The precision measurement and discovery potential of a neutrino factory based on boosted radioactive ions in a storage ring ("β-beam") is re-examined. In contrast with past designs, which assume ion γ factors of ∼ 100 and baselines of L = 130 km, we emphasize the advantages of boosting the ions to higher γ and increasing the baseline proportionally. In particular, we consider a "medium-"γ scenario (γ ∼ 500, L ∼ 730 km) and a "high-"γ scenario (γ ∼ 2000, L ∼ 3000 km). The increase in statistics, which grow linearly with the average beam energy, the ability to exploit the energy dependence of the signal and the sizable matter effects at this longer baseline all increase the discovery potential of such a machine very significantly.

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A β-beam with maximum γ = 150 (for 6 He ions) or γ = 250 (for 18 Ne) could be achieved at the CERN-SPS. We study the sensitivity to θ 13 and δ of such a beam as function of γ, optimizing with the baseline constrained to CERN-Frejus (130 km), and also with simultaneous variation of the baseline. These results are compared to the standard scenario previously considered, with lower γ = 60/100, and also with a higher γ ∼ 350 option that requires a more powerful accelerator. Although higher γ is better, loss of sensitivity to θ 13 and δ is most pronounced for γ below 100.

In this Technical Design Report (TDR) we describe the NEXT-100 detector that will search for neutrinoless double beta decay (β β 0ν) in 136 Xe at the Laboratorio Subterráneo de Canfranc (LSC), in Spain. The document formalizes the design presented in our Conceptual Design Report (CDR): an electroluminescence time projection chamber, with separate readout planes for calorimetry and tracking, located, respectively, behind cathode and anode. The detector is designed to hold a maximum of about 150 kg of xenon at 15 bar, or 100 kg at 10 bar. This option builds in the capability to increase the total isotope mass by 50% while keeping the operating pressure at a manageable level. The readout plane performing the energy measurement is composed of Hamamatsu R11410-10 photomultipliers, specially designed for operation in low-background, xenon-based detectors. Each individual PMT will be isolated from the gas by an individual, pressure resistant enclosure and will be coupled to the sensitive volume through a sapphire window. The tracking plane consists in an array of Hamamatsu S10362-11-050P MPPCs used as tracking pixels. They will be arranged in square boards holding 64 sensors (8 × 8) with a 1-cm pitch. The inner walls of the TPC, the sapphire windows and the boards holding the MPPCs will be coated with tetraphenyl butadiene (TPB), a wavelength shifter, to improve the light collection.

NEXT-100 is an electroluminescent high-pressure xenon gas time projection chamber that will search for the neutrinoless double beta (0νββ) decay of 136 Xe. The detector possesses two features of great value for 0νββ searches: energy resolution better than 1% FWHM at the Q value of 136 Xe and track reconstruction for the discrimination of signal and background events. This combination results in excellent sensitivity, as discussed in this paper. Material-screening measurements and a detailed Monte Carlo detector simulation predict a background rate for NEXT-100 of at most 4 × 10 −4 counts keV −1 kg −1 yr −1 . Accordingly, the detector will reach a sensitivity to the 0νββ-decay half-life of 2.8 × 10 25 years (90% CL) for an exposure of 100 kg · year, or 6.0 × 10 25 years after a run of 3 effective years.