-
Alexander Kusenko (UCLA) Q2C
Discovering dark matter in space
• The evidence for dark matter
• The candidates, and the detection strategies
• The opportunities in space
• Sterile neutrinos as dark matter:
1
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Alexander Kusenko (UCLA) Q2C
Dark matter
The evidence for dark matter is very strong:
• galactic rotation curves cannot be explained by the disk alone
• cosmic microwave background radiation
• gravitational lensing of background galaxies by clusters is so strong thatit requires a significant dark matter component.
• clusters are filled with hot X-ray emitting intergalactic gas; some(merging) clusters show displacement of dark and baryonic matter
2
-
Alexander Kusenko (UCLA) Q2C
Galactic rotation curves
3
-
Alexander Kusenko (UCLA) Q2C
Cosmic microwave background radiation (CMBR)
At redshift zdec = 1089 ± 1, the atoms formed and the universe becametransparent to radiation. Radiation emitted at that time, tdec = (379 ±8) kyr, has been red-shifted into the microwave range. Fluctuations havebeen measured first by COBE, and later by BOOMERANG, MAXIMA, ...,WMAP:
4
-
Alexander Kusenko (UCLA) Q2C
Cosmic microwave background radiation (CMBR)
At redshift zdec = 1089 ± 1, the atoms formed and the universe becametransparent to radiation. Radiation emitted at that time, tdec = (379 ±8) kyr, has been red-shifted into the microwave range. Fluctuations havebeen measured first by COBE, and later by BOOMERANG, MAXIMA, ...,WMAP:
5
-
Alexander Kusenko (UCLA) Q2C
These fluctuations can be represented in the form of a power spectrum Cl.First, one expands in spherical harmonics:
∆T
T=∑
l,m
almYlm(θ, φ)
And then one plots
Cl ≡1
2l + 1
l∑
m=−l
|alm|2
6
-
Alexander Kusenko (UCLA) Q2C
Power spectrum measured by WMAP
7
-
Alexander Kusenko (UCLA) Q2C
Power spectrum measured by WMAP
shows angular sizeof the horizon at the time of recombination
densities
sensitive to the ratio ofordinary to dark matter
WMAP: Ωmatter = 0.234 ± 0.035; Ωb = 0.0446 ± 0.0016
8
-
Alexander Kusenko (UCLA) Q2C
Gravitational lensing: seeing the invisible
9
-
Alexander Kusenko (UCLA) Q2C
Foreground cluster CL0024+1654 produces multiple images of a bluebackground galaxy in the HST image (left). Mass reconstruction (right).
10
-
Alexander Kusenko (UCLA) Q2C
None of the known particles can be dark matter
11
-
Alexander Kusenko (UCLA) Q2C
None of the known particles can be dark matter
12
-
Alexander Kusenko (UCLA) Q2C
Dark matter ⇒ new physics
13
-
Alexander Kusenko (UCLA) Q2C
Dark matter: what is it?
Can take guesses based on...
• ...compelling theoretical ideas
• ...simplicity
• ...observational clues
Dark matter properties determine the detection strategy
14
-
Alexander Kusenko (UCLA) Q2C
Dark matter: compelling theoretical ideas
SUSY is an appealing theoretical idea
Dark matter comes as part of the package as one of the following:
• Lightest supersymmetric particle, stable because of R-parity
– neutralino
– gravitino
– axino
• SUSY Q-balls, stable for gauge-mediated SUSY breaking, thanks to the baryon
number and energy conservation.
Theoretically motivated! Mass vs cross section OK, even ”natural”.
By no means minimal. No experimental evidence so far, but the search isunder way.
15
-
Alexander Kusenko (UCLA) Q2C
SUSY
WIMP
Direct detection
Weakly interacting particles (WIMP)can penetrate through rock; they can bedetected in underground detectors, such asZeplin and others, screened by the rockfrom cosmic rays (unwanted background)
16
-
Alexander Kusenko (UCLA) Q2C
Indirect detection of WIMPs
Annihilations of WIMPs in the center of MW galaxy can be detected bygamma ray and x-ray telescopes in space
17
-
Alexander Kusenko (UCLA) Q2C
Axion
�
�
�
�Strong CP problem: QCD vacuum is a superposition |θ〉 =
∑
n exp{−inθ}|n〉of topologically distinct vacuum states |n〉.
LQCD = Lpert + θg2
32π2F F̃
θ̄ = θ + arg det M
Experiment: |θ̄| ≪ 10−10 !
18
-
Alexander Kusenko (UCLA) Q2C
Possible solution: Peccei-Quinn symmetry
An elegant solution: θ̄ relaxes to zero dynamically, by a VEV of the axion .
Additional U(1) symmetry, Peccei-Quinn symmetry is spontaneouslybroken by instantons ⇒ axion has small mass.
Axion is a weakly interacting particle ⇒ dark matter
19
-
Alexander Kusenko (UCLA) Q2C
Axion dark matter
20
-
Alexander Kusenko (UCLA) Q2C
Axion dark matter
Various searches:
• axion-photon conversion in magnetic field
• detection of solar axions
• long-range forces; possible advantage in space: drag-free environment
21
-
Alexander Kusenko (UCLA) Q2C
Sterile dark matter: a simple (minimalist) solution
Need one particle ⇒ add just one particle
If a fermion, must be gauge singlet (anomalies)
Interactions only through mixing with neutrinos
⇒ sterile neutrino
Small mass and, therefore, stability! No symmetries required.
22
-
Alexander Kusenko (UCLA) Q2C
Sterile neutrinosThe name ”sterile” was coined by BrunoPontecorvo in a paper [JETP, 53, 1717 (1967)],which also discussed
• lepton number violation
• neutrinoless double beta decay
• rare processes (e.g. µ → eγ)
• vacuum neutrino oscillations
• detection of neutrino oscillations
• astrophysical neutrino oscillations
23
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Alexander Kusenko (UCLA) Q2C
Pontecorvo: neutrino oscillations can”convert potentially active particles into
particles that are, from the point of view
of ordinary weak interactions, sterile, i.e.practically unobservable, since they have
the ”incorrect” helicity” [JETP, 53, 1717(1967)]
24
-
Alexander Kusenko (UCLA) Q2C
Sterile neutrinos with a small mixing to active neutrinos
{
|ν1〉 = cos θ|νe〉 − sin θ|νs〉|ν2〉 = sin θ|νe〉 + cos θ|νs〉 (1)
The almost-sterile neutrino, |ν2〉 was never in equilibrium. Production ofν2 could take place through oscillations.
The coupling of ν2 to weak currents is also suppressed, and σ ∝ sin2 θ.The probability of νe → νs conversion in presence of matter is
〈Pm〉 =1
2
[
1 +
(
λosc
2λs
)2]−1
sin2 2θm, (2)
where λosc is the oscillation length, and λs is the scattering length.
25
-
Alexander Kusenko (UCLA) Q2C
Sterile neutrinos in the early universe
Sterile neutrinos are produced in primordial plasma through
• off-resonance oscillations. [Dodelson, Widrow; Abazajian, Fuller; Dolgov,Hansen; Shaposhnikov et al.]
• oscillations on resonance, if the lepton asymmetry is non-negligible[Fuller, Shi]
26
-
Alexander Kusenko (UCLA) Q2C
Mixing is suppressed at high temperature [Dolgov, Barbiieri; Stodolsky]
sin2 2θm =(∆m2/2p)2 sin2 2θ
(∆m2/2p)2 sin2 2θ + (∆m2/2p cos 2θ − V (T ))2, (3)
For small angles,
sin 2θm ≈sin 2θ
1 + 0.79 × 10−13(T/MeV)6(keV2/∆m2)(4)
Production of sterile neutrinos peaks at temperature
Tmax = 130 MeV
(
∆m2
keV2
)1/6
27
-
Alexander Kusenko (UCLA) Q2C
The resulting density of relicsterile neutrinos in conventionalcosmology, in the absence of alarge lepton asymmetry:
Ων2 ∼ 0.3(
sin2 2θ
10−8
)
(
ms
keV
)2
1e-11 1e-10 1e-09 1e-08 1e-07
sin2θ
1
10
m s [
keV
]
Ω > 0.3
Ω = 0.3
s
s
dark matter
[Dodelson, Widrow; Dolgov, Hansen; Fuller, Shi; Abazajian, Fuller, Patel]
28
-
Alexander Kusenko (UCLA) Q2C
Lyman-α forest: a look at the small-scale structure
29
-
Alexander Kusenko (UCLA) Q2C
The resulting density of relicsterile neutrinos in conventionalcosmology, in the absence of alarge lepton asymmetry:
Ων2 ∼ 0.3(
sin2 2θ
10−8
)
(
ms
keV
)2
Lyman-α forest clouds showsignificant structure on smallscales. Dark matter must be coldenough to preserve this structure.
1e-11 1e-10 1e-09 1e-08 1e-07
sin2θ
1
10
m s [
keV
]
Ω > 0.3
Ω = 0.3
s
s
dark matter
too warm
Seljak et al.
Viel et al.
30
-
Alexander Kusenko (UCLA) Q2C
Cold or warm dark matter?
CDM works well, but...
There are problem problems with cold dark matter on small scales
31
-
Alexander Kusenko (UCLA) Q2C
Some CDM problems eliminated by WDM• overproduction (by an order of magnitude!) of the satellite halos for galaxies of the
size of Milky Way.
• WDM can reduce the number of halos in low-density voids. [Peebles]
• observed densities of the galactic cores (from the rotation curves) are lower than whatis predicted based on the ΛCDM power spectrum. [Dalcanton et al.; van den Bosch etal.; Moore; Abazajian]
• The “angular-momentum problem”: in CDM halos, gas should cool at very early timesinto small halos and lead to massive low-angular-momentum gas cores in galaxies.[Dolgov]
• disk-dominated (pure-disk) galaxies are observed, but not produced in CDM becauseof high merger rate. [Governato et al.; Kormendy et al.]
• observations of dwarf spheroidal galaxies ⇒ m ∼ keV [Gilmore et al.]
32
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Alexander Kusenko (UCLA) Q2C
Radiative decay
Sterile neutrino in the mass range of interest have lifetimes longer thanthe age of the universe, but they do decay:
ν2 W+ ν1
l -l -
γ
ν2 l - ν1
W+W+
γ
Photons have energies m/2: X-rays. Large lumps of dark matter emit someX-rays. [Abazajian, Fuller, Tucker; Dolgov, Hansen; Shaposhnikov et al.]
33
-
Alexander Kusenko (UCLA) Q2C
X-ray observations
Virgo cluster image from XMM-Newton
34
-
Alexander Kusenko (UCLA) Q2C
Chandra, XMM-Newton can see photons: νs → νeγ
[Abazajian et al; Hansen et al.; Boyarsky et al.; Watson et al.]
35
-
Alexander Kusenko (UCLA) Q2C
Chandra, XMM-Newton can see photons: νs → νeγ
dSphs (favored)
[Abazajian et al; Hansen et al.; Boyarsky et al.; Watson et al.]
36
-
Alexander Kusenko (UCLA) Q2C
Emission of sterile neutrinos from a supernova
• Sterile neutrino emission from a supernova is anisotropic
• Sterile neutrinos with masses and mixing angles consistent with darkmatter can explain the pulsar velocities
[AK, Segrè; Fuller, AK, Mocioiu, Pascoli]
37
-
Alexander Kusenko (UCLA) Q2C
The pulsar velocities.
Pulsars have large velocities, 〈v〉 ≈ 250 − 450 km/s.[Cordes et al.; Hansen, Phinney; Kulkarni et al.; Lyne et al. ]
A significant population with v > 700 km/s,about 15 % have v > 1000 km/s, up to 1600 km/s.[Arzoumanian et al.; Thorsett et al. ]
38
-
Alexander Kusenko (UCLA) Q2C
A very fast pulsar in Guitar Nebula
HST, December 2001HST, December 1994
39
-
Alexander Kusenko (UCLA) Q2C
Map of pulsar velocities
40
-
Alexander Kusenko (UCLA) Q2C
Proposed explanations:
• asymmetric collapse [Shklovskii] (small kick)
• evolution of close binaries [Gott, Gunn, Ostriker] (not enough)
• acceleration by EM radiation [Harrison, Tademaru] (kick small, predictedpolarization not observed)
• asymmetry in EW processes that produce neutrinos [Chugai; Dorofeev,Rodinov, Ternov] (asymmetry washed out)
• “cumulative” parity violation [Lai, Qian; Janka] (it’s not cumulative )
41
-
Alexander Kusenko (UCLA) Q2C
Asymmetric collapse
“...the most extreme asymmetric collapsesdo not produce final neutron star velocities above 200km/s” [Fryer ’03]
42
-
Alexander Kusenko (UCLA) Q2C
Supernova neutrinos
Nuclear reactions in stars lead to a formation of a heavy iron core. Whenit reaches M ≈ 1.4M⊙, the pressure can no longer support gravity. ⇒collapse.
Energy released:
∆E ∼ GNM2Fe core
R∼ 1053erg
99% of this energy is emitted in neutrinos
43
-
Alexander Kusenko (UCLA) Q2C
Pulsar kicks from neutrino emission?
Pulsar with v ∼ 500 km/s has momentum
M⊙v ∼ 1041 g cm/sSN energy released: 1053 erg ⇒ in neutrinos. Thus, the total neutrinomomentum is
Pν; total ∼ 1043 g cm/s�
�
�
�a 1% asymmetry in the distribution of neutrinos
is sufficient to explain the pulsar kick velocities
But what can cause the asymmetry??
44
-
Alexander Kusenko (UCLA) Q2C
Magnetic field?
Neutron stars have large magnetic fields. A typical pulsar has surfacemagnetic field B ∼ 1012 − 1013 G.Recent discovery of soft gamma repeaters and their identification asmagnetars
⇒ some neutron stars have surface magnetic fields as high as1015 − 1016 G.
⇒ magnetic fields inside can be 1015 − 1016 G.Neutrino magnetic moments are negligible, but the scattering of neutrinosoff polarized electrons and nucleons is affected by the magnetic field.
45
-
Alexander Kusenko (UCLA) Q2C
Core collapse supernova
Onset of the collapse: t = 0
Fe
core
46
-
Alexander Kusenko (UCLA) Q2C
Core collapse supernova
Shock formation and “neutronization burst”: t = 1 − 10 ms
PNSburst
shock
ν
Protoneutron star formed. Neutrinos are trapped. The shock wave breaksup nuclei, and the initial neutrino come out (a few %).
47
-
Alexander Kusenko (UCLA) Q2C
Core collapse supernova
Thermal cooling: t = 10 − 15 s
PNSthermal
ν
Most of the neutrinos emitted during the cooling stage.
48
-
Alexander Kusenko (UCLA) Q2C
Electroweak processes producing neutrinos (urca),
p + e− ⇀↽ n + νe and n + e+ ⇀↽ p + ν̄e
have an asymmetry in the production cross section, depending on the spinorientation.
σ(↑ e−, ↑ ν) 6= σ(↑ e−, ↓ ν)The asymmetry:
ǫ̃ =g2
V− g2
A
g2V
+ 3g2A
k0 ≈ 0.4 k0,
where k0 is the fraction of electrons in the lowest Landau level.
49
-
Alexander Kusenko (UCLA) Q2C
In a strong magnetic field,
20 30 40 50 600
0.1
0.2
0.3
0.4
0.5
0.6
0.7
B=10 G16
B=3x10 G15
0K
20 30 40 50 60
µ, MeV
B=3x10 G16
k0 is the fraction of electrons in the lowest Landau level.
Pulsar kicks from the asymmetric production of neutrinos?[Chugai; Dorofeev, Rodionov, Ternov]
50
-
Alexander Kusenko (UCLA) Q2C
Can the weak interactions asymmetry cause an
anisotropy in the flux of neutrinos due to a large
magnetic field?
No
eνeνeν
eνeν
eν
eν
eν
eν
eνeν
eν
eνeν e
ν
Neutrinos are trapped at high density.
51
-
Alexander Kusenko (UCLA) Q2C
Can the weak interactions asymmetry cause an
anisotropy in the flux of neutrinos due to a large
magnetic field?
NoRescattering washes out the asymmetry
In approximate thermal equilibrium the asymmetries in scattering amplitudesdo not lead to an anisotropic emission. Only the outer regions, nearneutrinospheres, contribute (a negligible amount). [Vilenkin,AK, Segrè]
However, if a weaker-interacting sterile neutrino was produced inthese processes, the asymmetry would, indeed, result in a pulsarkick!
52
-
Alexander Kusenko (UCLA) Q2C
Sterile neutrinos leave the star without scattering. Hence, they give thepulsar a kick.
eνeν
νsνs
νs
eν
νs
eν
νs
eν
B
53
-
Alexander Kusenko (UCLA) Q2C
Allowed range of parameters (time scales, fraction of total energy emitted):
1e-11 1e-10 1e-09 1e-08 1e-07
sin2θ
1
10
m s [
keV
]
�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������
�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������
Ω = 0.3
Ω > 0.3
ν
ν
dark matter
pulsar kick
oscillations)(off−resonance
[Fuller, AK, Mocioiu, Pascoli]
54
-
Alexander Kusenko (UCLA) Q2C
Resonant active-sterile neutrino conversions in matter
Matter potential:
V (νs) = 0
V (νe) = −V (ν̄e) = V0 (3Ye − 1 + 4 Yνe)
V (νµ,τ) = −V (ν̄µ,τ) = V0 (Ye − 1 + 2 Yνe) + cZ
L
~k · ~Bk
cZ
L=
eGF√2
(
3Ne
π4
)1/3
[D’Olivo, Nieves, Pal]
55
-
Alexander Kusenko (UCLA) Q2C
The magnetic field shifts the position of the resonance because of the~k· ~B
kterm in the potential:
µν
νs
νs
νs
νs
νµµν µν
µν
µν
In the absence of magnetic field, νs escape isotropically
56
-
Alexander Kusenko (UCLA) Q2C
The magnetic field shifts the position of the resonance because of the~k· ~B
kterm in the potential:
µν
νsνs
νs
νs
νsνs
µνµν
µν eν
µν
µνµν
B
Down going neutrinos have higher energies
of the
57
-
Alexander Kusenko (UCLA) Q2C
The range of parameters (resonance, adiabaticity, weak damping):
1e-11 1e-10 1e-09 1e-08 1e-07
sin2θ
1
10
m s [
keV
]
�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������
�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������
Ω = 0.3
Ω > 0.3
ν
ν
pulsar kick(resonant oscillations)
dark matter
2
58
-
Alexander Kusenko (UCLA) Q2C
Resonance & off-resonance oscillations
1e-11 1e-10 1e-09 1e-08 1e-07
sin2θ
1
10
m s [
keV
]
����������������������������������������
����������������������������������������
2
Ω > 0.3s
1
3
�������������������������������������������������
�������������������������������������������������pulsar kick
pulsar kick and dark matter (L=0)
too warm
[ A.K., Segrè, PL B396, 197 (1997); Fuller, A.K.,Mocioiu,Pascoli, PR D 68, 103002 (2003)]
59
-
Alexander Kusenko (UCLA) Q2C
Other predictions of the pulsar kick mechanism
• Stronger supernova shock [Fryer, AK, ApJ, in press; astro-ph/0512033]
iii
iii
ConvectionRegion
60
-
Alexander Kusenko (UCLA) Q2C
Other predictions of the pulsar kick mechanism
• Stronger supernova shock [Fryer, AK, ApJ, in press; astro-ph/0512033]
61
-
Alexander Kusenko (UCLA) Q2C
Other predictions of the pulsar kick mechanism
• Stronger supernova shock [Fryer, AK, ApJ, in press; astro-ph/0512033]
• No B − v correlation is expected because
– the magnetic field inside a hot neutron star during the first ten seconds is verydifferent from the surface magnetic field of a cold pulsar
– rotation washes out the x, y components
• Directional ~Ω − ~v correlation is expected, because
– the direction of rotation remains unchanged– only the z-component survives
B
62
-
Alexander Kusenko (UCLA) Q2C
Reionization
WMAP, three years of data, reionization redshift: zr = 10.9+2.7−2.3.
(This improves the one-year WMAP result, zr = 17 ± 5.)
Observations of distant quasars: reionization must be completed by z = 6.
First stars can ionize gas, but can they form so early?
WMAP 3 yrs ⇒ new challenge: can one end reionization by z = 6 withoutexceeding the optical depth τ
WMAP= 0.10 ± 0.03?
Small halos collapse first and start ionizing gas. If reionization is to becompleted by z = 6, small halos shine to early, too bright, and exceedτ
WMAP.
63
-
Alexander Kusenko (UCLA) Q2C
Need suppression of star formation in small halos by an order ofmagnitude: [Haiman, Bryan, astro-ph/0603541]
Warm dark matter (gravitinos) could suppress small structure and butthey would also delay the star formation.
What about sterile neutrinos?
• they are warm ⇒ small halos suppressed
• they decay and produce x-rays, and x-rays can ionize gas!
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Alexander Kusenko (UCLA) Q2C
Photons from radiative decays
Sterile neutrino in the mass range of interest have lifetimes longer thanthe age of the universe, but they do decay:
ν2 W+ ν1
l -l -
γ
ν2 l - ν1
W+W+
γ
Photons have energies m/2: X-rays. X-rays can ionize gas.
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Alexander Kusenko (UCLA) Q2C
Sterile neutrino decays: an increase in ionization fraction
10 20 50 100 200 500 1000
0.001
0.01
0.1
1
xe
z
The ions too few to explain the WMAP results [Ferrara, Mapelli]...
...but it’s a much higher fraction than in the absence of sterileneutrinos. Ionization catalyzes formation of molecular hydrogen [AK;P.L. Biermann]...
production of molecular hydrogen speeds up gas cooling, halocollapse and star formation
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Alexander Kusenko (UCLA) Q2C
[Tegmark, et al., ApJ 474, 1 (1997) ]
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Alexander Kusenko (UCLA) Q2C
Molecular hydrogen
H + H → H2 + γ − very slow!In the presence of ions the following reactions are faster:
H+ + H → H+2 + γ,H+2 + H → H2 + H+.
H+ catalyze the formation of molecular hydrogen![Biermann, AK, PRL 96, 091301 (2006)]
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Alexander Kusenko (UCLA) Q2C
The fraction of molecularhydrogen f
ḟ ≈ km(t) nH(t) xe(t),
where km is the rate shown
0 200 400 600 800 10000
0.5
1
1.5
2
z
mk , 1
0 cm
/s
−16
3
End result: H2 production is enhanced at z ∼ 100Sterile neutrino decays can precipitate the early star formation. Stars canreionize the universe by redshif zr = 11 ± 3 (WMAP) while avoiding theminihalo problem
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Alexander Kusenko (UCLA) Q2C
Clues of sterile neutrinos
The sterile neutrinos can explain:
• the light neutrino masses
• dark matter
• baryon asymmetry via leptogenesis [ Asaka, Shaposhnikov; Akhmedovet al. ]
• pulsar kicks
• reionization
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Alexander Kusenko (UCLA) Q2C
Clues of sterile neutrinos
Depending, of course, on how far down it goes.This could be the greatest discovery of the century.
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Alexander Kusenko (UCLA) Q2C
Summary
• Dark matter is out there and needs to be discovered!
• NASA can make the discovery in space using x-ray and gamma-ray telescopes
• A sterile neutrino with keV mass is a viable dark matter candidate, free of CDMproblems, consistent with dSphs observations.
• The same neutrino is emitted from a supernova with a sufficient anisotropy to explainthe pulsar velocities
• The same neutrino can boost the production of molecular hydrogen and precipitate arapid early star formation.
• A rather minimal extension of the Standard Model, the addition of three sterileneutrinos explains all the present data, including dark matter, the baryonasymmetry of the universe, the pulsar velocities, and reionization
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