The Muon programme g-2, EDM and lepton flavour violation Joost Vossebeld - Thanks for inputs from: Mark Lancaster, Becky Chislet, Joe Price ...

The Muon programme
g-2, EDM and lepton flavour violation

Joost Vossebeld
Thanks for inputs from: Mark Lancaster, Becky Chislet, Joe Price,
Graziano Venanzoni, Yoshi Uchida

 Joost Vossebeld IOP APP-HEP April 2023 1

Fermilab g-2 experiment
Muons in the g-2 storage ring undergo precession of their spin direction

 1 Ԧ × 
 = − + 2 − 
 −1 

Second term reduces to 0 when muon momentum is 3.1 GeV.
Muon spin precession leads to a rate fluctuation in (forward) emitted electrons.
ωa is extracted from the oscillation in the rate plot.
Muon anomalous momentum is then obtained from

UK plays a major role on the experiment
• Hardware: Straw Tracker & DAQ
• Operation and offline sofware
• Analysis: Magnetic field, g-2 and EDM

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Fermilab g-2: status April 7th 2021: First results

• Run1 was only 5% of total statistics
 PRL 126 (2021) 14, 141801 (>1000 citations)
• Analysis of Run2/3 in progress (x4 Run1 data). Expecting a 27/02/23: TDR Goal of 21 x BNL dataset reached
 factor two reduction in the uncertainty (462ppb → 230 ppb).
 Release expected after 1st June.

• Run6 data taking is in progress (mostly for systematic studies)

• Analysis Run4/5/6 started. Results expected for 2025

 3

g-2: theory - experiment comparison
4.2σ discrepancy compared to “dispersive approach calculations, but lattice
calculations much nearer experimental result. Further theory updates expected this
year.
Worldwide programme to improve precision on both measurement and theory
predictions to resolve or confirm the discrepancies.
Experimentally:
1. more results from the FNAL g-2 experiment.
2. independent g-2/EDM experiment at J-PARC (early 2030’s).
3. Measurements to constrain the hadronic vacuum polarisation used in dispersive
 theory predictions: MUonE and (re-)analysis of existing e+e- data
4. Measurement of other muon properties: cLFV programme and EDM
 measurements.
Strong contributions from the UK across this programme!

Recently boosted with a £4.3M investment through the Leverhulme Trust International Professorship, awarded to
Graziano Venanzoni to come to Liverpool (g-2 data analysis and theory, contributions to MUonE experiment and
analysis of e+e- data; future muon EDM measurements).

 Joost Vossebeld IOP APP-HEP April 2023 4

Muon EDM @ FNAL g-2
 1 Ԧ × 2 
 =− + 2 − + + Ԧ × 
 −1 ℏ 

• Muon EDM causes a tilt in precession plane.
• Asymmetry in vertical decay angle of positrons, which can be
 measured by the UK built tracking detectors.
• This was also measured at the BNL g-2 experiment.
 (dμ < 1.9 × 10-19e.cm (95% C.L.)

Status FNAL EDM measurement:
• Run 1 analysis still blinded.
• Assuming zero signal, expected limit is: dμ < 2.0 × 10-19e.cm (95% C.L.)
• Further factor ~10 improvement expected with full statistics, and tracking improvements.

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 5 Joost Vossebeld IOP APP-HEP April 2023

µEDM experiment at PSI
 1 Ԧ × 2 B
 =− + 2 − + + Ԧ × 
 −1 ℏ 
 
With a suitable radial electric field ( ≅ 2) the g-2 precession can be removed,
leaving only the out of plane precession due to .

Proposed experiment:
• Highly polarised anti-muons from pion decays on production target, injected into
 solenoid via super-conducting channel and locked in orbit with a magnetic pulse.
• Radial E field from thin electrodes. = 3 
• Pixel detector for positron tracking,

 6
 Joost Vossebeld IOP APP-HEP April 2023 + @ 125MeV/ 

µEDM experiment at PSI
Stage 1
• = 28 MeV/ ; B=3T; E=0.3MV/m
• Demonstration of EDM frozen spin techniques

 ~20 cm
• Sensitivity d(µ) ~ 3 × 10−21 cm
• To be completed before 2027 HiMB upgrade

Stage 2
• = 125 MeV/ ; B = 3T; E = 2.0MV/m
 = 3 

• Sensitivity ~ 6 × 10−23 cm
 Brookhaven g-2
• HV-MAPS positron tracker
 Prospect FNAL g-2
• Start early 2030s
 Prospect
 PSI muEDM

UK focus on general experiment development and positron tracker

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 Joost Vossebeld IOP APP-HEP April 2023 7

MUonE: Alternative approach to measuring the hadronic
contribution to g-2
Alternative to extraction hadronic contribution from e+e- → hadrons (optical theorem).
The leading order hadronic contribution to aµ can also be determined from the hadronic
contribution to the running of electro-magnetic coupling.
Muon electron scattering is a clean way to measure this.
MUonE exploits 150 GeV muons at CERN to hit electrons at rest in a low Z target (Beryllium).
• Challenging kinematics (θμ < 5 mrad, θe < 30 mrad, Ee > 1 GeV) require excellent angular resolution and a
 measurement of the incoming muon.
• Multiple thin targets with associated tracking stations.
UK contributions
• Silicon modules for tracking station (based on CMS 2S strip modules)
• Low thermal expansion support structures for tracker stations
• Development of an upstream beam spectrometer
• Physics analysis
MUonE Schedule:
• Demonstration in 2023 (2 or 3 stations) Tracker station, CFRP
 Tracker station, invar frame
• First measurements with ~10 stations before LS3 frame
• Full experiment (40 stations) after LS3 (target 0.3% uncertainty on aμHLO)
 Joost Vossebeld IOP APP-HEP April 2023 8

Charged Lepton Flavour violation in Muon decays
Heavily suppressed in the SM due to low neutrino mass. SM

Charged lepton flavour violation can appears naturally in NP theories.
Any observation of CLFV is evidence of NP.

The long lifetime and clean decay modes make muon decays ideal to look for rare
CLFV decays!

If charged lepton flavour is not conserved we expect these neutrinoless muon
decays:
•  → eγ,
•  → eee
• N → eN

 Joost Vossebeld IOP APP-HEP April 2023 9

CLVF -decays

 Best limits Projected sensitivities (90%CL)
 →e < 4.3x10-13 MEG (PSI) 4x10-14 MEG II (PSI)
 →eee < 1.0x10-12 SINDRUM (PSI) 4x10-15 Mu3e I (PSI)
 1x10-16 Mu3e II (PSI)
 N→eN < 7.0x10-13 SINDRUM II (PSI) 6x10-17 Mu2e (FNAL)
  Au → e Au 7x10-15 COMET I (J-PARC)
 6x10-17 COMET II (J-PARC)

New experiments will push → e sensitivity by up to four orders of magnitude over the next 5-10 years.

 Joost Vossebeld IOP APP-HEP April 2023 10

Mu3e at PSI
Experiment design
• DC proton beam produces pions on target.
• Muons from pion decay collected in πe5 beamline
 (serving both MEG-II and Mu3e)
 HV-MAPS outer tracking layers (UK)

Inside mu3e
• Muons stopped on thin mylar target
• Decay positrons and electrons are tracked in ultra
 low mass tracker
• Excellent time and vertex resolution to reduce
 Scintillating fibres
 combinatorics
• Excellent momentum resolution to reject near- HV-MAPS vertex layers
 endpoint +→e+e+e−  decays Scintillating tiles

Sensitivity target
• Phase I: 108 µ/s → BR(→eee) < 2x10-15
• Phase II (after HiMB upgrade) → BR(→eee) < 10-16

 Joost Vossebeld IOP APP-HEP April 2023 11

Mu3e: Experiment design/status
MUPIX low mass (~1.1‰ X0) HV-MAPS tracking Interconnect circuit
layers
• Final production chip signed off and in production
• Production for HV-MAPS tracker in 2023/2024. 15 µm kapton
 50 µm
• Cooling plant for gaseous Helium cooling HV-MAPS (outer layers only)

 qualified and partial system in operation.

Similar schedule for scintillating fibres and tiles.

Off-detector infrastructure largely in place.

Schedule:
Commissioning with full central detector in 2024.
Physics with complete detector in 2025.

HiMB upgrade in 2027-2028, deploying large
capture solenoids to achieve 2x109 µ/s on target
(formal approval 2024)

Mu3e-II operation from 2023.

 Joost Vossebeld IOP APP-HEP April 2023 12

Mu2e (FNAL): N→ eN conversion
Experiment design
• Pulsed proton beam produce pions on target.
• Muons from pion decay captured with graded
 solenoid
• Curved transport solenoid to reduce backgrounds

 signal
• Muons stop on Aluminium target
• N → eN leads to monochromatic electron emission
 (Ee=104.97 MeV)
• Hollow tracker in detector solenoid → no acceptance
 for electrons from Michel decays, full acceptance for
 signal electrons.

Schedule:
• Construction to complete in 2025
• First physics in 2026
• Long shutdown 2027-
• Long physics programme from 2029

 Joost Vossebeld IOP APP-HEP April 2023 13

MU2e – STM (UK)
UK delivers the stopping target monitor and DAQ. to determine the muon-
on-target rate. HPGe detector and LaBr3 calorimeter to measure the
gamma ray spectrum.

Successful test beam in ELBE FZDR facility
 HPGe Resolution not degraded in presence of beam
• Demonstration of capability of both LaBr and HPGe detectors
• Employed DAQ hardware/firmware/software for first time
• HPGe - can take data at 100 kHZ at required resolution

 Joost Vossebeld IOP APP-HEP April 2023 14

Comet (J-PARC)
PHASE I (R(N→ eN) < 10-14)
• 3.2 kW proton beam
• Capture and transport solenoids to extract muons from pion decays.
• Muon target inside cylindrical detector in detector solenoid. Detector only sees the
 monochromatic electron from the LFV conversion process.
• Targeted start in 2024

PHASE II (R(N→ eN) < 10-16)
• 57 kW proton beam
• First bend to achieve clean muon beam
• Second bend acts as electron spectrometer followed by ECAL

Just completed first beamline commissioning run “Phase-α”:
• strong UK involvement UK in the planning and operations
• Data analysis ongoing

 Joost Vossebeld IOP APP-HEP April 2023 15

Summary
Exciting programme on precision muon physics programme (FNAL, PSI, J-PARC) with growing UK involvement.

• FNAL g-2 experiment nearing completion of data taking, but much of the analysis work still to be completed for both muon
 g-2 and EDM.
 • Next set of results expected this summer.
• Much UK work on g-2 theory and on measurements to constrain HPV in dispersive calculations.
 • Theory updates this summer
 • Development of MUonE experiment to take data after LHC LS3.

• Progress towards a dedicated muon EDM experiment at PSI.
 • Full experiment foreseen ~2030

• Lepton flavour experiments (mu3e, mu2e, Comet) are progressing towards start-up in mid/late 2020s.
 • 4 order of magnitude improvement on sensitivity previous experiments
 • Results will place tight constraints on NP models.

Many new results to look forward to over the next 5 to 10 years, with sensitivity to PeV-scale NP

 Joost Vossebeld IOP APP-HEP April 2023 16