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2 | Authors: J.E.Campagne,M.Maltoni,M.Mezzetto,T.Schwetz |
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3 | |
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4 | Title: Physics potential of the CERN-MEMPHYS neutrino oscillation project |
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5 | |
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6 | |
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7 | We thank the referee for his/her positive evaluation or our work, the |
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8 | detailed reading of the manuscript, and the critical comments. Below |
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9 | we answer to the criticisms, and list the corresponding changes in the |
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10 | text. We hope that with these changes our work is suitable for |
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11 | publication. |
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12 | |
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13 | |
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14 | REFEREE: (1) One of the most surprising feature of the results |
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15 | obtained in this paper is that the mass hierarchy can be resolved up |
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16 | to sin22theta13 =0.03 just by combining SPL and |
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17 | betabeam(Fig.16). However, the authors seem to fail to place enough |
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18 | emphasis on this unexpected result. I understood that some explanation |
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19 | is already offered by the authors in page 25. What are the key |
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20 | experimental features which enable the resolution? |
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21 | |
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22 | ANSWER: As we explain on p. 26 (previously p. 25), this interesting |
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23 | effect appears because of a delicate interplay of the (tiny) matter |
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24 | effect in all four CP and T conjugate channels. This can also be seen |
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25 | from Fig. 5, where the solutions with the wrong hierarchy appear at |
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26 | slightly different locations for BB and SPL. We belief that the |
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27 | explanation given on p. 26 is clear enough. A more detailed |
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28 | investigation of this effect goes beyond the scope of the paper. |
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29 | |
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30 | |
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31 | |
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32 | REFEREE: (2)The authors view of the current status of the ideas for |
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33 | future projects seems to be a bit outdated. Now another option of T2K |
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34 | upgrade, which is called as T2KK, is available. In this option people |
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35 | claimed that all the eight-fold parameter degeneracy can be lifted in |
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36 | situ without relying on combination with atmospheric neutrinos, |
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37 | thereby reserving the latter as a redundant cross check. Also they use |
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38 | superbeam only, avoiding a costly beta beam construction. From |
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39 | outsiders view it looks like the better option within the scope of |
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40 | megaton class water Cherenkov than CERN-MEMPHYS. Even though the |
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41 | authors do not fully treat the T2KK setting in their analysis, it |
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42 | would be nice if the authors comments on comparison between these two |
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43 | strategies. |
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44 | |
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45 | ANSWER: In our work we have confined ourselves to the standard T2HK |
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46 | setup, since the porpose of our work is not a T2K optimization study |
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47 | investigating various configurations for that experiment. In contrast, |
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48 | here T2HK mainly serves as a point of reference to which we compare |
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49 | the CERN--MEMPHYS experiments. For this aim we prefer to stick to the |
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50 | ``minimal'' one-detector configuration at a relatively short baseline, |
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51 | since two-detector setups with very long baselines clearly represent a |
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52 | different class of experiments whose consideration goes beyond the |
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53 | scope of the present work. We have added a paragraph to say this |
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54 | explicitly on p. 4. |
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55 | |
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56 | Moreover, we have added at several places in the text comments on the |
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57 | sensitivities of T2KK obtained in Refs. [34,35], which are the |
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58 | previous Ref. [60] and the more recent T2KK publication which we have |
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59 | added to the references. Specifically, we comment on T2KK on p. 22 |
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60 | (CPV), on p. 26 (mass hierarchy) and on p. 27 (octant degeneracy). In |
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61 | addition we mention also the the wide band beam idea in relation to |
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62 | the mass hierarchy deterimation, referring to the analysis [74]. |
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63 | |
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64 | |
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65 | |
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66 | REFEREE: (3)The most serious problem in appearance neutrino experiment |
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67 | with water Cherenkov detector is the background issue, in particular |
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68 | pi0 rejection. While the authors refer Ref. [51] for 'tighter cut' |
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69 | the reference is not informative at all. Also, one of the advantage of |
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70 | the present analysis, as compared to theprevious SPL studies, is that the |
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71 | authors take into account the spectrum informations. But, at such low |
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72 | energies it is highly nontrivial to reconstruct neutrino energy |
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73 | because of Fermi motion. The authors description of the procedure is |
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74 | too brief to allow understanding such high performance of the |
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75 | method. The reasonably informative description should be provided |
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76 | for both aspects. |
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77 | |
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78 | ANSWER: We quoted reference [53] (previous [51]) because it's the only |
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79 | paper where the SPL analysis has been described. It had been performed |
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80 | by D. Casper from the SuperKamiokande collaboration. We don't have |
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81 | today all the details of this analysis, but the pi0 background can be |
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82 | understood from general principles. We have added the following text |
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83 | in sec. 3.3: |
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84 | |
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85 | "Following Ref. [53], the $\pi^o$ background is reduced using a tighter |
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86 | PID cut compared to the standard Super-Kamiokande analysis used in |
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87 | K2K, but the cuts are looser than for T2K. Indeed, at SPL energies the |
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88 | $\pi^o$ background is less severe than for T2HK. This is because the |
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89 | resonant cross section is suppressed, and the produced pions have an |
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90 | energy where the angle between the two gammas is very wide, leading to |
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91 | a small probability that the two gamma rings overlap. This results in |
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92 | a higher signal efficiency of SPL compared to T2HK (60\% against 40\%) |
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93 | and a smaller rate of $\pi^o$ background." |
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94 | |
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95 | |
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96 | Regarding the energy reconstruction, we think to have fully |
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97 | illustrated the problem with Figure 2 and the related discussion in |
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98 | sec. 3.2. We are using the same lepton momentum smearing function as |
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99 | SK and we are using the most updated version of the Nuance neutrino |
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100 | generator, that is worldwide considered the reference for neutrino |
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101 | generators, and Fermi motion is taken into account. In ref. [34] |
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102 | (previous [60]) the authors quote an energy resolution of 80 MeV for |
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103 | QE events. By applying our methods we have 77 MeV resolution in their |
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104 | energy range. |
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105 | |
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106 | |
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107 | |
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108 | REFEREE: (4)The Dm2 sensitivity of T2HK as presented in Table 4 |
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109 | appears too good to be true. It is strongly believed that it cannot be |
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110 | less than 2 |
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111 | |
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112 | ANSWER: We have added the following text to the discussion of Tab. 4: |
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113 | |
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114 | "In the interpretation of the numbers given in Tab. 4 one should |
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115 | consider that at accuracies below 1\% systematics might become |
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116 | important, which are not accounted for here. We do include the most |
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117 | relevant systematics (see Secs. 2 and 3), however, at that level |
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118 | additional uncertainties related to, for example, the spectral shapes |
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119 | of signal and/or background, or the energy calibration might |
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120 | eventually limit the accuracy." |
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121 | |
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122 | |
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123 | |
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124 | REFEREE: (5) CP sensitivity of T2HK obtained by the authors is |
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125 | significantly different from that estimated by T2KK people. With the |
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126 | same 5% error the former is up to sin22theta13 =2x10-3,but the latter |
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127 | extends to much smaller theta13 region, as shown in one of T2KK paper, |
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128 | Fig.7 in Ref.[60]. The authors of the paper include SK |
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129 | experimentalists and hence their treatment of the errors might be more |
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130 | appropriate than the present analysis. Therefore, at least some |
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131 | comments must be made on this discrepancy. |
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132 | |
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133 | ANSWER: In Ref. [34] (previously Ref. [60]) the systematical errors |
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134 | are taken to be correlated between neutrino and antineutrino data (see |
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135 | eqs. 3 and 4 of [34]), whereas in our calculation they are assumed to |
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136 | be uncorrelated. We have verified that this has a notable impact on |
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137 | the CPV sensitivity and can explain the different results. We added a |
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138 | corresponding remark in the text on p. 22. Note that also for the |
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139 | CERN-MEMPHYS experiment we assume the errors to be uncorrelated, so we |
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140 | compare all experiments on equal footing. |
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141 | |
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142 | |
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143 | REFEREE: (6)It is not obvious which figure is nu only and which |
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144 | combines nu and anti-nu. A comment in each caption might be helpful to |
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145 | the readers. |
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146 | |
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147 | ANSWER: We added nu and anti-nu running times in all figure captions. |
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148 | |
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149 | REFEREE: (7)The fact that the spectral information solves the |
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150 | intrinsic degeneracy is noticed for T2K I about 2 years ago in |
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151 | Ref. [60]. |
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152 | |
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153 | ANSWER: We do not claim to have discovered this here for the first |
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154 | time. We add references to [30,32,34] in the discussion of Fig. 6. |
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155 | |
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156 | |
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157 | REFEREE: (8) The authors do not explain the reason why theta13 |
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158 | sensitivity has a sharp minimum at delta = pi, as seen in Fig.9; It is |
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159 | unusually abrupt drop of more than an order of magnitude. |
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160 | |
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161 | ANSWER: We add the following paragraph to the discussion of Fig. 9: |
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162 | |
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163 | "The peak of the sensitivity curves around $\delCP \approx \pi$ |
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164 | appears due to the interplay of neutrino and antineutrino data. For |
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165 | the Super Beams neutrino (antineutrino) data are most sensitive in the |
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166 | region $\pi \lesssim \delCP \lesssim 2\pi$ ($0 \lesssim \delCP |
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167 | \lesssim \pi$), and opposite for the \BB, compare also Fig. 14 in |
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168 | Sec. 6.1. The particular shape of the sensitivity curves emerges from |
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169 | the relative location of the corresponding curves for neutrino and |
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170 | antineutrino data, which is controlled by the $L/E_\nu$ value where |
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171 | the experiment is operated and the value of $|\Delta m^2_{31}|$. The |
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172 | fact that the peak is most pronounced for the \BB\ follows from the |
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173 | somewhat smaler $L/E_\nu$ of the \BB\ compared to the Super Beams, |
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174 | whereas the shapes for SPL and T2HK are similar because of the similar |
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175 | $L/E_\nu$ values. |
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176 | |
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