Light stop decays: implications for LHC searches
Eur. Phys. J. C (2015) 75:420
DOI 10.1140/epjc/s10052-015-3626-z
Regular Article - Theoretical Physics
Light stop decays: implications for LHC searches
R. Gröber1,a , M. Margarete Mühlleitner2,b , E. Popenda3,c , A. Wlotzka2,d
1 Sezione di Roma Tre, Istituto Nazionale di Fisica Nucleare (INFN), 00146 Roma, Italy
2 Institute for Theoretical Physics, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany
3 Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland
Received: 3 October 2014 / Accepted: 17 August 2015 / Published online: 9 September 2015
© The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract We investigate the flavour-changing neutral current decay of the lightest stop into a charm quark and the
lightest neutralino and its four-body decay into the lightest
neutralino, a down-type quark and a fermion pair. These are
the relevant stop search channels in the low-mass region. The
SUSY-QCD corrections to the two-body decay have been
calculated for the first time and turn out to be sizeable. In
the four-body decay both the contributions from diagrams
with flavour-changing neutral current couplings and the mass
effects of final state bottom quarks and τ leptons have been
taken into account, which are not available in the literature so
far. The resulting branching ratios are investigated in detail.
We find that in either of the decay channels the branching
ratios can deviate significantly from 1 in large parts of the
allowed parameter range. Taking this into account, the experimental exclusion limits on the stop, which are based on
the assumption of branching ratios equal to 1, are considerably weakened. This should be taken into account in future
searches for light stops at the next run of the LHC, where the
probed low stop mass region will be extended.
1 Introduction
With the discovery of a new scalar particle by the LHC experiments ATLAS and CMS [1–4] we have entered a new era
of particle physics. The investigation of its properties, like
spin and CP quantum numbers and couplings to other standard model (SM) particles, have identified it as the longsought Higgs particle predicted by the Higgs mechanism [5–
9]. The absence of any discovery of new particles beyond the
SM, however, leaves the question of the underlying dynamics
a e-mail:
b e-mail:
c e-mail:
d e-mail:
of the mechanism of electroweak symmetry breaking open.
Models with the Higgs boson emerging as composite bound
state from a strongly coupled sector [10–17] are compatible
with the LHC data, as well as extensions like supersymmetry
(SUSY) [18–32] based on a weakly interacting theory. One
of the main goals of the LHC is therefore the search for new
particles and the subsequent investigation of their properties
in order to pin down the true nature of the discovered Higgs
boson.
Among the plethora of beyond the SM extensions, SUSY
is one of the most extensively studied models. It requires the
introduction of at least two complex Higgs doublets, leading
in its most economic version, the minimal supersymmetric
extension of the SM (MSSM) [33–38], to five Higgs bosons,
among which the lightest CP-even state h can be identified
with the recently discovered SM-like boson. Within SUSY
models the hierarchy problem can be solved by the symmetry
between bosonic and fermionic degrees of freedom. Assuming SUSY to be softly broken, the Higgs mass corrections
grow logarithmically with the square of the SUSY scale m S .
The loop corrections from the top loops and their SUSY partners, the stops, are crucial in order to shift the mass of the
lightest SUSY Higgs boson above the upper tree-level bound
set by the Z boson mass M Z . With the SUSY scale given by
the average stop mass, m 2S = m t˜1 m t˜2 , and the stop mixing
parameter X t , the mass squared of the lightest Higgs boson
including the leading corrections in the SM limit, is given by
m 2S
3m 4t
X t2
2
2
2
2
,
Mh = M Z cos 2β + 2π 2 v 2 log m 2 + X t 1 − 12
t
(1)
with m t denoting the top-quark mass, v the vacuum expectation value (VEV) with v ≈ 246 GeV and
Xt =
At − μ cot β
.
mS
(2)
The ratio of the two VEVs of the neutral components of the
MSSM Higgs doublets is given by tan β and At denotes the
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420 Page 2 of 21
soft SUSY breaking trilinear coupling in the stop sector. A
large Higgs boson mass of around 125 GeV can hence be
obtained either through a large stop mixing X t or through
heavy stops. Naturalness arguments suggest at least one of
the two stop mass eigenstates to be light, since the amount
of fine-tuning of the electroweak scale is significantly driven
by the stop sector [39]. The maximal mixing scenario, with
X t2 ≈ 6 and m S ≈ 500 GeV leading to the observed Higgs
mass value, therefore optimally reduces the amount of finetuning [40]. In most SUSY models a light stop arises naturally
due to the mixing being proportional to the large Yukawa
coupling, which leads to a large mass splitting between the
stop mass eigenstates.
Light stops not only play a special role in view of the Higgs
mass and naturalness arguments. A light stop can also lead to
the correct relic density through co-annihilation, in particular
for mass differences between the stop and the lightest neutralino χ̃10 of 15–30 GeV [41–46]. Moreover, light stops allow
for successful baryogenesis within the MSSM [47–59].1
Despite the LHC searches pushing the limits on the
coloured sparticles above the 1–1.5 TeV range for the first
two generations [65,66], the lightest stop can still be rather
light, with masses below the respective kinematical thresholds for the decay into a top and a lightest neutralino χ̃10 ,
t˜1 → t χ̃10 , and for the decay t˜1 → χ̃10 W b into a neutralino,
a W boson and a bottom quark b. Assuming the lightest
stop to be the next-to-lightest supersymmetric particle and
the χ̃10 to be the lightest SUSY particle (LSP), the light stop
can then decay into the LSP and a charm quark c or an up
quark u, t˜1 → (u/c)χ̃10 [67,68]. Another possible decay
channel is the four-body decay t˜1 → χ̃10 b f f¯ [69], with f
and f denoting generic light fermions. The two-body decay
into charm/up and neutralino is flavour-violating (FV). The
MSSM in general exhibits many sources of flavour violation, so that the decay can already occur at tree level. High
precision tests in the sector of quark flavour violation and
limits on flavour-changing neutral currents from K , D and
B meson studies put stringent constraints on the amount of
possible flavour violation [70–72]. In order to solve this new
physics flavour puzzle the framework of minimal flavour violation (MFV) has been proposed [73–77], which requires all
sources of flavour and CP violation to be given by the SM
structure of the Yukawa couplings. The hypothesis of MFV is
not renormalisation group invariant [76], however, inducing
flavour off-diagonal squark mass terms through the Yukawa
couplings, which results in tree-level FCNC couplings. If the
FV stop-neutralino-up (...truncated)
