing beamline)

• 10^-16 in phase II (new beamline)

• 4 orders of magnitude over previous experiment (SINDRUM 1988 - 10^-12)

The Goal: 10

^-16

1940 1960 1980 2000 2020

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

Mu3e Phase II Comet/Mu2e

(Updated from W.J. Marciano, T. Mori and J.M. Roney, Ann.Rev.Nucl.Part.Sci. 58, 315 (2008))

(32)

• Observe more than 10¹⁶ muon decays:

2 Billion muons per second

• Suppress backgrounds by more than 16 orders of magnitude

• Be sensitive for the signal

The Challenges

(33)

Muons from PSI

Paul Scherrer Institute in Villigen, Switzerland

(34)

Muons from PSI

Paul Scherrer Institute in Villigen, Switzerland World’s most intensive proton beam

2.2 mA at 590 MeV: 1.3 MW of beam power

(35)

• Rotating carbon wheel as target

• Hit with proton beam

• Produce pions

Pion production

(36)

Pion decay

• Pions decay to muons

• 2-body decay: Fixed muon momentum

• Currently: 10⁸ muons/s, more possible

(37)

Building the

Mu3e Experiment

(38)

Stop muons, let them decay

muon beam

target

(39)

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

(40)

• 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

^-

(41)

• Allowed radiative decay with internal conversion:

μ

⁺

→ e

⁺

e

^-

e

⁺

νν

• Only distinguishing feature:

Missing momentum carried by neutrinos

Internal conversion background

Branching Ratio

m_μ - E_tot (MeV)

0 1 2 3 4 5 6

10^-12

10^-16 10^-18 10^-13

10^-17 10^-15 10^-14

10^-19

• Need excellent μ3e

momentum resolution

(R. M. Djilkibaev, R. V. Konoplich, Phys.Rev. D79 (2009) 073004)

(42)

• Apply magnetic field (e.g. 1 Tesla)

• Measure curvature of particles in field

• Limited by detector resolution and scattering in detector

Momentum measurement

(43)

• Limited by detector resolution and scattering in detector

Momentum measurement

(44)

2 Billion Muon Decays/s

50 ns, 1 Tesla field

(45)

• High granularity (occupancy)

• Close to target (vertex resolution)

• 3D space points (reconstruction)

• Minimum material

(momenta below 53 MeV/c)

Detector Technology

(46)

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

(47)

• Small active region, fast charge collection via drift

Fast and thin sensors: HV-MAPS

P-substrate N-well

Particle

E field

(48)

• 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 )

(49)

HV-MAPS

3 m m

(50)

HV-MAPS

3 m m

Pixels with amplifier

40 x 32 pixels

80 x 103 μm pixel size

(51)

HV-MAPS

3 m m

Pixels with amplifier

40 x 32 pixels

80 x 103 μm pixel size

Comparator and digital pixel logic

(52)

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

(53)

Introduction

Y

• X

(54)

Introduction

Y

• X

(55)

Introduction

Y

• X

(56)

Position resolution given by pixel size

Position Resolution

(57)

Hit efficiency above 99% without tuning

Efficiency

(58)

Hit timestamp resolution better than 17 ns

(significant setup contribution in the measurement)

Time resolution

-400 -200 0 400

500 1000 1500 2000 2500 3000

Difference between trigger and timestamp [ns]200

σ = 16.6 ns

Hits per 10 ns bin Timestamp frequency 100 MHz

(59)

(60)

Building a detector thinner than a hair

(61)

Introduction

Y

• X

(62)

• 50 μm silicon

• 25 μm Kapton™ flexprint with aluminium traces

• 25 μm Kapton™ frame as support

• Less than 1‰ of a radiation length per layer

Mechanics

(63)

(64)

(65)

(66)

• Add no material:

Cool with gaseous Helium (low scattering, high mobility)

• ~ 150 mW/cm² - total 2 kW

• Simulations: Need ~ several m/s flow

Cooling

• Full scale heatable prototype built

• 36 cm active length

• No visible vibrations

• Can add local cooling

(67)

Introduction

Y

• X

(68)

Cooling tests

Global helium stream

Local helium stream

(69)

• 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

(70)

Precision vs. Acceptance

50 MeV/c 25 MeV/c 12 MeV/c B^→

33 cm

(71)

Precision vs. Acceptance

(72)

Precision vs. Acceptance

(73)

Precision vs. Acceptance

(74)

Precision vs. Acceptance

Ω ~ π MS

θ_MS

B

(75)

Detector Design

muon beam

target

(76)

Detector Design

muon beam

target

(77)

Detector Design

muon beam

target

inner pixel layers

(78)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers

(79)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers recurl pixel

layers

recurl pixel layers

(80)

Performance Simulations: Mass reconstruction

2] Reconstructed Mass [MeV/c

96 98 100 102 104 106 108 110

0 10000 20000 30000 40000 50000 60000 70000

Mu3e Phase Ib; 3 recurling tracks Mu3e Phase Ib; 3 recurling tracks Efficiency 13.44 %

Efficiency 13.44 % RMS 0.91 MeV/c2²

RMS 0.91 MeV/c 0.56 MeV/c2

σ 0.56 MeV/c² σ

Work in progress

(81)

Need suppression of accidental background:

Timing

(82)

Pixels: O(50 ns)

Timing measurements

Scintillating fibres O(1 ns);

Scintillating tiles O(100 ps)

(83)

Detector Design

scintillating ﬁbres

outer pixel layers

muon beam

target

inner pixel layers

(84)

Detector Design

outer pixel layers

muon beam

target inner pixel layers recurl pixel

layers

recurl pixel layers

Scintillating tiles

(85)

• 3-5 layers of 250 μm scintillating fibres

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

• Timing resolution O(1 ns)

(measured with sodium source)

Timing Detector: Scintillating Fibres

Single photon Efficiency > 98%

(≥ 2 photons)

(86)

Timing Detector: Scintillating tiles

• ~ 0.5 cm³ scintillating tiles

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

• KIP Heidelberg

Scin ator Tiles

SiPM Readout

Electronics

(87)

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

(88)

Performance Simulations: Signal & Background

96 98 100 102 104 106 108 110

2 Events per 100 keV/c

10-4

10-3

10-2

10-1

1

10 Internal Conversion Background

eee at 10-12

→ µ

eee at 10-13

→ µ

eee at 10-14

→ µ

eee at 10-15

→ µ

µ/s on Target; 108 15 µ

⋅ 10

Mu3e Phase Ib; 1 ⋅ 10¹⁵ µ on Target; 10⁸ µ/s Mu3e Phase Ib; 1

+ Michel e+

e-

Bhabha e+

Work in progress

(89)

Performance Simulations: Signal & Background

96 98 100 102 104 106 108 110

10-4

10-3

10-2

10-1

1 10

102 Internal Conversion Background

eee at 10-12

→ µ

eee at 10-13

→ µ

eee at 10-14

→ µ

eee at 10-15

→ µ

eee at 10-16

→ µ

⋅ 10

Mu3e Phase Ib; 1 ⋅ 10¹⁶ µ on Target; 10⁸ µ/s Mu3e Phase Ib; 1

+ Michel e+

e-

Bhabha e+

Work in progress

(90)

Data Acquisition

(91)

• 280 Million pixels (+ fibres and tiles)

• No trigger

• ~ 1 Tbit/s

• Need to find and fit billions of tracks/s

Data Acquisition

(92)

• 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

(93)

Sensitivity

Phase IA: Starting 2017

Target Inner pixel layers

Outer pixel layers μ Beam

(94)

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

Sensitivity

Phase IB: 2018+

1∙10⁸ μ/s

(95)

Sensitivity

Phase II: 2020+

New Beam Line

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

(96)

• 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 2017

• 2 billion muons/s not before 2020

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

Mu3e Phase II

(97)

Backup Material

(98)

Radiation Hardness

• Requirements not as strict as at LHC

• Irradiation at PS

• After 380 MRad (8×10¹⁵ n_eq/cm²)

• Chip still working

(Courtesy Ivan Perić, RESMDD 2012)

(99)

MUPIX electronics

(100)

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)

(101)

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)

(102)

Detector Design

outer pixel layers

muon beam

target inner pixel layers recurl pixel

layers

recurl pixel layers

Scintillating tiles

(103)

The hunt for

charged lepton flavour violation in μ-decays

(104)

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) < 5.7 ∙ 10

^-13

(2013) B(μ

^-

Au → e

^-

Au) < 7 ∙ 10

^-13

(2006) B(μ

⁺

→ e

⁺

e

^-

e

⁺

) < 1.0 ∙ 10

^-12

(1988)

(105)

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) < 5.7 ∙ 10

^-13

(2013) B(μ

^-

Au → e

^-

Au) < 7 ∙ 10

^-13

(2006) B(μ

⁺

→ e

⁺

e

^-

e

⁺

) < 1.0 ∙ 10

^-12

(1988)

upgrading Mu2e/Comet Mu3e

(106)

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

(107)

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

(108)

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

(109)

• DPNC, Geneva University

• Physics Institute, Heidelberg University

• KIP, Heidelberg University

• IPE, Karlsruhe Institute of Technology

• Paul Scherrer Institute

• Physics Institute, Zürich University

• Institute for Particle Physics, ETH Zürich

• Institute for Nuclear Physics, JGU Mainz

The Mu3e Collaboration

(110)

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

(111)

Muons from PSI

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

(112)

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

(113)

Performance Simulations: Background

96 98 100 102 104 106 108 110

10-4

10-3

10-2

10-1

1

Internal Conversion Background

eee at 10-12

→ µ

eee at 10-13

→ µ

eee at 10-14

→ µ

⋅ 10

Mu3e Phase Ia; 1 ⋅ 10¹⁴ µ on Target; 10⁷ µ/s Mu3e Phase Ia; 1

+ Michel e+

e-

Bhabha e+

Work in progress

(114)

Performance Simulations: Background

96 98 100 102 104 106 108 110

10-4

10-3

10-2

10-1

1 10

Internal Conversion Background

eee at 10-12

→ µ

eee at 10-13

→ µ

eee at 10-14

→ µ

eee at 10-15

→ µ

⋅ 10

Mu3e Phase Ia; 1 ⋅ 10¹⁵ µ on Target; 10⁸ µ/s Mu3e Phase Ia; 1

+ Michel e+

e-

Bhabha e+