Dark Matter Particles? WIMPs?
Graciela Gelmini - UCLA
DE-SC0009937

Cosmic Controversies, Chicago Oct. 5-8, 2019

Graciela Gelmini-UCLA• Dark Matter Particles or Not
Has two parts:-Dark Matter or not? So far there is no alternative to dark matter...
-Particles or not? Could be PBH, ... (Juan Garcia Bellido)• WIMPs: dead?, alive? in good shape?
The meaning of WIMP was not well deﬁned since its inception, but not evenin the most restrictive sense of the word WIMPs are dead... Words evolve- let
us not be held up by the word. Many models rejected, many others not.
There are mature background free techniques (based on Xe and Ar) to getto the multi-tonne scale in Direct Detection- we should continue with themwithout giving a bad rap to WIMP searches.... But also diversify, there aremany beautiful ideas for lighter DM... (Elena Aprile, Rouven Essig)

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Dark Matter Particles or Not

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What we know about DM:• 1- Attractive gravitational interactions and lifetime >> 𝑡𝑈• 2- DM is non-baryonic and is not observed to interact with lighti.e. it is either neutral or with a very small electromagnetic coupling such as:“Milli-Charged DM” which can be part of “Atomic DM”, with dark protons
and dark electrons forming dark atoms or “Mirror DM” whose Lagrangian isa copy of that of the SM, but for the mirror particles,or “electric or magnetic dipole DM”, or“anapole DM”

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What we know about DM:• 1- Attractive gravitational interactions and lifetime >> 𝑡𝑈• 2- DM is non-baryonic and is not observed to interact with light• 3- So far DM and not modiﬁed dynamics + only visible matter

After 2013, no proposed “alternative to Dark Matter” explained
the CMB anisotropy spectrum and the BAO
From any theory that claims to “replace Dark Matter” we should
ask for its prediction of the matter power spectrum and the CMB
angular power spectrum (stop discussing endlessly only about galaxy
rotation curves!) (Constantinos Skorkis)
Eloquently expressed in Dan Hooper’s talk “In Defense of Dark Matter” KITP 4/30/2018- in debate with Eric Verlinde (“emergent gravity” is a theorywithout cosmology)
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What we know about DM:• 1- Attractive gravitational interactions and lifetime >> 𝑡𝑈• 2- DM is non-baryonic and is not observed to interact with light• 3- So far DM and not modiﬁed dynamics + only visible matter• 4- The bulk of the DM must be nearly dissipationless but ≤10%of it could be dissipative.
• 5- DM has been mostly assumed to be collisionless, but huge selfinteraction upper limit 𝝈𝐬𝐞𝐥𝐟 /𝐦 ≤ 2 barn/GeV(by comparison e.g. 235U-neutron capture cross section is a few barns!)
Self Interacting DM (SIDM) just below limit (WIMPs: 𝜎𝐷𝑀−𝑝 <10−46cm2/10GeV)

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Graciela Gelmini-UCLA• 6- The mass of the major component of the DM has only been
constrained within some 90 orders of magnitude.
10−31GeV≤M ≤ 10−10M⊙ = 1047GeV= 2 1020kg (window ≃10M⊙ =1058GeV??)
Lower limit:“Fuzzy DM”, boson with de Broglie wavelength 1 kpc Hu, Barkana, Gruzinov, 2000
Upper limit on MACHOS (Massive Astrophysical Compact Halo ObjectS): Moniez
0901.0985, Yoo, Chaname, Gould, ApJ601, 311, 2004; Griest, Cieplak and Lehner 1307.5798, Niikura et al. 1701.02151

Problem with MACHOS: how would they form? Could be Primordial Black Holes

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Dark Matter: could be Primordial Black Holes (PBH)?
PBH are a hypothetical type of black hole not formed by the gravitational collapseof a large star but in an early phase transition, before BBN (thus non-baryonic)
Zel’dovich and Novikov, 1966; Hawking, 1971; Carr and Hawking, 1974

Many limits exclusively applying to BH:- 𝑀 > 1015g = 6 × 1038 GeV, lighter would have evaporated by now- 𝑀 > 1017g or evaporating BH would have been observed (by EGRET and Fermi)- 5 1017g< 𝑀 < 1020g excluded by non-observation of “femtolensing” of GRB 1204.2056
Revised: wave eﬀects (wavelength larger than Schwartzschild radius) and ﬁnite source size eﬀects- 1016g< 𝑀 < 1022g excluded- its accretion in stars would destroy compact remanent 1209.6021- 3 1018g<𝑀< 5 1024g excluded- its accretion in n stars in GC would destroy them (NS limit)
1301.4984 Revised: required a too high density in GC1807.11495- 𝑀 > 100 M⊙= 2 1035g excluded by absence of CMB spectral distortions 0709.0524

Many limits revised after LIGO’s BH-BH mergers events! Bird etal. 1603.00464, Clesse&GarciaBellido 1501.07565, 1603.05234

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Could Dark Matter be
Primordial Black Holes(PBH)?
compilation of bounds on PBH DM
density fraction 𝑓 for single mass
PHB (dashed limits can be avoided
with special assumptions)
Carr, Tenkanen and Vaskonen 1706.03746 + modiﬁed(M⊙ = 1057GeV)
Could LIGO BH ∼10’s M⊙ be most of the DM?
Bird etal. 1603.00464, Clesse&Garcia-Bellido 1603.05234, 1501.07565; and before the LIGO events Frampton 0905.3632, Frampton,Kawasaki, Takahashi &Yanagida 1001.2308

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Could Dark Matter be
Primordial Black Holes(PBH)?
compilation of bounds on PBH DM
density fraction 𝑓 for single mass(M⊙ = 1057GeV)New Subaru/HSC camera limit Niikura et al. Nat. Astron. 3, no. 6, 524, 2019 (1701.02151)
They also have a candidate PBH event consistent of a PBH of mass 10−7 M⊙.

Cosmic Controversies, Chicago Oct. 5-8, 20199

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What we know about DM:• 1- Attractive gravitational interactions and lifetime >> 𝑡𝑈• 2- DM is non-baryonic and is not observed to interact with light• 3- So far DM and not modiﬁed dynamics + only visible matter• 4- The bulk of the DM must be nearly dissipationless• 5- DM has been mostly assumed to be collisionless, but huge selfinteraction upper limit 𝝈𝐬𝐞𝐥𝐟 /𝐦 ≤ barn/GeV• 6- Mass within 90 orders of magnitude.
• 7- The bulk of the DM is Cold or Warm (NR or quasi when 𝑇 ∼ keV)
• 8- Particle DM requires Beyond Standard Model physicsIn the SM neutrinos are part of the DM, but Hot DM

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WIMPs: dead?, alive?
in good shape?

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Graciela Gelmini-UCLA

What are WIMPs? When in doubt...consult wikipidia

Agree with 1st two sentences- but WIMP not necessarily produced thermally...
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What are WIMPs? WIMP name invented in 1985 by Steigman and TurnerJune 1985 M. Turner’s lectures:Cosmic Controversies, Chicago Oct. 5-8, 201913

Graciela Gelmini-UCLA

What are WIMPs? WIMP name invented in 1985 by Steigman and Turner
June 1985 M. Turner’s lectures:

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What are WIMPs? WIMP changed meaning very fast

However WIMP not restricted to 1 to 100 GeV in mass
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What are WIMPs?

WIMPs have allowed themselves to be expelled from whole regions of parameter
space! WIMPs now...

Words evolve, but reducing WIMPs to the “WIMP miracle” WIMPs
is too much!! WIMPs could always be produced in other ways
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What are WIMPs? June 1988 M. Turner’s talk:Cosmic Controversies, Chicago Oct. 5-8, 201917

Graciela Gelmini-UCLA

The original GeV thermal-WIMP 2GeV for Ω𝐷𝑀 =1, now 4GeV for 0.25

Cosmic Controversies, Chicago Oct. 5-8, 201918

Graciela Gelmini-UCLA“Thermal WIMPs”
Standard calculations: start at 𝑇 > 𝑇𝑓.𝑜. ≃ 𝑚𝜒 /20 and assume that- WIMPs reach equilibrium while
Universe is radiation dominated- No particle asymmetry- Chemical decoupling (freeze-out) whenΓ𝑎𝑛𝑛 = ⟨𝜎𝑣⟩ 𝑛 ≤ 𝐻 ,- No entropy change in matter+radiation−263×10𝑐𝑚3/𝑠
2
Ω𝑠𝑡𝑑 ℎ ≈ 0.2⟨𝜎𝑣⟩
Weak annihilation cross section
𝜎𝑎𝑛𝑛𝑖ℎ𝑣 ≃ 𝐺𝐹2 𝑚2 ≃ 3 × 10−26𝑐𝑚3/𝑠
𝑚 ≃ GeV is enough to getΩ = Ω𝐷𝑀 ≃ 0.2! “WIMP Miracle”

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The rumors of WIMPs death are greatly exaggerated!
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WIMPs dead? Not to be held up by the word...
• Are WIMPs coupled to the W/Z bosons dead? Mostly- not entirely• Are “WIMP miracle” WIMPs dead? NO, but constrained...
(Notice that here I use “WIMP miracle” WIMPs to indicate the usual set of assumptions
made when deﬁning them. I do not call them “thermal WIMPs” because WIMPs produced
thermally which decouple before the temperature of the Universe was 5 MeV, during anepoch yet untested, could have a diﬀerent cross section to be the dark matter, if the standard
assumptions we make about cosmology are not right)• Why consider only “WIMP miracle” WIMPs? NO REASON......
• Are SUSY WIMP models dead? NO, many rejected many not...

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Upper limit on “WIMP miracle” from Fermi ST 95% CL upperlimits, 6 y of Fermi Large Area Telescope (LAT)- 15 stacked dwarfs 1611.03184

Shown are model for the GC excess. They are in tension with the upper limit.
“WIMP miracle” annihilation at decoupling cannot be smaller than the benchmark annihilation
cross section (< 𝜎𝑎𝑛𝑛𝑖ℎ𝑣 >≃ 2 × 10−26 cm3/s).

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Indirect limits on “WIMP miracle” cross sections
1- FermiLAT observations of stacked dwarf galaxies
Constrain annihilation at present of DM particles bound to galactic haloes

2- Positron spectrum measured by AMS-02 (more precise that earlier
measurements) Constrain annihilation at present of DM particles close to Earth
3- CMB anisotropy precision measurements (Planck and others)
Constrain DM annihilations (or decays) at recombination or after

They all imply < 𝜎𝑣 >< 3 × 10−26 cm3/s for WIMP 𝑚 < 𝑂(10) GeV (exactlimit depends on annih. mode). This rejects thermal WIMPs 𝑚 < 𝑂(10) GeV ifannihilation is in s-wave:< 𝜎𝑣 > is 𝑣 independent.
Not constraining for p-wave annihilation: < 𝜎𝑣 >∼ 𝑣2, because at freeze-out𝑣 ≃ 𝑐/3 and 𝑣 is much smaller at recombination and within galaxies

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Indirect limits on s-wave “WIMP miracle” cross sections“GeV-Scale Thermal WIMPs: Not Even Slightly Dead” Leane, Slatyer, Beacom and Ng,1805.10305
s-wave 2→2 annihilation, standard cosmological history, conservative limits

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Caveats to Thermal WIMPs as Dark Matter• Asymmetric DM We owe our very existence to a particle-antiparticle asymmetry so why
not also the DM? (Requires non-self conjugated DM candidates- neutralinos are Majorana
particles instead) (Nussinov 85; Gelmini, Hall, Lin 87; Kaplan 92; Barr, Chivukula, Fahri 90; Enkvist, MacDonald 98;
Gudnason, �Kouvaris , �Sannino 05; Kaplan, Luty, Zurek 09; Cohen et al 10; Frandsen, Sarkar, Sannino 10; Cheung, Zurek 11;
Del Nobile, Kouvaris, Sannino 11....among others)• Non-Standard Pre-Big bang Nucleosynthesis (pre-BBN) cosmology WIMP relic abundance isﬁxed before BBN, a moment in the Universe from which we have so far no data. (See e.g. Gelmini
et al hep-ph/0605016, or Gelmini, Gondolo 1009.3690 and refs. therein)

𝑇𝑓.𝑜. ≃ (𝑚/20) > 5 MeV for 𝑚 > 100 MeV!
Salas et al “Bounds on very low reheating scenarios after Planck” 1511.0067• WIMPs may be unstable and decay into the dark matter (Super-WIMP scenario).

(Feng,Rayaraman, Takayama 03; Feng, Smith 04�)

• WIMPs can be produced in decays of other particles (Sigurdson, Kamionkowski 04; Kaplinghat 05�)
If DM is Warm, cold WIMP DM could be a subdominant DM component orWIMPs could be WDM but in this case particle models must be very diﬀerent from usual- e.g.
WIMPs are created late in decays and never reach kinetic equilibrium with the thermal bathCosmic Controversies, Chicago Oct. 5-8, 201925

Graciela Gelmini-UCLA

Neutralino models to be found in DD! Roszkowski, Sessolo and Trojanowski, Rept.
Prog. Phys. 81, no. 6, 066201 (2018)

SUSY ”light vanilla” models rejected by LHC- but those remaining could be seenin DD. Green region allowed (notice tend to be heavy WIMPs)
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Proceed phenomenologically- consider all types of WIMP couplings
Context: The scope of DM particle models has changed:- 1980’s: DM candidates were an afterthought. Models proposed exclusively
to solve problems in Standard Model, such as SUSY, Technicolor, Peccei-Quinn
symmetry, neutrino masses - which also contain DM candidates (WIMPs, axions,sterile neutrinos)- 1990’s: DM candidates were mandatory. Models required to have a DM
candidate in SM extensions.
- Since 2000’s: DM model independent of the SM. Models made to ﬁt DM hints
and/or predict novel DM signals and experiments to detect them, without regard
for completion of the SM (but may have implications for colliders e.g. search forlight mediators, displaced vertices...)Lead to all types of DM interactions, to “dark sectors” seen through “portals”
i.e. very small couplings (with photons, with neutrinos, with the Higgs....)
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An example - the relevance of the neutrino ﬂoor:
Future of Direct Detection and its neutrino ﬂoor- from Snowmass 2013This is for Spin-Independent interactions Gelmini, Takhistov and Witte, 1804.01638

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Future of non-directional Direct Detection
Consider all types of WIMP interactions
Diﬀerential rates can be very diﬀerent than for SI
Fig. from Gluscevic, Gresham, McDermott, Peter and Zurek 1506.04454

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Future of Direct Detection The relevance of the neutrino ﬂoor
depends on the WIMP interaction

The Xe discovery limit of heavy 𝑞 𝑥 (𝑞 is the momentum exchanged) interacting
DM is not aﬀected by the neutrino ﬂoor (for exposures ≤ 100 tonne y) but require
experiments to extend the energy range and change their data analysis!
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Graciela Gelmini-UCLA+ Diversify into unexplored domains light DM, dark photons andother light mediators, boosted DM. “Dark Sector Workshop”1608.08632; “U.S. Cosmic
Visions: New Ideas in DM” 3/2017- KITP 2018 workshop “HEP at the Sensitivity Frontier” e.g.
for sub-GeV “Light Dark Matter” direct detection (Rouven Essig)

Scattering oﬀ e [e−] or inelastic scatt. on nuclei [N] (𝛾 emission in nuclear recoil, breaking chemicalbonds in molecules/crystals, multi-phonon processes in superﬂuid He or insulator crystals)
Axions: besides ADMX G2- with cavities at KAIST, Korea; New ideas: ”dielectric haloscope”
in MADMAX (𝑚 =40 to 300𝜇eV), CASPER and ABRACADABRA (< .𝜇 eV)...
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Continue strong diversiﬁed program of DM searches
Consider each proposed experiments on its merits

Direct detection with several target materials and strategies (continue maturebackground free multitonne experiments with Xe and Ar, axion cavity searches, and explore andexploit new ideas for lighter particles, such as SENSEI, FASER, CASPER, ABRACADABRA...)

Indirect detection with multi-messenger approach (although there areexceptions, due to astrophysical uncertainties may constitute a conﬁrmation but not a discoveryof DM) (Tracy Slatyer)

Collider searches LHC, DUNE, ﬁxed target...

(but if DM candidate isdiscovered, will need to conﬁrm it is the DM)

for WIMPs, they are as interesting as ever..., sub GeV DM, axions/ALPs/ WISPs,
sterile neutrinos,...
for all types of interactions, production models, mass ranges...
Very vibrant ﬁeld, will continue to be so for many years to comeCosmic Controversies, Chicago Oct. 5-8, 201932