Prospects for Observing Cosmic Neutrino Background Jen-Chieh Peng - - PowerPoint PPT Presentation

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Prospects for Observing Cosmic Neutrino Background

Jen-Chieh Peng Journal Club, Academia Sinica, January 3, 2011

• Properties of the cosmic neutrino

background (relic neutrinos)

• Brief review of previous proposed ideas for

detection

• Recent development

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Expected properties of the (yet unobserved) cosmic neutrino background (CNB) versus the cosmic microwave background (CMB)

• CNB took a snapshot of the Universe at a much earlier epoch than

CMB nν

• Since Δm21

2 = (8.0±0.3) x 10-5 ev2, and |Δm32 2| = (1.9 →3.0) x 10-3

ev2, at least two of the three neutrinos have masses higher than 10-2 ev, and these two types of CNB are non-relativistic (β<<1) CMB CNB Relation Temperature 2.73K 1.9 K (1.7 x 10-4 ev) Tν/Tγ = (4/11)1/3 =0.714 Decouple time 3.8 x 105 years ~ 1 sec Density ~ 411 / cm3 ~ 56 / cm3 (per flavor, nν = nν-bar ) nν = (3/22) nγ

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Non-standard cosmic neutrino background

8MeV agrees with standard p In inflationary models, C rediction NB density depends on the " 5MeV drops to ~90% of the standard prediction reheating temperatu 2M : eV re"

R R R R

T n n T T T n

ν ν ν

≥ ⇒ = ⇒ = ⇒

• Non-standard models allow

Non-standard mo drops to ~3% of the standard prediction ( ) ( ) ( ) ( ) ( ) at production dels also allow (flavor oscillation would have removed

e

n n n n n

μ τ

ν ν ν ν ν

• ≠

≠

• ≠

this asymmetry)

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Incomplete list of proposed searches for CNB

1) Coherent ν-nucleus scattering (effect of order GF2)

(Zeldovich and Khlpov, 1981; Smith and Lewin, 1983; Duda, Gelmini, Nussinov, 2001)

2 2 63 2 F 2 2 2 56 2 F 4 4 2

( -nucleon) G / 5 10 (Relativistic) G / 10 (No nuc n Relativistic) leus coherent scattering enhancement factor of A 10 coherence over For CNB, 10 ev, 2.4mm T E cm m m cm ev

ν ν ν ν ν

σ ν π ν λ π

− − −

× ⎛ ⎞ − ⎜ ⎟ ⎝ ⎠

• −

⇒ ≈

• ∼
• ∼

20 3 20

CNB wavelength enhancement factor of ~10 (coherence over a volume of ( ) containing ~10 nuclei)

ν

λ ⇒

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Isotropic CNB flux net force = 0 From COBE dipole anisotropy 369 2.5km/s (CNB is non-isotropi net acceleration due to "neutrino wind c, just like " ~10 cm/s on grain of si the dark ma ) z tter e

sun

v

ν

λ ⇒ ⇒ ⇒ = ±

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2) “Neutrino Optics” (effect linear in GF)

(R. Opher, 1974; R. Lewis, 1980) 3

Later, Cabibbo and Total reflection or Maiani showed that Index of refraction of CNB o re ( ) ( ) Effect is only due to the gradient of ( ), and n a fla frac t surf tion, ace / , nd 1 a a

F A F

F n p p n G G d x x n x n x

ν ν

ρ ∇ ′ = −

∫

• ∼

∼ gain negligible

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3) Torque exerted on a polarized target (effect linear in GF)

(Stodolsky, 1974)

For a polarized target (magnetized iron), there is an energy split of the two spin states

• The split is proportiona

f electron in the sea of (no the effec l to t for ) CNB

v v

n n n n

ν ν

= −

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5) Capture of CNB on radioactive nuclei

A very old idea: S. Weinberg, 1962

3 3 3 3 3 3

Consider tritium beta-decay: This is a 3-body -decay with -value ( ) ( ) ( ) wher

• f

Now consider the CNB c e ( ) refers to mass of particle apture reaction ) ) : ( )

a e e e

Q M H M He M e a H He e M b H He e Q M x x β ν ν ν

− − −

→ + + + → = − + − −

3 3

( This is a 2-body reaction with the -value of It foll ) 2 (

• ws that

( ) ( ) ( ) )

b e b a

Q M M Q Q M He M M Q H e ν ν

−

= + = + − −

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5) Capture of CNB on radioactive nuclei (continued)

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For massless neutrinos, ( ) 0, and we have Note that the conventional definition of Q-value for the -decay, Q , assumes ( ) 0, hence The maximal energy for electrons from the ( )

a b a

M H He Q Q Q e Q M M

β β

ν ν β ν

−

= → = + + = = +

• decay is the end-point energy (ignoring recoil energy)

Electrons from CNB capture reaction are mono-energetic: (Q = 18.6 KeV for tritium -decay) It follows tha ( ( ) ( ) t ) 2

e a a b b b a

T Q Q M T Q Q M T T M

β β β

β β ν ν ν ν = = − = = + = +

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5) Capture of CNB on radioactive nuclei (continued)

To check the feasibility of separating the CNB capture peak from the end-point, one need to conside Neutrino masses Experimental energy resolution Any local clustering of CNB due to grav r ity? C

• 32

45 2

Capture rate per tritium at aptur 10 / ( 7.6 1 e cross section on radioactive nuclei

• m:

Note that for exotherma Size of l reactio the tritium sourc n, is constant for small e R v n s v c v m v

ν ν ν ν ν

σ σ σ

− −

= × × ×

• ×

⋅ ×

• )

c

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5) Capture of CNB on radioactive nuclei (continued)

( )=1ev (mass degeneracy of three neutrinos) = Neutrino masses: Experimental energy resolution : Any local clustering of CNB due to gravity? Siz 0.5ev / 50 100 gram e of the tritium source: M n n

ν ν

ν Δ <

• >
• =
• s

Lazauskas, Vogel, Volpe, J. Phys. G. 35 (2008) 025001

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5) Capture of CNB on radioactive nuclei (continued)

( )=0 ev (for the lightest neutrino, assuming inverted mass hierarchy, the other two massive neutrinos are nearly degen Neutrino masses: Experimental energy resolut erate) =0.03 ( ion 0.0 : 6) ev M ν

• Δ

/ Any local clustering of CNB due to gravity? Size of the tritium s 1 100 g

• ur

ra s ce: m n n

ν ν

<

• >=
• M. Blennow, Phys. Rev. D 77 (2008) 113014

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Summary

• Observation of Cosmic Neutrino Background

would have tremendous impact on our knowledge of Universe at the very early stage.

• It would also have important impact on our

knowledge on neutrino physics (mass hierarchy, Dirac versus Majorana), as well as developing techniques to detect very low energy neutrinos from other sources (solar, supernova, geo, reactor…).

• Many interesting ideas have been proposed in

the past. None of them proved to be viable.

• The recent proposal of “capture on radioactive

nuclei” seems promising. More study is required.

• It remains a great challenge to come up with new

idea for observing the CNB.

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