The Mu3e Experiment

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Mu3e Experiment The

Niklaus Berger

PI Palaver,

April 2013

(2)

• The Challenge:

Finding one in 10

¹⁶

muon decays

• The Technology:

High Voltage Monolithic Active Pixel Sensors

• The Mu3e Detector:

Minimum Material, Maximum Precision

Overview

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Niklaus Berger – PI Palaver, April 2013 – Slide 3

All there, works beautifully, but...

• Why three generations?

• Why the mixing patterns between generations?

• Is there more to it?

(the dark universe...)

The Standard Model of Elementary Particles

e

^-

ν

_e

μ

^-

ν

_μ

τ

^-

ν

_τ

u c t

d s b

γ g Z W

^+/-

Higgs

(4)

All there, works beautifully, but...

• Why three generations?

• Why the mixing patterns between generations?

• Is there more to it?

(the dark universe...)

The Standard Model of Elementary Particles

e

^-

ν

μ

^-

ν

τ

^-

u c t

d s b

γ g Z W

^+/-

Higgs

Leptons

(5)

Lepton Bookkeeping

Normal Muon Decay:

μ

^-

e

^-

ν

_e

ν

_μ

(6)

Lepton Bookkeeping

Normal Muon Decay:

μ

^-

Muon number 1

e

^-

Electron number 1

ν

_e

Electron number -1

ν

_μ

Muon number 1

(7)

Lepton Bookkeeping

Normal Muon Decay:

μ

^-

Muon number 1

e

^-

Electron number 1

ν

_e

Electron number -1

ν

_μ

Muon number 1 Before:

Muons 1 Elektrons 0

After:

Muons 1

Electrons 0

(8)

Cooked books?

ν

_e

Electron number 1 ν

_μ

Muon number 1

(9)

Cooked books?

How about charged leptons (Muons)?

μ

⁺

Muon number -1

e

⁺

Electron number -1

Before:

Muons -1 Electrons 0

After:

Muons 0 Electrons -1

e

^-

Electron number 1 e

⁺

Electron number -1

(10)

This

(charged lepton flavour violation)

has never been seen

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• Neutrinos have mass

• Leptons do change flavour

• However: Standard Model

branching ratio for μ → eee < 10^-50

Charged Lepton Flavour Violation

µ ⁺ e ⁺

W +

ν _µ ν _e

γ

e ^- e ⁺

*

(12)

• Neutrinos have mass

• Leptons do change flavour

• However: Standard Model

branching ratio for μ → eee < 10^-50

• Can be much bigger with new physics

Charged Lepton Flavour Violation

µ ⁺ χ ~ ⁰ e ⁺

µ e~

~

γ

e ^- e ⁺

*/Z

(13)

• 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

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

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

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Search with SINDRUM (1988)

Less than one in 10

¹²

muon decays is to three electrons Corresponding to one gray hair in the population of

Baden-Württemberg

(15)

Our goal

Check whether more than one in 10

¹⁶

muon decays is to three electrons

Corresponding to one gray hair in all humans that ever

lived

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• Observe more than 10

¹⁶

muon decays:

2 Billion muons per second

• Suppress backgrounds by more than 16 orders of magnitude

• Be sensitive for the signal

The Challenges

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Muons from PSI

• The Paul Scherrer Institut (PSI) in Villigen, Switzerland has the world’s most

powerful DC proton beam (2.2 mA at 590 MeV)

• Pions and then muons are produced in rotating carbon targets

(18)

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

^-

(20)

• 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 - E (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,

(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)

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

• Invented by Ivan Peric at ZITI Mannheim

(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:

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

(25)

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MUPIX3 Set-up

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Introduction

Y

• X

(28)

• 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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(30)

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(32)

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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

(34)

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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

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Precision vs. Acceptance

50 MeV/c 25 MeV/c 12 MeV/c B^→

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Precision vs. Acceptance

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Precision vs. Acceptance

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Precision vs. Acceptance

(40)

Precision vs. Acceptance

Ω ~ π MS

θ_MS

B

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Detector Design

Target μ Beam

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Detector Design

Target Inner pixel layers

μ Beam

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Detector Design

Outer pixel layers μ Beam

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Detector Design

Scintillating fibres

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Detector Design

Outer pixel layers Recurl pixel layers

μ Beam

(46)

Detector Design

Scintillator tiles

μ Beam

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• 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

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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

(49)

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

(50)

Sensitivity

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Sensitivity

Phase IB: 2016+

Scintillator tiles

μ Beam

(52)

Sensitivity

Phase II: 2017+

Scintillator tiles

μ Beam

(53)

A collaboration has formed and submitted a research proposal to PSI

• University of Geneva

• University of Heidelberg: PI and KIP

• Paul Scherrer Institut (PSI)

• University of Zurich

• ETH Zurich

Also in contact with other interested groups

Collaboration

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

• First large scale use of HV-MAPS

• Build detector layers thinner than a hair

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

• Start taking first data in 2015

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

(55)

Backup Material

(56)

• 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)

Comparison with μ → eγ

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

µ⁺ χ~⁰ e⁺

µ e~

~

γ

e^- e⁺

*/Z µ⁺

e⁺ e^-

e⁺ Z’

(57)

• Z-Penguins can be important

• Lots of ongoing theory activity

Comparison with μ → eγ

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

µ⁺ χ~⁰ e⁺

µ e~

~

γ

e^- e⁺

*/Z

(58)

Radiation Hardness

• Requirements not as strict as at LHC

• Irradiation at PS

• After 380 MRad (8×10¹⁵ n_eq/cm²)

• Chip still working

(59)

MUPIX electronics

(60)

• Inductively heated sample

• Helium flow cooling

More on Cooling

(61)

• 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

s1

: 0.71 MeV/c2

s2

: 0.37 MeV/c2

sav

(62)

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)

• March test beam data (DESY, electrons) currently being analysed

• Next beam week in June Resolution for 30 × 40 μm pixels

(63)

• 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