Wino LSP detection in the light of recent Higgs searches at the LHC
ABSTRACT Recent LHC data showed excesses of Higgs-like signals at the Higgs mass of
around 125GeV. This may indicate supersymmetric models with relatively heavy
scalar fermions to enhance the Higgs mass. The desired mass spectrum is
realized in the anomaly-mediated supersymmetry breaking model, in which the
Wino can naturally be the lightest superparticle (LSP). We discuss
possibilities for confirming such a scenario, particularly detecting signals
from Wino LSP at direct detection experiments, indirect searches at neutrino
telescopes and at the LHC.
arXiv:1112.3123v2 [hep-ph] 16 Jan 2012
Wino LSP detection in the light of recent Higgs searches
at the LHC
Takeo Moroi and Kazunori Nakayama
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
Recent LHC data showed excesses of Higgs-like signals at the Higgs mass of
around 125GeV. This may indicate supersymmetric models with relatively heavy
scalar fermions to enhance the Higgs mass. The desired mass spectrum is realized
in the anomaly-mediated supersymmetry breaking model, in which the Wino can
naturally be the lightest superparticle (LSP). We discuss possibilities for confirming
such a scenario, particularly detecting signals from Wino LSP at direct detection
experiments, indirect searches at neutrino telescopes and at the LHC.
Higgs mass contains very important information about low-energy supersymmetry
(SUSY) models, which is well motivated because it provides a viable candidate of dark
matter (DM) and also because it realizes the gauge coupling unification. In particular, in
the minimal SUSY standard model (MSSM), the lightest Higgs boson cannot be heavier
than the Z-boson at the tree level, while a sizable radiative correction may enhance the
Higgs mass . The size of the radiative correction depends on the masses (and other
parameters) of superparticles. The lightest Higgs mass becomes larger as superparticles
(in particular, stops) become heavier. Thus, once the lightest Higgs mass is known, mass
scale of superparticles is constrained.
Recently, the ATLAS collaboration reported 3.6σ local excess of the standard model
(SM) Higgs-like event at mh ≃ 126 GeV . In addition, the CMS collaboration also
showed more than 2σ local excess at mh ≃ 124 GeV .1In order to achieve such a
value of the lightest Higgs mass in the MSSM, relatively large values of the superparticle
masses are required; the typical scale of the sfermion masses to realize mh≃ 125GeV is
10TeV–103TeV [4, 5]. Then, if the masses of all the superparticles are of the same order,
it is difficult to find experimental signals of low-energy SUSY and the existence of SUSY
is hardly confirmed.
Although the sfermion masses are much larger than the electroweak scale, gauginos
may be much lighter than sfermions and within the reach of collier and other experiments.
One interesting possibility is the model in which the SUSY breaking scalar masses are
from direct coupling to the SUSY breaking field while the gaugino masses are generated by
the anomaly-mediation mechanism [6, 7]; in this letter, we call such a model as anomaly-
mediated SUSY breaking (AMSB) model. Even in the AMSB model, however, if the pure
anomaly-mediation relation holds among the gaugino masses, gluino mass is about 8 times
larger than the mass of Wino. Thus, if the Wino mass is a few hundred GeV, which is the
lower bound on it from astrophysical and cosmological considerations as will be reviewed
later, the gluino mass becomes multi-TeV; with such a heavy gluino, the discovery of the
SUSY signal at the LHC becomes challenging because we consider the case that all the
squarks are extremely heavy.
1The excesses based on global probabilities, which take account of the look-elsewhere effect, are 2.3σ
(ATLAS) and 1.9σ (CMS).
Even so, there still exist possibilities of discovering signals of the AMSB scenario.
In particular, in the present framework, the neutral Wino is the lightest superparticle
(LSP) and may be DM. In such a case, pair annihilation cross section of the LSP and
the scattering cross section of the LSP off the nuclei are both enhanced compared to the
Bino LSP case, which has significant implications to direct and indirect detection of DM.
Because the search of the superparticles at the LHC may be difficult, it is important to
pursue these possibilities and explore how well we can study the AMSB scenario with
In this letter, motivated by the recent Higgs searches at the LHC, we discuss the
detectability of the signals of AMSB scenario. We pay particular attention to the case
of the Wino LSP. We focus on direct/indirect detection of the Wino DM at underground
laboratories and neutrino telescopes. We also comment on the LHC reach for the direct
Wino production. Since superparticles except for gauginos are heavy, standard methods
for SUSY searches may not work. Even in this case, we will show that there are some
windows for the confirmation of the SUSY.
Let us first briefly discuss important properties of the AMSB scenario. We assume
that the soft SUSY breaking scalar masses are generated by the direct coupling between
the scalars and the SUSY breaking hidden sector field, while the gaugino masses are
generated by the anomaly mediation mechanism. Adopting the pure AMSB relation, the
gaugino masses are given by [6, 7]
where ga(a = 1–3) are gauge coupling constants of the SM gauge groups, m3/2 is the
gravitino mass, and (b1,b2,b3) = (11,1,−3). Then, the Wino becomes the lightest among
the gauginos, and gaugino masses largely separate: m˜B: m˜ W: m˜ g≃ 3 : 1 : 8. Although
the AMSB relation may be affected by Higgs and Higgsino loop diagrams [7, 9], we adopt
the pure AMSB mass relation. With the gaugino masses being of O(100) GeV–O(1) TeV,
the gravitino mass becomes of O(10) TeV–O(100) TeV. The sfermion masses are expected
to be of the same order of the gravitino mass, which is preferred from the point of view
2The heavy SUSY particle spectrum and their detectability were discussed in a different context in
of realizing mh≃ 125 GeV. In particular, if the scalar masses are (almost) equal to the
gravitino mass, mh≃ 125 GeV requires relatively small value of tanβ ∼ a few (where
tanβ is the ratio of the vacuum expectation values of up- and down-type Higgs bosons)
Before discussing the detectability of the signals of AMSB model, we comment on
the supersymmetric Higgs mass parameter (so-called µ-parameter). In the present setup,
the soft SUSY breaking scalar mass parameters of up- and down-type Higgs bosons are
expected to be of O(10) TeV–O(100) TeV. In order to have viable electroweak symmetry
breaking, the µ-parameter (as well as heavy Higgs boson masses) is also expected to be of
the same order; then, the Higgsinos become extremely heavy and the Wino becomes the
LSP. Thus, we pay particular attention to the case of Wino LSP in the following. In some
of our following analysis, however, we consider the case with µ ∼ O(100) GeV–O(1) TeV
taking account of the possibility of an accidental tuning of the parameters. This is because
detection rates of some of signals (in particular, the direct detection rates) strongly depend
on the value of µ.
Taking account of the radiative correction due to the gauge boson loops, the neutral
Wino becomes lighter than the charged one. Therefore, we focus on the case of neutral
Wino LSP. In addition, we assume that the LSP (i.e., the neutral Wino) is the dominant
component of DM. The Wino LSP accounts for the present DM density for m˜ W≃ 3TeV
if it is produced only from thermal bath . In the AMSB scenario, however, the Wino
LSP can be non-thermally produced from the gravitino or moduli decay [7, 11]. If the
reheating temperature takes an appropriate value, for example, the decay of gravitino
produces the Wino LSP with correct relic density , while thermal leptogenesis 
works successfully . Thus the Wino is a good DM candidate in the present setup.
Hereafter, we assume that the right amount of Wino is somehow produced in the early
universe to be DM.
We start with discussing direct detection experiments of DM. The scattering cross
section of the Wino LSP off the nucleon significantly depends on µ. Since all scalars except
for the lightest Higgs boson are expected to be heavy enough, it is only the lightest Higgs
boson that mediates the spin-independent (SI) scattering. The DM-proton scattering
cross section is given by 
m˜ χ0 + mN
(npfp+ nnfn)2+ 4J + 1
(ap?sp? + an?sn?)2
where the first and the second term in the bracket are the contributions of SI and spin-
dependent (SD) interaction, respectively. Here m˜ χ0is the LSP mass, mN is the mass of
the target nucleus, np(nn) is the number of proton (neutron) in the target nucleus, J is
the total nuclear spin, apand anare the effective DM-nucleon SD couplings, and ?sp(n)?
are the expectation values of the spin content of the proton and neutron groups within
the nucleus. The effective DM-proton coupling, fp, is given by
where fTG= 1−?
Since the DM-Higgs coupling is proportional to the magnitude of Wino-Higgsino mixing,3
Tq, mpand mqdenote the proton and quark masses, respectively,
q is the effective DM-quark coupling obtained by the exchange of the Higgs boson.
the cross section is enhanced if the Wino-Higgsino mixing is large. In Fig. 1 we plot
the Wino-proton SI and SD scattering cross section. In this plot we have used following
values for the quark contents in the proton  : f(p)
and taken tanβ = 3 and tanβ = 20. The XENON100 experiment  most severely
constrains the SI cross section. The sensitivity is improved by a few orders of magnitude
for the next generation 1 ton scale detectors, and then broad parameter regions up to
m˜ W∼ µ ∼ 1TeV will be explored. The IceCube searches for neutrino events arising from
the DM annihilation in the Sun. Since the efficiency for the DM trapping into the Sun
depends on the DM-proton scattering cross section, the high-energy neutrino observations
give limits on it. For the SD cross section, the IceCube gives the most stringent limit,
and it will be further improved by about one order of magnitude with the DeepCore
We have also calculated the detection rate at the IceCube DeepCore, arising from
high-energy neutrinos produced by the Wino annihilation at the Galactic Center (GC).
3In the limit of pure Wino DM, the Wino-nucleon scattering cross section is too small to be de-