Anomalies observed by different experiments, the most significant ones being the ∼3.8 sigma νe appearance in a ∼50 MeV νµ beam from muon decay at rest observed by the LSND experiment and the ∼3.8 ...sigma νe and ν ¯ e appearance in a ∼1 GeV neutrino beam from pion decay in flight observed by MiniBooNE, suggest the existence of sterile neutrinos. The Short Baseline Neutrino (SBN) program at Fermilab aims to perform a sensitive search for sterile neutrinos by performing analyses of νe appearance and νµ disappearance employing three Liquid Argon Time Projection Chambers (LAr-TPCs) at different baselines. The VALOR neutrino fitting group was established within the T2K experiment and has led numerous flagship T2K oscillation analyses, and provided sensitivity and detector optimisation studies for DUNE and Hyper-K. The neutrino oscillation framework developed by this group is able to perform fits of several samples and systematic parameters within different neutrino models and experiments. Thus, VALOR is an ideal environment for the neutrino oscillation fits using multiple LAr-TPC detectors with proper treatment of correlated systematic uncertainties necessary for the SBN analyses.
The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 1964
, and CP ...violation in the weak interactions of quarks was soon established
. Sakharov proposed
that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter-antimatter disparity through a process called leptogenesis
. Leptonic mixing, which appears in the standard model's charged current interactions
, provides a potential source of CP violation through a complex phase δ
, which is required by some theoretical models of leptogenesis
. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments
. Until now, the value of δ
has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δ
that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δ
, which is cyclic and repeats every 2π, is -3.41, -0.03 for the so-called normal mass ordering and -2.54, -0.32 for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter-antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.
The T2K experiment reports updated measurements of neutrino and antineutrino oscillations using both appearance and disappearance channels. This result comes from an exposure of 14.9 (16.4) × 1020 ...protons on target in neutrino (antineutrino) mode. Significant improvements have been made to the neutrino interaction model and far detector reconstruction. An extensive set of simulated data studies have also been performed to quantify the effect interaction model uncertainties have on the T2K oscillation parameter sensitivity. T2K performs multiple oscillation analyses that present both frequentist and Bayesian intervals for the Pontecorvo-Maki-Nakagawa-Sakata parameters. For fits including a constraint on sin 2θ13 from reactor data and assuming normal mass ordering T2K measures sin 2θ23 = 0.53 +0.03 −0.04 and Δm232 = (2.45 ± 0.07) × 10−3 eV2 c−4. The Bayesian analyses show a weak preference for normal mass ordering (89% posterior probability) and the upper sin 2θ23 octant (80% posterior probability), with a uniform prior probability assumed in both cases. The T2K data exclude CP conservation in neutrino oscillations at the 2σ level.
This paper reports on the search for heavy neutrinos with masses in the range 140<MN<493 MeV/c2 using the off-axis near detector ND280 of the T2K experiment. These particles can be produced from kaon ...decays in the standard neutrino beam and then subsequently decay in ND280. The decay modes under consideration are N→ℓα±π∓ and N→ℓα+ℓβ−ν(−)(α,β=e,μ). A search for such events has been made using the Time Projection Chambers of ND280, where the background has been reduced to less than two events in the current dataset in all channels. No excess has been observed in the signal region. A combined Bayesian statistical approach has been applied to extract upper limits on the mixing elements of heavy neutrinos to electron-, muon- and tau- flavored currents (Ue2, Uμ2, Uτ2) as a function of the heavy neutrino mass, e.g., Ue2<10−9 at 90% C.L. for a mass of 390 MeV/c2. These constraints are competitive with previous experiments.
We perform a search for light sterile neutrinos using the data from the T2K far detector at a baseline of 295 km, with an exposure of 14.7(7.6)×1020 protons on target in neutrino (antineutrino) mode. ...A selection of neutral-current interaction samples is also used to enhance the sensitivity to sterile mixing. No evidence of sterile neutrino mixing in the 3+1 model was found from a simultaneous fit to the charged-current muon, electron and neutral-current neutrino samples. We set the most stringent limit on the sterile oscillation amplitude sin2θ24 for the sterile neutrino mass splitting Δm412<3×10−3 eV2/c4.
Neutrino- and antineutrino-oxygen neutral-current quasielasticlike interactions are measured at Super-Kamiokande using nuclear deexcitation γ rays to identify signal-like interactions in data from a ...14.94(16.35)×1020 protons-on-target exposure of the T2K neutrino (antineutrino) beam. The measured flux-averaged cross sections on oxygen nuclei are ⟨σν−NCQE⟩=1.70±0.17(stat.)−0.38+0.51(syst.)×10−38 cm2/oxygen with a flux-averaged energy of 0.82 GeV and ⟨σν¯−NCQE⟩=0.98±0.16(stat.)−0.19+0.26(syst.)×10−38 cm2/oxygen with a flux-averaged energy of 0.68 GeV, for neutrinos and antineutrinos, respectively. These results are the most precise to date, and the antineutrino result is the first cross section measurement of this channel. They are compared with various theoretical predictions. The impact on evaluation of backgrounds to searches for supernova relic neutrinos at present and future water Cherenkov detectors is also discussed.
This paper reports the first T2K measurement of the transverse kinematic imbalance in the single- π+ production channel of neutrino interactions. We measure the differential cross sections in the ...muon-neutrino charged-current interaction on hydrocarbon with a single π+ and at least one proton in the final state, at the ND280 off axis near detector of the T2K experiment. The extracted cross sections are compared to the predictions from different neutrino-nucleus interaction event generators. Overall, the results show a preference for models that have a more realistic treatment of nuclear medium effects including the initial nuclear state and final-state interactions.
Muon neutrino and antineutrino disappearance probabilities are identical in the standard three-flavor neutrino oscillation framework, but CPT violation and nonstandard interactions can violate this ...symmetry. In this work we report the measurements of sin2θ23 and Δ$m$$^{2}_{32}$ independently for neutrinos and antineutrinos. The aforementioned symmetry violation would manifest as an inconsistency in the neutrino and antineutrino oscillation parameters. The analysis discussed here uses a total of 1.97 × 1021 and 1.63 × 1021 protons on target taken with a neutrino and antineutrino beam respectively, and benefits from improved flux and cross section models, new near-detector samples and more than double the data reducing the overall uncertainty of the result. No significant deviation is observed, consistent with the standard neutrino oscillation picture.
We report measurements by the T2K experiment of the parameters θ23 and Δm322, which govern the disappearance of muon neutrinos and antineutrinos in the three-flavor PMNS neutrino oscillation model at ...T2K's neutrino energy and propagation distance. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, muon-like events from each beam mode are used to measure these parameters separately for neutrino and antineutrino oscillations. Data taken from 1.49×1021 protons on target (POT) in neutrino mode and 1.64 × 1021 POT in antineutrino mode are used. The best-fit values obtained by T2K were sin2 (θ23) = 0.51−0.07+0.06(0.43−0.05+0.21) and Δm322=2.47−0.09+0.08(2.50−0.13+0.18)× 10−3 eV2/c4 for neutrinos (antineutrinos). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed. An analysis using an effective two-flavor neutrino oscillation model where the sine of the mixing angle is allowed to take nonphysical values larger than 1 is also performed to check the consistency of our data with the three-flavor model. Our data were found to be consistent with a physical value for the mixing angle.
Abstract
The T2K experiment presents new measurements of neutrino oscillation parameters using
$$19.7(16.3)\times 10^{20}$$
19.7
(
16.3
)
×
10
20
protons on target (POT) in (anti-)neutrino mode at ...the far detector (FD). Compared to the previous analysis, an additional
$$4.7\times 10^{20}$$
4.7
×
10
20
POT neutrino data was collected at the FD. Significant improvements were made to the analysis methodology, with the near-detector analysis introducing new selections and using more than double the data. Additionally, this is the first T2K oscillation analysis to use NA61/SHINE data on a replica of the T2K target to tune the neutrino flux model, and the neutrino interaction model was improved to include new nuclear effects and calculations. Frequentist and Bayesian analyses are presented, including results on
$$\sin ^2\theta _{13}$$
sin
2
θ
13
and the impact of priors on the
$$\delta _{\textrm{CP}}$$
δ
CP
measurement. Both analyses prefer the normal mass ordering and upper octant of
$$\sin ^2\theta _{23}$$
sin
2
θ
23
with a nearly maximally CP-violating phase. Assuming the normal ordering and using the constraint on
$$\sin ^2\theta _{13}$$
sin
2
θ
13
from reactors,
$$\sin ^2\theta _{23}=0.561^{+0.021}_{-0.032}$$
sin
2
θ
23
=
0
.
561
-
0.032
+
0.021
using Feldman–Cousins corrected intervals, and
$$\varDelta {}m^2_{32}=2.494_{-0.058}^{+0.041}\times 10^{-3}~\text {eV}^2$$
Δ
m
32
2
=
2
.
494
-
0.058
+
0.041
×
10
-
3
eV
2
using constant
$$\varDelta \chi ^{2}$$
Δ
χ
2
intervals. The CP-violating phase is constrained to
$$\delta _{\textrm{CP}}=-1.97_{-0.70}^{+0.97}$$
δ
CP
=
-
1
.
97
-
0.70
+
0.97
using Feldman–Cousins corrected intervals, and
$$\delta _{\textrm{CP}}=0,\pi $$
δ
CP
=
0
,
π
is excluded at more than 90% confidence level. A Jarlskog invariant of zero is excluded at more than
$$2\sigma $$
2
σ
credible level using a flat prior in
$$\delta _{\textrm{CP}},$$
δ
CP
,
and just below
$$2\sigma $$
2
σ
using a flat prior in
$$\sin \delta _{\textrm{CP}}.$$
sin
δ
CP
.
When the external constraint on
$$\sin ^2\theta _{13}$$
sin
2
θ
13
is removed,
$$\sin ^2\theta _{13}=28.0^{+2.8}_{-6.5}\times 10^{-3},$$
sin
2
θ
13
=
28
.
0
-
6.5
+
2.8
×
10
-
3
,
in agreement with measurements from reactor experiments. These results are consistent with previous T2K analyses.
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