Searching for Lepton-Flavour Violation with the Mu3e Experiment

Searching for Lepton-Flavour Violation with the Mu3e Experiment

Ann-Kathrin Perrevoort

Physics Institute, Heidelberg University

NuFACT Uppsala September 25, 2017

Charged Lepton Flavour Violation in µ → eee

Expectation from neutrino mixing:

BR_µ→eee ∼ (^∆m_m2^2ν

W )² <10⁻⁵⁴

Observation of µ →eee is a clear sign for New Physics

Signal and Background

Signal Background

Signalµ⁺ → e⁺e⁻e⁺ Combinatorial background

Internal conversion µ⁺ → e⁺e⁻e⁺νµνe

• Common vertex

• Coincident

• ∑Ee=m_µ

• ∑p⃗e=0

• No common vertex

• Not coincident

• ∑Ee≠m_µ

• ∑p⃗e≠0

• Common vertex

• Coincident

• ∑Ee<m_µ

• ∑p⃗e≠0

The Mu3e Experiment

SINDRUM Mu3e

BRµ→eee<1.0⋅10⁻¹² at 90%CL [1988]

Sensitivity of one in 10¹⁵(10¹⁶) µ decays

High muon stopping rates

▸ Phase I: 10⁸µ/s

▸ Phase II:>10⁹µ/s

Background suppression

▸ Very good vertex and time resolution

▸ Excellent momentum resolution

Muon Beam

Paul-Scherrer Institute

2.2 mA proton beam with 590 MeV Secondary beamlines:

sub-surface µ⁺ with 28 MeV

10⁸muons/s at existing beamline πE5 10⁹muons/s at future beamline HiMB

(under investigation)

Experimental Area

Target E Infrastructure platform I Infrastructure

platform II

Access walkway

Detector control and filter farm barracks Controlled

access door

MEG II

Existing πE5 front access

Mu3e Removable access stairway

Experimental Area

Multiple Coulomb Scattering

Ω MS

θMS

B

• Decay electrons have low momentum<53 MeV/c

• Momentum resolution is dominated by multiple scattering

σp

p ∼^θMS_Ω with θ_MS∝ ¹_p√ _x

X0

→ reduce material thickness x

→ increase opening angleΩ

Multiple Coulomb Scattering

Ω ~ π MS

θMS

B

• Decay electrons have low momentum<53 MeV/c

• Momentum resolution is dominated by multiple scattering

σp

p ∼^θMS_Ω with θ_MS∝ ¹_p√

x X0

→ reduce material thickness x

→ increase opening angleΩ atΩ≈π ⇒ ^σ_p^p ∼O(θ²_MS)

Multiple Coulomb Scattering

• Decay electrons have low momentum<53 MeV/c

• Momentum resolution is dominated by multiple scattering

σp

p ∼^θMS_Ω with θ_MS∝ ¹_p√

x X0

→ reduce material thickness x

→ increase opening angleΩ atΩ≈π ⇒ ^σ_p^p ∼O(θ²_MS)

The Detector in Phase I

Target Inner pixel layers

Outer pixel layers Recurl pixel layers

Scintillator tiles μ Beam

Tracking detector:

Thin Si pixel sensors (HV-MAPS) Stopping rate of 10⁸µ/s

B-field of 1 T

+ Timing detector:

Scintillating fibres and tiles Length: 110 cm

Diameter: 18 cm

Pixel Tracker

Measure low momentum electron tracks with excellent precision Minimize material to reduce multiple Coulomb scattering:

• Thin Si pixel sensors

• Flexible printed circuit boards

• Kapton support structure

→ 1.16 radiation lengths

• Cooling with gaseous helium

Pixel Sensors: HV-MAPS

High Voltage Monolithic Active Pixel Sensors

• AMS 180 nm HV-CMOS process

• N-well in p-substrate

• Reverse bias of∼80 V

▸ Fast charge collection via drift

▸ Depletion zone of∼ (10−20)µm Thinning possible (≲50µm)

• Integrated readout electronics

▸ Signal amplification and shaping in N-well

▸ Digitisation and zero-suppression in periphery

• Pixel size 80×80µm² Sensor size 2×2cm²

P-substrate N-well

Particle E field

I. Peri´c, NIM A 582 (2007)

Pixel Sensors: MuPix Prototype

MuPix7: HV-MAPS prototype for Mu3e

• 32×40 pixels `a 103×80µm²

• 2.9×3.2mm² of active area

• 50µm thin

• ‘System-on-chip’

• Zero-suppressed hit addresses and timestamps

Pixel Sensors: MuPix Prototype

MuPix7: HV-MAPS prototype for Mu3e

• 32×40 pixels `a 103×80µm²

• 2.9×3.2mm² of active area

• 50µm thin

• ‘System-on-chip’

• Zero-suppressed hit addresses and timestamps

Efficiency >99%

Timing resolution <20 ns

Threshold [V]

0.7 0.71 0.72 0.73 0.74 0.75

Efficiency

0.95 0.96 0.97 0.98 0.99 1

Efficiency Noise 99 %

Preliminary

Noiserate per pixel [1/s]

−1 10 1 10 102 103 104

600

− −500 −400 −300 −200 −100 0

Entries [1/run]

102 103 104

Time diff erence between hit and scintillator time [ns]

σ= 14.3 ns

Time difference between hit and scintillator time [ns]

Pixel Sensors: MuPix Prototype

Latest prototype: MuPix8

→ Arrived in August

• First large MuPix sensor: 2×1cm²

• 128×200 pixels `a 81×80µm²

• Analogue pulse information

• Different substrates:

20Ωcm and 80Ωcm

Pixel Sensors: MuPix Prototype

column 0 20 40 60 80 100 120

row

0 20 40 60 80 100 120 140 160 180

0 200 400 600 800 1000 1200

Preliminary hitmap of a Sr-90 source

Latest prototype: MuPix8

→ Arrived in August

• First large MuPix sensor: 2×1cm²

• 128×200 pixels `a 81×80µm²

• Analogue pulse information

• Different substrates:

20Ωcm and 80Ωcm

Pixel Sensors: MuPix Prototype

Latest prototype: MuPix8

→ Arrived in August

• First large MuPix sensor: 2×1cm²

• 128×200 pixels `a 81×80µm²

• Analogue pulse information

• Different substrates:

20Ωcm and 80Ωcm MuPix9 submitted in August

• Small-scale prototype

• Slow control, serial powering

Cooling

Cooling with gaseous helium Power consumption of Si pixel sensors is 250 mW/cm²

SciFi

Pixel Layers

Scintillating Tiles

water cooled beam pipe water cooled beam pipe

Scintillating Tiles Pixel Layers

gaseous helium

Cooling

Cooling with gaseous helium Power consumption of Si pixel sensors is 250 mW/cm²

Timing Detector

Suppression of combinatorial background by a factor of 100

200 300 400 500 600 700 800 900 1000 1100 fibre detector time resolution [ps] 

0 20 40 60 80 100 120

timing background (Bhabha+Michel) suppression

both timing detectors only fibres

Scintillating Fibres

Entries 24772

Mean 0.05009 ±0.008864 Std Dev 1.394 ±0.006268 Integral 2.474e+04

/ ndf

χ2 327.1 / 54

p0 191.7 ±7.7

p1 0.09037 ±0.03579 p2 2.363 ±0.047

p3 2596 ±26.8

p4 0.05652 ±0.00492 p5 0.5724 ±0.0056

10

− −5 0 5 10

Counts

0 500 1000 1500 2000 2500

3000 Entries 24772

Mean 0.05009 ±0.008864 Std Dev 1.394 ±0.006268 Integral 2.474e+04

/ ndf

χ2 327.1 / 54

p0 191.7 ±7.7

p1 0.09037 ±0.03579 p2 2.363 ±0.047

p3 2596 ±26.8

p4 0.05652 ±0.00492 p5 0.5724 ±0.0056

• 3 layers of fibres with ∅∼250µm and length of 28 to 30 cm

• Round and square fibres under investigation

• Photon detection at both ends with LHCb SiPM column array

• Readout with custom-designed MuTRiG

• Prototype with 3 layers of square multiclad fibres:

σ_t= (572±6)ps and ϵ≳95%

Scintillating Tiles

Mezzanine Board Connector 448 Channel

Module

Endring Cooling

Pipe

Scintillator Tiles

MuTRiG PCB SiPM Flexprint

Time Difference [ps]

-400 -200 0 200 400

# Entries

0 5 10 15 20 25 30

103

×

TWC No TWC ) ps

× 2 = (56 σ

) ps

× 2 = (70 σ

• 6.5×6.5×5.0mm³ tiles with individual SiPMs

• Custom-designed MuTRiG:

TDC ASIC for SiPM readout

• Prototype yields time resolution

∼70 ps and efficiencyϵ≳99.7%

Simulation Results for Phase I

2 Events per 0.2 MeV/c

−4

10

−3

10

−2

10

−1

10 1 10 102

at 10-12

→ eee µ at 10-13

→ eee µ at 10-14

→ eee µ at 10-15

→ eee µ ν

ν

→ eee µ

Bhabha +Michel

muons/s muon stops at 108

1015

Mu3e Phase I

Simulation Results for Phase I

Data taking days

0 50 100 150 200 250 300

eee)→µBR(

−15

10

−14

10

−13

10

−12

10

−11

10

10-15

× 2

SES 90% C.L. 95% C.L.

Mu3e Phase I 10⁸ muon stops/s 19.7% signal efficiency

SINDRUM 1988

Summary

Mu3e Search for LFV decayµ →eee with a sensitivity of BR<10⁻¹⁶ Low-material tracking detector

▸ High muon rates

▸ Thin Si pixel sensors

▸ Scintillating fibres and tiles

Phase I Prospected single-event sensitivity of 2⋅10⁻¹⁵ in 300 days of data taking Phase II Ultimate sensitivity with detector upgrade

and high intensity muon beamline

Status

Finalizing detector design for phase I

Preparing for construction and commissioning Pixel MuPix8 is first large scale prototype

In the lab and running

MuPix10 could be used for module building (2^nd half of 2018)

Timing Very successful prototypes for tiles and fibres

MuTRiG is being characterized

Mechanics Challenging due to tight spacial constraints Integration well advanced

Magnet Expected 1^st half of 2019

Technical design report to be published soon

Mezzanine Board Connector 448 Channel

Module Cooling

Pipe

Appendix

The Phase II Detector

Target Inner pixel layers

Outer pixel layers Recurl pixel layers

Scintillator tiles μ Beam

Thin timing detector

Increase muon stopping rate to 2⋅10⁹µ/s

Additional recurl stations increase acceptance for recurler Smaller beam profile ⇒smaller target radius

Appendix

History of cLFV Searches in µ and τ Decays

1940 1960 1980 2000 2020

Year

90%–CL bound

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

e 3e

N eN

3

10^–16

SINDRUM SINDRUM II MEG

MEG II Mu3e Phase I

Mu3e Phase II Comet/Mu2e μ

μ μ

μ μ

γ

γ τ τ

Appendix

Charged Lepton Flavour Violation

O_ee^SLL= (eP_Lµ)(eP_Le)

Crivellin, Davidson, Pruna, Signer [arXiv:1611.03409]

-10^-7 -10^-8 -10^-9 -10^-10 10^-9

10^-8 10^-7 10^-6 10^-5 10^-4 10^-3 10^-2

10^-1 SINDRUM (1988) Mu3e (BR≤10⁻¹⁵) Mu3e (BR≤10⁻¹⁶)

10^-10 10^-9 10^-8 10^-7 10^-9 10^-8 10^-7 10^-6 10^-5 10^-4 10^-3 10^-2 10^-1 MEG (2016)

MEG II (BR≤4·10⁻¹⁴)

C^DL

C^SLLee

Λ =m_Z/GeV

O_L^D=emµ(eσ^µνPLµ)Fµν

Appendix

Tracking in MS-dominated Environment

Appendix

Signal Decay µ → eee

Signature forµ decay at rest Common vertex

Coincident in time

∑E_e=m_µc²

∑p⃗_e=0

Ee= (0−53)MeV

Multiple Coulomb scattering limits momentum resolution σ_p∝√

x

Appendix

Background: Combinatorial Background

e⁺

e⁺ e^-

e⁺

e^- e⁺

+

Overlays of Michel decay µ → eνν, Bhabha scattering, photon conversion, . . .

No common vertex Not coincident

∑E_e≠m_µc²

∑p⃗_e≠0

Increases with beam intensity

Appendix

Background: µ → eeeνν

BR_µ+→e⁺e⁻e⁺νµνe = (3.4±0.4)⋅10⁻⁵[Nucl.Phys.B260, 1985]

Common vertex Coincident in time

∑E_e<m_µc²

∑p⃗_e≠0

→Missing energy due to neutrinos Need very good momentum resolution

Appendix

Background: µ → eeeνν

0 5 10 15 20 40 60

10^-18 10^-16 10^-14 10^-12 10^-10 10^-8

0.85 0.9 0.95 1

E//MeV

dB dE/ Kfactor

no cuts onE/ E/ ≤20 MeV E/ ≤10 MeV E/ ≤5 MeV K factor

NLO calculations for µ → eeeνν: Pruna, Signer, Ulrich [arXiv:1611.03617]

Appendix

Background: µ → eeeνν

10⁻¹⁵ 10⁻¹⁴ 10⁻¹³ 10⁻¹² 10⁻¹¹ 10⁻¹⁰ 10⁻⁹ 10⁻⁸

0.6 0.7 0.8 0.9

101 102 103 104 105

error band×10 dB/dm123[MeV−1]

µ⁻→e⁻(e⁺e⁻)νµν¯e

BNLO/BLO

m123[MeV]

10⁻²⁰ 10⁻¹⁸ 10⁻¹⁶ 10⁻¹⁴ 10⁻¹² 10⁻¹⁰ 10⁻⁸ 10⁻⁶ 10⁻⁴

0.7 0.8 0.9

1 10 100

error band×10

B(

/ E^max

)

µ⁻→e⁻(e⁺e⁻)νµν¯e

BNLO/BLO

E/max[MeV]

NLO calculations for µ → eeeνν: Fael, Greub arXiv:[1611.03726]

Appendix

Magnet and Detector Cage

Appendix

Target

Extended hollow double-cone target made of 75µm to 85µm mylar foil 10 cm long with a radius of 19 mm High stopping muon stopping rate Vertex separation over a large surface Low distortion for ‘escaping’ electrons

Appendix

Lightweight Mechanics

• 50µm silicon sensor

• 80µm Flexible printed circuit board (FPC)

• 25µm Kapton support structure

→ ∼0.1%of radiation length

Aluminium 14 um

Aluminium 14 um Glue 5 um Glue 5 um Polyimide 25 um Polyimide 10 um

Polyimide 10 um MuPix 50 um

Kapton frame 25 um

Bus signals Power distribution Sensor signals Ground SpTAB pads Dielectric spacer layer

{

FPC

Mechanical support

Sensor

Glue 5 um Glue 5 um

Appendix

Pixel Sensors: HV-MAPS

Pixel Periphery State Machine

readout state machine

VCO

&

PLL

8b/10b

encoder serializer LVDS ...

other pixels

sensor CSA

comparator tune

DAC

threshold baseline source

follower

test-pulse injection

readout 2^nd amplifier

integrate charge

amplification

line driver

digital output AC coupling

via CR filter per pixel threshold adjustment

Hit finding, digitisation, zero-suppression and readout on-chip Continuous and fast readout at 1.25 Gbit/s

Appendix

Data Acquisition

2844 Pixel Sensors

up to 45 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

...

12 FPGAs

6272 Tiles

FPGA FPGA

...

14 FPGAs

Data Mass

Gbit Ethernet

Switching

Board Switching

Board Switching

Board

Front-end(inside magnet)

Switching Board

Appendix

Data Acquisition

Triggerless data acquisition Front-end board

▸ Slow control

▸ Buffer and merge data

▸ Time-sorting Readout board

▸ Switch between front-end and filterfarm

▸ Merge data of sub-detectors GPU filterfarm

▸ Fast track finding and online reconstruction

▸ Reduce data rate by a factor of∼ 80

~3000 Pixel Sensors

up to 45 1.25 Gbit/s links

FPGA FPGA FPGA

...

Switching Boards

Data Collection

Server

Mass Storage Gbit Ethernet

86 FPGAs

1 6 Gbit/s link each

12 10 Gbit/s links per board

8 Inputs each

GPU PC

GPU PC

GPU 12 PCs PC Front-end boards

Switching boards

Filterfarm

Appendix

MuPix Telescope

• Tests of new prototypes and system integration

• 4 or 8 planes of MuPix7

• Scintillating tiles

• Readout via Altera Stratix IV development boards

• Test beam at PSI, DESY, SPS, MAMI

Appendix

MuPix7 Results

Testbeam at DESY: 4 GeV e⁺beam; using DESY Duranta telescope

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

Appendix

MuPix7 Results

Testbeam at DESY: 4 GeV e⁺beam; DUT rotated by 60° wrt to beam axis

Efficiency

0.984 0.986 0.988 0.99 0.992 0.994 0.996 0.998 1

Efficiency Noise 99 %

Preliminary

Noiserate per pixel [1/s]

1 10 102

Appendix

Timing Information

Tracks expected within readout frame of 50 ns

Matching with time information of scintillating fibres and tiles

Appendix

Reconstruction

h1

ϕ01 ϕ12

x y

d01

d12

ϕM S

c1 c2

rT ,01

rT ,12

h0

h2

• 3D multiple scattering fit for track reconstruction

• Spatial uncertainties of hit positions are ignored as MS dominates

• Hits in 3 layers form a ‘triplet’

• Join triplets by minimizing MS angles

• Subsequent vertex fit with 3 trajectories of correct charge

Appendix

Short Tracks: 4 Hits

Appendix

Short Tracks

[rad]

λ

−1.5 −1 −0.5 0 0.5 1 1.5

p [MeV]

0 10 20 30 40 50 60

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

[MeV/c]

pmc

0 10 20 30 40 50

[MeV/c]pσ

0 0.5 1 1.5 2 2.5 3

Appendix

Long Tracks: 6 Hits

Appendix

Long Tracks: 8 Hits

Appendix

Long Tracks

[rad]

λ

−1.5 −1 −0.5 0 0.5 1 1.5

p [MeV]

0 10 20 30 40 50 60

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Efficiency.

[MeV/c]

pmc

0 10 20 30 40 50

[MeV/c]pσ

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Momentum resolution.

Appendix

Vertex Resolution and Mass Resolution of Signal Events

[mm]

target

2 d

− −1.5 −1−0.5 0 0.5 1 1.5 2 0

20 40 60 80 100 120

103

×phase I, 3 recurlers

RMS = (0.4272 ± 0.0003) mm µ = (-0.0030 ± 0.0003) mm σ = (0.2778 ± 0.0002) mm

2] [MeV/c mrec

96 100 104 108 112

0 50 100 150 200 250

103

×

phase I, 3 recurlers

RMS = (1.2105± 0.0007) MeV/c² µ = (105.299± 0.003) MeV/c² σ = (0.618± 0.003) MeV/c²

Appendix

µ → eee: Phase Space

2] [MeV

2ee

m

0 2 4 6 8 10 12×10³

]2[MeV

2 ee

m

0 2 4 6 8 10 12

103

×

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 103

×

Generated

2] [MeV

2ee

m

0 2 4 6 8 10 12×10³

]2[MeV

2 ee

m

0 2 4 6 8 10 12

103

×

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 103

×

Truth information

after reconstruction and vertex fit

Appendix

µ → eee: Effective Operator em

A

µ

σ

e

F

2] [MeV

2ee

m

0 2 4 6 8 10 12×10³

]2[MeV

2 ee

m

0 2 4 6 8 10 12

103

×

1 10 102

103

Generated

2] [MeV

2ee

m

0 2 4 6 8 10 12×10³

]2[MeV

2 ee

m

0 2 4 6 8 10 12

103

×

1 10 102

103

Truth information

Appendix

µ → eee: Effective Operator ( µ

e

)( e

e

)

2] [MeV

2ee

m

0 2 4 6 8 10 12×10³

]2[MeV

2 ee

m

0 2 4 6 8 10 12

103

×

0 0.2 0.4 0.6 0.8 1 1.2

103

×

Generated

2] [MeV

2ee

m

0 2 4 6 8 10 12×10³

]2[MeV

2 ee

m

0 2 4 6 8 10 12

103

×

0 0.2 0.4 0.6 0.8 1 1.2

103

×

Truth information

after reconstruction and vertex fit

Appendix

µ → eee: Effective Operators

Efficiency for reconstructing a µ → eee decay Dipole operator

emµALµLσ^µνeRFµν

Dipole Phase Space Scalar 4-Fermion

Efficiency

0 0.05 0.1 0.15 0.2 0.25 0.3

4-fermion scalar operator

(µ_Le_R)(e_Le_R)

Appendix

Searching for µ → e X with Mu3e

• Emission of unobserved neutral, light boson in µ⁺ → e⁺X⁰

• Familon: Goldstone boson of SSB of flavour symmetry [Wilczek, 1982]

• Search for a peak on the e⁺ momentum spectrum

μ⁺

e⁺

X⁰

[MeV]

Ee

10 20 30 40 50

0 0.2 0.4 0.6 0.8 1

Michel spectrum μ->eX signal

Appendix

Searching for µ → e X with Mu3e

Sensitivity to µ → e X for 1⋅10¹⁵ muon stops using a toy MC study

Familon Mass [MeV]

30 40 50 60 70 80 90

Branching Fraction

−9 10

−8 10

−7 10

−6 10

−5 10

−4

10 90% CL

4-hit 6-&8-hit TWIST

TWIST results by courtesy of R. Bayes [arXiv:1409.0638]

Appendix

Mu3e Collaboration

Founding members:

University of Geneva Heidelberg University

Karlsruhe Institute of Technology JGU Mainz

Paul Scherrer Institute ETH Z¨urich

University of Z¨urich In the process of joining:

University of Bristol University of Liverpool University College London University of Oxford