Abstract
Recent data from long-baseline neutrino oscillation experiments have provided new information on \theta_{13}, hinting that 0.01\lesssim sin^2 2\theta_{13} \lesssim 0.1 at 2 sigma C.L. Confirmation of this result with high significance will have a crucial impact on the optimization of the future long-baseline oscillation experiments designed to probe the neutrino mass ordering and leptonic CP violation. In this context, we expound in detail the physics reach of an experimental setup where neutrinos produced in a conventional wide-band beam facility at CERN are observed in a proposed Giant Liquid Argon detector at the Pyh\"asalmi mine, at a distance of 2290 km. This particular setup would have unprecedented sensitivity to the mass ordering and CP violation in the light of large \theta_{13}. With a 10 to 20 kt `pilot' detector and just a few years of neutrino beam running, the mass hierarchy could be determined, irrespective of the true values of \delta_{CP} and the mass hierarchy, at 3 sigma (5 sigma) C.L. if sin^2 2\theta_{13}(true) = 0.05 (0.1). With the same exposure, we start to have 3 sigma sensitivity to CP violation if sin^2 2\theta_{13}(true) > 0.05, in particular testing maximally CP-violating scenarios at a high C.L. After optimizing the neutrino and anti-neutrino running fractions, we study the performance of the setup as a function of the exposure, identifying three milestones to have roughly 30%, 50% and 70% coverage in \delta_{CP}(true) for 3 sigma CP violation discovery. For comparison, we also study the CERN to Slanic baseline of 1540 km. This work demonstrates that an incremental program, staged in terms of the exposure, can achieve the desired physics goals within a realistically feasible timescale, and produce significant new results at each stage.
Highlights
Introduction and motivationOver the last thirteen years or so, marvellous data from world-class experiments involving neutrinos from the sun [1,2,3,4,5,6,7], the Earth’s atmosphere [8,9], nuclear reactors [10,11], and accelerators [12,13] have firmly established the phenomenon of neutrino oscillations
For long-baseline experiments, the measurement of the mass hierarchy is easier than a measurement of δCP because matter effects enhance the separation between the oscillation spectra, and the event rates, of a normal hierarchy’ (NH) and an inverted hierarchy’ (IH)
We will discuss the CP violation (CPV) discovery potential of the facility, showing that whilst a larger exposure is needed in order to obtain a reasonable sensitivity than for a measurement of the mass hierarchy, it is still feasible to cover a significant portion of the parameter space with a moderate exposure
Summary
Over the last thirteen years or so, marvellous data from world-class experiments involving neutrinos from the sun [1,2,3,4,5,6,7], the Earth’s atmosphere [8,9], nuclear reactors [10,11], and accelerators [12,13] have firmly established the phenomenon of neutrino oscillations. A non-zero, and in particular a large value of θ13 will provide a boost to the field of neutrino oscillation physics, making it possible to discover leptonic CP violation (CPV) if the Dirac CP phase, δCP, is not equal to 0◦ or 180◦, and allowing for a direct determination of sgn(∆m231) aka, the neutrino mass hierarchy1 Given their relatively short baselines, narrow band beams and limited statistics, the present generation T2K and NOνA experiments have a limited reach in probing CPV and the neutrino mass hierarchy [23], even for the large values of θ13 indicated by the recent global fits [16, 17]. The information obtained about sgn(∆m231) at the early stage of the experiment plays a vital role in maximizing the sensitivity of the setup to CPV - the right choice of the mass hierarchy is essentially a mandatory input for discovering CPV Possibilities to determine it with atmospheric neutrinos [44,45] have been explored, but are less efficient and more prone to systematic errors than the direct method considered here.
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