Status of COMET~µ-e Conversion Search in J-PARC~
Yuki Nakai(Kyushu University)
on Behalf of the COMET Collaboration
15/08/2017

Flavour Physics Conference

1The History and TheoryN of Charged Lepton Flavour(CLFV)
χ̃µ Violationµ
ee0

CLFV and µ-e Conversion
µ

e

W−

µ

e

H0

γ

q

q

(d) Heavy Neutrinos

q

q

γ

µ

e

˜l−

e

H0

γ

Z′

γ
q

q

(e) Exotic Higgs

CLFV processes are good probes for BSM search.

qqq(f ) Supersymmetry(a)ExoticHiggs
photonic

Figure 1.6: Feynman diagrams that produce µ-e conversion through New Physics models.
The
N
upper three diagrams ((a) to (c)) all connect to the nucleus via some massiveµ (f)) allW−
exchange particle, whereas the lower three diagrams ((d) toconnect e-15 to interactions with the quarks, since
via an exchanged photon. In additionµ-e conversion interacts with the whole nucleus, there are also modelsγwhere theqqinteraction involves external gluon -50
lines.

BSM : O(10 )SM : < O(10 )4qq(b) Z-prime

four-fermion
µ

e

γ
PRISM

H0
q

e

q
γ

q

(d) Heavy Neutrinos(e) Exotic Higgs

1.3.2 Muon CLFV Channels< Current Limits of CLFV (90%C.L.) >
1.6 shows a variety of Feynman diagrams for µ-e conversion involving new particlesµ ! e + : < 4.2 x 10Fig.and-13couplings(MEG)
predicted by many BSM theories. The large variety of models to whichconversion
be sensitive makes this a particularly attractive search channel for-13 would(SINDRUMII)µ + N ! e + N : < 7 xµ-eNew10
MEG
Physics [29].

COMET Phase-II

10

Figure 1.6: Feynman diagrams that produce µ-e conversion th
upper three diagrams ((a) to (c)) all connect t
exchange particle, whereas
the Phase-I
lower three dia
COMET
via an exchanged photon. (extended)
In addition to inteµ-e conversion interacts with the whole nucleus,interaction involves external gluon lines.

1.3.2 Muon CLFV Channels
It can also be seen how complementary the diﬀerent muon
3 CLFV channels will be.
10
MEG(Previous)
SINDRUM-II
In the case of leptoquarks for example, shown in Fig. 1.6c, one can
expectµ-e conversion
Fig.
1.6
shows
a
varietyof
Feynmandiagrams
for µ-e conv
to take place at tree level, whilst generating a signal in a µ+ → e+ e− e+ experiment canand couplings
only occur via loop diagrams. Similarly, the relative sensitivities
between predictedµ+ → e+ γby many BSM theories. The larconversion
beissensitive makes this a particularl
searches and µ-e conversion searches can be used to pin down µ-e
what
the New would
Physics
New Physics
in the case of a positive observation, or heavily constrain numerous
diﬀerent[29].
models in
the case of a null measurement. This is apparent from the fact that
It canNew
alsoPhysics
be seencan
how complementary the diﬀerent
be classed as photonic (such as the lower three diagrams in Fig.
1.6)
or
as
a
four-Fermi
In the case of leptoquarks for example, shown in Fig. 1.6c,
2contact interaction (as in the upper three diagrams in Fig. 10
1.6).toThephysics,
takenew
place
at treewhichlevel, whilst generating a signal in a
0.01
0.1
1
10
100
‘switches on’ at some new mass scale, is integrated away to leaveaneﬀective,
low-energy
only occur via loop diagrams. Similarly, the relative sen
field theory.
searches and µ-e conversion searches can be used to pin dµ -> eγ : photonicµN -> eN : photonic & four-fermi
Combined result can distinguishphysics models.
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Photonic

Four-Fermi

2µ-e Conversion Searchmonochromatic ew/ 105 MeV/cµ-e conversion

⌫¯e

⌫µ

endpoint at ~105 MeV/c-> main physics BG

Decay In Orbit(DIO)< Experimental Merits of the µ-e conv. > 0.18

DIO

(1) Signal electron has 105 MeV/c-> Low physics BG(2) No coincidence is required

Counts per 0.1 MeV/c

0.16

signal

0.14
0.12
0.1
0.08
0.06
0.04

-> No accidental BG

0.02
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102

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103

104
105
106
Electron Momentum [MeV/c]

3

Proton Beam at J-PARC

JAPAN

Tokai

Proton beam with the highest intensity
Powerful proton beam : 8 GeV, 3.2 kW

J-PARCHigh statistics

Main Ring(0.75 MW)

3 GeV
Rapid Cycle

HadronExperimental
Facility

Linac(330m)(Log. scale)

Materials and Life science
experimental Facility

Neutrino to
Kamiokande

Pulsed beam & time window
Lifetime of muonic Aluminum : 864 ns
Time window : 700-1170 ns

Beam related BG free
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4COMET Experiment< Phase-I >ProtonProtonPhase-II

Pion
Production
TargetPion

Phase-I

Muon

Muon
Stopping
Target15/08/2017(1) Beam measurement-> study of the new beam line
(2) Physics measurementwith sensitivity, O(10-15)(improvement of ~100)< Phase-II > start in 202X
Physics measurementwith the highest sensitivity, O(10-17)(improvement of 10,000)

Muon
Stopping
TargetDetector Systemstart in 2018 or 2019

検出器
Detector
System

Flavour Physics Conference5Status of the Facility & Magnet
Beam Line

ProtonPion
Production
Targetinstallation of the Transport Solenoid
Experimental Hall
(new building)Phase-I

Experimental Area

Muon
Stopping
TargetDetector System

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6Status of the Detector Systems

Straw Tube Trackers + ECAL
(StrECAL)

physics measurement

Cylindrical Detector System(CyDet)15/08/2017

beam measurement
Flavour Physics Conference7Detector System (StrECAL)
Beam measurement (in Phase-I)
Physics measurement (in Phase-II)
Straw Tube Tracker
extremely low material tube whose thickness < 20 um
momentum resolution : < 200 keV/c<Performance for 105 MeV electron> requirement
Spatial resolution : 143 um 200 um

EM Calorimeter (ECAL)
20 x 20 x 120 mm3 LYSO crystals

Straw Tube Tracker(magnetic field : 1 T)
EM Calorimetermeasures energy, event timing, hit position-> triggers signal events<Performance for 105 MeV electron >requirements
Energy resolution : 4.22%
5%
Position resolution : 7.6 mm 10 mm
1 ns
Time resolution : 0.4 ns
Figure 11.33: Full-scale prototype; (Left) Partially completed without vacuum wall, (Right) Whole

Final Prototypes
view of the completed full-scale prototype
R&D has almost finished ! (including combined test)
Mass production will start in near future
2016, with the various momentum electron beam. The setup for the beam test is schematishown in Figure 11.34 (Left), and its photo is also shown in Figure 11.34 (Right). Here
Flavour PhysicscallyConference
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Detector System (CyDet)ePhysics measurement (in Phase-I)
Cylindrical Drift Chamber (CDC)
Cylindrical Drift Chamberconsists of 20 stereo layers~15,000 field wires, ~5,000 sense wires
momentum resolution : < 200 keV/c<Performance of the prototype>requirement
Spatial resolution : < 200 um at 1 T 200 umµ

Completed construction! (June 2016)
Under Commissioning with cosmic ray
Cylindrical
Cylindrical Trigger Hodoscopes (CTH)
Trigger Hodoscopes
plastic-scintillators and Cherenkov detectors
measures event time
identifies electron events<Performance of the prototype> requirement
Time resolution : < 0.8 ns
1 ns
prototype of the CTH
S/N ratio : S/N > 100
20
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Hodoscopes

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10Background Estimation

Table 20.8: Summary of the estimated background events for a single-event sensitivity of 3 ◊ 10≠15 in
COMET Phase-I with a proton extinction factor of 3 ◊ 10≠11 .

Type
Physics

BackgroundEstimated events
Muon decay in orbit
0.01
Radiative muon capture
0.0019Neutron emission after muon capture< 0.001
Charged particle emission after muon capture< 0.001
Prompt Beam
* Beam electrons* Muon decay in flight* Pion decay in flight* Other beam particles
All (*) Combined
Æ 0.0038
Radiative pion capture
0.0028
Neutrons≥ 10≠9Delayed Beam Beam electrons≥0
Muon decay in flight≥0
Pion decay in flight≥0
Radiative pion capture≥0
Anti-proton induced backgrounds
0.0012†OthersCosmic rays
< 0.01
Total
0.032

15/08/2017† This estimate is currently limited by computing resources.

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11Single Event Sensitivity
1
B(µ + Al ! e + Al) =Nµ · fcap · fgnd · Aµ e

Nµ : # of total muons stopped in the target (1.5 x 1016)-4 muons per 8 GeV proton
muon yield: 4.7x10
20.1.5Net
Signal Acceptance

3.2 kW and 1.26x107 sec operation (~146 days)
Thus thenet acceptancefor thetoµ≠econversionof Aµ-e =0.041 is obtained for T1 = 700
fcap : Fractionof captured
muons
total
muonssignal
on target(0.61)
ns and T2 =1170 ns, where appropriate numbers of the online event selection (see Section 16.1.3),
fgnd : Fractionof µ-e conversion to the ground state in the final state (0.9)
the offline track finding efficiency (See Section 13.5.1) and DAQ efficiency (assumed) are conAµ-e : Netsidered.
signalThe
acceptance(0.041)breakdown of
the acceptance is shown in Table 20.2.
Event selection
Online event selection efficiency
DAQ efficiency
Track finding efficiency
Geometrical acceptance + Track quality cuts
Momentum window (Ámom )Timing window (Átime )Total

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Value
0.9
0.9
0.99
0.18
0.93
0.3
0.041

Comments
Section 3
16.1.3x 10-15 (S.E.S.)

or< 7x10-15 (90% C.L.)

Section 13.5.1

103.6 MeV/c < Pe <106.0 MeV/c
700 ns < t < 1170 ns

Flavour Physics Conference12Summaryµ-e conversion is a good probe for BSM search.
COMET Phase-I searches for µ-e conversion with a sensitivity of O(10-15).
The highest intensity proton beam in J-PARC achieves high statistics.
Pulsed beam and time window suppress beam related BG.
Experimental hall and so on have been built and transport solenoid have been installed.
We have 2 detector systems; StrECAL and CyDet.
Both prototypes showed good performances.
Especially, CDC construction completed and commissioning is on going
S.E.S : 3 x 10-15, which corresponds to < 7x10-15 (90% C.L.)
Estimated BG events : 0.032 events
2018

2019

MagnetDetector &Beam line~half year run

Physics

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COMET Collaborators(176 collaborators)

Beam Line

CDC wires (in construction phase)
our mascot, COMET-chan

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backup slides

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Beam Structure
timing window method

suppress beam related BG

extinction factor(requirement : < 10-9)
good enough!

Figure 4.7: Extinction levels measured at the MR abort line with single bucket filling with tof protons equivalent to that of 3.2 kW operation, as a function of the applied RF voltage ducirculation after acceleration.

for the COMET experiment. We will optimise the voltage for long-term operations inkeep the RF cavity temperatures stable to within the capabilities of the water cooling

Main Ring is operated in “COMET mode”

No protons in the time window
15/08/2017

Another study was performed to understand how protons leak from a bucket during
circulation (flat-top) period. The extinction was measured at the abort line at differen
by adjusting the time at which the protons are kicked out from the MR. Figure
the extinction measured when applying 160 kV RF voltage as a function of the time
between the beginning and end of the flat-top period. A time-dependence in proto
can be seen. Further studies will continue in collaboration with the J-PARC accelera
to understand the mechanism behind this effect and to ensure delivery of the beamnecessary for the COMET experiment.

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16

aluminium for COMET. We took data with 100-µm-thick and 50-µm-thick aluminium target
The detector was placed about 30 Transition
cm far from thetargetposition.
The Intensity
absolute (%)
detectio
Energy(keV)
Relative≠4
efficiency ( ◊ Á) of the 2p æ 1s 2p
line
be 3.56 ◊ 10 . 79.8
One±measure
æ was
1s determined
346.828 ± to
0.002
08spectrum is shown in the Fig. 14.2 where
the
346.8
keV
line
is
clearlyvisible.
The
3pæ
3d æ 2p66.11 ± 0.02
67.6 ± 1.71s lin
is also recognisable around 410 keV 3p
region.
The
4p
æ
1s
line
is
howeverdominatedby doubæ 1s
412.87 ± 0.05
7.62 ± 0.15
peaks, 431 and 438 keV, from the leadshielding.
4p æ
1s
435.96 ± 0.10
4.87 ± 0.10

2p æ 1s346.828 ± 0.002
3d æ 2p66.11 ± 0.02
3p æ 1s412.87 ± 0.05
100
4pestimatedæ 1s# of muonic atoms isby 435.96 ± 0.10

79.8 ± 08
67.6 ± 1.7
7.62 ± 0.15
4.87 ± 0.10

2p->1s muonic X-ray detected byGermanium detector

Count/1keV

Muonic X-ray Measurement
80
60
40
20
0
300

350

400

450
keV
Figure 14.1: Germanium detector and its dimensions.

Figure 14.2: Muonic X-ray spectra taken at PSI. The 2p æ 1s muonic X-ray of 346.8 keV is clear
aluminium for COMET. We took data with 100-µm-thick and 50-µm-thick alu
seen.

Count/1keV

The detector was placed about 30 cm far from the target position. The abÁ) of the 2p
æ 1s will
linebewas
determined
to metrebe 3.56◊ 10≠4 .
In the COMETefficiency
Phase-I, (the◊germanium
detectorplacedabout 3.5
downstream
is shown
in the14.2inwhere
the 346.8
line
is clearlyvisible.
of the centre ofspectrum
the detector
solenoid,
asFig.
shown
Fig. 14.3,
since keV
there
will
be no open
spacT
is also recognisable around 410 keV region. The 4p æ 1s line is however domi
peaks, 431 and 438 keV, from the
198 lead shielding.
100
80
60

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40

17re, the determination of these SUSY contributions would enable us to study thethe slepton mass matrix, and then more importantly the “SUSY soft breaking”
origin of SUSY particle masses. It should be noted that the slepton mixing
to study, as precisely as in cLFV studies, at high energy collider experiments
The toHistory
and Theory of Charged Lepton Flavour Violation (CLFV)
LHC. Hence, studies of cLFV would provide a unique opportunity
study
ixing. In the following, the SUSY contributions to LFV are presented in moreµ->eγ and µN->eN

q

31

q
µ

µ

e

H0
q
(a) Exotic Higgsqqq(b) Z-prime

One of the diagrams of SUSY contributions to a µ to e transition (µ → e “γ”).
CHAPTER 2. PHYSICS MOTIVATION
ates the magnitude of the slepton mixing.

q

L

Z′

q

in BSM (SUSY model)

µ−

e

e−
(c) Leptoquarks
χ̃0

N

q

q

µ

W−

e

odels with the seesaw mechanism
γ

µ

e
H0

q

e

µ

˜l−

e

γ

γ

everal mechanisms which give the conditions in Eq. (2.3). Among these, gravityq
q
q
q
q
q
γ
oft breaking is the most often employed. It gives the conditions in Eq. (2.3)
ty scale (=Planck, ∼ 1019 GeV) or grand unified scale (∼ 1016 GeV). Non-zero(d) Heavy Neutrinos(e) Exotic Higgs(f ) Supersymmetry
matrix elements can then be induced by radiative corrections from MG to the(∼ 102 GeV).
Figure 1.6: Feynman diagrams
that produceµ-econv.
conversion
through New Physics models. Th
diagramsof µ-eprocess
oduce lepton mixing and the neutrino masses, the seesaw mechanism[42] hasd most extensively. In this mechanism, three right-handed neutrinos Ni are upper three diagrams ((a) to (c)) all connect to the nucleus via some massiv
exchange particle, whereas the lower three diagrams ((d) to (f)) all connec
for theofneutrinos
areneutrino
given by the
Yukawa couplingf Massthe terms
diagrams
massive
contributions
to between
a µ to thee transition
blets (Lα ) and the right-handed neutrinos and the Majorana mass term for the via an exchanged photon. In addition to interactions with the quarks, sincd neutrinos:µ-e conversion interacts with the whole nucleus, there are also models where th

in SM

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18Status of the CDC
Prototype study has finished.
Construction of the CDC already completed. (June 2016)

Now under commissioning with cosmic rayfrom upstream

CDC (under cosmic ray test)

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CDC wires (in construction phase)19Performance of the Trigger Hodoscopes(tdc0[0]-31.1/sqrt(padc[0]-14.9)) - (tdc1[0]-23.2/sqrt(padc[1]-Time Resolution2.0)) {((tof[1]-16.83/sqrt(tof_padc[1]-25.70) + tof[0]-16.37/sqrt(tof_padc[0]-25.11))/2.< -19.0 && (tof[1]-16.83/sqrt(tof_padc[1]-25.70) + tof[0]-16.37/sqrt(tof_padc[0]-25.11))/2.> -19.9)&&(tof_tdc1[0] > -100.0 && tof_tdc1[0] <-90.0 && tof_tdc0[0] > -100.0 && tof_tdc0[0] <-90.0)}

tdc

4000

EntriesMean
RMS
χ2 / ndf
Constant
Mean
Sigma

3500
3000
2500

92227
1.62
1.195
120.2 / 20
4055 ± 21.6
1.912 ± 0.004
0.7905 ± 0.0055

2000
1500
1000

r~45 cm500
0-5<Cherenkov Det.>
Mean
S/N ratio：170(1) 3.873

-1

10

RMS

23282

-2

Pedestal
-1

10

-1

0

1

2

3

4

5

Δt [ns]Sc<Scintillator>
Entries
23282
Mean
4.677
S/N ratio : 114.6
(7)

1.064

RMS

0.9468

-2

-2

10

-3

10

-4

10

10

-3

10

Signal-4

10

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Entries

EntriesCh-3

padc[0] {drs_padc<10 || drs_padc<10 || drs_padc>20 || drs_padc>>20}

padc[1] {drs_padc<10 || drs_padc<10 || drs_padc>20 || drs_padc>>20}

Entries

-4

100

200

300

400
500
Pulse Hight [mV]0

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100

200

300

400
500
Pulse Hight [mV]20

Electronics of the CyDet

Front-end board for CTH

Front-end board for CDC
analog+input+from+CDC+48ch

Slit
ASDs
ADCs

Slit
FPGA

Trigger/IF
DAQ/IF+
SiTCP

SFP+slot

Power
JTAG

LVDS

Trigger board (prototype)15/08/2017

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ed to amplify the APD signal. We have developed a prototype preamplifier, whoseagram is shown in Figure 12.3. Figure 12.4 shows the prototype preamplifier board,ads out seven APDs.

Electronics of the StrECAL
Figure 12.7: Theboard
next prototype
preamplifier
installed
the crate (E-ROS)
structure.
Mezzanine
+ WFD
board
forinECAL

Figure 12.3: A circuit diagram of a prototype preamplifier.

ECAL Preamplifier

Figure 12.8: The mezzanine board is in the left side and is connected to the EROS digitizer board.

12.4.

Module, Readout and Mechanical Structure

The basic unit of the ECAL is a 2◊2 crystal matrix module. In the ECAL system, 480(=1920/4) modules are needed to cover the full cross-section of the detector region. Figure 12.9
shows the schematic layout of the prototype module structure together with 4 APDs, Teflon tapeand a Al-Mylar case for wrapping and a preamplifier board. This module structure is designed
to achieve a good light collection efficiency and its uniformity with low material budget in frontof the crystals and between them. In Figure 12.9, the preamplifier board is located inside themodule structure. However, in the ECAL system, the preamplifier board is separated from the
Figure 12.4: A prototype preamplifier board.
module structure and is located outside the vacuum vessel as shown in Figure 12.6 for a better
WFD
board
the straw
tube tracker (ROESTI)
thermalmanagement
at an foracceptablenoise level.
plifier output is designed to be differential so that the signal can be transmittedovery long distances without suffering from noise. The receiver circuit of the preamplifier
A prototype module constructed (without the preamplifier board) is shown in Figure 12.10.
designed to convert the differential signal to the single-ended signal and toTheproducepolished crystals were wrapped with two layers of 76 µm think high reflection Teflon
signal with an analogue adder. The single-ended signals are passed to a tape
waveformfrom Saint-Gobain. One HAMAMATSU S8664-55 APD is attached to each crystal, using

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22Straw Tube Tracker
Align straw tube trackers along X&Y axis. (= a “station”)
5 stations measure a track of incident charged particle.
→ Reconstruct momentum of the charged particle.
Readout waveforms with 1 GSPS by a Front-end board.
→ Can distinguish pileup events.

position resolution

X-T plot
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ECAL
LYSO crystals（20 x 20 x 120 mm3）+ APD-> placed cylindrically(1) energy
requirements(2) event timing
energy resolution：5%(3) hit position on ECAL
time resolution：1 ns(4) supplying trigger
position resolution：10 mm(5) particle ID（use momentum measured by straw tube tracker）
Record waveforms with 1 GSPS
LYSO

silicone rubber(EJ-560)

reflectors (2 layer)outer side : Teflon (BC-642)
inner side : ESR

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APD(S8664-1010)

spacercoated with reflector

connectors
for slow control

APD board<ECAL Structure>

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Prototype24

Trigger Hodoscopes
triggers only electrons with high momentum<upstream side>

PrototypeLight Guide

90 mm
FM PMT(H8409-70) 156 mm

300 mm...

Preamplifier(front-end board)...
read-out &triggering board(COTTRI)

r~45 cm

DAQ system

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COMET Phase-II

3.1. INTRODUCTION TO THE COMET EXPERIMENT

31

Pion Capture Section• Spectrometer transports electrons with highmomentum to the detector system.

Production
TargetA section to capture pions with a largesolid angle under a high solenoidal
magnetic field by superconducting
maget

Detector Section
A detector to search formuon-to-electron conversion processes.

• measure momentum and energy with the
tracker and ECAL respectively.

Stopping
Target103

6.5. ELECTRON TRACKER

Pion-Decay andMuon-Transport Section
A section to collect muons fromdecay of pions under a solenoidal magnetic field.

Figure 3.1: Schematic layout of the muon beamline and detector for the proposed searchfor µ− −e− conversion, the COMET experiment.
the occurrence of beam-related background events, a pulsed proton beam utilizing abeam extinction system is proposed. Since muons in muonic atoms have lifetimes ofthe order of 1 µsec, a pulsed beam with beam buckets that are short compared withthese lifetimes would allow removal of prompt beam background events by allowing
measurements to be performed in a delayed time window. As will be discussed below,
there are stringent requirements on the beam extinction during the measuring interval.
Tuning of the proton beam in the accelerator ring as well as extra extinction devices
need to be installed to achieve the required level of beam extinction.
• Curved solenoids for charge and momentum selection: The captured pions
decay to muons, which are transported with high eﬃciency through a superconducting
solenoid magnet system. Beam particles with high momenta would produce electron
background events in the energy region of 100 MeV, and therefore must be eliminatedwith the use of curved solenoids. The curved solenoid causes the centers of the helicalmotion of the electrons to drift perpendicular to the plane in which their paths are
curved, and the magnitude of the drift is proportional to their momentum. By using
this eﬀect and by placing suitable collimators at appropriate locations, beam particles
of high momenta can be eliminated.

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