Physics Letters B 753 (2016) 664–669
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Single top partner production in the Higgs to diphoton channel in the
Littlest Higgs Model with T -parity
Ning Liu
a
, Lei Wu
b
, Bingfang Yang
a
, Mengchao Zhang
c
a
College of Physics & Electronic Engineering, Henan Normal University, Xinxiang 453007, China
b
ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics, The University of Sydney, NSW 2006, Australia
c
State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Academia Sinica, Beijing 100190, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received
5 October 2015
Received
in revised form 13 December 2015
Accepted
19 December 2015
Available
online 31 December 2015
Editor:
J. Hisano
The top partner as a hallmark of the Littlest Higgs Model with T -parity (LHT model) has been extensively
searched for during the Large Hadron Collider (LHC) Run-1. With the increasing mass limits on the top
partner, the single production of the top partner will be dominant over the pair production. Under the
constraints from the Higgs data, the electroweak precision observables and R
b
, we find that the mass of
T -even top partner (T
+
) has to be heavier than 730 GeV. Then, we investigate the observability of the
single T -even top partner production through the process pp → T
+
j with the sequent decay T
+
→th in
the di-photon channel in the LHT model at the LHC. We find that the mass of T
+
can be excluded up to
800 GeV at 2σ level at 14 TeV LHC with the integrated luminosity L =3ab
−1
.
© 2015 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
The discovery of a 125 GeV Higgs boson at the LHC Run-1
[1,2] is a major step towards elucidating the electroweak symmetry
breaking mechanism and marks a great triumph of the Standard
Model (SM). However, without protection by a symmetry in the
SM, the Higgs mass is quadratically sensitive to the cutoff scale
via radiative corrections, rendering the theory with m
h
rather
unnatural. This electroweak naturalness problem is widely consid-
ered
as a major motivation for new physics beyond the SM.
Among
many extensions of the SM, the Littlest Higgs Model
with T -parity (LHT model) is one of the most promising candi-
dates
that can successfully solve the electroweak naturalness prob-
lem
[3–5]. It is based on a nonlinear sigma model with a global
SU(5) symmetry, which is broken down to SO(5) by a vacuum con-
densate
f ∼ /4π TeV. At the same time, the gauged subgroup
[SU(2) × U (1)]
2
is broken to its diagonal subgroup SU(2) × U (1)
that is identified as the SM electroweak gauge group. All quadrati-
cally
divergent one-loop contributions to the Higgs mass only first
appear at two-loop level, but their values are suppressed by an ad-
ditional
loop factor and thus are sufficiently small to prevent the
little hierarchy problem from being re-introduced at the TeV scale.
E-mail addresses: wlln@mail.ustc.edu.cn (N. Liu), leiwu@physics.usyd.edu.au
(L. Wu),
yangbingfang@htu.cn (B. Yang).
Due to the implementation of the T symmetry, the top quark sec-
tor
is enlarged, which leads to the abundant phenomenology of
top quark sector in the LHT model. In particular, the vector-like
top partner that is related to the mechanism of canceling the large
quadratic divergences in the Higgs mass from the SM top quark
loop has been widely studied [6].
Since
the top partner plays an important role in understanding
the electroweak naturalness problem, the ATLAS and CMS collabo-
rations
have performed the searches for the vector-like top partner
through the pair or single production with different final states
bW , tZ and th during the LHC Run-1. They have excluded the top
partner with the mass less than about 700 GeV [7,8]. However,
their bounds strongly depend on the assumptions on the decay
branching ratios and the properties of the top partner, in particular
its group representations. On the other hand, the indirect searches
for the top partners through their contributions to the electroweak
precision observables (EWPOs) [5], Z -pole observables [9,10] and
the flavor physics [11] have been extensively investigated. The non-
observation
of the top partners, in conjunction with the EWPOs
and the recent discovery of a 125 GeV Standard Model-like (SM-
like)
Higgs boson, has tightly constrained the parameter space of
the LHT model where the top partner can be light [12–15]. Theo-
retically,
the LHT model with the top partner at TeV scale usually
suffers from a higher degree of fine tuning. But phenomenologi-
cally,
such a TeV top partner can be well probed at the LHC and
future high energy colliders.
http://dx.doi.org/10.1016/j.physletb.2015.12.066
0370-2693/
© 2015 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
.