SciPost Phys. Proc. 1, 040 (2019)

MEG final results and progress towards MEG II
Toshiyuki Iwamoto1? on behalf of the MEG II collaboration
1 ICEPP, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan? iwamoto@icepp.s.u-tokyo.ac.jp
Proceedings for the 15th International Workshop on Tau Lepton Physics,
Amsterdam, The Netherlands, 24-28 September 2018
doi:10.21468/SciPostPhysProc.1

Abstract
The MEG experiment, which is to search for lepton flavor violating muon decay, had beensuccessfully finished in 2013. The final sensitivity of the experiment was 5.3 × 10−13 , andsince the experiment did not find any signal, the upper limit of the branching ratio of
the µ+ → e+ γ was set to be 4.2 × 10−13 at 90% CL. The MEG II experiment will improve
the sensitivity by an order of magnitude with three years data taking, and the target sensitivity is 6 × 10−14 . In 2018 after the detector integration, the muon beam data taking
will be planned with limited number of readout channels. In 2019, the engineering run
and the physics run will be started.
Copyright T. Iwamoto et al.
This work is licensed under the Creative Commons
Attribution 4.0 International License.
Published by the SciPost Foundation.

Received 10-12-2018
Accepted 17-01-2019
Check forupdates
Published 21-02-2019
doi:10.21468/SciPostPhysProc.1.040

The standard model (SM) of elementary particle physics can explain most experimental
results, and there is no strong evidence that indicates new physics beyond the SM up to now.
However, the SM can not explain why we have three generations, the existence of dark matter,
many parameters contained in the model itself, gravity, and we consider that the model is alow energy appproximation of a more fundamental theory. Many physicists in the world arelooking for such new physics beyond the SM.
One method is to use the highest energy accelerator like the Large Hadron Collider toproduce new particles which belong to the new physics. The other method is to use a highintensity beam to detect rare phenomena which can happen in new physics. One example isto look for lepton flavor violation (LFV) phenomena. LFV is practically forbidden in the SM,
but in the neutrino sector, it is already seen in the neutrino oscillation phenomena. In thecharged lepton sector, it has never been detected although there is no theoretical reason why
it is forbidden in the SM. On the other hand, many new physics scenarios like SUSY-GUT or
SUSY-seesaw predict large branching ratio of µ+ → e+ γ.
The MEG experiment was designed to search for such regions where new physics like
SUSY-GUT, SUSY-seesaw predict through µ+ → e+ γ, and it had a real chance to discover newphysics. The data taking had been performed between 2009 and 2013, and the final sensitivity
was 5.3×10−13 . Unfortunately the MEG experiment did not find any signal, and the upper limitof the branching ratio of the µ+ → e+ γ is set to be 4.2×10−13 [1].
The MEG II experiment will improve the sensitivity by an order of magnitude with three
years data taking, and the target sensitivity is 6×10−14 [2].
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Figure 1: The schematic view of the MEG detector

1

MEG Experiment

The MEG experiment was performed at the Paul Scherrer Institute (PSI) in Switzerland whichhas a world most intense 590 MeV proton accelerator with a current of 2.2mA. The accelerator
can produce DC muon beam up to 1×108 /s, but the MEG experiment used 3×107 /s to reduce
the accidental background, and thus to maximize the experimental sensitivity. Positive Muons
are stopped at the thin target located in the center of the magnetic spectrometer. We willmeasure a positron and a γ emitted from it at the same time, back-to-back, with the energy of
52.83MeV by the MEG detector (Fig. 1). Positrons are bent by the superconducting magnet,and the trajectory is tracked by the segmented drift chambers. Positron times are measured bya timing counter which consists of scintillation bars read out by PMTs. γs are measured with900L liquid xenon γ detector with 846 PMTs. There are two main backgrounds to search forµ+ → e+ γ decays. One is an accidental background of a usual Michel decay e+ and a randomγ from radiative muon decay (RMD) or annihilation in flight. The other is a radiative muon
decay background, and a γ is emitted in addition to the normal Michel decay. The time ofthe e+ and γ is coincident. Our dominant background is the accidental background. The total
number of accidental background (Nacc ) is proportional to2Nacc ∝ R2µ × ∆Eγ2 × ∆Ee × ∆Θeγ
× ∆t eγ × T,

where Rµ is a muon rate, ∆Eγ is a resolution of γ energy, ∆Ee is a resolution of e+ energy,∆Θeγ is a resolution of opening angle between e+ and γ, ∆t eγ is a timing resolution between
e+ and γ, and T is total measurement time. The number of the accidental background isproportional to the square of the muon beam intensity, and the lower instantaneous muon
beam rate is better to suppress the accidental background. The DC muon beam such as PSI
proton accelerator is suitable for this purpose. Better detector resolutions are also importantto reduce the accidental background.
The accumulated number of muons stopped on the target between 2009 and 2013 is
7.5×1014 µ+ , which is the full dataset for the MEG experiment. Five observables of Eγ , Ee ,teγ , θeγ , and φeγ are used for the analysis, and the blind analysis is adopted in Eγ , teγ plane.
After opening the blinded region, the maximum likelihood analysis is applied for those events
within the analysis window.
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Figure 2: The best functions of each variable projected from the likelihood results
for the full dataset, and the relative signal likelihood, Rsi g with full data points. The
blue solid line is the total fit result, the magenta dash line is the number of accidental
backgrounds, and the red dot-dash line is the number of RMD.
Fig. 2 shows the likelihood results projected into each variable. Rsi g is the relative signal
likelihood, defined by Rsi g = log10 (Lsi g /(0.07·LRM D +0.93·LAC C )). From the likelihood fit, thenumbers of signal, RMD, and accidental background are extracted. The green hatched histograms show the signal PDFs magnified with 100 times of Nsi g upper limit. All PDFs are wellconsistent with the data, and the result was consistent with no signal. The best fit values for
NAC C and NRM D were 7684 ± 103, 663 ± 59, respectively. The upper limit of the branchingratio of the µ+ → e+ γ is set to be 4.2×10−13 at 90%C.L.

2MEG II Experiment

The final sensitivity of the MEG experiment was limited by the accidental background. In orderto improve the MEG sensitivity further, we need to perform the detector upgrade to reduce thedominant accidental background. If the detector performance is improved sufficiently, we canmake use of the maximum beam intensity at PSI. To reduce the multiple scattering and the γ
background produced in the target, thinner target will be used in the MEG II. The stopping
rate will be 7×107 /s, which is more than twice intensity of the MEG. The drawing of the MEG
II detector is described in Fig. 3.
The positron spectrometer will reuse the superconducting magnet which has a special gradient magnetic field. In order to track 52.8MeV positron and to reconstruct vertex, angle, andmomentum, a new single volume cylindrical drift chamber (CDCH) is built for the MEG II
positron tracker. This has smaller stereo cells, and thus has more hits resulting in better momentum, angular resolutions. The total length of the drift chamber is 2m, and the chamber gas
will be He:iC4 H10 =85:15. A new pixelated positron timing counter (TC) is prepared to detect
52.8MeV positron time precisely with multiple hits down to 30 ps. 256 scintillator plates with40.3

SciPost Phys. Proc. 1, 040 (2019)

Figure 3: Drawing of the MEG II detector
a dimension of 12×(4 or 5)×0.5cm3 readout by SiPM are installed into both upstream anddownstream side. The higher positron detection efficiency is realized by less drift chambermaterial, high transparency towards timing counter.
The inner face of the liquid xenon γ detector (LXe), which was covered by 216 2-inch
PMTs, is now replaced with 4092 12×12mm2 VUV-sensitive MPPCs newly developed in cooperation with Hamamatsu [3]. This enables better granularity and uniformity of the detected
photoelectrons independent of the γ incident positions, and better energy, position resolutions
will be achieved.
High energy γs from radiative muon decays can be the dominant accidental backgroundsource for γ side because the material inside the drift chamber is reduced, and annihilationin flight can be reduced which is another dominant γ background. A tagging detector of theRMD to detect low momentum positron with high energy γ detection in the LXe detector canreduce the RMD background events. The new device for this, called Radiative Decay Counter(RDC), is newly introduced at the downstream side in the MEG II detector. Plastic scintillators
readout by SiPM are mainly to detect positron timing, and LYSO crystals with SiPMs are tomeasure positron energy.
Many detectors in the MEG II experiment have finer granularity with SiPMs and more
readout channels are required. Experiments like MEG II under high rate environment require
waveform digitizers to reduce pileup events. To fulfill these requirements, compact electronics with more functionality including waveform digitizer, simple trigger, bias voltage supply
for SiPM, amplifier, called WaveDREAM (Waveform digitizer with DRS4 readout module) aredeveloped at PSI. The online trigger is important to manage high event rate and backgroundsuppression, and the FPGA based trigger system is prepared for the MEG II experiment.
Fig. 4 shows the MEG II sensitivity as a function of the DAQ time.
Conservatively 20 DAQ weeks are assumed for a year. In a few weeks DAQ, the current
upper limit will be exceeded by the MEG II experiment, and the sensitivity will reach 6×10−14in three years.

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Figure 4: Sensitivity plot as a function of the DAQ time

3Current Status

CDCH construction is finished at PISA in July 2018, and sent to PSI. The HV conditioning wasperformed once outside the experimental site, and currently the CDCH detector is installedinside the superconducting magnet. TC construction has been fisniehd in 2017, and basic
performance test was already done with muon beam [4]. LXe detector construction has also
been finished in 2017. Detector commissioning is in progress [5]. RDC downstream detector
is constructed in 2017, and the performance test with muon beam was finished in 2017 [6].
The electronics development is in the final stage. Last year, 96 prototype WaveDREAM boards(1536 channels) were produced, and used for the beam time in 2017. This year, all the detectors are integrated in the experimental hall, and the prototype boards are used for the beamtime, too. Based on the final test this year, the mass production will be started next year [7].

4Conclusion

The MEG experiment finished physics data taking in 2013, and the final result was published in2016, which set the most stringent upper limit of the µ+ → e+ γ branching ratio, 4.2×10−13 at
90% C.L. In order to improve the sensitivity by an order of magnitude, the MEG II experiment,
which is to upgrade all the detectors of the MEG experiment, is proposed, and is accepted atPSI. The MEG II detector integration is now in progress. The pilot run in 2018 will have all thedetectors with limited number of electronics channels, and each detector performance will beevacuated carefully. The muon beam time will be started soon in December 2018. The mass
production of the readout electronics will be finished next year, and the engineering run will
be started followed by physics run. The sensitivity of the MEG II experiment will exceed thecurrent limit with a few month data, and will be improved by one order of magnitude withthree years data.

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Acknowledgements
This work is supported by MEXT/JSPS KAKENHI Grant Numbers JP22000004, JP26000004,
and JSPS Core-to-Core Program, A. Advanced Research Networks.

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