Neutrinos as Hot or Warm Dark Matter (Active + Sterile)

Neutrinos as Hot or Warm Dark Matter(Active + Sterile)Zhi-zhong Xing(IHEP, Beijing)Neutrinos are neutral, long-lived and weakly-interacting--- reasonably good candidates for hot/warm dark matter@ Matter to the Deepest, 12—18 September 2011, Ustron

A Naïve (Why Not) PictureHow dark is dark?HOTWARMCOLDHot dark matter: CB is guaranteed but not significant.Cold dark matter: most likely? At present most popular.Warm dark matter: suppress the small-scale structures.

Sub-eV or keV Neutrinos?A purely phenomenological argument to support sub-eV +keV sterile ’s in the FLAVOR DESERT of the SM (Xing 09).Sterile LandReally nothing in?Fertile SoilFLAVORDESERT

HOT DARK MATTER CB Formation Direct Detection Sterile Component

As T ~ a few MeV in the Universe, the survival relativistic particleswere photons, electrons, positrons, neutrinos and antineutrinos.Electroweak reactions:Formation of CBNeutrinos decoupled from matter:Weak interactionsNumber density of 6 relic ’s:Hubble expansion’s in thermal contact with cosmic plasmaneutrino and photontemperatures (blue)neutrino decoupling’s not in thermal contact with matterarrow of time

CMB and LSS: the existence of relic neutrinos had an impact on theepoch of matter-radiation equality, their species and masses couldaffect the CMB anisotropies and large scale structures (Wong’s talk)Timeline of the Big Bang:Witness / ParticipantAt the time of recombination (t ~ 380 000 yrs):The CB contribution to the total energy density of the Universe todayrelativisticnon-relativistic

CB Must Be ThereToday’s matter & energy densities in the Universe (Dunkley et al 09;Komatsu et al 09; Nakamura et al 10): 5-year WMAP + CDM modelThe CMB (t ~ 380 000 years) is already measured todayIs it likely to detect the CB (t ~ 1 s) in the foreseeablefuture? ---- important to test the standard cosmology.

Detection of CBWay 1: CB-induced mechanical effects on Cavendish-type torsion balance;Way 2: Capture of relic ’s on radioactive -decaying nuclei (Weinberg 62);Way 3: Z-resonance annihilation of UHE cosmic ’s and relic ’s (Weiler 82).Temperature todayRelic neutrino capture on -decaying nucleiMean momentum todayAt least 2 ’s cold todayHow to detect ULE ’s ?(Irvine & Humphreys, 83) no energy threshold on incident ’s mono-energetic outgoing electrons

ExampleSalient feature: the cross section of a capture reaction scales withso that the number of events converges to a constant for :e.g.(Cocco et al 07, Lazauskas et al 08).Capture rate: (1 MCi = 100 g =tritium atoms)Background: (the tritium -decay)Energy resolution (Gaussian function) :

IllustrationnormalhierarchyTarget mass: 100 g tritium atomsInput (13) : 10 degreesNumber of events per year: ~ 8Flavor effects are quite important(Blennow 08; Li, Xing 10, 11)0.24invertedhierarchyapproximatedegeneracy7.838.07

Cosmic anti-Background?OverdensitiesRelic antineutrino capture onEC-decaying Ho-163 nuclei.(Lusignoli, Vignati 11; Li, Xing 11)30 kg Ho-163~1 event/yr

Sub-eV Sterile ’s?BBN: current data only allow one sub-eV sterile neutrino;CMB: current data can allow two sub-eV sterile neutrinos.BBNG. Mangano,P. Serpico,arXiv:1103.1261(95% C.L.)The sharp cut-offis due to a He-4abundance upperbound (

Hamann et al, arXiv:1006.5276; 1108.4136Giusarma et al, arXiv:1102.477495%95%68%99%68%99%CMB68%95%68%95%

(3+1) SchemeThe LSND + MiniBooNE anomalies & the reactor antineutrino anomalyalso hint at 1 or 2 sub-eV sterile ’s (Yasuda’s talk).They could be thermally excited in the early Universe via oscillationsor collisions with active ’s; they are now non-relativistic; and theirnumber density per species is expected to equal that of active ’s.Input: (Li, Xing, Luo 10)

OverdensitiesGravitational clustering: Those cosmic ’s with velocities smaller thanthe escape velocity of a given structure can be bound to it. So largerGC effects for heavier ’s around the Earth (Ringwald, Wong 04).The (3+2) scheme:Input:Overdensity

WARM DARK MATTER Why keV ’s? Direct Detection Summary + Remarks

keV sterile Dark MatterThere is not any strong prior theoretical motivation for keV sterile ’s.(Asaka et al 05; Kusenko et al 10; Lindner et al 11; Barry et al 11; ....)A purely phenomenological argument to support keV sterile ’s in theFLAVOR DESERT of the standard model (Xing 09).

keV sterile Dark MatterThere is not any strong prior theoretical motivation for keV sterile ’s.keV sterile ’s(Asaka et al 05; Kusenko et al 10; Lindner et al 11; Barry et al 11; ....)A purely phenomenological argument to support keV sterile ’s in theFLAVOR DESERT of the standard model (Xing 09).

keV sterile Dark MatterProduction: via active-sterile oscillations in the early Universe, etc;Salient feature: warm DM in the form of keV sterile ’s can suppressthe formation of dwarf galaxies and other small-scale structures.Bounds on 2-flavor parameters:(Abazajian, Koushiappas, 2006)For simplicity, we assume only onetype of keV sterile neutrinos:Standard parameterization of V:6 mixing angles & 3 (Dirac) or 6(Majorana) CP-violating phases.

Decay RatesDominant decay mode [C = 1 (Dirac) or 2 (Majorana)]:Lifetime (the Universe’s age ~ 10^17 s):Radiative decay: X-ray and Lymanalphaforest observations.

Detection in the LabThe same method as the detection of the CB in the lab.Capture rate with a Gaussian energy resolution:Assumption: the number density of sterile’s is equivalent to the total amount of DMin our galactic neighborhood.Half-life effect of target nuclei (Li, Xing, 11)Two sources (Liao, 10; Li, Xing, 11):This method & the X-ray detection probe different parameter space.

IllustrationFor illustration: solid (dotted) curves with (without) half-life effects.Number of events per year: pink1.11.7Dim and remote observability of keV sterile neutrino DM in this way:--- tiny active-sterile neutrino mixing angles (main problem)--- background: keV solar neutrinos or scattering.

Summary----- CB: a test of cosmology as early as t ~ 1 s after theBig Bang, but a direct measurement is extremely difficult.----- Weinberg’s idea works in principle, but in practice itsuffers from small target mass, low number density, …..e.g., KATRIN’s tritium mass 0.1 mg, toooooo small----- Sterile ’s as hot dark matter (sub-eV) might not beimpossible, and they could be detected in the same way.----- Cosmic anti- background could also be detected byusing the same idea and EC-decaying nuclei as the target.In practice, this seems more difficult.----- Warm dark matter in the form of keV sterile ’s maybe detected in the same way, but it is more challenging!

Really Hopeless?keV ’s?ULE ’sobservedexpectedUHE ’s