The Mu3e Experiment

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The Mu3e Experiment

Niklaus Berger

Physics Institute, University of Heidelberg Charged Lepton Flavour Violation Workshop,

Lecce, May 2013

(2)

• The Question:

Can we observe charged lepton flavour violation?

• The Challenge:

Finding one in 10

¹⁶

muon decays

• The Mu3e Detector:

Minimum Material, Maximum Precision

Overview

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The hunt for

charged lepton flavour violation

(4)

History of LFV experiments

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,

(5)

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

⁺

) < 5.7 ∙ 10

^-13

(1988)

running

(6)

LFV Muon Decays: Standard Model

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Branching ratios suppressed by ∝ m (Δm

²_ν

)

²

≈ 10

^-50

(7)

LFV Muon Decays: Susy Loops

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Coherent conversion in

nucleus field for Q

²

(γ) ~ 0 Suppressed by extra

vertex w.r.t. μ → eγ

(8)

LFV Muon Decays: Susy Loops

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Coherent conversion in Suppressed by extra

SUSY - like many BSM models - naturally induces LFV

(9)

LFV Muon Decays: Susy Loops

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Coherent conversion in

nucleus field for Q

²

(γ) ~ 0 Suppressed by extra vertex w.r.t. μ → eγ

SUSY - like many BSM models - naturally induces LFV LFV in μ

⁺

→ e

⁺

γ implies LFV also in μ

^-

N → e

^-

N and

μ

⁺

→ e

⁺

e

^-

e

⁺

(10)

LFV Muon Decays: Tree diagrams

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

e.g. Leptoquarks e.g. extra Z’, LFV Higgs etc.

Not allowed

(11)

from dimensional analysis:

BR ∝ m_μ⁴ λ⁴

Z-Penguin diagrams in μ

⁺

→ e

⁺

e

^-

e

⁺

BR ∝ m_μ⁴ m_Z⁴

No decoupling in some models

(12)

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

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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

Kinematics

• 3-body decay

• Σ p_i = 0 Background

• Radiative decay

(14)

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

Kinematics

• 3-body decay

• Σ p_i = 0 Background

• Radiative decay

Con tinuous Be am

Con tinuous Be am Pul sed Be

am

(15)

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...

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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)

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Searching for

μ

⁺

→ e

⁺

e

^-

e

⁺

at the 10

^-16

level

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The Mu3e experiment at PSI

Search for μ

⁺

→ e

⁺

e

^-

e

⁺

Aim for sensitivity

• 10^-15 in phase I

• 10^-16 in phase II

Project approved in January 2013

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• DPNC, Geneva University

• Physics Institute, Heidelberg University

• KIP, Heidelberg University

• ZITI Mannheim, Heidelberg University

• Paul Scherrer Institute

• Physics Institute, Zürich University

• Institute for Particle Physics, ETH Zürich

The Mu3e Collaboration

(20)

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

• 4 orders of magnitude over previous experiment (SINDRUM 1988)

The Goal: 10

^-16

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

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

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• 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

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

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

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Muons from PSI

DC muon beams at PSI:

• πE5 beamline: ~ 10⁸ muons/s

(MEG experiment, Mu3e phase I)

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Muons from PSI

DC muon beams at PSI:

• πE5 beamline: ~ 10⁸ muons/s

(MEG experiment, Mu3e phase I)

• SINQ (spallation neutron source) target could even provide

~ 5 × 10¹⁰ muons/s High intensity muon beamline (HIMB) proposal

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

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• Muon rates in excess of 10¹⁰/s in acceptance

• 2∙10⁹/s needed for μ → eee at 10^-16

• Not before 2017

The High-Intensity Muon Beamline (HIMB)

Protons

Muons

SINQ Target Neutra Area

Access Shaft Access

Shaft

Pb + Zr + D₂O

Vacuum Al

D₂O

25 cm

(25)

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

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• 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

^-

(27)

• Allowed radiative decay with internal conversion:

μ

⁺

→ e

⁺

e

^-

e

⁺

νν

• Only distinguishing feature:

Missing momentum carried by neutrinos

Internal conversion background

µ⁺ ν_μ

e⁺

e^- e⁺ ν_e

γ*

W⁺

}

^E^miss

}

^E^tot

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)

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Building the

Mu3e Experiment

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• 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

(30)

High voltage monolithic active pixel sensors

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

• Use a high voltage commercial process (automotive industry)

• Small active region, fast charge collection via drift

• Can be thinned down to < 50 μm

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

Fast and thin sensors: HV-MAPS

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HV-MAPS chips: AMS 180 nm HV-CMOS

• MUPIX2:

Characterization during 2012

Single pixel Time-Over-Threshold Binary pixel matrix

• MUPIX3:

Just bonded

Column logic with address generation Extensive test beam campaign 2013

The MUPIX chips

MUPIX2

36 x 42 pixels

30 x 39 μm pixel size 1.8 mm² active area MUPIX3

40 x 32 pixels

80 x 92 μm pixel size 9.4 mm² active area For Mu3e:

256 x 256 pixels

80 x 80 μm pixel size 4 cm² area, 95% active

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• Measurements with ⁵⁵Fe source

• Good energy measurement

• Very good signal to noise

Details in theses:

A.K. Perrevoort: Characterization of HV-MAPS for Mu3e (Master thesis, 2012)

H. Augustin: Charakterisierung von HV-MAPS (Bachelor thesis, 2012)

MUPIX 2 Results

ToT [µs]

0 1 2 3 4 5

10-4

10-3

10-2

10-1

1 55Fe peak

SNR

5 10 15 20 25 30 35 40

Signal to Noise

(33)

• Measurements with LED pulses

• High-Voltage important for fast signal

• Amplification above ~70 V

Details in theses:

A.K. Perrevoort: Characterization of HV-MAPS for Mu3e (Master thesis, 2012)

H. Augustin: Charakterisierung von HV-MAPS (Bachelor thesis, 2012)

available from www.psi.ch/mu3e

MUPIX 2 Results

HV [V]

0 20 40 60 80

Latency [ns]

300 350 400 450 500 550 600 650 700

ToT [µs]

4 5 6 7 8 9 10

11 12

HV [V]

0 20 40 60 80

(34)

MUPIX 2 results

• Test beam at CERN SPS (170 GeV/c pions)

• Timepix telescope

• 2 hours data taking

• Mostly single pixel clusters

• Resolution as expected (pixel size/√12)

• More test beam data under study Resolution for 30 × 40 μm pixels

(35)

Introduction

Y

• X

(36)

• 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

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• Add no material:

Cool with gaseous Helium

• ~ 150 mW/cm² - total 2 kW

• Simulations: Need ~ 1 m/s flow

• First measurements: Need several m/s

• Full scale prototype on the way

Cooling

Details in thesis:

M. Zimmermann: Cooling with Gaseous Helium for the Mu3e Experiment

(Bachelor thesis, 2012)

available from www.psi.ch/mu3e

(40)

Introduction

Y

• X

(41)

• 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

(42)

Precision vs. Acceptance

50 MeV/c 25 MeV/c 12 MeV/c B^→

(43)

Precision vs. Acceptance

(44)

Precision vs. Acceptance

(45)

Precision vs. Acceptance

(46)

Precision vs. Acceptance

Ω ~ π MS

θ_MS

B

(47)

Detector Design

Target μ Beam

(48)

Detector Design

Target Inner pixel layers μ Beam

(49)

Detector Design

Target Inner pixel layers

Outer pixel layers μ Beam

(50)

Detector Design

Scintillating fibres

(51)

Detector Design

Outer pixel layers Recurl pixel layers

μ Beam

(52)

Detector Design

Scintillator tiles

μ Beam

(53)

• 3-5 layers of 250 μm scintillating fibres

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

• Timing resolution O(1 ns)

Timing Detector: Scintillating Fibres

x hitpos [mm]

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

y hitpos [mm]

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

0 10 20 30 40 50 60 70 80 90

(54)

Timing Detector: Scintillating tiles

• ~ 1 cm³ scintillating tiles

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

• Timing resolution O(100 ps)

p

(55)

• 280 Million pixels (+ fibres and tiles)

• No trigger

• ~ 1 Tbit/s

• FPGA-based switching network

• O(50) PCs with GPUs

Data Acquisition

Pixel Sensors

up to 108 800 Mbit/s links

FPGA FPGA FPGA

...

RO Boards 1 3 Gbit/s

link each

GPU

PC GPU

PC

GPU ... PC

12 10 Gbit/s ...

links per RO Board 4 Inputs each

Data Collection

Server

Mass Storage Gbit Ethernet

Pixel DAQ

(56)

Online software filter farm

• Continuous front-end readout (no trigger)

• ~ 1 Tbit/s

• PCs with FPGAs and 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

(57)

• 3D multiple scattering track fit

• Simulation results:

280 keV single track momentum 520 keV total mass resolution

Simulated Performance

Hits fitted per track

0 1 2 3 4 5 6 7 8 9

103

104

Reconstructed Momentum [MeV/c]

0 10 20 30 40 50 60

1 10 102

103

Rec. Momentum - Gen. Momentum [MeV/c]

-3 -2 -1 0 1 2 3

1 10 102

103

104 RMS: 0.28 MeV/c

Reconstructed track polar angle

0 0.5 1 1.5 2 2.5 3

1 10 102

103

2] Reconstructed Mass [MeV/c

1020 103 104 105 106 107 108 109 110 200

400 600 800 1000 1200 1400 1600

RMS: 0.52 MeV/c2

: 0.31 MeV/c2

σ1

: 0.71 MeV/c2

σ2

: 0.37 MeV/c2

σav

(58)

Simulated Performance

2] Reconstructed Mass [MeV/c

101 102 103 104 105 106

Events per muon decay and 0.1 MeV

10-20

10-19

10-18

10-17

10-16

10-15

10-14

10-13

10-12

10-11

10-10 _µ_→_eee_ν_ν_generated

simulated ν

ν

→ eee µ

Signal BF 10-12

Signal BF 10-13

Signal BF 10-14

Signal BF 10-15

Signal BF 10-16

Signal BF 10-17

(59)

Sensitivity

Phase IA: Starting 2015

(60)

Sensitivity

Scintillator tiles

μ Beam

(61)

Sensitivity

Phase II: 2017+

New Beam Line

Scintillator tiles

μ Beam

(62)

• 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 2015

• 2 billion muons/s from HIMB after 2017

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

(63)

Backup Material

(64)

Radiation Hardness

• Requirements not as strict as at LHC

• Irradiation at PS

• After 380 MRad (8×10¹⁵ n_eq/cm²)

• Chip still working

(65)

MUPIX electronics

(66)

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,