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

Low mass dark matter

Christopher McCabe

Effective Theories and Dark Matter, Mainz – 19th March 2015

• 1. General considerations
• 2. A peculiar neutralino model ¡

Christopher McCabe GRAPPA - University of Amsterdam

Results from: Boehm, Dolan, CM, Increasing Neff 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

• 1. General considerations:

How low is low mass? ¡

Christopher McCabe GRAPPA - University of Amsterdam

Low-mass dark matter candidates ¡

Christopher McCabe GRAPPA - University of Amsterdam

eV keV MeV GeV

Axion Gravitino Sterile neutrino WIMP

Low-mass dark matter candidates ¡

Christopher McCabe GRAPPA - University of Amsterdam

eV keV MeV GeV

Axion Gravitino Sterile neutrino WIMP

How light can we make WIMPs?

What is a WIMP? ¡

Christopher McCabe GRAPPA - University of Amsterdam

• Weak scale mass…
• Weak scale cross-section: ~0.1-10 pb
• Abundance from thermal freeze-out mechanism:

WIMP mass? ¡

• SM fermion get mass from the Higgs vev...

…yet most are below a GeV

• Lee-Weinberg argument
• On dimensional grounds:
• If , for
• Light WIMPs are sub-GeV
• Light WIMPs require a light mediator

Christopher McCabe GRAPPA - University of Amsterdam

hσvi = m2

DM

m4

weak

mweak = 100 GeV hσvi ⇡ 1 pb mDM ≥ 1 GeV ⇒

The thermal bath ¡

• ‘Freeze-out’ from what? Need a thermal bath of particles
• Kept in equilibrium with annihilations
• could be SM states or BSM states

χ χ f f f

Christopher McCabe GRAPPA - University of Amsterdam

e+ e− ν ¯ ν γ n p

SM BSM ? ? ? ? ?

Two cases ¡

• I’ll consider when WIMP in equilibrium with SM particles
• Case 1: In equilibrium with neutrinos
• Case 2: In equilibrium with electrons/photons

Christopher McCabe GRAPPA - University of Amsterdam

e+ e− ν ¯ ν γ n p

SM

Reminder: The usual Timeline ¡

e+ e− ν ¯ ν γ n p

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

• Plasma of particles in a thermal bath:

Tγ

Christopher McCabe GRAPPA - University of Amsterdam

Timeline: Neutrino decoupling ¡

e+ e− γ n p ν ¯ ν

• Species remain in thermal equilibrium until
• Neutrinos decouple at ~2.3 MeV

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

Events: decoupling

ν

Γ = nσv ∼ H

Tγ Tν = Tγ

Christopher McCabe GRAPPA - University of Amsterdam

Timeline: Big Bang Nucleosynthesis ¡

n p

4He++

H+ D+

3He++ 7Li+++

e+ e− γ e+ e− γ

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

BBN Events: decoupling

ν

Tγ Tγ

Christopher McCabe GRAPPA - University of Amsterdam

Timeline: Photon reheating ¡

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

BBN Events: decoupling

ν

me/3 MeV

reheating

γ

• When electrons and positrons become non-relativistic, they

transfer their entropy to photons

• Photon thermal bath heated

relative to neutrino bath:

Tν Tγ = ✓ 4 11 ◆1/3

Christopher McCabe GRAPPA - University of Amsterdam

Timeline: CMB formation ¡

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

BBN Events: decoupling

ν

me/3 MeV

reheating

γ

decoupling

γ

• Electrons recombine with protons:
• Photons decouple from matter: cosmic microwave

background is formed

H+ + e− → H + γ

Christopher McCabe GRAPPA - University of Amsterdam

Timeline: Today ¡

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

today BBN Events: decoupling

ν

me/3 MeV

reheating

γ

• Today we have (at least) two thermal relics:
• 1. CMB with (measured)
• 2. Cosmic neutrino background

with (not measured)

Tγ = 2.725 K Tν = 1.945 K

decoupling

γ

Christopher McCabe GRAPPA - University of Amsterdam

New timeline: With light dark matter ¡

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

• Plasma of particles in a thermal bath, including , which is

in equilibrium with the neutrinos

e+ e− ν ¯ ν γ n p Tγ

χ χ

Christopher McCabe GRAPPA - University of Amsterdam

New timeline: Neutrino decoupling ¡

e+ e− γ n p ν ¯ ν

• Neutrinos and decouple at ~2.3 MeV

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

Events: decoupling

Tγ Tν = Tγ

χ χ

ν, χ

Christopher McCabe GRAPPA - University of Amsterdam

New timeline: Neutrino heating ¡

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

BBN Events:

me/3 MeV

reheating

γ

• transfers entropy to

neutrinos heating them

• Neutrinos hotter at end of BBN:

m /3 MeV

χ

decoupling

ν, χ

reheating

ν Tν Tγ = ✓ 4 11 ◆1/3 3 + F(mχ/2.3 MeV) 3 + F(mχ/Tγ) 1/3

χ

Christopher McCabe GRAPPA - University of Amsterdam

New timeline: Today ¡

10 MeV 1 MeV 0.1 MeV 1 meV 1 eV

today Events:

• Today we have (at least) two thermal relics:
• 1. CMB with (measured)
• 2. Cosmic neutrino background now warmer:

Tγ = 2.725 K

decoupling

γ

BBN

me/3 MeV

reheating

γ

m /3 MeV

decoupling

ν, χ

reheating

ν

decoupling

ν

χ

Tν = 1.945 K ·  1 + F(mχ/2.3 MeV) 3 1/3 (not measured) ¡

Christopher McCabe GRAPPA - University of Amsterdam

Changes to BBN? ¡

• 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 ( )

Christopher McCabe GRAPPA - University of Amsterdam

νe + n ↔ p + e

Kolb, Turner, Phys.Rev. D34 (1986) Raffelt, Serpico, Phys.Rev. D70 (2004) Steigman, Nollett, arXiv:1312.5725

Changes to abundances ¡

• We implemented the changes into PArthENoPE BBN code

Christopher McCabe GRAPPA - University of Amsterdam

Yp = 0.2465 ± 0.0097 D/H = (2.53 ± 0.04) × 10−5

arXiv:0705.0290

PDG values

In equilibrium with EM particles ¡

• We implemented the changes into PArthENoPE BBN code

Christopher McCabe GRAPPA - University of Amsterdam

arXiv:0705.0290

Yp = 0.2465 ± 0.0097 D/H = (2.53 ± 0.04) × 10−5 PDG values

CMB: Neff changes ¡

• Higher neutrino temperature increases Neff

Neff = 3.046 " Tν Tγ ,✓ 4 11 ◆1/3#4

Christopher McCabe GRAPPA - University of Amsterdam

Planck TT,TE,EE +lowP+BAO (2015)

Neff = 3.04 ± 0.18

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 McCabe GRAPPA - University of Amsterdam

• 2. A peculiar neutralino model

Christopher McCabe GRAPPA - University of Amsterdam

How light can we make the neutralino? ¡

• The answer might be surprising:

it can be as light as we like - even massless

• Certain conditions are required…
• Bino-like
• Selectrions and squarks are reasonably heavy
• (some tuning of the parameters)

Explored in a series of papers by Dreiner and others

Christopher McCabe GRAPPA - University of Amsterdam

How light can we make neutralino dark matter? ¡

• In the MSSM, difficult to go below ~10 GeV:

10

• 6

10

• 5

10

• 4

10

• 3

10

• 2

10

• 1

10 10

1

mχ [GeV] 10

• 2

10

• 1

10 10

1

10

2

10

3

10

4

10

5

10

6

10

7

Ωstdh

2

Optimistic Limit tanβ = 50 tanβ = 5 Ω

std

h

2

~ 1 6 . 5 ( m

χ

/ k e V ) Profumo arXiv:0806.2150

Observed value

Christopher McCabe GRAPPA - University of Amsterdam

Solution is clear: need another light superpartner ¡

• Introduce sterile rhd sneutrino that mix with lhd sneutrino
• Light mass eigenstates are mostly rhd

with

Vsoft ⊃ m2

˜ νL|˜

νLi|2 + m2

˜ n|˜

ni|2 + Aijhu · ˜ Li˜ nj + h.c. ˜ ν1 = − sin θ1 ˜ ν↵

L + cos θ1 ˜

n↵? tan 2θi = 2Aiv sin β m2

˜ νL − m2 ˜ n

∼ 0.1 .

Christopher McCabe 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:

hσvi ⇡ 7 pb ✓sin θ 0.1 ◆4 ✓ m˜

χ0

1

5 MeV ◆2 ✓35 MeV m˜

ν1

◆4

Christopher McCabe 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):
• No usual indirect detection signal:

dominant annihilation is to low energy neutrinos σe ≈ 3 × 10−46 cm2 ✓195 GeV m˜

e

◆4

Is this WIMP invisible? ¡

Christopher McCabe GRAPPA - University of Amsterdam

Consequence: Neff is larger ¡

• Recall: Higher neutrino temperature increases Neff
• We now have a way to probe a light neutralino

Neff = 3.046 " Tν Tγ ,✓ 4 11 ◆1/3#4

Christopher McCabe 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 McCabe GRAPPA - University of Amsterdam