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We give a brief review of the current constraints and prospects for detection of higgsino dark matter in low-scale supersymmetry. In the first part we argue, after performing a survey of all potential dark matter particles in the MSSM, that the (nearly) pure higgsino is the only candidate emerging virtually unscathed from the wealth of observational data of recent years. In doing so by virtue of its gauge quantum numbers and electroweak symmetry breaking only, it maintains at the same time a relatively high degree of model-independence. In the second part we properly review the prospects for detection of a higgsino-like neutralino in direct underground dark matter searches, collider searches, and indirect astrophysical signals. We provide estimates for the typical scale of the superpartners and fine tuning in the context of traditional scenarios where the breaking of supersymmetry is mediated at about the scale of Grand Unification and where strong expectations for a timely detection of higgsinos in underground detectors are closely related to the measured

From the particle physics point of view, the simplest, most popular, and arguably most robust mechanism leading to the correct amount of cold dark matter (DM) in the early Universe is thermal freeze-out (see, e.g., [

As is well known, in the context of the freeze-out mechanism the measurement of the relic abundance provided by WMAP and Planck,

Since the 1990s, expectations about the scale of the new physics beyond the SM (BSM) have been driven by the theorists’ discomfort with the hierarchy problem. This is the well-known fact that, in a low-energy effective theory that includes one or more light fundamental scalars (as likely is the SM with a Higgs boson), one expects enormous quantum corrections to the scalar’s mass from the physics in the UV (the Planck scale, in the absence of anything else). Given the broad separation between the characteristic energies in play, this means that in order to get electroweak symmetry breaking (EWSB) one should fine tune the fundamental (unknown) Lagrangian parameters at the level, again in the absence of anything lighter than the Planck scale, of one part in

Remarkably, simply on dimensional grounds, if one of these expected TeV-scale BSM particles were to be the DM, its coupling to the SM extracted from the freeze-out mechanism would be of the size of the electroweak coupling constant,

Arguably the most complete and well motivated of the known BSM theories still remains low-scale supersymmetry (SUSY) (see, e.g., [

In this review we give a compact summary of the subject of DM in the traditional MSSM. After briefly surveying the particles with the potential of providing a good DM candidate, we argue that the nearly pure higgsino neutralino survives to these days as perhaps the only one that is not in substantial tension with any phenomenological constraint. Interestingly, it does so in a relatively model-independent way, without the need of resorting to narrow or secluded regions of the parameter space. We will thus review the higgsino’s prospects for detection in direct underground DM searches, indirect searches for DM in gamma-ray, and neutrino telescopes and at the LHC. Incidentally we will show that, in those models where SUSY breaking is transmitted to the visible sector at the scale of Grand Unification (GUT), the detection prospects of higgsino DM become tightly bound to the typical mass of the sfermions in the spectrum and, as a direct consequence, to the size of the Higgs boson mass.

In recent months several comprehensive reviews on the status of WIMP dark matter have appeared in the literature [

The structure of the review is as follows. In Section

One of the features making the MSSM very attractive from a phenomenological point of view is that its gauge symmetry structure originates directly from the supersymmetrization of the SM itself. As such, the fundamental gauge symmetry is SU(3)

One of the consequences is that a potentially viable DM particle is also expected to interact with SM-like strength. Since cosmological observations have long excluded the possibility of DM particles being charged under color [

Before we proceed to briefly review these three groups individually, we remind the reader that in order to make the lightest SUSY particle (LSP) stable on cosmological time scales, one introduces in the MSSM an additional discrete symmetry, R-parity [

The only particles of the MSSM that are electrically and color-neutral are the neutrinos, their scalar superpartners, called

At the tree level, the neutralino mass matrix takes the following well-known form:

In the remainder of this section we give an overview of the mentioned DM candidates of the MSSM, highlighting the strongest phenomenological constraints that can be applied in each case. We will not, however, discuss the neutrinos. It has been long known [

The interactions of the bino-like neutralino with the SM fields are easily found by directly supersymmetrizing the SM gauge-fermion-fermion interaction and applying the R-parity conservation constraint. The resulting vertex takes the form bino-sfermion-fermion,

The pair-annihilation of bino-like neutralinos in the early Universe proceeds at the leading order through the

(a) The dominant early-Universe annihilation channel for a nearly pure bino-like neutralino. (b), (c) Examples of annihilation and coannihilation tree-level channels into gauge bosons for a predominantly higgsino-like neutralino.

The bulk has been long known to be strongly constrained by direct SUSY searches at colliders. To give a semiquantitative estimate of these constraints, let us assume that only selectrons and smuons belong to the light SUSY spectrum, a reasonable ansatz in light of the strong LHC bounds on particles with color [

If, instead of selectrons and smuons, the light sfermions happen to be status, the parameter space opens up a little,

A way to evade the strong collider bounds is provided, if the bino-like neutralino and some other sparticles (sfermions

However, without any guidance from the theory in the UV, coannihilation of the bino with other sparticles can only be achieved in narrow slices of the parameter space, which require some tuning of the initial parameters to engineer the desired coincidence of neutralino and sfermion mass. And in models that are instead defined in terms of a limited number of free parameters in the UV, like the CMSSM [

The phenomenology of right-handed sneutrinos as DM, however interesting, is very model-dependent. In traditional see-saw models with large-scale Majorana mass the right-handed sneutrino is too heavy to be the DM. On the other hand, for a sneutrino of the “Dirac” type, or, in alternative, Majorana but such that the bare mass is of the order of the superpartners’ mass [

On the other hand, the correct relic density can certainly be obtained thanks to the mixing with the left-handed sneutrino, and SUSY breaking can generate

We have seen that singlet DM candidates in the MSSM are accompanied by some uncomfortable features: they are either strongly constrained by collider bounds, are only viable in fine-tuned regions of the parameter space, or present a phenomenology that is highly model-dependent. We therefore move on to reviewing the next set of candidates, the SU(2) doublets.

As supersymmetry assigns a Weyl spinor to each complex state in the scalar Higgs doublets one counts four physical higgsino states, which, after EWSB, give rise to two Majorana neutralinos,

To correctly compute the thermally averaged effective cross section that yields the DM relic abundance, one must take into account all possible annihilations and coannihilations of higgsino states. For

The effective cross section can be obtained at the leading order in the limit of all four states being degenerate (see, e.g., [

For heavy, very pure higgsinos, one should include in the calculation of

One can see that the cross section is typically much larger than

As we shall see in the next sections,

The left-handed sneutrino is a complex scalar field with SU(2) × U(1) quantum numbers equal to the higgsino’s. Like the higgsino, it has charged and neutral current couplings to the

A very important constraint on left-handed sneutrinos as DM arises because they, unlike the Majorana higgsino-like neutralinos, are not their own antiparticle, so that their elastic scattering with nuclei in direct detection experiments proceeds also through

If the wino LSP is heavier than the electroweak gauge bosons, its dominant final state channel for annihilation (and coannihilation with charginos) in the early Universe is into

Unlike higgsinos, in the wino case mass splitting between the charged and neutral fermion component of the SU(2) multiplet is generated exclusively by radiative corrections,

The Sommerfeld enhancement induces more dramatic modifications of the effective DM annihilation cross section when the average kinetic energy of the WIMP corresponds to speeds of the order of

The four neutralinos of the MSSM are all Majorana fermions that, after EWSB, remain neutral under

When

To briefly set the issue on quantitative grounds, let us estimate the strength of the coupling with which neutralino admixtures of higgsino and gaugino contribute to the spin-independent cross section. We recall that, in the limit of the squarks and heavy Higgs bosons being much heavier than

The main interaction between the neutralino and heavy nuclei in underground detectors in the limit of squarks and heavy Higgs bosons being much heavier than

As the neutralino LSP-Higgs-neutralino LSP tree-level vertex directly stems from applying the gauge covariant derivative on the Higgs doublets, it is nonzero only for a gaugino/higgsino admixture. For

If

The spin-independent cross section of the neutralino with protons (nucleons),

We show in Figure

The neutralino-proton spin-independent cross section,

Since the purity of well-tempered higgsino-dominated neutralinos stays well below 90% in those models attempting to provide a satisfactory solution to the hierarchy problem while saturating the relic abundance [

To conclude this subsection, we finally recall that, in cases where

These mechanisms, often called

Note that the

Light and heavy Higgs boson funnels are less constrained from direct LHC SUSY searches than the

The discussion of Section

We begin in Figure

(a) Spin-independent neutralino-nucleon cross section

We also show in Figure

In Figure

The relic density and DM observables are here calculated with

We have chosen to show in Figure

Lower bound on

To qualitatively understand what is happening, let us recall from Section

In order to keep the Higgs doublet soft mass under control, so as to obtain a higgsino-like LSP after EWSB, and avoid tachyonic physical states, numerical scans are in this situation driven to large negative

There is no apparent lower bound on the scattering cross section if we relax the requirement of radiative EWSB from boundary conditions generated at the GUT scale. This is the case, for example, in models where the typical mass of scalar particles is by several orders of magnitude decoupled from the electroweak vev (see, e.g., [

This highly inaccessible part of the higgsino parameter space proves particularly tricky to probe. For underabundant higgsinos,

We finally show in Figure

(a) Indirect detection bounds and projections in gamma-ray searches in space and terrestrial telescopes for

For the

We show in Figure

We conclude with a few words about the expected scale of the supersymmetric particles associated with higgsino DM. In truth, little is known in this regard, as the issue is highly model-dependent and there is not one only way of inferring the scale of SUSY breaking.

Of course, expressions similar to (

Thus, without pretence of presenting any universally valid result, but to just show an example of a model where the measurement of the Higgs mass actually does provide predictions for the maximally allowed typical scale of the superpartners, we present in Figure

(a) A plot of

Finally, like all BSM models developed at least in part to deal with the hierarchy problem, after the first two runs of the LHC models with higgsino DM have become marred by a certain amount of EWSB fine tuning. The severity of this issue depends, of course, on the specific features of each model: how EWSB is obtained and the relation to the mass of the Higgs boson. In the context of the CMSSM, the fine tuning associated with higgsino DM is shown in Figure

However, we emphasize that a large fine tuning is by no means exclusive to the CMSSM, to higgsino DM, or even to SUSY in general (see, e.g., [

The appealing theoretical features of the MSSM have made it, through the years, a natural favorite among the theoretical frameworks incorporating a possible DM particle. In this review, we have given a summary of the current status of phenomenological constraints on the DM candidates of the MSSM and have highlighted the growing consensus that, although available parameter space remains open for most DM aspirant particles, only one of them, the higgsino-like neutralino, is almost entirely free of tension from the increasing amount of observational data.

Much of what makes higgsinos very attractive is the fact that the current constraints are not evaded with specific arrangements of some model parameters, but rather as a consequence only of the higgsino isospin quantum numbers, which lead to a fairly large mass to produce

We have thus reviewed the excellent prospects for detection of higgsinos in the traditional experimental venues of direct DM detection in underground searches, indirect detection from astrophysical observations, and collider accelerators, all of which show reasons for optimism. The prospects are particularly enticing in supergravity-inspired scenarios with radiative EWSB, where the overall consistency of the theoretical picture requires a lower bound on the spin-independent cross section for higgsinos, determined indirectly but convincingly by the measured value of the Higgs boson mass.

For those models that might instead be characterized by very large scales for the superpartners (in agreement with the

Overall, we hope this might serve as an agile but comprehensive report on the consistency of the higgsino DM picture, and on the multiple opportunities that arise for its observation in the not so distant future.

The authors declare that there are no conflicts of interest regarding the publication of this article.

The authors would like to thank Luc Darmé for his comments on the manuscript and discussions. The use of the CIS computer cluster at the National Centre for Nuclear Research in Warsaw is gratefully acknowledged.

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