Supernovae SN1987A OPERA Constraints on neutrino parameters. Supernova neutrinos. Ly Duong. January 25, 2012

Transcription

1 January 25, 2012

2 Overview Supernovae Supernovae Supernova types Core collapse model Neutrino properties Detection of neutrinos Data and analysis Experiment results Comparison with results Possible neutrino constraints

3 Supernova is a violent explosion of a dying star - like the pictured below.

4 From an observational point of view, supernovae are classified into many types and sub-types, since their light curves and spectra differ greatly.

5 Type II supernovae Supernovae Hydrogen in its spectrum Progenitor typically between [ ] M red giants Initial mass does not determine the type of spectrum.

6 Core collapse dynamics Iron is the last stage of fusion- the core contracts and the increased temperature causes photodissociation. e + N(Z) N(Z 1) + ν e (1) e + p n + ν e (2) The above processes reduce the electron pressure, until the Chandreskhar mass limit is reached. The electrons can no longer hold the weight and collapse begins.

7 Neutron star formation The electron neutrinos produced in the electron capture process can leave the core freely provided the density is low enough. When the density of the inner part of the core exceeds 3 x g cm 3, neutrinos are trapped in the collapsing material. Degenerate nucleon pressure stops collapse and inner core settles into hydrostatic equilibrium.

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9 Neutrino production Inside the core of proto-neutron star, neutrinos of all flavors are produced through pair annihilation, bremsstrahlung (e-n and n-n), plasmon decay and photo-annihilation. Electron neutrinos are produced by electron capture process. The only place where they can escape is where the mean free path is larger than the core radius - the neutrinosphere. Since neutrino interactions depend on flavor and energy, there are different energy-dependent neutrinospheres.typically range from km, with ν e at the outermost radius.

10 The averaged energies of neutrinos are roughly E νe = 10 MeV and E νx = 20 MeV These values are integrated and averaged over Lytime Duong and givensupernova by numerical neutrinos models.

11 Prompt shock Supernovae The electrons neutrinos initially produced are piled up behind the shock. The shock travels until it reaches a zone with lower density a few milliseconds after the bounce (shock breakout) and the neutrinos are released. If the shock still has enough energy to propagate through to the envelope and expel it within 100 ms of the core bounce, then we have a prompt explosion.

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13 Stalled shock Supernovae The shock loses energy in the photodissociation process and at about 300 km it stalls. Star mass becomes important here- material could back leading to runaway gravitational collapse. An explosion can still be achieved if the shock is revived by the energy deposition by the neutrino flux that is produced thermally in the proto-neutron star, or convection behind the shock.

14 Convection Supernovae Convective engine to drive heat from the core to behind the shock.

15 As the shock stalls, matter continues to fall behind it, and more neutrinos are created. Shock is far outside of neutrinospheres - neutrinos created here leave the star freely Creates a hump in the luminosity distribution To summarize shock mechanism: Prompt shock: characterized by peak of electron neutrinos, and then peak of neutrinos of all avours. Stalled shock: initial electron neutrino burst, then thermal neutrino with a prolonged luminosity maximum.

16 Core-collapse (delayed shock) is the current standard theory. The initial mass mass and metallicity of the star determines whether or not an explosion occurs.

17 detection Detection Four detectors report an unusual burst of activity- Kamiokande II, IMB, Baksan and LSD. LSD events were detected hours before the rest, Baksan did not initially report a burst. LSD and Baksan data are usually not used in analysis.

18 IMB Supernovae Detection IMB is a Cherenkov detector - energy and direction inferred by collected light. IMB s the background rate is negligible at 2 events per day. The registered events are due to inverse neutron decay, and the neutrino energy is given by: E νe = E e MeV (3)

19 Constructed energy spectrum Detection Measured neutrino energy (ν e ) = 15 MeV

20 Superluminal neutrinos The detectors mentioned in the previous section found the neutrino bursts within seconds of 7:35 UT Earliest sighting of light was at 10:38 UT - about a few hours later than the neutrino detection. Does this automatically indicate that neutrinos are superluminal?

21 Photon scattering It is expected that the envelope around the supernova is ionized Scattering occurs, impeding the photons Crab nebula - echos are detected about 7 ms after the pulse. Crab nebula density 10 3 cm 3. Material around SN1987a has density 10 9 cm 3. This means the photon delay would be = 7000 seconds 2 hours (4)

22 Contradictions with measurement? The LMC lies at a distance 1.6 x 10 5 lyrs away. The fraction difference between neutrinos and photons velocity can be at most δ = t d 10 9 (5) reports the lead time for neutrinos to be 60 ns over the 3 ms flight time. IF the results were to be correct, one would expect at least a factor of 1000 larger in the time difference.

23 Energy dependency Note that there is a huge difference in the energies between these neutrinos finds linear increase (1ns/2GeV). Extrapolating to the supernova energies, the ratio difference is still

24 Mass Supernovae Let us neglect oscillations to determine a model independent upperbound for the electron neutrino. v c 1 = 10 9 (6) E = γmc 2 E 1 ( ) 2 = mc 2 E = mc 2 mc ev

25 Conclusion Supernova explosion type Ib, Ic and II can be described by core collapse model (delayed shock revived by some mechanism - for example convection) Numerical models predict average energies E νe = 10 MeV and = 20 MeV E νx SN1987a results indicate energy of E νe = 15 MeV, quite close to model predictions An upper bound can be found for the mass of ν e as 10 2 ev, completely oscillation and model independent. SN1987a also provides an important test for the controversial case of the superluminal neutrino An energy dependence of neutrino velocity is proposed - result is not entirely impossible.