η and π0 Decay to Two Neutrinos

Ali R. Fazely

Department of Physics

Southern University

Miami Conference, 2007

In collaboration with

Richard L. Imlay,

Samvel Ter-Antonyan,

Kevin Muhammad

Department of Physics

Southern University

• Do η orπ0 decay into two neutrinos?

• Standard Model: π0 → νν or η → νν is forbidden, except for massive neutrinos.

• The experimental search for such purely NC decays would shed light on the nature of neutrinos and weak processes.

Weak Current

• The π+ → e + + νe has a BR of

1.23 × 10-4 (pdg)

The helicity is broken due to the mass of the electron consistent with the V-A interaction.

πo → νν

Forbidden in V-A Interactionπo is 0- and vacuum is 0+

For massless neutrinos 0- → 0+ transition isallowed only if Pseudoscalar (P) is present

Same for η → νν

The rate for A and P

• The decay rate for A interaction is proportional to (1 - β ). For massless neutrinos β = 1 and the rate is zero.

• The decay rate for P interaction is proportional to (1+ β ). For massless neutrinos β = 1 and the rate is maximum.

The rate for A and P

• A-coupling:

BR = π → eνe/π → μνμ = (me/mμ)2[1/(1- mμ2/mπ

2)2] = 1.275×10-4

• P-coupling:

BR = π → eνe/π → μνμ = 1/(1- mμ2/mπ

2)2 = 5.5

Experimental results on η → νν andπ0 → νν

Z0

l-q-

q l+

Within the SM, the diagram is a simpleqq annihilation to lepton pairs through aZ0 propagator.

Decay Rate

82222 103/4 −×=απ πfGF

222/1222222 /)/41(4)(/)( ππνπν αππννπ mmmfmGall F −=→Γ→Γ

Arnellos, Marciano, Parsa, Nucl Phy B196 (1982)

2/122228 )/41(/103)(/)( πνπνπννπ mmmmall −×=→Γ→Γ −

Experimental Results

600 107.1)(/)( −×=→Γ→Γ allee πννπ

6106.1)(/)( −×=→Γ→Γ allηννπ µµ

600 101.2)(/)( −×=→Γ→Γ allπννπ ττ

700 107.2)(/)( −×=→Γ→Γ allπννπ

pdg, all at 90% CL

LSND

• Observation of a muon-like, beam-excess above 160 MeV in the LSND detector

• 8.7 ± 6.3 (stat) ± 2.4 (syst) beam-excess events (pdg)

• The possible Branching Ratio of is:

__0

µµ ννπ →

70__

0 10)(3.3)(6.44.6)(/)( −×±±≈→Γ→Γ syststatallπννπ µµ

LSND Results PRL 92, 4, 091801-1, hep-ex/0310060

π0 → ντντ

A 95% CL, upper limit of <18.2 MeV was set by the ALEPH Collaboration on ντ mass

from 3 and 5-prong τ decay. (Eu J Phys, 1998, pdg).

However,

(mτ/mπ)2 < 331.24/(134.9)2 ≈ 0.0182

π0 → ντντ

Therefore,

Γ(π0 → ντντ)/Γ(π0 → all) < 5.0 × 10-10,

assuming no exotic mechanisms!

A possible laboratory to set limits on the ντ

mass!

η → νν

4106)(

)( −×⟨→Γ

→Γall

invisible

ηη

(90%CL)

pdg, BES2 collaboration, 06Q PRL 97 202002

Photons, from π0 and η muons neutronsElectrons/positrons

Estimates for the BR of the Km3

Detectors, e.g. IceCube

• Dominant interaction inside the ice is:

• The signature is an electron or τ cascade event

XeOe +→+ τν τ ,16,

Primary FluxSolid and dashed lines are those of Gaisser & Honda,

(hep-ph/0203272 (2002)). Symbols are from spectra of Wiebel, Bierman & Meyer, Ast. And Astrophys. 330 (1998), used in our CORSIKA simulations

Atmospheric Neutrino Flux, CORSIKA

Detected Spectra

Branching Ratio vs. Energy,5-year run

Branching ratio vs. Years

Conclusions

• Limits can be set for the first time on η→νν with KM3 detectors

∀ π→νν limits would not be competitive with existing limits.

• Dedicated experiments can be designed to look for π→ντντ yielding limits on

the ντ mass as well as any possible P interaction.