Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations
Nature Nature 580, 339–344 (2020)
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 19641, and CP violation in the weak interactions of quarks was soon established2. Sakharov proposed3 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 leptogenesis4. Leptonic mixing, which appears in the standard model’s charged current interactions5,6, provides a potential source of CP violation through a complex phase δCP, which is required by some theoretical models of leptogenesis7,8,9. 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 experiments10,11. Until now, the value of δCPhas 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 δCP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δCP, 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.
Measurement of the charged-current electron (anti-)neutrino inclusive cross-sections at the T2K off-axis near detector ND280
First combined measurement of the muon neutrino and antineutrino charged-current cross section without pions in the final state at T2K
This paper presents the first combined measurement of the double-differential muon neutrino and antineutrino charged-current cross sections with no pions in the final state on hydrocarbon at the off-axis near detector of the T2K experiment. The data analyzed in this work comprise 5.8×1020 and 6.3×1020 protons on target in neutrino and antineutrino mode respectively, at a beam energy peak of 0.6 GeV. Using the two measured cross sections, the sum, difference and asymmetry were calculated with the aim of better understanding the nuclear effects involved in such interactions. The extracted measurements have been compared with the prediction from different Monte Carlo generators and theoretical models showing that the difference between the two cross sections have interesting sensitivity to nuclear effects. Data release can found
Measurement of the muon neutrino charged-current single $\pi^+$ production on hydrocarbon using the T2K off-axis near detector ND280
We report the measurements of the single and double differential cross section of muon neutrino charged-current interactions on carbon with a single positively charged pion in the final state at the T2K off-axis near detector using 5.56 × 10^20 protons on target. The analysis uses data control samples for the background subtraction and the cross section signal, defined as a single negatively charged muon and a single positively charged pion exiting from the target nucleus, is extracted using an unfolding method. The model-dependent cross section, integrated over the T2K off-axis neutrino beam spectrum peaking at 0.6 GeV, is measured to be σ = (11.76 +- 0.44 (stat) +- 2.39 (syst) × 10^-40 cm^2/nucleon. Various differential cross sections are measured, including the first measurement of the Adler angles for single charged pion production in neutrino interactions with heavy nuclei target.
Measurement of neutrino- and antineutrino-oxygen neutral-current quasielastic-like interaction cross sections by detecting nuclear de-excitation gamma-rays at the far detector with Run1-9 FHC and RHC data
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) × 10^20 protons-on-target exposure of the T2K neutrino (antineutrino) beam. The measured flux-averaged cross sections on oxygen nuclei are 1.70 +- 0.17(stat)+-0.51(syst) × 10^-38 cm2/oxygen with a flux 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.
Published in Phys.Rev. D100 (2019) no.11, 112009
DOI: 10.1103/PhysRevD.100.112009
e-Print: arXiv:1910.09439 [hep-ex]
Measurement of the muon neutrino charged-current cross sections on water, hydrocarbon and iron, and their ratios, with the T2K on-axis detectors
We report a measurement of the flux-integrated νμ charged-current cross sections on water, hydrocarbon, and iron in the T2K on-axis neutrino beam with a mean neutrino energy of 1.5 GeV. The measured cross sections on water, hydrocarbon, and iron are
σ^{H2O}_{CC}=(0.840±0.010(stat.)+0.10−0.08(syst.))×10−38cm2/nucleon, σ^{CH}_{CC}=(0.817±0.007(stat.)+0.11−0.08(syst.))×10−38cm2/nucleon, and σ^{Fe}_CC=(0.859±0.003(stat.)+0.12−0.10(syst.))×10−38cm2/nucleon,
respectively, for a restricted phase space of induced muons: θμ<45∘ and pμ>0.4 GeV/c in the laboratory frame. The measured cross section ratios are σ^{H2O}_{CC}/σ^{CH}_CC=1.028±0.016(stat.)±0.053(syst.), σ^{Fe}_{CC}/σ^{H2O}_{CC}=1.023±0.012(stat.)±0.058(syst.), and σ^{Fe}_{CC}/σ^{CH}_{CC}=1.049±0.010(stat.)±0.043(syst.). These results, with an unprecedented precision for the measurements of neutrino cross sections on water in the studied energy region, show good agreement with the current neutrino interaction models used in the T2K oscillation analyses.
Search for neutral-current induced single photon production at the ND280 near detector in T2K
Neutrino neutral-current induced single photon production is a sub-leading order process for accelerator-based neutrino beam experiments including T2K. It is, however, an important process to understand because it is a background for electron (anti)neutrino appearance oscillation experiments. Here, we performed the first search of this process below 1 GeV using the fine-grained detector at the T2K ND280 off-axis near detector. By reconstructing single photon kinematics from electron-positron pairs, we achieved 95\% pure gamma ray sample from 5.738×10^{20} protons-on-targets neutrino mode data. We do not find positive evidence of neutral current induced single photon production in this sample. We set the model-dependent upper limit on the cross-section for this process, at 0.114×10^{−38} cm^2 (90\% C.L.) per nucleon, using the J-PARC off-axis neutrino beam with an average energy of ⟨Eν⟩∼0.6 GeV. This is the first limit on this process below 1 GeV which is important for current and future oscillation experiments looking for electron neutrino appearance oscillation signals.
DOI: https://dx.doi.org/10.1088/1361-6471/ab227d
ArXiv: https://arxiv.org/abs/1902.03848
Physics World: https://physicsworld.com/a/t2k-searches-for-single-gamma-rays-from-neutrino-interactions
Search for light sterile neutrinos with the T2K far detector Super-Kamiokande at a baseline of 295 km
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 sin^2\theta_{24} for the sterile neutrino mass splitting \Delta m^2_{41}<3\times 10^{-3} eV^2/c^4.
link to publication : https://journals.aps.org/prd/abstract/10.1103/PhysRevD.99.071103
There is a data release for this paper here
Search for heavy neutrinos with ND280
This paper (Phys. Rev. D 100, 052006 (2019)) 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→l_α π and N→l_α l_β 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 that 2 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- flavoured 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 390MeV/c2. These constraints are competitive with previous experiments.
There is a data release for this paper here.
Search for CP violation in Neutrino and Antineutrino Oscillations by the T2K experiment with 2.2E21 protons on target
The T2K experiment measures muon neutrino disappearance and electron neutrino appearance in accelerator-produced neutrino and antineutrino beams. With an exposure of 14.7(7.6)×1020 protons on target in (anti)neutrino mode, 89 electron neutrino candidates and 7 electron antineutrino candidates were observed while 67.5 and9.0 are expected for δCP=0 and normal mass ordering. The obtained 2σ confidence interval for the CP violating phase, δCP , does not include the CP-conserving cases (δCP=0, π). The best-fit values of other parameters are sin^2_θ23 = 0.526+0.032 −0.036 and ∆m^2_32 = 2.463 ± 0.065E−3 eV2/c4 .
Published in Phys.Rev.Lett. 121 (2018) no.17, 171802
DOI: 10.1103/PhysRevLett.121.171802
