The PIENU experiment at TRIUMF aims to measure the branching ratio of the pion decay modes Rπ=[π+→e+νe(γ)]/[π+→μ+νμ(γ)] with precision of &lt;0.1%. 

Precise measurement of Rπ provides a stringent test of electron-muon universality in weak interactions. The current status of the PIENU experiment and future prospects are presented

Aguilar-Arevalo, A.

Aoki, M.

Blecher, M.

Britton, D.I.

Bryman, D.A.

Chen, S.

Comfort, J.

Cuen-Rochin, S.

Doria, L.

Gumplinger, P.

Hussein, A.

Igarashi, Y.

Ito, S.

Kettell, S.

Kurchaninov, L.

Littenberg, L.

Malbrunot, C.

Mischke, R.E.

Numao, T.

Protopopescu, D.

Sher, A.

Sullivan, T.

Vavilov, D.

vom Bruch, D.

English

Enlighten

PoS(KMI2017)034Precision Measurement of the p+!e+ne BranchingRatio in the PIENU ExperimentS. Ito1, A. Aguilar-Arevalo2, M. Aoki3, M. Blecher4, D. I. Britton5, D. A. Bryman6, D.vom Bruch6, S. Chen7, J. Comfort8, S. Cuen-Rochin6, L. Doria9, P. Gumplinger9, A.Husseina, Y. Igarashib, S. Kettellc, L. Kurchaninov9, L. Littenbergc, C. Malbrunotd;6,R. E. Mischke9, T. Numao9, D. Protopopescu5, A. Sher9, T. Sullivan6, and D. Vavilov91Okayama University, Okayama-shi, Okayama, 700-8530, Japan2Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de Mexico, México3Osaka University, Toyonaka, Osaka, 560-0043, Japan4Virginia Tech., Blacksburg, VA 24061, USA5University of Glasgow, Glasgow, UK6University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada7Tsinghua University, Beijing, 100084, China8Arizona State University, Tempe, AZ 85287, USA9TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, CanadaaUniversity of Northern British Columbia, Prince George, B.C. V2N 4Z9, CanadabKEK, 1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, JapancBrookhaven National Laboratory, Upton, NY 11973-5000, USAdPresent address: CERN, Genéve 23 CH-1211, Switzerland1E-mail: s-ito@okayama-u.ac.jpThe PIENU experiment at TRIUMF aims to measure the branching ratio of the pion decay modesRp = [p+!e+ne(g)]=[p+!m+nm(g)] with precision of < 0:1%. Precise measurement of Rpprovides a stringent test of electron-muon universality in weak interactions. The current status ofthe PIENU experiment and future prospects are presented.The 3rd International Symposium on? Quest for the Origin of Particles and the Universe"5-7 January 2017Nagoya University, JapanSpeaker.c Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/PoS(KMI2017)034Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. Ito1. IntroductionThe pion branching ratio, Rp = [p+!e+ne(g)]=[p+!m+nm(g)] is one of the most preciselycalculated in the Standard Model (SM). The most recent theoretical prediction including radiativecorrections [1, 2] gives RpSM = (1:23520:0002)10 4. The p+!e+ne branching fraction is sup-pressed to the order of 10 4, which is well known as “helicity suppression". A precise measurementof Rp provides a sensitive test of the SM hypothesis of electron-muon universality. The previousexperimental results at TRIUMF and PSI are respectively Rpexp = [1:22650:0034(stat)0:0044(syst)]10 4 [3] and Rpexp = [1:23460:0035(stat)0:0036(syst)]10 4 [4]. Both experimentsobtained comparable levels of statistical and systematic uncertainties; however there is a roomfor improvement by two orders of magnitude in precision to reach the precision of the theoreticalresult.The goal of the PIENU experiment is to improve the accuracy of Rp by a factor of 5, to <0.1%,which corresponds to 0.05% for the universality test. This precision also allows access to newphysics up to the mass scale of 1000 TeV/c2 for helicity unsuppressed pseudoscalar interactions[2]. Examples of the new pseudoscalar interactions probed include R-parity violating SUSY [5],leptoquarks [6], and the effects of charged Higgs bosons [2]. The p+!e+ne decay measurementis also sensitive to the existence of heavy sterile neutrino mixing [7] through the Rp measurement[8, 9] and a direct additional peak search [10].2. The PIENU ExperimentFigure 1 shows the schematic of the PIENU detector and the pion decay modes of interest.The PIENU detector was located at the TRIUMF M13 beam line [11]. The beam with momentum75 MeV/c was tracked by two multi-wire proportional chambers (WC1 and WC2) and two sets ofSi strip detectors (S1 and S2). After WC2, two thin plastic scintillators B1 and B2 were placed inorder to measure time and energy loss for pion identification. A plastic scintillator target B3 withthickness of 8 mm was immediately downstream of S2 and the beam pions decayed at rest in thecenter of the target.In order to reconstruct the decay-positron tracks and define their acceptance, another Si stripdetector (S3) and multi-wire proportional chamber (WC3) were employed. Two thin plastic scin-tillators (T1 and T2) were used to measure the time of decay positrons. These positrons entered alarge single crystal NaI(Tl) calorimeter surrounded by two rings of pure CsI crystals (97 total) todetect shower leakage. See Ref. [12] for more details of the PIENU detector.3. AnalysisThe decays p+!e+ne, and p+!m+nm followed by m+!e+nenm (p+!m+!e+ decay chain)have different energy and time characteristics that can be measured precisely and used to extractthe ratio Rp . The muon in p+!m+!e+ has a kinetic energy of 4.1 MeV and a range in plasticscintillators of about 1mm; the total energy of the decay positrons have a continuous distributionfrom 0.5 to 52.8 MeV. The decay time spectrum of p+!m+!e+ distributes as lplmlp lm (e lm t  e lp t), where lp and lm are the decay constants of the pion and muon, respectively. In contract, the1PoS(KMI2017)034Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. ItoFigure 1: Schematic of the PIENU detector and pion decays.decay p+!e+ne produces a mono-energetic positron at 69.8 MeV, and the decay time distributionis a simple exponential curve (lpe lp t). Rp can be obtained from the ratio of the positron yieldsfrom p+!e+ne decays and p+!m+!e+ decays by taking advantage of these differences.Figure 2 shows the energy spectrum of decay positrons in the NaI(Tl) plus CsI detectors.Two energy regions, above and below the vertical dashed black line in Figure 2 were used todistinguish the two pion decay modes. The time spectra in these two energy regions were fittedsimultaneously to a fitting function including backgrounds to extract raw branching ratio (Rpraw), towhich corrections were applied.The most important correction for Rpraw, which dominated the systematic uncertainties, wasthe p+!e+ne low-energy tail due to shower leakage from the calorimeters. The amount of thep+!e+ne low-energy tail below 52 MeV was estimated using two ways. First, the dominantp+!m+!e+ events were suppressed in the energy spectrum, and the p+!e+ne low-energy tailwas estimated by subtracting the remaining p+!m+!e+ suppressed spectrum. Second, a specialmeasurement of the energy spectrum using a mono-energetic positron beam was performed in orderto obtain the low-energy tail empirically.Additionally, the difference of the positron acceptance between p+!e+ne and p+!m+!e+events also required a significant correction. The uncertainty of the acceptance correction wasmuch improved compared to the previous TRIUMF experiment because the PIENU detector solidangle was about ten times larger than the previous experimental setup [3, 12]. See Refs. [8, 13] formore details of the analysis.4. Status and ConclusionThe PIENU experiment completed data taking in 2012, and the result of the partial data set2PoS(KMI2017)034Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. ItoEnergy in NaI + CsI [MeV]0 10 20 30 40 50 60 70 80 90 100Counts110210310410510+e→+µ→+pi eν+e→+piFigure 2: Measured energy spectrum of decay positrons without (solid black line) and with (dashed redline) p+!m+!e+ suppression. Dashed vertical line shows energy threshold at 52 MeV.analysis taken in 2010 was published in Ref. [8]. For this data set, the value of Rp including allcorrections is Rp = [1:23440:0023(stat)0:0019(syst)]10 4, which is consistent with the SMprediction. This result improves the test of electron-muon universality to gm=ge = 1:00040:0012for the charged current.A search for massive neutrino in the mass range of 0 130 MeV/c2 was also performed us-ing the 2009 and 2010 data sets. This analysis improved the 90% confidence-level limits on theneutrino mixing parameter Uei between the weak electron-neutrino eigenstate and a hypotheticalmassive eigenstate mni by a factor of up to four. Details of this analysis are described in Refs.[8, 9, 10].At present, analysis of the remaining data is in progress, and the 2011 data set was also ana-lyzed. By combining results of the 2010 and 2011 data sets, the uncertainty of Rp is improved toDRp = [0:0014(stat)0:0013(syst)]10 4, which corresponds to 0.15% in precision, an improve-ment by a factor of 3 [13]. Full data set analysis is expected to improve the precision of Rp to< 0:1% and will be finished in a few years.AcknowledgementsThis work was supported by the Natural Sciences and Engineering Research Council GrantSAPPJ 157985-2013 and TRIUMF through a contribution from the National Research Council ofCanada, and by the Research Fund for the Doctoral Program of Higher Education of China, byCONACYT doctoral fellowship from Mexico, and by JSPS KAKENHI Grants No. 18540274, No.21340059, and No. 24224006 in Japan. We are grateful to Brookhaven National Laboratory forthe loan of the crystals and to the TRIUMF operations, detector, electronics, and DAQ groups fortheir engineering and technical support.3PoS(KMI2017)034Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. ItoReferences[1] V. Cirigliano and I. Rosell, JHEP. 10 (2007) 005.[2] D. A. Bryman, W. Marciano, R. Tschirhart, and T. Yamanaka, Annu. Rev. Nucl. Part. Sci. 61, 331(2011).[3] D. I. Britton et al., Phys. Rev. Lett. 68, 3000 (1992) and Phys. Rev. D 49, 28 (1994).[4] G. Czapek et al., Phys. Rev. Lett. 70, 17 (1993).[5] M. J. Ramsey-Musolf et al., Phys. Rev. D76, 095017, (2007).[6] O. Shanker. Nuclear Physics B, 204(3):375-386. (1982).[7] T. Asaka et al., JHEP 05, 030 (2009).[8] A. Aguilar-Arevalo et al., Phys. Rev. Lett. 071801, 115 (2015).[9] S. Ito et al., Hyperfine Interact. 238 (2017) no.1, 1.[10] M. Aoki et al., Phys. Rev. D 052002, 84 (2011).[11] A. Aguilar-Arevalo et al., Nucl. Instrum. Methods. A609, 102 (2009).[12] A. Aguilar-Arevalo et al., Nucl. Instrum. Methods. A791, 38 (2015).[13] S. Ito, Ph.D thesis, Osaka University (2016).4

Precision Measurement of the π+→e+νe Branching Ratio in the PIENU Experiment

Enlighten: Publications

P
o
S(KMI2017)034
Precision Measurement of the p+!e+ne Branching
Ratio in the PIENU Experiment
S. Ito1, A. Aguilar-Arevalo2, M. Aoki3, M. Blecher4, D. I. Britton5, D. A. Bryman6, D.
vom Bruch6, S. Chen7, J. Comfort8, S. Cuen-Rochin6, L. Doria9, P. Gumplinger9, A.
Husseina, Y. Igarashib, S. Kettellc, L. Kurchaninov9, L. Littenbergc, C. Malbrunotd;6,
R. E. Mischke9, T. Numao9, D. Protopopescu5, A. Sher9, T. Sullivan6, and D. Vavilov9
1Okayama University, Okayama-shi, Okayama, 700-8530, Japan
2Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de Mexico, México
3Osaka University, Toyonaka, Osaka, 560-0043, Japan
4Virginia Tech., Blacksburg, VA 24061, USA
5University of Glasgow, Glasgow, UK
6University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada
7Tsinghua University, Beijing, 100084, China
8Arizona State University, Tempe, AZ 85287, USA
9TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada
aUniversity of Northern British Columbia, Prince George, B.C. V2N 4Z9, Canada
bKEK, 1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, Japan
cBrookhaven National Laboratory, Upton, NY 11973-5000, USA
dPresent address: CERN, Genéve 23 CH-1211, Switzerland
1E-mail: s-ito@okayama-u.ac.jp
The PIENU experiment at TRIUMF aims to measure the branching ratio of the pion decay modes
Rp = [p+!e+ne(g)]=[p+!m+nm(g)] with precision of < 0:1%. Precise measurement of Rp
provides a stringent test of electron-muon universality in weak interactions. The current status of
the PIENU experiment and future prospects are presented.
The 3rd International Symposium on“ Quest for the Origin of Particles and the Universe"
5-7 January 2017
Nagoya University, Japan
Speaker.
c Copyright owned by the author(s) under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/
P
o
S(KMI2017)034
Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. Ito
1. Introduction
The pion branching ratio, Rp = [p+!e+ne(g)]=[p+!m+nm(g)] is one of the most precisely
calculated in the Standard Model (SM). The most recent theoretical prediction including radiative
corrections [1, 2] gives RpSM = (1:23520:0002)10 4. The p+!e+ne branching fraction is sup-
pressed to the order of 10 4, which is well known as “helicity suppression". A precise measurement
of Rp provides a sensitive test of the SM hypothesis of electron-muon universality. The previous
experimental results at TRIUMF and PSI are respectively Rpexp = [1:22650:0034(stat)0:0044
(syst)]10 4 [3] and Rpexp = [1:23460:0035(stat)0:0036(syst)]10 4 [4]. Both experiments
obtained comparable levels of statistical and systematic uncertainties; however there is a room
for improvement by two orders of magnitude in precision to reach the precision of the theoretical
result.
The goal of the PIENU experiment is to improve the accuracy of Rp by a factor of 5, to <0.1%,
which corresponds to 0.05% for the universality test. This precision also allows access to new
physics up to the mass scale of 1000 TeV/c2 for helicity unsuppressed pseudoscalar interactions
[2]. Examples of the new pseudoscalar interactions probed include R-parity violating SUSY [5],
leptoquarks [6], and the effects of charged Higgs bosons [2]. The p+!e+ne decay measurement
is also sensitive to the existence of heavy sterile neutrino mixing [7] through the Rp measurement
[8, 9] and a direct additional peak search [10].
2. The PIENU Experiment
Figure 1 shows the schematic of the PIENU detector and the pion decay modes of interest.
The PIENU detector was located at the TRIUMF M13 beam line [11]. The beam with momentum
75 MeV/c was tracked by two multi-wire proportional chambers (WC1 and WC2) and two sets of
Si strip detectors (S1 and S2). After WC2, two thin plastic scintillators B1 and B2 were placed in
order to measure time and energy loss for pion identification. A plastic scintillator target B3 with
thickness of 8 mm was immediately downstream of S2 and the beam pions decayed at rest in the
center of the target.
In order to reconstruct the decay-positron tracks and define their acceptance, another Si strip
detector (S3) and multi-wire proportional chamber (WC3) were employed. Two thin plastic scin-
tillators (T1 and T2) were used to measure the time of decay positrons. These positrons entered a
large single crystal NaI(Tl) calorimeter surrounded by two rings of pure CsI crystals (97 total) to
detect shower leakage. See Ref. [12] for more details of the PIENU detector.
3. Analysis
The decays p+!e+ne, and p+!m+nm followed by m+!e+nenm (p+!m+!e+ decay chain)
have different energy and time characteristics that can be measured precisely and used to extract
the ratio Rp . The muon in p+!m+!e+ has a kinetic energy of 4.1 MeV and a range in plastic
scintillators of about 1mm; the total energy of the decay positrons have a continuous distribution
from 0.5 to 52.8 MeV. The decay time spectrum of p+!m+!e+ distributes as lplmlp lm (e lm t  
e lp t), where lp and lm are the decay constants of the pion and muon, respectively. In contract, the
1
P
o
S(KMI2017)034
Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. Ito
Figure 1: Schematic of the PIENU detector and pion decays.
decay p+!e+ne produces a mono-energetic positron at 69.8 MeV, and the decay time distribution
is a simple exponential curve (lpe lp t). Rp can be obtained from the ratio of the positron yields
from p+!e+ne decays and p+!m+!e+ decays by taking advantage of these differences.
Figure 2 shows the energy spectrum of decay positrons in the NaI(Tl) plus CsI detectors.
Two energy regions, above and below the vertical dashed black line in Figure 2 were used to
distinguish the two pion decay modes. The time spectra in these two energy regions were fitted
simultaneously to a fitting function including backgrounds to extract raw branching ratio (Rpraw), to
which corrections were applied.
The most important correction for Rpraw, which dominated the systematic uncertainties, was
the p+!e+ne low-energy tail due to shower leakage from the calorimeters. The amount of the
p+!e+ne low-energy tail below 52 MeV was estimated using two ways. First, the dominant
p+!m+!e+ events were suppressed in the energy spectrum, and the p+!e+ne low-energy tail
was estimated by subtracting the remaining p+!m+!e+ suppressed spectrum. Second, a special
measurement of the energy spectrum using a mono-energetic positron beam was performed in order
to obtain the low-energy tail empirically.
Additionally, the difference of the positron acceptance between p+!e+ne and p+!m+!e+
events also required a significant correction. The uncertainty of the acceptance correction was
much improved compared to the previous TRIUMF experiment because the PIENU detector solid
angle was about ten times larger than the previous experimental setup [3, 12]. See Refs. [8, 13] for
more details of the analysis.
4. Status and Conclusion
The PIENU experiment completed data taking in 2012, and the result of the partial data set
2
P
o
S(KMI2017)034
Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. Ito
Energy in NaI + CsI [MeV]
0 10 20 30 40 50 60 70 80 90 100
Co
un
ts
1
10
210
310
410
510
+e→+µ→+pi eν+e→+pi
Figure 2: Measured energy spectrum of decay positrons without (solid black line) and with (dashed red
line) p+!m+!e+ suppression. Dashed vertical line shows energy threshold at 52 MeV.
analysis taken in 2010 was published in Ref. [8]. For this data set, the value of Rp including all
corrections is Rp = [1:23440:0023(stat)0:0019(syst)]10 4, which is consistent with the SM
prediction. This result improves the test of electron-muon universality to gm=ge = 1:00040:0012
for the charged current.
A search for massive neutrino in the mass range of 0 130 MeV/c2 was also performed us-
ing the 2009 and 2010 data sets. This analysis improved the 90% confidence-level limits on the
neutrino mixing parameter Uei between the weak electron-neutrino eigenstate and a hypothetical
massive eigenstate mni by a factor of up to four. Details of this analysis are described in Refs.
[8, 9, 10].
At present, analysis of the remaining data is in progress, and the 2011 data set was also ana-
lyzed. By combining results of the 2010 and 2011 data sets, the uncertainty of Rp is improved to
DRp = [0:0014(stat)0:0013(syst)]10 4, which corresponds to 0.15% in precision, an improve-
ment by a factor of 3 [13]. Full data set analysis is expected to improve the precision of Rp to
< 0:1% and will be finished in a few years.
Acknowledgements
This work was supported by the Natural Sciences and Engineering Research Council Grant
SAPPJ 157985-2013 and TRIUMF through a contribution from the National Research Council of
Canada, and by the Research Fund for the Doctoral Program of Higher Education of China, by
CONACYT doctoral fellowship from Mexico, and by JSPS KAKENHI Grants No. 18540274, No.
21340059, and No. 24224006 in Japan. We are grateful to Brookhaven National Laboratory for
the loan of the crystals and to the TRIUMF operations, detector, electronics, and DAQ groups for
their engineering and technical support.
3
P
o
S(KMI2017)034
Precision measurement of the p+!e+ne branching ratio in the PIENU experiment S. Ito
References
[1] V. Cirigliano and I. Rosell, JHEP. 10 (2007) 005.
[2] D. A. Bryman, W. Marciano, R. Tschirhart, and T. Yamanaka, Annu. Rev. Nucl. Part. Sci. 61, 331
(2011).
[3] D. I. Britton et al., Phys. Rev. Lett. 68, 3000 (1992) and Phys. Rev. D 49, 28 (1994).
[4] G. Czapek et al., Phys. Rev. Lett. 70, 17 (1993).
[5] M. J. Ramsey-Musolf et al., Phys. Rev. D76, 095017, (2007).
[6] O. Shanker. Nuclear Physics B, 204(3):375-386. (1982).
[7] T. Asaka et al., JHEP 05, 030 (2009).
[8] A. Aguilar-Arevalo et al., Phys. Rev. Lett. 071801, 115 (2015).
[9] S. Ito et al., Hyperfine Interact. 238 (2017) no.1, 1.
[10] M. Aoki et al., Phys. Rev. D 052002, 84 (2011).
[11] A. Aguilar-Arevalo et al., Nucl. Instrum. Methods. A609, 102 (2009).
[12] A. Aguilar-Arevalo et al., Nucl. Instrum. Methods. A791, 38 (2015).
[13] S. Ito, Ph.D thesis, Osaka University (2016).
4


Shintaro Ito

A Aguilar-Arevalo

M. Aoki

M. Blecher

D I. Britton

D A. Bryman

D vom Bruch

S. Chen

J. Comfort

S Cuen-Rochin

L. Doria

P. Gumplinger

A. Husseina

Y. Igarashi

S. Kettell.

L. Kurchaninov

L. Littenberg

C. Malbrunot

R E. Mischke

T. Numao

D. Protopopescu

A. Sher

T. Sullivan

D. Vavilov

Crossref

Precision Measurement of the $\pi$$^+$$\rightarrow$$e^+$$\nu$$_e$ Branching Ratio in the PIENU Experiment

http://eprints.gla.ac.uk/157697/1/157697.pdf

Precision Measurement of the π+→e+νe Branching Ratio in the PIENU Experiment

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