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Register a new userHeavy neutrino mixing in the T2HK, the T2HKK and an extension of the T2HK with a detector at Oki Islands
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We study the discovery potential for the mixing of heavy isospin-singlet neutrinos in extensions of the Tokai-to-Kamioka (T2K) experiment: The Tokai-to-Hyper-Kamiokande (T2HK), the Tokai-to-Hyper-Kamiokande-to-Korea (T2HKK), and a plan of adding a new detector at Oki Islands to the T2HK. We parametrize the mixing of heavy neutrinos in terms of a non-unitary mixing matrix acting on active flavors. It is shown that in the T2HK and T2HKK, the sensitivity to heavy neutrino mixing deteriorates for some values of $CP$-violating phases in the standard and the non-unitary mixing matrices, but the deterioration is drastically mitigated if a detector at Oki Islands is added to the T2HK. We also consider the feasibility of measuring the mass hierarchy and the standard $CP$-violating phase $delta_{CP}$ in the presence of heavy neutrino mixing, by fitting experimental data with the standard oscillation parameters only, i.e., under the assumption of unitary mixing. It is revealed that such measurement can be performed to some accuracy in the T2HKK and the extension of the T2HK with a detector at Oki Islands, owing to the fact that data with largely varying $L/E$ are collected in these experiments, while it cannot be in the T2HK.
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T2HK and T2HKK are the proposed extensions of the of T2K experiments in Japan and DUNE is the future long-baseline program of Fermilab. All these three experiments will use extremely high beam power and large detector volumes to observe neutrino oscillation. Because of the large statistics, these experiments will be highly sensitive to systematics. Thus a small change in the systematics can cause a significant change in their sensitivities. To understand this, we do a comparative study of T2HK, T2HKK and DUNE with respect to their systematic errors. Specifically we study the effect of the systematics in the determination of neutrino mass hierarchy, octant of the mixing angle $theta_{23}$ and $delta_{CP}$ in the standard three flavor scenario and also analyze the role of systematic uncertainties in constraining the parameters of the nonstandard interactions in neutrino propagation. Taking the overall systematics for signal and background normalization, we quantify how the sensitivities of these experiments change if the systematics are varied from $1%$ to $7%$.
We investigate the performance of T2HK in the presence of a light eV scale sterile neutrino. We study in detail its influence in resolving fundamental issues like mass hierarchy, CP-violation (CPV) induced by the standard CP-phase $delta_{13}$ and new CP-phase $delta_{14}$, and the octant ambiguity of $theta_{23}$. We show for the first time in detail that due to the impressive energy reconstruction capabilities of T2HK, the available spectral information plays an important role to enhance the mass hierarchy discovery reach of this experiment in 3$ u$ framework and also to keep it almost intact even in $4 u$ scheme. This feature is also of the utmost importance in establishing the CPV due to $delta_{14}$. As far as the sensitivity to CPV due to $delta_{13}$ is concerned, it does not change much going from $3 u$ to 4$ u$ case. We also examine the reconstruction capability of the two phases $delta_{13}$ and $delta_{14}$, and find that the typical 1$sigma$ uncertainty on $delta_{13}$ ($delta_{14}$) in T2HK is $sim15^0$ ($30^0$). While determining the octant of $theta_{23}$, we face a complete loss of sensitivity for unfavorable combinations of unknown $delta_{13}$ and $delta_{14}$.
With the recent measurement of reactor mixing angle $theta_{13}$ the knowledge of neutrino oscillation parameters that describe PMNS matrix has improved significantly except the CP violating phase $delta_{CP}$. The other unknown parameters in neutrino oscillation studies are mass hierarchy and the octant of the atmospheric mixing angle $theta_{23}$. Many dedicated experiments are proposed to determine these parameters which may take at least 10 years from now to become operational. It is therefore very crucial to use the results from the existing experiments to see whether we can get even partial answers to these questions. In this paper we study the discovery potential of the ongoing NO$ u$A and T2K experiments as well as the forthcoming T2HK experiment in addressing these questions. In particular, we evaluate the sensitivity of NO$ u$A to determine neutrino mass hierarchy, octant degeneracy and to obtain CP violation phase after running for its scheduled period of 3 years in neutrino mode and 3 years in anti-neutrino mode. We then extend the analysis to understand the discovery potential if the experiments will run for (5$ u$+5$bar{ u}$) years and (7$ u$+3$bar{ u}$) years. We also show how the sensitivity improves when we combine the data from (3$ u$+3$bar{ u}$) years of NO$ u$A run with (3$ u$+2$bar{ u}$) years of T2K and (3$ u$+7$bar{ u}$) years of T2HK experiments. The CP violation sensitivity is marginal for T2K and NO$ u$A experiments even for ten years data taking of NO$ u$A. T2HK has a significance above 5$sigma$ for a fraction of two-fifth values of the $delta_{CP}$ space. We also find that $delta_{CP}$ can be determined to be better than $35^circ $, $21^circ $ and $9^circ $ for all values of $delta_{CP}$ for T2K, NO$ u$A and T2HK respectively.
In this paper, we perform a comparative analysis between the future proposed long-baseline experiments ESSnuSB and T2HK in measuring the leptonic CP phase $delta_{rm CP}$. In particular, we study the effect of the neutrino mass ordering degeneracy and the leptonic mixing angle $theta_{23}$ octant degeneracy in the measurement of leptonic CP violation and precision for both experiments. Since the ESSnuSB (T2HK) experiment probes the second (first) oscillation maximum to study neutrino oscillations, the effect of these degeneracies are significantly different in both experiments. Our main conclusion is that for the ESSnuSB experiment, the information on the neutrino mass ordering does not play a major role in the determination of $delta_{rm CP}$, which is not the case for the T2HK experiment. However, the information on the true octant compromises the CP sensitivity of the ESSnuSB experiment as compared to T2HK if $theta_{23}$ lies in the lower octant. These conclusions are true for both the 540~km and 360~km baseline options for the ESSnuSB experiment. In addition, we investigate the effect of different running times in neutrino and antineutrino modes and the effect of $theta_{23}$ precision in measuring $delta_{rm CP}$.
The muon decay-at-rest ($mu$-DAR) facility provides us with an ideal platform to probe purely muonic charged-current nonstandard neutrino interactions (NSIs). We propose to probe this class of NSI effects using antineutrinos from a $mu$-DAR source in conjunction with neutrinos from the future Tokai to Kamioka superbeam experiment with megaton Hyper Kamiokande detector (T2HK). Even though muonic NSIs are absent in neutrino production at T2HK, we show that our proposed hybrid setup comprising $mu$-DAR and T2HK helps in alleviating the parameter degeneracies that can arise in data. Analytic considerations reveal that the oscillation probability is most sensitive to the NSI parameter in the $mu$-e sector. For this parameter, we show that the $mu$-DAR setup can improve on the existing bounds down to around 0.01, especially when the data are combined with neutrino data from T2HK experiment due to the lifting of parameter degeneracies. The high precision with which $mu$-DAR can measure $delta_{rm{CP}}$ is shown to be robust even in the presence of the considered NSIs. Finally, we show that the combination of $mu$-DAR along with T2HK can also be used to put mild constraints on the NSI phase in the vicinity of the maximal CP-violating value for the chosen benchmark value of $varepsilon^{mu e}_{mu e}=0.01$.
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