Speaker
Shintaro Ito
(Osaka University, Toyonaka, Osaka, 560-0043, Japan)
Description
Study of rare decays is an important approach to search for new physics beyond the Standard Model. The branching ratio of charged pion decays, `$R={\Gamma}({\pi}^+{\rightarrow}e^{+}{\nu}_{e}({\gamma}))$/${\Gamma}({\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu}({\gamma}))$` is one of the most precisely calculated processes involving quarks with `$R_{SM}=(1.2352{\pm}0.0001)$` `${\times}10^{-4}$`. Precise measurement of `$R$` provides one of the most stringent tests of the hypothesis of electron-muon universality in weak interactions. The current experimental values of the branching ratio are `$R_{EXP}=(1.2265{\pm}0.0034(stat){\pm}0.0044(syst)){\times}10^{-4}$` (TRIUMF, 1992) and `$R_{EXP}=(1.2346{\pm}0.0035(stat){\pm}0.0036(syst)){\times}10^{-4}$` (PSI, 1993), which indicate that there is a room for improvement by two orders of magnitude in precision. The goal of PIENU experiment at TRIUMF is to improve the accuracy of the branching ratio measurement by a factor of 5, to $<0.1\%$. This precision allows potential access to new physics up to the mass scale of 1000 TeV for pseudoscalar interactions. Examples of the new physics probed include R-parity violating SUSY, heavy neutrino mixing, excited gauge bosons, leptoquarks, compositeness, and the effects of charged Higgs bosons.
The PIENU experiment collected about `$4.5{\times}10^5~{\pi}^+{\rightarrow}e^+{\nu}_e$` events, which corresponds to about 30 times higher statistics than in the previous TRIUMF experiment. The time and energy of decay positrons from decays of pions at rest were measured in a spectrometer employing plastic scintillators, tracking detectors, and NaI(Tl) and pure CsI crystals. The branching ratio is obtained from the ratio of the positron yields from `${\pi}^+{\rightarrow}e^+{\nu}_e$` decays `$(E_e=69.8 ~{\rm MeV})$` and `${\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu}$` decays followed by `${\mu}^+{\rightarrow}e^+{\nu}_e\overline{{\nu}_{\mu}}$` decays (`${\pi}^+{\rightarrow}{\mu}^+{\rightarrow}e^+, E_e=0.5$` to 52.8 MeV). Two energy regions, above and below the `${\pi}^+{\rightarrow}{\mu}^+{\rightarrow}e^+$` distribution were used to determine the time spectra of decay positrons. Data in the two regions are fitted simultaneously to extract the branching ratio to which corrections are applied. The most important corrections resulting in systemmatic uncertainties include the ${\pi}^+{\rightarrow}e^+{\nu}_e$ low energy distribution in the calorimeter due to shower leakage and the difference of positron acceptances between `${\pi}^+{\rightarrow}e^+{\nu}_e$` and `${\pi}^+{\rightarrow}{\mu}^+{\rightarrow}e^+$` events. The current analysis corresponds to approximately `$10\%$` of the statistics of the full data. Results will be presented.
Primary author
Shintaro Ito
(Osaka University, Toyonaka, Osaka, 560-0043, Japan)
Co-authors
Ahmed Hussein
(University of Northern British Columbia, Prince George, B.C. V2N 4Z9, Canada)
Aleksey Sher
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Alexis Aguilar Arevalo
(Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Mexico)
Chloe Malbrunot
(University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada)
Dan Protopopescu
(University of Glasgow, Glasgow, UK)
David Britton
(University of Glasgow, Glasgow, UK)
Dmitry Vavilov
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Doria Luca
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Dorothea vom Brunch
(University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada)
Douglas Bryman
(University of British Columbia, Vancouver, B.B. V6T 1Z1, Canada)
Joseph Comfort
(Arizona State University, Tempe, AZ 85287, USA)
Laurence Littenberg
(Brookhaven National Laboratory, Upton, NY 11973-5000, USA)
Leonid Kurchaninov
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Marvin Blecher
(Virginia Tech., Blacksburg, VA 24061, USA)
Masaharu Aoki
(Osaka University, Toyonaka, Osaka, 560-0043, Japan)
Peter Gumplinger
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Richard Mische
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Saul Cuen-Rochin
(University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada)
Shaomin Chen
(Tsinghua University, Beijing, 100084, China)
Toshio Numao
(TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada)
Tristan Sullivan
(University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada)
Youichi Igarashi
(KEK, 1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, Japan)
steve kettell
(Brookhaven National Laboratory, Upton, NY 11973-5000, USA)
