dark matter candidates: status and perspectives

PoS(ICRC2015)005

Dark Matter Candidates: Status and Perspectives

Tim M.P. Tait∗Department of Physics and AstronomyUniversity of California, IrvineIrvine, CA 92697E-mail: [email protected]

This review summarizes the talk on Dark Matter Candidates: Status and Perspectives during theplenary sessions of ICRC 15 in The Hague, Netherlands. It includes discussions of theoretically-motivated candidates which could play the role of dark matter, as well as some highlights of thecurrent status of direct, indirect, collider, and astronomical searches which aim to shed light onits properties and identify.

The 34th International Cosmic Ray Conference30 July- 6 August, 2015The Hague, The Netherlands

∗Speaker.

c© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/

mailto:[email protected]

PoS(ICRC2015)005

Dark Matter Candidates: Status and Perspectives Tim M.P. Tait

Theories of Dark Matter

mSUGRA

R-parityConserving

Supersymmetry

pMSSM

R-parityviolating

Gravitino DM

MSSM NMSSM

DiracDM

Extra Dimensions

UED DM

Warped Extra Dimensions

Little Higgs

T-odd DM

5d

6d

Axion-like Particles

QCD Axions

Axion DM

Sterile Neutrinos

LightForce Carriers

Dark Photon

Asymmetric DM

RS DM

Warm DM

?

HiddenSector DM

WIMPless DM

Littlest Higgs

Self-InteractingDM

Q-balls

T Tait

Solitonic DM

QuarkNuggets

Techni-baryons

Dynamical DM

Figure 1: (Incomplete) Venn diagrams of theories of dark matter.

1. Introduction

The evidence for dark matter is overwhelming [1], and points to the need for what is mostlikely a new quantum field which must supplement the Standard Model (SM) of particle physics.The identification of this field is thus of paramount importance in order to extend the StandardModel. Seeing how dark matter fits together with the Standard Model structure is a likely toprovide key insights into fundamental physics and may reveal new principles of Nature. A widevariety of experimental searches aimed at uncovering clues are underway. In this talk I provide anover-view of theoretical ideas for what could constitute the dark matter (Sec. 2) and discuss thecurrent status of experimental searches (Sec. 3). I apologize in advance that because each of theseareas are wide fields in themselves, my discussion will by necessity be somewhat personalized andincomplete. I must further apologize that references are largely to reviews or other talks at theconference, and are intended more as a starting point for an interested reader to learn more ratherthan a fair historical representation of the literature.

2. Candidates

There are a wide variety of the theoretical ideas as to what might constitute the dark matter(see Fig. 1). In terms of its particle physics properties, a viable dark matter candidate must satisfy

2

PoS(ICRC2015)005


the relatively modest requirements that it be:

• dark, not charged under either the SM’s electromagnetic U(1)EM or strong nuclear SU(3)C

interactions;

• cold, meaning that it was sufficiently non-relativistic at the time of structure formation; and

• stable, with a lifetime such that a significant fraction of it persists in the Universe today.

Nothing in the Standard Model itself possesses these properties without invoking additional ingre-dients, and a dizzying array of possibilities could represent the truth, or contain elements of truth.There are far too many to go through in detail, and so I limit my discussion to brief mention ofsterile neutrinos, axions, and weakly-interacting massive particles (WIMPs).

2.1 Sterile Neutrinos

Dark matter may be intimately linked to another manifestation of physics beyond the StandardModel: the fact that neutrinos are observed to have masses, leading to them mixing when propa-gating over long distances. The simplest extension of the SM engineering masses for the neutrinosis to add additional Weyl fermions which are singlets under the SM gauge symmetries (and hencesterile). Such particles are allowed by symmetries to have Majorana masses and to interact withthe combination of the Standard Model Higgs doublet together with the ordinary (active) neutri-nos. After the Higgs acquires its vacuum expectation value and the fields are rotated to the massbasis, the theory describes six Majorana fermions which are mixtures of the original active andsterile fermions. Measurements of neutrino oscillations indicate that three of these states must beoverwhelmingly composed of the active components, implying that the additional fermions aredominantly sterile.

These heavier states can decay into the lightest states (typically plus a photon) through theelectroweak interaction. The rate of this decay is proportional to the small quantity of active neu-trino that state contains, and for very small mixing a lifetime on the order of the age of the Universewould be viable with such a state playing the role of dark matter. For such a particle to be coldenough to be consistent with our understanding of structure formation requires that its mass bem & 10 keV [2].

2.2 Axions

The axion is a hypothetical particle which arises from the Peccei-Quinn solution to the strongCP problem [3, 4, 5]. The problem boils down to the fact that the strong nuclear force inherentlycontains a parameter θ̄ consistent with all known symmetries which violates the discrete CP sym-metry, and would induce an electric dipole moment (EDM) for the neutron. Increasingly sensitivemeasurements have failed to observe such a neutron EDM, and currently require θ̄ . 10−9. Sucha tiny value seems profoundly unnatural, and begs for a dynamical explanation, which the axionprovides by transforming it into a dynamical quantity which then relaxes to zero.

The fluctuations of this dynamical field appear as a pseudo-Goldstone boson whose inter-actions are characterized by 1/ fa and mass is roughly ma ∼ fπ/ fa mπ . Observations require

3

PoS(ICRC2015)005


fa & 109 GeV [2], indicating that the axion must be extremely light and extremely weakly cou-pled. The axion is also inherently unstable, but its tiny mass and coupling indicate that it can easilyhave a lifetime long enough for it to successfully play the role of dark matter.

2.3 Weakly Interacting Massive Particles

Particles with roughly electroweak scale masses and couplings occur in many popular exten-sions of the Standard Model, including supersymmetric versions [6, 7, 8], models with UniversalExtra Dimensions [9, 10, 11, 12, 13], and realistic little Higgs theories [14, 15]. Even on top oftheir purely theoretical motivation, they make for an attractive dark matter candidate because theirabundance in the early Universe can be understood from their freezing out from equilibrium. Sincetheir density is determined by when they fall out of equilibrium with the SM plasma, the observeddensity of dark matter implies an annihilation rate of (assuming that their average cross section〈σv〉 is s-wave, or velocity-independent) of 3× 10−26 cm3/s [16]. Given their large masses andrelatively strong coupling to the SM, it is generically necessary to impose a (perhaps very weaklybroken) symmetry such that they are stable or long-lived enough to be dark matter.

3. Probes

Each of these visions for dark matter imply that it has some kind of interaction with ordinarymatter. Thus, each leads to strategies for how it can be observed and identified. In this section, Ibegin with a quick review of the particle physics strategies for detecting dark matter, followed by adiscussion of how they fit together, and close with some comments about astronomical probes.

3.1 Direct Detection

Direct searches for dark matter seek to observe the presence of the ambient dark matter inthe solar system through its interaction with detectors on the Earth. As a result, such searches aresensitive to the precise density of dark matter in the neighborhood of the solar system, as well as tothe distribution of its velocity.

One class of experiments builds very sensitive detectors aimed at seeing the dark matter col-liding with a SM target (typically the nucleus of a heavy atom [17, 18], though scattering withelectronic structure is also being investigated [19, 20, 21]). Dark matter appears as an otherwiseundetected particle coming in, scattering with the detector and producing a signal, and then exit-ing again without being detected. For dark matter with masses in the range of ∼ 10− 100 GeV,remarkable advances in sensitivity spanning several orders of magnitude (see Fig. 2) have beenachieved in a short range of time, and have effectively covered scenarios in which the dark matterinteracts by exchange of a Z boson, and are now closing in on the possibility of exchange of thenewly discovered Higgs boson as well. Ultimately, such detectors will become sensitive to lowenergy neutrinos [22], which will slow down future increases in sensitivity. Another active targetregion is masses below ∼ 10 GeV, which require sensitivity to much smaller momentum transfer.

Ambient axons can also be detected, by using the fact that they typically couple to electricand magnetic fields, ~E ·~B with a coupling characterized by 1/ fa (but with some dependence on theUV details of the axion model). In an experiment with a background magnetic field, an incomingaxion can be converted by this interaction into a photon, whose presence can then be magnified and

4

PoS(ICRC2015)005

Dark Matter Candidates: Status and Perspectives Tim M.P. Tait10 Direct Detection Program Roadmap 39

1 10 100 1000 10410!5010!4910!4810!4710!4610!4510!4410!4310!4210!4110!4010!39

10!1410!1310!1210!1110!1010!910!810!710!610!510!410!3

WIMP Mass !GeV"c2#

WIMP!nucleoncrosssection!cm2 #

WIMP!nucleoncrosssection!pb#

7BeNeutrinos

NEUTRINO C OHER ENT SCATTERING

NEUTRINO COHERENT SCATTERING(Green&ovals)&Asymmetric&DM&&(Violet&oval)&Magne7c&DM&(Blue&oval)&Extra&dimensions&&(Red&circle)&SUSY&MSSM&&&&&&MSSM:&Pure&Higgsino&&&&&&&MSSM:&A&funnel&&&&&&MSSM:&BinoEstop&coannihila7on&&&&&&MSSM:&BinoEsquark&coannihila7on&&

8BNeutrinos

Atmospheric and DSNB Neutrinos

CDMS II Ge (2009)

Xenon100 (2012)

CRESST

CoGeNT(2012)

CDMS Si(2013)

EDELWEISS (2011)

DAMA SIMPLE (2012)

ZEPLIN-III (2012)COUPP (2012)

SuperCDMS Soudan Low ThresholdSuperCDMS Soudan CDMS-lite

XENON 10 S2 (2013)CDMS-II Ge Low Threshold (2011)

SuperCDMS Soudan

Xenon1T

LZ

LUX

DarkSide G2

DarkSide 50

DEAP3600

PICO250-CF3I

PICO250-C3F8

SNOLAB

SuperCDMS

Figure 26. A compilation of WIMP-nucleon spin-independent cross section limits (solid curves), hintsfor WIMP signals (shaded closed contours) and projections (dot and dot-dashed curves) for US-led directdetection experiments that are expected to operate over the next decade. Also shown is an approximateband where coherent scattering of 8B solar neutrinos, atmospheric neutrinos and di↵use supernova neutrinoswith nuclei will begin to limit the sensitivity of direct detection experiments to WIMPs. Finally, a suite oftheoretical model predictions is indicated by the shaded regions, with model references included.

We believe that any proposed new direct detection experiment must demonstrate that it meets at least oneof the following two criteria:

• Provide at least an order of magnitude improvement in cross section sensitivity for some range ofWIMP masses and interaction types.

• Demonstrate the capability to confirm or deny an indication of a WIMP signal from another experiment.

The US has a clear leadership role in the field of direct dark matter detection experiments, with mostmajor collaborations having major involvement of US groups. In order to maintain this leadership role, andto reduce the risk inherent in pushing novel technologies to their limits, a variety of US-led direct search

Community Planning Study: Snowmass 2013

Figure 2: Constraints (and future projections) on the cross section for dark matter to scatter with a nucleonas a function of the dark matter mass. From Ref. [17].

detected [2]. Already, interesting regions of parameter space have been ruled out by these searches,and new ideas [23, 24] hold the promise to push into previously unexplored regions of parameterspace.

3.2 Collider Searches

High energy colliders such as the LHC can hope to produce the dark matter directly. Since it isexpected to interact much too weakly to leave a trace in the detectors which examine the collisions,its existence must be inferred from an imbalance in the net momentum of the observed productsof the reaction. Since colliders do not rely on the dark matter being already present in the initialstate, a positive signal would not establish the production of dark matter (as opposed to some otherweakly interacting, but unstable state). Nonetheless, they have exquisite control over the initialstate and painstakingly developed control over background processes. A positive signal would behighly suggestive and would help sharpen the direction of future direct and indirect searches.

There are many different searches by the ATLAS and CMS experiments which are relevantfor theories of dark matter, including e.g. searches for supersymmetric particles [25, 26] andgeneric searches for anomalously large production of events with missing transverse momentum[27, 28]. In order to connect such searches to the properties of dark matter, a theoretical con-struct is required. A variety are employed, including the phenomenological Minimal Supersym-metric Standard Model (pMSSM) [8], simplified models describing the dark matter and a parti-cle mediating its interactions [29, 30], and effective field theories (EFTs) which are the universal

5

PoS(ICRC2015)005


in the signal region. No significant excess over the predicted background is observed, soupper limits on the b̃-pair production are set (see Fig. 2, right).

The monojet search [7] targets the direct t̃-pair production. As mentioned before, thereis a strong belief that the mass of the lightest t̃ is relatively low in the SUSY spectrum.Initial t̃ searches focused on the tt + Emiss

T signature, excluding direct t̃ pair production upto mt̃ ⇠ 700 GeV for a light LSP (see e.g. [8] for a combination of a single lepton and arazor inclusive CMS analyses). In the case 10 < mt̃ � m� < 80 GeV, the dominant decayis expected to be t̃ ! c�̃0

1. The signature would consist of a soft hadronic jet associatedto a limited amount of Emiss

T , that would not o↵er a handle for an e�cient separationfrom the SM backgrounds, unless it recoils against a hard Initial State Radiation (ISR) jet.CMS searches for this signature, selecting events with a hard ISR jet (and allowing for atmost one more soft jet). The modeling of the ISR is crucial for this analysis and for otheranalyses targeting compressed spectra: discrepancies between the data and the simulationare corrected for by using high statistics SM control samples (tt, Z+jets, ...). Exclusionlimits are presented on the left plot of Fig. 3; the sensitivity of the analysis is higher at lowvalues of mt̃ � m�, where the events appear to be more monojet-like.

Figure 3: Left: exclusion limits for the monojet search [7]. Right: exclusion limits forgluino pairs production, with gluinos decaying to di↵erent mixtures of bb�̃0

1, bt�̃01, and tt�̃0

1,obtained by the razor inclusive analysis [8].

Finally, more exclusion limits on gluino pair production have been obtained by the razorinclusive analysis [8]. The branching ratios of the gluino decays are varied from the mostfavorable final state bb�̃0

1 to the most challenging tt�̃01, see Fig. 3 (right). The di↵erence

between the exclusion limits of the two extreme cases (100-150 GeV for low �̃01 masses) gives

some feeling about the dependence of the results on the assumptions on which simplifiedmodels are relying.

4

(a) (b)

Figure 2: (a)Exclusion limits for the pair production of gluinos. (b) Exclusion limits forsquarks decaying via charginos. The dashed and solid lines show the expected and observedlimits, respectively, including all uncertainties except the theoretical signal cross sectionuncertainties.

performed on data collected by the ATLAS detector in 4.7 fb�1 at a center-of-mass energyps = 7 TeV and 20.3 fb�1 at

ps = 8 TeV. In the SUSY searches the full capabilities of

the ATLAS detector are used, as the targeted signatures include all reconstructed physicsobjects: muons, electrons, taus, photons, jets, b-tagged jets, transverse missing energy. Agood understanding of the SM processes is compulsory because the signal events are, manytimes, expected to be located in tails of distributions or as deviations over small SM eventyields. As no excess is seen over the SM expected yields, limits on the SUSY particlesmasses are set.

2 ATLAS SUSY searches strategy

With the many parameters (105) introduced by the Minimal Supersymetric Model (MSSM),additional constraints are needed when searching for SUSY. In the last years the strategyhas moved towards what is named Natural SUSY. In Natural SUSY the third generationand elecroweakinos are light, while the other supersymmetric particles can be heavy. Thecross-section for the direct production of supersymmetric particles at the LHC can be seenin Fig. 1.

Based on the type of decay the produced supersymmetric particles can decay promptlyor be long lived. Most of the ATLAS searches concentrate on searches for promptly decayingsupersymmetric particles but in Section 2.4 an example of a search for long lived particlesis presented. As the realisation of SUSY in nature can be R-parity conserving or violatingthis fact is taken into account both in the production and decay of the searched particles.

When considering full R-parity conserving models the LSP ( �̃01) will be produced in pairs

in cascade decays of directly produced sparticles. The signature of the LSP is the presence

2

Figure 3: Constraints on parameter space of supersymmetric theories (as indicated) derived from searchesfor the indicated channels involving missing momentum at the LHC [25, 26].

limit of all models when the mediator particles are heavy compared to the energies of interest[31, 32, 33, 34, 35, 36, 37, 38]. As two (of many possible) examples of the impact of such asearch, Fig. 3 shows regions of supersymmetric parameter space excluded by searches for missingmomentum plus jets of hadronic particles.

3.3 Indirect Detection

Indirect detection aims to look at the rare annihilation of two dark matter particles which isexpected in regions where the dark matter is over-dense, such as the in centers of galaxies. Whilemany of the products of such annihilation will themselves decay on their way from the annihi-lation point to the Earth, photons, neutrinos, and anti-matter may arrive, and could be distinctiveenough from astrophysical sources for one to reconstruct their origin as a signal of dark matter [39].Searches for dark matter annihilations resulting in gamma rays [40, 41, 42], neutrinos [44, 43],positrons [45, 46], and anti-protons [47] are all well underway. While charged particles are sub-ject to the galactic magnetic fields, GeV–TeV photons and neutrinos are expected to propagate ongalactic scales with relatively little directional or energy loss. As a result, their distribution in thesky allows the direction of their origin to be reconstructed.

Features appear in these searches which are not well-described by modeling of astrophysicalfore- and back-grounds. The positron excess above 10 GeV first observed by PAMELA [45] andsubsequently confirmed by AMS-02 [46] remains unexplained, with a dark matter interpretationrequiring a cross section which is somewhat shockingly large, though not outside of the realm oftheoretical engineering [48, 49]. More recently, analysis of gamma rays from the direction of thegalactic center region collected by the Fermi LAT show an excess at ∼ GeV energies [50, 51, 52,53, 54], which is also observed in analysis by the Fermi collaboration itself [55, 56, 57]. In Fig. 4is the spectrum of the excess for four different models of the astrophysical back- and fore-grounds.These spectra are roughly consistent with dark matter annihilation into hadronic final states when

6

PoS(ICRC2015)005


GeV Observations of the Galactic Centre 19

Figure 13. Differential fluxes for the 15� ⇥ 15� region about the GC of theNFW component with spectrum modelled with an exponential cut-off powerlaw. The envelopes include the fit uncertainties for the normalisation andspectral index. Hatch styles: Pulsars, intensity-scaled (red, vertical); Pulsars,index-scaled (black, horizontal); OBstars, intensity-scaled (blue, diagonal-right); OBstars, index-scaled (green, diagonal-left). Results from selectedother works are overlaid. Filled symbols: Hooper & Slatyer (2013), differentsymbols bracket the results obtained when different regions of the sky areconsidered in the fit; Angled crosses: Gordon & Macı́as (2013); Open sym-bols: Abazajian et al. (2014), front-converting events shown with triangles,front- and back-converting events shown with squares and circles, depend-ing on the modelling of the fore-/background. Stars : Calore et al. (2015a).Note: the overlaid results are rescaled to the DM content over the 15� ⇥ 15�region for an NFW profile with index �=1.

stant for each IEM, the interplay between the centrally peakedpositive residual template and the interstellar emission com-ponents is not surprising. Because the IC component is max-imally peaked toward the GC for all IEMs an additional tem-plate that is also peaked there will also be attributed someflux when fit. Over all IEMs the effect of including the NFWmodel for the residual results in an IC annulus 1 contributionthat is up to three times smaller and H I annulus 1 contributionthat is up to three times larger.

Note that even if a centrally peaked template is included asa model for the positive residual, it does not account for all ofthe emission. This can be seen in Fig. 14, which shows theresidual counts for the NFW template and IEM with the bestspectral residuals (Pulsars index-scaled). Qualitatively, the re-mainder does not appear distributed symmetrically about theGC below 10 GeV, and still has extended positive residualseven at higher energies along and about the plane.

5. DISCUSSION5.1. Interstellar Emission

This study is the first using the Fermi–LAT data that hasmade a separation between the large-scale interstellar emis-sion of the Galaxy and that from the inner ⇠ 1 kpc about theGC. The IC emission from annulus 1 is found to dominatethe interstellar emission from the innermost region, and rep-resents the majority of the IC brightness from this componentalong and through the line-of-sight toward the GC. The con-

Pulsars index-scaled IEM was tested by also setting them to the GALPROPpredictions and refitting for the annulus 1 interstellar emission, point sources,and residual model parameters. The normalisation and cut-off energy of theresidual model did not appreciably change, indicating that the majority of anyeffect related to the structured fore-/background from the index-scaled IEMsis likely from annulus 4.

tribution by the IC from annulus 1 to the total flux depends onthe IEM and whether the residual is fitted (Sec. 4.3). For thelatter case the IC from annulus 1 is still up-scaled comparedto the GALPROP predictions, but by a factor ⇠ 2 lower thanif fitted solely for the interstellar emission components andpoint sources. The remainder is distributed across the H I-related ⇡0-decay annulus 1 component and the template usedto fit the residual centred on the GC. For either case (residualtemplate used/not-used), the fitted fluxes attributed to the ICannulus 1 component across all IEMs are within a factor ⇠ 2– the flux and its range is the important quantity, instead ofthe individual (model-dependent) scaling factors.

The Pulsars intensity-scaled IEM with the residual tem-plate gives the minimal ‘enhanced’ flux for IC annulus 1.The average CR electron intensity & 5 GeV in the Galac-tic plane is estimated for this model within ⇠ 1 kpc of theGC as ⇠ 2.8 ± 0.1 ⇥ 10�4 cm�2 s�1 sr�1, where the uncer-tainty is statistical only. This energy range is used because itslower bound corresponds to the CR electron energies produc-ing ⇠ 1 GeV IC �-rays. This is ⇠ a factor of two higher thanthe local total CR electron density for this same energy rangefor the Pulsars baseline model. On the other hand, the OB-stars intensity-scaled IEM fitted without the residual compo-nent gives the maximal ‘enhanced’ flux for IC annulus 1. Theaverage CR electron intensity & 5 GeV in the Galactic planewithin ⇠ 1 kpc of the GC for this IEM is ⇠ 9.4 ± 0.1⇥ 10�4

cm�2 s�1 sr�1.Measurements of the interstellar emission at hard X-ray

energies to MeV �-rays by INTEGRAL/SPI (Bouchet et al.2011) show that the majority is due to IC scattering by ⇠GeVenergy CR electrons off the infrared component of the ISRF21.The GALPROP calculations, which follow the same “conven-tional” model normalisation condition to local CR measure-ments as used in this paper, made to interpret the SPI measure-ments indicate that IEMs with at least factor of 2 higher CRdensities toward the inner Galaxy are a plausible explanationfor the data. Another possible explanation is a higher intensityfor the radiation field energy density in the inner Galaxy thanused in the standard ISRF model of Porter et al. (2008); thesepossibilities are not tested here because they require detailedinvestigations that are beyond the scope of the current work.The higher CR electron densities obtained from this analy-sis are plausible given the same electrons are IC scatteringdifferent components of the ISRF to produce the interstellaremission & 1 GeV and at SPI energies.

The purpose for fitting the baseline IEMs to the datawas to obtain estimates for the interstellar emission fore-/background. However, the fit results for the individual ringsfor each IEM potentially give some information on the large-scale distribution of CRs througout the Galaxy. Tables 5 and 6in Appendix A.1 give the fit coefficients and fluxes for thescaled IEMs, while Fig.15 shows the integrated fluxes for the1–10 (top) and 10–100 GeV (bottom) energy ranges, respec-tively, over the 15�⇥ 15� region for the GALPROP-predictedand scaled version of each IEM for the Pulsars (left) and OB-stars (right) source distributions.

The fitting procedure generally increases the intensity ofeach annulus relative to the nominal model. The coeffi-cients for the intensity-scaled Pulsars and OBstars IEMs aremostly higher than the GALPROP predictions toward the in-ner Galaxy (annuli 2 � 3). Those for the OBstars IEM are

21 The majority of the IC �-rays in the energy range of this study areproduced by scattering off the optical component of the ISRF.

24 Fermi–LAT Collaboration

Figure 18. Same as in Figure 13, but with the spectrum of the NFW profilemodeled with a power-law per energy band over the 1 � 100 GeV range.The envelopes include the fit uncertainties for the normalisation and spectralindices.

through the line-of-sight to the GC.The IEM fitting interior to the solar circle uses the tangent

ranges for positive and negative longitudes to obtain parame-ters for the annuli 2 � 4 (Table 5). To examine the effect ofthe azimuthal averaging, fits to the tangent ranges were madefor positive and negative longitudes to gauge the difference inthe parameters for the IEMs obtained when considering eachseparately. The scaling factors for annulus 4 obtained whenfitting negative and positive longitude ranges were statisticallyconsistent 28 with those found when fitting both ranges com-bined. For annuli 2 and 3 the fits to the positive and nega-tive tangent longitude ranges result in scaling parameters thatdiffer by factors up to ⇠ 2 from each other, which is wellbeyond the statistical uncertainty; the average value obtainedby fitting both tangent ranges together is approximately in-between for the intensity-scaled IEMs over annuli 2 and 3.For the index-scaled IEMs the spectral parameters are harderor softer than the average when using the positive/negativetangent ranges individually for annuli 2 � 4. However, thereis no clear trend and the over/under-prediction is not confinedto a particular energy interval.

The uncertainty for the IEM fore-/background flux towardthe GC due to the azimuthally averaged IEMs is difficult toquantify precisely. A minimal estimate can be made from thestatistical uncertainty for the annulus 4 ⇡0-decay flux for eachIEM, because the fit results for the combined tangent rangesare within these uncertainties when fitted to the positive andnegative ranges individually. Above 1 GeV this is ⇠ 4⇥10�8

ph cm�2 s�1 for the 15�⇥15� region about the GC across allIEMs. This is comparable to the fitted flux from annulus 1⇡0-decay or the TS < 25 point sources over the same region.

Any analysis employing the Galactocentric annulus decom-position for the gas column densities is subject to the loss ofkinematic resolution for sight lines within l ⇠ ±12� of theGC/anti-GC. Appendix B of Ackermann et al. (2012a) detailsthe transformation of H I and CO gas-survey data into the col-umn density distributions over Galactocentric annuli used inthis analysis, and employed by many others. The assump-

28 The average statistical uncertainty for the normalisation of each inter-stellar emission component per annulus is ⇠ 10%, except for annuli 2 and 3;see Appendix A.

tions made in the transformation for the site lines over the15� ⇥ 15� region about the GC have an impact on the inter-stellar emission and point sources in the maximum-likelihoodfitting and consequently the spatial distribution of residuals.Approximations made interpolating the gas column densityacross the l ± 10� range can result in an incorrect gas densitydistribution along the line-of-sight. Spurious point sources inthe analysis and structure in residuals can result from this be-cause a higher/lower CR intensity compared to where the gasshould be placed is used in creating the interstellar emissiontemplates. The scaling procedure for the IEM then adjusts theindividual annuli potentially producing low-level artifacts dueto a combination of the effects described above.

To obtain an estimate of the uncertainties associated withmisplacement of the gas new maps of the column densityper annuli are created. 10% of the H I gas column densityis randomly displaced over the annuli and recombined withthe ⇡0-decay emissivity 29 in each annulus to create modifiedintensity maps for this process, which are summed to pro-duce new fore-/background intensity maps. The 68% frac-tional change per pixel from 100 such realisations for eachIEM is compared with the fore-/background resulting fromthe scaling procedure (Sec. 3.1). Depending on the IEM andenergy range, variations from 1% to 15% in the intensity perpixel for the fore-/background from the structured interstel-lar emission across the 15� ⇥ 15� region are obtained, withthe largest for OBstars index-scaled and smallest for the Pul-sar intensity-scaled IEM, respectively. Because of the some-what arbitrary choice of the precise fraction of H I columndensity30 that is redistributed over the annuli these variationsare illustrative rather than providing a true ‘systematic uncer-tainty’ associated with the gas misplacement. Note that theuncertainty is maximised toward the GC because it is furthestaway from the gas column density interpolation base points atl ⇠ ±12�.

6. SUMMARYThe analysis described in this paper employs specialised

IEMs that are fit to the �-ray data without reference to the15� ⇥ 15� region about the GC. Finding point-source seedsfor the same region using a method that does not rely on de-tailed IEMs, the source-seeds and IEMs are combined in amaximum-likelihood fit to determine the interstellar emissionacross the inner ⇠ 1 kpc about the GC and point sourcesover the region. The overwhelming majority of �-ray emis-sion from the 15� ⇥ 15� region is due to interstellar emissionand point sources. To summarise the results for these aspectsof the analysis:

• The interstellar emission over the 15� ⇥ 15� region is⇠ 85% of the total. For the case of fitting only ‘stan-dard’ interstellar emission processes and point sourcesthe fore-/background is ⇠ 80% with the remaining⇠ 20% mainly due to IC from the inner region. Thecontribution by the ⇡0-decay process over the inner re-gion is much less than the IC, with the relative contri-butions by the H I- and CO-related emission suppressedcompared to the GALPROP predictions.

29 The contribution by CO-related ⇡0-decay emission is the same as thatobtained from the scaling procedure.

30 Similar modifications of the CO column density distribution are notexplored because the detailed knowledge to make a truly informed estimateis not available.

Figure 4: Spectrum of the excess of gamma rays from the galactic center [57] for four different models ofthe back- and fore-grounds. The spectra in the right plot are fixed to the functional form of a power law withexponential cut-off, while those on the right are fit in energy bins.

the dark mass is between about 30 and 120 GeV [58], or even higher if the annihilation is intomediators which themselves decay into SM particles [59, 60, 61]. While tantalizing, it remainsunclear whether this excess is in fact the result of dark matter annihilation, as opposed to somemore prosaic explanation.

3.4 Complementarity of Particle Probes

A common theme among direct, indirect, and collider searches for dark matter is the factthat all three ultimately are testing the strength and nature of dark matter’s interactions with theStandard Model. Thus, in some sense they can be compared as far as their particular strengthsand weaknesses, leading to an understanding of how they complement one another [62, 63]. InFig. 5 we show the “current" reach (as of 2013) and future projections of all three types of searchestranslated into the annihilation cross section (normalized by the target cross section for a thermalrelic saturating observations). The translation is performed in the limit in which the mediatingparticles are heavy (compared to the energy of any of the experiments) such that the interactionis described by an effective field theory of the form indicated on the figure. This exercise revealsbroad trends, such as the fact that collider limits tend to be stronger on lighter dark matter particles,whereas indirect detection works best at larger masses and direct detection works best when theinteraction allows for spin-independent scattering with heavy nuclei. Similar features are observedin more complete theories such as the pMSSM [64].

3.5 Astronomical Probes

Astronomy offers a unique window to properties of the dark matter which are difficult to oth-erwise access. For example, the distribution of dark matter in galaxy cluster mergers was longago recognized to provide an upper limit on the cross section for dark matter to scatter elasticallywith itself [65]. More recently, self-interaction has been invoked to alleviate tensions (modulo sys-tematic uncertainties from astrophysics and structure formation simulations) in the observations ofgalaxies [66, 67]. The necessary cross sections are much larger than what would be expected for

7

PoS(ICRC2015)005

Dark Matter Candidates: Status and Perspectives Tim M.P. Tait15

FIG. 2: Dark matter discovery prospects in the (m�,�i/�th) plane for current and future direct detec-tion [152], indirect detection [95, 153], and particle colliders [154–156] for dark matter coupling to glu-ons [137], quarks [137, 157], and leptons [158, 159], as indicated.

is expected to be most promising, since high energy collisions readily produce light dark matterparticles with large momenta. Fig. 2 confirms that the di�cult low-mass region is e↵ectively coveredby searches at the LHC (in the case of gluon or quark couplings) or at LEP and ILC (in the case oflepton couplings). The sensitivity of both direct searches and colliders is increasingly diminished athigh masses, and this is where indirect detection probes play an important complementary role —in the case of couplings to quarks and leptons, CTA arrays are able to cover the relevant parameterregion in the mass range around 1 TeV.

B. Supersymmetry

The e↵ective theory description of the dark matter interactions with standard model particlesdiscussed above is an attempt to capture the salient features of the dark matter phenomenologywithout reference to any specific theoretical model. However, the complementarity between thedi↵erent dark matter probes illustrated in Fig. 2 is also found when one considers specific theoreticalmodels with WIMP dark matter candidates. Among the many other alternatives, low energysupersymmetry [160] and models with universal extra dimensions (UED) have been a popular andwidely studied extensions of the standard model. In the following, we will discuss the interplaybetween di↵erent dark matter detection strategies within the context of supersymmetric and UEDmodels.

1. The phenomenological MSSM (pMSSM)

Even within the general context of low-energy supersymmetry, there are many di↵erent scenariosthat can be considered. Philosophically, these scenarios tend to fall within either top-down orbottom-up approaches. In top-down models, the low-energy sparticle spectrum is derived from ahigh-energy theory, which relies on various theoretical assumptions, but typically requires relativelyfew input parameters. Alternatively, one can phenomenologically describe the properties of the low-energy sparticle spectrum with fewer theoretical assumptions, but with a greater number of inputparameters. We begin this discussion of complementarity within the context of supersymmetricdark matter by considering a phenomenological approach to the minimal supersymmetric standardmodel (MSSM), based on the results presented in [161], before turning our attention to more

Figure 5: Current exclusions (as of 2013) and future projections of the reach of different classes of exper-iments for three types of dark matter interactions, presented as a bound on the annihilation cross sectiondivided by the cross section leading to saturations of the observed relic density of dark matter. From [62].

a generic WIMP, more in line with hadron-hadron scattering rates. Already, this suggests inter-esting directions leading to such a large scattering cross section to explore, such as the possibilitythat the dark matter is a composite bound state of a hidden sector non-Abelian gauge symmetry[68, 69, 70, 71].

4. Outlook

The identity of the dark matter stands among the most pressing questions confronting particlephysics today. There is a bewildering plethora of theoretical ideas under discussion, which a widearray of experimental activity aims to confirm or rule out. In this data rich era, there is goodhope that we will soon discover clues as to the nature of dark matter. Uncovering those clues andcombining them into a cogent picture of dark matter will take collaboration between theorists andexperimentalists. If I had to guess, it will be confusing, surprising, and undoubtably a lot of fun!

Acknowledgements

I am happy to thank the organizers of ICRC 2015 for an excellent, stimulating meeting. Myresearch is supported in part by NSF grant PHY-1316792 and by a Chancellor’s Fellowship fromthe University of California, Irvine.

References

[1] G. Bertone, D. Hooper and J. Silk, Phys. Rept. 405, 279 (2005) doi:10.1016/j.physrep.2004.08.031[hep-ph/0404175].

[2] A. Kusenko and L. J. Rosenberg, arXiv:1310.8642 [hep-ph].

[3] R. D. Peccei and H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977). doi:10.1103/PhysRevLett.38.1440

[4] S. Weinberg, Phys. Rev. Lett. 40, 223 (1978). doi:10.1103/PhysRevLett.40.223

8

PoS(ICRC2015)005


[5] F. Wilczek, Phys. Rev. Lett. 40, 279 (1978). doi:10.1103/PhysRevLett.40.279

[6] G. Jungman, M. Kamionkowski and K. Griest, Phys. Rept. 267, 195 (1996)doi:10.1016/0370-1573(95)00058-5 [hep-ph/9506380].

[7] A. Arbey, M. Battaglia and F. Mahmoudi, Eur. Phys. J. C 72, 1847 (2012)doi:10.1140/epjc/s10052-011-1847-3 [arXiv:1110.3726 [hep-ph]].

[8] R. C. Cotta, J. S. Gainer, J. L. Hewett and T. G. Rizzo, New J. Phys. 11, 105026 (2009)doi:10.1088/1367-2630/11/10/105026 [arXiv:0903.4409 [hep-ph]].

[9] T. Appelquist, H. C. Cheng and B. A. Dobrescu, Phys. Rev. D 64, 035002 (2001)doi:10.1103/PhysRevD.64.035002 [hep-ph/0012100].

[10] G. Servant and T. M. P. Tait, Nucl. Phys. B 650, 391 (2003) doi:10.1016/S0550-3213(02)01012-X[hep-ph/0206071].

[11] H. C. Cheng, J. L. Feng and K. T. Matchev, Phys. Rev. Lett. 89, 211301 (2002)doi:10.1103/PhysRevLett.89.211301 [hep-ph/0207125].

[12] K. Kong and K. T. Matchev, JHEP 0601, 038 (2006) doi:10.1088/1126-6708/2006/01/038[hep-ph/0509119].

[13] F. Burnell and G. D. Kribs, Phys. Rev. D 73, 015001 (2006) doi:10.1103/PhysRevD.73.015001[hep-ph/0509118].

[14] H. C. Cheng and I. Low, JHEP 0309, 051 (2003) doi:10.1088/1126-6708/2003/09/051[hep-ph/0308199].

[15] J. Hubisz and P. Meade, Phys. Rev. D 71, 035016 (2005) doi:10.1103/PhysRevD.71.035016[hep-ph/0411264].

[16] For recent refinements and original references, see: G. Steigman, B. Dasgupta and J. F. Beacom,Phys. Rev. D 86, 023506 (2012) doi:10.1103/PhysRevD.86.023506 [arXiv:1204.3622 [hep-ph]].

[17] P. Cushman et al., arXiv:1310.8327 [hep-ex].

[18] For a review, see : Enactali Figueroa-Feliciano, these proceedings (2015).

[19] R. Essig, J. Mardon and T. Volansky, Phys. Rev. D 85, 076007 (2012)doi:10.1103/PhysRevD.85.076007 [arXiv:1108.5383 [hep-ph]].

[20] Y. Hochberg, Y. Zhao and K. M. Zurek, arXiv:1504.07237 [hep-ph].

[21] R. Essig, M. Fernandez-Serra, J. Mardon, A. Soto, T. Volansky and T. T. Yu, arXiv:1509.01598[hep-ph].

[22] J. Billard, L. Strigari and E. Figueroa-Feliciano, Phys. Rev. D 89, no. 2, 023524 (2014)doi:10.1103/PhysRevD.89.023524 [arXiv:1307.5458 [hep-ph]].

[23] P. W. Graham and S. Rajendran, Phys. Rev. D 88, 035023 (2013) doi:10.1103/PhysRevD.88.035023[arXiv:1306.6088 [hep-ph]].

[24] P. Sikivie, N. Sullivan and D. B. Tanner, Phys. Rev. Lett. 112, no. 13, 131301 (2014)doi:10.1103/PhysRevLett.112.131301 [arXiv:1310.8545 [hep-ph]].

[25] L. S. Ancu [ATLAS Collaboration], arXiv:1412.2784 [hep-ex].

[26] A. Gaz [CMS Collaboration], arXiv:1411.1886 [hep-ex].

9

PoS(ICRC2015)005


[27] G. Aad et al. [ATLAS Collaboration], JHEP 1304, 075 (2013) doi:10.1007/JHEP04(2013)075[arXiv:1210.4491 [hep-ex]].

[28] V. Khachatryan et al. [CMS Collaboration], Eur. Phys. J. C 75, no. 5, 235 (2015)doi:10.1140/epjc/s10052-015-3451-4 [arXiv:1408.3583 [hep-ex]].

[29] J. Abdallah et al., Phys. Dark Univ. 9-10, 8 (2015) doi:10.1016/j.dark.2015.08.001 [arXiv:1506.03116[hep-ph]].

[30] D. Abercrombie et al., arXiv:1507.00966 [hep-ex].

[31] M. Beltran, D. Hooper, E. W. Kolb and Z. C. Krusberg, Phys. Rev. D 80, 043509 (2009)doi:10.1103/PhysRevD.80.043509 [arXiv:0808.3384 [hep-ph]].

[32] M. Beltran, D. Hooper, E. W. Kolb, Z. A. C. Krusberg and T. M. P. Tait, “Maverick dark matter atcolliders,” JHEP 1009, 037 (2010) [arXiv:1002.4137 [hep-ph]].

[33] J. Goodman, M. Ibe, A. Rajaraman, W. Shepherd, T. M. P. Tait and H. -B. Yu, “Constraints on LightMajorana dark Matter from Colliders,” Phys. Lett. B 695, 185 (2011) [arXiv:1005.1286 [hep-ph]].

[34] Y. Bai, P. J. Fox and R. Harnik, “The Tevatron at the Frontier of Dark Matter Direct Detection,” JHEP1012, 048 (2010) [arXiv:1005.3797 [hep-ph]].

[35] J. Goodman, M. Ibe, A. Rajaraman, W. Shepherd, T. M. P. Tait and H. -B. Yu, “Constraints on DarkMatter from Colliders,” Phys. Rev. D 82, 116010 (2010) [arXiv:1008.1783 [hep-ph]].

[36] A. Rajaraman, W. Shepherd, T. M. P. Tait and A. M. Wijangco, “LHC Bounds on Interactions of DarkMatter,” Phys. Rev. D 84, 095013 (2011) [arXiv:1108.1196 [hep-ph]].

[37] P. J. Fox, R. Harnik, J. Kopp and Y. Tsai, “Missing Energy Signatures of Dark Matter at the LHC,”Phys. Rev. D 85, 056011 (2012) [arXiv:1109.4398 [hep-ph]].

[38] K. Cheung, P. Y. Tseng, Y. L. S. Tsai and T. C. Yuan, “Global Constraints on Effective Dark MatterInteractions: Relic Density, Direct Detection, Indirect Detection, and Collider,” JCAP 1205, 001(2012) [arXiv:1201.3402 [hep-ph]].

[39] For expanded discussion, see: Marco Cirelli, these proceedings (2015).

[40] M. Ackermann et al. [Fermi-LAT Collaboration], Phys. Rev. Lett. 115, no. 23, 231301 (2015)doi:10.1103/PhysRevLett.115.231301 [arXiv:1503.02641 [astro-ph.HE]].

[41] A. Abramowski et al. [HESS Collaboration], Phys. Rev. D 90, 112012 (2014)doi:10.1103/PhysRevD.90.112012 [arXiv:1410.2589 [astro-ph.HE]].

[42] T. Arlen et al. [VERITAS Collaboration], Astrophys. J. 757, 123 (2012)doi:10.1088/0004-637X/757/2/123 [arXiv:1208.0676 [astro-ph.HE]].

[43] S. Desai et al. [Super-Kamiokande Collaboration], Phys. Rev. D 70, 083523 (2004) [Phys. Rev. D 70,109901 (2004)] doi:10.1103/PhysRevD.70.083523, 10.1103/PhysRevD.70.109901 [hep-ex/0404025].

[44] M. G. Aartsen et al. [IceCube Collaboration], Phys. Rev. Lett. 110, no. 13, 131302 (2013)doi:10.1103/PhysRevLett.110.131302 [arXiv:1212.4097 [astro-ph.HE]].

[45] O. Adriani et al. [PAMELA Collaboration], Nature 458, 607 (2009) doi:10.1038/nature07942[arXiv:0810.4995 [astro-ph]].

[46] M. Aguilar et al. [AMS Collaboration], Phys. Rev. Lett. 110, 141102 (2013).doi:10.1103/PhysRevLett.110.141102

10

PoS(ICRC2015)005


[47] O. Adriani et al. [PAMELA Collaboration], Phys. Rev. Lett. 105, 121101 (2010)doi:10.1103/PhysRevLett.105.121101 [arXiv:1007.0821 [astro-ph.HE]].

[48] N. Arkani-Hamed, D. P. Finkbeiner, T. R. Slatyer and N. Weiner, Phys. Rev. D 79, 015014 (2009)doi:10.1103/PhysRevD.79.015014 [arXiv:0810.0713 [hep-ph]].

[49] D. Hooper and T. M. P. Tait, Phys. Rev. D 80, 055028 (2009) doi:10.1103/PhysRevD.80.055028[arXiv:0906.0362 [hep-ph]].

[50] D. Hooper and L. Goodenough, Phys. Lett. B 697, 412 (2011) doi:10.1016/j.physletb.2011.02.029[arXiv:1010.2752 [hep-ph]].

[51] K. N. Abazajian and M. Kaplinghat, Phys. Rev. D 86, 083511 (2012) [Phys. Rev. D 87, 129902(2013)] doi:10.1103/PhysRevD.86.083511, 10.1103/PhysRevD.87.129902 [arXiv:1207.6047[astro-ph.HE]].

[52] C. Gordon and O. Macias, Phys. Rev. D 88, no. 8, 083521 (2013) [Phys. Rev. D 89, no. 4, 049901(2014)] doi:10.1103/PhysRevD.88.083521, 10.1103/PhysRevD.89.049901 [arXiv:1306.5725[astro-ph.HE]].

[53] T. Daylan, D. P. Finkbeiner, D. Hooper, T. Linden, S. K. N. Portillo, N. L. Rodd and T. R. Slatyer,arXiv:1402.6703 [astro-ph.HE].

[54] F. Calore, I. Cholis and C. Weniger, JCAP 1503, 038 (2015) doi:10.1088/1475-7516/2015/03/038[arXiv:1409.0042 [astro-ph.CO]].

[55] For more detail, see: Simona Murgia, these proceedings (2015).

[56] For more detail, see: Troy Porter, these proceedings (2015).

[57] M. Ajello et al. [Fermi-LAT Collaboration], arXiv:1511.02938 [astro-ph.HE].

[58] P. Agrawal, B. Batell, P. J. Fox and R. Harnik, JCAP 1505, 011 (2015)doi:10.1088/1475-7516/2015/05/011 [arXiv:1411.2592 [hep-ph]].

[59] C. Boehm, M. J. Dolan, C. McCabe, M. Spannowsky and C. J. Wallace, JCAP 1405, 009 (2014)doi:10.1088/1475-7516/2014/05/009 [arXiv:1401.6458 [hep-ph]].

[60] M. Abdullah, A. DiFranzo, A. Rajaraman, T. M. P. Tait, P. Tanedo and A. M. Wijangco, Phys. Rev. D90, 035004 (2014) doi:10.1103/PhysRevD.90.035004 [arXiv:1404.6528 [hep-ph]].

[61] A. Martin, J. Shelton and J. Unwin, Phys. Rev. D 90, no. 10, 103513 (2014)doi:10.1103/PhysRevD.90.103513 [arXiv:1405.0272 [hep-ph]].

[62] D. Bauer et al. [Snowmass 2013 Cosmic Frontier Working Groups 1Ð4 Collaboration], Phys. DarkUniv. 7-8, 16 doi:10.1016/j.dark.2015.04.001 [arXiv:1305.1605 [hep-ph]].

[63] S. Arrenberg et al., arXiv:1310.8621 [hep-ph].

[64] M. Cahill-Rowley, R. Cotta, A. Drlica-Wagner, S. Funk, J. Hewett, A. Ismail, T. Rizzo and M. Wood,Phys. Rev. D 91, no. 5, 055011 (2015) doi:10.1103/PhysRevD.91.055011 [arXiv:1405.6716[hep-ph]].

[65] D. N. Spergel and P. J. Steinhardt, Phys. Rev. Lett. 84, 3760 (2000) doi:10.1103/PhysRevLett.84.3760[astro-ph/9909386].

[66] M. Boylan-Kolchin, J. S. Bullock and M. Kaplinghat, Mon. Not. Roy. Astron. Soc. 415, L40 (2011)[arXiv:1103.0007 [astro-ph.CO]].

[67] For recent discussion, see: M. Kaplinghat, S. Tulin and H. B. Yu, arXiv:1508.03339 [astro-ph.CO].

11

PoS(ICRC2015)005


[68] J. M. Cline, Z. Liu, G. Moore and W. Xue, Phys. Rev. D 90, no. 1, 015023 (2014)doi:10.1103/PhysRevD.90.015023 [arXiv:1312.3325 [hep-ph]].

[69] K. K. Boddy, J. L. Feng, M. Kaplinghat and T. M. P. Tait, Phys. Rev. D 89, no. 11, 115017 (2014)doi:10.1103/PhysRevD.89.115017 [arXiv:1402.3629 [hep-ph]].

[70] K. K. Boddy, J. L. Feng, M. Kaplinghat, Y. Shadmi and T. M. P. Tait, Phys. Rev. D 90, no. 9, 095016(2014) doi:10.1103/PhysRevD.90.095016 [arXiv:1408.6532 [hep-ph]].

[71] Y. Hochberg, E. Kuflik, H. Murayama, T. Volansky and J. G. Wacker, Phys. Rev. Lett. 115, no. 2,021301 (2015) doi:10.1103/PhysRevLett.115.021301 [arXiv:1411.3727 [hep-ph]].

12

dark matter candidates: status and perspectives

Documents