Optimization of neutrino oscillation facilities for large θ 13
Pilar Coloma
1
Enrique Fernandez-Martinez
0
Open Access
0
CERN Physics Department, Theory Division
, CH-1211 Geneva 23,
Switzerland
1
Departamento de Fsica Teorica & Instituto de Fsica Teorica, Universidad Autonoma de Madrid
, Campus de Cantoblanco,
28049 Madrid, Spain
Up to now, future neutrino beam experiments have been designed and optimized in order to look for CP violation, 13 and the mass hierarchy under the conservative assumption that 13 is very small. However, the recent results from T2K and MINOS favor a 13 which could be as large as 8. In this work, we propose a re-optimization for neutrino beam experiments in case this hint is confirmed. By switching from the first to the second oscillation peak, we find that the CP discovery potential of future oscillation experiments would not only be enhanced, but it would also be less affected by systematic uncertainties. In order to illustrate the effect, we present our results for a Super-Beam experiment, comparing the results obtained at the first and the second oscillation peaks for several values of the systematic errors. We also study its combination with a -beam facility and show that the synergy between both experiments would also be enhanced due to the larger L/E. Moreover, the increased matter effects at the longer baseline also significantly improve the sensitivity to the mass hierarchy.
1 Introduction 2 3 4
Summary and conclusions
Introduction
The discovery of neutrino oscillations demands some extension of the Standard Model of
particle physics leading to neutrino masses and flavour mixing in the lepton sector.
Despite the progress in our understanding of neutrino physics over the last years, we remain
ignorant of the mechanism behind neutrino masses and the full pattern of masses and
mixings is, as yet, incomplete. Two distinct regimes have been observed (see ref. [1] for
a recent global fit). Atmospheric neutrino data as well as long baseline experiments with
neutrino beams from accelerators require a mass splitting of m31 = 2.5 103 eV2 and
2
a nearly maximal mixing angle 23 45. Solar and reactor neutrino data, on the other
hand, show oscillations with much longer periods, corresponding to a smaller splitting of
m21 = 7.6 105 eV2 and a non-maximal, although large, mixing angle, 12 = 34. The
2
ordering of the neutrino masses, i.e. whether a normal or inverted hierarchy is realized in
nature, as well as the absolute neutrino mass scale remain unknown. Similarly, the third
mixing angle, 13, and the existence of leptonic CP violation have not yet been probed.
New results from the T2K experiment [2], MINOS [3] and Double-CHOOZ [4] favour
large values of 13, saturating the present constraints. A global fit to present data yields
a preference for non-zero 13 at 3 with a best fit at sin2 213 = 0.051 (0.063) for
normal (inverted) hierarchy [1], see also [5, 6]. If confirmed with larger statistics and by the
ongoing reactor searches [7, 8], this would imply that our ability to probe for leptonic CP
violation and determine the neutrino mass hierarchy are closer at hand than we dared hope
for. In such an event, we should evaluate the optimization of future oscillation facilities
to measure these two observables. Indeed, most neutrino oscillation experiment proposals
choose their energy and baseline so as to observe the e oscillations of neutrinos
2
and antineutrinos (or its T conjugates) at the first maximum of the atmospheric m31
oscillation. The vacuum oscillation probability for this channel, expanded up to second
Figure 1. Terms of the oscillation probability in vacuum as a function of L/E for 13 = 1 (left)
and 13 = 10 (right). Notice the different scales in the Y-axis between the two panels. The
terms driven by the atmospheric (green) and solar (red) oscillation frequencies as well as the
CP-violating interference (without the cos( 321 L ) term) between the two (blue) are shown.
Pe P ((e) ()) = s223 sin2 213 sin2
where the upper/lower sign in the formula refers to neutrinos/antineutrinos,
J c13 sin 212 sin 223 sin 213 and ij 2mEi2j . We will refer to the three terms
in eq. (1.1) as atmospheric, solar and CP interference terms, respectively.
In figure 1 the three terms in eq. (1.1) are depicted as a function of L/E. The left
panel shows the case of 13 = 1, while the right panel corresponds to 13 = 10 (close to
the best fit of T2K). For the CP-violating interference term only the coefficient in front of
cos 321 L has been shown. As can be seen, for 13 = 1 the choice of the first
oscillation peak is indeed very favorable for the exploration of CP violation, since the coefficient
multiplying the CP-violating term is larger than either the solar or the atmospheric
CPconserving terms. On the other hand, for 13 = 10 the first oscillation peak is dominated
by the atmospheric term whereas the CP interference term is only a subleading component
of the oscillation probability which could be missed unless the systematic error on the signal
(...truncated)