Antineutrinos

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Pauli proposed that two particles were emitted, and Fermi called the second one a neutrino, V. The complete process therefore is n — p -H e 9. Owing to the low probabiHty of its interacting with other particles, the neutrino was not observed until 1959. Before the j3 -decay takes place there are no free leptons, so the conservation of leptons requires that there be a net of 2ero leptons afterward. Therefore, the associated neutrino is designated an antineutrino, 9-, that is, the emitted electron (lepton) and antineutrino (antilepton) cancel and give a net of 2ero leptons. 

From the discussion on lepton conservation, it follows that the two emitted Ps would be accompanied by two antineutrinos. This case is denoted... 

In the last decade, neutrino experiments have demonstrated that neutrinos are massive particles which may oscillate among three autostates. Such experiments [77-82] have evidenced the mass difference between the autostates, but not the neutrino mass scale value. The only way to determine the neutrino mass is the knowledge of the shape of the end point of energy spectrum in beta decays. In the hypothesis of the Majorana neutrino (neutrino coincides with antineutrino and its rest mass is different from zero), the measure of the decay half-life in the neutrinoless double-beta decay (DBD) would be necessary. A number of recent theoretical interpretations of neutrino oscillation experiments data imply that the effective Majorana mass of the electron neutrino (as measured in neutrinoless DBD) could be in the range 0.01 eV to the present bounds. 

A nucleus (A Z) decays with the emission of two electrons (e ) and two antineutrinos (ve) following the equation ... 

Neutrinos Neutrinos and antineutrinos are formed whenever a positron particle is created in a radioactive decay they are highly penetrating. 

Neutrinos are also generated by purely nuclear processes involving weak interactions, e.g. in the Sun. Such neutrinos can be an important cause of energy losses in compact stars through the Urea process, in which an inverse / -decay is followed by a normal fS-decay resulting in a neutrino-antineutrino pair. 

During most of the first 0.1 second after the Big Bang (ABB), the relativistic particles are photons, electrons, positrons and Nv species of neutrinos and antineutrinos Nv is expected to be 3, from ve, vfl and vr. There is a sprinkling of non-relativistic protons and neutrons which make a completely negligible contribution to the energy density. The temperature is then given by... 

Because of charge neutrality, ye- t] B and is consequently negligible. That the same thing holds for yVe (and for yV/tI) is a postulate, but a very plausible one because otherwise we would have neutrino or antineutrino degeneracy with LV 2> B. The upshot is that the chemical potentials of neutrons and protons are equal and so from Eq. (2.43) one has a simple Boltzmann-type equilibrium ratio (n/p)eq = e-fa-mptflkT = g-1.29M eV/kT (4.36)... 

This is a process characteristic of nucleides with high n p ratios, and involving the loss of an electron from the nucleus, which is usually, but not invariably accompanied by the emission of y-photons. A detailed energy balance reveals that the simple picture cannot account for all the energy lost by the nucleus in the decay and the emission of an additional particle - the antineutrino, v is postulated to account for this. The general equation for a negatron emission is... 

The exact energy carried by an emitted negatron will depend upon the angle between its path and that of the antineutrino. As the angle can vary from atom to atom, so will the distribution of energy between the particles. Negatron spectra (Figure 10.3) thus do not have sharp peaks. 

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The merger remnant emits neutrinos in copious amounts. The total neutrino luminosities are typically around 2 1053 ergs/s with electron-type antineutrinos carrying away the bulk of the energy. Typical neutrino energies... 

Figure 4 The annihilation of neutrino-antineutrino pairs above the remnant of a neutron star merger drives relativistic jets along the original binary rotation axis (only upper half-plane is shown). The x-axis lies in the original binary orbital plane, the dark oval around the origin is the newly formed, probably unstable, supermassive neutron star formed in the coalescence. Color-coded is the asymptotic Lorentz-factor. Details can be found in Rosswog et al. 2003.

Energy release due to (anti)neutrino untrapping. The configurations for the quark stars are obtained by solving the Tolman-Oppenheimer-Volkoff equations for a set of central quark number densities nq for which the stars are stable. In Fig. 13 the configurations for different antineutrino chemical potentials are shown. The equations of state with trapped antineutrinos are softer and therefore this allows more compact configurations. The presence of antineutrinos tends to increase the mass for a given central density. 

A reference configuration with total baryon number Ni> = 1.51 Nq (where Nq is the total baryon number of the sun) is chosen and the case with (configurations A and B in Fig. 13) and without antineutrinos (/ in Fig. 13) are compared. A mass defect can be calculated between the configurations with trapped antineutrinos and without it at a constant total baryon number and the result is shown in Fig. 14). The mass defect could be interpreted as an energy release if the configurations A, B with antineutrinos are initial states and the configuration / without them is the final state of a protoneutron star evolution. 

Figure 13. Quark star configurations for different antineutrino chemical potentials r = 0, 100, 150 MeV. The total mass M in solar masses (MsUn = M in the text) is shown as a function of the radius R (left panel) and of the central number density nq in units of the nuclear saturation density no (right panel). Asterisks denote two different sets of configurations (A,B,f) and (A ,B ,f ) with a fixed total baryon number of the set.

After the collapse of a protoneutron star the star cools down by surface emission of photons and antineutrinos. Antineutrinos are trapped because they were generated by the direct /5-process in the hot and dense matter and could... 

Figure lJh Mass defect AM and corresponding energy release AE due to antineutrino untrapping as a function of the mass of the final state Mf. The shaded region is defined by the estimates for the upper and lower limits of the antineutrino chemical potential in the initial state = 150 MeV (dashed-dotted line) and = 100 MeV (dashed line), respectively. 

Figure 15. Left graph Quark star cooling by antineutrino and photon emission from the surface. Middle graph Two-phase structure developes due to the trapped antineutrinos a normal quark matter shell and a superconducting interior. Right graph Antineutrino untrapping and burst-type release of energy.

In the previous Section we noted that the typical temperature, above which the star becomes opaque to neutrinos is Topac 0.4 4- 3 MeV, where we ignore here the differences in the absorption/production properties of different neutrino flavors [45], Saying neutrino we actually will not distinguish neutrino and antineutrino, although their absorption/production could be different. If we assume an initial temperature of To < T%pac, the star radiates neutrinos directly from the interior region. For To > T"po/P the neutrino transport to the surface is operative and leads to a delay of the cooling evolution. 

Energy radiation for T > T ac. The neutrino/antineutrino collisions produce e e+ pairs outside the star, which efficiently convert to photons. We use the estimate of Ref. [45] for the vv —> e e+ conversion rate ... 

Apart from this phenomenon, gravitational collapse has another important effect. The tremendously hot neutron star in the making emits a copious supply of thermal neutrinos and antineutrinos. These transfer some 10 erg, that is, almost all the gravitational energy liberated by compaction of part of the original star into a neutron star with mass around 1.5 Mq and radius 10 km. 

This chain of events involved the so-called weak interaction, a puny and slow force compared with the strong and electromagnetic interactions. The weak interaction governs the conversion of protons into neutrons and vice versa, with creation of a neutrino (antineutrino). It thus determines the lifetime of free neutrons, which naturally decay into protons. In fact, neutrons have a life expectancy of around 10 minutes. However, before they disappear, they may have the opportunity to combine with protons, one which they readily accept. In that case, nuclear physics makes its appearance in the Universe. 

Beta-minus Beta-minus decay essentially mirrors beta-plus decay. A neutron converts into a proton, emitting an electron and an anftneutrino (which has the same symbol as a neutrino except for the line on top). Particle and antiparticle pairs such as neutrinos and antineutrinos are a complicated physics topic, so we ll keep it basic here by saying that a neutrino and an antineutrino would annihilate one another if they ever touched, but they re otherwise very similar. Again, the mass number remains the same after decay because the number of nucleons remains the same. However, the atomic number increases by 1 because the number of protons increases by 1 ... 

Is the neutron as we understand it today really a combination of a proton and electron as Rutherford envisioned it This is a philosophically interesting question. When neutrons decay, they produce a proton and an electron (and an antineutrino as well) however, these particles are not understood to have a real existence within an intact neutron. Indeed, the constituent parts of neutrons (and protons for that matter) are understood to be quarks. The phenomenon of neutron decay is explained by a transformation of one of its constituent quarks, turning the neutron into a proton the energy difference between the neutron and proton gives rise to the electron and antineutrino. 

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