Update from the Mu3e Experiment

Update from the Mu3e Experiment

Niklaus Berger

Physics Institute, University of Heidelberg Charged Lepton Working Group,

February 2013

• The Challenge:

Finding one in 10

muon decays

• The Technology:

High Voltage Monolithic Active Pixel Sensors

• The Mu3e Detector:

Minimum Material, Maximum Precision

Overview

• Neutrinos have mass

• Leptons do change flavour

• However: Standard Model

branching ratio for μ → eee < 10^-50

The Physics: Charged Lepton Flavour Violation

µ ⁺ e ⁺

W +

ν _µ ν _e

γ

e ^- e ⁺

*

• Neutrinos have mass

• Leptons do change flavour

• However: Standard Model

branching ratio for μ → eee < 10^-50

• Can be much bigger with new physics

The Physics: Charged Lepton Flavour Violation

µ ~

γ

e ^- e ⁺

*/Z

• Neutrinos have mass

• Leptons do change flavour

• However: Standard Model

branching ratio for μ → eee < 10^-50

• Can be much bigger with new physics

The Physics: Charged Lepton Flavour Violation

∝ ⁺ χ ~ ⁰ e ⁺

µ e~

~

γ

e ^- e ⁺

*/Z

µ

e

e

e

Z’

• Ratio κ between dipole and contact

• 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 )

Comparison with μ → eγ

L

= A m

μ

σ

e

F

+ (μ

γ

e

) (e

γ

e

) (κ+1)Λ

κ (κ+1)Λ

∝⁺ χ~⁰ e⁺

µ e~

~

γ

e^- e⁺

*/Z µ⁺

e⁺ e^-

e⁺ Z’

• Z-penguins could be important

• Lots of theory activity

Comparison with μ → eγ

∝⁺ χ~⁰ e⁺

µ e~

~

γ

e^- e⁺

*/Z

• 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

90%–CL bound

10^–10 10^–8 10^–6 10^–4 10^–2 10⁰

μ eγ

μ 3e

μN eN MEG

• Observe more than 10¹⁶ muon decays:

2 Billion muons per second

• Suppress backgrounds by more than 16 orders of magnitude

• Be sensitive for the signal

The Challenges

1940 1960 1980 2000 2020

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))

DC muon beams for particle physics at PSI:

• πE5 beamline: ~ 10⁸ muons/s (MEG experiment)

• SINQ (spallation neutron source) target could even provide

~ 5 × 10¹⁰ muons/s

• The μ → eee experiment (final stage) requires 2 × 10⁹ muons/s focused and collimated on a ~2 cm spot

Muons from PSI

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

• 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

• 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⁺

}

}

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

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

• 1 T magnetic field

• Resolution dominated by multiple scattering

• Momentum resolution to first order:

Σ

/P ~ θ

/Ω

• Precision requires large lever arm (large bending angle Ω) and

Momentum measurement

MS

θ

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

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

Fast and thin sensors: HV-MAPS

The MUPIX chips

• Pixel size 80 × 80 μm

• Goal for thickness: 50 μm

• 1 bit per pixel, zero suppression on chip

• Power: 150 mW/cm²

• Data output up to 3.2 Gbit/s

• Time stamps every 50 ns

ToT [µs]

0 1 2 3 4 5

10-4

10-3

10-2

10-1

1

SNR

30 35 40

55Fe peak

Introduction

Y

• X

• 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

• Add no material:

Cool with gaseous Helium

• ~ 150 mW/cm²

• Simulations: Need ~ 1 m/s flow

• First measurements: Need several m/s

• Full scale prototype on the way

Cooling

• 1 T magnetic field

• Resolution dominated by multiple scattering

• Momentum resolution to first order:

Σ

/P ~ θ

/Ω

• Precision requires large lever arm (large bending angle Ω) and low multiple scattering θ_MS

Momentum measurement

Ω MS

θ

B

Precision vs. Acceptance

50 MeV/c 25 MeV/c 12 MeV/c B^→

Ω ~ π MS

θ_MS

B

Detector Design

Target μ Beam

Detector Design

Target Inner pixel layers

μ Beam

Detector Design

Target Inner pixel layers

Outer pixel layers μ Beam

Detector Design

Target Inner pixel layers

Scintillating fibres

Outer pixel layers μ Beam

Detector Design

Target Inner pixel layers

Scintillating fibres

Outer pixel layers Recurl pixel layers

μ Beam

Detector Design

Target Inner pixel layers

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles

μ Beam

• 250 μm fibres - O(0.5 ns)

• 0.5 cm³ tiles - O(60 ps)

• Photosensor: SiPM;

high gain, high frequency

• Readout via switched capacitor array (PSI developed DRS5 chip) or

STiC ASIC developed in Heidelberg

Timing measurements

Online software filter farm

• Continuous front-end readout (no trigger)

• ~ 1 Tbit/s

• 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

• 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

Simulated Performance

V

10-18

10-17

10-16

10-15

10-14

10-13

10-12

10-11

10-10 _{µ→ eeeνν 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

Sensitivity

Target Inner pixel layers

Outer pixel layers μ Beam

Target Inner pixel layers

Scintillating fibres Outer pixel layers Recurl pixel layers

Scintillator tiles μ Beam

Target Inner pixel layers

Scintillating fibres

Outer pixel layers Recurl pixel layers

Scintillator tiles μ Beam

Phase Ia: Starting 2015

Phase Ib: 2016+

Phase II: 2017+

New Beam Line

• The Mu3e Research Proposal was approved by the PSI research committee in January

Proposal available on arXiv:1301:6113

• Phase I experiment mostly funded

• Aim for first measurements in 2015

• High-intensity beam line under study (earliest availability 2017+)

Current Status

Collaboration

Participating Institutes:

• University of Geneva

• University of Heidelberg (3 Institutes)

• Paul Scherrer Institut (PSI)

• University of Zurich

• ETH Zurich

Also in contact with other interested groups

Backup Material

Radiation Hardness

• Requirements not as strict as at LHC

• Irradiation at PS

• After 380 MRad (8×10¹⁵ n_eq/cm²)

• Chip still working

(Courtesy Ivan Perić)