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