[387] | 1 | \subsection{Supernova neutrinos} |
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| 2 | \label{sec:SN} |
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| 3 | |
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| 4 | \subsubsection{Core-collapse} |
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| 5 | The large mass of a MEMPHYS-type detector means that the sample of |
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| 6 | events collected during a supernova explosion would outnumber that |
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| 7 | of all other existing detectors. For instance, for a supernova at |
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| 8 | 10 kpc $\sim 2\times 10^5$ events would be observed, |
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| 9 | whereas Super-Kamiokande (22.5 kt) will see only 9,000 events (see Figure |
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| 10 | \ref{fig:SN}, from ref.~\cite{Fogli:2004ff}). |
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| 11 | These numbers |
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| 12 | are to be compared with the 19 (11 for Kamiokande and 8 for IMB) |
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| 13 | events coming from the SN1987A in the Large Magellanic Cloud (50 kpc). |
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| 14 | |
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| 15 | \begin{figure} |
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| 16 | \begin{center} |
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| 17 | \epsfig{figure=./figures/snburst.eps,width=8cm,height=8cm} |
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| 18 | \caption{\it % Figure to be redone for 440 kt! |
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| 19 | The number of events in a 400 kt water \v{C}erenkov detector (left scale) |
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| 20 | and in SK (right scale) in all channels and in the individual |
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| 21 | detection channels as a function of distance for a supernova |
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| 22 | explosion \cite{Fogli:2004ff}.} |
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| 23 | \label{fig:SN} |
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| 24 | \end{center} |
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| 25 | \end{figure} |
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| 26 | |
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| 27 | An estimated number of $3\pm 1$ supernovae occur in our galaxy and |
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| 28 | its satellites every century. |
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| 29 | A MEMPHYS-type detector would also be sensitive to supernovae |
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| 30 | occurring throughout |
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| 31 | the local group of galaxies. For a supernova explosion in Andromeda |
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| 32 | (730-890 kpc), |
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| 33 | the proposed detector will collect roughly the same amount of neutrinos |
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| 34 | detected for the SN1987A. |
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| 35 | A handful of events might be seen even at a distance as large as 3 Mpc. |
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| 36 | |
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| 37 | One of the unsolved problems in astrophysics is the mechanism of supernova |
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| 38 | core-collapse. |
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| 39 | Inverse beta decay events from the silicon burning phase preceding |
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| 40 | the supernova explosion have very low (sub-threshold) positron |
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| 41 | energies, and could only be detected through neutron capture by adding |
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| 42 | Gadolinium \cite{Beacom:2003nk}, |
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| 43 | provided that they can be statistically distinguished from background |
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| 44 | fluctuations. |
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| 45 | The silicon burning signal should then be seen with a statistical |
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| 46 | significance of 2$\div$8 standard deviations at a reference distance of 1 |
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| 47 | kpc. Unfortunately, at the |
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| 48 | galactic center ($\sim$10 kpc) the estimated silicon burning signal would |
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| 49 | be 100 times smaller and thus unobservable. |
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| 50 | |
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| 51 | There are better prospects to observe the neutronization burst from a |
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| 52 | galactic supernova |
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| 53 | by means of elastic scattering on electrons, including contributions |
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| 54 | from all flavors: a 0.4 Mton detector might observe such signal with a |
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| 55 | statistical significance at the level of 4 standard deviations. |
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| 56 | At the distance of the Large Magellanic Cloud, however, the |
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| 57 | sensitivity drops dramatically. |
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| 58 | |
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| 59 | Returning to the overall rate in the inverse beta channel, |
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| 60 | the high statistics available for a |
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| 61 | galactic supernova explosion will allow many possible spectral |
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| 62 | analyses, providing insight both on the properties of the collapse |
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| 63 | mechanism and on those of neutrinos. |
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| 64 | |
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| 65 | For the first topic, an example is given in~\cite{Fogli:2004ff} |
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| 66 | in the context of shock-wave |
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| 67 | effects, based on the comparison of arrival times in different energy bins. |
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| 68 | |
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| 69 | Concerning the spectral properties which depend on neutrino |
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| 70 | oscillation parameters, |
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| 71 | it has been shown in \cite{Minakata:2001cd} that a detector |
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| 72 | like the proposed one, considering the inverse-beta channel alone with |
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| 73 | the current best values of solar neutrino oscillation parameters, |
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| 74 | would allow the determination of the parameter $\tau_E$, defined as |
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| 75 | the ratio of the average energy of time-integrated neutrino spectra |
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| 76 | $\tau_E=\langle E_{\bar\nu_\mu}\rangle /\langle E_{\bar\nu_e}\rangle$, |
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| 77 | with a precision at the level of few percent, to be compared with a |
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| 78 | $\sim$20\% error possible at Super-Kamiokande. This would make it possible to |
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| 79 | distinguish normal from inverted mass hierarchy, if |
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| 80 | $\sin^2\theta_{13}>10^{-3}$ \cite{Lunardini:2003eh}. |
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| 81 | In the region $\sin^2\theta_{13}\sim (3\cdot 10^{-6}-3\cdot |
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| 82 | 10^{-4})$, measurements of $\sin^2\theta_{13}$ are possible with a |
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| 83 | sensitivity at least an order of magnitude better than planned |
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| 84 | terrestrial experiments \cite{Lunardini:2003eh}. |
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| 85 | |
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| 86 | Up to now we have investigate supernova explosions occurring in our galaxy, |
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| 87 | however the calculated rate of supernova explosions within a distance of |
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| 88 | 10 Mpc is about one per year. Although the number of events from |
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| 89 | a single explosion at such large distances would be small, the signal |
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| 90 | could be separated from the background with the |
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| 91 | request to observe at least two events within a time window |
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| 92 | comparable to the neutrino emission time-scale ($\sim$10 sec), |
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| 93 | together with the full energy and time distribution of the |
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| 94 | events \cite{Ando:2005ka}. |
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| 95 | In a MEMPHYS-type detector, |
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| 96 | with at least two neutrinos observed, a supernova could be identified |
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| 97 | without optical confirmation, so that the start of the light curve |
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| 98 | could be forecasted by a few hours, along with a short list of probable |
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| 99 | host galaxies. This would also allow the detection of supernovae |
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| 100 | which are either heavily obscured by dust or are optically |
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| 101 | dark due to prompt black hole formation. |
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| 102 | Neutrino detection with a time coincidence could therefore act |
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| 103 | as a precise time trigger for other supernova detectors |
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| 104 | (gravitational antennas or neutrino telescopes). |
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| 105 | |
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| 106 | Finally, one can notice that electron elastic scattering events would |
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| 107 | provide a pointing accuracy on the supernova |
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| 108 | explosion of about $1^\circ$. |
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| 109 | |
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| 110 | \subsubsection{Diffuse Supernova Neutrinos} |
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| 111 | |
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| 112 | An upper limit on the flux of |
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| 113 | neutrinos coming from all past core-collapse supernovae |
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| 114 | (the Diffuse Supernova Neutrinos~\footnote{We |
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| 115 | prefer to denote these neutrinos as ``Diffuse'' rahter than ``Relic'' |
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| 116 | to avoid confusion with the primordial neutrinos produced one second |
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| 117 | after the Big Bang.}, DSN) has been set by the |
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| 118 | Super-Kamiokande experiment \cite{Malek:2002ns}, |
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| 119 | however most of the estimates are below this limit and therefore |
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| 120 | DSN detection thorough inverse |
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| 121 | beta decay appears to be feasible at a megaton scale water \v{C}erenkov |
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| 122 | detector. |
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| 123 | |
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| 124 | Typical estimates for DSN fluxes |
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| 125 | (see for example \cite{Ando:2004sb}) predict an event |
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| 126 | rate of the order of 0.1$\div$0.5 cm$^{-2}$s$^{-1}$MeV$^{-1}$ |
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| 127 | for energies above 20 MeV, a cut |
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| 128 | imposed by the rejection of spallation events. |
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| 129 | After experimental selections analogous to the ones |
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| 130 | applied in the Super-Kamiokande analysis, such events are retained with an |
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| 131 | efficiency of about 47\% for energies between 20 and 35 MeV; this is |
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| 132 | to be considered as a very conservative estimate at MEMPHYS, where the |
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| 133 | bigger overburden will reduce the cosmic-muon induced background and |
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| 134 | less stringent selection criteria can be applied. |
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| 135 | Two irreducible backgrounds remain: atmospheric $\nu_e$ and $\bar\nu_e$, |
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| 136 | and decay electrons from |
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| 137 | the so called ``invisible muons'' generated by CC interaction of |
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| 138 | atmospheric neutrinos and having an energy below threshold for |
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| 139 | \v{C}erenkov signal. |
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| 140 | |
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| 141 | The spectra of the two backgrounds were taken from the |
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| 142 | Super-Kamiokande estimates |
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| 143 | and rescaled to a fiducial mass of 440 kton of water, while the |
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| 144 | expected signal was computed according to the model called LL |
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| 145 | in \cite{Ando:2004sb}. |
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| 146 | The results are shown in Fig.~\ref{fig:snr}: |
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| 147 | the signal could be observed with a statistical significance of about |
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| 148 | 2 standard deviations after 10 years. |
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| 149 | |
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| 150 | \begin{figure} |
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| 151 | \begin{center} |
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| 152 | \epsfig{figure=./figures/snrelic.eps,width=13cm} |
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| 153 | \caption{\it Diffuse Supernova Neutrino signal and backgrounds (left) |
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| 154 | and subtracted signal with statistical errors (right) in a 440 kt |
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| 155 | water \v{C}erenkov detector with a 10 years exposure. |
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| 156 | The selection efficiencies of SK were assumed; |
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| 157 | the efficiency change at 34 MeV is due to the spallation cut.} |
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| 158 | \label{fig:snr} |
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| 159 | \end{center} |
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| 160 | \end{figure} |
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| 161 | |
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| 162 | |
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| 163 | As pointed out in \cite{Fogli:2004ff}, |
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| 164 | with addition of Gadolinium \cite{Beacom:2003nk} |
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| 165 | the detection of the captured neutron |
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| 166 | would give the possibility to reject neutrinos other than $\bar\nu_e$ |
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| 167 | from spallation events and from atmospheric origin, and the |
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| 168 | detection threshold could be lowered significantly - to about 10 MeV - |
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| 169 | with a large gain on signal statistics. |
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| 170 | The tails of reactor neutrino spectra would |
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| 171 | become the most relevant source of uncertainty on the background. In |
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| 172 | such condition, not only would the statistical significance of the |
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| 173 | signal become much higher, but is would even be possible to |
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| 174 | distinguish between different theoretical predictions. For example, |
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| 175 | the three models considered in \cite{Ando:2004sb} |
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| 176 | would give 409, 303 and 172 events respectively above 10 MeV. |
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| 177 | An analysis of the expected DSN spectrum that would be observed |
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| 178 | with a Gadolinium-loaded water \v{C}erenkov detector has been carried out |
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| 179 | in \cite{Yuksel:2005ae}: the possible limits on the emission parameters of |
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| 180 | supernova $\bar\nu_e$ emission have been computed for 5 years running of |
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| 181 | a Gd-enhanced SuperKamiokande detector, which would correspond to 1 year |
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| 182 | of one MEMPHYS shaft, and are shown in Fig.~\ref{fig:sndpar}. |
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| 183 | Detailed studies on characterization of the backgrounds, however, |
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| 184 | are needed. |
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| 185 | |
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| 186 | \begin{figure} |
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| 187 | \begin{center} |
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| 188 | \epsfig{figure=./figures/sndpar.eps,width=8cm} |
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| 189 | \caption{\it Possible 90\% C.L. measurement of the emission parameters |
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| 190 | of supranova $\bar\nu_e$ emission after 5 years running of |
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| 191 | a Gd-enhanced Super-Kamiokande detector, which would correspond to 1 year |
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| 192 | of one MEMPHYS shaft. The points corespond to different assumptions on |
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| 193 | the average energy and integrated luminaosty: A,B,C are taken at the |
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| 194 | edge of the region excluded by SK, D is often regarded aas the |
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| 195 | canonical values for $\bar\nu_e$ emission before neutrino mixing. See |
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| 196 | \cite{Yuksel:2005ae}. } |
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| 197 | \label{fig:sndpar} |
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| 198 | \end{center} |
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| 199 | \end{figure} |
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| 200 | |
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| 201 | %\subsubsection{Gravitational trigger and GRBs (???)} |
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| 202 | |
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| 203 | |
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