The Mu3e Experiment: New Physics in Different Places?

The Mu3e Experiment:

New Physics in Different Places?

Moritz Kiehn Université de Genève

Département de physique nucléaire et corpusculaire

DPNC Seminar, Genève, February 2017

Overview 1

1. Charged lepton flavor violation 2. Signal and background

3. Detector concept

4. Technologies

5. Reconstruction

6. Summary

Flavor in the Standard Model 2

u

up 2.4 MeV/c

⅔

½

c

charm 1.27 GeV/c

⅔

½

t

top 171.2 GeV/c

⅔

½

down

d

4.8 MeV/c

-⅓

½

s

strange 104 MeV/c

½

-⅓

b

bottom 4.2 GeV/c

½ -⅓

ν e

<2.2 eV/c 0

½

ν μ

<2.2 eV/c² 0

½

ν τ

<2.2 eV/c² 0

½

e

electron 0.511 MeV/c -1

½

μ

muon 105.7 MeV/c

½

-1

τ

tau 1.777 GeV/c

½ -1

γ

photon 0 0 1

g

gluon 0 1 0

Z

91.2 GeV/c 0 1

80.4 GeV/c 1

±1 mass→

spin→

charge→

QuarksLeptons Gauge Bosons

I II III

name→

electron neutrino

muon neutrino

tau neutrino

Z boson

W boson Three Generations

of Matter (Fermions)

μ W

0

±

2 2 2

2 2 2

2

2 2 2 2

2

adapted from Wikipedia

Initially:

• Quark transitions via weak interaction

• Lepton flavor conserved

Neutrino Mixing

• LFV in neutral sector

• Charged sector?

Anything else?

Charged lepton flavor violation? 3

e ⁺ γ /Z W ⁺

ν _µ ν _e

e ⁺

µ ⁺

e ⁻

Example: µ ⁺ → e ⁺ e ⁻ e ⁺

In the Standard Model

• Via neutrino mixing

• Suppressed by ∼ _{∆ m}

m

2

• Expected BR( µ → eee ) 10 ^{− 50}

Importance

• Observable rate only from new physics

• Sensitive new physics search

Searches for charged lepton flavor violation 4

• Long history

• Multiple future experiments planned

Beyond the Standard Model 5

E.g at loop level

• Supersymmetry

• Seesaw

Contact-like

• Extra dimensions

• New heavy bosons

µ ⁺ e ⁺

e ⁺ γ /Z

e ⁻

e ⁺

e ⁺

µ ⁺ e ⁻

Effective theory 6

Dipole-like

µ ⁺ e ⁺

e ⁺ γ /Z

e ⁻

Contact-like

e ⁺ e ⁺

µ ⁺ e ⁻

Sensitive up to O( 1000 TeV )

Searches with muons 7

µ ⁺ → e ⁺ γ

MEG upgrade

µ ⁻ + Au → e ⁻ + Au

Comet/Mu2e

µ ⁺ → e ⁺ e ⁻ e ⁺

Mu3e: this talk

Current Limits 8

cLFV Process BR @ 90 %CL Experiment

µ ⁺ → e ⁺ e ⁻ e ⁺ < 1 × ₁₀ ^{− 12} _Sindrum

µ ⁺ → e ⁺ γ < 5 . 7 × 10 ^{− 13} MEG

µ ⁻ + Au → e ⁻ + Au < 7 × ₁₀ ^{− 13} _{Sindrum II}

The Mu3e experiment 9

Search for µ ⁺ → e ⁺ e ⁻ e ⁺

Planned sensitivity:

• Phase I: 2 in 10 ¹⁵ decays (existing beamline)

• Phase II: 1 in 10 ¹⁶ decays (future beamline)

4 orders of magnitude over previous experiment

(SINDRUM 1988)

The Mu3e collaboration 10

Paul Scherrer Institute Université de Genève ETH Zürich

University Zürich Heidelberg University

Karlsruhe Institute of Technology

Mainz University

Signal 11 e ⁺

e ⁺ e ^-

• Common vertex

• Same time

• ( Í

P _i ) ² = m ² _µ

• Í _p ® _i = 0 (muon at rest)

• p < 53 MeV

Internal conversion background 12

e ^- e ⁺

e ^{+ ν}

ν

• Common vertex

• Same time

• ( Í _P

i ) ² < m ² _µ

• Í

p ® _i , 0

• p < 53 MeV

→ Requires excellent momentum resolution

Internal conversion background 12

Djilkibaev, Konoplich, Phys.Rev.D79, 2009

• Common vertex

• Same time

• ( Í

P _i ) ² < m ² _µ

• Í

p ® _i , 0

• p < 53 MeV

→ Requires excellent momentum resolution

Combinatorial background 13

e ⁺

e ⁺ e ^-

• from Michel decay, Bhabba scattering, photon conversion, …

• No common vertex

• Not same time

→ Requires good vertex resolution

→ Requires good time resolution

Multiple scattering 14

Ω MS

θ _MS

B

θ _MS ∼ ¹ _p p

x / X ₀

Mu3e example

• p = 35 MeV / c

• 50 µm Si

• Ω R = 5 cm

→ ∆ y ≈ _{320 µm}

→ Scattering dominates

Detector requirements 15

Environment

• High rate: > 10 ⁹ µ ⁺ Decays / s

• Low momentum: p < 53 MeV

• Multiple scattering dominates

Detector

• Spatial resolution: < 100 µm

• Time resolution: < 1 ns

• Low mass: x / X ₀ ∼ _{1 ‰}

• Momentum resolution: 0 . 5 MeV

e ⁺

e ⁺

e ^-

Detector Layout 16

Question:

Acceptance vs. resolution

Detector Layout 16

Question:

Acceptance vs. resolution

Detector Layout 16

Question:

Acceptance vs. resolution

Detector Layout 16

Question:

Acceptance vs. resolution

Answer: both

Recurling tracks 17

Ω ~ π MS

θ

B

Momentum resolution dominated by multiple scattering

σ _p p ∼ ^θ _Ω ^MS

with θ _MS ∼ ¹ _p p

x / X ₀

Uncertainty vanishes at Ω ∼ π

(first order)

Detector Concept 18

Target

̀ Beam

• > 10 ⁹ µ ⁺ Decays / s

• Electrons p < 53 MeV

• Multiple scattering dominates

Detector Concept 18

Target Inner pixel layers

̀ Beam

• > 10 ⁹ µ ⁺ Decays / s

• Electrons p < 53 MeV

Detector Concept 18

Target Inner pixel layers

Scintillating fibres

Outer pixel layers

̀ Beam

• > 10 ⁹ µ ⁺ Decays / s

• Electrons p < 53 MeV

• Multiple scattering dominates

Detector Concept 18

Target Inner pixel layers

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

̀ Beam

• > 10 ⁹ µ ⁺ Decays / s

• Electrons p < 53 MeV

Paul Scherrer Institut 19

Paul Scherrer Institut

Villigen, Switzerland

Proton accelerator 20

Proton accelerator

2 . 2 mA at 590 MeV

Continuous beam

Experimental area and beamline 21

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

π E5 beamline

∼ 28 MeV surface muons

Shared with MEG

Experimental area and beamline 21

PiE5 Channel AST

Dipole ASC Dipole Split

Triplet Triplet I Triplet II

Mu3e

solenoid Mu3e

solenoid QSM Singlet ASK dipole

QSO Doublet

ASL dipole

Triplet II Collimator

4000 3500 3000 2500 2000 1500 1000 500 0

Normalized Rate Z- Branch Momentum Spectrum

25 26 27 28 29 30 31 32 33

Muon Momentum [MeV/c]

π E5 beamline

Target 22

100 mm

38 mm

19 mm 20.8°

m Mylar μ 5 8 Mylar

m μ 5 7

Simulated stopping distribution

z of muon stop [cm]

−40 −20 0 20 40

Entries

0 100 200 300 400 500 600 700 800 900

Thin, hollow, double-cone geometry

Optimized stopping power

Ultra-lightweight mechanics 23

• 50 µm Silicon sensor

• 75 µm Kapton flexprint

• 25 µm Kapton support frame

Ultra-lightweight mechanics 24

Outer layers Outer layer module

4mm

6mm HDI ~100µm

Mupix sensor 50µm tap-bonds

Mupix periphery polyimide 15µm

V-shaped groove for stability and cooling

Mechanical prototype 25

Silicon Pixel Sensors 26

Hybrid

Sensor 250μm Readout c

hip 180 μm

Pixel

Pixel electronics Connection via solder bump

Global logic and data driver

Monolithic Active Pixel Sensor

Monolith ic Sensor 50 μm

Pixel Pixel electronics

Global logic and data driver On-chip interconnect

• HV ∼ 700 V

• Sensor thickness ∼ _{250 µm}

• Extra material

• Complex, (expensive)

• HV ∼ 80 V (HV-MAPS)

• Thin active zone < 20 µm

• Cheap, commercial process

50 µm silicon 27

Monolithic Active Pixel Sensors 28

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

• HV ∼ 80 V (HV-MAPS)

• Fast charge collection by drift

• Thin active zone < 20 µm

• Fully integrated readout electronics

MuPix7 sensor prototype 29

• 103 × _{80 µm} ² _{pixel size}

• 3 . 8 × ₄ . 1 mm ² sensor size

• Zero-suppressed, binary hits

• Global threshold + per-pixel tune-dac

• Fully integrated trigger-less readout

• LVDS serial link 1 . 6 Gbit / s

Testbeam at DESY

External EUDET-type telescope

Testbeam at PSI

Mupix7 performance 32

0° incidence

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

60° incidence

Threshold [V]

0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 0.75

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

Measured at DESY 4 GeV electrons

− 85 V sensor bias

MuPix7 time resolution 33

600

− −500 −400 −300 −200 −100 0

Entries [1/run]

102 103 104

Time diffrence between hit and scintillator time [ns]

σ= 14.3 ns

• DESY test beam

• 4 GeV electrons

• Using external scintillator as reference

Next: MuPix8 34

• First full-size prototype

• 80 × _{80 µm} ² _{pixel size}

• Updated electronics

• 4x LVDS serial link 1 . 6 Gbit / s

• Joint submission with Atlas CMOS

• Submitted end of 2016, AMS 180 nm technology

Occupancy and timing 35

2 × 10 ⁹ decays, 50 ns integration 2 × 10 ⁹ decays, 1 ns resolution

Fibre detector 36

Thin ribbons

Square/round 250 µm scintillating fibres SiPM-based readout

Custom readout chip STiC/MuTrig

Fibre time resolution 37

Round fibre

−10 −5 0 5 10

events/98 ps

0 100 200 300 400 500 600 700 800

Square fibre

h

Entries 7255

Mean -0.0003309

RMS / ndf 2 1.18

χ 100.4 / 61

p0 901.5 ± 41.7

p1 0.05357 ± 0.01229

p2 0.5745 ± 0.0195

p3 526.3 ± 40.8

p4 -0.05916 ± 0.03307

p5 1.525 ± 0.056

-10 -5 0 5 10

Events/200 ps

0 100 200 300 400 500 600 700 800

h

Entries 7255

Mean -0.0003309

RMS / ndf 2 1.18

χ 100.4 / 61

p0 901.5 ± 41.7

p1 0.05357 ± 0.01229

p2 0.5745 ± 0.0195

p3 526.3 ± 40.8

p4 -0.05916 ± 0.03307

p5 1.525 ± 0.056

Tile detector 38

Mezzanine Board Connector 448 Channel

Module

Endring Cooling

Pipe

Scintillator

Tiles SiPM

Endring

Connector

Tile detector prototype 39

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

4x4 tile prototype

Cooling 40

SciFi

Pixel Layers

Scintillating Tiles

water cooled beam pipe water cooled beam pipe

Scintillating Tiles Pixel Layers

gaseous helium

Cooling with gaseous helium

Global and local flow

Thermal prototype

Cooling tests 42

Global/local cooling

0 5 10 15 20 25 30 35

10 15 20 25 30 35 40 45 50 55

∆TW[°C]

PostionW[cm]

LayerW3Ww/oWlocalWcooling LayerW4Ww/oWlocalWcooling LayerW3Ww/WlocalWcooling LayerW4Ww/WlocalWcooling P/AW=W250WmW/cm^2 v_global=W2.3Wm/s v_local=W20Wm/s

FEM simuations

T [°C]

0 10 20 50 40 30

Full phase I detector 43

Readout architecture 44

2928 Pixel Sensors

up to 36 1.25 Gbit/s links

FPGA FPGA FPGA

...

86 FPGAs

1 6 Gbit/s link each

GPU

PC GPU

PC GPU

PC 12 PCs 12 10 Gbit/s

links per

8 Inputs each

~ 3072 Fibre Readout Channels

FPGA FPGA

...

48 FPGAs

~ 3500 Tiles

FPGA FPGA

...

48 FPGAs

Data Collection

Server

Mass Storage Gbit Ethernet

Switching Board

Switching Board Switching

Board

Front-end(inside magnet)

Switching Board

• Trigger-less

• Full online reconstruction

• GPU-based filter farm

Tracking with multiple scattering 45

Dominating position

Position Resolution

σ

Θ

d

Dominating scattering

Position Resolution

σ

Θ

Reconstruction d

• Kalman filter

• General Broken Lines

• Anything else?

Triplet(s) track fit 46

Sensor 1 Sensor 2

Sensor 3

θ

θ

Assumptions:

• No position error

• No energy loss

• Thin scatterer at middle hit

Minimize:

χ _i ² ( R _{3 D} ) = ϕ _MS ( R _{3 D} ) ²

σ _ϕ ² + θ _MS ( R _{3 D} ) ² σ _θ ²

Problem: highly non-linear Solution: linearize around circle

Berger et al., NIM A844 135–140

Triplet(s) track fit 47

triplet 1

triplet 2 ^1. Define overlapping triplets

χ ² (¯ R _{3 D} ) = Õ χ _i ²

2a. Minimize χ ² globally

2b. Equivalent: minimize each triplet

¯ R _{3 D} =

Í w _i R _{3 D , i}

Í _w

i

Simplified simulation 48

Track resolution

2.5 3.0 3.5

Rel. momentum resolution / %

4 hits / = 70°

Single Helix Triplets GBL (Single Helix) GBL (Triplets)

0 5 10 15

resolution / mrad

20 30 40 50

Momentum / MeV/c 0

5 10 15

resolution / mrad

Layout and uncertainties

2 cm

Uncertainties increased by factor 5

Berger et al., NIM A844 135–140

Simplified simulation 48

Track resolution

0 1 2 3

Rel. momentum resolution / %

6 hits / = 70°

Single Helix Triplets GBL (Single Helix) GBL (Triplets)

0 5 10 15

resolution / mrad

10 15

Layout and uncertainties

10 cm

Uncertainties increased

by factor 5

Phase I full simulation and reconstruction 49

[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

−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

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

Tracking efficiency Momentum resolution

Only central tracker 4 hits

With recurl stations

6 hits

Phase I sensitivity 50

2

96 98 100 102 104 106 108 110

2

Events per 0.2 MeV/c

−3

10

−2

10

−1

10 1 10 10

at 10

→ eee µ

at 10

→ eee µ

at 10

→ eee µ

at 10

→ eee µ ν

ν

→ eee µ

muons/s muon stops at 10

10

Mu3e Phase I

Bhabha + Michel

Simulated signal and background Different signal branching ratios.

Expected background sources.

Phase I sensitivity 51

Data taking days

0 50 100 150 200 250 300

eee)  µ BR(

−15

10

−14

10

−13

10

−12

10

−11

10

10

× 2

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

Mu3e Phase I 10

muon stops/s 18.4% signal efficiency

SINDRUM 1988

Simulated sensitivity

Summary 52

Summary

• Search for µ ⁺ → e ⁺ e ⁻ e ⁺

• Phase I sensitivity: 2 in 10 ¹⁵ decays

Status

• Technical design report submitted (January 2017)

• Detector R&D

• First prototype in 2017/2018