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

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 1

Expected properties of the (yet unobserved) cosmic neutrino background (CNB) versus the cosmic microwave background (CMB) CMB CNB Relation Temperature 2.73K 1.9 K T ν /T γ = (4/11) 1/3 (1.7 x 10 -4 ev) =0.714 3.8 x 10 5 years Decouple time ~ 1 sec Density ~ 411 / cm 3 ~ 56 / cm 3 (per n ν = (3/22) n γ flavor, n ν = n ν -bar ) • CNB took a snapshot of the Universe at a much earlier epoch than CMB n ν 2 = (8.0±0.3) x 10 -5 ev 2 , and | Δ m 32 • Since Δ m 21 2 | = (1.9 → 3.0) x 10 -3 ev 2 , 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) 2

Non-standard cosmic neutrino background • In inflationary models, C NB density depends on the " reheating temperatu re" : T R ≥ ⇒ 8MeV agrees with standard p rediction T n ν R = ⇒ 5MeV drops to ~90% of the standard prediction T n ν R = ⇒ 2M eV drops to ~3% of the standard prediction T n ν R • Non-standard models allow ν ≠ ν ( ) ( ) n n • Non-standard mo dels also allow ν ≠ ν ≠ ν ( ) ( ) ( ) at production n n n μ τ e (flavor oscillation would have removed this asymmetry) 3

Incomplete list of proposed searches for CNB 1) Coherent ν -nucleus scattering (effect of order G F2 ) (Zeldovich and Khlpov, 1981; Smith and Lewin, 1983; Duda, Gelmini, Nussinov, 2001) − λ 4 For CNB, � 10 ev, � 2.4mm T ν ν σ ν π × − 2 2 63 2 ( -nucleon) ∼ G / � 5 10 (Relativistic) E cm ν F 2 ⎛ ⎞ m π − − 2 2 56 ν 2 ∼ G / � 10 (No n Relativistic) ⎜ ⎟ m cm ν F ⎝ ⎠ ev • ν − ⇒ 2 ≈ 4 nuc leus coherent scattering enhancement factor of A 10 • ⇒ 20 coherence over CNB wavelength enhancement factor of ~10 λ 3 20 (coherence over a volume of ( ) containing ~10 nuclei) ν ⇒ Isotropic CNB flux net force = 0 ⇒ = ± From COBE dipole anisotropy 369 2.5km/s (CNB is non-isotropi c, v sun just like the dark ma tter ) ⇒ -26 2 λ net acceleration due to "neutrino wind " ~10 cm/s on grain of si z e 4 ν

2) “Neutrino Optics” (effect linear in G F ) (R. Opher, 1974; R. Lewis, 1980) Total reflection or refraction of CNB o n a fla t surf ace ′ = − Index of re frac tion, / , a nd 1 ∼ n p p n G F Later, Cabibbo and Maiani showed that � � ∫ ρ ∇ 3 ∼ ( ) ( ) F G d x x n x ν F A Effect is only due to the gradient of ( ), n x ν and a gain negligible 5

3) Torque exerted on a polarized target (effect linear in G F ) (Stodolsky, 1974) For a polarized target (magnetized iron), there is an energy split of the two spin states o f electron in the sea of the CNB − The split is proportiona l to n n ν v = (no effec t for ) n n ν v 6

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5) Capture of CNB on radioactive nuclei A very old idea: S. Weinberg, 1962 Consider tritium beta-decay: → + − + ν 3 3 ) a H He e e β This is a 3-body -decay with -value of Q − = − − − ν 3 3 ( ) ( ) ( ) ( ) Q M H M He M e M a e wher e ( ) refers to mass of particle M x x Now consider the CNB c apture reaction : − ν + → + 3 3 ) b H He e e This is a 2-body reaction with the -value of Q − = ν + − − 3 3 ( ) ( ) ( ) ( ) Q M M H M He M e b e It foll ows that = + ν 2 ( ) Q Q M 8 b a

5) Capture of CNB on radioactive nuclei (continued) ν = For massless neutrinos, ( ) 0, and we have M = Q Q a b Note that the conventional definition of Q-value for β ν = the -decay, Q , assumes ( ) 0, hence M β = + ν ( ) Q Q M β a The maximal energy for electrons from the → + − + ν 3 3 H He e e β -decay is the end-point energy (ignoring recoil energy) = = − ν ( ) T Q Q M β a a Electrons from CNB capture reaction are mono-energetic: = = + ν ( ) T Q Q M β b b β (Q = 18.6 KeV for tritium -decay) β It follows tha t = + ν 2 ( ) T T M 9 b a

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 r • Neutrino masses • Experimental energy resolution • Any local clustering of CNB due to grav ity? • C aptur e cross section on radioactive nuclei • Size of the tritium sourc e Capture rate per tritium at om: − = σ × × 32 � 10 / R v n s ν ν σ × Note that for exotherma l reactio n, is constant for small v v ν ν − σ × × ⋅ 45 2 � ( 7.6 1 0 ) v c m c ν 10

5) Capture of CNB on radioactive nuclei (continued) • ν Neutrino masses: ( )=1ev (mass degeneracy of three neutrinos) M • Δ Experimental energy resolution : = 0.5ev • < > = Any local clustering of CNB due to gravity? / 50 n n ν ν • Siz e of the tritium source: 100 gram s Lazauskas, Vogel, Volpe, J. Phys. G. 35 (2008) 025001 11

5) Capture of CNB on radioactive nuclei (continued) • M ν Neutrino masses: ( )=0 ev (for the lightest neutrino, assuming inverted mass hierarchy, the other two massive neutrinos are nearly degen erate) • Δ Experimental energy resolut ion : =0.03 ( 0.0 6) ev • < >= Any local clustering of CNB due to gravity? / 1 n n ν ν • Size of the tritium s our ce: 100 g ra s m M. Blennow, Phys. Rev. D 77 (2008) 113014 12

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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. 16

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