Status of the Mu3e Experiment at PSI

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Ann-Kathrin Perrevoort

on behalf of the Mu3e Collaboration

Physikalisches Institut, Heidelberg

FCCP, September 12, 2015, Anacapri

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The Mu3e Experiment

Searching for the lepton flavour violating decayµ⁺ → e⁺e⁻e⁺ In this talk

• Introduction to Mu3e

• Experimental Concept

• Current Status and Outlook

A. Perrevoort (Heidelberg) Mu3e FCCP 2015 2 / 20

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Searching for New Physics in the Decayµ→eee

Lepton Flavour conserved in Standard Model

. . . butνoscillations

Expectation from lepton mixing: BR_µ_→eee∼(^∆_m^m_W^ν)⁴<10⁻⁵⁴

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Charged Lepton Flavour Violation

Observation ofµ →eee is a clear sign for New Physics

SUSY, extra heavy vector bosons (Z’), . . .

Current limit: BR_µ_→_eee<1.0⋅10⁻¹²at 90 % CL[SINDRUM, 1988]

Mu3e: New experiment sensitive to BR’s of 10⁻¹⁵(10⁻¹⁶)

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300 400 500 600 700 800 1000900 2000 3000 4000 5000 6000 7000

10^-2 10^-1 1 10 10²

!

" (TeV)

EXCLUDED (90% CL) B(µ # e$)=10^-13

B(µ # e$)=10^-14

B(µ # eee)=10^-14 B(µ # eee)=10^-16

LCLFV=[₍_κ^m₊₁^µ₎_Λ2µ_Rσ_µνe_LF^µν]

dipole-like+[₍_κ₊^κ₁₎_Λ2(µ_Lγ_µe_L)(e_Lγ^µe_L)]

four-fermion

A. Gouv ˆea, P. Vogel, Prog.Part.Nucl.Phys. 71 (2013)

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Signal and Background

Signal Background

Signalµ⁺ → e⁺e⁻e⁺ Common vertex Coincident

∑E_e=m_µ

∑⃗pe=0

Accidental combinations No common vertex Not coincident

∑E_e≠m_µ

∑p⃗e≠0

Internal conversion µ⁺ → e⁺e⁻e⁺νµνe

Common vertex Coincident

∑E_e<m_µ

∑⃗pe≠0

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Background

0 1 2 3 4 5 6

−19 10

−18 10

−17 10

−16 10

−15 10

−14 10

−13 10

−12 10

mμ-E_tot[MeV]

Branching Ratio

Internal conversion µ⁺ → e⁺e⁻e⁺νµνe

Common vertex Coincident

∑E_e<m_µ

∑⃗pe≠0

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Signal and Background

Signal Background

Detector requirements:

• Very good vertex (∼200 µm) and time resolution (∼100 ps)

• Excellent momentum resolution (∼0.5 MeV)

• Minimal material amount

+ High muon stopping rates (10⁸to 10⁹muons/s)

Thin silicon pixel sensors + Scintillating fibres/tiles

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Phase I: Detector Configuration A

Tracking detector with Si pixel sensors

Phase IA 10⁷muons/s

BR∼10⁻¹⁴ 2017

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Experimental Concept

Muon Beam

Paul-Scherrer Institute in Switzerland

2.2 mA proton beam with 590 MeV Secondary beamlines:µ⁺with 28 MeV 10⁸muons/s at existing beamline 10⁹muons/s at future beamline

under investigation

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High-Voltage Monolithic Active Pixel Sensors

P-substrate N-well

Particle E field

I. Peri´c, NIM A 582 (2007)

Pixel Periphery

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

• High voltage of>60 V

• Fast charge collection via drift

• Depletion zone of∼10 µm Thinning possible (≲50 µm)

• Integrated readout electronics

• Pixel size 80×80µm² Sensor size 2×2cm² Thin and granular

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Experimental Concept

High-Voltage Monolithic Active Pixel Sensors

Schwellwert [V]

0.6 0.8 1 1.2 1.4 1.6

Effizienz

0.85096 0.85098 0.851 0.85102 0.85104 0.85106

0.7 0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78

0.75 0.8 0.85 0.9 0.95

1 98 %

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Efficiency

Threshold [V]

Mean noise rate per pixel [1/s]

Latest prototype: MuPix7

• Pixel size 103×80µm² Sensor size 2.9×3.2mm²

• Zero-suppressed hit addresses and timestamps via fast serial link

• Successfully tested in lab and in testbeam campaigns

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Lightweight Mechanics

• 50 µm silicon sensor

• 100 µm Kapton flexprint with aluminum traces

• 25 µm Kapton support structure

→ ∼1 ‰ of radiation length

Cooling with gaseous helium

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Experimental Concept

Data Acquisition

Triggerless data acquisition Front-end board

• Buffer and merge data ofO(¹⁵)^sensors

• Time-sorting

• Slow control

Switching board

• Switch between front-end and filterfarm

• Merge data of sub-detectors

GPU filterfarm

• Fast track finding and online reconstruction

• Reduce data rate from

∼1 Tbit/s to∼^{100 MB/s}

~ 1000 Pixel Sensors

up to 45 1.25 Gbit/s links

FPGA FPGA FPGA

...

Switching Boards 1 6.4 Gbit/s link each

GPU PC

GPU 12 PCs PC ...

12 6.4 Gbit/s links per RO Board 4 Inputs each

Data Collection

Server

Mass Storage Gbit Ethernet

2 Switching boards

~40 FPGAs Front-end boards

Switching boards

Filterfarm

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Phase I

Increase muon rate to 10⁸muons/s Ð→ precise time measurement required

Tracks expected within readout frame of 50 ns

Matching with time information of scintillating fibres and tiles

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Experimental Concept

Phase I: Detector Configuration B

Scintillators improve time resolution

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Improve momentum resolution by measuring re-curling particles Increase acceptance for re-curlers

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Experimental Concept

Phase IB 10⁸muons/s

BR∼10⁻¹⁵ 2018

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Scintillating Fibres

-4 -3 -2 -1 0 1 2 3 4

Events/200ps

0 1000 2000 3000 4000 5000

σ = (412 x √2)ps

t1-t2[ns]

Time resolution of squared fibres

• ∼3 layers of fibres with diameter of 250 µm

• Round and squared fibres under investigation

• Photon detection at both ends with SiPM array

• Readout with custom-designed STiC chip

• Time resolution:

σ_round

√2 ≈1.5 ns

σ_squared

√2 ≤500 ps

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Experimental Concept

Scintillating Tiles

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

• Size∼1×1×1cm³

• Each tile has a SiPM

• Readout with custom-designed STiC chip

• Time resolution≲100 ps

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Phase II

Phase II 10⁹muons/s at future beamline BR∼10⁻¹⁶

2020+

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Sensitivity Studies

Reconstructed mass for signal and background events Phase IB

Reconstructed Mass [MeV]

96 98 100 102 104 106 108 110

Events per 100 keV

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

+ Michel e+

e-

Bhabha e+

Mu3e: 1·10¹⁵ μ on Target; Rate 10⁸ μ/s

SIMULATION

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Mu3e

Precision experiment searching for LFV decayµ → eee Aiming at a sensitivity of BR∼10⁻¹⁵(10⁻¹⁶)

Lightweight pixel detector made of HV-MAPS Precise timing by scintillating fibres/tiles Triggerless readout

Operated at 10⁸muons/s

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Status and Outlook

Tests of HV-MAPS prototype

Mechanical prototype

Current status

Research proposal approved in 2013

Technical design report in preparation(Q1 2016) Research and development of subsystems Preparation of detector construction Outlook

Commissioning and first data in 2017 Phase IA: BR∼10⁻¹⁴(2017)

Phase IB: BR∼10⁻¹⁵(2018) Phase II : BR∼10⁻¹⁶(2020+)

requires muon rates of 10⁹muon/s

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Appendix

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History of LFV 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

Adapted from Marciano et al. [Ann.Rev.Nucl.Part.Sci.58, 2008]

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Multiple Coulomb Scattering

Ω MS

θ

MS

B

Decay electrons have low momentum<53 MeV/c

Momentum resolution is dominated by multiple scattering

σ p ∼^θ_Ω^MS θ_MS∝ _β¹_cp√

x X₀

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Ω ~ π MS

θ_MS

B

Decay electrons have low momentum<53 MeV/c

Momentum resolution is dominated by multiple scattering

σ p ∼^θ_Ω^MS θ_MS∝ _β¹_cp√

x X₀

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Layout of MuPix7

Periphery Pixel Matrix

Readout Control

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...

4860 Pixel Sensors

up to 56 1250 Mbit/s links

FPGA FPGA FPGA

...

82 FPGAs

RO Board

RO Board 1 6 Gbit/s

link each

Group A Group B Group C Group D

GPU PC

GPU PC 12 PCs

Subfarm A ...

12 10 Gbit/s links per RO Board 8 Inputs each

GPU PC

GPU PC 12 PCs

Subfarm D 4 Subfarms

~ 4000 Fibres

FPGA FPGA

...

16 FPGAs

~ 7000 Tiles

FPGA FPGA

...

14 FPGAs

RO Board

RO Board Group A Group B Group C Group D

RO Board

RO Board Group A Group B Group C Group D

Data Collection

Server

Mass Storage Gbit Ethernet

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Mu3e Collaboration

DPNC, Geneva University KIP, Heidelberg University

Physics Institute, Heidelberg University IPE, Karlsruhe Institute of Technology Institute for Nuclear Physics, JGU Mainz Paul Scherrer Institute

Institute for Particle Physics, ETH Z ¨urich Physics Institute, Z ¨urich University

Status of the Mu3e Experiment at PSI

The Mu3e Experiment

Charged Lepton Flavour Violation

Signal and Background

Signal and Background

Experimental Concept

Experimental Concept

Experimental Concept

Experimental Concept

Experimental Concept

Experimental Concept

Sensitivity Studies

SIMULATION

Status and Outlook

Appendix

History of LFV Searches in µ and τ Decays

Multiple Coulomb Scattering

Ω MS

θ

B

Layout of MuPix7

Mu3e Collaboration

History of LFV Searches in µ ^and τ ^Decays