The T2K experiment observes indications of ${\nu }_{\mu }\to {\nu }_e$ appearance in data accumulated with $1.43\times {10}^{20}$ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with $|\Delta m_{23}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$, ${sin}^{2}2{\theta }_{23}=1$ and ${sin}^{2}2{\theta }_{13}=0$, the expected number of such events is $1.5\pm 0.3(\mathrm{syst})$. Under this hypothesis, the probability to observe six or more candidate events is $7\times {10}^{-3}$, equivalent to $2.5\sigma$ significance. At 90% C.L., the data are consistent with $0.03(0.04)<{sin}^{2}2{\theta }_{13}<0.28(0.34)$ for ${\delta }_{CP}=0$ and a normal (inverted) hierarchy.

The T2K experiment is a long baseline neutrino oscillation experiment. Its main goal is to measure the last unknown lepton sector mixing angle θ13θ13 by observing νeνe appearance in a νμνμ beam. It also aims to make a precision measurement of the known oscillation parameters, View the MathML sourceΔm232 and sin22θ23sin22θ23, via νμνμ disappearance studies. Other goals of the experiment include various neutrino cross-section measurements and sterile neutrino searches. The experiment uses an intense proton beam generated by the J-PARC accelerator in Tokai, Japan, and is composed of a neutrino beamline, a near detector complex (ND280), and a far detector (Super-Kamiokande) located 295 km away from J-PARC. This paper provides a comprehensive review of the instrumentation aspect of the T2K experiment and a summary of the vital information for each subsystem

The ForwArd Search ExpeRiment (FASER) is an approved experiment dedicated to searching for light, extremely weakly interacting particles at the LHC. Such particles may be produced in the LHC’s high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons; light $B-L$ and $L_i-L_j$ gauge bosons; axionlike particles that are coupled dominantly to photons, fermions, and gluons through nonrenormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.

The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of $7.3\sigma$ when compared to $4.92\pm 0.55$ expected background events. In the Pontecorvo-Maki-Nakagawa-Sakata mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles ${\theta }_{12}$, ${\theta }_{23}$, ${\theta }_{13}$, a mass difference $\mathrm{\Delta }{m}_{32}^2$ and a $CP$ violating phase ${\delta }_{CP}$. In this neutrino oscillation scenario, assuming $|\mathrm{\Delta }{m}_{32}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$, ${\sin }^2{\theta }_{23}=0.5$, and $\mathrm{\Delta }{m}_{32}^2>0$ ($\mathrm{\Delta }{m}_{32}^2<0$), a best-fit value of ${\sin }^{2}2{\theta }_{13}=0.140_{-0.032}^{+0.038}$ ($0.170_{-0.037}^{+0.045}$) is obtained at ${\delta }_{CP}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of ${\theta }_{13}$ from reactor experiments, some values of ${\delta }_{CP}$ are disfavored at the 90% C.L.

Hyper-Kamiokande will be a next-generation underground water Cherenkov detector with a total (fiducial) mass of 0.99 (0.56) million metric tons, approximately 20 (25) times larger than that of Super-Kamiokande. One of the main goals of Hyper-Kamiokande is the study of $$CP$$ asymmetry in the lepton sector using accelerator neutrino and anti-neutrino beams. In this paper, the physics potential of a long-baseline neutrino experiment using the Hyper-Kamiokande detector and a neutrino beam from the J-PARC proton synchrotron is presented. The analysis uses the framework and systematic uncertainties derived from the ongoing T2K experiment. With a total exposure of 7.5 MW $$\times 10^7$$s integrated proton beam power (corresponding to $$1.56 \times 10^{22}$$ protons on target with a 30 GeV proton beam) to a $$2.5^\circ$$ off-axis neutrino beam, it is expected that the leptonic $$CP$$ phase $$\delta _{CP}$$ can be determined to better than 19 degrees for all possible values of $$\delta _{CP}$$, and $$CP$$ violation can be established with a statistical significance of more than $$3\,\sigma$$ ($$5\,\sigma$$) for $$76{\%}$$ ($$58{\%}$$) of the $${\delta _{CP}}$$ parameter space. Using both $$\nu _e$$ appearance and $$\nu _\mu$$ disappearance data, the expected 1$$\sigma$$ uncertainty of $$\sin ^2\theta _{23}$$ is 0.015(0.006) for $$\sin ^2\theta _{23}=0.5(0.45)$$.

We report on measurements of neutrino oscillation using data from the T2K long-baseline neutrino experiment collected between 2010 and 2013. In an analysis of muon neutrino disappearance alone, we find the following estimates and 68% confidence intervals for the two possible mass hierarchies: $\text{normal hierarchy}:{\sin }^2{\theta }_{23}=0.514_{-0.056}^{+0.055}$ and $\mathrm{\Delta }{m}_{32}^2=(2.51\pm 0.10)\times 10^{-3}{\mathrm{eV}}^2/c^4$ and $\text{inverted hierarchy}:{\sin }^2{\theta }_{23}=0.511\pm 0.055$ and $\mathrm{\Delta }{m}_{13}^2=(2.48\pm 0.10)\times 10^{-3}{\mathrm{eV}}^2/c^4$. The analysis accounts for multinucleon mechanisms in neutrino interactions which were found to introduce negligible bias. We describe our first analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance to estimate four oscillation parameters, $|\mathrm{\Delta }{m}^2|$, ${\sin }^2{\theta }_{23}$, ${\sin }^2{\theta }_{13}$, ${\delta }_{CP}$, and the mass hierarchy. Frequentist and Bayesian intervals are presented for combinations of these parameters, with and without including recent reactor measurements. At 90% confidence level and including reactor measurements, we exclude the region ${\delta }_{CP}=[0.15,0.83]\pi$ for normal hierarchy and ${\delta }_{CP}=[-0.08,1.09]\pi$ for inverted hierarchy. The T2K and reactor data weakly favor the normal hierarchy with a Bayes factor of 2.2. The most probable values and 68% one-dimensional credible intervals for the other oscillation parameters, when reactor data are included, are ${\sin }^2{\theta }_{23}=0.528_{-0.038}^{+0.055}$ and $|\mathrm{\Delta }{m}_{32}^2|=(2.51\pm 0.11)\times 10^{-3}{\mathrm{eV}}^2/c^4$.