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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 1 , and CP violation in the weak interactions of quarks was soon established 2 . Sakharov proposed 3 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 4 . Leptonic mixing, which appears in the standard model’s charged current interactions 5 , 6 , provides a potential source of CP violation through a complex phase δ CP , which is required by some theoretical models of leptogenesis 7 – 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 experiments 10 , 11 . Until now, the value of δ CP 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 CP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3 σ ). The 3 σ confidence interval for δ CP , 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 constrains CP symmetry in neutrino oscillations, excluding 46% of possible values of the CP violating parameter at a significance of three standard deviations; this is an important milestone to test CP symmetry conservation in leptons and whether the Universe’s matter–antimatter imbalance originates from leptons.

Hyper-Kamiokande consists of two identical water-Cherenkov detectors of total 520 kt, with the first one in Japan at 295 km from the J-PARC neutrino beam with 2.5$^\\circ$ off-axis angles (OAAs), and the second one possibly in Korea at a later stage. Having the second detector in Korea would benefit almost all areas of neutrino oscillation physics, mainly due to longer baselines. There are several candidate sites in Korea with baselines of 1000–1300 km and OAAs of 1$^\\circ$–3$^\\circ$. We conducted sensitivity studies on neutrino oscillation physics for a second detector, either in Japan (JD $\\times$ 2) or Korea (JD + KD), and compared the results with a single detector in Japan. Leptonic charge–parity (CP) symmetry violation sensitivity is improved, especially when the CP is non-maximally violated. The larger matter effect at Korean candidate sites significantly enhances sensitivities to non-standard interactions of neutrinos and mass ordering determination. Current studies indicate the best sensitivity is obtained at Mt. Bisul (1088 km baseline, $1.3^\\circ$ OAA). Thanks to a larger (1000 m) overburden than the first detector site, clear improvements to sensitivities for solar and supernova relic neutrino searches are expected.

Measurements of multiplicity and transverse momentum fluctuations of charged particles were performed in inelastic p+p interactions at 20, 31, 40, 80, and 158  GeV / c beam momentum. Results for the scaled variance of the multiplicity distribution and for three strongly intensive measures of multiplicity and transverse momentum fluctuations Δ [ P T , N ] , Σ [ P T , N ] and Φ p T are presented. For the first time the results on fluctuations are fully corrected for experimental biases. The results on multiplicity and transverse momentum fluctuations significantly deviate from expectations for the independent particle production. They also depend on charges of selected hadrons. The string-resonance Monte Carlo models Epos and U r qmd do not describe the data. The scaled variance of multiplicity fluctuations is significantly higher in inelastic p+p interactions than in central Pb+Pb collisions measured by NA49 at the same energy per nucleon. This is in qualitative disagreement with the predictions of the Wounded Nucleon Model. Within the statistical framework the enhanced multiplicity fluctuations in inelastic p+p interactions can be interpreted as due to event-by-event fluctuations of the fireball energy and/or volume.

Results on two-particle Δ η Δ ϕ correlations in inelastic p + p interactions at 20, 31, 40, 80, and 158 GeV/c are presented. The measurements were performed using the large acceptance NA61/SHINE hadron spectrometer at the CERN Super Proton Synchrotron. The data show structures which can be attributed mainly to effects of resonance decays, momentum conservation, and quantum statistics. The results are compared with the Epos and UrQMD models.

A bstract The next generation neutrino observatory proposed by the LBNO collaboration will address fundamental questions in particle and astroparticle physics. The experiment consists of a far detector, in its first stage a 20 kt LAr double phase TPC and a magnetised iron calorimeter, situated at 2300 km from CERN and a near detector based on a highpressure argon gas TPC. The long baseline provides a unique opportunity to study neutrino flavour oscillations over their 1st and 2nd oscillation maxima exploring the L/E behaviour, and distinguishing effects arising from δ CP and matter. In this paper we have reevaluated the physics potential of this setup for determining the mass hierarchy (MH) and discovering CP-violation (CPV), using a conventional neutrino beam from the CERN SPS with a power of 750 kW. We use conservative assumptions on the knowledge of oscillation parameter priors and systematic uncertainties. The impact of each systematic error and the precision of oscillation prior is shown. We demonstrate that the first stage of LBNO can determine unambiguously the MH to > 5 σ C.L. over the whole phase space. We show that the statistical treatment of the experiment is of very high importance, resulting in the conclusion that LBNO has ~ 100% probability to determine the MH in at most 4-5 years of running. Since the knowledge of MH is indispensable to extract δ CP from the data, the first LBNO phase can convincingly give evidence for CPV on the 3 σ C.L. using today’s knowledge on oscillation parameters and realistic assumptions on the systematic uncertainties.

The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle $\\theta _{13}$ have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal $\\sin ^22\\theta _{23}$, the octant of $\\theta _{23}$, and the mass hierarchy, in addition to the measurements of $\\delta _{{\\rm CP}}$, $\\sin ^2\\theta _{23}$, and $\\Delta m^2_{32}$, for various combinations of $\\nu$-mode and $\\bar {\\nu }$-mode data-taking.With an exposure of $7.8\\times 10^{21}$ protons-on-target, T2K can achieve 1$\\sigma$ resolution of 0.050 (0.054) on $\\sin ^2\\theta _{23}$ and $0.040\\ (0.045)\\times 10^{-3}\\,\\rm {eV}^2$ on $\\Delta m^2_{32}$ for 100% (50%) neutrino beam mode running assuming $\\sin ^2\\theta _{23}=0.5$ and $\\Delta m^2_{32} = 2.4\\times 10^{-3}\\,\\hbox {eV}^2$. T2K will have sensitivity to the CP-violating phase $\\delta _{\\rm {CP}}$ at 90% C.L. or better over a significant range. For example, if $\\sin ^22\\theta _{23}$ is maximal (i.e. $\\theta _{23}=45^\\circ$) the range is $-115^\\circ \\lt \\delta _{\\rm {CP}}\\lt -60^\\circ$ for normal hierarchy and $+50^\\circ \\lt \\delta _{\\rm {CP}}\\lt +130^\\circ$ for inverted hierarchy. When T2K data is combined with data from the NO$\\nu$A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero $\\delta _{{\\rm CP}}$ is substantially increased compared to if each experiment is analyzed alone.

Abstract We report a measurement of the flux-integrated $\\nu_{\\mu}$ 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 $\\sigma^{\\rm{H_{2}O}}_{\\rm{CC}} = (0.840\\pm 0.010(\\mathrm{stat.})^{+0.10}_{-0.08}(\\mathrm{syst.}))\\times10^{-38}\\,\\mathrm{cm}^2$/nucleon, $\\sigma^{\\rm{CH}}_{\\rm{CC}} = (0.817\\pm 0.007(\\mathrm{stat.})^{+0.11}_{-0.08}(\\mathrm{syst.}))\\times10^{-38}\\,\\mathrm{cm}^2$/nucleon, and $\\sigma^{\\rm{Fe}}_{\\rm{CC}} = (0.859\\pm 0.003(\\mathrm{stat.})^{+0.12}_{-0.10}(\\mathrm{syst.}))\\times10^{-38}\\,\\mathrm{cm}^2$/nucleon, respectively, for a restricted phase space of induced muons: $\\theta_{\\mu}<45^{\\circ}$ and $p_{\\mu}>$0.4 GeV/$c$ in the laboratory frame. The measured cross section ratios are ${\\sigma^{\\rm{H_{2}O}}_{\\rm{CC}}}/{\\sigma^{\\rm{CH}}_{\\rm{CC}}} = 1.028\\pm 0.016(\\mathrm{stat.})\\pm 0.053(\\mathrm{syst.})$, ${\\sigma^{\\rm{Fe}}_{\\rm{CC}}}/{\\sigma^{\\rm{H_{2}O}}_{\\rm{CC}}} = 1.023\\pm 0.012(\\mathrm{stat.})\\pm 0.058(\\mathrm{syst.})$, and ${\\sigma^{\\rm{Fe}}_{\\rm{CC}}}/{\\sigma^{\\rm{CH}}_{\\rm{CC}}} = 1.049\\pm 0.010(\\mathrm{stat.})\\pm 0.043(\\mathrm{syst.})$. 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.

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.

A bstract The electron (anti-)neutrino component of the T2K neutrino beam constitutes the largest background in the measurement of electron (anti-)neutrino appearance at the far detector. The electron neutrino scattering is measured directly with the T2K off-axis near detector, ND280. The selection of the electron (anti-)neutrino events in the plastic scintillator target from both neutrino and anti-neutrino mode beams is discussed in this paper. The flux integrated single differential charged-current inclusive electron (anti-)neutrino cross-sections, dσ/dp and dσ/d cos( θ ), and the total cross-sections in a limited phase-space in momentum and scattering angle ( p > 300 MeV/c and θ ≤ 45°) are measured using a binned maximum likelihood fit and compared to the neutrino Monte Carlo generator predictions, resulting in good agreement.

Measurements of hadron production in p + C interactions at 31 GeV/ c are performed using the NA61/SHINE spectrometer at the CERN SPS. The analysis is based on the full set of data collected in 2009 using a graphite target with a thickness of 4 % of a nuclear interaction length. Inelastic and production cross sections as well as spectra of π ± , K ± , p, K S 0 and Λ are measured with high precision. These measurements are essential for improved calculations of the initial neutrino fluxes in the T2K long-baseline neutrino oscillation experiment in Japan. A comparison of the NA61/SHINE measurements with predictions of several hadroproduction models is presented.