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you might have suspected that my title is too timely , but as a matter of fact , it was a speculative one when it was proposed to one of the conveners in early may , much earlier than the announcement of @xmath1 appearance events at t2k @xcite .
the resulted six clean events which fulfill the criteria for electron - like events in sk strongly suggest that @xmath0 has a nonzero value .
their best fit value is around @xmath2 close to the chooz limit @xcite .
it was soon followed by the minos collaboration which also obtained a relatively large value of @xmath0 @xcite .
in fact , by taking account of these new data from t2k and minos into the global analysis the bari group concluded that it implies evidence for nonzero @xmath0 at confidence level ( cl ) higher than @xmath3 @xcite .
the best fit value is @xmath4 .
see also @xcite .
i believe it an urgent matter to discuss what are the consequences of suggested large values of @xmath0 though it must be confirmed by the further running of these experiments as well as the ongoing reactor @xmath0 experiments @xcite .
i stated in a talk in neutrino 2008 in christchurch that `` my personal prejudice is that @xmath0 is relatively large .
if it is true it further encourages us to proceed to search for lepton cp violation and determination of the mass hierarchy '' @xcite .
here is a brief recapitulation of the very simple reasoning which led me to the above statement .
the lepton flavor mixing matrix , the mns matrix @xcite , @xmath5 , is a `` relative matrix '' @xmath6 , where @xmath7 and @xmath8 denote the unitary matrices which diagonalize the charged lepton and the neutrino mass matrices , respectively .
the mns matrix is composed of the three angles @xmath9 ( @xmath10 ) and a cp violating phase @xmath11 , the lepton analogue of the quark kobayashi - maskawa phase @xmath12 @xcite .
two of the angles are known to be large , @xmath13 and @xmath14 .
then , it is highly unlikely that only the remaining angle @xmath0 is extremely small , unless there is a good reason for tuning only @xmath0 into a small value .
that is , if a symmetry exists such that @xmath0 vanishes at the symmetry limit , there is a natural reason why only @xmath0 is small among the three mixing angles .
there exists many examples for such symmetry including @xmath15 symmetry and discrete symmetries . for a review
see , e.g. @xcite .
it is my prejudice that the large @xmath0 suggested by t2k and minos measurement , if confirmed , would give a tough time for these symmetries which aims at explaining small @xmath0 , unless a large correction to the symmetry limit is shown to be naturally induced .
without having compelling reasons for large @xmath0 , one had to take into account the possibility that @xmath0 is small in thinking about ways for measurement of cp phase and determination of the neutrino mass hierarchy .
that is why many of the existing proposals for such apparatus naturally include those that have sensitivities to a tiny value of @xmath0 with varying limiting values .
they include , for example ( sorry for my arbitrary choice ) , neutrino factory @xcite , beta beam @xcite , superbeam enriched with two detectors @xcite , and their combination @xcite . for their sensitivities see e.g. , @xcite .
i wonder , however , that racing for @xmath0 sensitivity could have produced an atmosphere that the better facility is the one which has sensitivities to smaller values of @xmath0 . in seeing the t2k and minos indication for large @xmath0
, i believe that it is the time to start thinking about changing the gear to more relevant things .    what is good if @xmath0 is as large as @xmath16 ?
my first answer is that a wide range of possibilities becomes open for strategies and setups aiming at exploring lepton cp violation through neutrino oscillation .
i believe that in the context of conventional superbeam @xcite unambiguous evidence for lepton cp violation requires a megaton scale water cherenkov detector , or a @xmath17 kton liquid ar detector @xcite . yet , the suggested large value of @xmath0 , still need to be confirmed and assumed here , stimulates to examine other possibilities such as reactor - accelerator combined method @xcite , combining different accelerator measurement @xcite , or the one adding one more new facility which may or may not require an extensive cost @xcite .
some more exotic options are discussed in @xcite .
it is notable that the cern - memphys project , despite an almost vacuum setting , has a reasonable sensitivity to the mass hierarchy to @xmath18 before combining with atmospheric neutrino data @xcite .
sometime ago the cp sensitivity achievable by the reactor - accelerator combined method was examined in detail @xcite .
the results obtained in this analysis indicate that combining data taken and to be taken by all planned experiments is not enough to guarantee discovery of cp violation in @xmath11 coverage of more than 30% in most of the allowed region of @xmath0 even though one assumes that the mass hierarchy is known .
it should be noticed , as observed and emphasized in @xcite , that the cp sensitivity is severely damaged by the parameter degeneracy due to unknown neutrino mass hierarchies @xcite .
for a recent overview of the parameter degeneracy see @xcite .
i believe that the one of the most important implications of the assumed large value of @xmath0 is that one can think of `` all in one '' ( _ obento _ in japanese , everything in a lunch box ) approach . as we saw in the above , a traditional approaches for cp and the mass hierarchy entailed in aggressive proposals @xcite to include a very small value of @xmath0 within the scope . though powerful and
the redundancy is of course welcome , they may not be the optimal cost - effective apparatus for large @xmath0 .
to my opinion , what is good in living in the world with large @xmath0 is that one can achieve the same goal by less expensive `` all in one '' setting .
more concretely , i mean by it a single detector ( such as a megaton water cherenkov detector ) assuming prior existence of intense neutrino beam .
timely enough the loi of hyper - kamiokande ( hyper - k ) project has just appeared @xcite .
therefore , let us briefly discuss the spirit of the `` all in one '' approach by taking this concrete setting .    with intense neutrino beam from 1.66 mw proton driver at j - parc it is demonstrated that hyper - k with 0.56 megaton fiducial mass has a superb performance for discovery of cp violation , which covers most of the region of @xmath0 allowed at 3@xmath19 cl @xcite .
see fig .
24 of @xcite which assumes the known mass hierarchy and for 1.5 ( 3.5 ) years of neutrino ( antineutrino ) running .
it shows a remarkable accuracy of determination of @xmath11 with 1@xmath19 error smaller than 20 degree for @xmath20 .
however , if the mass hierarchy is unknown the analysis suffers from the sign-@xmath21 degeneracy @xcite .
while allowing clean measurement of cp violation , a drawback of setting with such short baseline is that it is difficult to determine the mass hierarchy , another important goal of future neutrino experiments .
that is why many proposals for determining all the unknowns in neutrino parameters exploit baselines of several thousand km .
if @xmath0 is large , however , the mass hierarchy can be determined by high - statistics observation of atmospheric neutrinos , the idea discussed by many authors .
see e.g. , @xcite for early references .
the analysis presented in @xcite indicates that the sensitivity to the mass hierarchy depend very much on which octant @xmath22 lives but it can be carried out by @xmath23 years running of hyper - k for large @xmath0 .
it should also be mentioned that having other capabilities such as proton decay discovery is of crucial importance .
i emphasize that the realization of `` all in one '' approach is , of course , not limited to this particular version discussed above .
but , it is certainly encouraging to see the concrete example appeared which is armed with the realistic simulations based on their long - term experiences at super - k .
is theoretical treatment of neutrino oscillation well oiled when a large value of @xmath0 close to the chooz limit is established ?
for example , does the well known treatment of the degeneracy @xcite need modification ?
if yes , to what extent ?     computed numerically as a function of energy ( green dashed line ) , the one calculated by the cervera _
_ formula ( blue dash - dotted line ) , and with our formula with large @xmath0 corrections ( red solid line ) .
the left and the right panels are for baselines @xmath24 km and @xmath25 km for which the matter density is taken as 2.8 g/@xmath26 and 3.6 g/@xmath26 , respectively . , title="fig:",width=241 ]   computed numerically as a function of energy ( green dashed line ) , the one calculated by the cervera _
_ formula ( blue dash - dotted line ) , and with our formula with large @xmath0 corrections ( red solid line ) . the left and the right panels are for baselines @xmath24 km and @xmath25 km for which the matter density is taken as 2.8 g/@xmath26 and 3.6 g/@xmath26 , respectively .
, title="fig:",width=241 ]    to answer these questions a framework dubbed as `` @xmath27 perturbation theory '' ( @xmath28 ) has been formulated @xcite by assuming @xmath29 , which roughly corresponds to the chooz limit . by doing
so one can systematically compute the large-@xmath0 corrections to the cervera _
et al . _ formula @xcite
. while large corrections arise in certain limited region of energy and baseline as indicated in fig .
[ fpemu_1000 - 4000 ] , one can show on general ground that the correction terms are of order @xmath30 .
( by general argument one can show that they are the terms proportional to either @xmath31 or @xmath32 @xcite . ) therefore , the large-@xmath0 correction to the degenerate solutions obtained with the cervera _
_ formula , generally speaking , is not sizable .
see @xcite for more detail .
assuming that the t2k and minos hints for large @xmath0 will be confirmed , i argued that we are at the turning point of thinking of our strategy of how to explore the remaining unknowns in the lepton flavor mixing . in particular , it allows us to rely on `` all in one '' approach .
i hope that the personal view can be strengthened by further exploration toward nufact 2012 .
this work was supported in part by kakenhi , grant - in - aid for scientific research no .
23540315 , japan society for the promotion of science .
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as indicated by the recent data obtained by the t2k and the minos experiments @xmath0 can be large , even be comparable to the chooz limit . assuming that it will be confirmed by ongoing reactor and accelerator experiments i discuss its possible implications in the context of how to explore the remaining unknowns in the lepton flavor mixing . while it opens wide range of possibilities to explore cp and the mass hierarchy , 
 i emphasize that the large @xmath0 allows us to take `` all in one '' ( everything in a lunch box ) approach .