Right-handed sneutrino as cold dark matter of the universe
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Right-handed sneutrino as cold dark matter
of the universe
Takehiko Asaka (EPFL Niigata University)
Refs: with Ishiwata and MoroiPhys.Rev.D73:061301,2006Phys.Rev.D75:065001,2007
@ TAUP2007 (11/09/2007, Sendai)
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I. Introduction
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Content of the universe
What is dark matter??? No candidate in SM New Physics !!! One attractive candidate
Dark Matter
Dark energy (74%) Baryon (4%)
Dark matter (22%)
LSP in supersymmetric theories
[WMAP ’06]
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LSP Dark Matter
R-parity:ordinary SM particles: R-parity even (+1)additional superparticles: R-parity odd (-1)
Lightest superparticle (LSP) is stable LSP is a good candidate of DM if it is neutral
What is the LSP DM? Lightest neutralino (= combination of neutral gauginos and higgsinos)
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Other candidates for LSP DM
The lightest neutralino is NOT the unique candidate for the LSP DM In supergravity, “gravitino” In superstring, “modulino” With Peccei-Quinn symmetry, “axino” …
Now, we know that the MSSM is incomplete accounting for neutrino oscillations
alternative candidate for the LSP DM
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Introduce RH neutrinos to explain neutrino masses In supersymmetric theories, RH neutrino + RH sneutrino
If neutrino masses are purely Dirac-type, Masses of RH sneutrinos come from SUSY breaking
— Lightest RH sneutrino can be LSP, LSP RH sneutrino is a good candidate for CDM
(i.e., can be realized)
In this talk,
fermion (Rp=+1) scalar (Rp=-1)
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II. Right-handed sneutrino as dark matter
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Model
MSSM + three right-handed (s)neutrinosassuming neutrino masses are purely Dirac-type
Yukawa couplings are very small
Small Yukawa couplings are natural in ‘tHooft’s sense— chiral symmetry of neutrinos is restored in the limit of vanishing Yu
kawa couplings
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Model (2)
LSP = only suppressed interaction:
NLSP = MSSM-LSP MSSM-LSP can be charged rather long-lived:
—typically
Our claim: LSP as CDM
How are produced in the early universe???
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Production of RH sneutrino
is not thermalized in the early universe!!! Interaction rate of is very small:
— Typically,
How are produced in the early universe???
A. is effectively produced by superparticle decay
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Production by superparticle decay
Two distinct contributions: decay of superparticle in chemical equilibrium (CE) decay of NLSP after freeze-out (FO)
(CE)(FO)
time
T~m
NLSP
T~TF~mNLSP/20
sparticle
t~NLSP
“freeze-out”
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Relic density from sparticle in CE
Boltzmann equation
Dominant production occurs at T~mx— Present abundance is insensitive to thermal history fo
r T >> 100GeV
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Higgsino decay
In this case, the abundance is too small: But, the production is enhanced in some cases !
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Wino decays
DM can be realized with a mild degeneracy between and
Light will be a good target of collider exp.
(1) Enhance left-right mixing
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Larger neutrino mass enhances the production of since
Neutrino mass bound: From CMBR
— m<1.8eV m<0.60eV [WMAP ’06]
CF. if we include other data from large scalestructure/Ly-alpha, the bound becomes severer
DM can be realized when m~O(0.1) eV Scenario with degenerate neutrino masses will be tested
in future astrophysical observations
(2) Degenerate neutrinos
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NLSP decay after freeze-out
NLSP (=MSSM-LSP) decays into after freeze-out
is “would-be” relic density of NLSP
When , DM can be realized different parameter space from the standard neutralino D
M since Present abundance is insensitive to thermal history for T
>> 100GeV depends strongly on MSSM params
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Example
mSUGRA:
AG=0
AG=0
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III. Summary
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Summary
We discussed MSSM with three RH (s)neutrinos assuming neutrino masses are purely Dirac-type Lightest RH sneutrino can be LSP LSP RH sneutrino can be a good candidate for DM
— can be realized is insensitive to physics at T >> 100 GeV MSSM-LSP can be charged
The list of LSP DM Neutralino, Gravitino, Axino, … , RH sneutrino
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Comments:
Other production mechanism: by inflaton decay / as coherent oscillation
— depends on physics at high energy by new interaction
— extra U(1)’ [Lee, Matchev, Nasri]
When Majorana masses are present,
Yukawa couplings become larger:
[See also Gopalakrishna, de Gouvea, Porod]
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Sneutrinos
Mass squared matrix of sneutrinos
Very small left-right mixing of sneutrinos
RH sneutrino masses come from SUSY breaking
LSP can be the lightest RH sneutrino
suppressed by m
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Implication of RH sneutrino DM
Parameter space of RH sneutrino DM is different from the standard neutralino DM
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3. Cosmological constraints
NLSP (=MSSM-LSP) decays around or after the BBN would spoil success of BBN put constraints on
[Kawasaki, Kohri, Moroi]
lifetime of NLSP
• hadronic branching ratio• visibile energy of decay products• yield of NLSP
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NLSP decays
Bino-like neutralino Main decay mode:
— no visible energy Subdominant modes:
hadronic branching ratio is small
Stau Main decay mode:
hadronic branching ratio is large
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BBN constraint
mSUGRA point
Bino-like NLSP
Stau NSLP
BBN
hierarchical neutrinosdegenerate neutrinos
AG=0
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BBN constraints on NLSP decay
Bino-like NLSP almost harmless
Stau NLSP severely restricted
— even more stringent from recent obs.– 6Li production is enhanced [Pospelov / Hamaguchi et al]
lifetime should be shorter— degenerate neutrinos— large left-right mixing of sneutrino
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