Physics Letters B 798 (2019) 134980
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Search for heavy neutrinos in π → μν decay
A. Aguilar-Arevalo
a
, M. Aoki
b
, M. Blecher
c
, D.I. Britton
d
, D. vom Bruch
e,1
, D.A. Bryman
e,f
,
S. Chen
g
, J. Comfort
h
, L. Doria
f,2
, S. Cuen-Rochin
e,f
, P. Gumplinger
f
, A. Hussein
i,f
,
Y. Igarashi
j
, S. Ito
b,∗,3
, S.H. Kettell
k
, L. Kurchaninov
f
, L.S. Littenberg
k
, C. Malbrunot
e,4
,
R.E. Mischke
f
, T. Numao
f,∗
, D. Protopopescu
d
, A. Sher
f
, T. Sullivan
e,5
, D. Vavilov
f
a
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de Mexico, D.F. 04510, Mexico
b
Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
c
Physics Department, Virginia Tech., Blacksburg, VA 24061, USA
d
School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
e
Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1, Canada
f
TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3, Canada
g
Department of Engineering Physics, Tsinghua University, Beijing, 100084, China
h
Physics Department, Arizona State University, Tempe, AZ 85287, USA
i
University of Northern British Columbia, Prince George, British Columbia, V2N 4Z9, Canada
j
KEK, 1-1 Oho, Tsukuba-shi, Ibaraki 305-0801, Japan
k
Brookhaven National Laboratory, Upton, NY 11973-5000, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
5 April 2019
Received
in revised form 14 August 2019
Accepted
24 September 2019
Available
online 27 September 2019
Editor:
M. Doser
Keywords:
Pion
decay
Heavy
neutrino
In the present work of the PIENU experiment, heavy neutrinos were sought in pion decays π
+
→ μ
+
ν
at rest by examining the observed muon energy spectrum for extra peaks in addition to the expected
peak for a light neutrino. No evidence for heavy neutrinos was observed. Upper limits were set on the
neutrino mixing matrix |U
μi
|
2
in the neutrino mass region of 15.7–33.8 MeV/c
2
, improving on previous
results by an order of magnitude.
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Neutrino oscillations indicate that at least two of the known
neutrinos are massive, requiring the original Standard Model to
be expanded. The existence of additional heavy neutrinos (mostly
sterile, without direct coupling in weak interactions) is still an
open question. A wide range of motivations [1]for sterile neutrinos
come from cosmological and astrophysical arguments of baryoge-
*
Corresponding authors.
E-mail
addresses: s-ito@okayama-u.ac.jp (S. Ito), toshio@triumf.ca (T. Numao).
1
Present address: LPNHE, Sorbonne Université, Paris Diderot Sorbonne Paris Cité,
CNRS/IN2P3, Paris, France.
2
Present address: PRISMA Cluster of Excellence and Institut für Kernphysik, Jo-
hannes
Gutenberg-Universität Mainz, D 55128, Germany.
3
Present address: Faculty of Science, Okayama University, 3-1-1 Tsushimanaka,
Kita-ku, Okayama, 700-8530, Japan.
4
Present address: CERN, 1211 Geneva 21, Switzerland.
5
Present address: Department of Physics, Queen’s University, Kingston, Ontario,
K7L 3N6, Canada.
nesis [2], large scale structure formation [3] and big bang nucle-
osynthesis
[4], and from the tension in the Hubble constant [5]. In
the Neutrino Minimal Standard Model (νMSM) three sterile neu-
trinos
are introduced which include a stable dark matter candidate
[6]. MeV neutrinos could be accommodated in the νMSM to ex-
plain
the
7
Li abundance [7]or, with the addition of a MeV scalar,
to obtain consistent results for anomalies in neutrino experiments
[8]. In many scenarios, the mass scale of sterile neutrinos is not
constrained to be light [9,10], opening up many possibilities and
potential observables in particle physics, astrophysics and cosmol-
ogy.
Heavy
neutrinos mixing with the muon neutrino in the mass
region 1–400 MeV/c
2
have been sought by accelerator-based ex-
periments
studying pion [11–14] and kaon decays [15–17]. The
ratio of the π
+
→ μ
+
ν
H
decay rate to the normal π
+
→ μ
+
ν
decay rate can be written as
(π
+
→ μ
+
ν
H
)
(π
+
→ μ
+
ν)
=|
U
μi
|
2
ρ(m
H
) (1)
https://doi.org/10.1016/j.physletb.2019.134980
0370-2693/
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.