θ₁₃, δ and the neutrino mass hierarchy at a γ = 350 double baseline Li/B β-beam

Abstract

We consider a β-Beam facility where 8 Li and 8 B ions are accelerated at γ = 350, accumulated in a 10 Km storage ring and let decay, so as to produce intenseν e and ν e beams. These beams illuminate two magnetized iron detectors located at L ≃ 2000 Km and L ≃ 7000 Km, respectively. The physics potential of this setup is analysed in full detail as a function of the flux. We find that, for the highest flux considered (10 × 10 18 ion decays per year per baseline), the sensitivity to θ 13 reaches sin 2 2θ 13 ≥ 1 × 10 −4 ; the sign of the atmospheric mass difference can be identified, regardless of the true hierarchy, for sin 2 2θ 13 ≥ 3 × 10 −4 ; and, CP violation can be discovered in 70% of the δ-parameter space for sin 2 2θ 13 ≥ 10 −3 , having some sensitivity to CP violation down to sin 2 2θ 13 ≥ 2 × 10 −4 for |δ| ∼ 90 • . IntroductionThe • ; for the leptonic CP-violating phase δ we have no information whatsoever (see, however, Ref. [20]). We have no clue on the ordering of the neutrino mass eigenstates, either, i.e. on the sign of the atmospheric mass difference ∆m 2 atm (of which only the absolute value has been measured). It must be stressed that neutrinos being hierarchically ordered (∆m The full understanding of the leptonic mixing matrix constitutes, together with the discrimination of the Dirac/Majorana character of neutrinos and with the measurement of their absolute mass scale, the main neutrino-physics goal for the next decade. In the recent past, most of the experimental breakthroughs in neutrino physics have been achieved by exploiting the so-called "disappearance channels": by observing a deficit in the neutrinos that reach the detector with respect to those expected to be emitted from the source, a positive and eventually unambiguous signal of neutrino oscillations has been established. New-generation experiments have been proposed to look for the fleeting and intimately related parameters θ 13 and δ through "appearance channels" such as ν e ↔ ν µ (the "golden channel") [22] and ν e → ν τ (the "silver channel") [23]. However, strong correlations between θ 13 and δ [24] and the presence of parametric degeneracies in the (θ 13 , δ) parameter space, [25]-[27], make the simultaneous measurement of the two variables extremely difficult. A further problem arises from our present imprecise knowledge of atmospheric parameters, whose uncertainties are far too large to be neglected when looking for such tiny signals as those expected in appearance experiments [28].Most of the proposed solutions to these problems imply the combination of different experiments and facilities, such as reactors (Double Chooz [29] should start data taking in 2008), Super-Beams (of which T2K [30] is the first approved one), β-Beams [31] or the Neutrino Factory [32,33]. To compare different options and to understand which of them is the one to be pursued as a future neutrino facility, a list of observables to be measured has been defined: θ 13 ; the CP-violating phase δ; the sign of the atmospheric mass difference (hereaft...

LeptonicCPviolation

“…Alternatively, moderate values of γ ∼ 100 could be appropriate if ions with higher endpoint kinetic energy like 8 Li or 8 B are used. Because of its larger baseline, the HEββ setup would be sensitive to sgn(∆m 2 31 ) (Agarwalla et al, 2007;Coloma et al, 2008;Donini et al, 2005;Huber et al, 2006b;Meloni et al, 2008).…”

Exceptional sensitivity to neutrino parameters with a two-baseline Beta-beam set-up

“…The θ 13 and hierarchy sensitivity reach of this "magical" set-up is therefore superior to that of most other rival proposals involving Beta-beams [16,28,29,30,31,32,33,34,35,36,37,38,39], and is almost comparable to that possible with a Neutrino Factory [40] at the magic baseline. This tremendous sensitivity of the CERN-INO Beta-beam project to θ 13 and hierarchy comes from a combination of being close to the magic baseline as well as from being sensitive to near-maximal matter effects.…”

“…It is of interest to compare the sensitivity reach of our two-detector Beta-beam set-up with that chosen in [31], where the authors used L = 2000 km for the shorter baseline magnetized iron calorimeter detector in addition to another at the magic baseline, and 8 B and 8 Li ions as sources for both. While they use a smaller boost factor of 350 for the beam, they require larger number of ion decays than we have used for our standard luminosity.…”

“…The optimal baseline for CP studies with 8 B and 8 Li was seen to be around 1000-2000 km in [41]. In [31], the authors proposed a set-up with this scenario where they 5 With the existing facilities at CERN, one will be able to accelarate 6 He to only about γ = 150. This corresponds to a boost factor of about 250 for 18 Ne and 280 for 8 B, due to the different charge to mass ratios for these ions.…”

“…We end by summarizing the results for our combined CERN-INO and CERN-LNGS set-ups and make a comparative discussion of this set-up vis-a-vis the combined CERN-INO and CERNBoulby set-up. We also compare our set-up against the one studied in [31] and discuss the pros and cons of the two proposals. Some remarks about the beam related as well as atmospheric neutrino backgrounds are collected in the Appendix.…”