Should T2k Run in Dominant Neutrino Mode to Detect Cp Violation ?



Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan

The main aim of the T2K experiment in Japan is to discover CP violation in the leptonic sector by measuring the Dirac phase . For that purpose T2K has already started collecting data in both neutrino and antineutrino mode. But in this work we will show that, in T2K the main role of the antineutrinos is to resolve the octant degeneracy. If the octant is known then the pure neutrino run of T2K is capable to give the maximum CP sensitivity. On the otherhand in the experiment like NOA, antineutrinos are still useful even when octant is known. Thus we propose that let T2K run in the dominant neutrino mode whereas the antineutrino component of the other experiments can resolve the octant degeneracy in T2K. As an example we show that if T2K is combined with the experiments NOA and ICAL@INO, then T2K will have the potential to discover CP violation with maximum sensitivity in the dominant neutrino mode.

1 Introduction

Neutrino oscillation is a quantum mechanical interference phenomena in which neutrinos with one flavour evolve into another flavour over microscopical distance and time. In standard three flavour scenario, mathematically neutrino oscillations can be described by three mixing angles (, , ), two mass squared differences (, ) and one Dirac type CP phase . The measurement of and comes from solar and KamLAND data whereas the parameters and are measured by the accelerator and atmospheric neutrino experiments . The The smallest mixing angle has been measured quite recently by the short-baseline reactor experiments . At present the task of the current /next generation experiments is to measure the following unknown parameters: (i) neutrino mass hierarchy i.e, so called normal hierarchy (NH) or so called inverted hierarchy (IH), (ii) the octant of the mixing angle i.e., so called lower octant (LO) or so called higher octant (HO) and (iii) the value of the Dirac CP phase . In this paper we will study the role of antineutrinos in T2K to discover CP violation in the leptonic sector.

2 The T2K experiment

T2K is a long-baseline neutrino oscillation in Japan . In this experiment muon neutrinos which are produced in the JPARC facility are detected at Kamioka after traveling a distance of 295 km. This experiment has already seen events in both neutrino and antineutrino mode of running and now it is collecting more data to establish the CP violation in the leptonic sector on a firm footing. The CP sensitivity of the T2K experiment comes from the electron neutrino appearance channel given by:


where , , , and , where eV is the Wolfenstein matter term. The above equation is for neutrinos. The probability of the antineutrinos can be obtained by replacing and . As the sign of is opposite in neutrinos and antineutrinos, we understand that it is very important to have data from antineutrino run of T2K to establish CP violation in the leptonic sector on a firm footing. But apart from that one also needs to understand how antineutrinos help in the improvement of CP sensitivity. We know that CP sensitivity of T2K is suffered by parameter degeneracy . In parameter degeneracy, two sets of oscillation parameter give rise to equal value in the neutrino oscillation probability which makes it difficult to determine neutrino oscillation parameters uniquely. In recent data, there are two types of parameter degeneracy: (a) hierarchy- degeneracy and (ii) octant- degeneracy . It is well known that hierarchy- degeneracy behaves similarly in neutrinos and antineutrinos but the dependence of octant- degeneracy in neutrinos is different than antineutrinos. Thus we expect that antineutrinos will play a very important role in resolving the octant- degeneracy. In the next section we will study how antineutrinos help in the CP sensitivity by ruling out the wrong octant solutions.

3 Sensitivity of T2K

The CP violation (CPV) discovery is defined as the capability of an experiment to distinguish a true value of other than and .

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