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Exploring Leptonic CP Violation by Reactor and neutrino Superbeam Experiments
Nuclear Physics B (Proc. Suppl.) 143 (2005) 545 www.elsevierphysics.com
Exploring Leptonic CP Violation by Reactor and neutrino Superbeam Experiments Hiroaki Sugiyamaa∗ Theory Group, KEK, Tsukuba, Ibaraki 305-0801, Japan
The measurement of the CP-violating phase δ in the lepton sector is one of the most important goal in future neutrino oscillation experiments. The conventional method to achieve that is the comparison of P (νµ → νe ) and P (ν µ → ν e ) to be measured in future superbeam experiments. The measurement of the oscillation probability with ν µ seems, however, to be a difficulty of the method because it needs rather long exposure time after the measurement of P (νµ → νe ); For example, 2yr measurement with νµ and following 6yr measurement with ν µ are planed for T2K experiment phase II (4MW beam and 540kt fiducial volume of the detector) which we take as a superbeam experiment throughout this talk. In this talk, we see the possibility to extract information of δ quantitatively with the reactor neutrino measurement and the measurement of P (νµ → νe ) in the superbeam experiment. This method is interesting because the reactor experiment can run without waiting for the finish of the superbeam experiment with νµ beam in contrast with the measurement of P (ν µ → ν e ). The superbeam experiment with νµ beam gives a δ-depending allowed region of θ13 . On the other hand, the reactor experiment puts a bound on θ13 independently of δ. Then, the value of δ can be restricted by the combined analysis of those two measurements. The result of our analysis is shown in the figure. In the figure, we assumed best ∆m231 > 0 and θ23 = π/4 for simplicity. θ13 best and δ are experimental best-fit points or the values that nature chooses. GWth ·t·yr is the unit for the size of the reactor experiment, namely the ∗ This talk is based on [1] and supported by the Research Fellowship of Japan Society for the Promotion of Science for young scientists.
0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2005.01.210
10 GWth⋅t⋅yr 3
90% CL ( 1 d.o.f. ) ∆m231 >0 4MW,540kt,2yr
104 GWth⋅t⋅yr
0.16 0.14 0.12
sin22θbest 13
a
0.1 0.08 0.06 0.04 0.02 0
-0.4
-0.2
0
0.2
0.4
δbest/π thermal power times the detector volume times the exposure time. Insides of ellipses in the figure show the regions consistent with the hypothesis δ = 0 at 90%CL. Therefore, we can conclude that CP is violating in the lepton sector if nature chooses the values outside of the envelope of those ellipses. Although the sensitivity region is rather smaller than that of the conventional method which uses a long-term measurement with ν¯ superbeam, the early information about δ with the analysis will be helpful for later precision measurements with the conventional method. REFERENCES 1. H. Minakata and H. Sugiyama, Phys. Lett. B 580, 216 (2004) [arXiv:hep-ph/0309323], and references therein.
Exploring Leptonic CP Violation by Reactor and neutrino Superbeam Experiments Hiroaki Sugiyamaa∗ Theory Group, KEK, Tsukuba, Ibaraki 305-0801, Japan
The measurement of the CP-violating phase δ in the lepton sector is one of the most important goal in future neutrino oscillation experiments. The conventional method to achieve that is the comparison of P (νµ → νe ) and P (ν µ → ν e ) to be measured in future superbeam experiments. The measurement of the oscillation probability with ν µ seems, however, to be a difficulty of the method because it needs rather long exposure time after the measurement of P (νµ → νe ); For example, 2yr measurement with νµ and following 6yr measurement with ν µ are planed for T2K experiment phase II (4MW beam and 540kt fiducial volume of the detector) which we take as a superbeam experiment throughout this talk. In this talk, we see the possibility to extract information of δ quantitatively with the reactor neutrino measurement and the measurement of P (νµ → νe ) in the superbeam experiment. This method is interesting because the reactor experiment can run without waiting for the finish of the superbeam experiment with νµ beam in contrast with the measurement of P (ν µ → ν e ). The superbeam experiment with νµ beam gives a δ-depending allowed region of θ13 . On the other hand, the reactor experiment puts a bound on θ13 independently of δ. Then, the value of δ can be restricted by the combined analysis of those two measurements. The result of our analysis is shown in the figure. In the figure, we assumed best ∆m231 > 0 and θ23 = π/4 for simplicity. θ13 best and δ are experimental best-fit points or the values that nature chooses. GWth ·t·yr is the unit for the size of the reactor experiment, namely the ∗ This talk is based on [1] and supported by the Research Fellowship of Japan Society for the Promotion of Science for young scientists.
0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2005.01.210
10 GWth⋅t⋅yr 3
90% CL ( 1 d.o.f. ) ∆m231 >0 4MW,540kt,2yr
104 GWth⋅t⋅yr
0.16 0.14 0.12
sin22θbest 13
a
0.1 0.08 0.06 0.04 0.02 0
-0.4
-0.2
0
0.2
0.4
δbest/π thermal power times the detector volume times the exposure time. Insides of ellipses in the figure show the regions consistent with the hypothesis δ = 0 at 90%CL. Therefore, we can conclude that CP is violating in the lepton sector if nature chooses the values outside of the envelope of those ellipses. Although the sensitivity region is rather smaller than that of the conventional method which uses a long-term measurement with ν¯ superbeam, the early information about δ with the analysis will be helpful for later precision measurements with the conventional method. REFERENCES 1. H. Minakata and H. Sugiyama, Phys. Lett. B 580, 216 (2004) [arXiv:hep-ph/0309323], and references therein.