Matter effects at the T2HK and T2HKK experiments

Determining the neutrino mass hierarchy and measuring the CP-violating phase $\delta_{CP}$ are two of the main aims in neutrino physics today. The upcoming T2HK (with small matter effects and high statistics) and DUNE (with large matter effects) experiments have been shown to have excellent sensitivity to $\delta_{CP}$ and the neutrino mass hierarchy, respectively. The recent T2HKK proposal aims to improve the hierarchy sensitivity of T2HK by placing one of the two tanks of the HK detector at a site in Korea, to collect data at $\sim 1100$ km baseline. In light of the fact that DUNE will anyway collect data at $\sim 1300$ km, we explore whether it is advantageous to collect additional long-baseline data as proposed with T2HKK, or to enhance the $\delta_{CP}$-precision with the `conventional' T2HK by keeping both detector tanks in Japan. We do this by comparing the physics reach of these two options in conjunction with DUNE. We find that DUNE+T2HKK is better at excluding the wrong hierarchy, reaching $\Delta\chi^2 > 175$ irrespective of the true parameters. While DUNE+T2HK can measure $\delta_{CP}$ with more precision in some parts of the parameter space, both DUNE+T2HK and DUNE+T2HKK perform equally well near the current best-fit point, giving a $\delta_{CP}$ width of around $15^\circ$. The T2HKK setup allows us to correlate and constrain the systematic errors between the two detectors collecting data from the same source, which can increase the sensitivity of the experiment by up to 25\%. Such a reduction of the systematic errors is crucial for determining the oscillation parameters with greater significance.Comment: Version accepted for publication in Phys. Rev. D (Originally titled "T2HK and T2HKK: Does more matter matter?", changed at insistence of the editorial team.

Getting the best out of T2K and NOvA

We explore the combined physics potential of T2K and NOvA in light of the moderately large measured value of theta13. For sin^2 2*theta13 = 0.1, which is close to the best fit value, a 90% C.L. evidence for the hierarchy can be obtained only for the combinations (Normal hierarchy, -170 <= deltaCP <= 0) and (Inverted hierarchy, 0 <= deltaCP <= 170), with the currently planned runs of NOvA and T2K. However, the hierarchy can essentially be determined for any value of deltaCP, if the statistics of NOvA are increased by 50% and those of T2K are doubled. Such an increase will also give an allowed region of deltaCP around its true value, except for the CP conserving cases deltaCP = 0 or 180. We demonstrate that any measurement of deltaCP is not possible without first determining hierarchy. We find that comparable data from a shorter baseline (L ~ 130 km) experiment will not lead to any significant improvement.Comment: Version published in Phys. Rev.

A hybrid setup for fundamental unknowns in neutrino oscillations using T2HK (ν\nu) and μ\mu-DAR (νˉ\bar{\nu})

Neutrino mass hierarchy, CP-violation, and octant of $\theta_{23}$ are the fundamental unknowns in neutrino oscillations. In order to address all these three unknowns, we study the physics reach of a setup, where we replace the antineutrino run of T2HK with antineutrinos from muon decay at rest ($\mu$-DAR). This approach has the advantages of having higher statistics in both neutrino and antineutrino modes, and lower beam-on backgrounds for antineutrino run with reduced systematics. We find that a hybrid setup consisting of T2HK ($\nu$) and $\mu$-DAR ($\bar\nu$) in conjunction with full exposure from T2K and NO$\nu$A can resolve the issue of mass hierarchy at greater than 3$\sigma$ C.L. irrespective of the choices of hierarchy, $\delta_{\mathrm{CP}}$, and $\theta_{23}$. This hybrid setup can also establish the CP-violation at 5$\sigma$ C.L. for $\sim$ 55% choices of $\delta_{\mathrm{CP}}$, whereas the same for conventional T2HK ($\nu + \bar\nu$) setup along with T2K and NO$\nu$A is around 30%. As far as the octant of $\theta_{23}$ is concerned, this hybrid setup can exclude the wrong octant at 5$\sigma$ C.L. if $\theta_{23}$ is at least $3^{\circ}$ away from maximal mixing for any $\delta_{\mathrm{CP}}$.Comment: 18 pages, 28 pdf figures, 1 table. Minor changes in the text. A new footnote added. Accepted in JHE

Texture zeros of low-energy Majorana neutrino mass matrix in 3+1 scheme

In this work we revisit the zero textures in low energy Majorana neutrino mass matrix when the active neutrino sector is extended by a light sterile neutrino in the eV scale i.e., the 3+1 scheme. In 3+1 scenario, the low energy neutrino mass matrix ($m_\nu$) has ten independent elements. Thus in principle one can have minimum one-zero texture to maximum ten-zero texture. We summarize the previous results of one, two, three and four-zero textures which already exist in the literature and present our new results on five-zero textures. In our analysis we find that among six possible five-zero textures, only one is allowed by the present data. We discuss possible theoretical model which can explain the origin of the allowed five-zero texture and discuss other possible implications of such a scenario. Our results also concludes that in 3+1 scheme, one can not have more than five-zeros in $m_\nu$.Comment: 12 pages, 3 figures. Version published in PR

Synergies between neutrino oscillation experiments: An `adequate' configuration for LBNO

Determination of the neutrino mass hierarchy, octant of the mixing angle theta_{23} and the CP violating phase delta_{CP} are the unsolved problems in neutrino oscillation physics today. In this paper our aim is to obtain the minimum exposure required for the proposed Long Baseline Neutrino Oscillation (LBNO) experiment to determine the above unknowns. We emphasize on the advantage of exploiting the synergies offered by the existing and upcoming long-baseline and atmospheric neutrino experiments in economising the LBNO configuration. In particular, we do a combined analysis for LBNO, T2K, NOvA and INO. We consider three prospective LBNO setups -- CERN-Pyhasalmi (2290 km), CERN-Slanic (1500 km) and CERN-Frejus (130 km) and evaluate the adequate exposure required in each case. Our analysis shows that the exposure required from LBNO can be reduced considerably due to the synergies arising from the inclusion of the other experiments.Comment: 22 pages, 14 figures, 2 tables Version published in JHE

Can atmospheric neutrino experiments provide the first hint of leptonic CP violation?

The measurement of a non-zero value of the 1-3 mixing angle has paved the way for the determination of leptonic CP violation. However the current generation long-baseline experiments T2K and NOvA have limited sensitivity to delta_{CP}. In this paper we show for the first time, the significance of that atmospheric neutrino experiments in providing the first hint of CP violation in conjunction with T2K and NOvA. In particular, we find that adding atmospheric neutrino data from the ICAL detector at the India-based Neutrino Observatory (INO) to T2K and NOvA results in a two-fold increase in the range of delta_{CP} values for which a 2 sigma hint of CP violation can be obtained. In fact in the parameter region unfavorable for the latter experiments, the first signature of CP violation may well come from the inclusion of atmospheric neutrino data.Comment: 5 pages, 4 figures. Version accepted for publication in Phys. Rev. D (Rapid Communication