Precision Measurement of the π +→e+νe Branching Ratioin the PIENU Experiment
S. Ito For the PIENU Collaboration
Okayama University, Japan (E-mail: s-ito@okayama-u.ac.jp)
KMI2017 at Nagoya University, January 5-7, 2017

The Pion Branching Ratio Measurement and the PIENU Experiment
Study of rare decays is an important approach to search for new physics beyond the Standard Model. In the SM, the ratio of the charged
pion decay branching ratios, Rπ = Γ[π + →e+ νe (γ)]/Γ[π + →µ+ νµ (γ)] is precisely calculated with RπSM = (1.2352±0.0002) ×10−4 . While the previous
experimental values are Rπexp = [1.2265±0.0034(stat)±0.0044(syst)]×10−4 (1992, TRIUMF) and Rπexp = [1.2346±0.0035(stat)±0.0036(syst)]×10−4(1993, PSI), which indicate that there is a room for improvement by two orders of magnitude in precision. The PIENU experiment at TRIUMF
πwas designed to measure Rexp
more presicely than 0.1%.
Precise measurement of Rπ provides one of the most stringent tests of the hypothesis of electron-muon universality in weak interactions.
Examples of the new physics inducing universality violation are R-parity violating SUSY, heavy neutrino mixing, leptoquarks, and the eﬀects
of charged Higgs bosons. By measuring Rπ with precision of < 0.1%, it allows potential accsess up to the pseudoscalar mass scale of 1000 TeV.

The Rπ Measurement

The PIENU Detector

Decay positrons from π + →e+ νe decays (Ee = 69.8 MeV) andπ + →µ+ νµ decays followed by µ+ →e+ νe ν µ decays (π + →µ+ →e+ ,Ee = 0.5∼52.8 MeV) were detected by NaI(Tl). In order to detect
shower leakage, pure CsI crystals surrounded the NaI.
The Rπ was extracted by time spectra ﬁtting with some corrections(e.g. π + →e+ νe low energy tail). The amount of low energy tail was estimated by suppressing dominant π + →µ+ →e+ decays (ﬁlled energy spectrum), and using mono-energetic 70 MeV positron beam. The amountof the low energy π + →e+ νe tail was estimated to be (3.07±0.12)%.
Another systematic correction came from the diﬀerent relative acceptance of π + →e+ νe and π + →µ+ →e+ events. The uncertainty of acceptance correction was estimated within 0.03%.
107

Energy and Time Spectra

610(a)

5Counts/(0.25 MeV)

104(a) π→µ→e(b) π→eνCounts/bin

10105

31010-300

102

104-200-100

0100

200

300

Counts/bin

1010 20 30 40 50 60 70 80 90 100
Energy in NaI + CsI (MeV)

310102

101-300-200-100

0100

200

Rπexp = [1.2344±0.0023(stat)±0.0019(syst)]×10−4 (0.24%)
which is consistent with previous work and the SM prediction. The
present result improves the test of electron-muon universality compared to previous experiments by a factor of two: gµ /ge = 1.0004±0.0012
for charged current. This result was already published inA. Aguilar-Arevalo et al., Phys. Rev. Lett. 115, 071801 (2015).
The analysis of the remaining data is in progress and another data
set taken in 2011 was also completed. The combined result of 2010 and2011 data sets is∆Rπexp = [0.0014(stat)±0.0013(syst)]×10−4 (0.15%).
The statistical uncertainty by full data set analysis is expected to be
improved to 0.0007. It is anticipated that the systematic uncertainty
will also be substantially improved.

300

400500600Time[ns]

Discussion & Conclusion• Precision of 0.24% allowed potential access to new physics up to500 TeV/c2 , and 820 TeV/c2 by current result.
• Some theories beyond the SM were constrained (e.g. Massive neutrino search in π + →e+ νe decay. )S. Ito et al., Hyperﬁne Interact (2017) 238: 1.
• Full data set analysis and massive neutrino search are in progress.
• It is anticipated that analysis of full statistics will be ﬁnished in2017, and precision of Rπ will be improved to < 0.1%.
10-4|Uei|2

Data taking of the PIENU experiment was completed in 2012, andthe analysis of a partial data set taken in 2010 has been completed.
The partial data set corresponds to about 4×105 π + →e+ νe decay events.
For this data set with all corrections including low energy tail is

500
600
Time[ns]

π+→µ +→e+(b)

A.A.Aguilar-Arevalo et al.,Nucl.Instrum.Meth.A791.38-46(2015)

Results and Current Status

400

π+→e+ν e

Background events

10B1, B2, B3, T1, T2, V1, V2, and V3: Plastic scintillator

π+→µ+→e+Muon events
πDIF events

2103

WC1−3:Wire Chamber S1 − 3:Silicon strip

410

Result in 1992
PIENU Result10-5

10-6

10-710-80

20406080
100120
Neutrino mass [MeV/c2]