Dark Matter @

Charged lepton flavour violation experiments

Niklaus BergerInstitut für Kernphysik, Johannes-Gutenberg Universität Mainz

Dark Matter @ LHC Heidelberg, April 2018

Niklaus Berger – DM@LHC, April 2018 – Slide 2

Charged lepton flavour violation experiments:

• What do we have, what do we expect?

Beyond the standard channels:

• Exotics with Mu3e: μ → e X and Dark Photons

Overview

11

LXe Calorimeter

with higher granularity.

Muon Beam More than twice intense beam

Radiative Decay Counter Identify muon radiative-decays

Timing Counter Higher time resolution with highly segmented detector

Drift chamber Higher tracking performance with long single tracking volume

Target Thinner targetActive target option

outer pixel layers

muon beam

target

inner pixel layers

recurl pixellayers

recurl pixellayers

scintillating fibres

Scintillatingtiles

Niklaus Berger – DM@LHC, April 2018 – Slide 3

Standard Model branching fractions of

10-50ish

Lepton flavour violation experiments

Only limited by number of muons and background suppression:

Experimental/technical challenge

Niklaus Berger – DM@LHC, April 2018 – Slide 4

History of cLFV experiments

1940 1960 1980 2000 2020

Year

90%

–CL b

ound

10–14

10–12

10–10

10–8

10–6

10–4

10–2

100

e3e

N eN

3

10–16

SINDRUM SINDRUM II

MEG

MEG IIMu3e Phase I

Mu3e Phase IIComet/Mu2e

μμμ

μμ

γ

γττ

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

Niklaus Berger – DM@LHC, April 2018 – Slide 5

LFV Muon Decays

μ+ → e+γ μ-N → e-N μ+ → e+e-e+

Niklaus Berger – DM@LHC, April 2018 – Slide 6

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) < 4.2 ∙ 10-13

(2016)B(μ- Au → e-Au) < 7 ∙ 10-13

(2006) relative to nuclear capture

B(μ+ → e+e-e+) < 1.0 ∙ 10-12

(1988)

Niklaus Berger – DM@LHC, April 2018 – Slide 7

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

Niklaus Berger – DM@LHC, April 2018 – Slide 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

Kinematics• Quasi 2-body decay• Monoenergetic e-

• Single particle detected

Niklaus Berger – DM@LHC, April 2018 – Slide 9

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

Kinematics• Quasi 2-body decay• Monoenergetic e-

• Single particle detected

Kinematics• 3-body decay• Invariant mass constraint• Σ pi = 0

Niklaus Berger – DM@LHC, April 2018 – Slide 10

Searching for μ → eγ with

MEG

Niklaus Berger – DM@LHC, April 2018 – Slide 11

The MEG Detector

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

Niklaus Berger – DM@LHC, April 2018 – Slide 12

• 2009-2013 data

• Blue: Signal PDF, given by detector resolution

• No signal seen

• Upper limit at 90% CL:

BR(μ→eγ) < 4.2 × 10-13

A. M. Baldini et al. Eur.Phys. J. C76 (2016) no.8, 434

MEG Results

(MeV)+eE50 51 52 53 54 55 56

(MeV

)γ

E

48

50

52

54

56

58

γ+eΘcos1− 0.9995− 0.999− 0.9985−

(ns)

γ+ et

2−

1.5−

1−

0.5−

0

0.5

1

1.5

2

Niklaus Berger – DM@LHC, April 2018 – Slide 13

MEG Upgrade

11

LXe Calorimeter

with higher granularity.

Muon Beam More than twice intense beam

Radiative Decay Counter Identify muon radiative-decays

Timing Counter Higher time resolution with highly segmented detector

Drift chamber Higher tracking performance with long single tracking volume

Target Thinner targetActive target option

Ryu Sawada, SUSY 2014

MEG II

Niklaus Berger – DM@LHC, April 2018 – Slide 14

Angela Papa (Mainz Seminar)

MEG II

Niklaus Berger – DM@LHC, April 2018 – Slide 15

Searching for μ → e conversion with

Mu2e, DeeMee, COMET, PRISM

Niklaus Berger – DM@LHC, April 2018 – Slide 16

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

Niklaus Berger – DM@LHC, April 2018 – Slide 17

• Separate muon production and conversion target

• Not shown: cosmic ray veto and absorbers

• Straw tubes in vacuum

• Outside of radius of Michel electrons

Experimental layout - Mu2e

Conversion Target

Mu2e CDR

Niklaus Berger – DM@LHC, April 2018 – Slide 18

• J-PARC: Comet/DeeMee/Prism Fermilab: Mu2e

• Comet Phase I and DeeMee might get to ~10-14 as early as 2019

• 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/PIP-II explore paths to 10-18

Conversion: Expected sensitivities

Niklaus Berger – DM@LHC, April 2018 – Slide 19

Searching for μ+ → e+e-e+ with

Mu3e

Niklaus Berger – DM@LHC, April 2018 – Slide 20

• μ+ → e+e-e+

• Two positrons, one electron

• From same vertex

• Same time

• Σ pe = mμ

• Maximum momentum: ½ mμ = 53 MeV/c

The signal

Niklaus Berger – DM@LHC, April 2018 – Slide 21

• Combination of positrons from ordinary muon decay with electrons from: - photon conversion, - Bhabha (electron-positron) scattering, - Mis-reconstruction

• Need very good timing, vertex and momentum resolution

Accidental Background

Niklaus Berger – DM@LHC, April 2018 – Slide 22

• Allowed radiative decay with internal conversion:

μ+ → e+e-e+νν • Only distinguishing feature:

Missing momentum carried by neutrinos

Internal conversion background

• Need excellent momentum resolution Br

anch

ing

Ratio

10-12

10-16

10-18

10-14

e+e-e+ mass (MeV/c2)105 106104103102101

Internal conversionbackground

Signal

Niklaus Berger – DM@LHC, April 2018 – Slide 23

• 1T magnetic field

• Up to 108 μ/s

• Ultra-thin, fast pixels (HV-MAPS)

• Timing from scintillating fibres and tiles

• Measure all positrons/electrons down to 10 MeV pT

Detector Design: Phase I

Target

Inner pixel layers

Scintillating �bres

Outer pixel layers

Recurl pixel layers

Scintillator tiles

μ Beam

• No trigger

• ~ 1 Tbit/s

• FPGA-based switching network

• O(10) PCs with GPUs

• Only save things that look like μ+ → e+e-e+

• Or: Additional selection

Data Acquisition2844 Pixel Sensors

up to 45 1.25 Gbit/s links

FPGA FPGA FPGA

...

86 FPGAs

1 6 Gbit/slink each

GPUPC

GPUPC

GPUPC12 PCs

4 12 Gbit/slinks per

16 Inputseach

3072 Fibre Readout Channels

FPGA FPGA

...

12 FPGAs

6272 Tiles

FPGA FPGA

...

14 FPGAs

DataCollection

Server

MassStorage

Gbit Ethernet

SwitchingBoard

SwitchingBoard

Front-end(inside m

agnet)

Switching Board

SwitchingBoard

SwitchingBoard

Niklaus Berger – DM@LHC, April 2018 – Slide 25

Sensitivity - Mu3e Phase I

Data taking days0 50 100 150 200 250 300

eee

)→

µB

R(

15−10

14−10

13−10

12−10

11−10

-15 10×2

SES90% C.L.

95% C.L.

Mu3e Phase I muon stops/s81019.7% signal efficiency

SINDRUM 1988

• Start 2020

• Phase II with a high intensity muon beam line at PSI under study

Niklaus Berger – DM@LHC, April 2018 – Slide 26

Beyond μ+ → e+e-e+:

μ → eX and

Dark Photons

Thesis Ann-Kathrin Perrevoort

Niklaus Berger – DM@LHC, April 2018 – Slide 27

• Spontaneously broken flavour symmetry: Goldstone boson(s) called familons

• Can be a light dark matter candidate

• Lead to μ → eX, where X a familon

• μ → eX can also show up in other models, search for it with the large muon decay data set at Mu3e

Familons in Mu3e

Niklaus Berger – DM@LHC, April 2018 – Slide 28

• Signal: Two-body decay: Monoenergetic positron

• Background: All other positrons, dominated by Michel decay, smooth momentum distribution

• Bump hunt on the positron spectrum (all tracks...)

Signature and Background

10 20 30 40 500

0.2

0.4

0.6

0.8

1

Michel spectrumμ→eX signal (mX=60MeV)

pe [MeV]

entri

es/d

p e [a

.u.]

Niklaus Berger – DM@LHC, April 2018 – Slide 29

• Not possible to save all tracks: Use histograms from online reconstruction

• Baseline: Use only outgoing part of tracks (short/4 hits)

• Potential farm upgrade: Use also recurling part (long, 6/8 hit)

Search strategy

Target

Inner pixel layers

Scintillating �bres

Outer pixel layers

Recurl pixel layers

Scintillator tiles

μ Beam

Niklaus Berger – DM@LHC, April 2018 – Slide 30

• TWIST at TRIUMF

• Limits on the μ → eX BF in the few 10-6 region

Previous experiment: TWIST

Niklaus Berger – DM@LHC, April 2018 – Slide 31

Results

[MeV]Xm0 10 20 30 40 50 60 70 80 90

Bra

nchi

ng F

ract

ion

at 9

0% C

L

9−10

8−10

7−10

6−10

5−10

4−10

3−10 stopsµ1510×Mu3e Phase I SIM: 2.6TWIST 2014Short tracks (ext.calib.) Short tracks (sim.calib.)Long tracks (ext.calib.) Long tracks (sim.calib.)

Niklaus Berger – DM@LHC, April 2018 – Slide 32

Dark photon can be radiated, wherever a photon can be radiated Three cases:

• Dark photon is long-lived/decays to dark particles

• Dark photon goes to e+e- immediately

• Dark photon goes to e+e- at a displaced vertex (under study)

Dark Photons in Mu3e

Niklaus Berger – DM@LHC, April 2018 – Slide 33

μ → eΝΝA’ is a four-body decay...

• Shift to Michel spectrum

Invisible dark photons Generated

Niklaus Berger – DM@LHC, April 2018 – Slide 34

μ → eΝΝA’ is a four-body decay...

• Shift to Michel spectrum

Invisible dark photons Generated

Reconstructed

Niklaus Berger – DM@LHC, April 2018 – Slide 35

μ → eΝΝA’ is a four-body decay...

• Shift to Michel spectrum

• Can also come from detector misalignment

• Not really promising

Invisible dark photons Generated

Reconstructed

Niklaus Berger – DM@LHC, April 2018 – Slide 36

μ → eΝΝ(A’→ee) has the same visible final state as our signal: Will not be filtered away

Background is internal conversion decay μ+ → e+e-e+νν

Two e+e- combinations

Dark Photons in e+e-

Niklaus Berger – DM@LHC, April 2018 – Slide 37

Branching Fraction Limits

Niklaus Berger – DM@LHC, April 2018 – Slide 38

And on the mA’-ε plane

• Phase I: Detailed MC study

• Phase II: Assume same detector, 20x rate (and background)

Niklaus Berger – DM@LHC, April 2018 – Slide 39

• Exciting range of experiments going on-line: New lepton flavour violation limits upcoming

• Mu3e very competitive for μ → eX searches

• Improve by 2-3 orders of magnitude relative to TWIST in phase I

• Can access currently uncovered dark photon parameters

• Displaced vertices currently under study

Summary