Low mass dark matter Christopher M c Cabe Effective Theories and - PowerPoint PPT Presentation

Low mass dark matter Christopher M c Cabe Effective Theories and Dark Matter, Mainz – 19 th March 2015

1. General considerations 2. A peculiar neutralino model ¡ Results from: Boehm, Dolan, CM, Increasing N eff with particles in thermal equilibrium with neutrinos - arXiv:1207.0497 A lower bound on the mass of cold dark matter from Planck - arXiv:1303.6270 Christopher M c Cabe GRAPPA - University of Amsterdam

1. General considerations: How low is low mass? ¡ Christopher M c Cabe GRAPPA - University of Amsterdam

Low-mass dark matter candidates ¡ WIMP GeV MeV Sterile neutrino keV Gravitino eV Axion Christopher M c Cabe GRAPPA - University of Amsterdam

Low-mass dark matter candidates ¡ WIMP GeV MeV Sterile neutrino keV Gravitino eV Axion How light can we make WIMPs? Christopher M c Cabe GRAPPA - University of Amsterdam

What is a WIMP? ¡ - Weak scale mass… Weak scale cross-section: ~0.1-10 pb - - Abundance from thermal freeze-out mechanism: Christopher M c Cabe GRAPPA - University of Amsterdam

WIMP mass? ¡ SM fermion get mass from the Higgs vev... • …yet most are below a GeV Lee-Weinberg argument • h σ v i = m 2 DM On dimensional grounds: - m 4 weak If , for h σ v i ⇡ 1 pb - m weak = 100 GeV m DM ≥ 1 GeV ⇒ Light WIMPs are sub-GeV • Light WIMPs require a light mediator • Christopher M c Cabe GRAPPA - University of Amsterdam

The thermal bath ¡ ‘Freeze-out’ from what? Need a thermal bath of particles • Kept in equilibrium with annihilations • f χ f χ could be SM states or BSM states • f SM BSM e − ν p ? ? ? e + ? ¯ γ ν n ? Christopher M c Cabe GRAPPA - University of Amsterdam

Two cases ¡ I’ll consider when WIMP in equilibrium with SM particles • SM e − ν p e + ¯ γ ν n Case 1: In equilibrium with neutrinos • Case 2: In equilibrium with electrons/photons • Christopher M c Cabe GRAPPA - University of Amsterdam

Reminder: The usual Timeline ¡ 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV Plasma of particles in a thermal bath: • T γ e − ν p ¯ ν e + γ n Christopher M c Cabe GRAPPA - University of Amsterdam

Timeline: Neutrino decoupling ¡ 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV Events: decoupling ν Species remain in thermal equilibrium until • Γ = n σ v ∼ H Neutrinos decouple at ~2.3 MeV • T γ T ν = T γ e + γ ν e − ¯ ν n p Christopher M c Cabe GRAPPA - University of Amsterdam

Timeline: Big Bang Nucleosynthesis ¡ BBN 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV Events: decoupling ν T γ T γ 4 He ++ D + n γ 7 Li +++ H + p 3 He ++ e + e − e − γ e + Christopher M c Cabe GRAPPA - University of Amsterdam

Timeline: Photon reheating ¡ BBN 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV m e /3 MeV Events: decoupling reheating γ ν When electrons and positrons become non-relativistic, they • transfer their entropy to photons ✓ 4 ◆ 1 / 3 Photon thermal bath heated • T ν = relative to neutrino bath: T γ 11 Christopher M c Cabe GRAPPA - University of Amsterdam

Timeline: CMB formation ¡ BBN 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV m e /3 MeV Events: decoupling reheating decoupling γ γ ν H + + e − → H + γ Electrons recombine with protons: • Photons decouple from matter: cosmic microwave • background is formed Christopher M c Cabe GRAPPA - University of Amsterdam

Timeline: Today ¡ BBN 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV m e /3 MeV Events: decoupling reheating decoupling today γ γ ν Today we have (at least) two thermal relics: • 1. CMB with (measured) T γ = 2 . 725 K 2. Cosmic neutrino background with (not measured) T ν = 1 . 945 K Christopher M c Cabe GRAPPA - University of Amsterdam

New timeline: With light dark matter ¡ 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV Plasma of particles in a thermal bath, including , which is • χ in equilibrium with the neutrinos T γ e − ν p ¯ ν e + γ n χ Christopher M c Cabe GRAPPA - University of Amsterdam

New timeline: Neutrino decoupling ¡ 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV Events: decoupling ν , χ Neutrinos and decouple at ~2.3 MeV • χ T γ T ν = T γ e + γ ν e − ¯ ν χ n p Christopher M c Cabe GRAPPA - University of Amsterdam

New timeline: Neutrino heating ¡ BBN 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV m e /3 MeV m /3 MeV χ Events: decoupling reheating reheating γ ν , χ ν transfers entropy to • χ neutrinos heating them Neutrinos hotter at end of BBN: • ✓ 4 ◆ 1 / 3  3 + F ( m χ / 2 . 3 MeV) � 1 / 3 T ν = T γ 11 3 + F ( m χ /T γ ) Christopher M c Cabe GRAPPA - University of Amsterdam

New timeline: Today ¡ BBN 10 MeV 1 MeV 0.1 MeV 1 eV 1 meV m e /3 MeV m /3 MeV χ Events: decoupling reheating reheating decoupling decoupling today γ γ ν , χ ν ν Today we have (at least) two thermal relics: • 1. CMB with (measured) T γ = 2 . 725 K 2. Cosmic neutrino background now warmer: � 1 / 3  1 + F ( m χ / 2 . 3 MeV) (not measured) ¡ T ν = 1 . 945 K · 3 Christopher M c Cabe GRAPPA - University of Amsterdam

Changes to BBN? ¡ Kolb, Turner, Phys.Rev. D34 (1986) Raffelt, Serpico, Phys.Rev. D70 (2004) Steigman, Nollett, arXiv:1312.5725 A new light particle can contribute to the energy • density (if it is still relativistic during BBN) A different neutrino-photon temperature ratio changes: • 1. Neutrino energy density higher 2. Change to the weak interaction rates for proton <-> neutron conversion ( ) ν e + n ↔ p + e Christopher M c Cabe GRAPPA - University of Amsterdam

Changes to abundances ¡ We implemented the changes into PArthENoPE BBN code • arXiv:0705.0290 PDG values Y p = 0 . 2465 ± 0 . 0097 D/H = (2 . 53 ± 0 . 04) × 10 − 5 Christopher M c Cabe GRAPPA - University of Amsterdam

In equilibrium with EM particles ¡ We implemented the changes into PArthENoPE BBN code • arXiv:0705.0290 PDG values Y p = 0 . 2465 ± 0 . 0097 D/H = (2 . 53 ± 0 . 04) × 10 − 5 Christopher M c Cabe GRAPPA - University of Amsterdam

CMB: N eff changes ¡ Higher neutrino temperature increases N eff • ,✓ 4 ◆ 1 / 3 # 4 " T ν N e ff = 3 . 046 T γ 11 Planck TT,TE,EE +lowP+BAO (2015) N e ff = 3 . 04 ± 0 . 18 Christopher M c Cabe GRAPPA - University of Amsterdam

Mini-conclusion ¡ Assumptions: • - Light WIMPs are sub-GeV If in equilibrium with SM particles… - Then…MeV mass particles can show up through BBN • and CMB through effects on the neutrino-photon temperature relation Christopher M c Cabe GRAPPA - University of Amsterdam

2. A peculiar neutralino model Christopher M c Cabe GRAPPA - University of Amsterdam

How light can we make the neutralino? ¡ The answer might be surprising: • Explored in a series of papers it can be as light as we like - even massless by Dreiner and others Certain conditions are required… • Bino-like • Selectrions and squarks are reasonably heavy • (some tuning of the parameters) • Christopher M c Cabe GRAPPA - University of Amsterdam

How light can we make neutralino dark matter? ¡ In the MSSM, difficult to go below ~10 GeV: • 7 10 Profumo 6 arXiv:0806.2150 10 5 10 ) V e k 4 / 10 m χ ( 5 . 6 1 3 ~ 10 2 2 h Ω std h Ω std 2 10 1 10 0 Optimistic Limit 10 tan β = 50 tan β = 5 -1 10 -2 10 -6 -5 -4 -3 -2 -1 0 1 10 10 10 10 10 10 10 10 m χ [GeV] Observed value Christopher M c Cabe GRAPPA - University of Amsterdam

Solution is clear: need another light superpartner ¡ Introduce sterile rhd sneutrino that mix with lhd sneutrino • ν Li | 2 + m 2 n i | 2 + A ij h u · ˜ V soft ⊃ m 2 ν L | ˜ n | ˜ L i ˜ n j + h . c . ˜ ˜ Light mass eigenstates are mostly rhd • tan 2 θ i = 2 A i v sin β with ν ↵ n ↵? ∼ 0 . 1 . ν 1 = − sin θ 1 ˜ ˜ L + cos θ 1 ˜ m 2 ν L − m 2 ˜ ˜ n Christopher M c Cabe GRAPPA - University of Amsterdam

Solution is clear: need another light superpartner ¡ Neutralino remains in equilibrium with neutrinos: • Freeze-out happens as usual with a weak scale cross-section: • ◆ 4 ✓ m ˜ ◆ 2 ✓ 35 MeV ◆ 4 ✓ sin θ χ 0 h σ v i ⇡ 7 pb 1 0 . 1 5 MeV m ˜ ν 1 Christopher M c Cabe GRAPPA - University of Amsterdam

How can we test this? ¡ No collider constraints • Not visible in Z , h or meson decays • No direct detection (from electron scattering): • ◆ 4 ✓ 195 GeV σ e ≈ 3 × 10 − 46 cm 2 m ˜ e No usual indirect detection signal: • dominant annihilation is to low energy neutrinos Is this WIMP invisible? ¡ Christopher M c Cabe GRAPPA - University of Amsterdam

Consequence: N eff is larger ¡ Recall: Higher neutrino temperature increases N eff • ,✓ 4 ◆ 1 / 3 # 4 " T ν N e ff = 3 . 046 T γ 11 We now have a way to probe a light neutralino • Christopher M c Cabe GRAPPA - University of Amsterdam

Conclusions ¡ WIMPs can be light • …need a light mediator Usual detection strategies may fail • …direct/indirect/collider Can still have observable consequences • …BBN and CMB are sensitive probes of new physics Christopher M c Cabe GRAPPA - University of Amsterdam