Physics Letters B 760 (2016) 214–219
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Non-abelian gauge extensions for B-decay anomalies
Sofiane M. Boucenna
a
, Alejandro Celis
b
, Javier Fuentes-Martín
c,∗
, Avelino Vicente
c
,
Javier Virto
d
a
INFN, Laboratori Nazionali di Frascati, C.P. 13, 100044 Frascati, Italy
b
Ludwig-Maximilians-Universität München, Fakultät für Physik, Arnold Sommerfeld Center for Theoretical Physics, 80333 München, Germany
c
Instituto de Física Corpuscular, Universitat de València – CSIC, E-46071 València, Spain
d
Theoretische Elementarteilchenphysik, Naturwiss.-techn. Fakultät, Universität Siegen, 57068 Siegen, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
25 April 2016
Received
in revised form 1 June 2016
Accepted
27 June 2016
Available
online 30 June 2016
Editor:
B. Grinstein
We study the generic features of minimal gauge extensions of the Standard Model in view of recent
hints of lepton-flavor non-universality in semi-leptonic b → s
+
−
and b → cν decays. We classify the
possible models according to the symmetry-breaking pattern and the source of flavor non-universality.
We find that in viable models the SU(2)
L
factor is embedded non-trivially in the extended gauge group,
and that gauge couplings should be universal, hinting to the presence of new degrees of freedom sourcing
non-universality. Finally, we provide an explicit model that can explain the B-decay anomalies in a
coherent way and confront it with the relevant phenomenological constraints.
© 2016 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
Low-energy experiments have been crucial in the development
of the current Standard Model (SM) of electroweak interactions,
based on the gauge group SU(2)
L
⊗ U(1)
Y
. The structure of the
SM electroweak theory was beautifully revealed by a large variety
of experimental observations at low energy, together with require-
ments
of a proper high-energy behavior of the theory. In particular,
the intermediate vector bosons W
±
, Z were predicted theoretically
before their experimental discovery. Precision experiments at low
energies continue providing important information about the pos-
sible
ultraviolet (UV) completions of the SM, and New Physics (NP)
might be revealed again first through the precision frontier.
Currently
there are two sets of interesting tensions in B-physics
data:
1. In
2012 the BaBar Collaboration reported deviations from
lepton universality at the 25% level in the exclusive semileptonic
b → c decays, through a measurement of the ratios
R(D) =
(
B → Dτν)
(B → Dν)
SM
= 0.297 ±0.017 , (1)
R
(D
∗
) =
(
B → D
∗
τν)
(B → D
∗
ν)
SM
= 0.252 ±0.003 , (2)
*
Corresponding author.
E-mail
address: javier.fuentes@ific.uv.es (J. Fuentes-Martín).
with = e or μ. The measured values by BaBar [1], R(D) = 0.440 ±
0.072 and R(D
∗
) = 0.332 ± 0.030, show an excess with respect
to the SM of 2.0 σ and 2.7 σ respectively [1–3]. The Belle Col-
laboration reported
a measurement of these ratios in 2015 which
showed a slight enhancement with respect to the SM, R(D) =
0.375 ± 0.069 and R(D
∗
) = 0.293 ± 0.041 [4]. The LHCb Collab-
oration also
measured R(D
∗
) = 0.336 ± 0.040 [5], representing a
deviation from the SM at the ∼ 2 σ level. Very recently, the Belle
Collaboration has presented a new independent determination of
R(D
∗
) [6] which is 1.6 σ above the SM and is compatible with all
the previous measurements: R(D
∗
) =0.302 ± 0.032.
2. The
LHCb Collaboration has provided as well hints for flavor
non-universality (FNU) in b → s
+
−
transitions. The ratio [7]
1
R
K
=
(
B → K μ
+
μ
−
)
(B → Ke
+
e
−
)
SM
= 1 +O(m
2
μ
/m
2
b
), (3)
was measured in the low-q
2
region q
2
∈[1, 6] GeV
2
obtaining
R
K
= 0.745
+0.090
−0.074
± 0.036 [8], which represents a 2.6 σ devia-
1
We note that electromagnetic corrections to this ratio are expected to be of
order α log(m
2
e
/m
2
μ
) ∼ 8%. These logarithmic terms could also be enhanced by non-
perturbative
effects of order log(/m
B
), and/or large “accidental” numerical factors.
The experimental analysis takes into account part of the final-state radiation, but
a consistent study of electromagnetic effects is still lacking. In addition, in the
presence of flavor-non-universal new physics, hadronic uncertainties in R
K
are not
suppressed by m
2
μ
/m
2
b
, but only by (1 − R
NP
K
).
http://dx.doi.org/10.1016/j.physletb.2016.06.067
0370-2693/
© 2016 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
.