4 research outputs found
A phenomenological outlook on three-flavor atmospheric neutrino oscillations
The recent observations of atmospheric nu events from the Super-Kamiokande
experiment are compatible with three-flavor neutrino oscillations, occurring
dominantly in the nu_mu<--->nu_tau channel and subdominantly in the
nu_mu<--->nu_e channel. We present an updated analysis of the three-flavor
mass-mixing parameters consistent with the present phenomenology, including the
latest 45 kTy data sample from Super-Kamiokande. A comparison with our previous
results, based on 33 kTy data, shows that the oscillation evidence is
strengthened, and that the neutrino mass-mixing parameters are constrained in
smaller ranges
Neutrino mass hierarchy and precision physics with medium-baseline reactors: impact of energy-scale and flux-shape uncertainties
Nuclear reactors provide intense sources of electron antineutrinos,
characterized by few-MeV energy E and unoscillated spectral shape Phi(E).
High-statistics observations of reactor neutrino oscillations over
medium-baseline distances L ~ O(50) km would provide unprecedented
opportunities to probe both the long-wavelength mass-mixing parameters (delta
m^2 and theta_12) and the short-wavelength ones (Delta m^2 and theta_13),
together with the subtle interference effects associated to the neutrino mass
hierarchy (either normal or inverted). In a given experimental setting - here
taken as in the JUNO project for definiteness - the achievable hierarchy
sensitivity and parameter accuracy depend not only on the accumulated
statistics but also on systematic uncertainties, which include (but are not
limited to) the mass-mixing priors and the normalizations of signals and
backgrounds. We examine, in addition, the effect of introducing smooth
deformations of the detector energy scale, E -> E'(E), and of the reactor flux
shape, Phi(E) -> Phi'(E), within reasonable error bands inspired by
state-of-the-art estimates. It turns out that energy-scale and flux-shape
systematics can noticeably affect the performance of a JUNO-like experiment,
both on the hierarchy discrimination and on precision oscillation physics. It
is shown that a significant reduction of the assumed energy-scale and
flux-shape uncertainties (by, say, a factor of two) would be highly beneficial
to the physics program of medium-baseline reactor projects. Our results shed
also some light on the role of the inverse-beta decay threshold, of geoneutrino
backgrounds, and of matter effects in the analysis of future reactor
oscillation data
Neutrino mass hierarchy and electron neutrino oscillation parameters with one hundred thousand reactor events
Proposed medium-baseline reactor neutrino experiments offer unprecedented
opportunities to probe, at the same time, the mass-mixing parameters which
govern oscillations both at short wavelength (delta m^2 and theta_{12})
and at long wavelength (Delta m^2 and theta_{13}), as well as their tiny
interference effects related to the mass hierarchy (i.e., the relative sign of
Delta m^2 and delta m^2). In order to take full advantage of these
opportunities, precision calculations and refined statistical analyses of event
spectra are required. In such a context, we revisit several input ingredients,
including: nucleon recoil in inverse beta decay and its impact on energy
reconstruction and resolution, hierarchy and matter effects in the oscillation
probability, spread of reactor distances, irreducible backgrounds from
geoneutrinos and from far reactors, and degeneracies between energy scale and
spectrum shape uncertainties. We also introduce a continuous parameter alpha,
which interpolates smoothly between normal hierarchy (alpha=+1) and inverted
hierarchy (alpha=-1). The determination of the hierarchy is then transformed
from a test of hypothesis to a parameter estimation, with a sensitivity given
by the distance of the true case (either alpha=+1 or alpha=-1) from the
undecidable case (alpha=0). Numerical experiments are performed for the
specific set up envisaged for the JUNO project, assuming a realistic sample of
O(10^5) reactor events. We find a typical sensitivity of ~2 sigma to the
hierarchy in JUNO, which, however, can be challenged by energy scale and
spectrum shape systematics, whose possible conspiracy effects are investigated.
The prospective accuracy reachable for the other mass-mixing parameters is also
discussed
PINGU and the neutrino mass hierarchy: Statistical and systematic aspects
The proposed PINGU project (Precision IceCube Next Generation Upgrade) is
expected to collect O(10^5) atmospheric muon and electron neutrino in a few
years of exposure, and to probe the neutrino mass hierarchy through its imprint
on the event spectra in energy and direction. In the presence of nonnegligible
and partly unknown shape systematics, the analysis of high-statistics spectral
variations will face subtle challenges that are largely unprecedented in
neutrino physics. We discuss these issues both on general grounds and in the
currently envisaged PINGU configuration, where we find that possible shape
uncertainties at the (few) percent level can noticeably affect the sensitivity
to the hierarchy. We also discuss the interplay between the mixing angle
theta_23 and the PINGU sensitivity to the hierarchy. Our results suggest that
more refined estimates of spectral uncertainties are needed in next-generation,
large-volume atmospheric neutrino experiments