Beyond the WIMP Alternative dark matter candidates. Riccardo Catena. Chalmers University

Transcription

1 Beyond the WIMP Alternative dark matter candidates Riccardo Catena Chalmers University April 3, 2017

2 Outline WIMPs and the thermal production mechanism Generalized WIMPs Other candidates / production mechanisms - freeze-in - misalignment mechanism - asymmetric dark matter Summary and open questions

3 Standard WIMPs: basic properties Weak interactions Non-relativistic at chemical decoupling Present cosmological density via freeze-out mechanism O(1) GeV m χ O(100) TeV - lower bound: Ω WIMP 1 Lee and Weinberg, upper bound: Ω WIMP 1 and partial wave unitarity Griest and Kamionkowski, 1989

4 Freeze-out mechanism Boltzmann equation for WIMP number density: dn χ dt +3Hnχ = σannv ( n 2 χ n eq χ 2 ) RHS accounts for number changing reactions: χ + χ SM + SM m n(x)/n eq (x=1) [GeV] GeV 10 2 GeV 1 GeV σv weak σv em σv strong Equilibrium x=m/t Steigman, Dasgupta and Beacom,

5 Neutralino MSSM field content: spin 0 spin 1/2 SU(3) C, SU(2) L, U(1) Y (ũ L dl ) (u L d L ) ( 3, 2, 1 6 ) ũ R u R ( 3, 1, 3 2 ) d R d R ( 3, 1, 1 3 ) ( ν ẽ L ) (ν e L ) ( 1, 2, 1 2 ) ẽ R e R ( 1, 1, 1) (H u + H0 u ) ( H u + H u 0 ) ( 1, 2, ) (Hd 0 H d ) ( H d 0 H d ) ( 1, 2, 1 2 ) spin 1/2 spin 1 SU(3) C, SU(2) L, U(1) Y g g ( 8, 1, 0) W ± W 0 W ± W 0 ( 1, 3, 0) B 0 B 0 ( 1, 1, 0) Neutralino mass matrix: L mass = 1 2 ψt M χ 0ψ + h.c. M χ 0 = M 1 0 g v d / 2 g v u/ 2 0 M 2 gv d / 2 gv u/ 2 g v d / 2 gv d / 2 0 µ g v u/ 2 gv u/ 2 µ 0

6 Prospects for neutralino dark matter detection Arrenberg et al., Snowmass 2013 CF4 Working Group Report, XENON1T Survives DD, ID, and LHC Excluded by LHC but not DD or ID Excluded by DD and ID Excluded by ID but not DD Excluded by DD but not ID 10 7 R σsi (pb) m( χ 0 1 ) (GeV) Open question: How much is the fate of WIMP dark matter related to the one of SUSY?

7 Generalized WIMPs Canonical WIMPs only couple to nucleon and nucleon spin densities, i.e. SI and SD interactions Canonical WIMPs are collision-less after kinetic decoupling Two alternatives: - generalized WIMP-nucleon interactions - self-interacting WIMPs

8 Generalized WIMP-nucleon interactions Example: Non relativistic effective field theory of dark matter-nucleon interactions (NREFT) Fan et al., ; Fitzpatrick et al., NREFT is based upon two assumptions: - there is a separation of scales: q /m V 1, where m V is the mediator mass - dark matter is non-relativistic: v/c 1 It follows that the Hamiltonian for dark matter-nucleon interactions is Ĥ(r) = c τ kôk(r) t τ τ=0,1 k Ô k (r) are Galilean invariant operators; t 0 = 1 isospin, t 1 = τ 3

9 Generalized WIMP-nucleon interactions Inspection of the operators Ôk(r) shows that at linear order in the transverse relative velocity ˆv, they only depend on 5 nucleon charges and currents: 1 N Ŝ N ˆv ˆv Ŝ N ˆv Ŝ N This structure leads to 8 independent nuclear response functions (if nuclear ground states are CP eigenstates) For example: Ô 1(r) = 1 N 1 χδ 3 (r) and Ô4(r) = Ŝ N Ŝ χδ 3 (r) are the SI and SD interactions, respectively

10 Generalized WIMP-nucleon interactions Kavanagh, Catena and Kouvaris, Catena, Ibarra and Wild, N σ max 7σ 6σ 5σ 4σ 3σ 3 DD + Super-K (τ + τ /W + W ) Only DD DM with non-zero spin mχ [GeV] Q Na = 0.3, Q I = 0.09 Npert/Nfree π 4 π 2 O1 O8 O12 γ = cos 1 (ˆvχ ˆrdet) Atten. only Atten.+Defl. 3π 4 π

11 Self-interacting WIMPs Example: Thermal WIMP with self-interactions mediated by a light vector messenger L int = g χ χγ µ V µχ g ν νγ µ V µν Van den Aarssen, Bringmann and Pfrommer, Bringmann et al., The main motivation for this model is to clarify three problematic aspects of ΛCDM Cosmology: - cusp vs. core - too big to fail - missing satellites The three aspects are explained via neutrino-induced late kinetic decoupling of dark matter

12 Self-interacting WIMPs The small scale problems of ΛCDM mainly rest on the outcome of dark matter only N-body simulations In principle, ordinary astrophysics can reconcile theory and observations (Read, Agertz, and Collins, ), but a general consensus has not been reached E.g., energy injection via supernovae explosion is expected to play a key role in this context Open question: Canonical WIMPs are collision-less and cold. Is this the framework currently favored by Cosmology?

13 A critical view on the thermal production mechanism CMB is a thermal relic Cosmological neutrinos are a thermal relic Present cosmological baryon density determined by: 1) the initial baryon-antibaryon asymmetry; 2) the freeze-out of week interactions; 3) BBN Dark energy?? Open question: Is the thermal production mechanism guiding or misleading us?

14 Freeze-in mechanism It applies to Feebly Interacting Massive Particles (FIMPs), X, of negligible initial abundance Hall et al.,

15 Freeze-in mechanism Consider for example the interaction λxb 1B 2, where B 1 and B 2 are thermal bath particles If m B1 > m B2 + m X, n X obeys the equation dn X dt An approximate solution is + 3Hn X = gb 1m 2 B 1 Γ B1 T K 1(m B1 /T ) 2π Ω Xh g B1 g s g ρ m XΓ B1 m 2 B 1 Since Γ B1 λ 2 m B1, Ω Xh implies λ (m B1 /m X) 1/2

16 Freeze-in mechanism Notice that λ v/m, where M GeV This suggests various implementations of the freeze-in mechanism. For example: - Moduli with weak scale supersymmetry - Dirac neutrino masses within weak scale supersymmetry - FIMPs from kinetic mixing - Very heavy FIMPs and extra dimensions Hall et al., FIMPs typically have an unstable partner called Lightest Observable Sector Particle (LOSP) with weak scale interactions Key experimental signature: production and decay of LOSPs at the LHC = displaced vertices

17 Asymmetric dark matter Kaplan, Luty and Zurek, Freeze-out and -in mechanisms naturally explain the observed order of magnitude of the dark matter relic abundance, but not why this is close to the baryon abundance Asymmetric dark matter: the dark matter density arises from a dark matter particle-antiparticle asymmetry related to the (B L) asymmetry leading to baryogenesis. This mechanism predicts n DM n B, and therefore Ω DM (m DM/m B)Ω B

18 Asymmetric dark matter ADM may modify energy transport in the Sun, changing local neutrino production Lopes and Silk, The formation of stable compact dark matter objects is predicted Kouvaris and Nielsen, Direct, indirect and LHC signals possible, but model dependent and non specific do ADM

19 Asymmetric dark matter Maximum mass of dark stars (Chandrasekhar limit) for self-interacting ADM Kouvaris and Nielsen,

20 Misalignment mechanism It is relevant for dark matter candidates that can be described by a classical scalar field θ This is a good approximation in the limit of large occupation number for the θ quanta In this case, the cosmological evolution of θ determines the present density of its quanta If V (θ) = m 2 a(t )f 2 a(1 cos θ), then where θ 0 is the zero mode of θ The solution is θ 0 + 3H θ 0 + m 2 aθ 0 = 0 ( Ω θ0 h where θ is the initial misalignment f a GeV ) 7 6 θ 2

21 Misalignment mechanism Popular examples of dark matter candidates produced via misalignment mechanism are - Axions - Axion-like particles (ALPs) - Wave (or Fuzzy) dark matter Axions (a = f aθ) couple to ordinary matter as follows L aγγ = gaγγ 4 af µν Fµν L a ff = i C f m f a f fγ 5f a

22 Misalignment mechanism Axion searches at ADMX via inverse Primakoff effect Stern et al.,

23 Primordial Black Holes Bird et al., There is a window for PBHs to be DM if the BH mass is in the range 20M M 100M Lower masses are excluded by microlensing surveys. Higher masses would disrupt wide binaries if DM consists of 30M BHs, then the rate for mergers of such PBHs falls within the merger rate inferred from LIGO Green, Microlensing and dynamical constraints exclude all the dark matter being in PBHs, when for PBHs mass function produced by inflation models are considered

24 Summary and open questions Is the thermal production mechanism guiding or misleading us? How much is the fate of WIMP dark matter related to the one of SUSY? Canonical WIMPs are cold and collision-less. Is this the framework currently favored by Cosmology? If dark matter will not be detected in the next 5-10 years, is WIMP dark matter ruled out? If so, what is the next priority?