Light Thermal Dark Matter & Hidden Sectors Philip Schuster - PowerPoint PPT Presentation

Light Thermal Dark Matter & Hidden Sectors Philip Schuster (SLAC) FNAL Dark Matter Workshop June 4, 2019

Outline This is intended to be a non-technical introduction to sub-GeV thermal dark matter & hidden sectors • The WIMP paradigm & thermal dark matter • Beyond WIMPs • Thermal sub-GeV hidden sector dark matter • Comments on thermal freeze-out/in parameter space w.r.t. direct detection experiments See 2017 Cosmic Visions (1707.04591) report for many good references 2

Dark Matter Halo (Dark Matter) Disk Bulge CMB Power Spectrum Lensing Rotation curves We know there is new physics in the form of dark matter! But what is it? 3

A Strong Candidate: WIMP DM Simple, familiar particle content g W g SM weak force new matter DM with thermal freeze-out origin Simple, predictive cosmology Motivated mass range MeV GeV TeV WIMP

A Thermal Origin A big lesson of 20th century cosmology — The Universe evolved from an era of hot thermodynamic equilibrium in an expanding space-time!

A Thermal Origin Dark Matter interaction with familiar matter would (very) likely bring DM into thermodynamic equilibrium

A Thermal Origin Simple and predictive Boltzmann equation governs evolution of number density “n” (equilibrium number density) Dilution from Particle interactions expanding Universe provide thermal contact

A Thermal Origin Simple and predictive Boltzmann equation governs evolution of number density “n” As Universe cools below DM mass, density decreases as e -m/T Eventually dark matter particles can’t find each other to annihilate freeze-out occurs

A Thermal Origin Near freeze-out: A DM abundance (determined by interaction!) is left over to the present day

WIMPs and a Thermal Origin Larger cross-section ⇒ later freeze-out ⇒ lower density Correct DM density for: ___ q DM Z _ q DM Thermal origin suggests Dark Sector interactions and mass in the vicinity of the weak-scale 10

Compelled to Move Beyond WIMPs Basic weak-scale DM scenarios have been significantly constrained by the LHC, direct & indirect detection Existing experimental program will corner remaining WIMP models over the next few years What are we missing? 11

Logical Next Step Beyond WIMPs? Simple, familiar particle content g W g SM weak force new matter DM with thermal freeze-out origin Simple, predictive cosmology Motivated mass range MeV GeV TeV WIMP What attractive features can remain?

Lessons From Data Simple, familiar particle content g W g SM weak force new matter The ingredient most at odds with data underlying WIMPs is that interactions are mediated by W/Z bosons.

Lessons From Data Simple, familiar particle content g D g SM new force? new matter The ingredient most at odds with data underlying WIMPs is that interactions are mediated by W/Z bosons. Dark matter could be charged under a new force! (in keeping with the history of particle physics)

New Forces Interacting with The Standard Model Simple, familiar particle content g D g SM new force new matter Standard Model symmetries allow two types of (dim. 4) interactions with new force carriers at low-energy 1 2 � Y F Y µ ν F 0 µ ν Vector Mixing � h | h | 2 | ⇥ | 2 Higgs Mixing + a few other closely related possibilities…(see 1707.04591)

New Forces Interacting with The Standard Model Simple, familiar particle content g D g SM new force new matter Standard Model symmetries allow two interactions with new force carriers at Most compatible with low-energy cosmology & simple dark 1 2 � Y F Y µ ν F 0 µ ν matter models, and Vector Mixing illustrates much of the essential physics � h | h | 2 | ⇥ | 2 Higgs Mixing will be focus of many talks Increasingly constrained by LHC (though other scalar couplings less constrained)

New Forces Interacting with The Standard Model Simple, familiar particle content g D g SM new force new matter Standard Model symmetries allow two interactions with new force carriers at low-energy γ A 0 2 � Y F Y 1 µ ν F 0 µ ν Vector Mixing γ A 0 X g SM ∼ (10 − 6 − 10 − 2 ) e Mediator particle with naturally small (loop-level) Standard Model couplings...would have missed such physics without dedicated search!

Hidden Sector Dark Matter Simple, familiar particle content g D g SM new force new matter Dark Matter charged under a new force Provides a familiar and simple explanation for dark matter stability (i.e. lightest charged particle is stable!) Mediator mixing gives interaction with Standard Model

(Thermal) Hidden Sector DM Simple, familiar particle content g D g SM new force new matter Simple, predictive cosmology ?? Mass range ??

What About Thermal Abundance? < σ v > = y/ ( m DM ) 2 ∼ 1 / (20TeV) 2 m DM ∼ √ y × 20 TeV << TeV Very weakly coupled thermal dark matter should have a mass below the TeV-scale to obtain measured relic density (Direct) Thermal freeze-out works just fine down to ~MeV!

Ultra-Small Coupling & New Possibilities If coupling is large enough for DM to thermalize, then detailed balance results χ e A 0 / γ γ χ e

Ultra-Small Coupling & New Possibilities If coupling is large enough for DM to thermalize, then detailed balance results χ e A 0 / γ γ χ e But if coupling is too small for thermalization to occur, then DM is still produced through occasional SM reactions

Ultra-Small Coupling & New Possibilities “Freeze-in” through a vector mediator Y DM ( χχ → ee ) ( Works well above ~MeV Freeze-Out χ e A 0 / γ γ χ e Freeze-In Can work below ~MeV ) ( ee → χχ ) ◆ 1/ T T ⇠ m e em q 2 T , n χ ⇠ n e ( Γ /H ) , ⇢ DM ⇠ T eq T 3 Γ ( ee ! �� ) ⇠ ↵ 2 ⇥ ✓ m e T eq ◆ 1 / 2 ◆ 1 / 2 ✓ MeV 1 ⇠ 10 − 11 = ) q ⇠ m χ m pl m χ ↵ em

(Thermal) Hidden Sector DM Simple, familiar particle content g D g SM new force new matter DM with thermal freeze-out/in origin Simple, predictive cosmology Mass range ??

the Vicinity of the Weak Scale SM Matter Dark Matter? TeV For decades: look here! M W Generic mass scale for matter with O(1) coupling to origin of EWSB M proton ∼ M large e − # GeV ...but where do we expect (accidentally close to weak scale) hidden sector matter – with only small couplings to SM matter (generated radiatively)? m e ∼ small # × M W MeV 25 (derived from weak scale)

the Vicinity of the Weak Scale SM Matter Dark Matter? TeV Generic mass scale for matter with O(1) coupling M W to origin of EWSB Where do we expect hidden- sector matter? M proton ∼ M large e − # GeV ∼ M W × e − # (accidentally close to weak scale) (e.g. “hidden valley” scenario: ~conformal to weak scale, then confining) (e.g. dark sector scalar mixing with SM higgs) m e ∼ small # × M W small # × M W MeV 26 (derived from weak scale)

the Vicinity of the Weak Scale SM Matter Dark Matter? TeV Moving beyond WIMPs, the broad vicinity of Generic mass scale for matter with O(1) coupling the weak scale is still an excellent place to M W to origin of EWSB focus on: Expect hidden sector matter • An important scale! in the vicinity of – but naturally below – weak scale M proton ∼ M large e − # GeV ∼ M W × e − # • Below the weak scale is natural for very (accidentally close to weak scale) weakly coupled new physics • Thermal DM works well here! m e ∼ small # × M W small # × M W MeV 10 27 (derived from weak scale)

A Strong Candidate: (Thermal) Hidden Sector DM Simple, familiar particle content g D g SM new force new matter DM with thermal freeze-out/in origin Simple, predictive cosmology Motivated (broader) mass range MeV GeV TeV Thermal DM WIMP Dark/Hidden sector

Predictions Early universe thermal freeze-out cross-section bounded by DM abundance ¯ e + χ A ʹ χ e − ⇤ v ∼ � D ⇥ 2 � × m 2 χ x (velocity factors) m 4 A 0 But we can’t do this without precisely Want to use annihilation cross-section to infer coupling choosing a dark matter current! This strength, as a function of mass will fix the velocity (and spin) factors We need to consider the spin & interaction structure (i.e. the form of the dark matter current) for thermal dark matter framework to become quantitatively predictive

Predictive Models For a given choice of spin & parity, form of the current is determined by Lorentz invariance. Structure of mass terms also important. Different Low-Energy Phenomenology! Particle Type Dark Matter Current 30

Predictive Models For a given choice of spin & parity, form of the current is determined by Lorentz invariance. Structure of mass terms also important. Different Low-Energy Phenomenology! Particle Type Dark Matter Current Just like neutralino WIMP candidates Just like sneutrino or Dirac neutrino WIMP candidate Obvious similarity to WIMP phenomenology! 31

Coupling “Predictions” Thermal Relic Targets 10 - 4. 10 - 6. y = e 2 a D H m c ê m A' L 4 10 - 8. Elastic & Inelastic Scalar Relic Targets 10 - 10. Pseudo - Dirac Fermion Relic Target t e g r a g T n c i i l l p e 10 - 12. R u o a n c a r r o e j a k M a e W ⟷ M 10 - 14. D r e t h g i L 10 - 16. 10 2 10 3 1 10 m c @ MeV D 32