Niklaus Berger

(1)

Charged Lepton Flavour Violation Experiments

Niklaus Berger

Institute of Nuclear Physics,

Johannes Gutenberg-University Mainz

Zürich Phenomenology Workshop,

January 2015

(2)

Standard Model branching fractions of

10 ^-50ish

(3)

Niklaus Berger – Zürich, January 2015 – Slide 3

Only limited by number of muons (taus)

and background suppression

(4)

History of LFV experiments

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 II Mu3e I

Mu3e II

Comet II/Mu2e DeeMee/

Comet I

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

Ann.Rev.Nucl.Part.Sci. 58, 315 (2008))

(5)

History of LFV experiments

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 II Mu3e I

Mu3e II

Comet I

(6)

LFV Muon Decays

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

(7)

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)

relative to nuclear capture

B(μ

⁺

→ e

⁺

e

^-

e

⁺

) < 1.0 ∙ 10

^-12

(1988)

upgrading

(8)

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

(9)

LFV Muon Decays: Experimental signatures

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Kinematics

• 2-body decay

• Back-to-back

Kinematics

• Quasi 2-body decay

• Monoenergetic e^-

• Single particle detected

(10)

LFV Muon Decays: Experimental signatures

μ

⁺

→ e

⁺

γ μ

^-

N → e

^-

N μ

⁺

→ e

⁺

e

^-

e

⁺

Kinematics

• 2-body decay

• Back-to-back

Kinematics

• Single particle detected

Kinematics

• 3-body decay

• Invariant mass constraint

• Σ p_i = 0

(11)

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

• Radiative decay

Kinematics

• Single particle detected Background

• Decay in orbit

• Antiprotons, pions, cosmics

Kinematics

• 3-body decay

• Σ p_i = 0 Background

• Internal conversion decay

(12)

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

• Antiprotons, pions

Kinematics

• 3-body decay

• Σ p_i = 0 Background

• Radiative decay

Con tinuous Be am

Con tinuous Be am Pul sed Be

am

(13)

Searching for μ → eγ with

MEG

(14)

Muons from PSI

Paul Scherrer Institute in Villigen, Switzerland

(15)

Muons from PSI

Paul Scherrer Institute in Villigen, Switzerland World’s most intensive proton beam

2.2 mA at 590 MeV: 1.3 MW of beam power

(16)

Muons from PSI

DC muon beams at PSI:

• πE5 beamline: ~ 10⁸ muons/s

(MEG experiment, Mu3e phase I)

• Surface muons with about 27 MeV/c

• Higher rates, need magnetic elements closer to production target

(17)

• Muon lifetime 2.2 μs

• Single muon in target experiments limited to < 450’000 μ/s

• Corresponds to few 10¹² μ decays a year

• New experiments operate at 10⁷++ μ/s

• Many muons on target at any time

Rates and accidentals

(18)

MEG Signal and background

μ

⁺

→ e

⁺

γ

Kinematics

• 2-body decay

• Back-to-back

(19)

MEG Signal and background

μ

⁺

→ e

⁺

γ

Kinematics

• 2-body decay

• Back-to-back

Accidental Background

• Not exactly in time

• Not exactly same vertex

• e⁺, γ energies somewhat off

• Not exactly back-to-back

(20)

MEG Signal and background

μ

⁺

→ e

⁺

γ

Kinematics

• 2-body decay

• Back-to-back

Accidental Background

• Not exactly in time

• Not exactly same vertex

Radiative Decay

(21)

The MEG Detector

J. Adam et al. EPJ C 73, 2365 (2013)

(22)

COBRA Magnet

J. Adam et al. EPJ C 73, 2365 (2013)

Gradient field gives constant bending radius independent of angle

Fast sweep of curlers

(23)

• 2009-2011 data

• Blue: Signal PDF, given by detector resolution

• No signal seen

• Upper limit at 90% CL:

BR(μ→eγ) < 5.7 × 10

^-13

J. Adam et al. PRL 110, 201801 (2013)

MEG Results

(MeV) Ee

50 51 52 53 54 55 56

(MeV)γE

48 50 52 54 56 58

γ

Θe

cos

-1 -0.9995 -0.999 -0.9985 (nsec)γet

-2 -1.5

-1 -0.5

0 0.5

1 1.5

2

(24)

• 2012 & 2013 data are being analysed

MEG - Data

0 2 4 6 8

2009 2010 20112012+2013 double the statistics

k factor = SES-1 (1012 )

• Further improvements need detector improvements - upgrade ongoing

Ryu Sawada, SUSY 2014

(25)

MEG Upgrade

11

LXe Calorimeter with higher granularity.

Muon Beam More than twice intense beam

Radiative Decay Counter Identify gammas from muon radiative-decays

(optional) Timing Counter

Higher time resolution with highly segmented detector Drift chamber

Higher tracking performance with long single tracking

volume

Target

Thinner target

Active target option

(26)

MEG II sensitivity projection

0 12.5 25 37.5 50

Upgrade Statistics

k factor = SES^-1 ( 10¹²)

2012+2013 2016 2017 2018

2011 2010

2009

weeks

0 20 40 60 80 100

Branching ratio

10-14

10-13

10-12

90% C.L. MEG 2011

90% C.L. MEG 2013

Upgraded MEG in 3 years Discovery σ

5

Discovery σ

3

90% C.L. Exclusion

Sensitivity prospect

5 × 10

^-14

sensitivity in 3 years DAQ

(27)

Searching for μ → e conversion with

Mu2e, DeeMee, COMET,

PRISM

(28)

• Re-use part of the Tevatron infrastructure

• Proton pulses every 1700 ns

• > 10¹⁰ μ/s

• Project X would give another 2 orders of magnitude at an energy below the

antiproton threshold

Muons from Fermilab...

(29)

... and J-PARC

• 10¹¹ μ/s from 8 GeV/c protons

(30)

Backgrounds:

Anything that can produce a 105 MeV/c electron

• Primary proton beam

• Decay in Orbit (DIO)

• Nuclear capture (AlCap effort at PSI)

• Cosmics

Conversion Signal and Background

μ

^-

N → e

^-

N

• Single 105 MeV/c electron observed

(31)

• Proton beam produces pions, photons, (antiprotons) etc.

• Wait until things become better...

Beam induced background

(32)

• Nuclear recoil allows for electron energies above m_μ/2

• Calculation by Czarnecki, Garcia i Tormo and Marciano, Phys. Rev. D84 (2011)

• Requires excellent momentum resolution

Deacy-in-orbit background

100 101 102 103 104 105

10 ²⁰ 10 ¹⁸ 10 ¹⁶ 10 ¹⁴

E_e MeV

1 0

d dEeMeV1

Without recoil With nuclear

recoil

(33)

Experimental concept - DeeMee

Yohei Nakatsugawa, NuFACT2014

(34)

• Expect 2.1×10^-14 single event sensitivity for one year running

Sensitivity - DeeMee

Yohei Nakatsugawa, NuFACT2014

Momentum [MeV/c]

Cou nt s [/0.2 M eV/c ]

(35)

Capture most pions produced in target Shielding of superconducting magnet

very challenging

Production target inside a solenoid

(36)

• Separate muon production and conversion target

• Not shown: cosmic ray veto and absorbers

Experimental layout - Mu2e

Conversion Target

Mu2e CDR

(37)

• Straw tubes in vacuum

• Outside of radius of Michel electrons

Mu2e Tracker

Mu2e CDR

(38)

Experimental layout - COMET Phase I

Stopping Target

Production Target

Detector Section Pion-Decay and

Muon-Transport Section Pion Capture Section

A section to capture pions with a large solid angle under a high solenoidal magnetic field by superconducting maget

A detector to search for muon-to-electron conversion processes.

A section to collect muons from decay of pions under a solenoidal magnetic field.

Comet CDR

High solenoidal field

Capture pions with large

solid angle

(39)

Experimental layout - COMET Phase II

Detector Section

Pion-Decay and

Muon-Transport Section

Pion Capture Section

A section to capture pions with a large solid angle under a high solenoidal magnetic field by superconducting maget

A detector to search for muon-to-electron conversion processes.

A section to collect muons from decay of pions under a solenoidal magnetic field.

Stopping Target Production

Target

Comet CDR

Separate muon decay and detector region

One more bend

(40)

Add a muon storage ring

Further steps: Prism/Prime

5 m

Capture Solenoid

Matching Section Solenoid

RF Power Supply RF AMP

RF Cavity

C-shaped FFAG Magnet Ejection System Injection System

FFAG ring Detector

(41)

• Comet Phase I and DeeMee might get to ~10

^-14

as early as 2016

• Both Comet Phase II and Mu2e will start around 2020

• Should get single event sensitivities well below 10

^-16

• Prism/Prime and Mu2e with Project X explore paths to 10

^-18

Conversion: Expected sensitivities

(42)

• Models can be discriminated using Z-dependence

• However: low lifetime at high Z

Z-dependence

0 0.5 1 1.5 2 2.5

0 10 20 30 40 50 60 70 80 90 100

BµN->eN(Z) / BµN->eN(Z=13)

Z dipole

scalar vector

(43)

Searching for μ

⁺

→ e

⁺

e

^-

e

⁺

with

Mu3e

(44)

• μ⁺ → e⁺e^-e⁺

• Two positrons, one electron

• From same vertex

• Same time

• Σ p_e = m_μ

• Maximum momentum: ½ m_μ = 53 MeV/c

The signal

(45)

• 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

(46)

• Allowed radiative decay with internal conversion:

μ

⁺

→ e

⁺

e

^-

e

⁺

νν

• Only distinguishing feature:

Missing momentum carried by neutrinos

Internal conversion background

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

• Tree-level calculation; could one loop corrections be big?

(R. M. Djilkibaev, R. V. Konoplich, Phys.Rev. D79 (2009) 073004)

(47)

2 Billion Muon Decays/s

50 ns, 1 Tesla field

(48)

• High granularity (occupancy)

• Close to target (vertex resolution)

• 3D space points (reconstruction)

• Minimum material

(momenta below 53 MeV/c)

Detector Technology

(49)

• High granularity (occupancy)

• Close to target (vertex resolution)

• 3D space points (reconstruction)

• Minimum material

(momenta below 53 MeV/c)

• Gas detectors do not work (space charge, aging, 3D)

• Silicon strips do not work (material budget, 3D)

• Hybrid pixels (as in LHC) do not work (material budget)

Detector Technology

(50)

High voltage monolithic active pixel sensors - Ivan Perić

• Use a high voltage commercial process (automotive industry)

• Small active region, fast charge collection via drift

• Can be thinned down to < 50 μm

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

• Logic on chip: Output are

zero-suppressed hit addresses and timestamps

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

Fast and thin sensors: HV-MAPS

P-substrate N-well

Particle E field

(51)

Introduction

Y

• X

(52)

(53)

• 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

(54)

(55)

• 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

(56)

Precision vs. Acceptance

50 MeV/c 25 MeV/c 12 MeV/c B^→

33 cm

(57)

Precision vs. Acceptance

(58)

Precision vs. Acceptance

(59)

Precision vs. Acceptance

(60)

Precision vs. Acceptance

Ω ~ π MS

θ_MS

B

(61)

Detector Design

muon beam

target

(62)

Detector Design

muon beam

target

(63)

Detector Design

muon beam

target

inner pixel layers

(64)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers

(65)

Detector Design

scintillating ﬁbres

outer pixel layers

muon beam

target

inner pixel layers

(66)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers recurl pixel

layers

recurl pixel layers

(67)

Detector Design

outer pixel layers

muon beam

target

inner pixel layers recurl pixel

layers

recurl pixel layers

Scintillating tiles

(68)

Detector Design

outer pixel layers

muon beam

target inner pixel layers recurl pixel

layers

recurl pixel layers

Scintillating tiles

(69)

Pixels: O(50 ns)

Timing measurements

Scintillating fibres O(1 ns);

Scintillating tiles O(100 ps)

(70)

• 3 layers of 250 μm scintillating fibres

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

• Timing resolution O(1 ns)

(measured with sodium source)

Timing Detector: Scintillating Fibres

Single photon Efficiency > 98%

(≥ 2 photons)

(71)

Timing Detector: Scintillating tiles

• Test beam with tiles, SiPMs and readout ASIC

• Timing resolution ~ 80 ps

Time Difference [ps]

-7500 -500 -250 0 250 500 750

2000 4000 6000 8000 10000

σ = 79.2 ps

Front

Back

3.5 cm

(72)

• 280 Million pixels (+ fibres and tiles)

• No trigger

• ~ 1 Tbit/s

• FPGA-based switching network

• O(50) PCs with GPUs

Data Acquisition

1116 Pixel Sensors

up to 45 800 Mbit/s links

FPGA FPGA FPGA

...

38 FPGAs

RO Boards 1 6.4 Gbit/s

link each

GPU

PC 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 RO Boards Pixel DAQ

(73)

Online software filter farm

• 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

• What to save?

Events with three tracks from one vertex Histogram of all tracks

Online filter farm

(74)

Sensitivity

Phase IA: Starting 2017 10⁷ μ/s

Target Inner pixel layers

Outer pixel layers μ Beam

(75)

Sensitivity

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

Phase IB: 2018+ 10⁸ μ/s

(76)

Sensitivity

Phase II: 2020++ >10⁹ μ/s New Beam Line

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

(77)

• Exciting times ahead in searches for LFV muon decays

• MEG aims for another order of magnitude for μ→eγ

• DeeMee/Comet I aim for two orders on μ→e conversion

• Mu3e Phase I aims for two orders on μ→eee

• Mu2e/Comet II aim for < 10^-16 for μ→e conversion and Mu3e Phase II for < 10^-16 for μ→eee

• Ideas for 10^-18 are around

Summary

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 II Mu3e I

Mu3e II Comet II/Mu2e

DeeMee/

Comet I

(78)

Wish list

• Many models with BR predictions for all three processes

• Bonus points for conversion Z-dependence and μ → eee Dalitz plot

• One-loop calculation of μ → eeeνν

• Other ideas for what to do with 10¹⁶+ muon decays

(79)

Backup Material

(80)

MEG Upgrade - Calorimeter

• ~4000 VUV sensitive SiliconPMs on entry face (new development with Hamamatsu)

• Better position and energy resolution

• Better efficiency

(81)

MEG Upgrade - Drift Chamber

• New single volume drift chamber

• Lower Z gas mixture

• More space points per track

• Better rate capability

• Less material in front of timing counters

(82)

MEG Upgrade - Timing Counter

• Many small scintillators

• Read-out by SiliconPMs

• On average eight counters hit by track

• 30 ps timing resolution per track

(83)

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,

Rev.Mod.Phys. 73 (2001) 151)

(84)

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)

(85)

History of LFV experiments

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 II Mu3e I

Mu3e II

Comet I

(86)

Y

• X

Lepton flavour violating Τ-decays

LHCb

(87)

Y

• X

Belle II at Super KEKB

Expect 5× 10

¹⁰

Τ pairs - branching fractions of 10

^-9

achievable

(88)

• 3D multiple scattering track fit

• Simulation results:

280 keV single track momentum 520 keV total mass resolution

Simulated Performance - Mu3e Phase II

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

(89)

Simulated Performance - Mu3e Phase II

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

Niklaus Berger

Charged Lepton Flavour Violation Experiments

Niklaus Berger

Institute of Nuclear Physics,

Johannes Gutenberg-University Mainz

Zürich Phenomenology Workshop,

January 2015

Standard Model branching fractions of

10 -50ish

Only limited by number of muons (taus)

and background suppression

History of LFV experiments

History of LFV experiments

LFV Muon Decays

μ

→ e

γ μ

N → e

N μ

→ e

e

e

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

(2013) B(μ

Au → e

Au) < 7 ∙ 10

(2006)

relative to nuclear capture

B(μ

→ e

e

e

) < 1.0 ∙ 10

(1988)

upgrading

LFV Muon Decays: Experimental signatures

μ

→ e

γ μ

N → e

N μ

→ e

e

e

LFV Muon Decays: Experimental signatures

μ

→ e

γ μ

N → e

N μ

→ e

e

e

LFV Muon Decays: Experimental signatures

μ

→ e

γ μ

N → e

N μ

→ e

e

e

LFV Muon Decays: Experimental signatures

μ

→ e

γ μ

N → e

N μ

10 ^-50ish