Elastically Decoupling Relic (ELDER) Dark Matter Maxim Perelstein, Cornell U.S. Cosmic Visions: New Ideas in Dark Matter March 24 2017 Kuflik, MP , Rey-Le Lorier, Tsai, 1512.04545 (PRL) + work in progress
Thermal Relic DM � • Thermal Relic: DM in thermal and chemical equilibrium with SM plasma at high temperatures (=early times) • Predictive: DM-SM Scattering cross section decoupling time present density • “Non-Relativistic” Decoupling: due to exponential drop in equilibrium density of DM particle once • Relic density: • WIMP Miracle: when ( ) 1
“Light” Thermal Relic DS SM � • No definite discovery of weak-scale new physics so far motivates thinking about DM at different mass scales • What if the DM particle mass is at ~QCD scale? • Confining dynamics at ~QCD scale in the dark sector appears naturally in “mirror SM”/“twin-Higgs” models • “Dark pions” can be a natural DM candidate, if stable • Can adjust mediator mass and couplings to obtain the correct relic density via annihilation to SM, but no “miracle”! 2
The SIMP Miracle • A big “WIMP assumption”: DM annihilation to SM is the only relevant process • Obviously, only DM-number changing processes are relevant* • What about non-DM-number-conserving self-interactions? (NB: in QCD pion number not conserved, e.g. WZW term) • Strongly Interacting Massive Particle: process remains in equilibrium after decouples • Relic density determined by • SIMP Miracle: when [Hochberg, Kuflik, Volansky, Wacker, ’14] • “SIMP Assumption”: Elastic SM-DM scattering maintains the two sectors at the same temperature until freeze-out 3
Riding Down the Hill � annihilations to SM self-annihilations • Equilibrium NR number decouple here decouple here density: • SIMP follows the trajectory due to 3-to-2 self-annihilations • This process releases kinetic energy: � • Elastic SM-DM scattering must be fast enough to transfer this energy to the SM plasma, allow them to remain at same T � • “Elastic Decoupling”: 4
Beware: Cannibals! • Self-annihilations decoupling: @ • SIMP scenario: freeze-out before kinetic decoupling • Our work: what if ? • At , DM gas is in chemical equilibrium with no chemical potential (due to active self-annihilations), BUT • DM temperature determined by DM entropy conservation: � • “Cannibal” phase: Kinetic energy released in self-annihilations is used to “keep warm” in an expanding Universe [Carlson, Machacek, Hall, ’92] • DM density changes as log(scale factor) during this phase! 5
Thermal History 10 - 3 6 Chemical Equilibrium 5 10 - 4 4 10 - 5 3 Thermalized Thermalized 2 10 - 6 100 200 500 1000 2000 with SM with SM 10 - 7 Cannibalization Cannibalization Freeze - out Freeze - out Decoupling Decoupling numerical solution 10 - 8 of Boltzmann eqs. Frozen Frozen 10 - 9 10 - 10 Inset Inset 10 - 11 10 1 10 2 10 3 1 Equilibrium at • Eventually, self-annihilations decouple, DM density frozen-in 6
Meet the ELDER Non-perturbative 10 1 self-interactions ELDER ELDER 1 SIMP SIMP 10 - 1 WIMP WIMP Observational constraints 10 - 2 10 - 8 10 - 7 10 - 6 10 - 5 Exponential sensitivity to elastic cross section! • Relic density: ELastically DEcoupling Very weak sensitivity Relic (ELDER) to self-annihilation cross section 7
Observational Constraints CMB spectrum distortions from [similar bound from indirect detection] 10 - 7 WMAP WMAP SIMP SIMP Entropy ejected into photons/electrons after neutrinos decouple Planck Planck ELDER ELDER N eff N eff Must trap in the core 10 - 8 Supernova Supernova 10 - 2 10 - 1 • DM coupling to photons only assumed here • Similar constraints if DM coupling is primarily to electrons; weaker constraints if coupled to neutrinos (only 3 choices!) 8
Explicit Model [a la Choi, Lee, 1601.0356] • Consider a simple renormalizable model: � • Global U(1) ensures stability of the DM particle , but allows 3-to-2 self-annihilations: χ ∗ χ χ S � χ ∗ χ ∗ χ S S χ χ ∗ � χ χ • DM can be coupled to electrons via dark photon exchange: � • Resonant enhancement of self-annihilation for 9
Relic Density 10 Non-perturbative self-interactions ELDER ELDER SIMP SIMP Observational constraints 1 10 - 4 10 - 3 • Viable ELDER DM for - nice target for dark photon searches • ELDER target is the lower boundary of the SIMP range: 10
Elastic Self-Interaction � • Strong DM self-annihilation would generically be accompanied by strong DM elastic self-scattering • Small-scale simulation “issues” possibly hint at � • Constraint (Bullet cluster, halo shapes): • Constraint is stronger at low DM masses, becomes difficult to satisfy for in our model • Similar lower bound on from CMB ( bound), BBN 11
ELDER in Dark Photon Searches 2 − 10 K → π ν ν ε (g-2) ± 5 σ µ B A B AR 2017 favored 3 − 10 SIMP (g-2) NA64 e ELDER 4 − 10 − 3 2 1 − − 10 10 10 1 10 m (GeV) A' [BaBar, 1702.03327] • Since , the Dark Photon decays invisibly to DM pairs • A factor of 10 improvement in sensitivity would explore preferred SIMP/ELDER parameter space 12
ELDER in Direct Detection 10 - 38 CsI NaI Supercond. GaAs 10 - 39 Graphene Ge Si Superconduc. 10 - 40 SIMP � e [ cm 2 ] future sensitivities [from Dark Sectors 2016 report] 10 - 41 ELDER 10 - 42 10 - 43 10 20 30 40 50 m � [ MeV ] • Relic density constraint completely fixes direct detection cross section as a fn. of mass! Interesting range for future experiments. • Again, the ELDER curve is the lower boundary of the SIMP region 13
Conclusions • Considered a thermal relic with ~QCD-scale mass, number-changing self-annihilation process • Two regimes: SIMP and ELDER (with unusual thermal history involving “cannibalization” epoch) • ELDER relic abundance determined dominantly by the cross section of elastic scattering of DM on SM (not a number-changing process!) • Interesting predictions for DM direct detection and dark photon searches 14
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