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Neutrinos from Supernovae

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Neutrinos from Supernovae Proc Indian Natn Sci Acad, 70, A No. 1, January 2004, pp. 123–133 c Printed in India. Neutrinos from Supernovae a † SANDHYA CHOUBEY ¡ AND KAMALES KAR ¡ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton S017 1BJ (UK) †Theory Group, Saha Institute of Nuclear Physics, Kolkata 700 064 (India) (Received 10 January 2003; Accepted 17 July 2003) In this review, the effect of flavor oscillations on the neutrinos released during supernova explosion after core collapse is described. In some scenarios there is a large enhancement of the number of events compared to the no oscillation case. Various other features associated with supernova neutrinos are also discussed. Key Words: Core Collapse Supernova; Neutrino Mass and Mixing; Neutrino Detection 1 Introduction Sudbury Neutrino Observatory (SNO) through elec- tron scattering and charged/neutral current dissocia- February 23, 1987 saw the birth of a new era in tion of heavy water respectively. The recent results an- astrophysics-extra-solar system neutrino astronomy. nounced by the KamLAND reactor experiment gives The supernova explosion in the Large Magellanic for the first time conclusive evidence for neutrino os- Cloud (LMC) at a distance of about 50 kpc was not cillation using a terrestrial neutrino source and con- only the closest visual supernova since Kepler but was firms the Large Mixing Angle (LMA) solution to the also the source of neutrinos detected at the terrestrial solar neutrino problem. Thus the present day interest detectors of Kamiokande (KII) and IMB giving rise to in supernova neutrinos lies around the question: if you 11 and 8 events respectively. have a galactic supernova event today what would be The next few years saw great excitement in this the number of events and their time and energy dis- field. Astrophysics interacted with particle physics tributions in the large number of neutrino detectors intimately. From the number and the energy dis- in operation. The other related question is whether tribution of the observed neutrinos one tried to ex- one can get a signature of neutrino oscillation mecha- tract information about the stellar core and check nism from the observed data and also how other neu- them with model predictions. On the other hand trino properties get constrained. Information about the these neutrinos also gave particle physics constraints mechanisms of the supernova explosion is also an area on neutrino properties. In the last few years inter- of huge interest. est in this area got rejuvenated by the finding that In this review we survey some of these issues. neutrinos do have non-zero mass and the flavors do In section 2 we give a brief overview of the physics mix when they travel. This conclusion was reached of type II supernovae and the emission of neutrinos through the analysis of the atmospheric neutrinos de- from them. Section 3 introduces the subject of neu- tected at the Superkamioka (SK) along with their trino oscillation and the impact of vacuum and matter zenith angle dependence and the observation of the enhanced oscillation on the supernova neutrinos from deficit of detected solar neutrinos by the Chlorine the core. Section 4 describes the expected number of and Gallium radiochemical detectors and at SK and events in the terrestrial detectors for the different mass and mixing scenarios. Finally section 5 briefly states a Now at INFN Sezione di Trieste and Scuola Internazionale the other connected issues of supernova neutrino de- Superiore di Studi Avazanti, I-34014, Trieste, Italy. tection. 124 SANDHYA CHOUBEY AND KAMALES KAR 2 Type II Supernovae and Neutrino Emission and so the νe “neutrinosphere” has the largest radius and smallest temperature. In this article we shall as- Stars of masses larger than 8M ¢ after burning for sume that the three types of neutrino gas have Fermi- millions of years collapse when the nuclear reac- Dirac distributions with temperatures 11, 16 and 25 tions in the core stop with matter consisting mostly MeV for νe, ν¯e and νx respectively. of 56Fe like nuclei. This collapse proceeds very fast An important role played by neutrinos in type (timescale of the order of tens of milliseconds) and II supernovae is in the process of “delayed neutrino stops in the central region when its density goes be- heating”5 . In almost all simulations for large mass yond the nuclear matter density with a strong shock stars one sees that the shock wave moving outward starting to travel outward1. This shock wave, even- fast loses energy in dissociating the nuclei in the over- tually hitting the outer mantle in a few seconds and lying matter and soon becomes an accretion shock. supplying the explosion energy of a few times 1051 This shock gets revitalized over the much longer ergs, is believed to be the cause of type II supernova timescale of seconds through the absorption of a small explosion. During this process, the binding energy re- fraction of the thermal neutrinos that radiate out with leased comes out almost completely as neutrinos and each neutrino depositing energy of the order of 10 antineutrinos of three different flavors (e, µ and τ) in MeV. Large convection in the central regions also the “cooling phase” with the total energy release of helps this process. the order of 1053 ergs. Let us discuss the emission of the neutrinos in some more detail. Firstly during 3 The Neutrino Oscillation Probabilities the early stage of the collapse (densities less than 1012 g/cc) neutrinos are produced through neutronization να The flavor eigenstate created inside the super- ν nova can be expressed as a linear superposition of the £¥¤§¦ ¨ © ¦ ¤ ¨ ©¤ e N Z N 1 Z 1 e (1) ν ν α ∑ mass eigenstates such that i Uαi i , where U is the unitary mixing matrix and the sum is over N ν α ν neutrino states. After time t, the initial evolves £¤ ¤ e p n e (2) iEit ¦ © ν ν α ∑ £ to t i e Uαi i where Ei is the energy of th where only ν (not ν¯) are produced. At lower densi- the i mass eigenstate. Then the probability of find- e e ν ν ties these neutrinos have mean free path much larger ing a flavor β in the original α beam after traveling than the core radius and hence escape. But the to- a distance L in vacuum is given by tal energy of these neutronization neutrinos is much δ Pαβ αβ smaller than that in the cooling phase. Even then it ∆ 2 is possible to detect them for nearby galactic super- 2 mi jL ¨ 2 4 ∑ UαiUβiUα jUβ j sin 1 27 (3) novae at distances within 1 kpc . These neutrinos can E j i give information about the temperature and composi- ∆ 2 2 2 tion of the core. where mi j mi m j is the mass squared difference. The main neutrino emission is during the cooling Over the last few years the idea that neutrinos are phase where the thermal ν/ν¯ are produced through not massless but have small masses has become estab- pair production and other processes3 . Out of these lished as a result of Super-Kamiokande (SK) and SNO νµ , ντ , ν¯µ and ν¯τ , called collectively as νx, interact which have firm evidence for atmospheric and solar 6 11 with matter only through neutral current whereas νe neutrino oscillations . The SK atmospheric neu- 2 3 2 ν ∆ and ¯ have both charged current and neutral current trino data demand m 3 10 £ eV and almost e 32 interaction with matter. As the matter is neutron-rich maximal mixing (sin2 2θ 1) 6 while the global so- 23 ν ν ∆ 2 the ’s interact more with matter than the ¯’s. These lar neutrino data is best explained if m 6 1 e e 21 5 2 2 θ 10, 12 £ neutrinos deep inside the core are in equilibrium with 10 eV with large mixing angles (tan 0 41) . the ambient matter and their energy distributions are Very recently the KamLAND reactor antineutrino dis- close to Fermi-Dirac as seen through simulations and appearance experiment 13 provided conclusive con- through the analysis of 1987A neutrinos4 . As the stel- firmation of the LMA solution to the solar neutrino lar core has a strong density gradient, electron type problem, with mass and mixing parameters absolutely neutrinos can stay in equilibrium upto larger radius consistent with the solar neutrino results. The global NEUTRINOS FROM SUPERNOVAE 125 14 n ' ρ & analysis of the solar and the KamLAND data gives very well approximated by a power law r Cr " ∆ 2 5 2 2 θ 18 ! " m 7 17 10 eV and tan 0 44. The with n 3 in the core . 2 CHOOZ reactor experiment restricts sin θ 0 1 for Neutrino oscillation probabilities may also be sig- 13 # ∆ 2 $ 3 215 m 10 " eV . The only other positive signal for nificantly affected inside Earth as the neutrinos tra- neutrino oscillations comes from the accelerator ex- verse the Earth matter19. Thus the neutrino oscillation periment LSND which requires ∆m2 % eV2 and mix- probability is given by 2 θ % 3 ing angle small (sin 2 10 " ). To include LSND N in the framework of oscillation one needs to extend m P ∑ Pα P + β (6) the number of neutrino generations to four, or in other αβ i i , i * 1 words, include a sterile neutrino. However with the latest SNO data on solar neutrinos and the final data where from SK on atmospheric neutrinos, both the “2+2” N 2 2 m m m and “3+1” 4-generation scenarios fail to explain the Pα ∑ Uα ν ν (7) 1 i j i 0 j - - - - global neutrino data.
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