Asymmetric Dark Matter: Theories, Signatures, and Constraints

TL;DR: In this paper, the authors review theories of asymmetric dark matter (ADM), their cosmological implications and detection, highlighting the few common features and important mechanisms for generation and transfer of the matter-anti-matter asymmetry between dark and visible sectors.

Abstract: We review theories of Asymmetric Dark Matter (ADM), their cosmological implications and detection. While there are many models of ADM in the literature, our review of existing models will center on highlighting the few common features and important mechanisms for generation and transfer of the matter-anti-matter asymmetry between dark and visible sectors. We also survey ADM hidden sectors, the calculation of the relic abundance for ADM, and how the DM asymmetry may be erased at late times through oscillations. We consider cosmological constraints on ADM from the cosmic microwave background, neutron stars, the Sun, and brown and white dwarves. Lastly, we review indirect and direct detection methods for ADM, collider signatures, and constraints.

Summary

II. MECHANISMS FOR SHARING

• The second class naturally evades precision electroweak constraints by making use of higher dimension operators to transfer a primordial chemical potential in either the visible or DM sectors.
• The transfer mechanism explicitly relates the B and L asymmetries to the DM asymmetry via the interactions through the higher dimension operator.
• When the operator decouples as the universe cools, asymmetries freeze in separately in each sector.
• Making a natural connection between models with higher dimension and renormalizable interactions.
• The authors now review these two classes of models for asymmetry transfer.

B. Higher Dimension Operators and Renormalizable Interactions

• All of the issues with electroweak sphaleron mediated ADM transfer mechanisms can be evaded if the transfer mechanism involves no SM quantum numbers.
• At high temperatures, this barrier is not visible to the interactions, and the asymmetry is freely shared between the two sectors.
• The lowest dimension operators in the supersymmetric theory take the form EQUATION.
• The results depend on whether the operator Eq. ( 14) decouples above or below the electroweak phase transition, since the equations being solved change .
• The renormalizable interaction LH u X has been effectively utilized in the context of asymmetric neutrino or sneutrino DM [45] [46] [47].

III. MECHANISMS FOR GENERATING A PRIMORDIAL DARK AND BARYON ASYMMETRY

• Since ADM models may simply transfer a pre-existing asymmetry, it can naturally be embedded with the standard mechanisms for generating the cosmological lepton or baryon asymmetry.
• For the purposes of this review, darkogenesis and cogenesis are distinguished by whether the model has distinct mechanisms for transferring and generating the asymmetry.
• The usual Sakharov criteria for baryogenesis is that the mechanism must 5.
• In some of these models a Majorana mass for the neutrino was employed.
• The SM rather famously fails to meet the Sakharov criteria sufficiently to create the observed baryon asymmetry, both because CP is not amply violated in the SM, and because the electroweak phase transition does not feature a suitably strong departure from thermal equilibrium.

1. Decay

• The first of the Sakharov criteria is satisfied by the lifetime of the particle, τ , exceeding the age of the universe (set by the Hubble expansion) when the universe's temperature drops below the mass of the decaying particle.
• N need not generate a lepton asymmetry alone, however.
• Other interactions can be used in out-of-equilibrium decay scenarios From [53] .
• Related models [76, 77] start with an asymmetry in P , which annihilates away its thermal symmetric abundance.

2. Affleck-Dine Mechanisms

• Supersymmetric ADM models via the higher dimension operators of Eq. ( 14) make a natural playing field for Affleck-Dine (AD) baryogenesis [19] that simultaneously generates the DM and baryon asymmetries.
• The net asymmetry that is generated during re-heating is EQUATION where T R is the re-heat temperature and ρ χ is the inflaton density when Affleck-Dine baryogenesis occurs.
• Rather than making use of Q-balls to store some of the baryon asymmetry until after electroweak sphalerons have shut off the B and L violation in the SM, in some cases the condensate density may be of the same size as the baryon or lepton asymmetry generated by the Affleck-Dine mechanism [95] .

3. Electroweak Cogenesis

• Electroweak baryogenesis makes use of a first order phase transition in the Higgs sector to provide the needed out-of-equilibrium dynamics, interactions of the Higgs boson to accommodate C, CP violation, and electroweak sphalerons to contribute B violation, thus satisfying all the Sakharov criteria (see [4, 6] for a review of electroweak baryogenesis).
• The electroweak phase transition may then trigger a restoration of the D symmetry, and if this transition is sufficiently first order, the DM asymmetry is frozen in.
• While this simultaneously generates a DM and baryon asymmetry at the electroweak phase transition, the phases and dynamics of the electroweak and S 2 phase transitions need not be identical, implying different asymmetries in the two sectors.

4. Cogenesis and the WIMP Miracle

• As discussed in the introduction, the WIMP miracle is the observation that freeze-out through weak scale annihilation cross-sections gives rise to the observed relic abundance.
• ADM in most cases forgoes this miracle, instead driving the DM density via a baryogenesis related mechanism.
• In these models, the DM is not necessarily asymmetric, though baryon violating interactions in annihilations set the DM density.
• In [104] , the WIMP miracle was preserved via a metastable particle, which freezes out having its relic density set by annihilation in the usual way.

2. Spontaneous Dark Baryogenesis

• Spontaneous baryogenesis can be carried out instead in the DM sector and then transferred to the visible sector, typically via the higher dimension operators of Sec. II B.
• This mechanism was called "spontaneous cogenesis," [113, 114] though the asymmetry is driven by the DM sector and then transferred, rather than being driven by both sectors.
• If the DM sector decouples from the transfer operator before spontaneous baryogenesis has completed in the hidden sector, different asymmetries in the dark and visible sectors may result.

IV. ANNIHILATING THE THERMAL RELIC ABUNDANCE

• This thermal abundance must be efficiently removed through some mechanism.
• There are predominantly two ways to do this: through direct annihilation to light mediators, as in the left panel of Fig. 5 , or through mediators that are heavier than the DM, as in the right panel.
• When the DM carries a substantial asymmetry, the usual freeze-out calculation for the DM relic abundance is modified.
• How much of the symmetric component remains depends on the details of the freeze-out calculation when the non-zero asymmetry is included.
• Obtaining such large cross-sections is challenging consistent with current constraints, if the annihilation goes through a heavy mediator.

B. Dark Forces

• These states may be stable, which will remain as dark radiation today [124] .
• Since cosmological data is consistent with three neutrino species [7] , stringent constraints must be met by the dark sector.
• If m M > m X (but not much heavier), then annihilation can also proceed through an s-channel exchange of the mediator EQUATION For ADM, the CMB places a potentially important lower bound [116] on the annihilation cross-section, to sufficiently remove the symmetric component such that ionizing radiation is not injected into the spectrum at late time.
• If the ionizing efficiency is lower (e.g. for annihilation to pairs of τ 's), the constraints are weaker.
• The authors review these constraints in more detail below in Sec. VII.

V. ADM HIDDEN SECTORS

• In fact, as the authors saw in the previous section, light particles in the hidden sector may be important for annihilating the symmetric relic abundance of DM, leaving only the asymmetric component.
• As the authors will see in this section, dark sectors with structure may also set the mass scale in the DM sector, as well as give rise to self-interacting DM.
• Before the authors proceed, they note that while dark hidden sectors provide useful functions in the context of ADM, they are not unique to ADM.

A. Hidden Sector Models

• More predictive models of ADM arise when taking more ambitious goals for the DM sector than simply annihilating away the symmetric abundance of DM.
• The supersymmetry breaking effects in the DM sector are suppressed by a loop factor in comparison to the weak scale, naturally explaining why the DM may have a mass around the GeV scale.
• In the left figure, SUSY breaking is communicated to the visible (SM) sector, which is then weakly communicated to the hidden sector via matter or gauge interactions, as in [129] .
• The dark forces mediate scattering not only with nuclei, but also with the DM itself.

B. Self-interacting Dark Matter Through Dark Forces

• On the other hand, with the introduction of light dark forces, for example to annihilate the thermal relic abundance, as in Sec. IV, or to set the mass scale in the DM sector, as the authors just described above, self-interactions appear naturally.
• Indeed, there have long been questions about whether DM self-interactions can help alleviate some of the apparent inconsistencies between the standard collisionless Cold Dark Matter (CCDM) paradigm and the observations of structure.
• The brightest dwarf spheroidal galaxies in the Milky Way are expected to be hosted by the most massive DM sub halos in the galaxy.
• The simulations have to this point mostly assumed a constant scattering cross-section (with recent exceptions [165, 174] ), but when light dark forces are introduced, a wide variety of dynamics can ensue [147, 175, 176] .
• While DM self-interactions arise naturally within ADM models, they are not a smoking gun signature for ADM.

VI. ASYMMETRY WASH-OUT VIA OSCILLATIONS

• The phenomenology of ADM in the universe today depends in large part on whether the DM retains its primordial asymmetry until today.
• The authors follow the analysis of [180] , which fully accounted for all these effects, including scattering and the sensitivity to the type of interaction (scalar or vector), in the evolution.
• Wash-out does not necessarily occur, however, once oscillations commence.
• Coherence of the flavor wave function, as well as the flavor sensitivity of the interactions, is extremely important.
• The basic results can be summarized as follows: Rapid scatterings prevent oscillations from occurring.

VII. EVOLUTION OF ASTROPHYSICAL OBJECTS

• If the DM retains its cosmological asymmetry late into the universe, unique "smoking gun" signatures for ADM may be identified.
• If ADM retains its asymmetry until today, it may accumulate in large numbers in the center of astrophysical objects.
• The amount of ADM that accumulates 42 is proportional to the local density of DM, the scattering cross-section of the DM off the baryons in the object, and the depth of the gravitational potential well of the astrophysical objects.
• The authors consider the specific cases of constraints from neutron stars, the Sun, and brown and white dwarves.

B. Indirect Detection in the Milky Way

• While, naively, asymmetric DM is not self-annihilating, as the authors discussed in Sec. VI, its asymmetry can be washed out via oscillations after the relic density has already been set by its primordial chemical potential.
• Thus annihilations may recouple today, and give rise to an annihilation rate enhanced relative to that expected from a thermal relic, as in the models of Refs. [177, 206] .
• If the authors put aside the constraint from the CMB, the presence of a significant symmetric (sub-)component of ADM could thus give rise to a larger annihilation rate from ADM than would be expected from thermal DM [207] , even when the DM density is dominated by the asymmetric component.
• For the higher dimension operator models of Sec. II B, four fermion operators arise, and the DM may be long lived if the scale of the operator M is high.
• It is also possible that DM may scatter off of charged cosmic rays and give rise to energetic photons [208] , though the cross-sections need to be very large to produce a detectable signal.

C. Direct Detection

• The ADM mechanism itself, through the operators Eq. 15, need not provide a direct detection (DD) signal, though in some cases, depending on the UV completion, it will.
• Models with dark forces, as discussed in Sec. V, may also provide a direct detection mediator.
• Thus direct detection experiments could rule out an ADM sector thermalized with the visible sector through a light hidden gauge boson.

IX. COLLIDER SIGNATURES AND CONSTRAINTS

• A. Monojets and Extended Supersymmetric Cascade Decays ADM leads to a potentially rich array of DM signals at high energy colliders.
• The x-axis schematically represents inaccessibility and the y-axis represents energy.
• This barrier may either be massive states (mass at the electroweak scale or above) that couple to both the SM and to the ADM sector, or light states that couple weakly to the SM sector so that they escape constraints from lower energy machines.
• As the LOSP decays into the DM, the extended supersymmetric cascade decay chains imply a higher multiplicity of visible final state particles than for the same MSSM spectrum.

B. Flavor Constraints

• As the authors saw in Eqs. 78-80, the scale M also correlates with whether the LOSP has prompt decays at the LHC.
• The UV completion of this operator may include terms such as the following: EQUATION where i, j are flavor indices, and D is a new heavy state being integrated out to generate the operator in Eq. 15.

56 X. SUMMARY AND OUTLOOK

• In the last several years, Asymmetric Dark Matter has become a flourishing subfield of DM research.
• This is driven in part by a purely theoretical motivation to explore wellmotivated models outside the standard WIMP parameter space, and in part by anomalies in Direct and Indirect Detection.
• One of the most important implications of ADM is phenomenological, since theories of DM inform the types of experiments that will be designed and utilized for hunting DM.
• Lastly, the signals from the CoGeNT and CDMS experiments in the 7-10 GeV mass window increase the interest in ADM theories, where the scattering rate observed is predicted by scattering through a light dark force, as in Sec. VIII.
• Many of the tools of this review will be crucial along that path.

Figures

Table I: A minimal dark messenger sector. Anomaly cancellation is achieved via mirror fermions with the same U(1)D charge, but opposite hypercharge. There are two sterile states, X̄ 1,2 M .

Figure 10: left: The Hertzsprung-Russell diagram for a 1 M star, with σSD = 10 −37 cm2. For a moderate amount of DM accumulation, the star compensates for nuclear energy being lost in the core by increasing it outside the core, making the star hotter and more luminous. This is shown by the darkest blue curve with ρDM = 10 3 GeV/cm3. Eventually this is no longer possible, and the star contracts and cools, which is shown by the evolution along the green and orange curves. This yields a dramatic change in the usual evolution of the star that may be observable in a DM dense region. From [194]. right: Impact of ADM on brown dwarves. Solid curves show the standard evolution of low mass stars between 0.05 and 0.11M . The main sequence is reached as t increases when the luminosity stabilizes to a constant value, indicating that hydrogen burning has been ignited. In the standard case, this occurs for M > 0.08 M . By contrast, when DM is added (with boost ΓB = 10 3 in comparison to the usual collection rate in the Sun for the same DM parameters, so that enough DM is collected), the red and blue curves result. Stars between 0.08 and 0.1M , that entered the main sequence in the standard case, no longer enter the main sequence and instead become brown dwarves. From [199].

Figure 6: Constraints on the scale Λ of a higher dimension operator through which the DM can annihilate to SM quarks. The black curve corresponds to the minimum annihilation cross-section necessary to remove the symmetric component to 1%. The solid curves represent constraints from various direct detection experiments, while the dashed curves show constraints from ATLAS and CMS monojet searches. Only the grey shaded region is consistent with sufficiently annihilating the symmetric abundance and satisfying all the constraints. In this region the DM mass is well outside the natural ADM window of 1-10 GeV. See [120] for more details of the analysis.

Figure 7: Schematics of two means of communicating supersymmetry breaking to the DM sector. In the left figure, SUSY breaking is communicated to the visible (SM) sector, which is then weakly communicated to the hidden sector via matter or gauge interactions, as in [129]. In the right figure [136] SUSY breaking is directly communicated to both hidden and visible sectors via gauge interactions, but the hidden sector is lighter because SUSY breaking is communicated more weakly to the DM than the baryons.

Figure 1: left: ρDM/ρB versus the DM mass to DM-number violation decoupling temperature ratio, mX/TD. This plot is designed to illustrate how Boltzmann statistics with mX/TD > 1 can suppress the DM energy density, giving rise to the observed ρDM/ρB even for heavy DM mass. For decoupling of DM number violation at 200 GeV, the DM must be at least 2 TeV. From [29]. right: In the absence of a large DM density suppression from a large DM to decoupling temperature ratio, cancellations in the sphaleron conserved quantities B − L and B − 3D can be used to achieve the correct density. The dashed, solid and dot-dashed (red, blue, green) lines correspond to ΩDM/ΩB = (1, 5, 25), with mX/TD = 0.25 and DM mass mX = 200 GeV. From [30].

Figure 2: A schematic of higher dimension ADM models [18]. The x-axis represents the inaccessibility of the sector (the visible or dark sector) and the y-axis its energy, with the set-up inspired by Hidden Valley models [40]. The mediator of the higher dimension operator, ODOB−L, presents a barrier between the two sectors. At high temperatures the barrier is effectively removed though high energy interactions, allowing the B − L and D number to be shared between the two sectors. At low t mperatures, the barrier effectively fre zes in the asymmetry between th two sectors.

Figure 3: A schematic of leptogenesis ADM models. Out-of-equilibrium and CP violating decays of a sterile neutrino into both the SM and DM sectors give rise to an asymmetry in both sectors. From [53].

Figure 9: Excluded scattering cross-section off a nucleon from Bose-Einstein (and eventual black hole) formation in the neutron star J0437-4715. In the hatched region constraints are lifted because the mini-black hole evaporates. For comparison, the constraint from CDMS-II is shown. From the analysis of [184].

Figure 5: Typical processes for removing the thermal symmetric (in X and X̄) abundance. X − X̄ can annihilate to light force mediators φ (whether they be scalar or vector), or through a heavier state directly to SM ff̄ .

Figure 12: Diagrams for the decay of the lightest ordinary supersymmetric particle (LOSP) to the scalar X̃ and two SM fermions, if the LOSP is a squark (left) or neutralino (right).

Figure 11: The red curve shows the lower bound on the annihilation cross-section times ionizing efficiency of annihilation, f〈σv〉, as a function of the DM mass. The diagonal lines show isocontours of relic symmetric component, r ≡ X̄/X, while the horizontal isocontours show the constraint from the CMB for each choice of r. Where the two isocontours meet is the CMB constraint. From [116].

Figure 8: The effects of scattering and the “flavor” sensitivity of the annihilation process. The first panel shows the relative rates of scattering, oscillation and Hubble expansion H. Once H drops below ωosc ∼ δm, oscillations commence. If the interaction is flavor blind, scatterings are irrelevant, and annihilations proceed as soon as oscillations commence. This is shown in the upper right panel. On the other hand, when interactions are flavor sensitive, the decoherence effect of scattering is crucial. This is shown in the lower two panels. When there are no scatterings (κ = 0, left panel), coherence is maintained, but annihilation cannot proceed through flavor sensitive interactions, because det([H0, Y ]) = 0. When scatterings are present (right panel), coherence is lost, and annihilations proceed as soon as the scattering rate drops below H. Figure from Ref. [180].

Figure 4: Schematic of darkogenesis models. Darkogenesis makes use of dynamics in the dark sector (with a dark Higgs and dark sphalerons) to generate a dark asymmetry that is then transferred to the baryons via a connector sector. Figure from [109].

Full text

Asymmetric Dark Matter: Theories, Signatures, and Constraints
Kathryn M. Zurek
1
1
Michigan Center for Theoretical Physics, Department of Physics,
University of Michigan, Ann Arbor, Michigan 48109 USA
∗
We review theories of Asymmetric Dark Matter (ADM), their cosmological impli-
cations and detection. While there are many models of ADM in the literature, our
review of existing models will center on highlighting the few common features and
important mechanisms for generation and transfer of the matter-anti-matter asym-
metry between dark and visible sectors. We also survey ADM hidden sectors, the
calculation of the relic abundance for ADM, and how the DM asymmetry may be
erased at late times through oscillations. We consider cosmological constraints on
ADM from the cosmic microwave background, neutron stars, the Sun, and brown
and white dwarves. Lastly, we review indirect and direct detection methods for
ADM, collider signatures, and constraints.
∗
Electronic address: kzurek@umich.edu
arXiv:1308.0338v2 [hep-ph] 19 Nov 2013

2
Contents
I. Motivation: What is Asymmetric Dark Matter? 4
II. Mechanisms for Sharing a Primordial Dark or Baryon Asymmetry 8
A. Sphalerons 8
B. Higher Dimension Operators and Renormalizable Interactions 12
III. Mechanisms for Generating a Primordial Dark and Baryon Asymmetry 15
A. Cogenesis 16
1. Decay 16
2. Affleck-Dine Mechanisms 19
3. Electroweak Cogenesis 22
4. Cogenesis and the WIMP Miracle 23
B. Dark Baryogenesis 23
1. Dark “Electroweak Baryogenesis” 24
2. Spontaneous Dark Baryogenesis 26
IV. Annihilating the Thermal Relic Abundance 27
A. Heavy Mediators 29
B. Dark Forces 31
V. ADM Hidden Sectors 31
A. Hidden Sector Models 32
B. Self-interacting Dark Matter Through Dark Forces 36
VI. Asymmetry Wash-Out Via Oscillations 38
VII. Evolution of Astrophysical Objects 41
A. Neutron Stars 42
B. The Sun 45
C. Brown and White Dwarves 47
VIII. Indirect and Direct Detection 48

3
A. Indirect Detection via the Cosmic Microwave Background 49
B. Indirect Detection in the Milky Way 50
C. Direct Detection 51
IX. Collider Signatures and Constraints 53
A. Monojets and Extended Supersymmetric Cascade Decays 53
B. Flavor Constraints 55
X. Summary and Outlook 56
Acknowledgments 57
A. Distribution of baryon, lepton and dark matter asymmetries 57
References 59

4
I. MOTIVATION: WHAT IS ASYMMETRIC DARK MATTER?
The dark matter (DM) and baryon abundances are very close to each other observa-
tionally: ρ
DM
/ρ
B
≈ 5 [1]. In the standard Weakly Interacting Massive Particle (WIMP)
paradigm, however, these quantities are not a priori related to each other. The DM density
in the WIMP freeze-out paradigm is fixed when the annihilation rate drops below the Hubble
expansion [2, 3]:
n(T
fo
)hσ
ann
vi < H(T
fo
), (1)
where T
fo
is the temperature when DM annihilation freezes-out, n(T
fo
) is the DM number
density, and hσ
ann
vi is a thermally averaged annihilation cross-section. Thus the macroscopic
quantity of the DM number density in the universe today is related to the microscopic
quantity of the annihilation cross-section. On the other hand in baryogenesis [4–6], the
baryon density is set by CP-violating parameters and out-of-equilibrium dynamics (such as
order of the electroweak phase transition) associated with baryon number violating processes.
Since the quantities setting the baryon density and the DM density are unrelated to each
other in these scenarios, it seems surprising that the observed energy densities are so close to
each other. While it is possible that this is an accident, or that this ratio is set anthropically,
dynamics may also play a role. The theory of DM may, in fact, tie the DM density to the
baryon density.
The connection between the DM and baryon densities arises naturally when the DM has
an asymmetry in the number density of matter over anti-matter similar to baryons.
1
The
DM density is then set by its asymmetry, which can be directly connected to the baryon
asymmetry, rather than by its annihilation cross-section. Thus we have
n
X
− n
¯
X
∼ n
b
− n
¯
b
, (2)
where n
X
, n
¯
X
are the DM and anti-DM number densities, and n
b
, n
¯
b
are the baryon and
anti-baryon asymmetries. The asymmetry is approximately one part in 10
10
in comparison
1
In some theories connecting the DM and baryon densities, the DM does not have a matter-anti-matter
asymmetry. Even though the DM is not asymmetric in these cases, we discuss these models in this review
where appropriate.

5
to the thermal abundance, since
η ≡
n
B
n
γ
=
n
b
− n
¯
b
n
γ
≈ 6 ×10
−10
, (3)
with the last relation being obtained most precisely from Cosmic Microwave Background
(CMB) data [7]. Since ρ
DM
/ρ
B
∼ 5, the relation of Eq. 2 suggests m
X
∼ 5m
p
' 5 GeV.
The natural asymmetric DM mass may differ from this value by a factor of a few due to the
details of the model.
2
Furthermore, since this scale is not far from the weak scale, in some
models the DM mass may be related to weak scale dynamics, reducing the question of why
the baryon and DM densities are close to each other to the question of why the weak scale
is close to the QCD confinement scale. In other models, the DM mass scale is set by the
proton mass scale itself.
The idea that the DM and baryon asymmetries might be related to each other dates
almost from the time of the WIMP paradigm itself [8, 9]. The initial motivation for a
DM asymmetry was to solve the solar neutrino problem, by accumulating DM that affects
heat transport in the Sun, as pointed out by [10]. The subsequent development of DM
models with an asymmetry focused on electroweak sphalerons to relate the baryon and
DM asymmetries [11–15], though such models usually involve electroweak charged DM, and
have become highly constrained by both LEP and the LHC. In other cases decay mechanisms
were utilized [16, 17]. The Asymmetric Dark Matter (ADM) paradigm [18] provided a robust
framework to relate the baryon and DM number densities via higher dimension operators;
it encompasses many realizations and easily evades all experimental constraints. With this
paradigm as a sound and flexible framework, significant activity and development of ADM
models and phenomenology ensued. This development is the subject of this review. More
generally, the ADM mechanism
3
works as follows.
2
This natural relationship is broken in two instances. First, if DM-number violating process creating the
DM asymmetry decouples (at a temperature T
D
) after the DM becomes non-relativistic, in which case
there is a Boltzmann suppression in the asymmetry which scales as e
−m
X
/T
D
, where m
X
is the DM mass.
Thus the DM can be much heavier than 5 GeV. Second, if the DM and baryon setting mechanism yields
very different asymmetries in the visible and dark sectors, the DM may be much heavier or lighter than
5 GeV. We will review models that realize both cases, with the former occurring most prominently in
sphaleron models, and the latter occurring most prominently in decay models.
3
While the name “Asymmetric Dark Matter” was introduced in [18] to describe the higher dimension
operator models proposed there, we use the name “ADM” in this review to describe all models where the
dark matter density is set via its chemical potential.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Asymmetric dark matter: theories, signatures, and constraints" ?

The authors review theories of Asymmetric Dark Matter ( ADM ), their cosmological implications and detection. The authors consider cosmological constraints on ADM from the cosmic microwave background, neutron stars, the Sun, and brown and white dwarves. Lastly, the authors review indirect and direct detection methods for ADM, collider signatures, and constraints. 

Q2. What is the key piece of evidence for physics beyond the standard model?

The existence of the baryon asymmetry and dark matter (DM) are key pieces of evidence for physics beyond the standard model (SM). 

Q3. How can the constraints be mapped in the direct detection plane?

Since the formation of the black hole is ultimately dependent only on the accumulation of bosonic ADM, and hence the DM-nucleon scattering cross-section, the constraints can be mapped in the direct detection plane. 

Q4. What is the sym-metric abundance of the dark sector?

If the dark sector was thermalized in the process of asymmetry generation, the sym-metric abundance (which is 1010 times larger than the asymmetric component, since the cosmological baryon asymmetry is η ≈ 6× 10−10) must efficiently annihilate away. 

Q5. What is the net effect of Ltrans Lnuc?

The net effect is that for moderate amounts of captured DM (i.e. Ltrans < Lnuc but trans > nuc), the star becomes larger and more luminous. 

Q6. What is the simplest way to constrain DM at a collider?

Perhaps the simplest direct way to constrain DM at a collider is through initial state radiation plus missing energy [235, 236], where the missing energy (MET) signifies DM production. 

Q7. Why are sphaleron models subject to precision electroweak constraints?

Because of the need to extend the electroweak content of the SM, these models are often subject to severe precision electroweak constraints. 

Q8. What is the asymmetry generated by the Affleck-Dine mechanism?

Rather than making use of Q-balls to store some of the baryon asymmetry until after electroweak sphalerons have shut off the B and L violation in the SM, in some cases the condensate density may be of the same size as the baryon or lepton asymmetry generated by the Affleck-Dine mechanism [95]. 

Q9. What is the relative abundance of D, B and L?

The relative abundance of D, B and L depends on linear chemical equilibrium equations, which can be easily solved using the methods outlined in [31], and which the authors review in appendix A. 

Q10. What is the mechanism for sharing the asymmetry between the two sectors?

As the temperature in the early universe drops, the barrier becomes visible, and the interactions through the heavy mediator decouple, separately freezing in an asymmetry in each of the two sectors. 

Q11. How can the authors see the evolution of the particle-anti-particle asymmetry?

The evolution of the particle-anti-particle asymmetry can be seen explicitly by writing out the mass matrices and considering the Boltzmann equation in the absence of collisions. 

Q12. What is the asymmetry generated when the pseudo particle is forced out of its false minimum?

Using an impulse approximation when H = |f |m/|g| to calculate the asymmetry generated as the pseudo particle is forced out of its false minimum, the authors find [81]− 

Q13. What is the asymmetry transferred to the visible and DM sectors?

The asymmetry is then transferred to the visible and DM sectors via decay of heavy states that obtained asymmetries via the phase transition for G.12 

Q14. What is the significance of the UV completion of the operators?

For models of ADM mediated through the higher dimension operators reviewed in Sec. II B, there may be important implications for flavor physics, depending on the scale, M , of the UV completion of the operators. 

Q15. How is the asymmetry transferred to the visible sector?

This asymmetry is transferred to the visible sector by a messenger sector, which can be via renormalizable or higher dimension interactions, or via electroweak sphalerons. 

Q16. What is the ratio of the asymmetries in each sector?

In fact, the asymmetries that are generated in each sector are proportional to the CP violation in the decays of Ni to each sector [53]:χ = Γ(N1 → χφ)−