Athar, Husain (1995) NEUTRINO SPIN-FLIP EFFECTS IN COLLAPSING STARS AND IN THE EARLY UNIVERSE. Doctoral thesis, Quaid-i-Azam University, Islamabad.

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We study the spin-flavor conversions of active as well as sterile neutrinos (Ve - Vu" Ve - VS, etc.) in the magnetic fields of collapsing stars and the early Universe. In collapsing stars, for the neutrino mass squared difference ∆m2~ (10-10 - 10) eV2 these conversions take place in almost isotopically neutral region of the star, where the effective matter density for active to active neutrino conversions is suppressed up to 3 - 4 orders of magnitude. This suppression is shown to increase the sensitivity of the neutrino burst studies to the magnetic moment of neutrino u by 1.5 - 2 orders of magnitude and for realistic magnetic field the observable effects may exist for u ~ (2-3) X 1O-14 UB (UB is the Bohr magneton). In the isotopically neutral region the jumps of the effective potential for active to active neutrino conversions exist which influence the corresponding probabilities of conversions. The experimental signatures of the spin-flavor conversions between active to active and active to sterile neutrinos are discussed. In particular, in the case of direct mass hierarchy, the spin-flip effects result in a variety of modifications of the ve spectrum for both types of conversions. Taking this into account, we estimate the upper bounds on UB from the SN 1987 A data. In the isotopically neutral region the effects of possible twist of the magnetic field on the way of neutrinos can be important, inducing distortion in the neutrino energy spectra and further increasing the sensitivity to UB. However, if the total rotation angle is restricted by ∆Ǿ< π the absolute change of probabilities is small. In the early Universe, a conceivable twist in the primordial magnetic field is shown to affect oppositely the average neutrino and antineutrino conversion probabilities in contrast to astrophysical systems. In particular, for neutrino conversions, it enhances the effect thus relaxing the resonant ∆m2 values to achieve the same average conversion probability as without primordial field twist. Implications of this behavior for conversions between active to sterile neutrinos in the temperature range 1 MeV .≤ T. ≤100 MeV are discussed. Next, we study the effects of resonant (Ve - Vu) spin-flavor conversions of reheated neutrinos. For 0.7 MeV ≤ T. ≤ 2 MeV, these conversions affect the primordial Helium abundance predictions. We estimate the effect to be less than 1% (i.e ∆Y. ≤ 1.26 x 10-3). We point out that these conversions may occur for u as small as u ~ 1O-20 UB in a primordial magnetic field of electroweak origin. The effects of resonant spin-flavor as well as flavor conversions on relic neutrino energy spectra are discussed.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: neutrino spin-flip effects, collapsing stars, spin-flavor conversions, neutrinos, neutrino conversions, early universe
Subjects: Q Science > QC Physics
Depositing User: Muhammad Khan Khan
Date Deposited: 25 Oct 2016 10:00
Last Modified: 25 Oct 2016 10:00

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