LHC tests of light neutralino dark matter without light sfermions
Lorenzo Calibbi
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Jonas M. Lindert
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Toshihiko Ota
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Yasutaka Takanishi
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Open Access
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c The Authors.
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Wintherturerstrasse 190
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CH-8057 Zu rich
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Switzerland
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Bld du Triomphe
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CP225, B-1050 Brussels
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Belgium
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Physik-Institut, Universita t Zu rich
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Department of Physics, Saitama University
We address the question how light the lightest MSSM neutralino can be as dark matter candidate in a scenario where all supersymmetric scalar particles are heavy. The hypothesis that the neutralino accounts for the observed dark matter density sets strong requirements on the supersymmetric spectrum, thus providing an handle for collider tests. In particular for a lightest neutralino below 100 GeV the relic density constraint translates into an upper bound on the Higgsino mass parameter in case all supersymmetric scalar particles are heavy. One can define a simplified model that highlights only the necessary features of the spectrum and their observable consequences at the LHC. Reinterpreting recent searches at the LHC we derive limits on the mass of the lightest neutralino that, in many cases, prove to be more constraining than dark matter experiments themselves.
1 Introduction 2 3 4
Resonant neutralino annihilations
Constraints and viable parameter space
Direct and indirect DM searches
LHC phenomenology
A Neutralino masses and mixing
Employing the data collected at 7 and 8 TeV of center of mass energy, the LHC experiments
have recently published the results of an impressive number of searches for electroweak
production of new physics. In many cases, they were able to set constraints on the masses
of new electroweakly-interacting particles above the previous best bounds from LEP. This
is the case in particular for the electroweak sector of the minimal supersymmetric standard
model (MSSM), as well as of any of its extensions. The exact bounds depend on the details
of the spectrum, especially on the mass hierarchy controlling the decay chains, and there is
a generic loss of sensitivity in the regime of low mass splittings. However, it is remarkable
that, in the most favourable cases, the limits in the MSSM are up to 300 GeV for the
sleptons [1, 2] and up to 700 GeV for the charginos and neutralinos [2, 3].
The above mentioned searches have a crucial role in testing supersymmetric Dark
Matter (DM) scenarios as they allow to probe the relevant parameter space independently
of the colored sector of the theory, which might in principle be too heavy to be directly
accessed by the LHC experiments. The cardinal idea is the following: the measurements of
the DM relic density based on Cosmic Microwave Background (CMB) observations set
nontrivial requirements on the supersymmetric spectrum, thus providing an handle for collider
tests. This is true in particular if the lightest supersymmetric particle (LSP) is a bino-like
neutralino, whose weak interactions typically lead to overproduction in the early universe,
unless an efficient annihilation mechanism is at work. Since a limited set of supersymmetric
particles and parameters is involved in the computation of the neutralino annihilation cross
section, and hence of its relic density, one can define simplified models that highlight only
the necessary features of the spectrum and their observable consequences at the LHC.
The above sketched procedure has been recently employed by us to answer the question
on how light the MSSM neutralino is still allowed to be by direct searches for
electroweaklyinteracting supersymmetric particles at the LHC [4, 5]. Other related studies on light
neutralino Dark Matter have been recently published in [69]. For neutralinos lighter than
about 30 GeV, the typical spectrum selected by the relic density constraints features rather
light staus and Higgsinos, with masses smaller than few hundred GeV [10]. The electroweak
production of these particles and the following decays lead to events with multiple taus
and missing transverse momentum. Employing an ATLAS search for such a signature in
combination with the limits on the decay rate of the Higgs into neutralinos, we could set
a lower bound on the DM mass at about 24 GeV. Remarkably, with the above exercise, we
showed that electroweak LHC searches are at the moment more powerful than direct and
indirect searches in testing light neutralino Dark Matter. For early works addressing limits
on light neutralino Dark Matter, see e.g. [1113], and for limits on (very) light neutralinos
without cosmological bounds we refer to refs. [14, 15] and references therein.
In the present paper, we want to extend our previous work to the case where no light
sfermions are in the spectrum, i.e. scenarios with only neutralinos and charginos lighter
than few hundreds GeV. A motivation for such an exercise is that light Higgsinos are the
minimal tree-level requirement posed by naturalness arguments. A Higgsino-like LSP can
not however account for the full amount of the observed Dark Matter, unless (...truncated)