Background in the Mu3e Experiment Searching for Lepton Flavour Violation

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Background in the Mu3e Experiment

Searching for Lepton Flavour Violation

Ann-Kathrin Perrevoort

on behalf of the Mu3e Collaboration

Physikalisches Institut, Heidelberg

International School of Subnuclear Physics, Erice 2015

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The Mu3e Experiment

Indirect search for the lepton flavour violating decayµ⁺ → ê⁺ê⁻ê⁺

In this talk

▸ Introduction to Mu3e

▸ Detector Concept

▸ Background Studies

A. Perrevoort (Heidelberg) Mu3e ISSP Erice 2015 2 / 14

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The Mu3e Experiment

Charged Lepton Flavour Violating Decayµ⁺ →ê⁺ê⁻ê⁺

Lepton Flavour conserved in Standard Model

. . . butνoscillations

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

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The Mu3e Experiment

Charged Lepton Flavour Violating Decayµ⁺ →ê⁺ê⁻ê⁺

Observation ofµ →eee is a clear sign for New Physics

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

Mu3e is sensitive to one in 10¹⁵µdecays

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

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Signal Decay µ → ^eee

Signature forµdecay at rest Common vertex

Coincident in time

∑^E^e=^mµc²

∑⃗p_e =⁰

Ee=(⁰−⁵³)^MeV

Multiple Coulomb scattering limits momentum resolution

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Background

Accidental Combinations

e⁺

e⁺ e^-

e⁺

e^- e⁺

(e⁺)

Overlays of Michel decay, Bhabha scattering, photon conversion, . . .

No common vertex Not coincident

∑^Ee≠^mµc²

∑⃗p_e ≠⁰

Increases withbeam intensity

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Background

Internal Conversion Decayµ →^eeeνν

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

Common vertex Coincident in time

∑^Ee<^mµc²

∑⃗pe ≠⁰

→Missing energy due to neutrinos Need very good momentum resolution

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The Mu3e Detector

Tracking detector:

50 µm Si pixel sensors (HV-MAPS) + Lightweight mechanics

+ Timing detector:

+Scintillating fibres and tiles

Target Inner pixel layers

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

Paul-Scherrer Institute (CH) Polarizedµbeam with 10⁸^µ/^s

Full Geant4-based simulation

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The Mu3e Detector

Tracking detector:

50 µm Si pixel sensors (HV-MAPS) + Lightweight mechanics

+ Timing detector:

+Scintillating fibres and tiles

Target Inner pixel layers

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

Paul-Scherrer Institute (CH)

Polarizedµbeam with 10⁸^µ/^s Full Geant4-based simulation

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Mu3e Simulation

Physics Processes

Background decays

Michel decayµ → êνν Radiative decayµ → êγνν Internal conversionµ → eeeνν Signalµ → êee

3-body decay Other effects

Multiple Coulomb scattering Bhabha scattering

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Internal Conversion Decay µ → ^eee νν in Simulation

Γ_µ_→_eeeνν∝∣^Tµ→eeeνν∣²ρ

Matrix element by Djilkibaev and Konoplich[Phys.Rev.D79, 2009]

Only unpolarized muons

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Internal Conversion Decay µ → ^eee νν in Simulation

−1−0.8−0.6−0.4−0.2 0 0.2 0.4 0.6 0.8 1 0

0.02 0.04 0.06 0.08 0.1

Polarized Unpolarized

cos

Acceptance cut:

mvis>90MeV; pT>10MeV;|cos |<0.8

d / (d|cos |=0.1)

High-energy positrons in acceptance

θ

e

μ polarisation μ beam

μ

Γ_µ_→_eeeνν∝∣^Tµ→eeeνν∣²ρ

New calculations by A. Signer et al. (PSI) take polarisation into account

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Internal Conversion Decay µ → ^eee νν in Simulation

Branching Ratio

m - 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

Djilkibaev, Konoplich Phys.Rev.D79(2009)

Suppressµ → ^eeeννby cuts on electron energy E_tot=∑^Ee

µ→eee

ÐÐÐÐ→^mµc²

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Internal Conversion Decay µ → ^eee νν in Simulation

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]

Branchi ng Rati o

Internal Conversion Decay A. Signer et al.

µ→eee

ÐÐÐÐ→^mµc²

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Internal Conversion Decay µ → ^eee νν in Simulation

Branching Ratio

m - 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

Djilkibaev, Konoplich Phys.Rev.D79(2009)

µ→eee

ÐÐÐÐ→^mµc²

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Sensitivity Studies

Reconstructed mass for signal and background events

2] Reconstructed Mass [MeV/c 96 98 100 102 104 106 108 110

2 Events per 100 keV/c

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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Summary

Mu3e

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

Simulation

Full description of the experiment All background processes considerµ polarization

Next steps

Higher order corrections for background Sensitivity studies for different models beyond SM

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Status

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

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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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Loop and Tree Level Contributions

L_LFV=[₍_κ₊^m₁^µ₎_Λ2µ_Rσ^µνe_LF_µν]

γpenguin+[₍_κ₊^κ₁₎_Λ2(µ_Lγ^µe_L)(^eLγ_µe_L)]

tree

Adapted from A. de Gouv ˆea [Nucl.Phys.B188 2009]

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Mu3e Simulation

Radiative Muon Decayµ →^eγνν

BR_µ_→_eγνν=(¹.4±⁰.4)^{% for}^Eγ^min>^{10 MeV} Use BR calculated by Kuno et al. [Rev.Mod.Phys 73, 2001]

0 0.2 0.4 0.6 0.8 1

10-5 10-4 10-3 10-2 10-1 1 10 102 103 104

105 _1.0/y

Polarized BR Unpolarized BR

Generated distribution

histo for test1

y dBrad

Distribution of photon momentumy=^2p_m_µ^γ

Divergence forEγ →0 Generateγ momentum distributed according to∼ _E¹_γ Accept/reject events based on BR

Assign minimumE_γ^min, typ.

10 MeV

Scale BR using MC integration forE_γ^min≠10 MeV

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Pixel Sensors

P-substrate N-well

Particle E field

I. Peri´c, NIM A 582 (2007)

High Voltage Monolithic Active Pixel Sensors

• High voltage of>^{50 V}

• Fast charge collection via drift

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

• Integrated readout electronics

• Pixel size 80×^80µm² Thin and highly granular

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Lightweight Mechanics

• 50 µm silicon sensor

• 25 µm Kapton flexprint with aluminum traces

• 25 µm Kapton support structure

→ ∼1 ‰ of radiation length

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Muon Beam at PSI

Paul-Scherrer Institute in Switzerland 2.2 mA proton beam 590 MeV

Secondary beamlines: µ⁺with 28 MeV/c

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

→Phase I

→^{Phase II}

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Phase II Detector

Reach BR∼10⁻¹⁶with a muon rate of 10⁹^µ/^s

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Simulation of 50 ns of Beam Time (Phase II)

Tracks per readout frame of 50 ns Exploiting time resolution of scintillating fibres (1 ns) and tiles (0.1 ns)

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Readout Concept

...

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

Background in the Mu3e Experiment Searching for Lepton Flavour Violation