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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