Neutrinos From Massive Stars And Gamma-Ray Bursts

Neutrino is another kind of elementary particle,just like electron,photon and quark.All these particles,along with their interactions between have shaped this colorful world.With a unique property,neutrino is distinctive from other particles and play a special in the fundamental world.As a fermion,neutrinos are neutral particles and only participate in weak and gravita-tional interactions,making them radically different from electrons.Since neutrinos interact very weakly with ordinary matter,they are hard to catch in experiments.There are three flavors of neutrinos,and each flavor can transform into one another.The experimentally establishment of neutrino oscillation within three flavors indicates that neutrinos have none-zero masses,which provides the first clue for the existence of physics beyond the standard model(BSM).Besides being tightly linked to the development of particle physics,neutrino also plays an important role in astronomy and cosmology.Neutrinos with different energies can be produced in great numbers at the time of Big Bang,during the expansion of the universe,and from the evo-lution of stars and many violent astrophysical processes such as core-collapse supernova(CCSN)explosions and Gamma-Ray Bursts(GRBs),etc.On one hand,neutrinos help to determine the evolution of universe and affect the relevant astrophysical processes in a straightforward way.On the other hand,by detecting these neutrino signals,we can not only better study cosmology and understand the mechanisms of different astrophysical processes,but also study the properties of neutrino itself.The detection of solar neutrinos and neutrinos from SN1987a open a brand new era for neutrino astronomy.The detection of solar neutrinos help to confirm the existence of neutrino oscillation.What’s more,the solar neutrinos,which are precisely detected nowadays,have become one of the most important probes for the solar interior.It can be expected that,the detection of neutrinos from a nearby CCSN can greatly deepen our understanding of explosion mechanism and the nature of neutrinos.Same as solar neutrinos,neutrinos can also be made inside massive stars through various weak processes.Stellar core in massive stars can reach high temperature(～109-10K)and high densities(～106-9 g/cm3),and lots of e± pairs and photons exist.In these high temper-ature and density conditions,neutrinos can be produced in staggering numbers through many thermal processes,including e± pair annihilation,plasmon decay,photo-neutrino emission and electron-nucleus bremsstrahlung,etc.Since the rates of thermal processes grow rapidly with temperature and density,the corresponding thermal neutrino emissions from massive stars can be much more intense than that of solar neutrinos from pp fusions.Especially at the very late stages,neutrino flux from massive stars will be sizable and we are able to detect these signals if the star is close enough to the Earth.In Chapter 2,we will for the first time calculate and compare the energy-differential rates of neutrino emission in different thermal processes at a wide range of temperature and density;and then based on these rates and profile of massive stars,we discuss the possibilities of detecting these signals using terrestrial neutrino detectors.Our calculations show that during and after the C burning in the center of massive stars,e± pair annihilation always contributes over 99%of the pre-supernova neutrino signals.For a 20 M(?)star located 200 pc away from the Earth,about 800(for NH case)and about 200(for IH case)ve events are expected at JUNO.These signals can act as a probe of stellar model and relevant fun-damental physics,can be used to determine neutrino mass order,and meanwhile,can provide a pre-warning of supernova explosion.The studies of solar neutrinos and neutrinos from massive stars and SN explosions,with typical energies of 0.1-100 MeV are one of the main subjects in neutrino astronomy.Apart from these MeV scale neutrinos,TeV-PeV neutrinos can also be produced in many astrophysical objects.Especially in recent few years,as more and more high energy(HE)neutrino events have been observed by the IceCube telescope located at the South pole,to understand the origin and to study the relevant astrophysical processes have become hot topics of growing concern today.SN explosions and GRBs are the most violent astrophysical processes in the universe,and are considered as promising sources for HE neutrinos.Generally relativistic jets are thought to power GRBs,and may also occur during the explosions of CCSNe.In these astrophysical environment,jets can create collisionless shocks,which are able to accelerate charged particles to very high energies via the Fermi mechanism.These accelerated protons can collide with stellar matter or electromagnetic radiations,and produce π± and K± which then decay to HE neutrinos.We will discuss these issues in more details in Chapter 3,and focus on the production of HE neutrinos in CCSNe and GRB s.Since HE neutrinos are produced by similar mechanisms,studying the characteristics of HE neutrinos from different astrophysical sources will be of great importance for us to pin down the origin of IceCube events.Jets always emerge along with accretion disks(ADs)in CCSNs and GRBs.With high temperatures and high densities,large amounts of neutrinos with Ev～10 MeV are produced in ADs and emitted.Under proper conditions,these AD neutrinos may meet with the HE neutrinos produced in jets/shocks and annihilate them.In Chapter 4,we will investigate the effects of AD neutrinos on HE neutrinos.Our studies show that with proper parameters for AD neutrinos,such as effective temperature and luminosity,the spectrum of HE neutrinos emerging from CCSNe/GRBs can be softened with an index change up to 0.4-0.5,which indeed agrees well the IceCube observations.Depending on different oscillation scenar-ios for AD neutrinos,the flavor content of HE neutrinos are also evidently affected.What’s more,there is an one-to-one correspondence between spectral change and flavor change.With more and more HE neutrino events accumulated at IceCube,the signatures above could be well tested,and could be of great significance for probing ADs as well as for understanding the origin of HE neutrinos at IceCube.Neutrino studies now are in the ascendant.The Nobel Prize in physics 2015 was rewarded to experimentalists Takaaki Kajita and Arthur B.McDonald,for their contributions in leading the experiments SuperK and SNO to confirm the existence of neutrino oscillation.The next-generation large neutrino detectors,such as JUNO and HyperK are now under construction.They are powerful machines to explore neutrino properties,and meanwhile provide perfect op-portunities to study neutrinos from the sky.As more and more HE neutrino events are accumu-lated at IceCube,significant progress will be made in HE neutrino astronomy in the near future.With great expectations,let’s start our wonderful journey into the area of neutrino astronomy.