Muon to electron conversion

TheCOMET [J-PARC] & Mu2e [Fermilab]Experiments

 Positioning the 1st COMET transport solenoid

Phillip Litchfield(COMET)Charged lepton flavour violation
We already know that lepton flavour is not conserved• Weak mixing mechanism & non-degenerate neutrino masses• Neutrino (lack of) mass & charge means this is easiest to observein neutrino oscillations, but can also lead to CLFV:
𝝁𝒆
𝝂𝑾𝝂
𝝁𝑾

Neutrino oscillation𝒆𝑾
𝝁 – 𝒆 transition(without radiation)•The basic SM amplitudes can be related to the neutrino oscillation
parameters, but requires some radiation to conserve energy &momentum.

•The 𝜇 – 𝑒 system is particularly simple because the radiated ‘mass’must be neutral, and lighter than a muon.

Options for decaying muons
The most obvious candidate for the transition
to radiate is a photon, and the branching ratio is:Γ 𝜇 → 𝑒𝛾∝Γ 𝜇 → 𝑒νν𝑚𝑖2 ∗2 𝑈𝜇𝑖 𝑈𝑒𝑖𝑚𝑊𝑖2•

𝝁~ O 10−54𝒆𝑾𝜸

𝝁 → 𝒆𝜸 (see note)

For a free muon, 𝛾 (or 𝑒𝑒) is the only option…
…but in a muonic atom the radiation can be virtual
The nucleus absorbs it, and recoils slightly.
•𝝂

Because of the relatively large nuclear mass,
the electron is effectively mono-energetic.
Because the process does not require a‘real’ photon, other diagrams are possible…

𝝂𝝁𝒆
𝑾

𝜸𝑨

𝑨
𝝁 – 𝒆 conversion

Note: The 𝜸 can connect
anywhere, not just in the loop

New physics
Similar processes exist in a wide variety of new physics scenarios.
• Muon decay is at low energy, so reduce to effective operators:
𝝁𝑿
𝝁𝒆𝜸𝒆𝑿′𝜸𝝁𝝁 → 𝒆𝜸

Dipole coupling𝒆𝑚𝜇ℒd ~ 2 𝜇𝜎𝜇ν 𝑒 ∙ 𝐹𝜇νΛ𝜸𝒀𝝁𝒆𝒒𝒒

𝝁𝒆𝒀′

1ℒ4 ~ 2 𝜇 𝛾𝜇 𝑒 ∙ 𝑞𝛾𝜇 𝑞
Λ𝝃𝒒 (𝑨)𝒒 (𝑨)𝝁 – 𝒆 conversion

𝒒𝒒

Four-fermion coupling𝜇𝑁 → 𝑒𝑁 and 𝜇 → 𝑒𝛾•/ Tev••

𝟏𝟏
𝜿𝜇νℒ𝜇𝑒 ~ 𝟐𝑚𝜇 𝜇 𝜎𝜇ν 𝑒 ∙ 𝐹 +𝜇 𝛾𝜇 𝑒 ∙ 𝑞 𝛾𝜇 𝑞
𝜦 𝜿+𝟏
𝜿+𝟏
New physics  CLFV in rare muon decays.
Energy scale 𝜦 affects the1 2
rate of all such processes.
Rate~ 2ΛParameter 𝜿 depends on thenature of the new physics

Both 𝝁 → 𝒆𝜸 and 𝝁 – 𝒆 conversion
are sensitive to dipole terms, but
𝝁 – 𝒆 conv. is also sensitive to4-femion terms.
• More sensitive to some models.
• (If signal seen) the comparison
allows discrimination between models

COMET Phase-I(𝝁𝑵 → 𝒆𝑵 on Al) <10-14

𝜿Basics of a (modern)𝝁 – 𝒆 conversion experiment
Cartoon: Flip Tanedo http://www.particlebites.com/

Muon decays
Muons allowed stop in suitable target.
• Initially Aluminium, but other materials (Ti) under study.
• Conversion from 1s orbital: 𝜇𝑁 → 𝑒𝑁 gives a𝜇mono-energetic electron at 105MeV (≈ 𝑚𝜇 − 𝐵1𝑠 )Al‘Normal’ decays are backgrounds• Nuclear muon capture: 𝜇𝑁(𝑍) → ν𝑁(𝑍 − 1)
• Decay in Orbit [DIO]:
𝝁𝑵 → 𝒆𝝂𝝂𝑵

For a free muon, cuts off at
1𝑚 , but bound state has a
2 𝜇small tail up to 𝑚𝜇𝝁

Backgrounds
Three main background processes:• Decay in orbit, as before Energy resolution!
• Decay in flight:
Electrons from energetic freemuons can be boosted
to 105MeV.

Results from SINDRUM-II(BR <𝟕 × 𝟏𝟎−𝟏𝟑 @ 90%CL)**Muon DIO– Use momentum selection inmuon transport (see later)e energy /MeV

• Beam backgrounds:
Significant number of prompt 𝑒 − and 𝜋 − produced by beam. Can
eliminate this with timing if we have reliably beam-free time
windows.

Beam-free windows?
Naïvely, this sounds easy, but…
• High intensity pulsed muon beams are uncommon  new facilities• Need 𝝉𝝁 ≫ 𝝈𝐏𝐮𝐥𝐬𝐞 so choose stopping targets with long lifetime
 low Z, conflicts with high A preferred for coherent signal.
Aluminium (𝒁 = 𝟏𝟑, 𝝉𝝁 = 𝟎. 𝟖𝟖𝛍𝐬) preferredCartoon from
0.08

Protonpulse at production target
POT
pulse
-−
𝜋 arrival/decay
arrival/decaytime
time ( ´ 1M )p-−
𝜇 arrival
arrivaltime
time ( ´ 400 )m-−
𝜇 decay/capture-atom lifetime time ( ´ 400 )
m

0.07
0.06
0.05
0.04
0.03
0.02

Time selection window(at tracker plane)

0.01
00200

400

600

800

1000

1200

1400

1600
1800
Time (ns)Really beam-free windows?
Synchrotrons have stable acceleration buckets• Even if you don’t inject protons intothem, stray protons can remain
in stable acceleration.
RCSThe signal process is rare,
so requirements on the extinction
between pulses is very strict

Intermediate bucket BG
would end up here!

Main Ring
4/9 buckets filledMuon source
Main driver of sensitivity: Need lots of low energy muons!
• Use dedicated high-power pulsed proton beam lines (~8 GeV)• Resonant slow extraction onto pion production target
••

•

•
•

Collect backward-going
pions with capture solenoid
Can use gradient field toreflect low-energy forward
pions. (Mu2e )Maximise field at target togive larger solid angle apperture (COMET)Primary p

Pions decay to muons en-route to stopping target.
Many neutrons produced, requires careful shielding. The curved
transport line helps to eliminate direct line-of sight.

Muon transport

Muon transport is a curved solenoid• Particles are channelled in spiralpaths [solenoid], which naturally tend
up/down [curvature] depending on
momentum and charge• Gives charge sign and momentum
selection, which can be enhanced byusing a collimator.
• Use to eliminate high momentum
muons, other particles.
• Eliminates line-of-sight fromproduction target

Mu2e

 3D printed model of Mu2e solenoids

Mu2e overview
1.0T

2.5T

1.0T

4.7T
2.0T

•
•

S-shape and off-centre collimators that can rotate for BG studies
Stopping target is 17 × 0.2mm Al foils•

Target & detector surrounded by solenoid– Electron transport– Magnetic mirror•

Electrons spiral from target to tracker and EM calorimeterMu2e Tracker•••

Tracker made from straw tubes tomeasure particle momentum
Minimum radius of 380mm corresponds
to momentum ~ 60MeV‘Complete’ tracks need mom > 90 MeV

Mu2e calorimeter•••

Inorganic scintillator crystals
Provides fast response for trigger
Energy measurement combined withmomentum from tracker gives excellent
𝜇 𝑒 discrimination

COMET View from (newly furnished) COMET Counting Room

Two phases of COMET

Phase IPhase II

In time-revered order: Phase II…
• Muon transport is 180°  larger dispersion.
• COMET transport coils use compensating
dipole so selected tracks stay level.
• COMET uses a second curved solenoid as
an electron spectrometer. This filters out‘low’ momentum and +ive backgrounds• Final detector is tracker / EM calorimeter(like Mu2e) but full plane – thanks tospectrometer.

…and Phase I

Phase I has 2 goals:• Investigate backgrounds for phase II• Perform search at 100× sensitivity of
SINDRUM-II
For Phase I measurement use a cylindricaldrift chamber around the stopping target.
• Triggering by auxillary hodoscopes
Also include prototypes/partial elementsof Phase II detectors for development andcharacterising backgrounds at low current

Muon tracks

COMET developments
 COMET hall construction
completed!

 Magnets laid-out fornew beam switchyard

Transport Solenoid
Corrective dipoles
Completed 90° muontransport arc (includingoctagonal return yoke)

Extinction test

RCS

Main Ring

Requirement

Comet requires extinction:
𝑁𝐸𝑚𝑝𝑡𝑦𝐸=< 10−9𝑁𝐹𝑖𝑙𝑙𝑒𝑑

Important test in May 2014:
Excellent results!

Timelines
You are here

2013 2014 2015 2016 2017 2018 2019 2020 2021
COMET
Phase-I

Construction

COMET
Phase-II
Mu2eConstruction

Construction

2014: 𝟕 × 𝟏𝟎−𝟏𝟑 90% U.L. [ SINDRUM-II ] (since 2004)~2017: 𝟑 × 𝟏𝟎−𝟏𝟓 S.E.S. [ COMET Phase-I ] (~ 6mo)
~2021: 𝟑 × 𝟏𝟎−𝟏𝟕 S.E.S. [ COMET Phase-II & Mu2e ]beyond 2021: PRISIM /PRIME @J-PARC? [Goal 𝟑 × 𝟏𝟎−𝟏𝟗 ]Mu2e × ProjectX @FNAL?

Summary•

Charged lepton flavour violation is ‘natural’ in new physics
scenarios.

•The (arguably) SM process is driven by neutrino mixing, and ishugely suppressed: 𝐎 𝟏𝟎−𝟓𝟒•

Complimentary approaches from 𝜇 → 𝑒𝛾 𝜇 → 𝑒𝑒𝑒 and𝝁 – 𝒆 conversion•

Current limit is 𝐎 𝟏𝟎−𝟏𝟐 from SINDRUM-II•

COMET & Mu2e can improve on this by 4 orders of magnitudein 5~10 years•

Information on what the NP is through similar experiments.

Reserves

Production target
Phase-I baseline (unlikely to change):
60cm × 2cm dia. graphite (IG-43) target.
Higher Z is better for pion production,but graphite is a ‘safer’ choice:• IG-43 is used for T2K target(FX, >200kW beam) so is known tobe capable of handling our beam.
• Lower irradiation of target and shield makes removal and storage
safer in case of replacement in Phase-II• At Phase-I power, radiative cooling is sufficient for this target.

Capture Solenoid
Comet needs low energy pions
so collect from back and sidesof target.

Gradient field converts
transverse momentum into
longitudinal momentum.
• Effectively increases thesolid angle aperture into thetransport solenoid.

Cooling and shielding
A 5T solenoid is (unsurprisingly?) superconducting.
• And therefore cryogenically cooled…
But there is a high power beam hitting a target in the middle!
• Phase I: this heating is estimated up to•Phase II: heating can be[c.f. other sources ~15W]

Shielding is needed, for radiation and thermal heating.
• Copper and tungsten shieldCoils• Cooled with water• Will probably need upgrade forPhase II, gets very (radioactively) hot.
Non-trivial engineering challenge!

Phase-I Detector
Phase-I will have a dedicateddetector for μ  e conversion
measurements.
Because of the charged
particle tracks in the centre
channel, a co-axial cylindergeometry is used.
 CyDetThe detector and capture
target will sit within a 1T
solenoid field.

Low momentum particles
do not reach the detector

Referencedesign(close to latest)
Alternatives

CyDet
The main part of the detector
is a coaxial drift chamber• Helium-based gas mixture
to reduce multiple scattering.
– Resolution ~ 200 keV• z measurement by stereo layers• Large inner radius to reduce DIO hit rate
– Dim: 150cm × 84cm(outer) // 50cm(inner)• 19 concentric sense layers• Triggering from hodoscopes at ends

Drift chamber progress
Electron track [~3Hz]
Proton tracks[~7 in 500ns]

 Event display showing
event projection
 Stringing wires and CR test of
prototype section

Beam extinction

Intermediate bucket BG
would end up here!

J-PARC MR has 9 stable acceleration buckets• Need to maintain RF during extraction,
so that bunch structure remains.
• If RF is not strong enough,RCS
protons will ‘leak’ into empty
buckets.
Signal process is rare so even*currently prefer
3/9 buckets
a small leak is a major background

Main Ring
4/9 buckets*
filledExtinction measurement
COMET design requires that we can achieve
an extinction:
𝑁𝐸𝑚𝑝𝑡𝑦𝐸=< 𝟏𝟎−𝟗𝑁𝐹𝑖𝑙𝑙𝑒𝑑
Extinction can be improved by increasing RFvoltage, but this heats the cavities.
(And there is a limit…)
Accelerate Maintain RFP4
(End of flat-top)2012 test at 30 GeVdemonstrated this is possible
for RF > 120kV,
But study time at 8 GeV not
allocated until this year…