Tuesday, 2 July 2013

Neutrinos- Massive After All

Italian speaks of the 'small neutral one'. And the recent discovery that neutrinos have mass comes as a revelation for particle physics; they are particles produced by trillions of times over by our sun, play a subtle role in continental drift and are released vigorously by dying stars through supernovae. Pauli first proposed them in the 30s to account for energy conservation in beta decay (whereby a neutron becomes a proton and simultaneously emits an electron), he suggested they have a spin of ħ/2. Then in the 50s, Goldhaber computed the negative helicity of the neutrino (its spin along the direction of motion), also called 'handedness' by the electron capture of europium 152 when it decays into a neutrino and samarium 152, which emits a gamma ray. Firing both the neutrino and gamma ray adjacent to one another reveals the left handedness of the neutrino (as a conservation of angular momentum). In accordance with special relativity, an onlooker moving at luminal speed can overtake a massive neutrino and observe it spinning in the opposite direction; but since right-handed neutrinos were never detected, it was inferred that they were massless. Or so it seemed...Particles gain mass by interacting with the Higgs boson, and quantum field theory teaches that seemingly 'vacuous' nature of the vacuum is in fact teeming with Higgs bosons and when a particle interacts with the Higgs (a spinless, scalar field), it changes its handedness (Lorentz invariance). Popular science writers like to describe the Higgs field as a sort of ''molasses' which slows particles to endow them with mass but such is a flawed analogy, in fact fields don't slow particles and the quantum vacuum has no 'stickiness'. Naturally, its obvious to think that the known left-handed neutrinos can interact with the Higgs and become massive and right-handed; but again, since no right handed neutrinos have been detected, it was again inferred that neutrinos are massless. But recent developments from neutrino oscillations (whereby electron, muon and tau neutrinos convert into each other as they travel) from Japan's Super-K observatory attests to the fact that neutrinos do have mass. Since particles may behave like waves, oscillating neutrinos are a mixture of the three neutrino waves (or flavours) which can only oscillate if the component waves combine and form 'beats' in the waveform, such beats are the outcome of mass; thus if we can see neutrinos oscillating (which we do), then they have mass. But the standard model runs into trouble if it tries to accomodate massive neutrinos, so a means of renormalising the theory is necessary; enter the Dirac and Majorana neutrinos. The Dirac neutrino posits the reason why right handed neutrinos are so elusive is because their interaction is so weak by about 30 orders of magnitude, another similar idea comes from string theory where right handed neutrinos are stuck in extra compactified dimensions. But Majorana neutrinos require a lack of differentiation between antimatter and matter (neutrinos and antineutrinos are the same thing); thus don't rely on weak interactions to expound mass. Going back to the onlooker travelling at light speed, what if a right handed antineutrino is observed? Then neutrinos can acquire mass via a 'see-saw mechanism': when a left-handed neutrino interacts with a Higgs, it is granted mass and becomes a right-handed neutrino (which violates the law of energy conservation), however due to the Uncertainty principle ∆t~h/Mc2 such a quantum state can last for a period ∆t; subsequently becoming a left-handed neutrino again by interacting with the Higgs once more. This has a lot of implications for big bang cosmology, especially the leptogenesis and the conservation of lepton number to create the lepton asymmetry; as the early universe cooled down, massive right handed neutrinos ceased to transform into light left-handed ones and thus, since Majorana neutrinos are both matter and antiparticles simultaneously, they decayed into right- handed antineutrinos and left-handed neutrinos along with Higgs bosons.

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