Physics Letters B 785 (2018) 354–361
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Neutrinoless double-beta decay with massive scalar emission
Kfir Blum
a,b,∗
, Yosef Nir
a,∗
, Michal Shavit
a,∗
a
Weizmann Institute, Department of Particle Physics and Astrophysics, Rehovot 7610001, Israel
b
CERN, Theoretical Physics Department, Switzerland
a r t i c l e i n f o a b s t r a c t
Article history:
Received
8 March 2018
Received
in revised form 11 July 2018
Accepted
15 August 2018
Available
online 23 August 2018
Editor:
W. Haxton
Searches for neutrino-less double-beta decay (0ν2β) place an important constraint on models where
light fields beyond the Standard Model participate in the neutrino mass mechanism. While 0ν2β
experimental collaborations often consider various massless majoron models, including various forms
of majoron couplings and multi-majoron final-state processes, none of these searches considered the
scenario where the “majoron” φ is not massless, m
φ
∼ MeV, of the same order as the Q -value of the
0ν2β reaction. We consider this parameter region and estimate 0ν2βφ constraints for m
φ
of order MeV.
The constraints are affected not only by kinematical phase space suppression but also by a change in the
signal to background ratio charachterizing the search. As a result, 0ν2βφ constraints for m
φ
> 0 diminish
significantly below the reaction threshold. This has phenomenological implications, which we illustrate
focusing on high-energy neutrino telescopes. The spectral shape of high-energy astrophysical neutrinos
could exhibit features due to resonant νν → φ → νν scattering. Such features fall within the sensitivity
range of IceCube-like experiments, if m
φ
is of order MeV, making 0ν2βφ a key complimentary laboratory
constraint on the scenario. Our results motivate a dedicated analysis by 0ν2β collaborations, analogous
to the dedicated analyses targeting massless majoron models.
© 2018 The Author(s). 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
Neutrinoless double beta (0ν2β) decay [1–4],
(A, Z ) → ( A, Z +2) +2e
−
, (1)
is a lepton number violating process. It is sensitive to the neutrino
mass parameter
m
ee
=
i
m
ν
i
U
2
ei
,
(2)
where m
ν
i
(i = 1, 2, 3) are the neutrino masses and U is the lepton
mixing matrix [5]. While the renormalizable Standard Model (SM)
has lepton number as an accidental symmetry and, consequently,
predicts that the neutrinos are massless, adding dimension-five
terms [6]
1,2
*
Corresponding authors.
E-mail
address: kfir.blum@weizmann.ac.il (K. Blum).
1
Repeated flavour indices are summed-over, and the bracket (HL) denotes con-
traction
to an SU(2) singlet.
2
See Ref. [7]for a recent discussion of 0ν2β in the SM effective field theory.
L
d=5
=−
Z
αβ
(HL
α
)(HL
β
), (3)
where H is the Higgs doublet field and L
α
(α = e, μ, τ ) are the
lepton doublet fields, leads to neutrino masses,
m
ν
=
v
2
Z
, (4)
with v =246 GeV.
We
do not know the beyond-SM origin of the dimension-five
terms in Eq. (3). It is possible that additional light particles accom-
pany
the neutrino mass mechanism and interact with SM fields in
various ways. If there exists a light gauge-singlet scalar φ, then the
dimension-six terms
L
d=6
=−
Y
αβ
2
φ(HL
α
)(HL
β
) (5)
are possible. The dimension-six terms lead to Yukawa couplings of
φ to neutrinos,
L ⊃−
1
2
G
αβ
φ ν
α
ν
β
+h.c., (6)
G =
v
2
2
Y. (7)
https://doi.org/10.1016/j.physletb.2018.08.022
0370-2693/
© 2018 The Author(s). 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
.